Graduation date: 2007
Adiabatic parcel models suggest specific relationships between cloud thickness and cloud properties. Such relationships govern cloud radiative forcing and thus cloud feedbacks in the climate system. Current remote sensing techniques work well at measuring these properties in low-level cloud systems that are overcast. Significant biases exist, however, when measuring cloud properties in regions of broken clouds. The operational MODIS cloud product (MOD06) assumes each cloudy 1-km imager pixel is overcast. The failure to account for partial cloudiness can result in substantially greater cloud top temperatures than in overcast regions and a bias in the subsequent optical properties. Such discrepancies are accounted for in a partly cloudy pixel retrieval scheme that takes into account the fractional cloud cover in pixels that are only partly cloud covered. To assess the cloud properties in the MOD06 product
and those obtained with the partly cloudy pixel retrievals, CALIPSO lidar returns were used to identify low-level, single-layered cloud systems over oceans collocated with 1-km MODIS Aqua imagery. The MODIS cloud product, CALIPSO, and the partly cloudy pixel retrieval scheme all show similar relationships between cloud thickness and cloud properties when MODIS pixels are overcast. Cloud altitudes in overcast regions show relatively little variation within 50-km scales. When marine stratus cloud layers are broken, cloud top temperatures from the MODIS cloud product are substantially greater for partly cloudy pixels than overcast pixels within the same region. The cloud product suggests that clouds in partly cloudy pixels are at significantly lower altitudes. Such clouds exhibit biases in their cloud properties when compared with those derived using the partly cloudy pixel retrieval scheme. CALIPSO and the partly cloudy retrievals show similar relationships for cloud altitudes and cloud properties retrieved in both broken and overcast regions. The results suggest that the MODIS cloud products should be used to assess relationships between cloud altitude and cloud properties only for regions that are extensively overcast by clouds in a single layer.