East Asia and Western Pacific METEOROLOGY AND CLIMATE

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cooling to a minimum from the falling rain. As the rain falls out the
updraft, it increases the density of the downdraft air below the tilted
updraft. The downdraft air tends to conserve its momentum, undercutting
the updraft air on the downshear side of the storm. As a result
of updraft-downdraft interaction in the lowest layers, the growth of
the new cell occurs ahead of the old cell and results in the storm
tilting into the shear instead of with it. The lifted air exits from
the updrafts in the middle and upper layers. It continues to move
westward, heading toward the trailing stratiform region of the squall
line as in the tropical squall line studied by Chong et al. (1987).
The rear-to-front cooler air enters from the western edge of the convective
region (left side of the figure) in the 2-4 km layer. Buoyancy
of the air is further decreased due to evaporative cooling and precipitation
loading in the region immediately west of the high reflectivity
core. As the descending air approaches the lowest layers, it spreads
out to form a diverging outflow in the boundary layer. Part of the
spreading air moves toward the east, colliding with the front-to-rear
low-level environmental inflow and forming a weak gust front in the
area slightly to the east of the 45 dBZ contour at x - -12 km. The
interaction between the front-to-rear environmental low-level inflow
and the rear-to-front midtropospheric flow is responsible for maintaining
the updraft-downdraft structure in the convective region. On the
other hand, the front-to-rear flow near the surface behind the leading
edge (i. e., x < -12 km) is quite similar to a shallow layer of cool,
saturated air near the surface reported in Zipser (1977). This flow is
the product of convective-scale saturated downdrafts (-24 < x < -19 km)
west of the main updraft. Upward motion dominates in the convective
region behind the gust front with downward motion in the region between
the main cell and the new cell and behind the main cell. Such downward
motion coincides with the relatively high reflectivity region, indicative
of the important contribution of precipitation loading to convective
downdrafts.
Vertical distributions of P * and T
f
along the same cross
section are shown in Fig. 8. Notice that high pressure forms on the
upshear side of the updraft with low pressure on the downshear side. A
mesohigh occurs in the boundary layer at x - -16 km behind the leading
edge. The mesolows underneath the main and secondary updrafts at x =
-15 km and x = -9 km, respectively, are hydrostatic in nature since the
updraft air, fed by the low-level environmental inflow, is warmer and
lighter than the surroundings. A deviation vertical pressure gradient
is directed upward from the lowest layer to 2 km and then reverses its
direction (downward) in the layers above. These deviation vertical
pressure gradients are needed in order to maintain the net vertical
acceleration within the updraft. The distribution of T ' is closely
related to the updraft-downdraft structure (Fig. 7b) and the reflectivity
pattern (Fig, 7a). In the updraft region, warming prevails due
to the release of latent heat by condensation. Conversely, cooling
dominates in the downdraft region. The cool area underneath the main
updraft with high reflectivities (-16 < x < -13 km) is attributed in
part to rain loading. This cool area extends downward to the lowest
layer behind the gust front, indicative of a cell's cold surface outflow.
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