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vendredi 20 décembre 2013

    Impacts of atmospheric radiative forcings on the tropical climate

The radiative effects of tropical low-clouds have long been recognized
as critical for climate sensitivity estimates due to their role in
modulating the Earth's radiative budget at the top of the atmosphere.
Within the atmosphere, they exert a strong radiative cooling, and have
thus the potential to affect also the large scale atmospheric
circulation and the global hydrological cycle. The motivation of this
thesis is to better understand the role of atmospheric cloud radiative
effects (ACRE) of low-clouds in tropical circulation and precipitation.
To investigate this issue, planetary boundary layer (PBL) clouds are
made transparent to radiation in a climate model. It is found that PBL
ACRE enhance tropical precipitation and large-scale circulation through
their  coupling with surface turbulent fluxes. These results are shown
to be robust across models and model configurations.
The time-scale of the climate response to PBL ACRE is investigated by
running the climate model in a numerical weather prediction mode. This
approach is shown to be useful for studying the fast climate response to
radiative perturbations. Short-term forecasts show that the coupling
between PBL ACRE and surface heat fluxes and its impacts on
precipitation and atmospheric circulation occur within days. Such a fast
response allows for the use of short-term forecasts to investigate the
impacts that changes in model parameterizations have on this coupling
and their implications for some model biases.
Finally, the role of PBL ACRE on climate change is investigated in a
multi-model framework. It is shown that in all models PBL ACRE decrease
the hydrological sensitivity (defined as the response of global mean
precipitation to an increase in surface temperature) and reduce the
spread estimates among models. On the other hand, regional patterns of
precipitation changes do not depend much on PBL ACRE, suggesting that
PBL clouds affect the patterns of precipitation changes more  through
their impact on climate sensitivity than circulation.

En présence du jury composé de:

    Dr Hervé Douville, CNRS-CNRM (Toulouse), Rapporteur
    Dr Mark Ringer, MetOffice (Exeter), Rapporteur
    Dr Jean-Louis Dufresne, LMD (Paris), Examinateur
    Dr Romain Roehrig, Meteo-France-CNRM (Toulouse), Examinateur
    Prof Bjorn Stevens, MPI (Hamburg), Examinateur
    Prof Laurence Picon, LMD (Paris), Examinatrice
    Dr Sandrine Bony, LMD (Paris), Directrice de thèse