Raindrop size distribution measurements on the Southeast Tibetan Plateau during the STEP project

Abstract

As part of the Second Tibetan Plateau Scientific Expedition and Research (STEP) field campaign, raindrop size distribution (DSD) measurements were conducted with a laser optical particle size velocity disdrometer in Motuo on the Tibetan Plateau (TP). The DSD characteristics of five different rain rate classes and convective and stratiform precipitation types were studied using the DSD data from July to September 2019, comprising a total of 47,774 1-min raindrop spectra. The average raindrop spectral width and number concentration of large drops increased with the rainfall intensity. Furthermore, the combination of small- and midsize drops dominated the precipitation in Motuo, and their contributions exceeded 99% of the number concentration. The convective precipitation in Motuo was identified as maritime-like precipitation and was characterized by a large normalized intercept parameter log10Nw = 4.0–4.4 and small mass-weighted mean diameter Dm = 1.1–1.4 mm. Empirical relations among the three parameters (intercept N0, shape μ, and slope Λ) of the gamma distribution model, Nw and Dm of the normalized gamma distribution were derived. We also obtained power-law relationships of Z = ARb for the stratiform and convective precipitation in Motuo, and the empirical relation of Z = 300R1.4 significantly underestimated the convective precipitation in Motuo. When the precipitation type transitioned from stratiform into convective precipitation, coefficient A decreased and exponent b increased. Finally, empirical relations between Dm and the radar equivalent reflectivity factor at the Ku and Ka bands were obtained to improve the rainfall retrieval algorithms of the dual-frequency precipitation radar on the southeast TP.