The Nocturnal Surface Inversion and Influence of Clear-Air Radiative Cooling

Abstract

Nocturnal boundary-layer data from the Wangara and Voves experiments are analyzed. The lower part of the nocturnal inversion layer normally appears to be turbulent but strongly stratified. The upper thicker part of the inversion layer is characterized by weaker stratification which appears to be almost completely generated by clear-air radiative cooling. The radiative cooling is associated primarily with moisture, but is only modestly sensitive to the particular moisture distribution and thus varies little from night to night for the data analyzed here. The radiatively cooled layer thickens significantly as the night proceeds. On the other hand, the thickness of the turbulent layer normally varies slowly during the night, but differs significantly from night to night. For the inversion layer as a whole, turbulent heat flux divergence and clear-air radiative cooling make comparable contributions to the heat budget. The stratification normally decreases with height corresponding to negative curvature of the potential temperature profile. However, with increasing relative importance of the turbulence, this negative curvature weakens and may even become positive as with nearly well-mixed flow.