Entrainment and detrainment in numerically simulated cumulus congestus clouds. Part II: Cloud budgets

This paper is the second in a three-part series in which a three-dimensional numerical cloud model is used to simulate cumulus congests clouds at high resolution in an effort to better understand the mechanisms associated with entrainment and detrainment. The prescribed environment is that associated with nonprecipitating summertime New Mexican cumulus clouds that formed on consecutive days. Using budgests of mass and moisture. the effects of the clouds and their environment are examined here with an emphasis on understanding the life cycle of the clouds and the production of narrow detrainment layers aloft. Results are compared with measurements obtained in simillar environments. The mass flux profiles indicate the presence of a strong, persistent thermally driven circulation within the boundary layer, with the cloud circulation being secondary. Collaspsing turrets appear to be responsible for the significant detrainment that occurs at mid levels within one simulated cloud. Transport by downdrafts is significant throughout the cloud and subcloud layers. The boundary layer and cloud circulations dry the subcloud layer, with significant detrainment of moisture occuring in the upper portion of the boundary layer. Strong apparent moistening in the upper half of one cloud is driven by mean vertical transport of moisture toward the detrainment layer aloft. alough detrainment of moisture at midlevels is a comparatively small component of the apparent moistening. Storage of moisture is found to be an important effect.