Interannual to Diurnal Variations in Tropical and Subtropical Deep Convective Clouds and Convective Overshooting from Seven Years of AMSU-B Measurements

Abstract

Abstract This study surveys interannual to diurnal variations of tropical deep convective clouds and convective overshooting using the Advanced Microwave Sounding Unit B (AMSU-B) aboard the NOAA polar orbiting satellites from 1999 to 2005. The methodology used to detect tropical deep convective clouds is based on the advantage of microwave radiances to penetrate clouds. The major concentrations of tropical deep convective clouds are found over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), tropical Africa, the Indian Ocean, the Indonesia maritime region, and tropical and South America. The geographical distributions are consistent with previous results from infrared-based measurements, but the cloud fractions present in this study are lower. Land–ocean and Northern–Southern Hemisphere (NH–SH) contrasts are found for tropical deep convective clouds. The mean tropical deep convective clouds have a slightly decreasing trend with −0.016% decade−1 in 1999−2005 while the mean convective overshooting has a distinct decreasing trend with −0.142% decade−1. The trends vary with the underlying surface (ocean or land) and with latitude. A secondary ITCZ occurring over the eastern Pacific between 2° and 8°S and only in boreal spring is predominantly found to be associated with cold sea surface temperatures in La Niña years. The seasonal cycles of deep convective cloud and convective overshooting are stronger over land than over ocean. The seasonal migration is pronounced and moves south with the sun from summer to winter and is particularly dramatic over land. The diurnal cycles of deep convective clouds and convective overshooting peak in the early evening and have their minima in the late morning over the tropical land. Over the tropical ocean the diurnal cycles peak in the morning and have their minima in the afternoon to early evening. The diurnal cycles over the NH and SH subtropical regions vary with the seasons. The local times of the maximum and minimum fractions also vary with the seasons. As the detected deep convective cloud fractions are sensitive to the algorithms and satellite sensors used and are influenced by the life cycles of deep convective clouds, the results presented in this study provide information complementary to present tropical deep convective cloud climatologies.