三维变分同化雷达视风速的改进方案及其数值试验

An improved method for the three-dimensional variational assimilation of the radar seeable velocity and its numerical experiments

用雨滴末速度改进了万齐林等2005年提出的一种直接同化多普勒雷达径向风和回波强度时空变化所包含的风场信息——视风速的变分同化方法。通过理论推导和尺度分析发现,中尺度系统中雨滴末速度具有较大的量级,故该项在雷达回波守恒方程中起着重要作用。在原方案的基础上考虑雨滴末速度的影响,可以避免云团的局地增长作用对视风速的干扰,视风速能更准确地表示雷达回波的平流信息。台风"风神"的雷达观测资料分析表明,改进方案计算的视风速分布紧密、有规则,能较好地表现台风云系内的中尺度辐合辐散结构。运用中国气象科学研究院研发的三维变分同化系统(GRAPES-3D-Var)进行了一组模拟观测试验表明,改进方案的水平风场增量和位势高度增量都与模拟观测减自由积分的差值场相似,呈偶极型环流,径向风的垂直变化也得到明显改进。改进的视风速与径向风共同反演出的大气水平风场更准确。原方案的模拟观测试验则不能表现出以上特征。使用改进方案同化广州雷达观测资料后,敏感性试验的初始场中台风的中心位置与实况观测更接近,初始风场的气旋性环流更明显。敏感性试验模拟的6h降水分布与实况观测更相似,降水中心强度较小,但优于控制试验。

With the terminal velocity of raindrops,this paper improves a variational method that directly assimilates wind field information,including the radial velocity and the movement of radar echo（seeable velocity）,as proposed in Wan et al（2005）.According to the theoretical derivation and scale analysis,the terminal velocity of raindrops of a mesoscale system has a relatively large order of magnitude,so this term is an important role in the equation of radar echo conservation.The improved method-using the terminal velocity of raindrops-avoids the interference of local growth of cloud clusters,and also the advection of radar echo is to be more accurately expressed by the improved seeable velocity.The analysis of the observational radar dataset of ＇Fengshen＇ demonstrates that the improved seeable velocity is regularly and reasonably distributed and distinctly shows the mesoscale convergence and divergence of the typhoon.With the application of the GRAPES-3DVar system developed by the China Meteorological Administration（CMA）,a group of observational system simulation experiments（OSSE） were operated.The results of improved method show that the increments of both the horizontal wind and the geopotential height assume a dipole flow pattern which is concord with the difference between simulated observations and free integrations,and the vertical variation of the meridional wind is also better.The structure of horizontal wind fields retrieved by the improved seeable velocity and radial velocity is more accurate.With the original method,the results of the OSSE show no these characteristics.With the observational radar data assimilated by the improved method,we conducted another group experiments.The initial field of sensitive experiment shows that the typhoon centre was adjusted and more contiguous with the observation and the horizontal wind fields had more obvious cyclonic rotation.Moreover,the distribution of 6-hourly accumulative precipitation of the sensitive experiment was more similar to the observation.The maximum rainfall intensity was less,but it was superior to the control experiment.