Improving the rainfall rate estimation in the midstream of the Heihe River Basin using raindrop size distribution

During the intensive observation period of the Watershed Allied Telemetry Experimental Research (WA- TER), a total of 1074 raindrop size distribution were mea- sured by the Parsivel disdrometer, the latest state-of-the-art optical laser instrument. Because of the limited observation data in Qinghai-Tibet Plateau, the modelling behaviour was not well done. We used raindrop size distributions to improve the rain rate estimator of meteorological radar in order to ob- tain many accurate rain rate data in this area. We got the rela- tionship between the terminal velocity of the raindrop and the diameter (mm) of a raindrop: v(D) = 4.67D 0.53 . Then four types of estimators for X-band polarimetric radar are exam- ined. The simulation results show that the classical estimator R (ZH) is most sensitive to variations in DSD and the estima- tor R (KDP,ZH,ZDR) is the best estimator for estimating the rain rate. An X-band polarimetric radar (714XDP) is used for verifying these estimators. The lowest sensitivity of the rain rate estimator R (KDP, ZH, ZDR) to variations in DSD can be explained by the following facts. The difference in the forward-scattering amplitudes at horizontal and vertical po- larizations, which contributes KDP, is proportional to the 3rd power of the drop diameter. On the other hand, the exponent of the backscatter cross-section, which contributes to ZH, is proportional to the 6th power of the drop diameter. Because the rain rate R is proportional to the 3.57th power of the drop diameter, KDP is less sensitive to DSD variations than ZH.

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