Analysis of field observations of an all-weather vibrating-wire precipitation gauge

There are now more than 70 operational sites of the U.S. Climate Reference Network (USCRN). These sites are located in the continental U.S., Alaska (2), and Canada (1). Among other instrumentation, each site employs a Geonor all-weather vibrating-wire precipitation gauge with 3 wires as the primary instrument for measuring precipitation.

Over two years of 1-minute accumulations from each of three wires in a Geonor gauge installed in a pit located on the north campus of the University of Oklahoma, Norman (not part of the USCRN) have been analyzed to answer the following questions: (1) what is the sensitivity of the output of a vibrating-wire to temperature on the time scale of a daily cycle? (2) is this temperature sensitivity related to bucket accumulation and/or mean daily temperature and, if so, how? (3) what is the effect of wind speed on the resonant frequency of a vibrating wire? (4) with regard to estimating accumulation, is there any advantage in using the method that counts the number of cycles over a fixed time interval compared to the method that measures the time for a fixed number of cycles to occur (instructions P3 and P27, respectively, in the CSI data logger)? (5) what is the lowest 1-minute rain rate that can be measured before system noise overwhelms the signal?

The results of the analysis show that the temperature sensitivity is negative and its magnitude depends on a particular vibrating-wire and accumulation in the bucket. Further results show that the noise in the output generated by wind speed is in proportion to the speed and, for determining 1-minute rain rates, it is advantageous to measure the time for a fixed number of cycles to occur (instruction P27). Examples of 1-minute rain rates in light drizzle will be shown.