Entrainment and Detrainment in Numerically Simulated Cumulus Congestus Clouds.

In order to better understand the mechanisms for cumulus entrainment, we have numerically simulated cumulus congestus clouds in three dimensions, using one level of grid nesting to achieve grid spacings as small as 50 m. The model does not allow precipitation or ice formation, and achieves saturation through bulk condensation. The environment is that of New Mexican cumulus clouds that were observed on 9-10 August 1987. The topographical surface heating is simulated by a Gaussian surface heat flux profile. The clouds are 1-4 km in vertical extent, and undergo episodes of growth and decay. Model results agree nicely with available observations. We find that cumulus entrainment is primarily the result not of plume-like processes or penetrative downdrafts, but rather is associated with the toroidal circulation of rising thermals. These thermals contain cores of undilute subcloud air that play a vital role in cumulus entrainment. Thermals that have exhausted their buoyancy due to entrainment or overshooting may continue upward for a significant distance due to their momentum. Their subsequent collapse is responsible for a large portion of the entrainment. Our findings are consistent with the shedding thermal model proposed by Blyth et al. (1988). A significant detrainment layer was found in one of the simulations. The height of the layer was correlated with the level at which the buoyancy of actual parcels decreased most rapidly with height. The presence and strength of detrainment layers is evidently quite sensitive to the vertical buoyancy profile. It is not possible to generate realistic convective clouds (of the scale considered here) from a quiescent model atmosphere; turbulent motion is needed in order to broaden the clouds and reduce their buoyancy. One means for accomplishing this is presented. Spatial resolution at least as fine as that used here (50 m) appears necessary in order to capture the details of cumulus entrainment. Clouds generated on coarser grids do not entrain at a sufficient rate, leading to updrafts that are too strong and cloud tops that are too high.