Localising and quantifying night-time cooling effects from sub-catchments in a mid-European low mountain area

We propose a novel method to analyse and quantify cooling effects provided to a settlement by source areas of nocturnal cold-air drainage. In an interdisciplinary approach, these source areas were defined as hydrological sub-catchments of a complex catchment area in a low mountain range. The cold-air drainage model KLAM_21 was used to exclude the energetic influence of the sub-catchments from the model area by surrounding them with artificial barriers. The outputs of these runs were then compared to a reference run without exclusion to derive the cooling effect of each source area. The results were evaluated at sample points along the main valley and for residential areas of a medium sized city and two smaller settlements. We find that in the complex terrain of the study area, also comparatively remote source areas can have a noticeable cooling effect on the residential neighbourhoods of the target settlements from the middle of the night. The strongest effect however, could be attributed to the sub-catchments in direct vicinity of the target areas. The results at the sample points along the main valley showed that the cooling effect decreased with increasing distance to the sub-catchments and usually gets stronger during the night. The variation in strength of cooling effect between different sub-catchments is likely due to their individual properties such as remoteness, size, terrain, land-cover situation and cold-air exchange with other sub-catchments through overflow effects.