10th Conference on Mountain Meteorology and MAP Meeting 2002


Some Common Ingredients for heavy Orographic Rainfall and their Potential Application for Prediction

Yuh-Lang Lin, North Carolina State University, Raleigh, NC; and S. Chiao, J. A. Thurman, D. B. Ensley, and J. J. Charney

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 tropical storm Rachel 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 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. In order to help predict the occurrence of orographic heavy rainfall or flooding, we propose to use the orographic vertical moisture flux model, which include three of the above mentioned essential ingredients in an index of the form: [V x Gradient(h)]q. By adopting mesoscale model simulated wind and moisture fields, the flux model predicted rainfall distribution and amounts compare reasonably well with observations for different orographic rainfall events, such as MAP IOP-2B and IOP-8 over Alps, tropical storm Rachel, Typhoon Bilis, and Typhoon Nari over Taiwan's Central Mountain Range. Improvement of the flux model will also be investigated.

extended abstract  Extended Abstract (224K)

Poster Session 2, Orographic Precipitation/Operational and Numerical Weather Prediction (with Coffee Break)
Wednesday, 19 June 2002, 9:15 AM-11:00 AM

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