Entrainment and detrainment required to explain updraft properties and work dissipation

Abstract

A one-dimensional thermodynamic entrainment-detrainment model is used to determine updraft virtual temperature excess, updraft velocity, and other updraft properties from sounding data. The model correctly predicts most updraft properties and explains how work of buoyancy is dissipated. The unique feature of the model is that fractional entrainment and detrainment are both functions of the virtual temperature excess of the updraft and independent of updraft mass or diameter. The updraft temperature and composition are rigorously determined before updraft velocity is considered. The entrainment and detrainment functions allow the flows in and out of the updraft to vary in a physically realistic way and are used from the base of the sounding to cloud top. The model limits the growth of cumulus under conditions of dry air aloft. The model shows that entrainment inhibits deep convection. The model predicts higher intensity for continental than for oceanic updrafts. High humidity at the bottom of the atmosphere decreases the intensity of the updrafts because it lowers the condensation level, the level at which evaporative cooling comes into play. High humidity aloft increases the intensity of the updrafts because it reduced evaporative cooling.