Evaluations of Temperature Lapse Rate in and Around the East-West Himalayan Chain: An Implication for Climate Understanding and Modeling

This study evaluated the monthly characteristics of TLRs based on the results derived from 18 observation stations in Bhutan (1996–2009), 56 stations in Nepal (1985–2004), and 53 observation stations in Pakistan (1971–2000). The study covers an elevation range of 5–3920 m above sea level, with various topography, climatic regimes, and geographical coordinates. Various empirical analysis techniques, including thermodynamics and hydrostatic systems, have been used to obtain the results. The annual cycle of TLR, such as a bi-modal pattern (two minima in the winter and summer and two maxima in the pre- and post-monsoon seasons), is a typical pattern throughout the study area. However, the forcing strengths, mechanisms, and processes for the monthly variations in TLR magnitude and diurnal range differed. Monsoon, orographic controls, and mountain barrier effects on TLR are more robust in summer, especially during the day, while the influences of inversions and mountainous microclimates are higher during the non-monsoon period, particularly in the winter and at night. A shallower TLR occurs in summer throughout the study area because of the intense heat exchange process within the boundary layer, corresponding to warm and moist monsoonal atmospheric conditions. Steeper values of TLR in the non-monsoon period, especially in the pre- and post-monsoon seasons, result from strong dry convective cooling at higher elevations owing to high thermal forcing effects at lower elevations. The winter shallower TLR is associated with westerly-driven cold air flow towards the lower elevations and radiative cooling, especially at night, excluding Bhutan. There are systematic differences in TLRs monthly and diurnal variations in magnitudes, such as the lowest gradient value in Pakistan observed in August and the highest in May, one month later than observed in Nepal and Bhutan, due to the late arrival of monsoon moisture in summer and intense thermal forcing effects following the wet early pre-monsoon months. In addition to the variation in magnitudes, the variation in the diurnal range from one place to another is also due to distinct differences in topoclimate, in addition to the effects of the synoptic regime, moisture amounts, elevations, geographical coordinates, variations in net radiation, surface roughness, vegetation coverage, and distance from the sea coast. The results of this study are useful for determining the local or regional climatic behavior or interactions, climate reconstruction, and temperature field in various glacial-hydro-climatic, ecological, and agricultural modeling.