Analysis of a 14-Year Record of Aboveground and Ground Level 1-Minute Rainfall Observations

The goal of this presentation is to show examples of and account for differences between rainfall measurements from aboveground and ground level gauges. The conclusion is there is systematic undercatch by aboveground gauges relative to ground level gauges in the presence of high wind speeds and high rain rates. Analysis of a 14-year record of 1-minute rainfall accumulations that have been and continue to be collected at an observing facility employing three rain gauges supports this conclusion. The facility is located at Norman, Oklahoma and includes an aboveground tipping-bucket gauge and an identical tipping-bucket gauge and a weighing-bucket gauge in a pit such that their orifices are at ground level. Various other meteorological variables are also observed at this facility.

Of particular interest is the response of each gauge to high wind speeds and high rain rates. The aboveground gauge is susceptible to undercatch due to both wind speeds greater than about 5 m/s at a height of 2 m and rain rates greater than about 50 mm/h. Undercatch by the tipping-bucket gauge in the pit is due mainly to high rain rates and the weighing-bucket gauge experiences minimal effect of both high wind speeds and high rain rates. Time series of comparisons of accumulations among the gauges during rain events will be shown that demonstrate the systematic occurrence of undercatch. One consequence of undercatch by aboveground gauges is that flash flood models that incorporate surface observations may underestimate flash flood potential in high-rain-rate-high-wind-speed thunderstorms unless such conditions are already included in their calibration.