Tuesday, 31 July 2001: 4:45 PM
Essential Ingredients for heavy Orographic Rainfall and their Potential Application for Prediction
By comparing the synoptic and mesoscale environments conducive to orographic flooding or heavy rainfall in both US mountains and the Alps, we found the following common features: (1) a sufficiently high enough mountain elevation to help release the instability, (2) a conditionally or potentially unstable airstream impinging on the mountain, (3) a very moist low-level jet, (4) a mid-troposphere shortwave trough approaching the threat area, (5) a synoptic system (such as a quasi-stationary pressure ridge) to help slow down or anchor the convective system over the threat area. We then investigated the orographic flooding or heavy rainfall associated with impinging tropical depressions in 1959, 1999, and the 2000 typhoon Bilis on Taiwan's Central Mountain Range, and a tropical depression on Japan's Kyushu Mountains, and found that the common synoptic and mesoscale environments are similar to those associated with US and Alpine orographic heavy rainfall except that a high CAPE is often observed. Based on an essential ingredients argument, we found that orographic flooding or heavy rainfall requires significant contributions from any combinations of the following: (1) high precipitation efficiency of the incoming airstream, (2) a low-level jet, (3) a steep mountain, (4) favorable mountain geometry and confluence flow field, such as a concave geometry, (5) strong synoptically forced upward motion, (6) high moisture flow upstream, (7) a large convective system, (8) slow movement of the convective system, and (9) an upstream conditionally or potentially unstable airstream. These ingredients are also used to help explain the synoptic and mesoscale environments observed in some orographic flooding and heavy rainfall events that occurred in New Zealand, China, and India. In order to help predict the occurrence of orographic heavy rainfall or flooding, we propose to use the orographic vertical moisture flux, which include three of the above mentioned essential ingredients in an index of the form: [V x Gradient(h)]q. The estimated index values from soundings of tropical depression cases are consistent with the maximum rainfall rates for the Taiwan and Japan cases. We have also simulated the 2000 typhoon Bilis using the NAVY COAMPS model and found that the orographic vertical moisture flux calculated from the model output coincides well with the observed rainfall data over the mountainous area. Over the ocean or flat terrain, the occurrence of heavy rainfall is well represented by the general vertical moisture flux, defined as wq.