Rain-time operation, not rainfall alone, sets wind turbine blade rain-erosion exposure: synchronized disdrometer and SCADA measurements

Abstract. Rain erosion of wind-turbine blade leading edges accumulates while the blades turn in rain, yet exposure assessments prescribe a fixed rotor speed and rarely test how fast the turbines actually turn while it rains. We coupled drop-resolved disdrometer rainfall to the operating records of two whole wind farms over their common observation period: one wetter (Guangxi, 513 mm of reconstructed rainfall) and one drier (Jiangsu, 242 mm). Rain-time operation does not merely refine the exposure estimate; it sets its size. The observed tip speed replaces the fixed 90 m s⁻¹ reference, the rounded value of the fleet's observed operating-speed ceiling. Coupling it in lowers the operation-coupled observed-period index (E_SCADA) to 18 % of its fixed-speed value in Guangxi and 47 % in Jiangsu, so that reference overstates exposure by factors of 2.1–5.6. The microphysical input matters far less: replacing the parameterized fall-velocity relation with the measured spectra moves the same index by only 28–38 %. The operating state can therefore change which site appears worse. Under the common reference speed the wetter farm is roughly twice as exposed (E₉₀ 0.140 versus 0.066); with operation coupled in, the two point estimates change order (0.0251 versus 0.0311), although their bootstrap intervals overlap and the ranking is not statistically separated. Exposure is concentrated in time: the 10 % most exposed rain intervals carry 75–80 % of the total and the five most exposed rain events 78–90 %. Capping the rain-time tip speed at 80 m s⁻¹ removes 42–58 % of the exposure while touching at most 48 % of rain time, within the model framework and before power-loss or control constraints. Both indices are normalized to a literature reference coating and are screening quantities rather than lifetime predictions, and the threshold relation is extrapolated below its 90–150 m s⁻¹ calibration range. They supply the operational loading term that previous assessments had to assume.