Model Assessments of Precipitation with a Unified Regional Circulation Rainfall and Hydrological Watershed Model

It is held self-evident that a watershed is among the best instruments with which to measure the spatial-temporal distribution of precipitation during heavy rainfall periods. Therefore, this paper describes an approach to discriminate physics of options of precipitation, normally built into meteorological models, using integrated meteorology and hydrology modeling. A hydrology model, WASH123D, was employed to calibrate and validate typhoon-induced hydrographs in Lanyang River basin using measured rainfalls as the driving force in a previously published paper. The calibrated hydrological parameters of the watershed include Manning's coefficients in rivers and land surface, river characteristics, infiltration capacity of subsurface media, and so on. For forecasting purposes, rainfalls generated using numerical weather prediction models are needed as these are not known a priori. The problem is which physics in meteorological models ought to be used to produce the spatial-temporal distribution of rainfalls. An integrated meteorological and hydrological model is employed to address this issue. To our knowledge, this is the first time that the discrimination of rainfall models is accomplished via hydrological modeling. First, rainfall amounts and distribution, simulated with different options of physics built into the weather research forecasting (WRF) model, are investigated and assessed. The assessments indicate that it is very difficult to decide which physics is applicable to special conditions in Taiwan. Then, the integrated WRF and WASH123D model is employed to model hydrographs. The contrast between the hydrographs, simulated using rainfalls predicted by WRF and the rain gauge observation, is discussed. The magnitude and time lag of flood peaks from the simulated flood hydrographs are compared with those observed. Model calibration of hydrographs using the integrated model makes it easier to determine which physics in WRF is applicable to Taiwan from our selected cases. It was found that the Betts-Miller-Janjic cumulus parameterization is superior to others in our simulations for the case when the typhoon moves directly into the study area from the Western Pacific Ocean.