East Asia and Western Pacific METEOROLOGY AND CLIMATE

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Figures 11 and 12 demonstrate the resulting surface wind speed and the Reynolds
stresses for these two cases. One can see that when the critical level is located at
2 ~ 2.5 km, the surface wind speed is enhanced very much because the Reynolds
stress grows consistently showing coherent interference through multiple reflections.
Table 2. The tested cases
Case
03
02
01
00
-1
-2
-3
Zo
(km)
4 0
3,3
3 0
2 47
2.2
2 . 0
• 5
Finally, numerical simulations are conducted for the Nov. 17 case, using the
corresponding Panchiao sounding data as the initial condition. The results are
shown in Fig. 13. The fact that strong surface wind is being set up together with
the consistently growing Reynolds stress is clearly shown.
3.3 Concluding Remarks* The Hengchun fall wind is the result of multiple
reflection of the mountain wave between the critical level and the ground. The
favorable conditions are: (l)The critical level is located at about 2 to 3 km with
local Richardson number smaller than or near 1; (2) Easterly-wise low level wind
with sufficient strength; (3)Stable lower atmosphere.
Acknowledgment. Researches related to this article were supported by various
NSC grants, including NSC73-Q202-MOQ8-09, NSC77-0202- M008-14, NSC78-0202-
M008-18. Special thanks are extended to Ms. C.-H. Shiax> for assistance in various
stages during the preparation of this paper.
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