Statistical Analyses of Satellite Cloud Object Data from CERES. PartI: Methodology and Preliminary Results of the 1998 El Nino/2000 LaNina

摘要

This study presents an objective classification methodology that uses Earth Observing System (EOS) satellite data to classify distinct 'cloud objects' defined by cloud-system types, sizes, geographic locations, and matched large-scale environments. This analysis method identifies a cloud object as a contiguous region of the earth with a single dominant cloud-system type. It determines the shape and size of the cloud object from the satellite data and the cloud-system selection criteria. The statistical properties of the identified cloud objects are analyzed in terms of probability density functions (PDFs) based upon the Clouds and the Earth's Radiant Energy System (CERES) Single Satellite Footprint (SSF) data. Four distinct types of oceanic cloud objects-tropical deep convection, boundary layer cumulus, transition stratocumulus, and solid stratus-are initially identified from the CERES data collected from the Tropical Rainfall Measuring Mission (TRMM) satellite for this study. Preliminary results are presented from the analysis of the grand-mean PDFs of these four distinct types of cloud objects associated with the strong 1997/98 El Nino in March 1998 and the very weak 2000 La Nina in March 2000. A majority of the CERES footprint statistical characteristics of observed tropical deep convection are similar between the two periods in spite of the climatological contrast. There are, however, statistically significant differences in some cloud macrophysical properties such as the cloud-top height and cloud-top pressure and moderately significant differences in outgoing longwave radiation (OLR), cloud-top temperature, and ice diameter. The footprint statistical characteristics of the three observed boundary layer cloud-system types are distinctly different from one another in all cloud microphysical, macrophysical, optical properties, and radiative fluxes. The differences between the two periods are not significant for most cloud microphysical and optical properties and the top-of-the-atmosphere albedo, but are statistically significant for some cloud macrophysical properties and OLR. These characteristics of the grand-mean PDFs of cloud microphysical, macrophysical, and optical properties and radiative fluxes can be usefully compared with cloud model simulations. Furthermore, the proportion of different boundary layer cloud types is changed between the two periods in spite of small differences in their grand-mean statistical properties. An increase of the stratus population and a decrease of the cumulus population are evident in the El Nino period compared to the very weak La Nina period. The number of the largest tropical convective cloud objects is larger during the El Nino period, but the total number of tropical convective cloud objects is approximately the same in the two periods.