Development of a Nocturnal Temperature Inversion in a Small Basin Associated with Leaf Area Ratio Changes on the Mountain Slopes in Central Japan

Abstract

Nocturnal temperature inversion (NTI) is an important factor characterizing the local climate in mountainous areas. In central Japan, most of the mountain slopes are covered by forests, but the effects of their leaf expansion/fall on the NTI variations in basins have not been clarified. According to a three-year leaf area index (LAI) observation in the mixed forest of the Sugadaira Highland (1320 m a.s.l.), Nagano Prefecture, Japan, we identified weakening of the NTI associated with leaf expansion and strengthening after leaf fall in a small basin. Using digital elevation and land-cover data, we defined the distribution of the deciduous and mixed forests in the catchment area of nocturnal cold air drainage. The estimated timings of leaf expansion/fall at the catchment scale based on the effective cumulative temperature almost coincided with the NTI changes. Micrometeorology observations showed that NTI at the forest floor and downslope winds at the adjacent grassland strengthened during the dormant (leafless) season in the nighttime when the radiative cooling is strong. Calm and clear nights were chosen during the spring dormant season and the summer growing season for 22 and 30 nights, respectively. The heat loss during the cold-air pool development was estimated, and converted to storage heat flux in the forest areas. The storage heat flux was 3.8 W m−2 more on average in the growing season than the dormant season, and it was less than that of forests estimated in previous studies (several 10 W m−2), indicating that an increase in storage heat flux of the forests with leaf expansion could cancel nocturnal radiative cooling and weaken gravity currents at the forest floor.