Statistical characteristic differences of raindrop size distribution during rainy seasons in the Pan Third Pole region

The Qilian Mountains, a series of marginal mountains in the northeastern part of the Tibetan Plateau (TP), are a vitally important ecological protection barrier in the northwestern arid areas. In order to improve understanding of the microstructure of precipitation, the characteristics of raindrop size distribution (DSD) were analyzed using the Second Tibetan Plateau Comprehensive Scientific Expedition of observations. The Qilian Mountains region has its own unique DSD characteristics with a smaller raindrop diameter and a higher number concentration (3.69 for log10Nw and 0.94 mm for Dm) comparing with eastern, southern, northern, and central China, but are similar to those of southeastern TP. For all rainfall events, the number concentrations of small and large raindrops in the interior and on the southern slopes were greater than on the northern slopes, but midsize raindrops were less. The DSD spectrum of the interior was more variable and differed significantly from that of the northern slopes. The differences in the normalized intercept parameters of the DSD for stratiform and convective rainfall were 8.3 % and 10.4 %, respectively, and those of the mass-weighted mean diameters were 10.0 % and 23.4 %, respectively, while the standard deviations of DSD parameters at interior sites were larger. The differences in the coefficient and exponent of the Z–R relationship were 2.5 % and 10.7 %, respectively, with an increasing value of the coefficient from the southern to the northern slopes for stratiform rainfall but the opposite for convective rainfall. In addition, the DSD characteristics and Z–R relationships were more similar at the ipsilateral sites and had smaller differences between the southern slopes and interior of the mountains