Comparative rainfall observations from pit and aboveground rain gauges with and without wind shields

Abstract

Abstract. A study of rain gauge observations was initiated in 1995 to assess the magnitude of undercatch due to wind and the effect of reducing undercatch using an Alter-type wind shield. The observation site was near the Norman, Oklahoma, airport with good exposure in all directions. The experimental setup comprised three tipping-bucket rain gauges, three weighing-bucket rain gauges, and anemometers at 1 and 2 m. For each type of gauge, one was placed in a pit with its orifice at ground level, and the other two were placed above ground, one with an Alter shield and one without. One-minute rainfall accumulations and wind speed averages were obtained from 101 rainfall events over 30 months. The results show that for typical rainfall events, the undercatch of the unshielded tipping-bucket rain gauges was 4% relative to the tipping-bucket pit gauge. The comparable figure for the unshielded weighing-bucket gauge was 5%. Both Alter-shielded gauges showed less than a 1% reduction in undercatch relative to the unshielded gauges for typical rainfall events. We discuss the sources of errors that can be expected in similar rain gauge measurements when the gauges are properly maintained. IntroductionRain gauges provide point estimates of precipitation for hydrologic, meteorological, and agricultural interests and are often used as "ground truth" for radar-estimated rainfall. Because of the lack of studies of rainfall undercatch in the southern Great Plains, a 30-month-long experiment was initiated in 1995 to compare rainfall from aboveground gauges with and without wind shields to rainfall from colocated "pit" gauges that have their collector openings, or orifices, at ground level. The goals of the experiment were to determine the magnitude of undercatch, assess the effectiveness of the commonly used Alter-style wind shield [Alter, 1937] The focus of this paper is entirely experimental. The rainfall data that were collected integrate the time-dependent effects of rain rate, wind speed, and drop-size distribution over the course of a rainfall event. The results of the data analysis were used to achieve our first two goals, which were to assess the magnitude of undercatch due to the wind and determine the effectiveness of an Alter-type wind shield to reduce undercatch for rainfall events typical of the southern Great Plains. Discussion of our third goal, which was to derive a relation between undercatch and wind speed, is given in section 6. Because we are dealing with field data, we place special emphasis on the limits of accuracy in rainfall measurements and attempt to explain the causes of these limits. 3253

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