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  • 51.
    Engeln, Axel von
    et al.
    Universität Bremen.
    Buehler, Stefan
    Kirchengast, Gottfried
    University of Graz.
    Kuenzi, Klaus
    Universität Bremen.
    Temperature profile retrieval from surface to mesopause by combining GNSS radio occultation and passive microwave limb sounder data1999Inngår i: Conference on Satellite Meteorology and Oceanography, American Meteorological Society, 1999, s. 240-243Konferansepaper (Fagfellevurdert)
  • 52.
    Engeln, Axel von
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Kirchengast, Gottfried
    University of Graz, Institute for Geophysics, Astrophysics and Meteorology.
    Kuenzi, Klaus
    Universität Bremen, Institute of Environmental Physics.
    Temperature profile retrieval from surface to mesopause by combining GNSS radio occultation and passive microwave limb sounder data2001Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 28, nr 5, s. 775-778Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A theoretical study on the combination of a passive microwave and a GNSS radio occultation instrument, mounted on the International Space Station (ISS), was performed. Both instruments allow to deduce the atmospheric temperature profile. We combined realistically simulated data of the two highly synergistic sensors with the optimal estimation method (OEM) and retrieved single joint temperature profiles from 0 to 90 km altitude for which we found accuracies < 1 K below 35 km and < 4 K in the mesosphere, respectively. In addition, we performed simultaneous tropospheric water vapor and temperature retrievals leading to a water vapor accuracy < 10 to 20 % at altitudes below 5 to 8 km. The OEM allows to optimally exploit the synergy in the data and maximizes retrieval accuracy over the full range from surface to mesopause. It seems worthwhile to install this type of instrument combination on the ISS or other low-Earth-orbit (LEO) platforms.

  • 53.
    Engeln, Axel von
    et al.
    Naval Research Laboratory, Washington, DC.
    Nedoluha, Gerald
    Naval Research Laboratory, Washington, DC.
    Kirchengast, Gottfried
    University of Graz, Institute for Geophysics, Astrophysics and Meteorology.
    Buehler, Stefan
    One-dimensional variational (1-D Var) retrieval of temperature, water vapor, and a reference pressure from radio occultation measurements: A sensitivity analysis2003Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 108, nr 11, s. 4337-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A 1-D Var retrieval study of simulated radio occultation measurements is presented. Temperature and a water vapor profile are retrieved along with a reference pressure to generate the pressure profile by applying the hydrostatic equation. High-resolution European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric fields are used by a ray tracing tool to calculate the exact positions of the tangent point. The 1-D atmospheric profiles following the calculated tangent point trajectory in the 3-D ECMWF fields are used to simulate bending angle measurements with a 1-D forward model. Assimilation of these bending angles in a 1-D Var tool employing the same 1-D forward model is performed. We analyze the sensitivity of the retrieval to changes in vertical resolution, horizontal smearing of the tangent point trajectory, and the assumption of hydrostatic equilibrium for a nonvertical atmospheric scan. We find that retrievals calculated without adequate vertical resolution can have significant errors in temperature and water vapor. Errors in the retrieval by assuming hydrostatic equilibrium for a nonvertical scan generally cause only minor errors in the retrieved profiles. A study into the occurrence of rays curving down toward the Earth surface indicates that about 5–10% of the profiles could experience so-called critical refraction at altitudes between 0.5 km and 2 km in case of the applied high-resolution ECMWF data.

  • 54.
    Engeln, Axel von
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Teixeira, Joao
    UCAR/VSP at Marine meteorology division of Naval research laboratory, Monterey, CA.
    Wickert, Jens
    GeoForschungsZentrum Potsdam, Department of Geodesy and Remote Sensing, Potsdam.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    CHAMP radio occultation detection of the planetary boundary layer top2006Inngår i: Atmosphere and climate: studies by occultation methods, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 2006Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 55.
    Engeln, Axel von
    et al.
    Meteorological Division, European Organization for the Exploitation of Meteorological Satellites, Darmstadt.
    Teixeira, Joao
    NATO Undersea Research Centre, La Spezia.
    Wickert, Jens
    GeoForschungsZentrum Potsdam.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Comment on "Monitoring the atmospheric boundary layer by GPS radio occultation signals recorded in the open-loop mode" by S. Sokolovskiy et al.2007Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, nr 2, s. L02806-Artikkel i tidsskrift (Annet vitenskapelig)
  • 56.
    Engeln, Axel von
    et al.
    UK Met Office, Exeter.
    Teixeira, Joao
    Marine Meteorology Division, Naval Research Laboratory, Monterey, California.
    Wickert, Jens
    GeoForschungsZentrum Potsdam, Department of Geodesy and Remote Sensing, Potsdam.
    Buehler, Stefan
    Using CHAMP radio occultation data to determine the top altitude of the planetary boundary layer2005Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 32, nr 6, s. L06815-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A simple approach to derive the Planetary Boundary Layer (PBL) top altitude from CHAMP (CHAllenging Minisatellite Payload) radio occultation (RO) data is presented. Our RO processing cuts off at an altitude, typically ≤4 km, below which the GPS signals are affected by tracking errors. This lowest processed altitude (LPA) is assumed to coincide with the PBL top. We average LPAs for the years 2001 to 2004 over 5 Degree latitude longitude boxes and compare them to ECMWF analysis data. The ECMWF PBL top was calculated from the relative humidity gradient with respect to altitude. Agreement between the data sets is good in terms of mean PBL height, especially over sea. The CHAMP data shows the major features of PBL height with a realistic transition from stratocumulus regions to shallow and deep cumulus areas. CHAMP also shows a substantial amount of PBL height variability that may prove useful to study PBL dynamics.

  • 57. English, S.J.
    et al.
    O'Keeffe, U.
    Sreerekha, T.R.
    Buehler, Stefan
    Emde, Claudia
    Doherty, A.M.
    A comparison of RTTOVSCATT with ARTS and AMSU observations, using the Met Office mesoscale model short range forecasts of cloud ice and liquid water2003Inngår i: 13th International TOVS study conference (ITSC-XIII), 2003Konferansepaper (Fagfellevurdert)
  • 58.
    Eriksson, P.
    et al.
    Chalmers University of Technology.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Davis, C.P.
    Meteorological Service of New Zealand.
    Emde, C.
    Meteorological Institute, Ludwig-Maximilians-Universität, Munchen.
    Lemke, Oliver
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    ARTS, the atmospheric radiative transfer simulator, version 22011Inngår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 112, nr 10, s. 1551-1558Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The second version of the atmospheric radiative transfer simulator, ARTS, is introduced. This is a general software package for long wavelength radiative transfer simulations, with a focus on passive microwave observations. The core part provides a workspace environment, in line with script languages. New for this version is an agenda mechanism that gives a high degree of modularity. The framework is intended to be as general as possible: the polarisation state can be fully described, the model atmosphere can be one- (1D), two- (2D) or three-dimensional (3D), a full description of geoid and surface is possible, observation geometries from the ground, from satellite, and from aeroplane or balloon are handled, and surface reflection can be treated in simple or complex manners. Remote sensing applications are supported by a comprehensive and efficient treatment of sensor characteristics. Jacobians can be calculated for the most important atmospheric variables in non-scattering conditions. Finally, the most prominent feature is the rigorous treatment of scattering that has been implemented in two modules: a discrete ordinate iterative approach mainly used for 1D atmospheres, and a Monte Carlo approach which is the preferred algorithm for 3D atmospheres. ARTS is freely available, and maintained as an open-source project.

  • 59.
    Eriksson, Patrick
    et al.
    Chalmers University of Technology.
    Buehler, Stefan
    Engeln, Axel von
    Universität Bremen.
    ARTS: a modular public domain radiative transfer program2001Inngår i: Atmospheric millimeter and sub-millimeter radiative transfer modelling. 2, Berlin: Dialogos Förlag, 2001, s. 57-112Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 60.
    Eriksson, Patrick
    et al.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Ekström, Mattias
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Melsheimer, Christian
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Efficient forward modelling by matrix representation of sensor responses2006Inngår i: International Journal of Remote Sensing, ISSN 0143-1161, E-ISSN 1366-5901, Vol. 27, nr 9-10, s. 1793-1808Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The polarization, frequency and spatial responses of the sensor can be considered by calculating the Stokes vector of monochromatic pencil beam radiances for a set of frequencies and viewing directions, and weight these radiances with the instrument responses. This paper presents a highly efficient solution for this calculation procedure. The basic idea is to pre-calculate a matrix that represents the mapping from polarisation, frequency and spatial values to measured data. Sensor impacts can then be included by a simple matrix multiplication. The full sensor matrix can be obtained by determining the response matrix for the sensor parts individually. Data reduction methods can also be incorporated. A simple method for optimizing the calculation grids is further presented. The described approach for sensor modeling has been implemented in two public available softwares for atmospheric radiative transfer simulations.

  • 61.
    Eriksson, Patrick
    et al.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jimenez, Carlos
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Qpack: a general tool for instrument simulation and retrieval work2005Inngår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 91, nr 1, s. 47-64Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Remote sensing requires a complete observation system, consisting of the instrument, a forward model and a retrieval environment. This paper presents a software tool to complement atmospheric sensors, with focus on passive instruments operating in the mm and sub-mm wavelength regions. The tool is of general character and offers a complete, flexible and fast calculation environment, demonstrated in both preparatory instrument studies and operational inversions. Its features include a rapid approach for modelling of sensor characteristics, several types of data reduction, simple definition of covariance matrices, a large number of retrieval and error quantities, inversion characterisation and random realisation of measurements. The software is freely available for scientific use.

  • 62.
    Eriksson, Patrick
    et al.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Murtagh, Donal
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    A Hotelling transformation approach for rapid inversion of atmospheric spectra2002Inngår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 73, nr 6, s. 529-543Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atmospheric observations from space often result in spectral data of large dimensions. To allow an optimal inversion of the observed spectra it can be necessary to map the data into a space of smaller dimension. Here several data reduction techniques based on eigenvector expansions of the spectral space are compared. The comparison is done by inverting simulated observations from a microwave limb sounder, the Odin-SMR. For the examples tested, reductions exceeding two orders of magnitude with no negative influence on the retrieval performance are demonstrated. The techniques compared include a novel method developed especially for atmospheric inversions, based on the weighting functions of the variables to be retrieved. The new method shows an excellent performance in practical tests and is both computationally more effective and more flexible than the standard Hotelling transformation.

  • 63.
    Eriksson, Patrick
    et al.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Murtagh, Donal
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Elgered, Gunnar
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Kuhn, Thomas
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Measurement of tropospheric/stratospheric transmission at 10–35 GHz for H2O retrieval in low Earth orbiting satellite links2003Inngår i: Radio Science, ISSN 0048-6604, E-ISSN 1944-799X, Vol. 38, nr 4, s. 8069-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Active microwave limb sounding is a possible technique for measuring water vapor in the upper troposphere and lower stratosphere, and here a first assessment of the retrieval capabilities of transmission measurements in the range 10–35 GHz is presented. The proposed observing system consists of a constellation of low Earth orbiters measuring atmospheric transmission at the frequencies 10.3, 17.2, and 22.6 GHz. The use of these relatively long wavelengths guarantees a minimal, for being a remote sensing technique, influence from scattering. The original objective of the measurements was to derive water vapor profiles, but the potential to retrieve the liquid water content of clouds was also identified during the study. Retrieval errors due to thermal noise, gain instability, and spectroscopic uncertainties were considered. With the assumed instrument characteristics a measurement precision for water vapor in the upper troposphere of 5–10% is obtained, with capability to observe through ice clouds and clouds with a low water content.

  • 64.
    Eriksson, Patrick
    et al.
    Chalmers University of Technology, Department of Earth and Space Sciences.
    Rydberg, B.
    Chalmers University of Technology, Department of Earth and Space Sciences.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    On cloud ice induced absorption and polarisation effects in microwave limb sounding2011Inngår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 4, nr 6, s. 1305-1318Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Microwave limb sounding in the presence of ice clouds was studied by detailed simulations, where clouds and other atmospheric variables varied in three dimensions and the full polarisation state was considered. Scattering particles were assumed to be horizontally aligned oblate spheroids with a size distribution parameterized in terms of temperature and ice water content. A general finding was that particle absorption is significant for limb sounding, which is in contrast to the down-looking case, where it is usually insignificant. Another general finding was that single scattering can be assumed for cloud optical paths below about 0.1, which is thus an important threshold with respect to the complexity and accuracy of retrieval algorithms. The representation of particle sizes during the retrieval is also discussed. Concerning polarisation, specific findings were as follows: Firstly, no significant degree of circular polarisation was found for the considered particle type. Secondly, for the +/- 45 degrees polarisation components, differences of up to 4 K in brightness temperature were found, but differences were much smaller when single scattering conditions applied. Thirdly, the vertically polarised component has the smallest cloud extinction. An important goal of the study was to derive recommendations for future limb sounding instruments, particularly concerning their polarisation setup. If ice water content is among the retrieval targets (and not just trace gas mixing ratios), then the simulations show that it should be best to observe any of the +/- 45 degrees and circularly polarised components. These pairs of orthogonal components also make it easier to combine information measured from different positions and with different polarisations

  • 65.
    Gasteiger, J.
    et al.
    Meteorologisches Institut, Ludwig-Maximilians-Universität.
    Emde, C.
    Meteorologisches Institut, Ludwig-Maximilians-Universität.
    Mayer, B.
    Meteorologisches Institut, Ludwig-Maximilians-Universität.
    Buras, R.
    Meteorologisches Institut, Ludwig-Maximilians-Universität.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Lemke, Oliver
    Representative wavelengths absorption parameterization applied to satellite channels and spectral bands2014Inngår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 148, s. 99-115Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Accurate modeling of wavelength-integrated radiative quantities, e.g. integrated over a spectral band or an instrument channel response function, requires computations for a large number of wavelengths if the radiation is affected by gas absorption which typically comprises a complex line structure. In order to increase computational speed of modeling radiation in the Earth׳s atmosphere, we parameterized wavelength-integrals as weighted means over representative wavelengths. We parameterized spectral bands of different widths (1 cm−1, 5 cm−1, and 15 cm−1) in the solar and thermal spectral range, as well as a number of instrument channels on the ADEOS, ALOS, EarthCARE, Envisat, ERS, Landsat, MSG, PARASOL, Proba, Sentinel, Seosat, and SPOT satellites. A root mean square relative deviation lower than 1% from a “training data set” was selected as the accuracy threshold for the parameterization of each band and channel. The training data set included high spectral resolution calculations of radiances at the top of atmosphere for a set of highly variable atmospheric states including clouds and aerosols. The gas absorption was calculated from the HITRAN 2004 spectroscopic data set and state-of-the-art continuum models using the ARTS radiative transfer model. Three representative wavelengths were required on average to fulfill the accuracy threshold. We implemented the parameterized spectral bands and satellite channels in the uvspec radiative transfer model which is part of the libRadtran software package. The parameterization data files, including the representative wavelengths and weights as well as lookup tables of absorption cross sections of various gases, are provided at the libRadtran webpage.In the paper we describe the parameterization approach and its application. We validate the approach by comparing modeling results of parameterized bands and channels with results from high spectral resolution calculations for atmospheric states that were not part of the training data set. Irradiances are not only compared at the top of atmosphere but also at the surface for which this parameterization approach was not optimized. It is found that the parameterized bands and channels provide a good compromise between computation time requirements and uncertainty for typical radiative transfer problems. In particular for satellite radiometer simulations the computation time requirement and the parameterization uncertainty is low. Band-integrated irradiances at any level as well as heating and cooling rates below 20 km can also be modeled with low uncertainty.

  • 66.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Mendrok, Jana
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kottayil, Ajil
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Optimised frequency grids for infrared radiative transfer simulations in cloudy conditions2012Inngår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 113, nr 16, s. 2124-2134Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper shows that radiometer channel radiances for cloudy atmospheric conditions can be simulated with an optimised frequency grid derived under clear-sky conditions. A new clear-sky optimised grid is derived for AVHRR channel . For HIRS channel 11 and AVHRR channel 5, radiative transfer simulations using an optimised frequency grid are compared with simulations using a reference grid, where the optimised grid has roughly 100–1000 times less frequencies than the full grid. The root mean square error between the optimised and the reference simulation is found to be less than 0.3 K for both comparisons, with the magnitude of the bias less than 0.03 K. The simulations have been carried out with the radiative transfer model Atmospheric Radiative Transfer Simulator (ARTS), version 2, using a backward Monte Carlo module for the treatment of clouds. With this module, the optimised simulations are more than 10 times faster than the reference simulations. Although the number of photons is the same, the smaller number of frequencies reduces the overhead for preparing the optical properties for each frequency. With deterministic scattering solvers, the relative decrease in runtime would be even more. The results allow for new radiative transfer applications, such as the development of new retrievals, because it becomes much quicker to carry out a large number of simulations. The conclusions are applicable to any downlooking infrared radiometer.

  • 67. Holl, Gerrit
    et al.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, B.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, C.
    Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, Centre National de la Recherche Scientifique, Observatoire de Paris.
    Collocating satellite-based radar and radiometer measurements: methodology and usage examples2010Inngår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 3, nr 3, s. 693-708Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Collocations between two satellite sensors are occasions where both sensors observe the same place at roughly the same time. We study collocations between the Microwave Humidity Sounder (MHS) on-board NOAA-18 and the Cloud Profiling Radar (CPR) on-board CloudSat. First, a simple method is presented to obtain those collocations and this method is compared with a more complicated approach found in literature. We present the statistical properties of the collocations, with particular attention to the effects of the differences in footprint size. For 2007, we find approximately two and a half million MHS measurements with CPR pixels close to their centrepoints. Most of those collocations contain at least ten CloudSat pixels and image relatively homogeneous scenes. In the second part, we present three possible applications for the collocations. Firstly, we use the collocations to validate an operational Ice Water Path (IWP) product from MHS measurements, produced by the National Environment Satellite, Data and Information System (NESDIS) in the Microwave Surface and Precipitation Products System (MSPPS). IWP values from the CloudSat CPR are found to be significantly larger than those from the MSPPS. Secondly, we compare the relation between IWP and MHS channel 5 (190.311 GHz) brightness temperature for two datasets: the collocated dataset, and an artificial dataset. We find a larger variability in the collocated dataset. Finally, we use the collocations to train an Artificial Neural Network and describe how we can use it to develop a new MHS-based IWP product. We also study the effect of adding measurements from the High Resolution Infrared Radiation Sounder (HIRS), channels 8 (11.11 mu m) and 11 (8.33 mu m). This shows a small improvement in the retrieval quality. The collocations described in the article are available for public use

  • 68.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    LERMA, CNRS, Observatoire de Paris, Paris, France.
    Collocating satellite-based radar and radiometer measurements –: methodology and usage examples2010Konferansepaper (Annet vitenskapelig)
  • 69.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, Centre National de la Recherche Scientifique, Observatoire de Paris.
    Collocating satellite-based radar and radiometer measurements: methodology and usage examples2011Inngår i: Proceedings of the ESA Living Planet Symposium, European Space Agency, ESA , 2011Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Collocations between two satellite sensors are occasions where both sensors observe the same place at roughly the same time. We study collocations between the Microwave Humidity Sounder (MHS) on-board NOAA-18 and the Cloud Profiling Radar (CPR) on-board CloudSat. We present some statistical properties of the collocations. For 2007, we find approximately two and a half million MHS measurements with CPR pixels close to their centrepoints. We present some possible applications. We use the collocations to validate an operational Ice Water Path (IWP) product from MHS measurements, produced by the National Environment Satellite, Data and Information System (NESDIS) in the Microwave Surface and Precipitation Products System (MSPPS). IWP values from the CloudSat CPR are found to be significantly larger than those from the MSPPS. Finally, we use the collocations to train an Artificial Neural Network and describe how we can use it to develop a new MHS-based IWP product. The collocations described in the article are available for public use.

  • 70.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    LERMA, CNRS, Observatoire de Paris, Paris, France.
    Collocating satellite-based radar and radiometer measurements: methodology and usage examples2010Konferansepaper (Annet vitenskapelig)
  • 71.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    LERMA, CNRS, Observatoire de Paris, Paris, France.
    Hunting ice clouds: quantifying ice from passive infrared and microwave measurements by collocating with the CloudSat radar2011Konferansepaper (Annet vitenskapelig)
  • 72.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Eliasson, Salomon
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Mendrok, Jana
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    SPARE-ICE: Synergistic ice water path from passive operational sensors2014Inngår i: Journal of Geophysical Research: Atmospheres, ISSN 2169-8996, Vol. 119, nr 3, s. 1504-1523Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This article presents SPARE-ICE, the Synergistic Passive Atmospheric Retrieval Experiment-ICE. SPARE-ICE is the first Ice Water Path (IWP) product combining infrared and microwave radiances. By using only passive operational sensors, the SPARE-ICE retrieval can be used to process data from at least the NOAA 15 to 19 and MetOp satellites, obtaining time series from 1998 onward. The retrieval is developed using collocations between passive operational sensors (solar, terrestrial infrared, microwave), the CloudSat radar, and the CALIPSO lidar. The collocations form a retrieval database matching measurements from passive sensors against the existing active combined radar-lidar product 2C-ICE. With this retrieval database, we train a pair of artificial neural networks to detect clouds and retrieve IWP. When considering solar, terrestrial infrared, and microwave-based measurements, we show that any combination of two techniques performs better than either single-technique retrieval. We choose not to include solar reflectances in SPARE-ICE, because the improvement is small, and so that SPARE-ICE can be retrieved both daytime and nighttime. The median fractional error between SPARE-ICE and 2C-ICE is around a factor 2, a figure similar to the random error between 2C-ICE ice water content (IWC) and in situ measurements. A comparison of SPARE-ICE with Moderate Resolution Imaging Spectroradiometer (MODIS), Pathfinder Atmospheric Extended (PATMOS-X), and Microwave Surface and Precipitation Products System (MSPPS) indicates that SPARE-ICE appears to perform well even in difficult conditions. SPARE-ICE is available for public use.

  • 73.
    Hong, Gang
    et al.
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Heygster, Georg
    Institute of Environmental Physics, University of Bremen.
    Notholt, Justus
    Institute of Environmental Physics, University of Bremen.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Interannual to diurnal variations in tropical and subtropical deep convective clouds and convective overshooting from seven years of AMSU-B measurements2008Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 21, nr 17, s. 4168-4189Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study surveys interannual to diurnal variations of tropical deep convective clouds and convective overshooting using the Advanced Microwave Sounding Unit B (AMSU-B) aboard the NOAA polar orbiting satellites from 1999 to 2005. The methodology used to detect tropical deep convective clouds is based on the advantage of microwave radiances to penetrate clouds. The major concentrations of tropical deep convective clouds are found over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), tropical Africa, the Indian Ocean, the Indonesia maritime region, and tropical and South America. The geographical distributions are consistent with previous results from infrared-based measurements, but the cloud fractions present in this study are lower. Land-ocean and Northern-Southern Hemisphere (NH-SH) contrasts are found for tropical deep convective clouds. The mean tropical deep convective clouds have a slightly decreasing trend with -0.016% decade(-1) in 1999-2005 while the mean convective overshooting has a distinct decreasing trend with -0.142% decade(-1). The trends vary with the underlying surface (ocean or land) and with latitude. A secondary ITCZ occurring over the eastern Pacific between 2 degrees and 8 degrees S and only in boreal spring is predominantly found to be associated with cold sea surface temperatures in La Nina years. The seasonal cycles of deep convective cloud and convective overshooting are stronger over land than over ocean. The seasonal migration is pronounced and moves south with the sun from summer to winter and is particularly dramatic over land. The diurnal cycles of deep convective clouds and convective overshooting peak in the early evening and have their minima in the late morning over the tropical land. Over the tropical ocean the diurnal cycles peak in the morning and have their minima in the afternoon to early evening. The diurnal cycles over the NH and SH subtropical regions vary with the seasons. The local times of the maximum and minimum fractions also vary with the seasons. As the detected deep convective cloud fractions are sensitive to the algorithms and satellite sensors used and are influenced by the life cycles of deep convective clouds, the results presented in this study provide information complementary to present tropical deep convective cloud climatologies.

  • 74.
    Hong, Gang
    et al.
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Yang, Ping
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Baum, Bryan A.
    Space Science and Engineering Center, University of Wisconsin, Madison, Wisconsin.
    Heymsfield, Andrew J.
    National Center for Atmospheric Research, Boulder, Colorado.
    Weng, Fuzhong
    Satellite Meteorology and Climatology Division, Center for Satellite Applications and Research, NOAA NESDIS, Camp Springs, Maryland.
    Liu, Quanhua
    Joint Center for Satellite Data Assimilation, NOAA NESDIS, Camp Springs, Maryland.
    Heygster, Georg
    Institute of Environmental Physics, University of Bremen.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Scattering database in the millimeter and submillimeter wave range of 100-1000 GHz for nonspherical ice particles2009Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, s. D06201-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The inference of ice cloud properties from spaceborne sensors is sensitive to the retrieval algorithms and satellite sensors used. To approach a better understanding of ice cloud properties, it is necessary to combine satellite measurements from multiple platforms and sensors operating in visible, infrared, and millimeter and submillimeter-wave regions of the electromagnetic spectrum. The single-scattering properties of ice particles with consistent ice particle models are the basis for estimating the optical and microphysical properties of ice clouds from multiple satellite sensors. In this study, the single-scattering properties (extinction efficiency, absorption efficiency, single-scattering albedo, asymmetry factor, and scattering phase matrix) of nonspherical ice particles, assumed to be hexagonal solid and hollow columns, hexagonal plates, 3D bullet rosettes, aggregates, and droxtals, are computed from the discrete dipole approximation method for 21 millimeter and submillimeter-wave frequencies ranging from 100 to 1000 GHz. A database of the single-scattering properties of nonspherical ice particles are developed for 38 particle sizes ranging from 2 to 2000 μm in terms of particle maximum dimension. The bulk scattering properties of ice clouds consisting of various ice particles, which are the fundamental to the radiative transfer in ice clouds, are developed on the basis of a set of 1119 particle size distributions obtained from various field campaigns.

  • 75.
    Houshangpour, A.
    et al.
    Universität Bremen, Institute of Environmental Physics.
    John, V. O.
    Institute of Environmental Physics, University of Bremen.
    Buehler, Stefan
    Retrieval of upper tropospheric water vapor and upper tropospheric humidity from AMSU radiances2005Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 5, nr 8, s. 2019-2028Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A regression method was developed to retrieve upper tropospheric water vapor (UTWV in kg/m2) and upper tropospheric humidity (UTH in % RH) from radiances measured by the Advanced Microwave Sounding Unit (AMSU). In contrast to other UTH retrieval methods, UTH is defined as the average relative humidity between 500 and 200hPa, not as a Jacobian weighted average, which has the advantage that the UTH altitude does not depend on the atmospheric conditions. The method uses AMSU channels 6-10, 18, and 19, and should achieve an accuracy of 0.48 kg/m2 for UTWV and 6.3% RH for UTH, according to a test against an independent synthetic data set. This performance was confirmed for northern mid-latitudes by a comparison against radiosonde data from station Lindenberg in Germany, which yielded errors of 0.23 kg/m2 for UTWV and 6.1% RH for UTH.

  • 76.
    Höpfner, M.
    et al.
    Karlsruhe Institute of Technology.
    Milz, Mathias
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Orphal, J.
    Karlsruhe Institute of Technology.
    Stiller, G.
    Karlsruhe Institute of Technology.
    The natural greenhouse effect of atmospheric oxygen (O2) and nitrogen (N2)2012Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, s. L10706-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of collision-induced absorption by molecular oxygen (O 2) and nitrogen (N 2) on the outgoing longwave radiation (OLR) of the Earth's atmosphere has been quantified. We have found that on global average under clear-sky conditions the OLR is reduced due to O 2 by 0.11 Wm -2 and due to N 2 by 0.17 Wm -2. Together this amounts to 15% of the OLR-reduction caused by CH 4 at present atmospheric concentrations. Over Antarctica the combined effect of O 2 and N 2 increases on average to about 38% of CH 4 with single values reaching up to 80%. This is explained by less interference of H 2O spectral bands on the absorption features of O 2 and N 2 for dry atmospheric conditions

  • 77.
    Isoz, Oscar
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Eriksson, Per
    Chalmers University of Technology, Department of Earth and Space Sciences.
    Intercalibration of microwave temperature sounders using radio occultation measurements2015Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, nr 9, s. 3758-3773Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This is a study of the usefulness of radio occultation (RO) data for intercomparing different microwave temperature (MWT) sounding instruments. The RO data used are from the Global Navigational Satellite System Receiver for Atmospheric Sounding on the Metop-A and B satellites. The MWT sounders investigated are the Advanced Microwave Sounding Unit-A instruments on the satellites NOAA 15, 16, and 18 and Metop-A. We collocate RO and MWT data and then use these collocations to study various aspects of the MWT instruments. In addition, two different versions of the MWT data are compared: standard operational data (OPR) and the NOAA Integrated Microwave Intercalibration Approach data (IMICA). The time series of monthly mean differences shows that there are robust patterns for each satellite and data version, which mostly drift only slowly over time. The intersatellite spread, measured by the standard deviation of the yearly mean values by all satellites, is between 0.1 and 0.4 K, depending on channel, with no significant differences between OPR and IMICA data. The only notable exception is Channel 8 of NOAA 16, which appears to have a time-varying offset of 0.5–1 K relative to the other instruments. At this point it is not clear whether this deviation is real or a sampling artifact, so further study is needed. Due to the large number of collocations used, it is possible to also investigate the scene brightness and scan angle dependence of the MWT bias (relative to RO). First results of such an analysis are presented and discussed. Particularly, the investigation of the scan angle dependence is novel, since this bias pattern is difficult to assess without RO data. Further work is needed on these angular dependences, before conclusions are robust enough to include in data recalibration efforts, but our overall conclusion is that RO collocations are a powerful tool for intercomparing MWT sounders.

  • 78.
    Isoz, Oscar
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kinch, Kjartan
    Niels Bohr Institute, Copenhagen University.
    Bonnedal, Magnus
    RUAG Space, Gothenburg.
    Akos, Dennis
    Luleå tekniska universitet, Institutionen för system- och rymdteknik.
    Interference from terrestrial sources and its impact on the GRAS GPS radio occultation receiver2014Inngår i: Radio Science, ISSN 0048-6604, E-ISSN 1944-799X, Vol. 49, nr 1, s. 1-6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    It is well known that terrestrial GPS/Global Navigation Satellite Systems (GNSS) receivers are vulnerable and have suffered from intentional and unintentional interference sources. Unfortunately, space-based GPS/GNSS receivers are not exempt from radio frequency interference originating from the Earth. This paper explores data recorded by the GNSS Receiver for Atmospheric Sounding (GRAS) instrument onboard MetOp-A in September 2007, which is assumed to be representative of the typical environment for GPS/GNSS instrumentation in LEO orbit. Within these data it is possible to detect both pulsed interference and variations in the background noise. One plausible source of the pulsed interference is identified. We also show that neither the pulsed interference nor the variations in the background noise degrades the performance of the higher level products from GRAS.

  • 79. Jarret, Mark
    et al.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Child, R. K.
    Jimenez, C.
    Courcoux, N.
    Emde, C.
    Heygster, G.
    Sreerekha, T. R.
    Eriksson, P.
    Rydberg, B.
    Foster, P. R.
    Rose, T.
    Davis, C.
    Evans, K. F.
    Heymsfield, A.
    Lohmann, U.
    Stubenrauch, C.
    Establishment of mission and instrument requirements to observe cirrus clouds at sub-millimetre wavelengths: Final report, ESTEC contract no. 19053/05/NL/AR2007Rapport (Annet vitenskapelig)
  • 80.
    Jimenez, C.
    et al.
    School of Geosciences, University of Edinburgh.
    Eriksson, P.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    John, V. O.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    A practical demonstration on AMSU retrieval precision for upper tropospheric humidity by a non-linear multi-channel regression method2005Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 5, nr 2, s. 451-459Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A neural network algorithm inverting selected channels from the Advance Microwave Sounding Unit instruments AMSU-A and AMSU-B was applied to retrieve layer averaged relative humidity. The neural network was trained with a global synthetic dataset representing clear-sky conditions. A precision of around 6% was obtained when retrieving global simulated radiances, the precision deteriorated less than 1% when real mid-latitude AMSU radiances were inverted and compared with co-located data from a radiosonde station. The 6% precision outperforms by 1% the reported precision estimate from a linear single-channel regression between radiance and weighting function averaged relative humidity, the more traditional approach to exploit AMSU data. Added advantages are not only a better precision; the AMSU-B humidity information is more optimally exploited by including temperature information from AMSU-A channels; and the layer averaged humidity is a more physical quantity than the weighted humidity, for comparison with other datasets. The training dataset proved adequate for inverting real radiances from a mid-latitude site, but it is limited by not considering the impact of clouds or surface emissivity changes, and further work is needed in this direction for further validation of the precision estimates.

  • 81.
    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 tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    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 ice2007Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, nr Suppl.2, s. 129-149Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 82. Jiménez, Carlos J.
    et al.
    Gulkis, Samuel L.
    Beaudin, Gérard
    Encrenaz, Th. H.
    Eriksson, Patrick
    Chalmers University of Technology.
    Kamp, Lucas W.
    Lee, S.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Sub-millimeter observations of the terrestrial atmosphere during an Earth flyby of the MIRO sounder on the Rosetta spacecraft2013Inngår i: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 82-83, s. 99-112Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Sub-millimeter spectra recorded by the MIRO sounder aboard the Rosetta spacecraft have been used at the time of an Earth flyby (November 2007) to check the consistency and validity of the instrumental data. High-resolution spectroscopic data were recorded in 8 channels in the vicinity of the strong water line at 557 GHz, and in a broad band continuum channel at 570 GHz. An atmospheric radiative transfer code (ARTS) and standard terrestrial atmospheres have been used to simulate the expected observational results. Differences with the MIRO spectra suggest an anomaly in the behavior of four spectroscopic channels. Further technical investigations have shown that a large part of the anomalies are associated with an instability of one of the amplifiers. The quality of the MIRO data has been further tested by inverting the spectra with an atmospheric inversion tool (Qpack) in order to derive a mesospheric temperature profile. The retrieved profile is in good agreement with the one inferred from the Earth Observing System Microwave Limb Sounder (EOS-MLS). This work illustrates the interest of validating instruments aboard planetary or cometary spacecraft by using data acquired during Earth flybys.

  • 83.
    John, V. O.
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Analysis of upper tropospheric humidity measurements by microwave sounders and radiosondes2005Inngår i: Proceedings of the UTLS OZONE / CWVC Workshop on 'Water Vapour in the UTLS', 2005Konferansepaper (Fagfellevurdert)
  • 84.
    John, V. O.
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Comparison of microwave satellite humidity data and radiosonde profiles: a survey of European stations2005Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 5, nr 7, s. 1843-1853Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A method to compare upper tropospheric humidity (UTH) from satellite and radiosonde data has been applied to the European radiosonde stations. The method uses microwave data as a benchmark for monitoring the performance of the stations. The present study utilizes three years (2001-2003) of data from channel 18 (183.31±1.00 GHz) of the Advanced Microwave Sounding Unit-B (AMSU-B) aboard the satellites NOAA-15 and NOAA-16. The comparison is done in the radiance space, the radiosonde data were transformed to the channel radiances using a radiative transfer model. The comparison results confirm that there is a dry bias in the UTH measured by the radiosondes. This bias is highly variable among the stations and the years. This variability is attributed mainly to the differences in the radiosonde humidity measurements. The analysis also shows a difference between daytime and nighttime soundings which is attributed to radiation error in the radiosonde data. The dry bias due to this error alone correspond to approximately 11% relative error in the UTH measurements.

  • 85.
    John, V. O.
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Scaling of microwave radiances to layer averaged relative humidity2004Inngår i: IASTA Bulletin, ISSN 0971-4510, Vol. 16, s. 293-296Artikkel i tidsskrift (Annet vitenskapelig)
  • 86.
    John, V. O.
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    The impact of ozone lines on AMSU-B radiances2004Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 31, nr 21, s. L21108-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The impact of ozone lines on Advanced Microwave Sounding Unit-B (AMSU-B) radiances was investigated using a line-by-line radiative transfer model. The impact is found to be the largest for channel 18 (183.31 ± 1.00 GHz), with a maximum up to about 0.5 K. The channels 17 (150 GHz) and 20 (183.31 ± 7.00 GHz) are also marginally affected by the ozone lines. The magnitude of the impact shows an interesting dependence on the channel 18 brightness temperature which implies that the effect is not just an offset to the brightness temperature.

  • 87.
    John, V. O.
    et al.
    University of Miami.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kuvatov, M.
    Universität Bremen.
    Soden, B. J.
    University of Miami.
    Sreerekha, T. R.
    Met Office Hadley Centre, Exeter.
    Toward a long-term homogenized UTH data set derived from satellite microwave measurements2006Inngår i: Microwave remote sensing of the atmosphere and environment. V. / [ed] Azita Valinia; Seiho Uratsuka; Tapan Misra, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2006, s. 64100K-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This article presents some ideas and issues related to the creation of a long-term upper tropospheric humidity (UTH) data set using satellite based microwave measurements. Polar orbiting satellites have been measuring UTH for more than a decade now. There are three microwave instruments which can measure UTH from Space: Special Sensor Microwave/Temperature-2 (SSM/T2), Advanced Microwave Sounding Unit-B (AMSU-B), and Microwave Humidity Sounder (MHS). These instruments have channels at 183.31±1.00GHz which are sensitive to UTH. Retrieval of UTH and cloud issues are discussed in detail. Advantage of microwave measurements of UTH over infrared measurements are demonstrated. Preliminary results on the inter-calibration of these instruments are also shown.

  • 88.
    John, Viju O.
    et al.
    Met Office Hadley Centre, Exeter.
    Allan, Richard P.
    Department of Meteorology, University of Reading.
    Bell, William
    Met Office Hadley Centre, Exeter.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kottayil, Ajil
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Assessment of intercalibration methods for satellite microwave humidity sounders2013Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, nr 10, s. 4906-4918Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three methods for ntercalibrating humidity sounding channels are compared to assess their merits and demerits. The methods use the following: (1) natural targets (Antarctica and tropical oceans), (2) zonal average brightness temperatures, and (3) simultaneous nadir overpasses (SNOs). Advanced Microwave Sounding Unit-B instruments onboard the polar-orbiting NOAA 15 and NOAA 16 satellites are used as examples. Antarctica is shown to be useful for identifying some of the instrument problems but less promising for intercalibrating humidity sounders due to the large diurnal variations there. Owing to smaller diurnal cycles over tropical oceans, these are found to be a good target for estimating intersatellite biases. Estimated biases are more resistant to diurnal differences when data from ascending and descending passes are combined. Biases estimated from zonal-averaged brightness temperatures show large seasonal and latitude dependence which could have resulted from diurnal cycle aliasing and scene-radiance dependence of the biases. This method may not be the best for channels with significant surface contributions. We have also tested the impact of clouds on the estimated biases and found that it is not significant, at least for tropical ocean estimates. Biases estimated from SNOs are the least influenced by diurnal cycle aliasing and cloud impacts. However, SNOs cover only relatively small part of the dynamic range of observed brightness temperatures

  • 89.
    John, Viju O
    et al.
    Met Office Hadley Centre, Exeter.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Allen, Richard P.
    Department of Meteorology, University of Reading.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Parker, David E.
    Met Office Hadley Centre, Exeter.
    Soden, Brian J.
    Rosenstiel School of Marine and Atmospheric Science, University of Miami.
    Clear-sky biases in satellite infrared estimates of upper tropospheric humidity and its trends2011Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use microwave retrievals of upper tropospheric humidity (UTH) to estimate the impact of clear-sky-only sampling by infrared instruments on the distribution, variability, and trends in UTH. Our method isolates the impact of the clear-sky-only sampling, without convolving errors from other sources. On daily time scales, IR-sampled UTH contains large data gaps in convectively active areas, with only about 20-30 % of the tropics (30 degrees S-30 degrees N) being sampled. This results in a dry bias of about -9 % RH in the area-weighted tropical daily UTH time series. On monthly scales, maximum clear-sky bias (CSB) is up to -30 % RH over convectively active areas. The magnitude of CSB shows significant correlations with UTH itself (-0.5) and also with the variability in UTH (-0.6). We also show that IR-sampled UTH time series have higher interannual variability and smaller trends compared to microwave sampling. We argue that a significant part of the smaller trend results from the contrasting influence of diurnal drift in the satellite measurements on the wet and dry regions of the tropics.

  • 90.
    John, Viju O.
    et al.
    Met Office Hadley Centre, Exeter.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Atkinson, Nigel
    Met Office Hadley Centre, Exeter.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Monitoring scan asymmetry of microwave humidity sounding channels using simultaneous all angle collocations (SAACs)2013Inngår i: Journal of Geophysical Research: Atmospheres, ISSN 2169-8996, Vol. 118, nr 3, s. 1536-1545Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simultaneous all angle collocations (SAACs) of microwave humidity sounders (AMSU-B and MHS) on-board polar orbiting satellites are used to estimate scan-dependent biases. This method has distinct advantages over previous methods, such as that the estimated scan-dependent biases are not influenced by diurnal differences between the edges of the scan and the biases can be estimated for both sides of the scan. We find the results are robust in the sense that biases estimated for one satellite pair can be reproduced by double differencing biases of these satellites with a third satellite. Channel 1 of these instruments shows the least bias for all satellites. Channel 2 has biases greater than 5 K, thus needs to be corrected. Channel 3 has biases of about 2 K and more and they are time varying for some of the satellites. Channel 4 has the largest bias which is about 15 K when the data are averaged for 5 years, but biases of individual months can be as large as 30 K. Channel 5 also has large and time varying biases for two of the AMSU-Bs. NOAA-15 (N15) channels are found to be affected the most, mainly due to radio frequency interference (RFI) from onboard data transmitters. Channel 4 of N15 shows the largest and time varying biases, so data of this channel should only be used with caution for climate applications. The two MHS instruments show the best agreement for all channels. Our estimates may be used to correct for scan-dependent biases of these instruments, or at least used as a guideline for excluding channels with large scan asymmetries from scientific analyses.

  • 91.
    John, Viju O.
    et al.
    Met Office Hadley Centre, Exeter.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Candy, Brett
    Met Office Hadley Centre, Exeter.
    Saunders, Roger W.
    Met Office Hadley Centre, Exeter.
    Perker, David E.
    Met Office Hadley Centre, Exeter.
    Understanding intersatellite biases of microwave humidity sounders using global simultaneous nadir overpasses2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. D02305-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simultaneous nadir overpasses (SNOs) of polar-orbiting satellites are most frequent in polar areas but can occur at any latitude when the equatorial crossing times of the satellites become close owing to orbital drift. We use global SNOs of polar orbiting satellites to evaluate the intercalibration of microwave humidity sounders from the more frequent high-latitude SNOs. We have found based on sensitivity analyses that optimal distance and time thresholds for defining collocations are pixel centers less than 5 km apart and time differences less than 300 s. These stringent collocation criteria reduce the impact of highly variable surface or atmospheric conditions on the estimated biases. Uncertainties in the estimated biases are dominated by the combined radiometric noise of the instrument pair. The effects of frequency changes between different versions of the humidity sounders depend on the amount of water vapor in the atmosphere. There are significant scene radiance and thus latitude dependencies in the estimated biases and this has to taken into account while intercalibrating microwave humidity sounders. Therefore the results obtained using polar SNOs will not be representative for moist regions, necessitating the use of global collocations for reliable intercalibration.

  • 92.
    John, Viju Oommen
    et al.
    University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Courcoux, N.
    A cautionary note on the use of Gaussian statistics in satellite-based UTH climatologies2006Inngår i: IEEE Geoscience and Remote Sensing Letters, ISSN 1545-598X, E-ISSN 1558-0571, Vol. 3, nr 1, s. 130-134Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This letter presents a cautionary note on the assumption of Gaussian behavior for upper tropospheric humidity (UTH) derived from satellite data in climatological studies, which can introduce a wet bias in the climatology. An example study using European Centre for Medium-Range Weather Forecasts reanalysis data shows that this wet bias can reach up to 6 %RH, which is significant for climatological applications. A simple Monte Carlo approach demonstrates that these differences and their link to the variability of brightness temperatures are due to a log-normal distribution of the UTH. This problem can be solved by using robust estimators such as the median instead of the arithmetic mean.

  • 93.
    John, Viju Oommen
    et al.
    University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, Miami.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Engeln, A. von
    EUMETSAT, Darmstadt.
    Eriksson, P.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Kuhn, Thomas
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. 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 measurements2006Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 132, nr 621, s. 2675-2691Artikkel i tidsskrift (Fagfellevurdert)
    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).

  • 94.
    John, V.O.
    et al.
    Universität Bremen, Institute of Environmental Physics.
    Buehler, Stefan
    Kuvatov, M.
    Universität Bremen, Institute of Environmental Physics.
    Comparison of AMSU-B brightness temperature with simulated brightness temperature using global radiosonde data2003Inngår i: 13th International TOVS study conference (ITSC-XIII), 2003Konferansepaper (Fagfellevurdert)
  • 95.
    John, V.O.
    et al.
    Universität Bremen.
    Kuvatov, M.
    Universität Bremen.
    Buehler, Stefan
    ARTS - a new radiative transfer model for AMSU2003Inngår i: The technical proceedings of the twelfth international TOVS Study Conference, Lorne, Australia, 27 February - 5 March 2002 / [ed] J.F. LeMarshall ; A. Apostolou., Melbourne: Bureau of Meteorology Research Centre , 2003Konferansepaper (Fagfellevurdert)
  • 96. Kerridge, B.
    et al.
    Buehler, Stefan
    Consideration of mission studying chemistry of the UTLS2004Rapport (Annet vitenskapelig)
  • 97. Kerridge, B.
    et al.
    Buehler, Stefan
    Consideration of mission studying chemistry of the UTLS2007Rapport (Annet vitenskapelig)
  • 98.
    Kottayil, Ajil
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    John, Viju O.
    UK Met Office, Exeter.
    Miloshevich, Larry M.
    Milo Scientific LLC, Lafayette, Colorado.
    Milz, Mathias
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    On the importance of Vaisala RS92 radiosonde humidity corrections for a better agreement between measured and modeled satellite radiances2012Inngår i: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 29, nr 2, s. 248-259Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A study has been carried out to assess the importance of radiosonde corrections in improving the agreement between satellite and radiosonde measurements of upper-tropospheric humidity. Infrared [High Resolution Infrared Radiation Sounder (HIRS)-12] and microwave [Advanced Microwave Sounding Unit (AMSU)-18] measurements from the NOAA-17 satellite were used for this purpose. The agreement was assessed by comparing the satellite measurements against simulated measurements using collocated radiosonde profiles of the Atmospheric Radiation Measurement (ARM) Program undertaken at tropical and midlatitude sites. The Atmospheric Radiative Transfer Simulator (ARTS) was used to simulate the satellite radiances. The comparisons have been done under clear-sky conditions, separately for daytime and nighttime soundings. Only Vaisala RS92 radiosonde sensors were used and an empirical correction (EC) was applied to the radiosonde measurements. The EC includes correction for mean calibration bias and for solar radiation error, and it removes radiosonde bias relative to three instruments of known accuracy. For the nighttime dataset, the EC significantly reduces the bias from 0.63 to −0.10 K in AMSU-18 and from 1.26 to 0.35 K in HIRS-12. The EC has an even greater impact on the daytime dataset with a bias reduction from 2.38 to 0.28 K in AMSU-18 and from 2.51 to 0.59 K in HIRS-12. The present study promises a more accurate approach in future radiosonde-based studies in the upper troposphere.

  • 99.
    Kottayil, Ajil
    et al.
    Advanced Centre for Atmospheric Radar Research, Cochin University of Science and Technology, Kerala.
    John, Viju O.
    EUMETSAT, Darmstadt 64295, Germany; Met Office Hadley Centre, Exeter EX1 3PB, UK.
    Buehler, Stefan A.
    Meteorological Institute, University of Hamburg, Hamburg 20146, Germany.
    Mohanakumar, Kesavapillai
    Advanced Centre for Atmospheric Radar Research, Cochin University of Science and Technology, Kerala.
    Evaluating the diurnal cycle of upper tropospheric humidity in two different climate models using satellite observations2016Inngår i: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 8, nr 4, artikkel-id 325Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The diurnal cycle of upper tropospheric humidity (UTH) is known to be influenced by such processes as convection and the formation of clouds which are parameterized in current global climate models. In this study, we evaluate the performance of two climate models, the Community Atmospheric Model version 5 (CAM-5) and the Global Atmosphere 3.0 (GA-3) model in simulating the diurnal cycle of UTH (represented by a combination of sinusoids of 12 and 24 h periods) by comparing with microwave and infrared (IR) measurements (where available). These comparisons were made over two convective land regions in South America and Africa, and over oceanic regions in the Atlantic, Indian and West Pacific for the month of January 2007. We analyzed how the diurnal cycles from IR and microwave instruments differ, and the reason for the differences. Our study suggests that the differences in the diurnal cycles of IR and microwave UTH result from sampling differences due to the presence of clouds. As noted by earlier studies, the models exhibit considerable discrepancies in diurnal amplitude and phase relative to observations, and these discrepancies have different magnitudes over land and ocean.

  • 100.
    Kottayil, Ajil
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    John, V.O.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Correcting diurnal cycle aliasing in satellite microwave humidity sounder measurements2013Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, nr 1, s. 101-113Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Microwave humidity measurements from polar orbiting satellites are affected by diurnal sampling biases which are caused by changes in the local observation time of the satellites. The long term data records available from these satellites thus have spurious trends, which must be corrected. Diurnal cycles of the microwave measurements have been constructed by combining data over the period 2001--2010 from five different satellite platforms (NOAA-15, -16, -17, -18, and MetOpA). This climatological diurnal cycle has been used to deduce and correct the diurnal sampling bias in AMSU-B and MHS measurements. Diurnal amplitudes for channels which are sensitive to surface temperature variations show a sharp land-sea contrast with the amplitudes exceeding 10 K for land regions, but less than one Kelvin for oceanic regions. The humidity channels sensitive to the upper and middle troposphere exhibit a seasonal variation with large diurnal amplitudes over convective land regions (often above 3 K) in comparison to oceanic regions. The diurnal peak times of these channels over land occur in the early mornings. The diurnal sampling bias correction has a greater impact over land regions when compared to oceanic regions due to the large diurnal amplitudes over land. The diurnal cycle of humidity generated as a part of this study could be used to evaluate diurnal cycles in climate models.

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