热带气旋奇异向量在GRAPES全球集合预报中的初步应用

The preliminary appliation of tropical cyclone targeted singular vectors in the GRAPES global ensemble forecasts

基于副热带奇异向量的初值扰动方法已应用于GRAPES(Global and Regional Assimilation PrEdiction System)全球集合预报系统,但存在热带气旋预报路径离散度不足的问题.通过分析发现,热带气旋附近区域初值扰动结构不合理导致预报集合不能较好地估计热带气旋预报的不确定性,是路径集合离散度不足的可能原因之一.通过建立热带气旋奇异向量求解方案,将热带气旋奇异向量和副热带奇异向量共同线性组合生成初值扰动,以弥补热带气旋区域初值扰动结构不合理这一缺陷,进而改进热带气旋集合预报效果.利用GRAPES全球奇异向量计算方案,以台风中心10个经纬度区域为目标区构建热带气旋奇异向量求解方案,针对台风"榕树"个例进行集合预报试验,并开展批量试验,利用中国中央气象台最优台风路径和中国国家气象信息中心的降水观测资料进行检验,对比分析热带气旋奇异向量结构特征和初值扰动特征,评估热带气旋奇异向量对热带气旋路径集合预报和中国区域24 h累计降水概率预报技巧的影响.结果表明,热带气旋奇异向量具有局地化特征,使用热带气旋奇异向量之后,热带气旋路径离散度增加,路径集合平均预报误差和离散度的关系得到改善,路径集合平均预报误差有所减小,集合成员更好地描述了热带气旋路径的预报不确定性;中国台风降水的小雨、中雨、大雨、暴雨各量级24 h累计降水概率预报技巧均有一定提高.总之,当在初值扰动的生成中考虑热带气旋奇异向量后,可改进热带气旋初值扰动结果,并有助于改善热带气旋路径集合预报效果.

The initial perturbation approach based on the subtropical singular vectors has been applied in the Global and Regional Assimilation PrEdiction System(GRAPES)global ensemble forecast system.However,the tropical cyclone track ensemble spread is not enough for this system.It is found that the initial perturbation near the tropical cyclone is not reasonable.As a result,the forecast ensemble could not well estiminate the forecast uncertainty of the tropical cyclone,which may be one reason for the small track ensemble spread.Through computing the tropical cyclone singular vectors(TCSVs)and linearly combining the TCSVs and subtropical singular vectors to generate initial perturbation,the problem of almost zero initial perturbation near the tropical cyclone is solved,and the tropical cyclone ensemble forecasts are improved.Based on the computation scheme for the GRAPES global singular vectors,the square area around the typhoon center with the side length of 10 degree is identified as the targeted area to construct the computation scheme for TCSVs.The ensemble forecasts experiments are conducted for the typical typhoon Banyan and other TC cases,and the best typhoon track at the National Meteorological Center of China Meteorological Administration(CMA)and precipitation observation data at National Meteorological Information Center of CMA are used to verify the forecasts.The structure characters of the TCSVs and the initial perturbations are analyzed,and the influence of TCSVs on the tropical cyclone track ensemble forecasts and 24 h accumulated precipitation probability forecast skill for the China area is analyzed.The experiments show that TCSVs are localized.When TCSVs are used to generate initial perturbations,the tropical cyclone track ensemble spread is increased;the relation between the track ensemble spread and the ensemble mean forecast error is improved;the track ensemble mean forecast error is decreased;the ensemble members can better describe the forecast uncertainty of the tropical cyclone track;the probability forecast skill of 24 h accumulated precipitation in China for the light rain,moderate rain,heavy rain and rainstorm is increased.In a word,the use of TCSVs is effective for improving the initial perturbations and may help improve the tropical cyclone track ensemble forecasts when there exists one or more TCs.