Estimation of Wind-Induced Error of Rainfall Gauge Measurements Using a Numerical Simulation

Abstract

A new method of estimating the wind-induced error of rainfall gauge measurements is presented. The method is based on a three-dimensional numerical simulation of the airflow around a precipitation gauge and subsequent computation of particle trajectories. Three-dimensional velocity and turbulence flow fields around a gauge are computed for wind speeds ranging between 1 and 12 m s?1 by employing the k?ε turbulence model. Two-dimensional measurements of the flow using a constant temperature anemometer are carried out in a wind tunnel. The measurement results are used to verify numerical flow simulations. Subsequently, the computed flow fields are used for raindrop trajectory simulations and assessment of wind-induced measurement errors related to a given unique drop diameter and wind speed. These errors are approximated by a gamma-type function and integrated over a gamma drop size distribution. The resulting wind-induced error is presented as a function of the rate of rainfall, wind speed, and drop size distribution parameters. The wind-induced measurement error is evaluated for three operational precipitation gauges. The results show an increase of the error with a decreasing rainfall rate, and increasing wind speed and fraction of smaller drops. The comparison of gauges also reveals differences. The computed wind-induced errors are compared with the errors derived from field rainfall measurements. The compared values show a relatively good agreement.