Cloud processes in extratropical cyclones

Most general circulation models exhibit a low bias in cloud cover in the southern oceans, which has a significant impact on the amount of absorbed solar radiation. This, in turn, has been proposed to affect various aspects of the global climate, for example an excess in southern hemisphere tropical precipitation, also known as the double inter-tropical convergence zone problem. The southern oceans are strongly influenced by extratropical cyclones, and it was found that the cold sector of these cyclones is one area where models, including reanalyses, do not produce enough clouds. Using multiple NASA A-train and Terra observations of cloud properties, precipitation, water vapor, temperatures and winds, we explore the processes that produce clouds in midlatitude cyclones. This study is two-fold; on the one hand we contrast northern and southern hemisphere cyclones to isolate those processes that are more dominant in one hemisphere than the other; while on the other we establish a series of constraints to help evaluate and improve models. We will present some recent results on the possible mechanisms that produce clouds in the cold sectors of extratropical cyclones and how this region of the cyclones differs between the two hemispheres. For this we use a cyclone database and two different techniques to detect cold and warm fronts in order to delineate the different cyclonic sectors. By using both passive and active observations, we are able to provide a three dimensional view of these mechanisms. With these results, we then test two GCMs, one that underestimates southern hemisphere clouds and the other that overestimates cloud amounts in this region.