Séminaire

In recent decades the uncertainty in climate sensitivity has been traced back to cloud-climate feedbacks; a fact reiterated by the Intergovernmental Panel on Climate Change (Solomon et al., 2007). The magnitude which clouds respond to radiative forcings effectively determines the extent to which the Earth’s temperature will amplify due to an increase in greenhouse gases. Boundary layer clouds, which reside in the lowest few kilometers of the atmosphere, are known to play a vital role in cloud-climate feedbacks. Currently, however, general circulation models (GCM) are unable to realistically represent the cloudy boundary layer well. The ECHAM5 GCM was also shown to significantly underestimate (Sub)Tropical low-level clouds. As such, three new boundary layer schemes were included within ECHAM5. They include: a standard convective mass-flux scheme (referred to as ECHAM5 Std; Tiedtke, 1989); a modified version of the Tiedtke (1989) scheme in which a new convective trigger alters the statistics of shallow convection (referred to as ECHAM5 Trig; Roeckner & Esch, 2010); a bulk parameterization of the effects of transient shallow cumulus clouds (referred to as ECHAM5 VSMF; Von Salzen & McFarlane, 2002; Isotta, 2010); and an eddy diffusivity Dual Mass Flux scheme adjusted to better represent shallow convection (referred to as ECHAM5 DMF; Neggers et al., 2009).

The CALIPSO and CloudSat satellite retrievals, as well as their simulators, were used to evaluate the four different boundary layer cloud parameterizations. The CALIPSO and CloudSat satellites host active lidar and radar instruments that provide global coverage of the vertical distribution of clouds and their properties. Thus allowing the questions of how well does ECHAM5 represent clouds and precipitation in the present climate and whether the different representations of boundary layer clouds perform better compared to the CALIPSO and CloudSat retrievals to be addressed. Additionally, cloud-climate feedbacks and their sensitivity to boundary layer clouds were assessed by running the different boundary layer parameterizations in a series of idealized climate change experiments.

MP Lefebvre (01 44 27 27 99)