Interactions between warm rain clouds and atmospheric preconditioning for deep convection in the tropics

摘要

Positive low- and midlevel moisture and heat anomalies have been observed in many previous studies to occur prior to deep convective events (DCEs) in the tropics. Shallow and midlevel convection has been presumed to supply some of the heat and moisture necessary for atmospheric preconditioning and the generation of convective available potential energy (CAPE) for the development of this deep convection. Other recent studies have also shown that warm precipitating cumulus convection is strongly sensitive to sea surface temperature (SST) and that variability in precipitation efficiency may lead to changes in the amount of cloud water available for moistening the troposphere. In this study, a previously developed multisensor retrieval algorithm for Tropical Rainfall Measuring Mission (TRMM) satellite measurements is applied to investigate the influence of warm rain clouds on atmospheric preconditioning for deep convection. DCEs at three locations across the Pacific Ocean are identified using precipitation maxima for warm seasons from 1998 to 2002. Composites of thermodynamic anomalies are compared to warm rain cloud liquid water path (LWP) anomalies to explore the effects of clouds on low-level temperature and humidity, as well as CAPE generation. Results show that positive anomalies in precipitating warm cloud LWP occur with positive low-level moisture and heat anomalies prior to DCEs. Events composited by the mean SST anomaly highlight the sensitivity of warm precipitating cloud properties to temperature and show corresponding changes in moisture. At anomalously warm SSTs, warm precipitating clouds are more efficient at producing precipitation, leaving less water to evaporate and moisten the atmosphere, which corresponds with observed lower moisture anomalies and decreased CAPE generation.