A-Train Observations in Extratropical Cyclones: A Comprehensive Tool for Model Evaluation

Abstract:

Most general circulation models (GCMs) 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 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.

By combining 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. We divide the cyclones into their different sectors and explore the dominant cloud types and their relation with the environment using compositing techniques. For this we use a cyclone database and two different algorithms to detect cold and warm fronts in order to delineate the different sectors of the cyclones. We use the same techniques to analyze model outputs, so we can directly compare observations and models.

We will present results on the mechanisms that control cloud production in the post cold frontal region of the extratropical cyclones. 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. We will show how the convection and planetary boundary layer parameterizations and the lower tropospheric moist stability interact and provide a key indicator of GCM post-frontal cloud content in these storms.