GPS可降水量资料应用于MM5模式的变分同化试验

VARIATIONAL ASSIMILATION OF GPS PRECIPITABLE WATER INTO MM5 MESOSCALE MODEL

利用建立在长江三角洲地区GPS观测网中13个站点的资料对2002年6月27～28日影响长江三角洲地区的降水过程进行了MM5背景误差调节和可降水量资料的三维变分同化试验.试验结果表明:背景误差对三维变分同化的效果起着关键作用,模式变量(u,v,T,p和q)误差的水平尺度与NMC方法的平均时间长度有直接的关系.利用NMC方法重新构建的背景误差更接近实际的背景误差.三维变分技术能有效地同化GPS可降水量资料.GPS可降水量资料的同化使用不仅能调整模式初始湿度场,而且也能相应地调整模式初始气压场、温度场和风场.GPS可降水量资料的同化有利于减小模式初始场对可降水量的分析误差,并且有利于减小模式积分初期(3～6 h)可降水量的预报误差.与没有进行GPS可降水量同化相比,通过GPS可降水量资料的三维变分同化,使MM5模式6 h和24 h累计降水能力得到提高,改善了MM5模式降水预报性能.总体上,GPS可降水量资料的变分同化有利于模式降水预报能力的提高.

The GPS precipitable water （PW） from 13 GPS sites in the Yangtze delta is explored to investigate the tune of background errors and the three dimensional variational assimilation of MM5 on rainfall event occurred from June 27 to 28,2002. The results show： Background errors （BE） play a key role in the three dimensional variational assimilation of MM5. The horizontal scalelength of model variables （ u, v, T, p and q） is closely related to the average time of NMC technique. The scalelength of model variables is different for each other, which value is associated with the vertical height of the variable on the MM5 level. The BE calculated by NMC technique reach the true BE more closely than that provided by MM5-3DVAR system. GPS PW data can be assimulated into MM5 by using 3DVAR technique. After GPS PW data assimulation, the initial humidity field can be reanalyzed while the initial temperature, pressure and wind fields also being modified. 3DVAR of GPS PW is benefitial to reduce analysis bias of PW in initial fields which can result in restraining PW prediction bias during the earlier period （3 - 6 h） of model integration so as to improve PW prediction. The PW prediction improvement is related to the GPS receiver location in the area covered by GPS networks. By comparing the results with no GPS IPW assimilation, we find that GPS PW assimilation can increase the accuracy of 6 h and 24 h accumulated precipitation prediction so as to improve the precipitation prediction ability of MM5. On the whole, GPS PW data assimilation will improve the precipitation prediction of MM5. However, there are several difficulties that will impact the GPS assimilation, which are how to get the true background errors and GPS PW errors at real time. It will be further researches to develop a new technique to calculate more reasonable background errors and GPS PW errors.