不同控制变量的AMSR2资料同化方法对台风“山神”预报的影响研究

Different background fields and covariance schemes in AMSR2 radiance data assimilation and their impacts on the forecast and analysis of Typhoon Son-tinh

以2012年生成于太平洋西岸的第23号台风"山神"(Son-tinh)为个例,在WRFDA-3DVar同化系统中结合新型AMSR2同化模块,探讨两种不同的同化背景误差协方差(CV5、CV7)和不同的模式背景场启动机制(冷,热启动)对台风预报影响的研究。结果表明:(1)同化系统的分析场相对于背景场,对AMSR2辐射率资料亮温的模拟,在观测残差、标准差、均方根误差方面均有不同程度的减小。可以很好地模拟出台风的热力学结构,提高模式分析场的质量,从而进一步提高台风预报的准确度;(2)总体而言使用UV风的控制变量方案(CV7)的效果略好于使用流函数和势函数的方案(CV5)。CV7方案的正向温度增量区域大于CV5,尤其是在台风中心区域附近,正向温度增量有助于台风暖心结构的形成;(3)热启动的spin-up过程可以改善模式的热力结构和水汽特征;CV7方案可以在更小尺度上将背景场和观测联系起来且在边界附近产生的误差更小。热启动结合CV7方案对于台风路径的预报最为有效。

Taking No. 23 typhoon Son-tinh generated in the West Pacific Ocean as a case study, we analyze the influence of two different background error covariance(CV5, CV7) and two different background starting mechanisms(cold-start, warm-start) on typhoon forecast using the WRFDA-3 DVar assimilation system with the newly developed AMSR2 assimilation module. The results show that the observation error, standard deviation and root-mean-square error is reduced by assimilating the AMSR2 radiance data. In addition, the thermodynamic structure of typhoon is can be well simulated and the quantity of is enhanced, which favors the improvement of typhoon forecast accuracy. The area of positive temperature increment in CV7 scheme is larger than that in CV5 scheme, especially near the area of typhoon. Positive temperature increment favors the formation of typhoon warm core, which results in the slightly better performance of CV7 in typhoon forecast compared to CV5. The spin-up process of warm-start can improve the thermodynamic structure and vapor property of the model. On the other hand, the CV7 scheme could associate background field with observation on a smaller scale and generate less errors near the boundary. As a result, warm-start combined with CV7 scheme is most effective for typhoon track forecast.