Forecasting of the Evening Transition in Light Winds

Forecasting of the evening transition in light winds raises interesting questions about boundary layer processes in rapidly changing conditions. Under clear skies surface cooling rates may reach 6 K per hour. In light winds, such as may occur in anticyclonic conditions in winter, the decrease in the near-surface air temperature (at 2m) can be significantly slower than that in the surface skin temperature. By contrast, numerical weather forecasting models show a more rapid decrease in the air temperature, suggesting that the surface and the air temperatures are too tightly coupled in such models. Examples of this phenomenon will be presented.

A number of different processes, including atmospheric radiation in very light winds, are relevant to such transitional conditions. Idealized models of the evening transition are used to explore some of the issues involved and to suggest appropriate time and length scales for such transitions, where high stability develops rapidly in the surface layer. Large-eddy modelling of these transitional boundary layers is, in principle, a useful way to investigate the role of turbulent processes in particular, but there are formidable computational challenges, which will be discussed.