THE EFFECTS OF LONGWAVE RADIATION IN A SMALL CUMULUS CLOUD

The effects of longwave radiation in a small cumulus cloud are investi gated by a combination of a three-dimensional radiative transfer model as well as a slab-symmetric cloud dynamics model. The calculations in dicate that longwave radiative cooling substantially enhances the maxi mum cloud water content. For a run in an environment without wind shea r, the maximum increase reaches 96%. The total cloud water content was also increased somewhat (maximum 20%). The effects of longwave coolin g at different stages of development of the simulated cloud were furth er examined and analyzed. In the initial stage of the development, the augmentation of cloud water content near the cloud top and sides is t raced mainly to the direct effect of longwave radiative cooling on clo ud microphysics (i.e., radiative cooling reduces the local temperature and hence the saturation water vapor pressure, which leads to additio nal condensation). In the mature stage of the cloud, the increase of t otal cloud water content arises from a combination of the effects of r adiation on microphysics and dynamics. The cooling from radiation and evaporation produces additional downward motion at the sides of the cl oud. The enhanced low-level convergence invigorates the updraft to pro mote further cloud development. In the decaying stage, the negative bu oyancy produced by cloud top radiative cooling and a higher liquid wat er 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 bec omes weaker. Longwave cooling also enhances the asymmetric characteris tics of the simulated cloud. In conjunction with horizontal momentum t ransport, radiative cooling results in a more negative temperature per turbation and a stronger downdraft on the downshear flank relative to the upshear side.