Particle Size Distributions and Influential Factors Derived from Aircraft Measurements in the Cold Orographic Clouds in the Qilian Mountains, Northeastern Tibetan Plateau

Abstract Particle size distributions (PSDs) are highly important in numerical models of cloud microphysical schemes. However, PSDs have large uncertainties due to complex ice particle habits and formation processes. The PSDs and influential factors were investigated based on aircraft measurements of cold stratiform orographic clouds on 16 and 29 August 2020, over the Qilian Mountains (QM), northeastern Tibetan Plateau. The results revealed that the orographic clouds displayed multiple vertically layered structures, the number concentration of large ice particles was greater in the mature stage, and the supercooled liquid water content (SLWC) was greater in the dissipating stage. The seeder–feeder mechanism was present in both orographic events. The upper cold clouds containing low-concentration and large-sized ice particles acted as seeding clouds to the low-level feeding clouds with higher SLWC and significantly broadened PSDs. The PSDs were mostly unimodal and were well fitted with gamma distributions. The gamma fitting parameters of the intercept N 0 , slope λ , and dispersion μ were clearly related to the temperature for the nonprecipitating particles, whereas there was no clear relationship for the precipitating particles. The λ increased with increasing SLWC for the nonprecipitating particles and decreased with the increasing ice water content (IWC) and maximum diameter D max for the precipitating particles, indicating the importance of riming and aggregation processes in cold orographic clouds. The terms N 0 , μ , and λ were not independent parameters, and their relationships can be well formulated.