Where is the level of neutral buoyancy for deep convection?

Abstract

This study revisits an old concept in meteorology-level of neutral buoyancy (LNB). The classic definition of LNB is derived from the parcel theory and can be estimated from the ambient sounding (LNB-sounding) without having to observe any actual convective cloud development. In reality, however, convection interacts with the environment in complicated ways; it will eventually manage to find its own effective LNB and manifests it through detraining masses and developing anvils (LNB-observation). This study conducts a near-global survey of LNB-observation for tropical deep convection using CloudSat data and makes comparison with the corresponding LNB-sounding. The principal findings are as follows: First, although LNB-sounding provides a reasonable upper bound for convective development, correlation between LNB-sounding and LNB-observation is low suggesting that ambient sounding contains limited information for accurately predicting the actual LNB. Second, maximum mass outflow is located more than 3km lower than LNB-sounding. Hence, from convective transport perspective, LNB-sounding is a significant overestimate of the "destination" height level of the detrained mass. Third, LNB-observation is consistently higher over land than over ocean, although LNB-sounding is similar between land and ocean. This difference is likely related to the contrasts in convective strength and environment between land and ocean. Finally, we estimate the bulk entrainment rates associated with the observed deep convection, which can serve as an observational basis for adjusting GCM cumulus parameterization. © 2012. American Geophysical Union. All Rights Reserved.