A study of orographic effects on mountain-generated precipitation systems under weak synoptic forcing |
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| Authors: | Chuan-Yao Lin Ching-Sen Chen |
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| Institution: | (1) Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan, TW |
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| Abstract: | Summary ?Mountains profoundly impact precipitation systems in Taiwan, particularly in areas occupying roughly two-thirds of the island’s
landmass. This study examines the terrain structures possibly affecting the formation of rainfall systems in northern Taiwan
by analyzing radar data, surface rainfall data, and simulation results from MM5 (Fifth-Generation NCAR/Penn State Mesoscale
Model) under a weak synoptic influence condition. More specifically, this study analyzes precipitation systems formed in three
different days with different ambient wind directions (i.e., southwesterly, southerly and south-southeasterly flows) in a
low Froude number regime in Mei-Yu (or Baiu) season.
The southwesterly (southerly) predominant wind was blocked by CMR (central mountain range) over southwestern (southern) Taiwan.
Consequently, the southwesterly (southerly) winds were diverted around southern Taiwan, traveled northward following the terrain
contour of CMR and then converged in northeastern (northern) Taiwan to produce a NE-SW (N-S) orientated convergence area.
As anabatic flow and onshore flow intensified in northern Taiwan and thus enhanced the existing convergence in the late morning
and early afternoon, the precipitation system appeared over slope first and then moved down the slope following the predominant
wind direction. Upwards motion persisted in this convergence region, and initiated a new precipitation system. Consequently,
rainfall accumulation was orientated in a NE-SW (N-S) direction in northern Taiwan. On the windward side of CMR in central
Taiwan, precipitation was first produced in the slope by anabatic flow and was generated in lower land because of the interaction
between down slope and onshore flow in the late afternoon. When the flow was predominantly from the south-southeast, the convergence
due to the splitting of the predominant over western Taiwan became weaken after onshore flow over west coast developed since
the direction of onshore flow was against the splitting predominant flow. Precipitation only appeared in the sloping areas
of northwestern and central Taiwan in the relatively dry environment resulting from the anabatic flow.
Several sensitivity tests indicated that the lee-side convergence in a low Froude number regime superimposed by anabatic flow
and onshore flow is important for producing rainfall in northern Taiwan. The prevailing wind direction determined the orientation
of the rainfall accumulation in northern Taiwan. The high relative humidity is important for precipitation to form in lower
elevations.
Received February 9, 2001; Revised November 23, 2001 |
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