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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

In this paper the authors evaluate an inexpensive and automatable method to estimate cloud type at a given location during daylight hours using the time series of irradiance from a pyranometer. The motivation for this investigation is to provide ground-based estimates of cloud type at locations where there are no human observations of sky condition. A pyranometer naturally measures the effect of intervening clouds along the solar beam path to the sensor. Because a daily time series of irradiance is nonstationary, it is appropriately scaled to yield a stationary time series. From the latter, the standard deviation and ratio of observed irradiance to clear-sky irradiance derived from a 21-min moving window are related to one of the following cloud types or conditions: cirrus, cumulus, cirrus and cumulus, stratus, precipitation or fog, no clouds, and other clouds. Comparisons with human observations at the Department of Energy Atmospheric Radiation Measurement Calibration and Radiation Testbed site in northern Oklahoma show that the pyranometer method and human observations are in agreement about 45% of the time. Many of the differences can be attributed to two factors: 1) the pyranometer method is weighted toward clouds crossing the sun's path, while the human observer can view clouds over the entire sky, and 2) the presence of aerosols causes the pyranometer to overestimate the occurrence of cirrus and cirrus plus cumulus. When attenuation of the solar beam by aerosols is negligible or can be accounted for, the pyranometer method should be especially useful for cloud-type assessment where no other sky observations are available.

Abstract. We have investigated differences in rainfall accumulations for seven high rain rate events from three gauges: a Geonor T-200B vibrating-wire weighing gauge and two MetOne tipping-bucket gauges. The Geonor gauge and one tipping-bucket gauge are located in a pit so that their collection orifices are at ground level. Thus their measured rainfall accumulations are minimally affected by wind speed. The other tipping-bucket gauge is located 105 m from the pit and is surrounded by an Alter-type slatted wind screen. Its collection orifice is positioned 1 m above ground level. The results from the seven events show that the tipping-bucket gauges noticeably underestimated storm event rainfall totals relative to the weighing-bucket gauge when 1-min rain rates exceeded about 50 mm/h (2 in/h). In addition, we conclude that observable wind induced undercatch by the aboveground tipping bucket gauge begins when the wind speed at a height of 2 m exceeds around 5 m/s. In this paper we show and discuss time series of rain rates, differences in rain rates, and wind speeds for two of the seven events in an attempt to account for the lower storm totals from the two tipping bucket gauges relative to the weighing-bucket gauge.