Investigations of vertical wind variations at a mountain top in the Himalaya using Doppler Lidar observations and model simulations

Abstract

Hight-resolved observations of vertical winds remain nearly non-existing over the Himalayas, despite of anticipated crucial role of vertical motions in transporting pollution across the Himalayan hills. The present study analyze the vertical wind observations from surface to 1 km above ground level over Manora Peak (29.4 degrees N; 79.5 degrees E; 1958 m amsl) in the Himalaya performed using a Doppler Lidar during the Ganges Valley Aerosol Experiment (GVAX). Vertical wind exhibited a pronounced diurnal variability at Manora Peak comprising of upward motions during the daytime (05-10 UT) and downward motions during nighttime typical of a mountain valley system. Mean vertical wind speeds are observed to be varying from -0.8 to + 0.8 ms(-1) during the study period with a variance of 0.1-1.5 m(2)s(-2), which is attributed to the thermally driven turbulence. Mean vertical winds are observed to be stronger in the Doppler Lidar profiles above Manora Peak (-0.8 to 0.8 ms(-1)) as compared to near surface measurements at this station using an ultrasonic anemometer (-0.4 to 0.4 ms(-1)), and low altitude stations in India. Daytime vertical wind speeds are observed to be higher during pre-monsoon (0.81 ms(-1)), as compared to post-monsoon (0.24 ms(-1)) and winter (0.33 ms(-1)). Average Black Carbon (BC) concentrations are significantly higher during strong upward vertical winds, which indicates efficient transport of polluted air mass from low-altitude regions to the Himalaya. Weather Research and Forecasting (WRF) model reproduces the observed diurnal pattern in the vertical wind at the observation site however the model underestimates the variability.