The effect of orifice rim thickness on the wind speed above precipitation gauges

Abstract

The body of a free exposed precipitation gauge distorts the wind field, causing the losses of precipitation catch. The losses depend on the gauge aerodynamic characteristics. The possible effect of the gauge orifice rim thickness is investigated in a wind tunnel. Two groups of three experimental precipitation gauges were used. One group had a thin orifice rim (3 mm) and the other a thick one (19 mm). The orifice areas of the gauges in each group were 113, 189 and 300 cm2. The maximum wind speed increase above the centre of orifice areas was 31–43% but it was greater for gauges with thick orifice rims than for those with thin ones. The orifice area seems to be less important, particularly for a thick rim. In contrast, the intensity of turbulence was slightly greater for thinner rims and decreased with increasing orifice area. This seems to conform the results of field intercomparison measurements, as reported by Sevruk and Tettamanti (In Proc. 8th Symp. on meteorological observations and instrumentation, pp. 323–324, 1993) who showed that the wind induced error of precipitation measurement is generally greater for gauges with a thicker orifice rim than for gauges with a thinner one.