Detection of tropical deep convective clouds from AMSU‐B water vapor channels measurements

Abstract

Methods to detect tropical deep convective clouds and convective overshooting from measurements at the three water vapor channels (183.3 ± 1, 183.3 ± 3, and 183.3 ± 7 GHz) of the Advanced Microwave Sounding Unit‐B (AMSU‐B) are presented. Thresholds for the brightness temperature differences between the three channels are suggested as criterion to detect deep convective clouds, and an order relation between the differences is used to detect convective overshooting. The procedure is based on an investigation of the influence of deep convective cloud systems on the microwave brightness temperatures at frequencies from 89 to 220 GHz using simultaneous aircraft microwave and radar measurements over two tropical deep convective cloud systems, taken during the Tropical Rainfall Measuring Mission (TRMM) Large Scale Biosphere‐Atmosphere Experiment (LBA) campaign. Two other aircraft cases with deep convective cloud systems observed during the Third Convection and Moisture Experiment (CAMEX‐3) are used to validate the criteria. Furthermore, a microwave radiative transfer model and simulated mature tropical squall line data derived from the Goddard Cumulus Ensemble (GCE) model are used to validate the procedures and to adapt the criteria to the varying viewing angle of AMSU‐B. These methods are employed to investigate the distributions of deep convective clouds and convective overshooting in the tropics (30°S to 30°N) for the four 3‐month seasons from March 2003 to February 2004 using the AMSU‐B data from NOAA‐15, ‐16, and ‐17. The distributions show a seasonal variability of shifting from the winter hemisphere to the summer hemisphere. The distributions of deep convective clouds follow the seasonal patterns of the surface rainfall rates. The deep convective clouds over land penetrate more frequently into the tropical tropopause layer than those over ocean. The averaged deep convective cloud fraction is about 0.3% in the tropics, and convective overshooting contributes about 26% to this.