Numerical Sensitivity Study of Rainband Precipitation and Evolution

Using a deep, two-dimensional rainband model, the authors examined the effect of different microphysical treatments on rain evolution and precipitation processes in an otherwise fixed environment. Not only rain evolution but cloud cell activity and heat release patterns also varied greatly depending on the microphysical processes used to drive the model. When maritime microphysics were used, in the early stage, models produced intense rainfall peaks at both the front and the rear of the emerging system. Then, between these two first peaks, new rain peaks successively appeared, forming a broad rainband. Heat was released rather uniformly throughout the cloud depth above the freezing level. Strong winds that blew near freezing level were a common feature. Updraft was enhanced during the intermittent development of new cloud cells. In contrast, when continental microphysics were used, in the early stage, models produced heavy rainfall at the rear of the emerging system and only weak rainfall at the front. Although new rain peaks appeared between these two first peaks, the intensity of the rear rain peak remained dominant throughout the life of the rainband. Heat was released in a rather narrow area above freezing level. Strong updraft was observed in the rear cloud cell during successive cloud cell development. Both maritime and continental models held in common the observation that without ice nuclei, the front rain peak dominated and the cloudband became shallow. In addition, models that included enhanced freezing produced rain peaks of nearly equal intensity at the front and rear of the cloudband. The authors conclude that in addition to stability and wind shear, the precipitation process is another controlling factor in the determination of rainband characteristics.