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  • 1.
    Johansson, Erik
    Stockholm University, Faculty of Science, Department of Meteorology . SMHI - Sveriges meteorologiska och hydrologiska institut.
    Improving the understanding of cloud radiative heating2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Clouds play an essential role in regulating Earth’s radiation budget by reflecting and absorbing energy at different spectra. As they interact with radiation, they can radiatively heat or cool the adjacent atmosphere and the surface. This heating effect can have a strong implication for the circulation and can change the surface properties by, for example, melting sea ice. The lack of high-resolution global observations has previously been a limitation for our understanding of the vertical structure of cloud radiative heating, and for evaluating the cloud radiative effect in climate models. In this thesis, we will investigate and document cloud radiative heating derived from space-based observations. We will focus on two regions, the Arctic and the Tropics, where cloud radiative heating plays an important, but fundamentally different role.

    In the Tropics, radiative heating at high altitudes influences the large scale circulation. Stratiform, deep convective, and cirrus clouds have a strong radiative impact in the upper troposphere. We found while investigating the Indian monsoon, that thick stratiform clouds will radiatively heat the upper troposphere by more than 0.2 K/day when the monsoon is most intense during June, July and August. Deep convective clouds cause considerable heating in the middle troposphere and at the same time, cool the tropical tropopause layer (TTL). These two thick cloud types will also cool the surface during the monsoon, weakening the temperature gradient between land and ocean. During these months, cirrus clouds are frequently located inside the TTL. We further find that in the Tropics, the climate model, EC-Earth, can capture the seasonal variations in cloud radiative heating seen in the satellite observations. However, the model overestimates the radiative heating in the upper region  and underestimates them in the middle region of the troposphere. This dissimilarity is caused by unrealistic longwave heating and low cloud fraction in the upper and middle of the troposphere, respectively.

    Radiative heating from cirrus, located inside the TTL, is considered to play an important role in the mass transport from the troposphere to the stratosphere. This heating generates enough buoyancy so that the air can pass the barrier of zero net radiative heating. We find that high thin single-layer clouds can heat the upper troposphere by 0.07 K/day. If a thick cloud layer is present underneath, they will radiatively suppress the high cloud, causing it to cool the adjacent air instead. The optical depth and cloud top height of the underlying cloud are two crucial factors that radiatively impact the high cloud above.

    Warm moist air is regularly transported from the mid-latitudes into the Arctic by low- and high-pressure systems. As the moist air enters the Arctic, it increases the cloudiness and warms the surface. This surface heating has the potential to affect the ice cover months after the intrusion. We find that during extreme moist intrusions, the surface temperature in the Arctic can rise by more than 5 K during the winter months with an increase in cloudiness by up to 30% downstream from the intrusion. These extra clouds radiatively heat the lower part of the atmosphere and cool the middle part, affecting the stability of the Arctic atmosphere.

  • 2.
    Johansson, Erik
    et al.
    Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, A.
    L'Ecuyer, T.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    The vertical structure of cloud radiative heating over the Indian subcontinent during summer monsoon2015In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, no 20, p. 11557-11570Article in journal (Refereed)
    Abstract [en]

    Clouds forming during the summer monsoon over the Indian subcontinent affect its evolution through their radiative impact as well as the release of latent heat. While the latter is previously studied to some extent, comparatively little is known about the radiative impact of different cloud types and the vertical structure of their radiative heating/cooling effects. Therefore, the main aim of this study is to partly fill this knowledge gap by investigating and documenting the vertical distributions of the different cloud types associated with the Indian monsoon and their radiative heating/cooling using the active radar and lidar sensors on-board CloudSat and CALIPSO. The intraseasonal evolution of clouds from May to October is also investigated to understand pre-to-post monsoon transitioning of their radiative heating/cooling effects. The vertical structure of cloud radiative heating (CRH) follows the northward migration and retreat of the monsoon from May to October. Throughout this time period, stratiform clouds radiatively warm the middle troposphere and cool the upper troposphere by more than +/- 0.2 K day(-1) (after weighing by cloud fraction), with the largest impacts observed in June, July and August. During these months, the fraction of high thin cloud remains high in the tropical tropopause layer (TTL). Deep convective towers cause considerable radiative warming in the middle and upper troposphere, but strongly cool the base and inside of the TTL. This cooling is stronger during active (-1.23 K day(-1)) monsoon periods compared to break periods (-0.36 K day(-1)). The contrasting radiative warming effect of high clouds in the TTL is twice as largeduring active periods than in break periods. These results highlight the increasing importance of CRH with altitude, especially in the TTL. Stratiform (made up of alto- and nimbostratus clouds) and deep convection clouds radiatively cool the surface by approximately -100 and -400Wm(-2) respectively while warming the atmosphere radiatively by about 40 to 150Wm(-2). While the cooling at the surface induced by deep convection and stratiform clouds is largest during active periods of monsoon, the importance of stratiform clouds further increases during break periods. The contrasting CREs (cloud radiative effects) in the atmosphere and at surface, and during active and break periods, should have direct implications for the monsoonal circulation.

  • 3.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, Abhay
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    How Does Cloud Overlap Affect the Radiative Heating in the Tropical Upper Troposphere/Lower Stratosphere?2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 10, p. 5623-5631Article in journal (Refereed)
    Abstract [en]

    Characterizing two-layer cloud systems has historically been difficult. These systems have a strong radiative impact on the composition of and the processes in the upper troposphere-lower stratosphere (UTLS). Using 4 years of combined spaceborne lidar and radar observations, the radiative impact of two-layer cloud systems in the tropical UTLS is characterized, and its sensitivity to the properties of top- and bottom-layer clouds is further quantified. Under these overlapping cloud conditions, the bottom-layer clouds can fully suppress the radiative heating caused by high clouds in the UTLS, by inducing strong longwave cooling. If the vertical separation between the layers is <4 km, the radiative heating of the high cloud changes sign from positive to negative. Furthermore, the radiative effect at the top of the atmosphere is investigated, and it is found that the characteristic net warming by cirrus with ice water path <50 g/m(2) is suppressed in the two-layered system.

  • 4.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute (SMHI), Sweden.
    Devasthale, Abhay
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    Response of the lower troposphere to moisture intrusions into the Arctic2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 5, p. 2527-2536Article in journal (Refereed)
    Abstract [en]

    Water vapor intrusions (WVIs) explain a significant fraction of total moisture transport and its variability in the Arctic. WVIs can precondition the Arctic atmosphere for accelerated melting of sea ice through effects on surface longwave radiation. Using data from the NASA's A-Train convoy of satellites to estimate the response of the lower troposphere to WVIs into the Arctic, we show that WVIs are associated with a surface warming of up to 5.3K (3.3K) in winter and 2.3K (1.6K) in summer, when averaged over the entire Arctic Ocean. The intrusions also lead to additional cloud radiative heating of up to 0.15K/d via up to 30% increased cloudiness in the vertical and also cause a weakening of the stability in the lower troposphere. The lower tropospheric and surface warming during winter and spring highlights the importance of understanding contribution of preconditioning to accelerated ice melt in the Arctic.

  • 5.
    Johansson, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Devasthale, Abhay
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    L'Ecuyer, Tristan
    Wyser, Klaus
    Vertical cloud radiative heating in the tropics: Confronting the EC-Earth model with satellite observationsManuscript (preprint) (Other academic)
    Abstract [en]

    Understanding the coupling of clouds to large-scale circulation is one of the grand challenges faced by the global climate community. In this context, realistically simulating the vertical structure of cloud radiative heating/cooling (CRH) is a key premise to understand these couplings using Earth system models. Here, we evaluate CRH in two versions of the European Community Earth System Model (EC-Earth) using retrievals derived from the combined radar and lidar data from the CloudSat and CALIPSO satellites. One model version is used with two different horizontal resolutions (high resolution and its standard counterpart, i.e. EC-Earth3P-HR and EC-Earth3P). The other model version, EC-Earth3, is the EC-Earth version used for the AMIP experiment for CMIP6 experiments. The study focuses on the tropical region and the vertical structure of CRH and cloud properties, as such an evaluation has not yet been carried out for EC-Earth. We begin by evaluating the large-scale intra-seasonal variability in CRH in the different model versions, followed by an investigation of the changes in CRH during different phases of the El Nino Southern Oscillation (ENSO), a process that dominates the interannual climate variability in the tropics.

    All versions of EC-Earth evaluated here generally capture both the intra-seasonal and meridional variability in CRH over the convectively active and stratocumulus regions, and the CRH during the positive and negative phases of ENSO. However, two key differences between all model simulations and satellite retrievals emerge. First, the magnitude of CRH over the convectively active zones is up to twice as large in the models compared to the satellite data. Further dissection of net CRH into its shortwave and longwave components reveals noticeable differences in their vertical structure. The shortwave component of the radiative heating is overestimated by all model versions in the lowermost troposphere and underestimated in the middle troposphere. These over- and underestimations of shortwave heating are partly compensated by an overestimation of longwave cooling in the lowermost troposphere and heating in the middle troposphere. The observed biases in CRH can be traced back to disagreements in the cloud amount and cloud water content. We observe no noticeable improvement in the simulation of CRH by purely increasing the horizontal resolution in the model.

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