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<br /> <br />"~:lr;!.~\o~'i.,..o:;,:.;;.~~,,-.~~....-UV;'..~~~f~llo-l"'''''"':>'''''''' ~. ,._,.,,'..c~,,__~"" " <br /> <br />INTERNATIONAL JOURNAL OF CLIMATOLOGY. VOL. 14.681-689 (1994) <br /> <br />551.508. 77:551,501.9:53.088 <br /> <br />CLASSIFICATION SYSTEM OF PRECIPITATION GAUGE SITE <br />EXPOSURE: EVALUATION AND APPLICATION <br /> <br />B. SEVRUK <br />Swiss Federal Institute oj Technology ETH-Zentrum, CH-8092, Zurich, Switzerland <br /> <br />AND <br /> <br />L. ZAHLAVOVA <br />Slovak Technical University, Faculty oj Civil Engineering, Bratislava, Czechoslovakia <br /> <br />Received 12 September 1992 <br />Accepted 23 October 1993 <br /> <br />ABSTRACT <br /> <br />The precipitation gauge site exposure is assessed from the station history records. Four classes and three interim classes <br />ranging from open to protected gauge sites are used. They are characterized by the average vertical angle of obstacles <br />around the gauge, as measured in eight directions of the wind rose. The accuracy of class estimates was checked in the <br />field, and the error for an experienced person was found to be approximately 1: 0,5 of a class. Applications of the <br />classification system for estimation of wind-induced error of precipitation measurement and for detection of in- <br />homogeneities of precipitation time-series are discussed. <br /> <br />KEY WORDS Precipitation measurement Wind effect Exposure Gauge site <br /> <br />INTRODUCTION <br /> <br />The problem with preCIpItation measurement techniques is that despite the simplicity of a common <br />precipitation gauge, the actual process of precipitation measurement is hampered by a number of systematic <br />errors. The most important error stems from the fact that an elevated precipitation gauge distorts the wind <br />field above the gauge orifice. Due to adverse wind action, some of the lighter precipitation particles are <br />borne away before reaching the precipitation gauge and are lost for the measurement. This wind-induced <br />error causes considerable losses of precipitation catch, generally amounting to 2-10 per cent for rain and <br />up to 50 per cent for snow, and even more in the mountains. <br />The published precipitation data do not include the losses. Therefore, the results of many hydrological, <br />meteorological, and climatological computations are to be accepted with caution. Better results can be <br />expected when precipitation data are corrected for the wind-induced error and for other systematic errors, <br />due to wetting, evaporation, splash, etc. <br />The complex problem of correction of precipitation data has been dealt with by two international <br />workshops (Sevruk, 1986a, 1989), and a lot of recent information can be found in the Proceedings. Sevruk <br />(1992) provides a detailed account of wind-induced error, and cites more recent literature. This paper <br />addresses one practical aspect of assessment of wind-induced error of precipitation measurement at a <br />particular gauge site, namely the assessment of the degree of gauge site exposure (GSE). The GSE can <br />fundamentally affect the magnitude of wind-induced error and its correction. <br /> <br />BACKGROUND <br /> <br />The wind-induced error in precipitation measurement depends on three groups of variables: instrumental, <br />observational, and environmental. Of the instrumental variables, the following seem to be most important: <br />the use of a wind shield, the shape of the precipitation gauge body, and, particularly, the shape and thickness <br /> <br />CCC 0899-8418/94/060681-09 <br />@ 1994 by the Royal Meteorological Society <br />