East Asia and Western Pacific METEOROLOGY AND CLIMATE

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Our study is to obtain and analyze the tropical cyclone's outflow
structure as simulated by the NRL model, and to examine whether the
hypothesized interactions between the upper-tropospheric systems and
the outflow are likely. For this purpose, a realistic steady-state
tropical cyclone structure is obtained by initializing the NRL regional
model with the mean tropical sounding and a Rankine vortex. In the
simulated tropical cyclone, it is found that the outflow layer is
dominated, in angular momentum transfer, by one or two jets, depending
on the zonal wind shear. It is also found that there are secondary
circulations around the outflow jet (Fig. 2). The secondary
circulations in the jet entrance region is a thermally direct
circulation, i.e., the rising branch is located on the anticyclonic
shear side of the jet toward the storm center, while the subsiding
branch is located on the cyclonic shear side of the jet away from the
center. In the jet exit region, the direction of the secondary
circulation is reversed and thermally indirect.
We found that these circum-jet secondary circulations play
important roles in the interaction of the outflow jet with uppertropospheric
systems. In a numerical experiment we found that when the
jet is accelerating, simulating the confluence of the outflow jet and
an approaching westerly trough, the secondary circulation is enhanced
and initiates deep convections in the area of upward branches near the
jet (Fig. 3). These findings will be reported in Shi et al. (1989).
IV. VERTICAL NORMAL MODE INITIALIZATION.
In order to filter out the spurious oscillations in the first few
hours in the forecast of the regional model, a new initialization
scheme is devised based on the vertical normal modes of the model. The
spurious oscillations are mainly caused by the existence of fast-moving
gravity waves, which arise due to the incapability of observing
atmospheric state on all scales and errors in the regional analysis.
The vertical normal mode method is to project the analysis into the
model's vertical eigen space* A steady state solution of the