Characteristics of Raindrop Size Distributions during Meiyu Season in Mount Lushan, Eastern China

Abstract

 Meiyu front precipitation makes the region prone to frequent floods, mudslides, landslides, and other disasters, and has been the focus of ongoing and challenging meteorological research. Raindrop size distribution (RSD) is a fundamental method for exploring the characteristics and physical processes of rainfall. This study investigated the precipitation characteristics in Lushan mountainous areas during the Meiyu season using laser disdrometer observed RSD data from 2016 to 2019. For the average spectra of five rain rate classes, the concentrations of large raindrops (> 0.5 mm) increased with rain rate (R), while the concentrations of small raindrops (< 0.5 mm) increased only under rain rates higher than 10 mm h⁻¹. The gamma distribution parameters of N₀ and Λ increased/decreased with rain rate, and μ exhibited negative values in different rain rate classes. The distribution pattern features were N (D) = 721D^−1.79e^−1.20D. Distributions of the frequency for mass-weighted mean diameter (D_m) and the logarithm of the generalized intercept parameter (log₁₀N_w) both showed a unique bimodal type, and an exceptionally high N_w (log₁₀N_w > 4.5) subset with small D_m was determined. The stratiform and convective rain of RSD were also investigated. D_m − R and N_w − R showed similar variations in two types of precipitation. The lower μ values resulted in higher primary and constant coefficients in the quadratic polynomial fitting for the μ − Λ relationship (Λ = 0.0347μ² + 1.180μ + 2.495). The Z − R relationship in stratiform precipitation characteristics was Z = 203R^1.59. Further investigations showed that high N_w values usually occurred in persistent precipitation. The RSD can be characterized as high concentrations of the first two diameter classes with narrow spectrum width (< 1 mm), which were captured during in-cloud rain with a low but continuous rain rate (< 5 mm h⁻¹). The mountainous topography plays an important role in reshaping the characteristics of RSD and the physical processes of precipitation.