Precipitation measurement intercomparison in the Qilian Mountains, north-eastern Tibetan Plateau

Abstract. An experimental field study of wind-induced bias in precipitation measurements was conducted from September 2010 to April 2015 at a grassland site (99°52.9´ E, 38°16.1´ N; 2980 m) in the Hulu watershed in the Qilian Mountains, on the north-eastern Tibetan Plateau, in China. The experiment included (1) an unshielded Chinese standard precipitation gauge (CSPG_UN; orifice diameter = 20 cm, height = 70 cm), (2) a single Alter shield around a CSPG (CSPG_SA), (3) a CSPG in a pit (CSPG_PIT) and (4) a Double-Fence International Reference (DFIR) with a Tretyakov-shielded CSPG (CSPG_DFIR). The catch ratio (CR) used the CSPG_DFIR as a reference (CR = CSPG_X/CSPG_DFIR, %; X denotes UN, SA or PIT). The results show that the CSPG_SA, CSPG_PIT and CSPG_DIFR caught 0.9, 4.5 and 3.4 % more rainfall; 7.7, 15.6 and 14.2 % more mixed precipitation (snow with rain, rain with snow); 11.1, 16.0 and 20.6 % more snowfall and 2.0, 6.0 and 5.3 % more precipitation (of all types), respectively, than the CSPG_UN from September 2012 to April 2015. The CSPG_PIT and CSPG_DFIR caught 3.6 and 2.5 % more rainfall; 7.3 and 6.0 % more mixed precipitation; 4.4 and 8.5 % more snowfall; and 3.9 and 3.2 % more total precipitation, respectively, than the CSPG_SA. However, the CSPG_DFIR caught 1.0 % less rainfall; 1.2 % less mixed precipitation; 3.9 % more snowfall and 0.6 % less total precipitation than the CSPG_PIT. From most to least precipitation measured, the instruments ranked as follows: for rain and mixed precipitation, CSPG_PIT > CSPG_DFIR > CSPG_SA > CSPG_UN; for snowfall, CSPG_DFIR > CSPG_PIT > CSPG_SA > CSPG_UN. The CR vs. 10 m wind speed for the period of precipitation indicated that with increasing wind speed from 0 to 8.0 m s⁻¹, the CR_UN/DFIR and CR_SA/DFIR for rainfall decreased slightly. For mixed precipitation, the wind speed showed no significant effect on CR_UN/DFIR and CR_SA/DFIR below 3.5 m s⁻¹. For snowfall, the CR_UN/DFIR and CR_SA/DFIR vs. wind speed showed that CR decreased with increasing wind speed. The precipitation measured by the shielded gauges increased linearly relative to that of the unshielded gauges. However, the increase in the ratio of the linear correlation should depend on specific environmental conditions. A comparison of the wind-induced bias indicates that the CSPG_PIT could be used as a reference gauge for rain, mixed and snow precipitation events at the experimental site. As both the PIT and DFIR effectively prevented wind from influencing the catch of the precipitation gauge, the CR_PIT/DFIR had no relationship with wind speed. Cubic polynomials and exponential functions were used to quantify the relationship between catch ratio and wind speed. For snow, for both event and daily scales, the CR_UN/DFIR and CR_SA/DFIR were significantly related to wind speed; while for rain and mixed precipitation, only the event scale showed a significant relationship.