Heat Budget Studies on Nocturnal Cooling in a Basin

Abstract

Heat budget observations were carried out at Akaigawa, Hokkaido, to investigate the mechanism of extreme nocturnal cooling in a basin. The results of vertically integrated heat budget studies of the NBL (nocturnal boundary layer) show that when the general current above the NBL is weak, about 80% of the observed cooling of the whole NBL is attributed to horizontal advective cooling which is due to downslope cold air from surrounding mountains. Cooling due to sensible heat flux divergence accounts for less than 10% of the observed cooling and can be dominant only in the lowest layer. However, when the wind speed of the general current exceeds a critical value (about 10m/s at a height of 1000m), advective cooling decreases and turbulent cooling begins to dominate the observed cooling. Cooling due to atmospheric radiative heat flux divergence remains unchanged during the night and varies little from night to night. It accounts for about 10% of the observed cooling under weak wind conditions and its relative importance increases under strong wind conditions when the observed cooling is reduced.