East Asia and Western Pacific METEOROLOGY AND CLIMATE

20
3. SWDPTiC ^mLYSIS WD DIA^DSTIC STUDIES
The detailed case study of the Mei-Yu frontal system in June 10-
15, 1975 presented by Chen and Tsay (35) was probably the first paper
in Taiwan belonging to the category of diagnostic study. The synoptic
conditions and the budgets on moisture, kinetic energy and vorticity
were studied to reveal the structure and dynamics of the Mei-Yu
system. It was found that cumulus convection played an important role
in the maintenance of the intensity of the Mei-Yu front (in terms of
vorticity) in addition to the mid latitude baroclinic processes. The
transport of the heat and moist rue by the low level jet was found to
be essential in maintaining the cumulus convection (7,12,27,36). The
mean vertical motion field for this case showed that the vertical
advection and twisting terms cannot be neglected in the vorticity
equation over mountain slopes and baroclinic zone (8). Results also
showed that the major moisture source for the Mei-Yu area was the Bay
of Bengal. The temperature and vertical motion fields showed that a
thermally direct circulation with ascending warm air and descending
cold air prevailed over the Mei-Yu area. This secondary circulation
was particularly strong over the baroclinic region especially in the
vicinity of Japan.
Chen (10) observed that there was a continuous cloud band
accompanying the Mei-Yu frontal system which extended from the
vicinity of Japan southwestward to Taiwan and southern China. Along
the Mei-Yu frontal system, a marked wind shear line at 850 mb and 700
mb coincided with the maximum gradient of mixing ratio, relative
humidity and equivalent potential temperature. To the south of the
wind shear line, a 700 mb low level jet was located over the area of
maximum mixing ratio and high equivalent potential temperature. Over
and to the south of the wind shear line, strong cyclonic vorticity,
horizontal convergence and upward motion prevailed. The strong
convection in the southern portion of the cloud band occurred over the
area of maximum value of these kinematic parameters. The vorticity and
kinetic energy budget studies by Chen and Tsay (37) showed that an
area of maximum vorticity at 850 mb was located between the Mei-Yu
trough and the area of maximum horizontal convergence (or maximum
upward motion). The generation of cyclonic vorticity due to horizontal
convergence was counteracted by negative vorticity advections and thus
led to a quasi -stationary state of the Mei-Yu front at surface and 850
mb, Kinetic energy budget showed that a major part of the kinetic
energy generated by cross-contour processes tended to dissipate in
situ over the Mei-Yu area while only a very small portion (15%) was
transported to the environment. The mesoscale analysis of this case
(13,36) showed that the mesoscale circulation system with horizontal
dimensions of 200-300 km was characterized by cyclonic vorticity,
horizontal convergence and upward motion in the boundary layer. This
mesoscale circulation system tended to organize and enhance the
mesoscale convective systems.