基于ECMWF模式预报的台风降水地形订正方法

A study of terrain correction method on typhoon precipitation based on ECMWF forecasts

为了解决复杂地形条件下ECMWF模式预报的台风降水较实况显著小的问题,对Smith 1979年提出的地形降水方程进行改进,提出以饱和湿层高度作为方程积分上限,针对不同高度地形设定不同的降水效率;以无量纲湿弗劳德数大于1作为有、无地形降水的判据;利用ECWMF细网格预报场,通过迎风坡地形降水估算方程来订正模式预报的台风降水。用该地形降水订正方法对1617号台风"鲇鱼"的降水进行了订正预报。结果表明,虽然在一些小尺度地形区域会产生明显的空报,但是对于大尺度地形区域的强降水有显著的订正效果。对1513、1521和1614台风的订正结果进一步表明,该地形降水订正方法对改进台风极端降水预报效果显著。需要指出的是,采用的地形降水订正方法仅考虑了稳定条件下的地形降水,对于其他情形下的地形降水订正方法尚待进一步的研究。

The orographic precipitation equation proposed by Smith(1979) is improved in the present study to solve the problem that the typhoon precipitation predicted by the ECMWF model is significantly smaller than actual precipitation under complex terrain condition. It is proposed that the height of the saturated moist layer is used as the upper bound of the equation integral, and different precipitation efficiencies are specified for different terrain elevations;the dimensionless wet Froude number greater than 1 is used as the criterion for orographic precipitation. The precipitation correction method is applied to correct the precipitation of Typhoon Megi(typhoon 1617) predicted by the ECMWF mode. The results show that although there exists obvious false prediction in some small-scale terrain areas, significant correction effect is found for strong precipitation in large-scale terrain areas. Further application of the correction method on precipitation forecast of typhoons 1513, 1521 and 1614 shows that the correction method of orographic precipitation on the windward slope can significantly improve the prediction of typhoon extreme precipitation, which has great implications for disaster prevention and mitigation.It is necessary to point out that the orographic precipitation correction method used in the present study only considers orographic precipitation under stable condition, and its application in other cases needs further study.