Regional scale studies

Clouds properties can be:

• macrophysical: altitude, spatial extension, water content
• optical: optical depth,
• microphysical: particle size, shape, orientation
• radiative: shortwave and longwave fluxes

All theses are strongly linked to each other, and are primarily driven by the atmospheric environment (thermodynamic and dynamic). As the atmospheric environment is different among regions (i.e. dry air in polar regions compared to the Tropics, deep convection at low latitude, storm-tracks at mid-latitude, interaction between surface and atmosphere above ocean and continent, etc.), the processes leading to the formation, maintenance and dissipation of clouds cannot be simply summarized in one picture that would be valid everywhere.

Here we take advantage of new satellite observations to examine specific cloud properties in different regions of the planet (Tropics, mid-latitude, polar regions) in relation with their environment to improve our understanding of cloud processes specificity in each region. The activities presented in this section include boundary layer clouds whose properties are depending on the interaction between the surface and the atmosphere, as well as upper tropospheric clouds composed of ice.

Global scale studies

Here we consider clouds at global scale (i.e. more directly relevant to climate) to explore large patterns such as the link between cloud occurrence (and properties) and the large scale atmospheric circulation, or the link between cloud properties and their global radiative impact. A particular emphasis is given to high-level optically-thin ice clouds which tend to trap infrared outgoing radiation, (the greenhouse effect, which produces a net heating), while their ability to reflect incoming solar shortwave radiation (the albedo effect, which induces a cooling) is limited by their semi-transparency (Stephens et al. 1990). The sign of the net radiative forcing of these high cirrus therefore depends on their layer optical depth and the vertical distribution of ice water content, concentration, effective radius, shape and orientation of their ice crystals. Another emphasis is given to the evaluation of the description of tropical boundary layer clouds in climate models, because they constitute a significant source of uncertainty for model-based estimates of future climate (Bony et al. 2004). Thanks to their ability to simultaneously document  different variables on a global scale, during several years, at high spatial resolution, A-train observations can lead to significant advances on these topics.