Nocturnal air and road surface temperature variations in complex terrain

Abstract

AbstractThis paper is part of an ongoing project dealing with the modelling of local climate for predicting temperature variations and risks of road slipperiness under various synoptic conditions. Temperature recordings from mobile measurements taken along road stretches have been analysed to determine the influence of valleys on the variation in air and road surface temperatures. During clear, calm nights, the variation in air temperature between valley bottoms and surrounding areas can be related to geometric properties of valleys, such as depth and width. It is seen that temperature differences between valleys and surrounding areas increase with increasing depth and width. The prevailing wind speed has been found to be a major factor controlling the variation in temperature. The variation in air temperature is totally smoothed out in situations in which there is an ambient wind speed exceeding 3 m s−1. In the interval 2–3 m s−1, the degree of wind exposure in valleys is of importance for the magnitude of the air temperature differences. Counter‐radiation from clouds has been found to significantly reduce the intensity of cold air pools. Under partly cloudy situations (3–5 octas) with no wind, the temperature difference between valley bottoms and surrounding areas differs by approximately 6°C in comparison with clear, calm situations. The relationship between differences in air and road surface temperature has also been studied. In general, an increase in cold air pool intensity of 1°C lowers the road surface temperature by 0·4°C.