Cloud-Top Evolution of Tropical Oceanic Squall Lines from Radar Reflectivity and Infrared Satellite Data

Abstract

Precipitation estimation over the tropical oceans is commonly performed using passive infrared (IR) measurements of cloud-top brightness temperature from geostationary satellites to infer the location of deep convection. It has been recognized in recent years that the majority of tropical precipitation is produced by mesoscale convective systems (MCSs). However, the relationship between the IR cloud-top patterns associated with MCSs and the underlying precipitation is not well understood. The assumption that the coldest cloud tops are associated with deep, active convection has been central to the characterization of cloud system motion and organization, and to many IR-based rainfall retrievals. Previous studies suggested that this view may be oversimplified when applied to propagating convective systems, such as squall lines. The goal of this study was to understand the evolution of the cold cloud associated with tropical oceanic squall line MCSs, and to discuss the implications for the retrieval of precipitation organization and rainfall rate from satellite IR data. Shipboard radar reflectivity and Geostationary Meteorological Satellite (Japan) brightness temperature data collected during the Tropical Oceans Global Atmospheres Coupled Ocean?Atmosphere Response Experiment have been used to study the evolution of two tropical oceanic squall line MCSs. Results suggested a complex, evolution-dependent relationship between the radar-derived precipitation pattern associated with mesoscale convective systems and the overlying cloud tops observed by the satellite. The coldest clouds formed in the wake of the leading edge of the propagating lines, following the intensification of deep convection on the leading edge. In an environment of deep tropospheric directional wind shear (e.g., during westerly wind bursts), the cold cloud shield became spatially decoupled from the source convection to form swaths of cold cloud. These cloud swaths were generally normal to the squall line orientation. As convection on the leading edge of the squall line weakened, the cold cloud shield expanded. As a result the coldest clouds were more closely associated with weaker surface precipitation. These results have implications for the interpretation of the location, shape, and motion of cold cloud features in the tropical western Pacific region. They may also help to explain the poor performance of IR rain retrieval algorithm when applied to instantaneous images. Results from this study may aid in the interpretation of twice-per-day ?snapshots ? of MCSs (from IR, microwave, and radar sensors) from the recently launched Tropical Rainfall Measuring Mission satellite.