梅雨期暴雨系统的流依赖中尺度可预报性

Flow-Dependent Mesoscale Predictability of Meiyu Heavy Rainfall

中尺度天气系统的初值敏感性,导致了中尺度系统预报极限的存在。中尺度系统的初始误差的快速增长及其中尺度可预报性依赖于系统流的特征。梅雨暴雨形成是多尺度天气系统共同作用的结果,决定了梅雨期暴雨的形成机制的多样性,也决定了其初值敏感性的差异性。本文重点对比分析了五种不同类型的梅雨暴雨的误差增长特征及其机制。冷空气抬升、低层涡旋、边界层冷池、重力波等在梅雨期暴雨系统的误差增长发挥着重要作用,它们通过不同方式促使初始误差由小尺度向大尺度传递。利用初始随机扰动和系统扰动方法构造的初始误差试验表明,系统扰动试验中误差的初始调整较小,能够快速到达误差最优增长尺度。冷槽推进型的误差扰动尺度大,冷空气抬升控制不稳定区域的误差尺度,可预报性强;西南涡移出型和北槽南涡型的低层涡旋平流作用抵消涡旋内的正负偏差,加快了误差能量向大尺度传递,提高了暖季降水的预报技巧;自组织型中重力波、冷池强迫抬升等拟连机制使得误差能量快速频散,出现误差在多个尺度上共同增长,且误差饱和尺度小于上述三种类型,可预报性略差;受局地地形和热力条件影响,非组织局地型的中小尺度误差能量很难饱和,可预报性最差。梅雨期暴雨的误差结构主要决定于暴雨形成中起主要作用的系统。冷空气抬升使得冷槽推进型暴雨中误差沿锋面垂直倾斜;冷空气侵入致使冷槽推进型和北槽南涡型的中层温度误差出现显著性增长;在西南涡移出型、北槽南涡型、自组织型、非组织局地类型中,冷池的形成导致出现了低层的高动量偏差;在所有的暴雨类型中,在对流发展后期,误差向高层传递并增大,导致在高层出现大误差带。

The initial condition sensitivity leads to a limit for the prediction of mesoscale weather systems.The rapid growth of initial errors and associated mesoscale predictability depend on the features of flow.The generation of Meiyu heavy rainfall is the result of interaction of many different-scale weather systems.Therefore,there are different mechanisms for the formation and the sensitivity to initial condition of Meiyu heavy rainfall.The mechanisms of error growth for five different types of Meiyu rainfall are investigated.It is the larger scale cold air lifting,low level vortex,boundary layer cold pool,and gravity wave that play important roles in the error growth of Meiyu rainfall systems.They promote the scale growth of initial errors from small to larger scales in different ways.The perturbation simulations with random or systematic initial errors demonstrate that the initial errors can quickly reach its optimal scales in the systematic perturbation experiment.The cold air lifting controls the scale of error in the moving trough type of Meiyu rainfall,as a result,the scale of error is rather large and the predictability is rather high.The low-level vortex advection inthe Meiyu rainfall of the moving-out Southwest China vortex type and the north-trough with south-vortex type can offset the deviation within the vortex,and accelerate the transport of error energy from the small scale to large scale,and then the forecast skill of warm-season heavy rainfall is improved.The self-organizing rainfall system has a smaller saturation scale of error and lower predictability than those of the first two types of rainfall,because the immanent coupling mechanism of gravity wave and the cold pool can accelerate the dispersion of error energy,which results in the co-growth of errors at many scales.It is difficult for small scale error to reach its saturation in the non-organizing local rainfall systems,which is usually influenced by local terrain and thermal-dynamic conditions,and therefore it has the lowest predictability.The structure of error in Meiyu heavy rainfall is mainly dependent on the sub-weather systems,which play the most important role in the formation of Meiyu rainfall.The error in the moving trough type slants vertically along the front.Cold air intruding in the moving-trough and north-trough with south-vortex types leads to a prominent increasing of temperature errorat the mid-levels.The genesis of cold pool in the moving-out Southwest China vortex,north-trough with south-vortex,self-organizing,and non-organizing local system types results in a high momentum difference at the lower levels.During the later stage of all types of rainfall,there will be a large error zone at the upper levels,due to the transportation from the lower levels to upper levels.