Some Common Ingredients for Heavy Orographic Rainfall

The purpose of this paper is to synthesize some common synoptic and mesoscale environments conducive to heavy orographic rainfall. Previous studies of U.S. and Alpine cases and new analyses of some Alpine and east Asian cases have shown the following common synoptic and mesoscale environments are conducive to heavy orographic rainfall: 1) a conditionally or potentially unstable airstream impinging on the mountains, 2) a very moist low-level jet (LLJ), 3) a steep mountain, and 4) a quasi-stationary synoptic system to slow the convective system over the threat area. A deep short-wave trough is found to approach the threat area in the U.S. and European cases, but is not found in the east Asian cases. On the other hand, a high convective available potential energy (CAPE) value is observed in east Asian cases, but is not consistently observed in the U.S. and European cases. The enhancement of low-level upward motion and the increase of instability below the trough by the approaching deep short-wave trough in the U.S. and Alpine events may partially compensate the roles played by high CAPE in the East Asian events. In addition, the concave mountain geometry plays an important role in helping trigger the convection in Alpine and Taiwanese cases. Based on an ingredient argument, it is found that a heavy orographic rainfall requires significant contributions from any combinations of the above four common synoptic and mesoscale environments or ingredients, and high precipitation efficiency of the incoming airstream, strong upward motion, and large convective system. These ingredients are also used to help explain the synoptic and mesoscale environments observed in some orographic flooding and heavy rainfall events in other regions, such as in New Zealand, China, and India. An index, U(∂h/∂x)q, where U is the flow velocity perpendicular to the mountain range, ∂h/∂x the mountain slope, and q the water vapor mixing ratio, is also proposed to help predict the occurrence of heavy orographic rainfall. Estimates of this proposed index indicate that it may serve as a good indicator for predicting east Asian heavy orographic rainfall events.