Using vibrating-wire technology for precipitation measurements

Abstract

Vibrating-wire technology applied to the measurement of precipitation was developed at the Norwegian Geotechnical Institute (NGI), Oslo in the early 1980s (Bakkehi et al. 1985). This application was an outgrowth of previous work at NGI involving the design and manufacture of instruments employing the vibrating-wire technique to measure strain and loads in concrete in bridges, earth pressures in dams, soil porewater pressure in boreholes under embankments and other geomechanical examples (Tunbridge and ien 1988). The vibrating-wire precipitation gauge that has been available for many years is called the NGI Geonor T-200B and is the gauge investigated in this Chapter. Figure 1 is an example of a T-200B with and without its case removed. The Geonor T-200B gauge is in the class of a weighing-recording gauge and is widely used at more than 1400 locations world-wide of which over 500 are in Canada and the United States (Brylawski 2007, personal communication). The purpose of this Chapter is to provide a summary of various field investigations involving the Geonor gauge that have been performed since 2000, mainly at Norman, Oklahoma, USA. The summary begins with a review of the essentials of gauge operation. This is followed by six Sections dealing with, in order, a description of the measurement site and data acquisition, advantages of using three versus two or one vibrating wires, calibration-verification of the wires, sensitivity of vibrating wire frequency to temperature, comparison of rain rates from the Geonor to rain rates from a disdrometer and observations of very low precipitation events. The last Section highlights the findings. This Chapter deals only with liquid precipitation, the primary reason being that frozen precipitation typically occurs only a few times annually in central Oklahoma and usually in the form of sleet or freezing rain. In addition, the gauge is unheated and, as will be seen later, its orifice is at ground level.