Nocturnal boundary layer cooling rates in valleys, basins, and over plains

For practical applications such as the prediction of minimum temperatures at a certain location and height, an understanding of the cooling rates in the nocturnal boundary layer is of crucial importance. Our knowledge and understanding of how topography affects these cooling rates is limited. Therefore, we used sequences of vertical temperature soundings over flat plains, and in a variety of valleys and basins of different size and shape to determine atmospheric cooling rates under clear, undisturbed weather conditions. A simple fit to the observed cooling rate curves allows us to calculate a cooling lag coefficient that can be considered characteristic of the valley, basin or plain topography. For most topography types, the lag coefficient is in the range from 3-4 hours. Some other characteristics of the cooling curves and the implications of our findings will be discussed in terms of physical understanding.