The Influence of Seasonally Varying Atmospheric Characteristics on the Intensity of Nocturnal Cooling in a High Mountain Hollow

Abstract

Seasonal differences in the longwave radiation balance, diurnal cycles of humidity and cloudiness, and ambient winds during the warm season were evaluated to determine their effect on the nocturnal cooling processes of a high mountain hollow in central Japan. This hollow is located at an elevation of 2230 m at the bottom, having a depth of 50?200 m with a diameter of approximately 1 km. One of the most marked seasonal changes in the atmosphere is a reduction in the downward longwave radiation from summer to autumn due to reduced water vapor. This reduction is larger than that of the upward longwave radiation due to a reduced surface temperature. This radiation balance resulted in larger (smaller) potential intensity of radiative cooling during autumn (summer). A composite analysis of diurnal variation for days in summer and autumn showed that the humidity and cloud conditions, differing between the seasons, determined the initial timing and development of nocturnal cooling. During summer (July and August), increased downward longwave radiation in the evening caused by a combination of increased water vapor and cloud cover over the mountains, suppresses and delays cooling. In contrast, dry and fair weather conditions throughout early autumn (September and October) causes strong and continuous cooling. Despite the high potential for radiative cooling during late autumn (after mid-October), nocturnal cooling was frequently disturbed by strong ambient winds exceeding 3.6 m s?1. The weak winds during summer and early autumn have a minor effect in determining the actual cooling intensity.