Characteristics of Tropical Midlevel Clouds Using A-Train Measurements



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Midlevel clouds are observed globally and impact the general circulation through their influence on the radiation budget and their precipitation production. However, because midlevel clouds occur less frequently than high and low clouds they are relatively understudied. Satellite observations from the MODIS, CALIPSO, and CloudSat instruments onboard the A-Train are combined to study midlevel cloud characteristics in the Tropical Western Pacific (TWP) between January 2007 and December 2010. Characteristic cloud and microphysical properties including cloud top height (CTH), geometric thickness, optical depth, effective radius, and liquid or ice water path (LWP or IWP), and environmental properties, including temperature and specific humidity profiles, are determined for precipitating and non-precipitating midlevel clouds.

In the study region, approximately 14% of all cloudy scenes are classified as midlevel clouds (4 km < CTH < 8 km). These are concentrated in areas of deeper convection associated with the Pacific warm pool, ITCZ, and SPCZ. Non-precipitating clouds dominate the region, accounting for approximately 70% of all single and two-layer midlevel clouds scenes. Midlevel clouds occur most frequently in three different scenarios: high over midlevel clouds (~65%), single-layer (~25%), and midlevel over mid- or low-level clouds (~10%). Environmental moisture appears to play a larger role than temperature in determining midlevel cloud distributions due to large variations in moisture between the different cloud scenarios.

In all scenes, a trimodal distribution in CTH frequency is found within the midlevel. Two of these peaks have been identified in previous studies; however a third midlevel mode is recognized here. CTHs occur most frequently in peaks between 5-6 km, 6-6.25 km, and 6.5-7.5 km. Although the past studies have only noted two midlevel peaks, this third mode is a robust feature in this dataset. Two types of clouds dominate these peaks: non-precipitating altostratus or altocumulus-like clouds less than 1 km thick and geometrically thick precipitating cumulus congestus clouds. Environmental temperature stable layers and dry maxima are found at each one of these peak frequency heights. Again, moisture seems to play a more dominant role in determining the height of the midlevel clouds due to larger variances between the moisture gradients associated with each peak.

Microphysical properties (optical depth, effective radius, and LWP or IWP) are characterized for single-layer clouds. Approximately 30% of all single-layer midlevel clouds are precipitating and these clouds tend to occur on the edges of the deep tropics. In general, precipitating clouds have greater optical depths, effective radii, and water path. This research implies that some past studies at single point locations can be representative of the broader tropics, whereas others are not.