The role of microphysical processes on the mesoscale simulation over the complex terrain , the Himalayas

Abstract

The objective of this study is to evaluate the impact of the four different cloud microphysical schemes (WSM3, WSM6, Morrison double moment and Lin scheme) within the Weather Research and Forecasting model (WRF), as part of simulations of mesoscale weather systems across complex terrain in the Nepalese Himalayas. The Himalayas is characterized by a complex and rugged topography, with altitudes varying e.g. 70m in Southeastern Nepal, to the highest peak of the world, 8850m (Mt. Everest), and which extends from West to East covering many South and Central Asian countries: Afghanistan, Pakistan, China, India, Nepal, Bhutan, and Myanmar. Circulation in such a complex environment is complicated due to obstruction of flows by mountain ranges which in turn have wide ranging effects on cloud and rain formation and distribution. Monsoon rain is intrinsically linked to people’s daily life across the South Asia since more than 80% people depend on agriculture and majority of the agricultural systems are rainfall dependent. Modeling of the key microphysical process in this complex terrain provides insight into the general understanding of the processes and their spatial patterns, however there are many uncertainties in general. These uncertainties are even more pronounced when such models are applied to the complex terrain characteristic of the Himalayas. Numerical experiments are designed using the WRF model, with three nested domains (27, 9 and 3 km grid spacing). The performance of the four categories of microphysical schemes is examined in model experiments for (i) monsoon onset, (ii) monsoon decay and (iii) winter rainfall. The simulated results are compared with limited observed meteorological parameters such as rainfall, temperature, wind speed and wind direction, from ground-based meteorological stations situated within the high resolution (3km x 3km) domain. Results show that a) Simulated rainfall is very sensitive to the chosen microphysical scheme with rainfall spatial and temporal characteristics being very different for each scheme. However, the majority of the WRF simulations showed similar general patterns with monsoon onset and subsequent maturation across the Southeast region of Nepal which then gradually moves Northwest over time before dissipating; b) It was found that strong moist convection caused by near surface convergence of wind is responsible for producing significant nocturnal maximum precipitation during the monsoon period. All the WRF simulations revealed that the continuous southerly moist monsoonal flow interacting with the South slope of the Himalayas and associated diurnal variation of ambient atmospheric state is the major cause of the nocturnal maximum rain generally across the region; c) The WSM6 microphysical scheme performed relatively better than the other schemes.