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  • 1.
    Arnqvist, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Bergström, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Flux-profile relation with roughness sublayer correction2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, 1191-1197 p.Article in journal (Refereed)
    Abstract [en]

    Calculation of momentum flux using Monin–Obukhov similarity theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this article is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux-profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for mesoscale modelling, scalar transport models, or sound propagation models.

  • 2. Bech, J.
    et al.
    Gjertsen, U.
    Haase, Gunther
    SMHI, Research Department, Atmospheric remote sensing.
    Modelling weather radar beam propagation and topographical blockage at northern high latitudes2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 626, 1191-1204 p.Article in journal (Refereed)
    Abstract [en]

    This paper describes a study to evaluate the variability of radio-propagation conditions and to assess their effects upon weather-radar beam blockage corrections for precipitation estimates. Radiosonde observations are examined in order to analyse the propagation conditions at several locations covered by the Nordic Weather Radar Network (NORDRAD). A beam-propagation model is used to simulate the interaction between the radar beam and the topography and to derive correction factors. The model is applied to correct yearly accumulations, assuming standard radio-propagation conditions, and is also used to examine case studies in detail under various propagation conditions. The correction reduces the bias between yearly radar precipitation estimates and gauge records by 1 dB for moderate blockages (1% to 50%), and by up to 3 dB for severe blockages (50% to 70%). The case studies indicate that HIRLAM forecasts show potential to predict the radar coverage and the associated ground- and sea-clutter patterns. This research aims at determining a beam-blockage-correction algorithm to be used within the NORDRAD quality-control system. This is particularly useful for obtaining radar precipitation estimates in environments with complex topography. Copyright (C) 2007 Royal Meteorological Society.

  • 3.
    Bengtsson, Lisa
    et al.
    SMHI, Research Department, Meteorology.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Impact of a stochastic parametrization of cumulus convection, using cellular automata, in a mesoscale ensemble prediction system2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 695, 1150-1159 p.Article in journal (Refereed)
    Abstract [en]

    A stochastic parametrization for deep convection, based on cellular automata, has been evaluated in the high-resolution (2.5 km) ensemble prediction system Hirlam Aladin Regional Mesoscale Operational NWP Ensemble Prediction System (HarmonEPS). We studied whether such a stochastic physical parametrization, whilst implemented in a deterministic forecast model, can have an impact on the performance of the uncertainty estimates given by an ensemble prediction system. Various feedback mechanisms in the parametrization were studied with respect to ensemble spread and skill, in both subgrid and resolved precipitation fields. It was found that the stochastic parametrization improves the model skill in general, by reducing a positive bias in precipitation. This reduction in bias, however, led to a reduction in ensemble spread of precipitation. Overall, scores that measure the accuracy and reliability of probabilistic predictions indicate that the net impact (improved skill, degraded spread) of the ensemble prediction system is improved for 6 h accumulated precipitation with the stochastic parametrization and is rather neutral for other quantities examined.

  • 4.
    Bengtsson, Lisa
    et al.
    SMHI, Research Department, Meteorology.
    Steinheimer, Martin
    Bechtold, Peter
    Geleyn, Jean-Francois
    A stochastic parametrization for deep convection using cellular automata2013In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, no 675, 1533-1543 p.Article in journal (Refereed)
    Abstract [en]

    A cellular automaton (CA) is introduced to the deep convection parametrization of the high-resolution limited-area model Aire Limitee Adaptation/Application de la Recherche a l'Operationnel (ALARO). The self-organizational characteristics of the CA allow for lateral communication between adjacent numerical weather prediction (NWP) model grid boxes and add additional memory to the deep convection scheme. The CA acts in two horizontal dimensions, with finer grid spacing than the NWP model. It is randomly seeded in regions where convective available potential energy (CAPE) exceeds a threshold value. Both deterministic and probabilistic rules, coupled to the large-scale wind, are explored to evolve the CA in time. Case studies indicate that the scheme has the potential to organize cells along convective squall lines and enhance advective effects. An ensemble of forecasts using the present CA scheme demonstrated an ensemble spread in the resolved wind field in regions where deep convection is large. Such a spread represents the uncertainty due to subgrid variability of deep convection and could be an interesting addition to an ensemble prediction system.

  • 5. Bromwich, David H.
    et al.
    Wilson, Aaron B.
    Bai, Le-Sheng
    Moore, George W. K.
    Bauer, Peter
    A comparison of the regional Arctic System Reanalysis and the global ERA-Interim Reanalysis for the Arctic2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 695, 644-658 p.Article in journal (Refereed)
    Abstract [en]

    The Arctic System Reanalysis version 1 (ASRv1), a high-resolution regional assimilation of model output, observations and satellite data across the mid- and high latitudes of the Northern Hemisphere, and the global European Centre for Medium Range Forecasting Interim Reanalysis (ERAI) are compared with atmospheric observations for the period December 2006 to November 2007. Results throughout the troposphere show observations to be well assimilated in the ASRv1, as monthly and annual near-surface (upper-level) temperature, dew-point (relative humidity), pressure (geopotential height) and wind-speed biases compared with surface stations and radiosondes are very small. These results are similar to the ERAI, although wind-speed biases are significantly smaller in the ASRv1. Despite the ASRv1’s use of a 3D-variational (Var) assimilation compared with the ERAI’s 4D-Var, similar results suggest that a regional approach with higher-resolution terrain and a detailed land-surface description forced by a global reanalysis may improve the assimilation of observations and help offset temporal information lost by the 3D-Var compared with the 4D-Var. However, the ASRv1 forecast field results compared with the ERAI are mixed. The ASRv1 and ERAI show negative precipitation biases during cool months compared with gauge observations, and too much precipitation falls in the ASRv1 during summer in the midlatitudes. Stations north of 60°N demonstrate smaller precipitation biases in the ASRv1 than the ERAI except during the summer, when the ASRv1 is very dry. Short-wave radiation compared with observations is much too large in the ASRv1, and both reanalyses show long-wave radiation deficits during most months. These results point to inadequacies in model physics in the ASRv1 (e.g. convective and radiation schemes) that will continue to be refined in subsequent versions of the ASR.

  • 6.
    Buehler, Stefan
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Jimenez, C.
    Evans, K. F.
    Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder.
    Eriksson, P.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Rydberg, B.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Heymsfield, A.
    NCAR, Boulder, Colorado.
    Stubenrauch, C.
    CNRS/IPSL - Laboratoire de Meteorologie Dynamique, Ecole Polytechnique, Palaiseau.
    Lohmann, U.
    ETH Zurich, Institute for Atmospheric and Climate Science.
    Emde, C.
    Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen.
    John, V. O.
    Met Office Hadley Centre, Exeter.
    Sreerekha, T. R.
    Met Office Hadley Centre, Exeter.
    Davis, C. P.
    School of Geosciences, University of Edinburgh.
    A concept for a satellite mission to measure cloud ice water path, ice particle size, and cloud altitude2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no Suppl.2, 109-128 p.Article in journal (Refereed)
    Abstract [en]

    A passive satellite radiometer operating at submillimetre wavelengths can measure cloud ice water path (IWP), ice particle size, and cloud altitude. The paper first discusses the scientific background for such measurements. Formal scientific mission requirements are derived, based on this background and earlier assessments. The paper then presents a comprehensive prototype instrument and mission concept, and demonstrates that it meets the requirements. The instrument is a conically scanning 12-channel radiometer with channels between 183 and 664 GHz, proposed to fly in tandem with one of the Metop satellites. It can measure IWP with a relative accuracy of approximately 20% and a detection threshold of approximately 2 g m-2. The median mass equivalent sphere diameter of the ice particles can be measured with an accuracy of approximately 30 µm, and the median IWP cloud altitude can be measured with an accuracy of approximately 300 m. All the above accuracies are median absolute error values; root mean square error values are approximately twice as high, due to rare outliers.

  • 7. Couvreux, F.
    et al.
    Roehrig, R.
    Rio, C.
    Lefebvre, M. -P
    Caian, Mihaela
    SMHI, Research Department, Climate research - Rossby Centre.
    Komori, T.
    Derbyshire, S.
    Guichard, F.
    Favot, F.
    D'Andrea, F.
    Bechtold, P.
    Gentine, P.
    Representation of daytime moist convection over the semi-arid Tropics by parametrizations used in climate and meteorological models2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 691, 2220-2236 p.Article in journal (Refereed)
    Abstract [en]

    A case of daytime development of deep convection over tropical semi-arid land is used to evaluate the representation of convection in global and regional models. The case is based on observations collected during the African Monsoon Multidisciplinary Analysis (AMMA) field campaign and includes two distinct transition phases, from clear sky to shallow cumulus and from cumulus to deep convection. Different types of models, run with identical initial and boundary conditions, are intercompared: a reference large-eddy simulation (LES), single-column model (SCM) version of four different Earth system models that participated in the Coupled Model Intercomparison Project 5 exercise, the SCM version of the European Centre for Medium-range Weather Forecasts operational forecast model, the SCM version of a mesoscale model and a bulk model. Surface fluxes and radiative heating are prescribed preventing any atmosphere-surface and cloud-radiation coupling in order to simplify the analyses so that it focuses only on convective processes. New physics packages are also evaluated within this framework. As the LES correctly reproduces the observed growth of the boundary layer, the gradual development of shallow clouds, the initiation of deep convection and the development of cold pools, it provides a basis to evaluate in detail the representation of the diurnal cycle of convection by the other models and to test the hypotheses underlying convective parametrizations. Most SCMs have difficulty in representing the timing of convective initiation and rain intensity, although substantial modifications to boundary-layer and deep-convection parametrizations lead to improvements. The SCMs also fail to represent the mid-level troposphere moistening during the shallow convection phase, which we analyse further. Nevertheless, beyond differences in timing of deep convection, the SCM models reproduce the sensitivity to initial and boundary conditions simulated in the LES regarding boundary-layer characteristics, and often the timing of convection triggering.

  • 8.
    Dahlgren, Per
    et al.
    SMHI, Research Department, Meteorology.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    A high-resolution regional reanalysis for Europe. Part 1: Three-dimensional reanalysis with the regional HIgh-Resolution Limited-Area Model (HIRLAM)2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 698, 2119-2131 p.Article in journal (Refereed)
  • 9. Dee, D. P.
    et al.
    Uppala, S. M.
    Simmons, A. J.
    Berrisford, P.
    Poli, P.
    Kobayashi, S.
    Andrae, U.
    Balmaseda, M. A.
    Balsamo, G.
    Bauer, P.
    Bechtold, P.
    Beljaars, A. C. M.
    van de Berg, L.
    Bidlot, J.
    Bormann, N.
    Delsol, C.
    Dragani, R.
    Fuentes, M.
    Geer, A. J.
    Haimberger, L.
    Healy, S. B.
    Hersbach, H.
    Holm, E. V.
    Isaksen, L.
    Kållberg, Per
    SMHI, Research Department, Meteorology.
    Koehler, M.
    Matricardi, M.
    McNally, A. P.
    Monge-Sanz, B. M.
    Morcrette, J. -J
    Park, B. -K
    Peubey, C.
    de Rosnay, P.
    Tavolato, C.
    Thepaut, J. -N
    Vitart, F.
    The ERA-Interim reanalysis: configuration and performance of the data assimilation system2011In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 137, no 656, 553-597 p.Article, review/survey (Refereed)
    Abstract [en]

    ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF. Copyright (C) 2011 Royal Meteorological Society

  • 10. Feser, F.
    et al.
    Barcikowska, M.
    Krueger, O.
    Schenk, Frederik
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Weisse, R.
    Xia, L.
    Storminess over the North Atlantic and northwestern Europe: A review2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 687, 350-382 p.Article, review/survey (Refereed)
    Abstract [en]

    This review assesses storm studies over the North Atlantic and northwestern Europe regarding the occurrence of potential long-term trends. Based on a systematic review of available articles, trends are classified according to different geographical regions, datasets, and time periods. Articles that used measurement and proxy data, reanalyses, regional and global climate model data on past and future trends are evaluated for changes in storm climate. The most important result is that trends in storm activity depend critically on the time period analysed. An increase in storm numbers is evident for the reanalyses period for the most recent decades, whereas most long-term studies show merely decadal variability for the last 100-150 years. Storm trends derived from reanalyses data and climate model data for the past are mostly limited to the last four to six decades. The majority of these studies find increasing storm activity north of about 55-60° N over the North Atlantic with a negative tendency southward. This increase from about the 1970s until the mid-1990s is also mirrored by long-term proxies and the North Atlantic Oscillation and constitutes a part of their decadal variability. Studies based on proxy and measurement data or model studies over the North Atlantic for the past which cover more than 100 years show large decadal variations and either no trend or a decrease in storm numbers. Future scenarios until about the year 2100 indicate mostly an increase in winter storm intensity over the North Atlantic and western Europe. However, future trends in total storm numbers are quite heterogeneous and depend on the model generation used.

  • 11. Graversen, Rune G.
    et al.
    Burtu, Mattias
    Stockholm University, Faculty of Science, Department of Meteorology .
    Arctic amplification enhanced by latent energy transport of atmospheric planetary waves2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 698, 2046-2054 p.Article in journal (Refereed)
    Abstract [en]

    The atmospheric northward energy transport plays a crucial role for the Arctic climate; this transport brings to the Arctic an amount of energy comparable to that provided directly by the sun. The transport is accomplished by atmospheric waves-for instance large-scale planetary waves and meso-scale cyclones-and the zonal-mean circulation. These different components of the energy transport impact the Arctic climate differently. A split of the transport into stationary and transient waves constitutes a traditional way of decomposing the transport. However this procedure does not take into account the transport accomplished separately by the planetary and synoptic-scale waves. Here a Fourier decomposition is applied, which decomposes the transport with respect to zonal wave numbers. Reanalysis and model data reveal that the planetary waves impact Arctic temperatures much more than do synoptic-scale waves. In addition the latent transport by these waves affects the Arctic climate more than does the dry-static part. Finally, the EC-Earth model suggests that changes of the energy transport over the twentyfirst century will contribute to Arctic warming, despite the fact that in this model the total energy transport to the Arctic will decrease. This apparent contradictory result is due to the cooling induced by a decrease of the dry-static transport by planetary waves being more than compensated for by a warming caused by the latent counterpart.

  • 12. Guemas, Virginie
    et al.
    Blanchard-Wrigglesworth, Edward
    Chevallier, Matthieu
    Day, Jonathan J.
    Deque, Michel
    Doblas-Reyes, Francisco J.
    Fuckar, Neven S.
    Germe, Agathe
    Hawkins, Ed
    Keeley, Sarah
    Koenigk, Torben
    SMHI, Research Department, Climate research - Rossby Centre.
    Salas y Melia, David
    Tietsche, Steffen
    A review on Arctic sea-ice predictability and prediction on seasonal to decadal time-scales2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 695, 546-561 p.Article in journal (Refereed)
    Abstract [en]

    Sea ice plays a crucial role in the Earth's energy and water budget and has a substantial impact on local and remote atmospheric and oceanic circulations. Predictions of Arctic sea-ice conditions a few months to a few years in advance could be of interest for stakeholders. This article presents a review of the potential sources of Arctic sea-ice predictability on these time-scales. Predictability mainly originates from persistence or advection of sea-ice anomalies, interactions with the ocean and atmosphere and changes in radiative forcing. After estimating the inherent potential predictability limit with state-of-the-art models, current sea-ice forecast systems are described, together with their performance. Finally, some challenges and issues in sea-ice forecasting are presented, along with suggestions for future research priorities.

  • 13. Guemas, Virginie
    et al.
    Garcia-Serrano, Javier
    Mariotti, Annarita
    Doblas-Reyes, Francisco
    Caron, Louis-Philippe
    SMHI, Research Department, Climate research - Rossby Centre.
    Prospects for decadal climate prediction in the Mediterranean region2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 687, 580-597 p.Article in journal (Refereed)
    Abstract [en]

    The Mediterranean region stands as one of the most sensitive to climate change, both in terms of warming and drying. On shorter time-scales, internal variability has substantially affected the observed climate and in the next decade might enhance or compensate long-term trends. Here we compare the multi-model climate predictions produced within the framework of the CMIP5 (Coupled Model Intercomparison Project Phase 5) project with historical simulations to assess the level of multi-year climate prediction skill in the Mediterranean region beyond that originating from the model accumulated response to the external radiative forcings. We obtain a high and significant skill in predicting 4-year averaged annual and summer mean temperature over most of the study domain and in predicting precipitation for the same seasons over northern Europe and sub-Saharan Africa. A lower skill is found during the winter season but still positive for temperature. Although most of this high skill originates from the model response to the external radiative forcings, the initialization contributes to the temperature skill over the Mediterranean Sea and surrounding land areas. The high and significant correlations between the observed Mediterranean temperatures and the observed Atlantic multidecadal oscillation (AMO) in the summer and annual means are captured by the CMIP5 ensemble which suggests that the added skill is related to the ability of the CMIP5 ensemble to predict the AMO. Such a link to the AMO seems restricted to western Africa and summer means only for the precipitation case.

  • 14. Guichard, F
    et al.
    Petch, J C
    Redelsperger, J L
    Bechtold, P
    Chaboureau, J P
    Cheinet, S
    Grabowski, W
    Grenier, H
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Kohler, M
    Piriou, J M
    Tailleux, R
    Tomasini, M
    Modelling the diurnal cycle of deep precipitating convection over land with cloud-resolving models and single-column models2004In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, no 604, 3139-3172 p.Article in journal (Refereed)
    Abstract [en]

    An idealized case-study has been designed to investigate the modelling of the diurnal cycle of deep precipitating convection over land. A simulation of this case was performed by seven single-column models (SCMs) and three cloud-resolving models (CRMs). Within this framework, a quick onset of convective rainfall is found in most SCMs, consistent with the results from general-circulation models. In contrast, CRMs do not predict rainfall before noon. A joint analysis of the results provided by both types of model indicates that convection occurs too early in most SCMs, due to crude triggering criteria and quick onsets of convective precipitation. In the CRMs, the first clouds appear before noon, but surface rainfall is delayed by a few hours to several hours. This intermediate stage, missing in all SCMs except for one, is characterized by a gradual moistening of the free troposphere and an increase of cloud-top height. Later on, convective downdraughts efficiently cool and dry the boundary layer (BL) in the CRMs. This feature is also absent in most SCMs, which tend to adjust towards more unstable states, with moister (and often more cloudy) low levels and a drier free atmosphere. This common behaviour of most SCMs with respect to deep moist convective processes occurs even though each SCM simulates a different diurnal cycle of the BL and atmospheric stability. The scatter among the SCMs results from the wide variety of representations of BL turbulence and moist convection in these models. Greater consistency is found among the CRMs, despite some differences in their representation of the daytime BL growth, which are linked to their parametrizations of BL turbulence and/or resolution.

  • 15.
    Gustafsson, Nils
    et al.
    SMHI, Research Department, Meteorology.
    Thorsteinsson, S.
    Stengel, M.
    Holm, E.
    Use of a nonlinear pseudo-relative humidity variable in a multivariate formulation of moisture analysis2011In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 137, no 657, 1004-1018 p.Article in journal (Refereed)
    Abstract [en]

    We present a reformulation of the humidity part of the HIRLAM (HIgh-Resolution Limited-Area Model) variational data assimilation. The purpose is to rectify some of the shortcomings of the present formulation which uses specific humidity, q, as an assimilation control variable with homogeneous and static covariances. One problem is that specific humidity forecast errors tend to have a non-Gaussian probability distribution, in particular near saturation and near zero humidity. In addition, the variance of the distribution tends to change in space and time due to the dependency of the water vapour saturation pressure on temperature. A modified pseudo-relative humidity variable has been adapted to the statistical balance background constraint, including the associated moisture balance formulation. Background-error statistics for the new moisture control variable and the moisture-related balances were derived, taking differences between forecasts valid at the same time as a proxy for background forecast errors. The background-error statistics were compared with the corresponding statistics for specific humidity as the moisture assimilation control variable. In connection with the nonlinearity of the change of the variable, it was noted that specified background-error standard deviations were chosen to be substantially reduced for nearly dry and saturated states, which can raise difficulties. The impact of the new moisture assimilation control variable is illustrated with simulated observation experiments as well as data assimilation experiments using real observations, for one summer month and one winter month in a 4D-Var assimilation cycle using two outer loop iterations in the 4D-Var minimization. The impact of the new formulation on forecast verification scores is small and essentially neutral, while using the second outer loop in the old formulation has a small positive impact. Copyright (C) 2011 Royal Meteorological Society

  • 16.
    Hanley, John
    et al.
    University College Dublin, Ireland.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Objective identification and tracking of multi-centre cyclones in the ERA-Interim reanalysis dataset2012In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, no 664, 612-625 p.Article in journal (Refereed)
    Abstract [en]

    We present a novel cyclone identification and tracking method that explicitly recognizes multicentre cyclones (MCCs), defined as a cyclonic system with two or three sea-level pressure minima within its outermost contour. The method allows for the recognition of cyclone merger and splitting events in a natural way, and provides a consistent measure of the cyclone extent. Using the ERA-Interim reanalysis dataset, we compute a climatology using this method and show that MCCs occur in about 32 of all cyclone tracks and are much more prevalent in more intense storms. We also show that the method permits reconnection of tracks that would have been spuriously split using a conventional method. We present spatial maps of cyclone mergers, splitting, genesis and lysis using the method and also compute statistics of precipitation falling within cyclones, showing that it is strongly concentrated in the most intense cyclones.

  • 17.
    Hannachi, Abdelwaheb
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Woollings, T.
    Fraedrich, K.
    The North Atlantic jet stream: a look at preferred positions, paths and transitions2012In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, no 665, 862-877 p.Article in journal (Refereed)
    Abstract [en]

    Preferred jet stream positions and their link to regional circulation patterns over the winter North Atlantic/European sector are investigated to corroborate findings of multimodal behaviour of the jet positions and to analyse patterns of preferred paths and transition probabilities between jet regimes using ERA-40 data. Besides the multivariate Gaussian mixture model, hierarchical clustering and data image techniques are used for this purpose. The different approaches all yield circulation patterns that correspond to the preferred jet regimes, namely the southern, central and the northern positions associated respectively with the Greenland anticyclone or blocking, and two opposite phases of an East Atlantic-like flow pattern. Growth and decay patterns as well as preferred paths of the system trajectory are studied using the mixture model within the delay space. The analysis shows that the most preferred paths are associated with central to north and north to south jet stream transitions with a typical time-scale of about 5 days, and with life cycles of 12 weeks. The transition paths are found to be consistent with transition probabilities. The analysis also shows that wave breaking seems to be the dominant mechanism behind Greenland blocking.

  • 18. Harnik, Nili
    et al.
    Dritschel, David G.
    Heifetz, Eyal
    Stockholm University, Faculty of Science, Department of Meteorology . Tel Aviv University, Israel.
    On the equilibration of asymmetric barotropic instability2014In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 140, no 685, 2444-2464 p.Article in journal (Refereed)
    Abstract [en]

    The conjunction of turbulence, waves and zonal jets in geophysical flows gives rise to the formation of potential vorticity staircases and to the sharpening of jets by eddies. The effect of eddies on jet structure, however, is fundamentally different if the eddies arise from barotropic rather than from baroclinic instability. As is well known, barotropic instability may occur on zonal jets when there is a reversal of potential vorticity gradients at the jet flanks. In this article we focus on the nonlinear stages of this instability and its eventual saturation. We consider an idealized initial state consisting of an anticyclonic potential vorticity strip sitting in the flanks of an eastward jet. This asymmetric configuration, a generalization of the Rayleigh problem, is one of the simplest barotropic jet configurations which incorporates many fundamental aspects of real flows, including linear instability and its equilibration, nonlinear interactions, scale cascades, vortex dynamics, and jet sharpening. We make use of the simplicity of the problem to conduct an extensive parameter sweep, and develop a theory relating the properties of the equilibrated flow to the initial flow state by considering the marginal stability limit, together with conservation of circulation and wave activity.

  • 19.
    Jimenez, Carlos
    et al.
    Observatoire de Paris, Laboratoire d'Etudes de Rayonnement et de la Matire en Astro-physique, Centre National de la Recherche Scientifique, Paris.
    Buehler, Stefan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Eriksson, Patrick
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Evans, K. F.
    Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder.
    Performance simulations for a submillimetre-wave satellite instrument to measure cloud ice2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no Suppl.2, 129-149 p.Article in journal (Refereed)
    Abstract [en]

    The performance of a conically scanning satellite instrument for the measurement of cloud ice was studied. The instrument measures radiances in 12 channels placed around the 183, 325 and 448 GHz water vapour lines and the 243, 664 and 874 GHz window channels, and is designed to provide estimations of ice water path (IWP), the equivalent sphere diameter (DME), and the median ice mass height (ZME). Overall median relative errors of around 20% for IWP, 33 µm for DME, and 240 m for ZME for a midlatitude winter scenario, and 17% for IWP, 30 µm for DME, and 310 m for ZME for a tropical scenario were found. Detection limits (relative retrieval error reaching 100%) of around 2 gm-2 were estimated for both scenarios. The performance of a five-receiver instrument, where either the 664 or 874 GHz channel is dropped, was close, but with increased errors for very thin and high clouds. A trade-off between having the 874 GHz receiver or two infrared channels at 10.7 and 12 µm emerged, as very similar performance was found between the six-receiver instrument and the five-receiver instrument with the infrared channels. Another trade-off between receiver selection and noise was also apparent, with some of the four-receiver selections operating at half noise levels being able to compete with the standard six-receiver instrument. Dual-polarized measurements were also tested, but they did not significantly improve the retrievals of IWP or DME.

  • 20.
    John, Viju Oommen
    et al.
    University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami.
    Buehler, Stefan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Engeln, A. von
    EUMETSAT, Darmstadt.
    Eriksson, P.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Kuhn, Thomas
    University of Köln, Physikalisches Institut.
    Brocard, E.
    University of Bern, Institue of Applied physics.
    Koenig-Langlo, G.
    Alfred Wegener Institute for Polar and Marine Research, Bremerhaven.
    Understanding the variability of clear-sky outgoing long-wave radiation based on ship-based temperature and water vapour measurements2006In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 132, no 621, 2675-2691 p.Article in journal (Refereed)
    Abstract [en]

    High-resolution radiative transfer model calculations with the Atmospheric Radiative Transfer Simulator (ARTS) were used to simulate the clear-sky outgoing long-wave radiative flux (OLR) at the top of the atmosphere. The unique set of radiosonde data collected by the research vessel Polarstern of the Alfred Wegener Institute for Polar and Marine Research during 27 expeditions in the years 1982 to 2003 was used to investigate the sources of clear-sky OLR variability for ocean areas in different climate zones and seasons. For this dataset, tropospheric temperature variations contribute approximately 33 W m(-2) OLR variability. tropospheric relative humidity variations 8.5 W m(-2), and vertical structure 2.3-3.4 W m(-2). Of these, 0.3-1.0 W m(-2) are due to structures on a vertical scale smaller than 4 km, which cannot be resolved by conventional remote-sensing instruments. It was also found that the poor absolute accuracy of current humidity data in the upper troposphere, approximately 40% relative error in relative humidity, leads to a significant uncertainty in OLR of about 3.8 W m(-2) (for a midlatitude summer atmosphere), which should be put in the context of the double CO2 effect of only 2.6 W m(-2) (for the same atmosphere).

  • 21.
    Kahnert, Michael
    et al.
    SMHI, Research Department, Air quality.
    Nousiainen, T.
    Raisanen, P.
    Mie simulations as an error source in mineral aerosol radiative forcing calculations2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 623, 299-307 p.Article in journal (Refereed)
    Abstract [en]

    The role of aerosols remains a major uncertainty for climate and climate change. For the direct radiative forcing by mineral aerosols, the uncertainty in the refractive index in has been regarded as the most important error source, while the impact of aerosol non-sphericity has been considered a minor issue and is neglected in climate models. Here, the errors caused by the spherical particle approximation (SPA) are evaluated by comparing radiative fluxes based on (i) Mie simulations and (ii) laboratory measurements of aerosol optical properties. Furthermore, they are contrasted with the errors related to the uncertainty in the refractive index. These two error sources are found to be of comparable magnitude, although they are strongly dependent on optical depth, surface albedo, and particle size. Thus, our results provide evidence that, contrary to common beliefs, the use of spherical model particles in radiative transfer simulations is probably among the major sources of error in quantifying the climate forcing effect of mineral aerosols. This stems from misrepresentation of the scattering phase function and the asymmetry parameter. Aerosol single-scattering computations based on non-spherical model particles are expected to reduce the shape-related errors and thus significantly improve the accuracy of radiative forcing simulations. Copyright (c) 2007 Royal Meteorological Society.

  • 22. Kral, ST
    et al.
    Sjöblom, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Nygård, T
    Observations of summer turbulent surface fluxes in a High Arctic fjord2014In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 140, 666-675 p.Article in journal (Refereed)
    Abstract [en]

    The summer atmospheric boundary layer over a fjord in the High Arctic has beeninvestigated during three consecutive years (2008–2010). Measurements of turbulentsurface layer fluxes of momentum and sensible heat using a sonic anemometer and slowresponseinstruments were taken from a tower on the coast of Isfjorden, Svalbard, andanalysed for seasonal variability and differing fetch conditions. The results resembled theatmospheric boundary layer characteristics previously found for ice-free winter conditions.The momentum flux was usually directed downwards, but for low wind speeds there was apossible contribution of swell, giving an upward directed momentum flux. The cross-windcomponent of the momentum flux sometimes contributed significantly to the total flux. Thesensible heat flux was very dependent on the origin of air, whether it had a long over-waterfetch, or originated from land areas with or without glaciers. In addition to non-stationarityof the flow due to the influence of the fjord’s shape and its surrounding topography, lowwind speeds questioned the validity of the Monin–Obukhov similarity theory, whereas theover-water fetch and off-wind angle were of minor importance. Moreover, some results,especially the off-wind angle, were very sensitive to how the raw data were treated, inparticular which coordinate rotation method was employed, the double rotation or theplanar fit method.

  • 23.
    Lahoz, William A.
    et al.
    University of Reading, Data assimililation research centre.
    Buehler, Stefan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Legras, Bernard
    Laboratoire de Météorologie Dynamique, Paris.
    The COST 723 Action2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no S2, 99-108 p.Article in journal (Refereed)
    Abstract [en]

    An overview is provided of the COST 723 Action, Data Exploitation and Modelling of the Upper Troposphere and Lower Stratosphere. The three working groups are introduced and a summary of Action activities within them is provided. The achievements of the Action are: three international workshops; the LAUTLOS humidity measurement campaign; dedicated meetings to discuss the quality of upper troposphere/lower stratosphere ozone and humidity measurements; two journal special issues; more than 90 papers in the peer-reviewed literature; one international summer school; and a successor COST Action which builds on COST 723. The recommendations made are: for COST to continue to support the short-term scientific missions instrument, as they are perceived to be value for money; to encourage the use of COST money to increase links between COST Actions and other scientific communities; and for the COST secretariat to recommend that Actions consider a summer school instead of a final workshop or meeting.

  • 24.
    Landelius, Tomas
    et al.
    SMHI, Research Department, Atmospheric remote sensing.
    Dahlgren, Per
    SMHI, Research Department, Meteorology.
    Gollvik, Stefan
    SMHI, Research Department, Meteorology.
    Jansson, A.
    Olsson, Esbjörn
    SMHI, Research Department, Meteorology.
    A high-resolution regional reanalysis for Europe. Part 2: 2D analysis of surface temperature, precipitation and wind2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 698, 2132-2142 p.Article in journal (Refereed)
  • 25. Lenderink, G
    et al.
    Siebesma, A P
    Cheinet, S
    Irons, S
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Marquet, P
    Muller, F
    Olmeda, D
    Calvo, J
    Sanchez, E
    Soares, P M M
    The diurnal cycle of shallow cumulus clouds over land: A single-column model intercomparison study2004In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, no 604, 3339-3364 p.Article in journal (Refereed)
    Abstract [en]

    An intercomparison study for single-column models (SCMs) of the diurnal cycle of shallow cumulus convection is reported. The case, based on measurements at the Atmospheric Radiation Measurement program Southern Great Plains site on 21 June 1997, has been used in a large-eddy simulation intercomparison study before. Results of the SCMs reveal the following general deficiencies: too large values of cloud cover and Cloud liquid water, unrealistic thermodynamic profiles, and high amounts of numerical noise. Results are also strongly dependent on vertical resolution. These results are analysed in terms of the behaviour of the different parametrization schemes involved: the convection scheme, the turbulence scheme, and the cloud scheme. In general the behaviour of the SCMs can be grouped in two different classes: one class with too strong mixing by the turbulence scheme, the other class with too strong activity by the convection scheme. The coupling between (subcloud) turbulence and the convection scheme plays a crucial role. Finally, (in part) motivated by these results several models have been successfully updated with new parametrization schemes and/or their present schemes have been successfully modified.

  • 26.
    Leung, Wing -Y. H.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Savre, J.
    Bender, Frida A. -M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Komppula, M.
    Portin, H.
    Romakkaniemi, S.
    Sedlar, J.
    Noone, Kevin
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sensitivity of a continental night-time stratocumulus-topped boundary layer to varying environmental conditions2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 700, 2911-2924 p.Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulation of a nocturnal stratocumulus-topped boundary layer in a continental midlatitude environment has been performed to examine the sensitivity of the cloud to a number of different environmental parameters. The simulations showed that the stratocumulus cloud was strongly affected by the presence of an overlying free tropospheric cirrus cloud (FTC), in agreement with previous studies of marine nighttime stratocumulus. When introducing an FTC with an optical thickness of 2, stratocumulus liquid water path decreased by 30%. Enhancing the optical thickness of the FTC to 8 further decreased the liquid water path by almost 10%. The presence of an FTC decreased the cloud-top radiative cooling which decreased the turbulent mixing in the boundary layer, so that the liquid water content and cloud depth were reduced. The sensitivity of the stratocumulus cloud to an overlying FTC was found to be affected by the moisture content in the free troposphere. When a clear positive or negative moisture gradient above the inversion was imposed, and an overlying FTC with an optical thickness of 8 was introduced, the stratocumulus cloud LWP decreased by more than 40%. Furthermore, the effect of changes in free tropospheric moisture content and an overlying FTC on the stratocumulus cloud properties was found to be nonlinear; the combined response was in general weaker than the two responses added together. The modeled response to changes in cloud condensation nuclei (CCN) concentrations was found to be non-significant, unless the CCN concentrations were so low that drizzle was induced (similar to 50 cm(-3))

  • 27.
    Linders, Torsten
    et al.
    Institutionen för geovetenskaper, Göteborgs Universitet.
    Saetra, Øyvind
    Meteorologisk Institutt, Norge.
    Bracegirdle, Thomas J.
    British Antarctic Survey, Cambridge, UK.
    Limited polar low sensitivity to sea-surface temperature2011In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 137, no 654, 58-69 p.Article in journal (Refereed)
  • 28.
    Lindskog, Magnus
    et al.
    SMHI, Research Department, Meteorology.
    Dee, Dick
    Tremolet, Yannick
    Andersson, Erik
    Radnoti, Gabor
    Fisher, Mike
    A weak-constraint four-dimensional variational analysis system in the stratosphere2009In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 135, no 640, 695-706 p.Article in journal (Refereed)
    Abstract [en]

    A weak-constraint four-dimensional variational (4D-Var) analysis system designed to correct stratospheric model errors has been evaluated. Verifications against upper-level radiosonde temperature observations and Stratospheric Sounding Unit (SSU) radiance data show that the addition of a weak constraint in the stratosphere call greatly reduce analysis bias. Both single-observation analysis experiments and extended assimilations have been performed to help us understand the impact of the model error covariance specifications required for the weak-constraint formulation. It is found that the use of multivariate balance constraints similar to those implemented in background-error covariances can be problematic. Copyright (C) 2009 Royal Meteorological Society

  • 29.
    Mahfouf, J.-F.
    et al.
    CNRM–GAME, Météo-France and CNRS.
    Birman, C.
    CNRM–GAME, Météo-France and CNRS.
    Aires, F.
    Estellus, Paris.
    Prigent, C.
    L'Observatoire de Paris-LERMA.
    Orlandi, E.
    University of Cologne.
    Milz, Mathias
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Information content on temperature and water vapour from a hyper-spectral microwave sensor2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 693, 3268-3284 p.Article in journal (Refereed)
    Abstract [en]

    This study examines the information content on atmospheric temperature and humidity profiles that could be provided by a future spaceborne microwave sensor with a few hundred radiances in the millimetre and submillimetre spectral domains (ranging from 7–800 GHz). A channel selection method based on optimal estimation theory is undertaken, using a database of profiles with associated errors from the European Centre for Medium-Range Weather Forecasts (ECMWF) numerical weather prediction model and the radiative transfer model Atmospheric Radiative Transfer Simulator (ARTS) under clear-sky conditions. The main results indicate that, by increasing the number of channels within the oxygen absorption band around 60 GHz and within the water-vapour absorption band at 183 GHz, the accuracy of temperature and humidity retrievals in the troposphere and stratosphere (for temperature) would be noticeably improved compared with present and planned microwave radiometers. The channels located in the absorption lines at 118 GHz and above 200 GHz do not bring significant additional information regarding atmospheric profiles under clear-sky conditions, partly due to greater radiometric noise. With a set of 137 selected channels that contribute to 90% of the total information content (measured by the degree of freedom for signal), it is possible to achieve almost the same performance in terms of variance error reduction as with 276 candidate channels. Sensitivity studies of various prescribed quantities defining the channel selection have been undertaken, in order to check the robustness of the conclusions. They show that none of the choices modifies the above findings.

  • 30.
    Mauritsen, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Enger, Leif
    On the use of shear-dependent turbulent length-scales2008In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 134, no 631, 539-540 p.Article in journal (Refereed)
    Abstract [en]

    We investigate the behaviour of the recently proposed shear-dependent turbulent length-scale in the neutrally stratified surface layer. It is found that close to ground, this shear-dependent length-scale is proportional to height, and thus, equivalent to the law of the wall. We speculate that the law of the wall could be just a special case of applying the shear-length to near-surface flows. 

  • 31. Mauritsen, Thorsten
    et al.
    Enger, Leif
    Uppsala University, Disciplinary Domain of Science and Technology, För teknisk-naturvetenskapliga fakulteten gemensamma enheter, International Science Programme (ISP).
    On the use of shear-dependent turbulent length-scales2008In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, Vol. 134, no 631, 539-540 p.Article in journal (Refereed)
    Abstract [en]

    We investigate the behaviour of the recently proposed shear-dependent turbulent length-scale in the neutrally stratified surface layer. It is found that close to ground, this shear-dependent length-scale is proportional to height, and thus, equivalent to the law of the wall. We speculate that the law of the wall could be just a special case of applying the shear-length to near-surface flows.

  • 32. Megner, L.
    et al.
    Tan, D. G. H.
    Körnich, Heiner
    SMHI, Research Department, Meteorology.
    Isaksen, L.
    Horanyi, A.
    Stoffelen, A.
    Marseille, G. -J
    Linearity aspects of the ensemble of data assimilations technique2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 687, 426-432 p.Article in journal (Refereed)
    Abstract [en]

    We examine the linearity of the Ensemble of Data Assimilations (EDA) technique with respect to the amplitude of the applied observation perturbations. We provide explicit examples to assess the linear relationship between such modifications of the observing system and the resulting changes in the EDA ensemble spread. The results demonstrate that, for a state-of-the-art numerical weather prediction (NWP) system, such linearity between the applied observation perturbations and the EDA ensemble spread holds well for temporal and spatial regimes relevant to global medium-range weather prediction: specifically, for forecast lead-times of up to approximately 5 days, in the vertical throughout the troposphere up to the lower and middle stratosphere and for broad horizontal scales.

  • 33.
    Megner, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tan, D. G. H.
    Kornich, H.
    Isaksen, L.
    Horanyi, A.
    Stoffelen, A.
    Marseille, G. -J
    Linearity aspects of the ensemble of data assimilations technique2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 687, 426-432 p.Article in journal (Refereed)
    Abstract [en]

    We examine the linearity of the Ensemble of Data Assimilations (EDA) technique with respect to the amplitude of the applied observation perturbations. We provide explicit examples to assess the linear relationship between such modifications of the observing system and the resulting changes in the EDA ensemble spread. The results demonstrate that, for a state-of-the-art numerical weather prediction (NWP) system, such linearity between the applied observation perturbations and the EDA ensemble spread holds well for temporal and spatial regimes relevant to global medium-range weather prediction: specifically, for forecast lead-times of up to approximately 5 days, in the vertical throughout the troposphere up to the lower and middle stratosphere and for broad horizontal scales.

  • 34.
    Messori, Gabriele
    et al.
    Department of Physics, Imperial College London, UK.
    Czaja, A.
    Department of Physics, Imperial College London, UK.
    On local and zonal pulses of atmospheric heat transport in reanalysis data2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 691, 2376-2389 p.Article in journal (Refereed)
    Abstract [en]

    The present study analyses large values (or pulses) of local and zonally integrated meridional atmospheric heat transport due to transient eddies. The data used is the European Centre for Medium-range Weather Forecasts ERA-Interim reanalysis with daily, 0.7 degrees latitude and longitude resolution. The domain of interest is the extratropics. First, the circulation associated with local pulses of heat transport is described. This is found to match many of the features found in warm conveyor belts, although important regional differences exist. The large values of heat transport are seen to be associated with co-varying meridional velocity and moist static energy anomalies. Next, it is shown that there exist strong pulses of meridional heat transport when a zonal integral around a given latitude circle is considered. These zonal pulses are only partly driven by the synchronized occurrence of a large number of local pulses. The existence of such pronounced variability in zonally integrated meridional heat transport can have important consequences for the energy balance of the high latitudes.

  • 35.
    Michelson, Daniel
    et al.
    SMHI, Core Services.
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Landelius, Tomas
    SMHI, Research Department, Atmospheric remote sensing.
    Collier, C G
    Haase, Gunther
    SMHI, Research Department, Atmospheric remote sensing.
    Heen, M
    'Down-to-Earth' modelling of equivalent surface precipitation using multisource data and radar2005In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 131, no 607, 1093-1112 p.Article in journal (Refereed)
    Abstract [en]

    The estimation of surface rainfall from reflectivity data derived from weather radar has been much studied over many years. It is now clear that central to this problem is the adjustment of these data for the impacts of vertical variations in the reflectivity. In this paper a new procedure (known as Down-to-Earth, DTE) is proposed and tested for combining radar measurements aloft with information from a numerical weather-prediction (NWP) model and an analysis system. The procedure involves the exploitation of moist cloud physics in an attempt to account for physical processes impacting on precipitation during its descent from the height of radar echo measurements to the surface. The application of DTE leads to increased underestimation in the radar measurements compared to precipitation gauge observations at short and intermediate radar ranges (0-120 km), but is successful at reducing the bias at further ranges. However the application of DTE does not lead to significant decreases in the random error of the surface rain rate estimate. No improvement is made when attempting to account for the precipitation phase measured by radar. It is concluded that further work on radar data quality control, along with improvements to the NWP model, are essential to improve upon results using such a physically based procedure.

  • 36.
    Nilsson, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Rutgersson, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Sullivan, Peter
    Smedman, Ann-Sofi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Convective boundary-layer structure in the presence of wind-following swell2012In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 138, no 667, 1476-1489 p.Article in journal (Refereed)
    Abstract [en]

    The marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area, where they were initiated by momentum transferred to the ocean wave field during storms. The atmospheric boundary layer during wind-following swell and various stability states has been investigated using large-eddy simulation (LES) data. The dominant energy-containing motions in the near-neutral atmospheric boundary layer over flat terrain are known to be dominated by near-ground shear-induced regions of high- and low-speed flow. Windfields and momentum fluxes from LES for swell-dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves. Statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary-layer winds and fluxes. A mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of LES data. Coherent detached eddies from the directly wave-induced motions near the surface are found to maintain an upward momentum transfer. This mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions. In this way, it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell. This has implication for the turbulence length-scales during wind-following swell.

  • 37.
    Nilsson, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Sahlée, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Rutgersson, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Turbulent momentum flux characterisation using extended multiresolution analysis2014In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 140, 1715-1728 p.Article in journal (Refereed)
    Abstract [en]

    The variability of turbulent momentum flux in neutral and unstable atmospheric boundary layers is characterised by analysing surface-layer measurements and data from large-eddy simulations (LES). The method involves multiresolution (MR) decomposition of vertical wind and advected variables into eddy fluctuations on different scales. It provides a measure of the amount of flux variability that stems from same-scale correlations and from combinations of different scale eddy fluctuations. Combining two analysis methods enabled MR component cospectra to be introduced in order to study the contribution of downward and upward flux on different scales. These component cospectra were used to investigate at which scales most of the upward and downward momentum flux occurs. By using MR spectra, cospectra, and flow visualisation this investigation provides insights into turbulence structure and fluxes in neutral and unstable stratification. It is shown that most of the flux variability in the lower part of the boundary layer can be characterised as a combination of larger scale streamwise elongated horizontal wind streaks and smaller scale vertical wind fluctuations. These streaks are found to account for a large part of downward momentum flux at relatively large, energy-containing scales. Most of the upward momentum flux is found to occur at smaller scales. This can be interpreted as showing that upward momentum flux in these conditions is caused by the generation of smaller scale secondary motions when larger scale turbulence elements break down and dissipate. Differences in the height dependence of turbulence structure and momentum flux for neutral and unstably stratified conditions are also investigated and related to the existence of wind streaks and horizontal rolls in these different conditions.

  • 38. Orsolin, Yvan J.
    et al.
    Nikulin, Grigory
    Umeå University, Faculty of Science and Technology, Space Science.
    A low-ozone episode during the European heat wave of August 20032006In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 132, no 615, 667-680 p.Article in journal (Refereed)
    Abstract [en]

    An intense low-ozone episode (LOE) was observed over Scandinavia and the North Sea in the middle of August 2003. The LOE occurred under exceptional meteorological conditions, associated with a severe heatwave over Europe. The column ozone minimum for summer (June, July and August) 2003 was reached during the event.

    Using meteorological analyses, satellite ozone observations from the Total Ozone Mapping Spectrometer and the Michelson Interferometer for Passive Atmospheric Sounding aboard the Environment Satellite, we demonstrate that the LOE results from the conjunction of a deep tropospheric blocking over Europe, and a displaced Arctic pool of low-ozone air in the stratosphere, above the anticyclone. The anticyclonic anomaly is part of a Rossby wave train that is apparent throughout the troposphere, and whose influence is felt up to 50 hPa. In the mid-stratosphere (e.g. 30 hPa) long-period westward propagating planetary waves dominate, with a ridge extending over northern Europe in mid-August.

    We band-pass filtered the geopotential field to isolate sub-monthly fluctuations, and calculated three-dimensional wave activity fluxes for quasi-stationary, quasi-geostrophic disturbances embedded in a zonally asymmetric basic state. Results clearly indicate that upward wave fluxes in the lower stratosphere originate from the Atlantic sector, upstream of the maturing blocking.

  • 39. Orsolini, Yvan J.
    et al.
    Karpechko, Alexey Yu.
    Nikulin, Grigory
    SMHI, Research Department, Climate research - Rossby Centre.
    Variability of the Northern Hemisphere polar stratospheric cloud potential: The role of North Pacific disturbances2009In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 135, no 641, 1020-1029 p.Article in journal (Refereed)
    Abstract [en]

    The potential of the Arctic stratosphere to sustain the formation of Polar Stratospheric Clouds (PSCs) is a key factor in determining the amount of ozone destroyed each winter, and is often measured as a 'PSC volume'. The latter quantity has been shown to closely follow a near-linear compact relationship with winter-averaged column ozone loss, and displays a high variability from monthly to decadal time-scales. We examine the connection between meteorological conditions in the troposphere and the variability of lower polar stratospheric temperatures over the last four decades, and specifically, conditions leading to a high PSC volume. In addition to the well-established connection between the North Atlantic Oscillation (NAO) and the polar vortex, we demonstrate the large influence of precursory disturbances over the North Pacific and the Far East, the region of maximum climatological upward wave activity flux. Namely, very high monthly PSC volume (in the top 12%) predominantly follows the development of positive tropospheric height anomalies over the Far East, which lead to a weakening of the background planetary wave trough, and lessened upward wave activity flux into the stratosphere. Precursory anomalies over the Far East are reminiscent of East Asian monsoon amplification episodes. Copyright (C) 2009 Royal Meteorological Society

  • 40.
    Persson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Is the Coriolis effect an "optical illusion'?2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 690, 1957-1967 p.Article in journal (Refereed)
    Abstract [en]

    The difference between the derivations of the Coriolis effect on a rotating turntable and on the rotating Earth is discussed. In the latter case a real force, the component of the earth's gravitational attraction, non-parallel to the local vertical, plays a central role by balancing the centrifugal force. That a real force is involved leaves open, not only the question on the inertial nature of the inertial oscillations', but also the way we tend to physically conceptualize the terrestrial Coriolis effect.

  • 41. Raisanen, P.
    et al.
    Haapanala, P.
    Chung, C. E.
    Kahnert, Michael
    SMHI, Research Department, Air quality.
    Makkonen, R.
    Tonttila, J.
    Nousiainen, T.
    Impact of dust particle non-sphericity on climate simulations2013In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, no 677, 2222-2232 p.Article in journal (Refereed)
    Abstract [en]

    Although mineral aerosol (dust) particles are irregular in shape, they are treated as homogeneous spheres in climate model radiative transfer calculations. Here, we test the effect of dust particle non-sphericity in the ECHAM5.5-HAM2 global aerosol-climate model. The short-wave optical properties of the two insoluble dust modes in HAM2 are modelled using an ensemble of spheroids that has been optimized to reproduce the optical properties of dust-like aerosols, thereby providing a significant improvement over spheres. First, the direct radiative effects (DRE) of dust non-sphericity were evaluated diagnostically, by comparing spheroids with both volume-equivalent and volume-to-area (V/A) equivalent spheres. In the volume-equivalent case, the short-wave DRE of insoluble dust at the surface and at the top of the atmosphere (TOA) was slightly smaller (typically by 3-4%) for spheroidal than for spherical dust particles. This rather small difference stems from compensating non-sphericity effects on the dust optical thickness and asymmetry parameter. In the V/A-equivalent case, the difference in optical thickness was virtually eliminated and the DRE at the TOA (surface) was approximate to 20% (approximate to 13%) smaller for spheroids than for spheres, due to a larger asymmetry parameter. Even then, however, the global-mean DRE of non-sphericity was only 0.055 W m(-2) at the TOA and 0.070 W m(-2) at the surface. Subsequently, the effects of dust non-sphericity were tested interactively in simulations in which ECHAM5.5-HAM2 was coupled to a mixed-layer ocean model. Consistent with the rather small radiative effects noted above, the climatic differences from simulations with spherical dust optics were generally negligible.

  • 42.
    Rydberg, Bengt
    et al.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Eriksson, Patrick
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Buehler, Stefan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Prediction of cloud ice signatures in submillimeter emission spectra by means of ground-based radar and in-situ microphysical data2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no Suppl.2, 151-162 p.Article in journal (Refereed)
    Abstract [en]

    Submillimetre down-looking radiometry is a promising technique for global measurements of cloud ice properties. There exist no observation data of sufficient size that can be used for detailed pre-launch studies of such an instrument and other means must be found to obtain data to optimise the instrument design and similar tasks. Several aspects of the observations make traditional retrieval methods not suitable and nonlinear multidimensional regression techniques (e.g. Bayesian Monte Carlo integration and neural networks) must be applied. Such methods are based on a retrieval database and to be successful the database must mimic relevant real conditions closely. A method to generate such databases of high quality is described here. Correct vertical distributions of cloud ice are obtained by basic data from ground-based radars. Cloud ice particle microphysical properties are generated randomly where statistical parameters are selected to mimic in situ measurement data closely. Atmospheric background fields from ECMWF are perturbed to account for variation on sub-grid scales. All these data, together with sensor characteristics, are fed into a state-of-the-art radiative transfer simulator (ARTS). The method was validated by a successful comparison with AMSU data.

  • 43.
    Savre, Julien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-eddy simulations of an Arctic mixed-phase stratiform cloud observed during ISDAC: sensitivity to moisture aloft, surface fluxes and large-scale forcing2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, 1177-1190 p.Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulation (LES) is used to examine the complex interactions between cloud properties and boundary-layer structure in Arctic low-level mixed-phase clouds using idealised conditions based on the Indirect and Semi-Direct Aerosol Campaign (ISDAC, April 2008). The persistence of steady mixed-phase conditions depends mostly on a balance between ice vertical redistribution and ice growth by vapour deposition in such a way that ice crystals cannot accumulate within the cloud layer to consume the available liquid water. An external source of water vapour is necessary to balance the net sink of total water in the cloud layer. Two main local sources of moisture are present: the initial moist surface layer and the free troposphere. In the studied case, the surface layer is found to be the dominant source of vapour to the cloud, the temperature inversion preventing significant entrainment from above. In most of the cases, the simulated boundary layer becomes rapidly well-mixed despite the stabilising effect of ice sublimation and latent cooling close to the surface. The minor effect of near-surface latent cooling on stability is connected to the initially moist surface layer limiting ice sublimation. Water vapour supply in the sub-cloud layer, resulting from entrainment of moisture from aloft, reduces ice sublimation above the surface layer and contributes to the maintenance of some degree of boundary-layer decoupling. In contrast, moisture surface fluxes reduce sublimation in the surface layer and accelerate cloud-surface coupling. Overall, the persistence of cloud-surface decoupling remains mostly driven by large-scale heat and moisture advection.

  • 44. Siebesma, A P
    et al.
    Jakob, C
    Lenderink, G
    Neggers, R A J
    Teixeira, J
    Van Meijgaard, E
    Calvo, J
    Chlond, A
    Grenier, H
    Jones, Colin
    SMHI, Research Department, Climate research - Rossby Centre.
    Kohler, M
    Kitagawa, H
    Marquet, P
    Lock, A P
    Muller, F
    Olmeda, D
    Severijns, C
    Cloud representation in general-circulation models over the northern Pacific Ocean: A EUROCS intercomparison study2004In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 130, no 604, 3245-3267 p.Article in journal (Refereed)
    Abstract [en]

    The EUROCS (EUROpean Cloud Systems study) project aims to improve the treatment of cloud systems in regional and global climate and weather prediction models. This paper reports an intercomparison study of cloud representation over the Pacific Ocean for nine climate and weather prediction models. The study consists of an analysis of a June/July/August 1998 period along an idealized trajectory over the Pacific Ocean that encompasses both the ascending and descending branch of the Hadley circulation. The three cloud types that are studied by EUROCS, stratocumulus, shallow cumulus and deep-convective cloud systems, do all occur in a persistent and geographically separated way, along this trajectory. The main focus of this study is on processes related to the hydrological cycle within the Hadley circulation. These include the large-scale dynamics (i.e. the strength of the up- and downwelling branches of the Hadley cell), the cloud processes (i.e. cloud cover, cloud amounts and precipitation), and the impact of these processes on the radiation budget both at the top of the atmosphere and at the ocean's surface. In order to make a quantitative assessment, special care has been taken to select reliable observational datasets. The main conclusions are that (1) almost all models strongly underpredicted both cloud cover and cloud amount in the stratocumulus regions while (2) the situation is opposite in the trade-wind region and the tropics where cloud cover and cloud amount are overpredicted by most models. These deficiencies result in an overprediction of the downwelling surface short-wave radiation of typically 60 W m(-2) in the stratocumulus regimes and a similar underprediction of 60 W m(-2) in the trade-wind regions and in the intertropical convergence zone (ITCZ). Similar biases for the short-wave radiation were found at the top of the atmosphere, while discrepancies in the outgoing long-wave radiation are most pronounced in the ITCZ.

  • 45.
    Smedman, Ann-Sofi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Högström, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Hunt, Julian C. R.
    Sahleé, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Heat/mass transfer in the slightly unstable atmospheric surface layer2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 622, 37-51 p.Article in journal (Refereed)
    Abstract [en]

    In very slightly unstable conditions, when the Obukhov length is much greater than the surface layer depth, it is observed that the structure of the surface layer turbulence does not accord with standard similarity theory. In particular the efficiency of the turbulent exchange of sensible and latent heat is observed to be more strongly enhanced than is consistent with the standard model. Also the profiles of dissipation of turbulent kinetic energy and temperature fluctuation variance are found to depend on the structure of the whole boundary layer (i.e. are non-local), indicating that a large-scale transport process is at work. At the same time, co-spectral analysis shows how the large-scale eddy motions that determine the heat transport process near the surface are typically 1/5 of the surface layer depth. All these features are found to be similar in measurements at a marine site and at a flat land site, indicating that they are determined by the dynamics of the whole boundary layer rather than being simply dependent on the surface boundary conditions. We hypothesize that in slightly unstable conditions there is a bifurcation in the structure of the large-scale eddy motions. This is a transition from quasi-steady longitudinal roll structures to detached eddies. In the particular regime identified here, the unsteady component dominates the heat transport at the surface. The observed enhancement in the surface layer of the eddy diffusivity of heat is observed to be significantly greater than for momentum. This is shown to be consistent with observations and with a model calculation of the interaction between the surface layer and the descending detached eddies.

  • 46.
    Smedman, Ann-Sofi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Högström, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Sahleé, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Johansson, Cecilia
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Air and Water Science.
    Critical re-evaluation of the bulk transfer coefficient for sensible heat over the ocean during unstable and neutral conditions2007In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 622, 227-250 p.Article in journal (Refereed)
    Abstract [en]

    A new analysis of the neutral heat transfer coefficient CHN on data from Östergarnsholm is presented, which is primarily based on a limited set of measurements with the very accurate MIUU (Meteorological Institute of the University of Uppsala) instrument, but with additional information from an extensive set of measurements with Solent sonic R2. Sonic data are, however, used with great caution, since for wind speed U above 10 m s-1, a strongly wind-speed-dependent correction is shown to be required. This error is roughly proportional to (U-10) for sea-air temperature differences less than 4-5 K. For a larger temperature difference, no correction appears to be necessary in the wind speed range 10-15 m s-1. We infer from our data that for conditions when unstable and near-neutral conditions prevail, measurements of the sea surface - air temperature difference are accurate to within 0.1 K at our site. This means that data for a range of relatively small temperature differences (0.5-1.5 K) which were often rejected in previous studies could be retained. It is observed that a rapid increase of CH and CHN occurs in that range. For wind speed above 10 m s-1, CHN is observed to increase rapidly with U10. During those conditions, the wave field at the site is known to have characteristics very similar to those in deep-sea conditions. In a previous analysis of data from Östergarnsholm, it was speculated that observed high CHN values could be due to spray. Calculations with a spray model showed, however, conclusively that for wind speeds less than 14 m s-1, the spray effect on the sensible heat flux is expected to be small. The high CHN values must instead be due to dynamic effects. It is demonstrated that when the Obukhov length L is less than about -150 m a regime with very specific characteristics ensues. This regime is dominated by surface-layer scale eddies, which cause Monin-Obukhov relations for the exchange of sensible heat to break down. The characteristics of this surface-layer regime are treated in detail in the companion paper. The rise of CHN with wind speed is shown to be closely related to a corresponding increase of ZOTwith roughness Reynolds number for winds above 10 m s-1. This means that during those conditions, traditional surface renewal theory for heat is no longer valid. It is suggested that this, in turn, is a result of increasing importance of wave-breaking with increasing wind and with a possible link to processes in near-surface atmospheric layers in the regime with -L > 150 m.

  • 47.
    Sotiropoulou, Georgia
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sedlar, Joseph
    Stockholm University, Faculty of Science, Department of Meteorology .
    Forbes, Richard
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Late Summer Arctic clouds in the ECMWF forecast model: an evaluation of cloud parameterization scheme2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 694, 387-400 p.Article in journal (Refereed)
    Abstract [en]

    Mixed-phase clouds are an integral part of the Arctic climate system, for precipitation and for their interactions with radiation and thermodynamics. Mixed-phase processes are often poorly represented in global models and many use an empirically based diagnostic partition between the liquid and ice phases that is dependent solely on temperature. However, increasingly more complex microphysical parametrizations are being implemented allowing a more physical representation of mixed-phase clouds.

    This study uses in situ observations from the Arctic Summer Cloud Ocean Study (ASCOS) field campaign in the central Arctic to assess the impact of a change from a diagnostic to a prognostic parametrization of mixed-phase clouds and increased vertical resolution in the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS). The newer cloud scheme improves the representation of the vertical structure of mixed-phase clouds, with supercooled liquid water at cloud top and ice precipitating below, improved further with higher vertical resolution. Increased supercooled liquid water and decreased ice content are both in closer agreement with observations. However, these changes do not result in any substantial improvement in surface radiation, and a warm and moist bias in the lowest part of the atmosphere remains. Both schemes also fail to capture the transitions from overcast to cloud-free conditions. Moreover, whereas the observed cloud layer is frequently decoupled from the surface, the modelled clouds remain coupled to the surface most of the time. The changes implemented to the cloud scheme are an important step forward in improving the representation of Arctic clouds, but improvements in other aspects such as boundary-layer turbulence, cloud radiative properties, sensitivity to low aerosol concentrations and representation of the sea-ice surface may also need to be addressed.

  • 48.
    Sotiropoulou, Georgia
    et al.
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Sedlar, Joseph
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Forbes, Richard
    Tjernström, Michael
    Stockholms universitet, Meteorologiska institutionen (MISU).
    Late Summer Arctic clouds in the ECMWF forecast model: an evaluation of cloud parameterization scheme2016In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 142, no 694, 387-400 p.Article in journal (Refereed)
    Abstract [en]

    Mixed-phase clouds are an integral part of the Arctic climate system, for precipitation and for their interactions with radiation and thermodynamics. Mixed-phase processes are often poorly represented in global models and many use an empirically based diagnostic partition between the liquid and ice phases that is dependent solely on temperature. However, increasingly more complex microphysical parametrizations are being implemented allowing a more physical representation of mixed-phase clouds.

    This study uses in situ observations from the Arctic Summer Cloud Ocean Study (ASCOS) field campaign in the central Arctic to assess the impact of a change from a diagnostic to a prognostic parametrization of mixed-phase clouds and increased vertical resolution in the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS). The newer cloud scheme improves the representation of the vertical structure of mixed-phase clouds, with supercooled liquid water at cloud top and ice precipitating below, improved further with higher vertical resolution. Increased supercooled liquid water and decreased ice content are both in closer agreement with observations. However, these changes do not result in any substantial improvement in surface radiation, and a warm and moist bias in the lowest part of the atmosphere remains. Both schemes also fail to capture the transitions from overcast to cloud-free conditions. Moreover, whereas the observed cloud layer is frequently decoupled from the surface, the modelled clouds remain coupled to the surface most of the time. The changes implemented to the cloud scheme are an important step forward in improving the representation of Arctic clouds, but improvements in other aspects such as boundary-layer turbulence, cloud radiative properties, sensitivity to low aerosol concentrations and representation of the sea-ice surface may also need to be addressed.

  • 49. Stengel, M.
    et al.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Unden, Per
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    The impact of cloud-affected IR radiances on forecast accuracy of a limited-area NWP model2013In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, no 677, 2081-2096 p.Article in journal (Refereed)
    Abstract [en]

    The impact of cloud-affected satellite radiances on numerical weather prediction (NWP) accuracy is investigated. The NWP model used is the HIgh Resolution Limited Area Model (HIRLAM). Its four-dimensional variational data assimilation (4D-Var) system was used to assimilate cloud-affected infrared (IR) radiances from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). Cloud parameters are modelled internally in the observation operator and used in the radiative transfer calculations. The interaction between the cloud parameters and the model control vector variables is incorporated in the adjoint version of the observation operator, which is used to derive cloud-affected Jacobians prior to the inner-loop minimization of the cost function. The developed framework supports an extensive usage of satellite observations with spatial coverage extended into cloudy regions, which therefore provides additional analysis increments and supports a more accurate description of the atmospheric state. In extended assimilation and forecast experiments the total number of assimilated satellite observations could be increased by approximately 10%. This was associated with a clear indication of a positive impact of cloud-affected radiances on the moisture and geopotential height fields of the NWP model analysis and forecast accuracy when used on top of clear-sky radiance observations. This is revealed by reduced analysis errors of the total integrated water vapour and by reduced forecast errors in the mid and upper troposphere.

  • 50. Stengel, M.
    et al.
    Lindskog, Magnus
    SMHI, Research Department, Meteorology.
    Unden, Per
    SMHI, Research Department, Meteorology.
    Gustafsson, Nils
    SMHI, Research Department, Meteorology.
    Bennartz, R.
    An extended observation operator in HIRLAM 4D-VAR for the assimilation of cloud-affected satellite radiances2010In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 136, no 649, 1064-1074 p.Article in journal (Refereed)
    Abstract [en]

    An extended observation operator for the direct assimilation of cloud-affected infrared satellite radiances in the High Resolution Limited Area Model (HIRLAM) is examined. The operator includes a simplified moist-physics scheme, which enables the diagnosis of cloudiness in itself using background values of temperature, moisture and surface pressure. Subsequently, a radiative transfer model provides simulated cloud-affected radiances to be used as background equivalents to the satellite observations. The observation operator was evaluated by using infrared observations measured by the Spinning Enhanced Visible and Infrared Imager (SEVIRI). An observation-screening procedure, which incorporates SEVIRI cloud-retrieval products, supports an improved selection of usable cloudy scenes, leading to good agreement between the observations and background equivalents. The tangent-linear observation operator was verified against finite differences from its nonlinear formulation. The increments revealed a near-linear behaviour for the selected channels for a large number of cases. The adjoint observation operator was used to derive brightness-temperature sensitivities with respect to temperature and moisture changes in the presence of radiance-affecting clouds. Differences from the clear-sky sensitivities were found in and below clouds. In a four-dimensional variational data assimilation experiment, cloud-affected SEVIRI observations were assimilated, resulting in additional increments in both moisture and wind fields. The corresponding analysis fields revealed a reduced deviation from the observations for the majority of all cloudy scenes and a reduced bias for wind and temperature in the upper troposphere against independent radiosonde observations. Overall, our results highlight the capability of this observation operator in the HIRLAM assimilation system and encourage its application for the extended usage of cloudy satellite observations in numerical weather prediction. Copyright (C) 2010 Royal Meteorological Society

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