The Numerical Simulations On Dynamical Extended Range Forecasting For Persistent Heavy Rainfalls

Using the NCEP/NCAR FNL reanalysis data, the Global Forecast System (GFS) data and the national surface weather stations rainfall dataset in this paper, the cases of persistent heavy rainfalls in south region of our country are selected and the wave spectrums of cares are analyzed, the predictability of these cases is also discussed by using the regional models (WRF3.4), the forecast experiments about the low-pass filtering of lateral boundary and the low frequency spectral nudging of inner region have designed by the evaluation and analysis of GFS forecasting data about two persistent heavy rainfalls over Southern China, for exploring the method which can improve the prediction capability of 10-15 days atmospheric circulation systems and extend the precipitation forecast period of validity. The main conclusions of this paper are summed up as:(1) According to the definition of persistent heavy rainfalls,22 cases are selected. These cases concentrated in May to August. Analyzing the wave spectrums of cares shows that the large scale waves of the high latitude have an adjustment before the process and then become more stable, the larger amplitude is mainly concentrated in 1 to 8 waves, the high latitude area has greater amplitude. In the process of persistent heavy rainfalls, the wave energy concentrated in 1 and 4 waves in the high latitude area.(2) The spectral characteristics of the rainy season in South China persistent heavy rainfalls are as follows:the high-latitude trough presses to the south and stable development, also has a same phase with the mid-latitude trough, the mid-latitude trough strengthens southward and cooperates with the stable maintenance subtropical high to form the persistent heavy rainfall, the mid-latitude trough is mainly contribution of 1-3 waves. For the persistent heavy rainfall of Meiyu period, the westerly trough in the range of 30～50°N which is mainly contribution of 1-4 waves slowly moves eastward and the subtropical high extends to the west at the same time. For the post-flood of South China persistent heavy rainfall, the trough in the range of 20～45°N which is mainly contribution of 1-2 waves stable maintenance and development, and, the subtropical high which is mainly contribution of 2～4 waves extends to the west.(3) For the transformation process of larger weather systems among the selected persistent heavy rainfalls process, the model has a similar prediction effect between the different leading times. The forecast leading times have an obvious influence on the forecasting effectiveness in the last 60 hours.(4) Through the analysis of the sensitivity test of model prediction ability, the longest forecasting effectiveness for the selected persistent heavy rainfalls is 8 days, and 10 days for the circulation pattern. The model forecast has smaller area and weaker intensity for the precipitation, and the circulation pattern of upper lever has better forecast than the lower.(5) The evaluation of GFS in the two persistent heavy rainfalls process indicates that the prognosis of upper lever better than the lower and the ultra-long wave (0-3) has a better forecast accuracy. The forecasting error is noticeably increasing from the 8th day forecasting.(6) The quantitative evaluation of the filtering and spectral nudging experiments shows that the low-pass filtering of lateral boundary has a stability improvement for the extended range forecasting, the function of the low frequency spectral nudging in inner region mainly occurs in the bigger forecast error region. The high frequency weaken of lateral boundary has better effect than the low-pass filter, especially combining the low frequency spectral nudging in inner region with it, it has a significant improvement for the average forecast from 10 to 15 days.(7) The qualitative evaluation of model experiments indicates that the low frequency of lateral boundary experiment reduces forecast error of the high latitude, the experiment which includes the low frequency spectral nudging in inner region and the lateral boundary low frequency improves obviously in the key area of height field and wind field.