Statistical Analyses of Satellite Cloud Object Data from CERES. Part IV: Boundary Layer Cloud Objects during 1998 El Niño

Abstract

Three boundary layer cloud object types?overcast, stratocumulus, and cumulus?that occurred over the Pacific Ocean during January?August 1998 are identified from the Clouds and the Earth?s Radiant Energy System (CERES) single scanner footprint data. Characteristics of each cloud object type matched with atmospheric states are examined for large regions in the tropics and subtropics and for different size categories. Stratocumulus cloud objects dominate the entire boundary layer cloud population in all regions and size categories. Overcast cloud objects, which have the largest average size, are more prevalent in the subtropics and near the coastal regions, while cumulus cloud objects are prevalent over the open oceans and the equatorial regions, particularly within the small-size categories. Cloud objects with equivalent diameters less than 75 km are excluded in the analysis. The differences between the tropical and subtropical statistical distributions of cloud properties are small for liquid water path (LWP), cloud optical depth, and top-of-the-atmosphere (TOA) albedo, but large for cloud-top temperature and outgoing longwave radiation (OLR), for each of the three cloud object types. The larger cloud objects have higher LWPs, cloud optical depths, TOA albedos, and OLRs, but lower SSTs and cloud-top heights for the stratocumulus and overcast types. Lower-tropospheric stability seems to be the primary factor for the differences in the distributions of cloud physical properties between the regions or between the size categories. Atmospheric dynamics also play a role in determining the differences in the distributions of cloud physical properties between the size categories, but not a significant role for those between the types or between the regions. The latter may be due to uncertainties in the matched vertical velocity data. When the three cloud object types are combined in small regions, lower-tropospheric stability determines the transition of boundary layer cloud types along a Pacific transect. The proportion of each type is the most important factor for diagnosing the combined cloud properties along this transect, such as LWP, cloud optical depth, and TOA albedo. Atmospheric dynamics also play complicated roles in determining the combined cloud properties along this transect.