Updraft Buoyancy within and Moistening by Cumulonimbi prior to MJO Convective Onset in a Regional Model

Abstract

rocesses responsible for widespread development of moderately deep cumulonimbi during a transition period before onset of two large-scale convective events associated with the Madden?Julian oscillation in late 2011 are investigated. A regional model (WRF) is capable of rapidly producing an approximately 3-day-long transition period prior to MJO convective onset similar to observed transition periods, during which moderately deep cumulonimbi were prevalent. During transition periods, evaporation in precipitating elements and horizontal advection of moisture away from the clouds in the nearby clear-air environment contributed to humidification below 400 hPa. Nonprecipitating clouds were present in the model mostly between 900 and 950 hPa and had no major impact on tropospheric moistening. Whether nonprecipitating cumuli grew into moderately deep cumulonimbi largely depended on the buoyancy of updrafts that extended into the 700?850-hPa layer. As mean environmental temperatures decreased, the mean cumulus updraft buoyancy in this layer became less negative. The start of two simulated transition periods were marked by rapid decreases in environmental temperature caused by reduction in environmental subsidence and/or increased cooling by advection or radiation. Small, widespread changes in the difference between 700- and 850-hPa environmental and updraft temperatures?on the order of 0.1 K and less than 0.4 K?had important ramifications for whether shallow clouds grew vertically into moderately deep clouds that moistened the troposphere and made it conducive to MJO convective onset.