Temperature lapse rate in complex mountain terrain on the southern slope of the central Himalayas

This study presents the first results of monthly, seasonal and annual characteristics of temperature lapse rate on the southern slope of the central Himalayas, based on 20 years record of surface air temperature at 56 stations in Nepal. These stations are located at a range of elevations between 72 and 3,920 m above sea level. It is proven that the lapse rate can be calculated with a linear regression model. The annual cycle of temperature lapse rate exhibits a bi-modal pattern: two maxima in the pre- and post-monsoon seasons respectively separated by two minima in winter and summer, respectively. This pattern is different from the findings from the other mountain regions and suggests different controlling factors in the individual seasons. The highest temperature lapse rate occurs in the pre-monsoon and is associated with strong dry convection (i.e., corresponding to the clear weather season and considerable sensible heat flux). The post-monsoon has the second highest lapse rate, and its cause is similar to the pre-monsoon season but with a relatively small thermal forcing effect after the rainy summer. The lowest lapse rate occurs in winter and is associated with strong radiative cooling and cold air flows over low-elevation areas. The summer lapse rate minimum is due to latent heating over the higher elevations and reduced solar heating over the lower elevations.