Computation of Rain Rates in Areas of Wind Farms

Deployment of wind turbines to meet requirements for clean energy presents challenges for weather radar users. Accurately estimating rain rate in the area of wind farm has become an urgent issue. An early study of Ryzhkov and Zrnic (1998) reveals that in areas where Z (reflectivity) is contaminated by clutter due to anomalous propagation the rain rate R can be obtained using specific differential phase KDP. This is achieved by taking the measured ΦDP (differential phase) between the last valid differential phase datum at range just ahead of the contaminated range interval and the first datum beyond the affected range interval. Herein a method is proposed to detect the range locations (gates) within the contaminated radial interval where the turbine clutter is not present. The method uses the correlation coefficient between the horizontally and vertically polarized returns and the local standard deviation of ΦDP. By scrutinizing the U.S. wind turbine database and our detection results, we found that there is commonly a sufficient space between turbines where radar returns are not contaminated. To recover the uncontaminated data within the wind farm area, a two-dimensional running median filter with the size smaller than the size of a wind farm has been designed and applied to a specific area. It utilizes the reflectivity Z from uncontaminated range locations to obtain a local median Z at each gate within the specified area. A rebuilt Z field is achieved by replacing the wind turbine contaminated Z with median Z and keeping uncontaminated Z. Then the relation R(A) between R and specific attenuation A is applied to estimate the rainfall amount in the area of a wind farm which requires the knowledge of a total span of differential phase over the wind farm area. The results are compared with rain gauge measurements and indicate that the performance of R(A) QPE with rebuilt Z is satisfactory.

Reference
Ryzhkov, A., and D. Zrnic, 1998: Polarimetric rainfall estimation in the presence of anomalous propagation. J. Atmos. Oceanic Technol., 15, 1321-1331.