A Mesoscale Analysis of Heavy Rain Caused by Frontal and Topographical Heterogeneities on Taiwan Island

The prevailing mesoscale model MM5 (V3) is used to simulate a heavy rain case caused by interaction between a move-in front and topographical heterogeneities on Taiwan Island. It is found that both thermodynamic and dynamic fields along the front are heterogeneous in time and space. The heterogeneity becomes more significant as the effect of topography is added on. The heterogeneous distribution of physical variables along the front is the main reason for the heterogeneous frontal rain band; the optimum cooperation of the low level and upper level jet is another reason for the development of the rain band.Topography can strengthen the rainfall and causes extremely heavy rain cells. Updraft induced by topography extends to a rather low level, while the uplifted air by frontal circulation can reach to higher levels.The quasi-steady topographic circulation overlaps the frontal circulation when the front moves over Taiwan Island; the advantageous cooperation of various mesoscale conditions causes the large upward velocity on the windward side of the island.