MJO活动轨迹对贵州区域强降水过程延伸期预报的影响分析

通过对贵州省主汛期季节内振荡(Intra-Seasonal Oscillation,ISO)活跃年进行低频对流场和降水的合成分析,确定了影响贵州主汛期ISO和降水的热带印度洋(Indian Ocean,IO)低频对流关键区和南海(South China Sea,SCS)低频对流关键区,并利用MJO活动轨迹对贵州区域强降水过程开展了延伸期预报试验。将贵州省主汛期ISO位相划分为发展、峰值、减弱、抑制、谷值和恢复6个位相,发现贵州主汛期ISO活跃年的降水与本地区低频对流具有较好的对应关系,即在峰值位相时低频对流最强、降水正异常强度最强;在谷值位相时低频对流最弱、降水负异常强度最强。同时,热带和副热带低频对流场在贵州主汛期ISO波动的第1、4位相、第2、5位相及第3、6位相均呈反位相特征。在热带印度洋低频对流发展、并东传的过程中,有两条传播路径分别激发了孟加拉湾西南季风ISO活跃和南海热带季风ISO活跃共同影响贵州主汛期降水;在贵州主汛期有3个低频对流活跃期,IO关键区和SCS关键区ISO都有3次提前的低频对流加强。基于上述研究,分析MJO活动轨迹对贵州主汛期区域强降水过程的影响,发现热带印度洋MJO活动中心强度在贵州区域强降水过程发生前15 d~前3 d具有较好的持续性预报信号,提前9 d时正相关性最好。与延伸期预报业务规定的预报时段(未来11~30 d)相结合,通过确定贵州典型区域强降水过程发生前(提前量为10 d)至过程结束时段的MJO活动轨迹在历年中的最相似时段,发现MJO活动中心轨迹和强度对贵州区域强降水过程的趋势预报具有较好的指示意义。

Extended-range forecast of the regional heavy rainfall process in Guizhou by studying MJO activity trajectory

Based on the composite analysis of low-frequency convective field and precipitation features in the active year of the Intra-Seasonal Oscillation (ISO) in the main flood period of Guizhou, key areas with low-frequency convection which affect the ISO and precipitation during main flood period were identified over tropical Indian Ocean (IO) and the South China Sea (SCS), respectively. Moreover, the extended-term forecast experiment of the heavy rainfall process in Guizhou was carried out by using MJO activity trajectory. The six ISO phases of the main flood period in Guizhou can be identified as:development, peak, weakening, suppression, valley and recovery. It is found that the precipitation in the ISO active year during the main flood period in Guizhou has a good correlation with the low-frequency convection occurred in the region, i.e. the strongest low-frequency convection occurred with the strongest positive precipitation intensity anomaly in the peak phase, and the weakest low-frequency convection occurred with the most negative precipitation anomaly in the valley phase. Besides, the 1st and 4th phases, the 2nd and 5th phases, and the 3rd and 6th phases of the ISO fluctuation show anti-phase characteristics of the tropical and subtropical low-frequency convective fields during the main flood period in Guizhou. During the development and eastward propagation of low-frequency convection in the tropical IO, there are two propagation paths that stimulate the ISO activities of the southwest monsoon over the bay of Bengal and tropical monsoon over the SCS, and the activities jointly affect the precipitation during main flood period in Guizhou eventually. There are three active phases of low-frequency convection during the main flood period of Guizhou, and the ISO in the IO key area and the SCS key area have three times of enhanced low-frequency convection in advance. By analyzing the impact of MJO activity trajectory on the regional heavy rainfall process in the main flood period of Guizhou, we found that characteristic of zonal eastward transmission of the ISO activity center intensity over the tropical Indian Ocean (the MJO activity center intensity) can be regarded as a good continuous forecast signal from 15 days to 3 days before the occurrence of heavy rainfall process in main flood period of Guizhou, and the best positive correlation are seen in the 9th day in advance. Determine the most similar period of the trajectories of MJO activity from the period of before (10 days in advance) the occurrence to the end of the process of heavy rainfall in Guizhou in the past year, and combine the results with the forecast period (11-30 days in the future) specified by the extended period forecast operation, we found that the trajectory and intensity of the MJO activity center trajectory have good indication significance for the trend forecast of the heavy rainfall process in Guizhou.