两种动力控制变量对比分析及其对台风同化和预报的影响

Comparative analysis of two dynamical control variables and their impacts on typhoon assimilation and prediction

目前常用的流函数-势函数(ψ-χ)和水平风分量(U-V)两种动力控制变量方案分别被认为适用于大尺度同化和对流尺度同化,而两种方案在台风尺度同化中的具体特征及其对台风预报的影响尚未明确。基于WRF-3DVarDA系统,分析不同动力控制变量方案的背景场误差协方差的统计特征,并进行了单点观测同化理想试验和5场台风过程的20组连续循环同化和预报批量试验。两种动力控制变量方案的背景场误差协方差的统计特征分析表明,ψ-χ方案背景场误差更小、长度尺度更大,而U-V方案的局地性特征更明显,方差更大;单点观测试验证实了两种方案的统计特征,也表明ψ-χ作为控制变量会导致虚假负增量的出现;批量试验表明,U-V方案的台风路径、最大风速以及台风强度的整体优于ψ-χ方案;诊断分析表明,U-V方案得到的更合理的热动力结构特征是提高台风预报的主要原因。

The role of data assimilation is to provide a superior initial field for application in numerical models by means of applying observation data.The three-dimensional variation method is widely used in operational forecasting,due to its rather low computational costs and strong assimilation ability.An important component of the three-dimensional variation method is background error covariance,which can directly affect data assimilation and numerical prediction results.Due to the fact that the background error covariance is very substantial,in practical application the analysis increment is often used to solve the minimization problem of the three-dimensional variational cost function.Different control variables have various assimilation and prediction effects,thus selecting appropriate control variables is an important prerequisite for building a reasonable assimilation system.At present,the two most frequently used dynamical control variable schemes are the stream function potential function(ψ-χ)scheme,and horizontal wind components(U-V)scheme.These are considered to be respectively suitable for large-scale assimilation and convective scale assimilation.However,neither the specific characteristics of the two schemes in typhoon scale assimilation nor their impact on typhoon forecasting have yet to be investigated.In this study,based on the WRF-3DVarDA system,we analyze the statistical characteristics of the background error covariance of the two different control variable schemes.Next,we carry out an ideal test of single point observation assimilation and 20 groups of continuous cycle assimilation and prediction experiments in regard to five typhoon events.The analysis results of the background error covariance statistical characteristics of the two schemes show that the background error is smaller and the length scale is larger in theψ-χscheme,while more obvious local features and larger variance are found in the U-V scheme.In addition,the results of a single point observation test confirm the statistical characteristics of the two schemes,and also reveals thatψ-χas a control variable can lead to a false negative increment.The results of batch experiments show that in general the U-V scheme is superior to theψ-χscheme,in terms of typhoon track error,maximum wind speed error and minimum sea level pressure error.The diagnostic analysis results show that the greater number of reasonable thermal and dynamic structural features in the U-V scheme are the source of improvement in the typhoon forecast.