How Circulation Adjustment Affects the Axial Error of the Precipitation Forecast

In this paper, Topical Cyclone (TC) Usagi is used as an example. The relationship between the precipitation distribution and middle-latitude circulation index during the extratropical transition (ET) of Usagi was analysed. The analysis results showed that the circulation pattern in the Asian region was changed from zonal to meridional, and this trend was continuously increased. In order to evaluate how such circulation adjustment affects the accuracy of precipitation forecast, the Contiguous Rain Area (CRA) method was modified to include rotation in the pattern matching and error decomposition. The mean squared error (MSE_total) is divided into 4 parts, i.e. MSE_{displacement 、}MSE_volume、MSE_rotation and MSE_pattern, wherein MSE_rotation is used to evaluate the axial error of the rainfall area. Finally, the 3-hourly precipitation forecast within 4 days before the ET of Usagi was verified using the modified CRA method. The results showed that the rotation error was relatively small when the background circulation was relatively stable, whereas the error increased during the circulation adjustment. It can be inferred that the prediction accuracy of background circulation, especially middle-latitude circulation adjustment was crucial for the precipitation forecast, especially the rotation error. If the systematic axial error of the model precipitation forecast can be counted, and the statistics results can be used to calibrate the forecast results, the forecast of rainfall area will be improved.