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

Journal of Climate, Sep 1, 2008 by Hong, Gang, Heygster, Georg, Notholt, Justus, Buehler, Stefan A

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^sup -1^ in 1999-2005 while the mean convective overshooting has a distinct decreasing trend with -0.142% decade^sup -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 8S and only in boreal spring is predominantly found to be associated with cold sea surface temperatures in La Nia 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.

1. Introduction

Deep convective clouds play an important role in the earth’s climate by transporting heat, moisture, and momentum from the lower to the upper troposphere. The tropical tropopause layer (TTL) between about 14 and 18 km (Sherwood and Dessler 2001; Alcala and Dessler 2002) is the important transition layer between the tropical troposphere and the stratosphere (Thuburn and Craig 2002; Liu and Zipser 2005). The convective overshooting penetrating the TTL affects the exchange between the troposphere and the stratosphere, thereby influencing the physical and chemical processes in the TTL and lower stratosphere (Gettelman et al. 2002; Notholt et al. 2003). Deep convective clouds also influence the earth’s radiation budget by interacting with incoming shortwave and outgoing longwave radiation (Hendon and Woodberry 1993; Hall and Yonder Haar 1999; Lin et al. 2006).

The seasonal and diurnal cycles of tropical deep convective clouds belong to the most fundamental variations of the climate system (Yang and Slingo 2001; Slingo et al. 2004). The ability to represent the seasonal and diurnal cycles is a key parameter when investigating climate models. Recently, Slingo et al. (2004) evaluated the representation of the diurnal cycle in the Hadley Centre Climate Model using simulations of the infrared radiances observed by Meteosat 7. They found that the diurnal cycle errors most probably arise from deficiencies in the convection scheme of the model. Significant differences between the measurements and model simulations were found by comparing basic cloud climatologies from 10 general circulation models (GCMs) with satellite measurements (Zhang et al. 2005).

Satellite observations provide a global survey of tropical deep convective clouds. Their advantages of high temporal sampling and wide spatial coverage have been extensively used to study the seasonal and diurnal cycles of tropical deep convective clouds (e.g., Yang and Slingo 2001; Belts and Jakob 2002; Slingo et al
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