Rainfall simulation associated with Typhoon Herb (1996) near Taiwan

Due to the presence of the high altitude and complicated mesoscale topography associated with Taiwan°¦s Central Mountain Range (CMR), significant variations in track and intensity occur as a typhoon approaches the island. The interaction of the typhoon circulation with the CMR also produces significant mesoscale variations in pressure, wind and precipitation distribution over Taiwan. The understanding of the dynamics of typhoon circulation and its interaction with the Taiwan terrain, and the forecasting of track, intensity, and mesoscale wind and precipitation distribution are some of the most important scientific and forecasting problems for meteorologists in Taiwan. In this paper we perform high-resolution (2.2 km) numerical integrations, using a non-hydrostatic mesoscale model (MM5), to study the rainfall simulation associated with Typhoon Herb (1996) near Taiwan.

High-resolution numerical simulations (with variable horizontal resolutions) and detailed analyses were carried out to examine the ability of the model in simulating the track, intensity change, and the detailed mesoscale wind and precipitation distributions associated with each typhoon, and to assess the impact of horizontal resolution and topography on rainfall simulation. It was found that the 6.7 or 10-km MM5 successfully simulated the mesoscale rainfall distribution associated with each extreme rainfall event. The ability of the model to successfully simulate the observed rainfall declined with the reduction of horizontal grid resolution. The ability of the model to capture the heavy precipitation and its associated mesoscale structure appears to hinge on its ability to predict accurate storm track and circulation, and to resolve the detailed topography of the CMR. The CMR was found to play a key role by both substantially increasing the total rainfall produced by each typhoon, and by focusing the heavy rainfall over the upwind slopes of the mountains. The results indicate that a high-resolution mesoscale model, such as MM5, can serve as a useful tool for providing the detailed mesoscale precipitation and wind distribution for typhoons near Taiwan.

Additional research is undergoing to assess the potential of such high-resolution mesoscale or typhoon models in order to improve the detailed wind and rainfall forecasts for typhoons near Taiwan. Detailed potential vorticity budget is being studied to understand the potential vorticity evolution associated with each typhoon under the influence of the diabatic heating and the topography. More experiments are carried out to investigate the model sensitivity to the uncertainty on the initial bogused vortex and the environmental flow, and to various cumulus parameterization schemes and the microphysics used, which are important issues affecting the simulation of typhoon track, structure, and intensity. A special comparison of the simulated precipitable water (PW) with that retrieved from surface GPS observations was also made. Work is under way to assimilate the PW and GPS/MET data and to evaluate the added-value of the special remote-sensing data on typhoon simulations. It is hoped that better observations from the satellites, Doppler radars, reconnaissance, unmanned aircrafts, specific field experiments, as well as the PW and GPS/MET data, can all be incorporated into high-resolution numerical models through advanced data assimilation techniques. Therefore, a systematic approach by combining the observational analyses and numerical model experiments will advance our understanding on the dynamics of the mountain effects on typhoons, possibly aiding in the forecast and understanding of the storm behavior near Taiwan.