East Asia and Western Pacific METEOROLOGY AND CLIMATE

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£ * ,-
(6)
dp dy
where u g is the geostrophic wind in x-direction. The third term and the
fourth term can be combined as
where u ag is the ageostrophic wind in the x-direction. R ag is called the
ageostrophic residue because it represents the departures of the x-velocity
and temperature fields from geostrophic balance (Orlanski and Ross,
1977).
Jiang and Kau (1987), Jiang and Jeng (1987) estimated the
magnitudes of each term using case studies and numerical model
simulations, respectively. They found that the first and the second terms
were negligibly small. The generation of horizontal vorticity depends
primarily upon the ageostrophic residue. Hence, knowing the behavior of
Rag we can easily predict the rate of generation of cross-stream vorticity
along the front.
5. CONCLUSION
In late-winter and early-spring, the Topographic effect of the
Tibetan Plateau induced a strong sub-geostrophic motion near the ground
in southern China. The ageostrophic residue is the main effect of the
transverse circulation which is important in the frontogenesis process.
Therefore, stationary surface frontal zones often existed in southern China
in late-winter and early-spring.
According to the three-cell circulation concept and the conservative
property of potential vorticity, it appears to exist a semi-permanent upper
sub-tropical front from which the high potential vorticity stratospheric air
folds into the troposphere.
It is well known that the formation mechanisms of the upper subtropical
front and the stationary surface front over southern China are
quite different. However, we do not quite understand the coupling effects
of these two frontal systems until now. More research about this is needed,
REFERENCES:
Eliassen A., "On the Vertical Circulation in the Frontal Zone". Geofys.
Publik., 24, 147-160 (1962).