Cloud vertical distribution in extratropical cyclones

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Description

Using two consecutive winters of CloudSat-CALIPSO observations, NCEP-2 reanalysis atmospheric state parameters over the northern and southern hemisphere oceans (30°–70° N/S) between November 2006 and September 2008, and an automated front detection algorithm, we examined how clouds are distributed along the vertical across warm and cold fronts in extratropical cyclones. These distributions generally resemble those from the original model introduced by the Bergen School in the 1920s, with the following exceptions: (1) Substantial low cloudiness which is present behind and ahead of the warm and cold fronts; (2) Ubiquitous high cloudiness, some of it very thin, throughout the warm-frontal region; (3) Upright convective cloudiness near and behind some warm fronts. One winter of GISS general circulation model simulations of fronts at 2°x2.5°x32L resolution gave similar cloud distributions but with much lower cloud fraction, a shallower depth of cloudiness, and a shorter extent of tilted warm-frontal cloud cover in the cold sector. A close examination of the relationship between the cloudiness and relative humidity fields indicated that upward transport of water vapor is too weak in modeled mid-latitude cyclones, and this is related to weak vertical velocities in the model. The model also produced too little cloudiness for a given value of vertical velocity or relative humidity. For global climate models run at scales coarser than tens of kilometers, we suggest that the current underestimate of modeled cloud cover in the storm track regions, and in particular the 50°–60°S band of the southern oceans, could be reduced with the implementation of a slantwise convection parameterization. We will discuss how these results can be expanded to investigate modeled cloud properties (e.g. LWC, IWC, optical thickness) within cold and warm fronts using long-term ground-based ASR observations at the Southern Great Plains site.