The Role of Cloud Top Entrainment in Cumulus Clouds

An attempt has been made to discover the role played by cloud tops in the entrainment process in cumulus clouds, as well as to determine whether observed downdraft magnitudes can be explained on the basis of evaporative cooling. An instrumented research aircraft was used to acquire thermodynamic, microphysical and dynamical data within, and near to, cumulus clouds over the high plains and midwestern U.S. The entrainment source regions found were either at or above the aircraft sampling level in 78 of the 87 cases examined, with the environment within 20 mb of the cloud top being the source of entrained air in these cases. Vertical velocity measurements acquired in these clouds were compared with those predicted by evaporative cooling, which is found to adequately explain the magnitude of the observed downdrafts in the 51 cases where a comparison between predicted and observed downdraft magnitude was possible.