A model simulation of“99. 6”Meiyu Front rainstorm by nonhydrostatic mesoscale modelMM5V3 is analyzed in an effort to study the mechanism of theMeiyu front. The results of trajectories analysis show that four airstreams converge into the low vortex. On low levels, the southwest Jet and the northeast flow increased convergence on low levels as the low meso -α vortexwith shear line develop ing, and the westly Jet and the eastly Jet increased divergence on high levels. It is this collocation from low levels to high levels that urged meso -α low vortex to develop. The diagnosis of convective momentum transport (CMT) shows that, the budget residual X of the horizontalmomentum has different effects in different periods of the low vortex with the shear line: On low levels, X strengthened the southwest flow to north on the occurring stage, on middle levels, X accelerated northwest flow behind EastAsia Trough to north and also accelerated northwest flow ahead of EastAsia Trough to the south.All these were in favor of deepening the EastAsia Trough and mingling cold and warm air, and gave favorable conditions to p roduce rainstorm. On the develop ing period, X has the reverse direction with southwest flow, which p revented southwest flow to the north. At the same time, it decelerated the cold (warm) flow behind ( ahead of) the East Asia Trough. on the level of 500 hPa. The diagnosis of CMT on the violently develop ing period of the mesoscale system shows that strong X always related to the meso -β systemswhich brought intense ascendingmotions.While the shear line launched out, X has strong decelerating effects for southwest flow. The result of diagnosis also revealed that large - scale environment field has overt energy interconversion with subgrid scale systems on the intense develop ing period of the mesoscale system, and energy p rimarilywas transported to microscale systems. Furthermore, energy exchanges on high levels were closely with the location of Upper Jet. The diagnoses of apparent heat source indicate that releasing of strong latent heat of condensation concomitant with intense develop ing of the low vortex. Onlymeso -β systemsA and B have better relation with apparent heat source. The characters of apparentmoist sink are analogous to that of apparent heat source.

Abstract:A model simulation of“2002.3”sandstorm by nonhydrostatic mesoscale model MM5 is analyzed in an effort to study the
mechanism of the sandstorm.The diagnosis of convective momentum transport (CMT) shows that, at every level,the maximum bud-
get residualXof the horizontal momentum is located behind the trough,whichXstrengthened the northwest cold air flow to south.
The angle between Xand Vwas smaller than 90°behind the trough,that means accelerated northwest cold air flow
to south and it is in favor of deepening the trough and moves to southeast. At the level of 300hPa, kinetic ener-
gy transferEhad clear belt distribution, and the maximum centers ofEwas mainly located on the each side of upper Jet, that
showed the upper Jet played a very important role in kinetic energy transfer. The distribution character of vorticity and divergency is
consistent with the trough that trigged the sandstorm.Model trajectories analyses indicated that there was airstream in the trough at
the upper level of 300hPa, and it flow to southeast at the level of 500hPa, there exist mountain-plain circulation to the north of the
Plateau. The sand transport routing can be determined by superimposing the sand concentration over the trajectories, theoretically,
but the forecasting of the trajectories is difficult at now.