Abstract
This study improves surface wind predictions in an unresolved valley using an artificial neural network (ANN). Forecasting winds in complex terrain with a mesoscale model is challenging. This study assesses the quality of 3-km wind forecasts by the Weather Research and Forecasting (WRF) Model and the potential of postprocessing by an ANN within the 1–2-km-wide Cadarache valley in southeast France. Operational wind forecasts for 110 m above ground level and the near-surface vertical potential temperature gradient with a lead time of 24–48 h were used as ANN input. Observed horizontal wind components at 10 m within the valley were used as targets during ANN training. We use the directional accuracy (DACC45, wind direction error ≤ 45°) and mean absolute error to evaluate the WRF direct model output and the ANN results. By postprocessing, the score for DACC45 improves from 56% in the WRF direct model output to 79% after applying the ANN. Furthermore, the ANN performed well during the day and night, but poorly during the morning and afternoon transitions. The ANN improves the DACC45 at 10 m even for poor WRF forecasts (direction bias ≥ 45°) from 42% to 72%. A shorter lead time and finer grid spacing (1 km) showed negligible impact, which suggests that a 3-km grid spacing and a 24–48-h lead time is effective and relatively cheap to apply. We find that WRF performs well in near-neutral conditions and poorly in other atmospheric stability conditions. The ANN posttreatment consistently improves the wind forecast for all stability classes to a DACC45 of about 80%. The study demonstrates the ability to improve Cadarache valley wind forecasts using an ANN as postprocessing for WRF daily forecasts.
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