East Asia and Western Pacific METEOROLOGY AND CLIMATE

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accumulated rainfall amount on the ground after five hours integration is shown
in Fig. 7, the upper solid curve. A comparison with the observations shows very
good agreement.
2.4 Conclusion. We have demonstrated above that upstream heavy rainfall is
quite possibly induced by uplifting due to upsteam retardation of airflow. The most
favorable conditions for upstream intensification or induction of strong convection
appear to be: (1) weak surface wind; (2) the atmosphere has a stable lower layer
and conditionally unstable layer aloft;(3) high mountain with large aspect ratio
especially on the windward side.
3. The Fall Wind over Hengchun Peninsula.
3.1 Introduction. The so-called 'fall wind' is a peculiar phenomenon which occurs
every year from October till April of the next year over the western coastal
area of the Hengchun Penisula (the southern tip of Taiwan). The phenomenon is
characterized by a strong, gusty wind storm. The surface gust can easily reach 25
m/s from calm condition in a couple of hours. One of the typical cases occurred
during the period from November 15 till 17, 1986. In the following we will use it
for evaluating the proposed theory.
The surface gusts recorded at Hengchun station were surveyed for the last ten
years. According to the record, we define the fall wind event to be the one having
maximum surface gust of over 15 m/s. Table 1 shows the averaged number of
events for each month from 1981 to 1986. One can see that the October-December
is the pre-dominant period. There was at least one event every two days.
Table 1. Number of cases of fall wind event
at Hengchun in 1981-1986 period
Month
No. of
cases
JAN
78
FEB
68
MAR
45
APR
.21
MAY
18
JUN
6
JUL
1
AUG
2
SEP
5
OCT. NQV
57 92
DEC
93
Averaged
No. of
cases/yr
"13.0
11.3
7.5
3.5
3.0
1,0
0.1
0.3
0.8
9,5 1.5.3
15,5
For most cases there appeared no distinct mechanism which can cause the
surface gusts (weather maps not shown here), except that only a few cases were
due to typhoon but occurred very rarely. However, when we examine the upper