Study on target observation of a heavy rainfall in Shanxi Province

A heavy rainfall process in southern Shanxi from May 20 to 21, 2023 was simulated and analyzed using the high-resolution Weather Research Forecasting model (WRF). On this basis, the target observation sensitive area of the heavy rainfall process was identified using the conditional nonlinear optimal perturbation (CNOP) method, and the physical significance of the sensitive area was analyzed. Finally, the effectiveness of the sensitive area identified by the CNOP method was investigated through the observation system simulation test (OSSE) and the reasons are analyzed. The results show that the WRF model can simulate the heavy rainfall in the south of Shanxi Province, but the local rainstorm has a deviation in location, a slightly lower intensity, and west-shift positions of moderate and heavy rains, while the range of light rain is relatively large; The sensitive area identified by CNOP method is mainly located near the center of the northeast cold vortex, reflecting the important influence of the northeast cold vortex on the rainstorm event. Further, according to the distribution of the local maximum energy of the CNOP type initial error, three sub sensitive regions are selected for comparative analysis. The three comparison regions reflect the influence of the Qinghai Tibet Plateau low pressure, the shear line in Shanxi region and the Western Pacific subtropical high, respectively. The results show that assimilating the simulated observation data in the sensitive region can maximize the TS score of heavy rain and rainstorm forecast, followed by assimilating the observation data in Shanxi and its surrounding areas. Assimilating the observation data in the low pressure area of the Qinghai Tibet Plateau and the northern area of the Western Pacific subtropical high can also improve the TS score of heavy rain and rainstorm forecast, but to a lesser extent. Further analysis revealed that the simulated observation data within the sensitive area has a significant impact on the wind forecast in southern Shanxi. With the help of the complex terrain in southern Shanxi, it would promote the convergence of wind in multiple regions, expanding the area of heavy rain, and thus improves the forecast technique for this heavy rainfall event.