East Asia and Western Pacific METEOROLOGY AND CLIMATE

Wu et al.(82) analyzed 43 cases of heavy rainfall in March-June
and found that there was often a distinct cyclonic shear or even a
closed circulation at 850 mb over the area of heavy rainfall. For all
the cases analyzed, a well developed low was usually located over
Manchuria, Korea or Japan with a deep trough extending southwestward
to the East China Sea or the Yangtze River estuary. For heavy
rainfall events in different seasons, a conrnon feature at 850 mb
and/or 700 mb is the existence of the meso-oc scale trough which
probably provides sufficient dynamical lifting for triggering off the
development of MIS (52, 53, 56, 63, 64, 68). Chen and Wi (41) studied
the relationships between trough and heavy rainfall using 35 cases of
heavy rainfall events in May-June of 1965-1984. It was found that all
the heavy rainfall events was accompanied by a short wave trough or a
shear line at 850mb and 700mb. The heavy rainfall tends to occur
when the short wave trough moves toward Taiwan from southern China.
Chen and Tsay(5) identified two types of synoptic conditions for heavy
rainfall based on the existence of the Manchuria low. Chu and
Jen(56) identified two types of surface Mei-Yu front during heavy
rainfall event. One is the front approaching Taiwan from the north,
and the other is the quasi-stationary front in the vicinity of Taiwan
with a wave formation.
Another feature accompanying heavy rainfall event in Taiwan is the
850 mb cold tongue which usually extends from the East China Sea
southwestward to southern China . Chu and Jen(56) observed that the
cold tongue over the southeastern China coast usually moves toward
Taiwan prior to the occurrence of the heavy rainfall. This is
accompanied by pronounced warm advect ion over the Taiwan area 12 h
before the heavy rainfall occurrence.
Potentially unstable condition favorable for the development of
MIS usually existed over the area to the south of thelVfei-Yu front (6,
28). Chu and Jen(56) observed the existence of a stable layer in the
lower troposphere (950-900 mb) for some heavy rainfall cases and
pointed out that the total index is more reliable than the K index in
identifying the heavy rainfall condition. Synoptic studies for the
MISs over Taiwan and southern China suggested that there are at least
six conditions favorable for the development of MIS. They are (1) warm
advect ion in the lower troposphere, (2) low-level convergence over the
low pressure and/or the front, (3) low-level jet, (4) short wave
trough in the lower to middle troposphere, (5) the middle and upper
troposphere diffluent flow and/or speed divergence, and (6) potential
instability in the lower and middle troposphere.
4.4 Triggering Mechanisms for the Mssoscale Convective Systems
Observational studies suggested that seme mesoscale circulation
systems are apparently responsible for the formation of the MISs over
Taiwan and southern China. These include the frontal secondary
circulation, low level jet, quasi-stationary mesolow, topography,
outflow boundary and local circulations^, 7, 11, 20, 28, 30, .32, 43,
51, 52, 53). The mesoscale circulation system appears to be an
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