The Effects of Longwave Radiation in a Small Cumulus Cloud

Abstract

The effects of longwave radiation in a small cumulus cloud are investigated by a combination of a three-dimensional radiative transfer model as well as a slab-symmetric cloud dynamics model. The calculations indicate that longwave radiative cooling substantially enhances the maximum cloud water content. For a run in an environment without wind shear, the maximum increase reaches 96%. The total cloud water content was also increased somewhat (maximum 20%). The effects of longwave cooling at different stages of development of the simulated cloud were further examined and analyzed. In the initial stage of the development, the augmentation of cloud water content near the cloud top and sides is traced mainly to the direct effect of longwave radiative cooling on cloud microphysics (i.e., radiative cooling reduces the local temperature and hence the saturation water vapor pressure, which leads to additional condensation). In the mature stage of the cloud, the increase of total cloud water content arises from a combination of the effects of radiation on microphysics and dynamics. The cooling from radiation and evaporation produces additional downward motion at the sides of the cloud. The enhanced low-level convergence invigorates the updraft to promote further cloud development. In the decaying stage, the negative buoyancy produced by cloud top radiative cooling and a higher liquid water load speeds up the decay process. The effect of wind shear was also studied. It was shown that, similar to the case of a calm environment, longwave cooling strengthens the secondary circulation and the cloud water content. However, shear suppresses convection and the cloud becomes weaker. Longwave cooling also enhances the asymmetric characteristics of the simulated cloud. In conjunction with horizontal momentum transport, radiative cooling results in a more negative temperature perturbation and a stronger downdraft on the downshear flank relative to the upshear side.