Factors determining the decrease in surface wind speed following the evening transition

Meteorological measurements were made by surface instrumentation at a flat rural site over a period of several years. These are analysed to determine any systematic factors governing the decrease in surface wind speed during the evening. The 10 m wind speed is found to depend on the surface cooling and gradient wind speed, with a larger decrease in the 10 m wind speed with surface cooling when the gradient wind is stronger. The relationships are relatively unaffected by cooling rate. Momentum and heat fluxes are also found to have well defined relationships with surface cooling and gradient wind, with downward heat fluxes reaching a maximum at a 2 degC cooling. These observations are compared with the predictions of a simple semi-analytical model which gives qualitatively similar results to the observations and shows that the greater 10 m wind speed decrease at higher gradient wind speeds is due to a thicker boundary layer. The model also predicts the formation of an upper inversion with an increase in 10 m wind later in the evening and this prediction is compared with balloon and surface observations made on a clear evening. A simple forecasting rule for the reduction in 10 m wind during the evening is given.