Estimation of drag coefficient at low wind speeds over the monsoon trough land region during MONTBLEX-90

Abstract

Using observational data acquired during the Monsoon Trough Boundary Layer Experiment (MONTBLEX-90), the drag coefficient C_D is estimated over two land stations, Jodhpur (arid region) and Kharagpur (moist region), by eddy correlation of velocity data from sonic anemometers. At low winds C_D varies with wind speed according to a power law, which at Jodhpur takes the form C_D =0.04 ̅U ₁₀ ^−1.06 (̅U ₁₀ being the mean wind speed at 10 m). The large values of the drag coefficient at low winds are associated with both near-neutral and strongly unstable situations. On the other hand, the C_D values at Kharagpur are less than at Jodhpur by nearly 50% for wind speeds > 3ms⁻¹, and cannot be explained by the different roughness lengths at the two sites.