The collapse of turbulence in the evening

B.J.H. van de Wiel, A.F. Moene, H.J.J. Jonker, P. Baas, S. Basu, J. Sun, A.A.M. Holtslag

Research output: Chapter in Book/Report/Conference proceedingConference paperAcademic

Abstract

A common experience in everyday weather is the fact that near-surface wind speeds tend to weaken in the evening, particularly in fair weather conditions. This cessation of wind usually coincides with the collapse of turbulence which leads to a quiet flow near the ground. As the absence of turbulent mixing leads to cold extremes, its prediction is vital for reliable winter temperature forecasts. It is, for example, well-known that unexpected frost events can completely deregulate winter traffic and enhances the risk on calamities (Fig. 1). Yet, the physical explanation behind this phenomenon remained unknown so far. Here, we present a mechanism to explain this intriguing phenomenon by combining extensive observational analysis with new theoretical insights. We detected a remarkable constraint in the kinetic energy of the flow in response to nocturnal cooling. In turn, this results in a limit on the maximum heat that can be transported towards the earth’s surface. When the actual heat loss at the surface exceeds this maximum turbulence cannot survive the intense density stratification. By using this insight, a simple predictive tool is developed. This modeling tool shows to be successful in predicting the cessation of turbulence as observed in a unique data set from a 200m meteorological tower covering 10-years of observations.
Original languageEnglish
Title of host publicationProceedings of the Workshop on Diurnal cycles and the stable boundary layer, Reading, UK, 7-10 November 2011
Place of PublicationReading, UK
PublisherEuropean Centre for Medium-Range Weather Forecasts
Pages197-204
Publication statusPublished - 2012
EventWorkshop on Diurnal cycles and the stable boundary layer, Reading, UK -
Duration: 7 Nov 201110 Nov 2011

Workshop

WorkshopWorkshop on Diurnal cycles and the stable boundary layer, Reading, UK
Period7/11/1110/11/11

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