Ground Level and Aboveground Observations of Record Rainfall in May 2015 in Oklahoma

The month of May 2015 in Oklahoma was remarkable in terms of the number of high intensity rain events. The statewide average was 366.52 mm (14.43 in) compared to the previous monthly record for any month of 273.05 mm (10.75 in) observed in October 1941. The focus of this work is a comparison of rainfall measurements at ground level with traditional aboveground measurements. Our first comparison comprises results from three gauges at the Norman Mesonet site: an aboveground tipping-bucket gauge, an identical tipping-bucket gauge in a pit, and a weighing-bucket gauge in the same pit. The gauges in the pit have their orifices at ground level. These three independent measurements provide different estimates of rainfall due to the effects of wind speed and rain rate. The aboveground Mesonet tipping-bucket gauge, which has an Alter shield, is, nevertheless, exposed to the wind. In addition, an increase in rain rate results in an increase in undercatch from tipping-bucket gauges. Measurements from the tipping-bucket gauge in the pit are impacted by rain rate similar to the aboveground gauge, but are minimally impacted by wind speed. Measurements from the weighing-bucket gauge are not impacted by rain rate and minimally impacted by wind speed and thus provide the best estimate of the actual rainfall. For the month of May 2015 the respective totals from the aboveground tipping-bucket gauge, the tipping-bucket gauge in the pit, and the weighing-bucket gauge were 592.33 mm (23.32 in), 617.72 mm (24.32 in), and 646.85 mm (25.47 in). The difference between the highest and lowest estimates is 8.4%. Individual rain events yielded differences ranging from 0% to greater than 10%. We will show how high wind speed and high rain rates result in undercatch of the two tipping-bucket gauges in our presentation. The second comparison is between a tipping-bucket gauge and a collocated NWS cooperative standard rain gauge at the National Weather Center and the third comparison is among numerous CoCoRaHS gauges distributed throughout the Norman area. The results from these two comparisons will be briefly discussed.

One conclusion from our analyses is that the official record of rainfall for May, which is based on aboveground gauges, is a significant underestimate of the actual rainfall. Another conclusion is that flash flood models applied during high-rain-rate-high-wind-speed events will underestimate flooding potential if rainfall amounts from aboveground gauges are used. The third conclusion is that there is little prospect for correcting aboveground measurements for undercatch due to wind speed and rain rate.