国家自然科学基金项目 42105008，国家自然科学基金项目 42192553
1.Key Laboratory, Meteorological Disaster of Ministry of Education (KLME) /Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science and Technology;2.Jiangsu Meteorological Observatory;3.Liangshan Meteorological Bureau;4.Shanghai Typhoon Institute, China Meteorological Administration
长江中下游地区的暖区暴雨过程易受复杂下垫面强迫的影响，具有较大的预报不确定性，尤其是其中的对流触发过程。为探讨此类过程的触发条件及揭示其可预报性受限制程度，本文针对2020年6月23日一次复杂地形包裹下的长江中下游暖区暴雨展开高分辨率的数值模拟和对流尺度集合模拟，通过Lagrange气块后向轨迹分析、去除地形和关闭热效应的敏感性试验以及集合敏感性分析等方法对此次过程的对流触发阶段展开分析。结果表明，此次过程被抬升气块的主要源地为1.5 km以下的边界层，仙霞岭和杉岭在正午时分因热力作用而驱动的出谷风是引发局地辐合抬升的动力源，高低层散度、湿位涡的垂直配置以及偶极型位涡异常对此次对流触发过程具有较好的指示意义。此外，该过程对前期近地面2 m温度和视热源具有较高的敏感性，该结果证实下垫面强迫的精确刻画对于提升暖区暴雨的预报效果至关重要。逐步减小初始场误差的初值敏感性试验进一步表明，此次暖区对流过程的可预报性显著低于北边的锋面过程，表现为锋面对流的偏差总能量能随初始误差的缩小持续性降低，而暖区对流的偏差总能量曲线则仍能增长至与原水平相近，呈现出非线性辐合收缩特征。因此，对于天气尺度强迫显著的锋面对流，或可优先考虑通过加强资料同化能力等手段降低初始场误差来减小预报误差；但对于复杂地形下的暖区暴雨对流触发过程，则需要更加强调通过集合预报来捕捉其不确定性。
Warm-sector rainstorm events over the middle-lower reaches of Yangtze River (MLRYZ) usually have large prediction uncertainties due to that they are easily affected by the complex underlying surface, especially in the convection initiation stage. In this paper, high-resolution numerical simulation and convection-permitting ensemble simulations are carried out for a warm-sector rainstorm over MLRYZ on June 23, 2020 that was affected by the complex terrain to investigate the trigger mechanism and reveal the limited predictability of this event. The Lagrangian backward trajectories analysis of air parcels, sensitivity experiments of removing terrain and closing thermal effect, and ensemble sensitivity analysis are used to analyze the convection initiation stage. Results show that the lifted air parcels mainly come from planet boundary layer below 1.5 km. The valley wind driven by thermal effect of Xianxia and Shan Mountains is the dominant dynamic source that triggered local convergence and lifting. The divergence of high and low levels, the vertical configuration of moist potential vorticity and dipole potential vorticity anomaly have a good indication of the convection initiation. In addition, this event is highly sensitive to the 2 m temperature and apparent heat source at low level, indicating the importance of accurate underlying forcing to the warm-sector rainstorm prediction. The initial condition sensitivity experiments which reduce the initial errors gradually suggest that the predictability of the warm-sector convective event is significantly lower than that of the frontal event occurred in the north. The RMDTE (Root Mean Difference Total Energy) of frontal convection can be decreased continuously with the reduction of the initial errors, while the RMDTE curves of warm-sector convection still reach original level, showing a nonlinear convergence characteristic. Therefore, for the frontal convection with strong synoptic forcing, data assimilation technique may be prioritized to reduce the initial errors so as to further reduce the forecast errors. However, for the convection initiation process of warm-sector rainstorm under complex terrain, more attentions should be paid on ensemble forecasts to represent its uncertainty.