Rainfall Kinetic Energy in Denmark: Relationship with Drop Size, Wind Speed, and Rain Rate

Rainfall kinetic energy is an important parameter to estimate erosion potential in connection to soil erosion or in the recent years to the erosion of the leading edges of wind turbine blades. Little is known about the seasonal drop size distribution and fall velocity dependence of rainfall kinetic energy as well as its relationship with wind speed. Therefore, 6 years of Thies Laser Precipitation Monitor disdrometer and wind measurements from Voulund, a field site in western Denmark, were analyzed. It was found that the rainfall kinetic energy was highest in summer due to higher drop concentrations and in autumn due to more time with rain. The rainfall kinetic energy peaked for drop diameters between 0.875 and 2.25 mm independent of the season. Rainfall kinetic energy decreased significantly with increasing wind speed, if considering the vertical fall speed of the drops for the calculation of the rainfall kinetic energy. However, it should be noted that the measurement uncertainty increases with increasing wind speed. As disdrometer observations are rarer than rain rate observations, the performance of empirical equations describing the relationship between rainfall kinetic energy rate and rain rate was investigated. It was found that an equation trained with an alternative method fulfilled the statistical requirements for linear regression and had a similar error compared to equations in the literature. Based on the analyses, it can be concluded that the erosion potential due to rainfall kinetic energy is highest between June and November at low wind
speeds and high rain rates.