Extended Abstract (480K)P2.26
Orographic Effects on a Conditionally Unstable Flow over an Idealized Three-Dimensional Mesoscale Mountain
Shu-Hua Chen, Univ. of California, Davis, CA; and Y. L. Lin
Based on idealized numerical simulations by using the Weather and Research Forecast (WRF) model, three flow regimes are identified for a conditionally unstable flow over a three-dimensional mesoscale mountain range: (I) regime with upstream-propagating convective system, (II) regime with quasi-stationary upslope and downslope convective systems, and (III) regime with stationary upslope convective system and downstream propagating convective system. The major differences from the same flow over a two-dimensional mountain range are that: (1) flow are allowed to go around the mountain, which helps to produce convective lines moving outward on the upstream side of the mountain and curve into the mountain on the lee side, and (2) relative large amount of rainfall can be produced over upslope, instead of at mountain peak, under a low-level jet, which is more consistent with observations. The moist flow deflects slightly rightward through the low pressure far upstream. The Coriolis force helps part of the upstream flow to make a transition from flow-around the mountain to flow-over the mountain by deflecting the southerly flow to become an east-southeasterly barrier jet. This induces the upward motion over the southern slope of the mountain ridge, which then triggers MCS and then produced orographic rainfall over upslope and/or upstream. Effects of concave mountain geometry on orographic precipitation are investigated by adopting an arc-shaped mountain. In addition to the upward motion over the southern slope of the east-west ridge, a region of upward motion is induced over the eastern slope of the western flank of the mountain. Thus, in addition to the mechanisms of ?localized LLJ? and ?inhomgeneous moisture distribution?, the formation of the convective systems and rainfall near a concave region of an arc-shaped mountain is attributed to the Coriolis-force deflected east-southeasterly jet over the inner slopes of the arc-shaped mountain. The transition from flow-around the mountain to flow-over the mountain occurs on both the east-west main ridge and the western flank of the mountain. In addition, the upward motion in the concave region is not produced by the convergence between the Coriolis-force deflected southeasterly jet and southwesterly flow from the tip of western flank. When CAPE decreases, it is found that the upstream moist flow tends to shift to that in a higher regime. We have estimated for MAP IOP-2B and IOP-8 from Milan soundings, which implies that both IOP-2B and IOP-8 belong to Regime I, although is much smaller for IOP-8. This may help partially explain the upstream extension of rainfall pattern for IOP-8 compared with that for IOP-2.
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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