It has been known that the convective clouds are related to the realization of potential (convective) instability of the warm and moist air flow blowing from the southwest, and the large scale dynamic forcing, especially the deformational frontogenetic forcing of the large scale flow between the North Western Pacific Subtropical High and the migrational highs at higher latitudes. However, the behavior and the formation mechanism of the convective clouds are not fully understood. In addition, the prediction of these convective storms in a numerical model is of great significance in the rainfall forecast during the Meiyu season.
Recently, in a series of numerical simulations of Meiyu front forced by an idealized deformational flow, the author has recognized development of some convective clouds, which have similar horizontal sizes and behaviors as the above mentioned convective clouds. In this study, to draw attention from both modelers and operational forcasters, some insights on the storm-scale prediction along the Meiyu front are presented based on the numerical simulation results.
The model used is the Advanced Regional Prediction System (ARPS) developed at the Center for Analysis and Prediction of Storms (CAPS), University of Oklahoma. It is a nonhydrostatic compressible model with microphysics of cloud and rain so that some convective clouds can be generated. The model is initialized with an environmental condition of a moderate baroclinity at the upper troposphere and an intense moisture contrast at the lower troposphere with the moist maritime air to the south and the dry continental air to the north.
The simulation with both surface drag and surface heat (sensible and latent) fluxes shows 3 meso-scale rain bands in a rain belt of 500km in width. The first is associated with the upward branch of the frontogenetic vertical circulation and the second is about 100km to the south. Both of them weaken and disappear after a new band begins to develop to the south. The third band, on the other hand, has a longer life span. It develops about 400km to the south of the second band and moves northwards. It reaches the maximum intensity at about 300km to the south of the first band and then deteriorates somehow but does not dissappear. It later redevelops at the location of the firtst band.
The experiment with surface drag but without surface heat fluxes shows the missing of the second band and the weakening of the first band. Instead, the third band disappears after a modest development and a little amount of movement while the first band is maintained at its original location. The third experiment with surface fluxes but no drag shows development of convection in a belt of 1000km in width to the south of the frontal zone without clearly defined meso-scale rain banads. The belt becomes smaller in its horizontal scale and shifts northward but never reaches the frontal zone.
Because the experiments are designed to investigate the synoptic scale structure of the Meiyu front and some computational mixing is employed, the model may not simulate the same convective clouds and rainbands as in the observed individual convective storms. Nevertherless, the model is indeed able to simulate some features of the convection with reliability because of relatively high resolution and small time step of integration. Therefore, the experiments suggest that the meso-scale convective rainbands are generated as a result of interactions among the large scale forcing, latent heat release and the surface processes. Especially, the role of surface drag in organizing the convection should be emphasized in the prediction of such rain bands. It is also shown that the ARPS has the potential ability to predict the meso-scale rainbands associated with the Meiyu front.
Because the heavy rainfall associated with the convective storms and meso-scale rainbands accounts for a large amount of the Meiyu precipitation, predicting the evolution of convective storms a few hours in advance using a numeric model is of great significance in the decision making of flood contral during the Meiyu season. The quasi-operational test of the ARPS in Oklahoma, U.S., indicated that the prediction of convective storms is feasible with the conventional observational network and a relatively dense surface network. The experiments discussed in this note suggest that models such as ARPS can be used in the prediction of convective storm of the Meiyu season. The central China area, especially the mid-reach of the Changjiang (Yangtze) River, is characterized by widespread hills and numerous lakes. This may make the prediction sensitive to the surface processes. As the surface drag and fluxes are important in the formation of the rainbands, great effort should be made to improve the package of the surface processes and the representation of the surface characteristics with higher resolution.