Observations of the site dependency of the morning wind and the role of gravity waves in the transitions

Values of 10 m wind speed have been plotted against screen temperature during morning warming and averaged over several years for a number of sites within Great Britain. When the temperature is referenced to that at the time of the coming evening transition the plots give consistent lines with a gradient dependent on geostrophic wind. However the gradient for a given geostrophic wind varies between sites. Comparison with similar plots made during evening cooling shows that the wind plot against temperature increases relatively abruptly at some stage during the early hours of the morning and then increases to its midday maximum with a reduced wind-temperature gradient to that in the evening. However, as with the evening plots, the gradients of the morning plots increase in hillier terrain. The effect of wind direction on the gradients during morning and evening is also described in order to verify that the effect of topography on the plots is consistent. A one-dimensional numerical model is used to simulate both transitions. It is shown that external drag factors such as form drag in the region of low hills are not sufficient to explain the observations. Only an external drag of the type given by a simple formulation of gravity wave generation and absorption within the stable boundary layer gives good agreement with the observations. This suggests that an important factor causing the wind to drop during the evening cooling is gravity wave drag and its relative absence, due to the near-surface convective layer, is a factor during the morning wind increase.