Preliminary study on the characteristics and causes of low-temperature, low-energy rainstorms in Panzhihua

Studying the characteristics and causes of low-temperature, low-energy rainstorms helps deepen the understanding of various types of rainstorms. Based on 338 rainstorm events in Panzhihua from 1980 to 2022, combined with conventional surface observations and ERA5 reanalysis data, this study determined the localized screening thresholds for low-temperature, low-energy rainstorms, analyzed their spatiotemporal characteristics, environmental conditions, and formation mechanisms. The results show that: (1) the occurrence frequency of low-temperature, low-energy rainstorms in Panzhihua exhibits a north-high and south-low spatial distribution. September is the most frequent month of occurrence, exhibit an obvious nocturnal rainfall feature, and the main rainfall period is from around 22:00 (Beijing Time, BT, the same below) to 12:00 the next day. (2) Low-temperature, low-energy rainstorms in Panzhihua mostly occur under when the air temperature at 14:00/20:00 (T₁₄/T₂₀), relative humidity (RH₁₄/RH₂₀), and water vapor pressure(e₁₄/e₂₀) exceed 20.7/21.2 ℃, 73%/79.8%, and 20.3/20.7 hPa, respectively. Conversely, when T₁₄/T₂₀, RH₁₄/RH₂₀, and e₁₄/e₂₀ fall below 19/20.2 ℃, 61%/69%, and 16.5/17.3 hPa, respectively, the occurrence of such rainstorm events can generally be ruled out. (3) Although the thermal conditions of low-temperature, low-energy rainstorm events are relatively weak, their dynamic and moisture conditions are stronger than those of non-low-temperature, non-low-energy rainstorms. Specifically, the peak vertical upward motion and low-level convergence intensity are higher, and the water vapor flux convergence covers a broader area. The intense low-level convergence and updraft, coupled with the persistent moisture transport from the Bay of Bengal, effectively compensated for the adverse conditions of low temperature and limited convective available potential energy, thereby establishing the critical circulation background for the occurrence of heavy rainfall. This study establishes localized diagnostic criteria for environmental parameters of low-temperature, low-energy rainstorms in Panzhihua City, reveals the compensatory mechanism of dynamic and moisture conditions for thermal deficiencies, which can help improve the forecasting and early warning capabilities for such rainstorms.