Characteristics of orographic raindrop size distribution in the Tianshan Mountains, China

Characteristics of orographic raindrop size distribution (DSD) in the Tianshan Mountains, China are studied based on second-generation OTT Particle Size Velocity disdrometer installed at the top (Tianchi, 43.88°N, 88.12°E, 1941.8 m above sea level) and the foot stations (Urumqi, 43.79°N, 87.65°E, 935 m above sea level) from June to August 2020 and 2021. For the overall rainfall, the concentration of drops of all sizes in Tianchi is greater than that in Urumqi. Furthermore, in both regions, small drops (diameter < 1 mm) primarily contributed to the total number concentration, and both small drops and mid-size drops (1 ≤ diameter ≤ 3 mm) had important contributions to the rainfall rate. For the DSD of different rain rate classes, the concentration of mid-size drops is higher in the first two rain rate classes in Urumqi than that in Tianchi, while the opposite is true when the rain rate exceeds 5 mm h−1. Meanwhile, the concentration of small drops is higher in the first two rain rate classes in Tianchi than that in Urumqi, while the opposite is true for small drops in partial diameter, as the rain rate class increases. In addition, two kinds of rainfall kinetic energy (rainfall kinetic energy flux: KEtime and the rainfall kinetic energy content: KEmm) and various rainfall kinetic energy-rainfall intensity relations (KEtime/KEmm –R relations) were derived based on DSD data. Further, the possible thermo-dynamical and microphysical processes that cause the dissimilarities in DSD between Urumqi and Tianchi are also discussed in this work. Affected by the difference in altitude between the top and the foot stations, the foot station had relatively hotter and drier conditions in the near-surface layer than the top station during the rainfall period, so the evaporation rate of small drops was higher at the foot station than that at the top station, resulting in fewer small drops at the foot station. Meanwhile, lower black body temperature, and stronger seeder-feeder mechanism at the top station during the rainfall period may be partly responsible for more mid-size drops and large size drops (diameter > 3 mm) at the top station