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

Using observational data acquired during the Monsoon Trough Boundary Layer Experiment (MONTBLEX-90), the drag coefficient C<SUB>D</SUB> 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<SUB>D</SUB> varies with wind speed according to a power law, which at Jodhpur takes the form C<SUB>D</SUB> =0.04 ̅U <SUB>10</SUB> <SUP>−1.06</SUP> (̅U <SUB>10</SUB> 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<SUB>D</SUB> values at Kharagpur are less than at Jodhpur by nearly 50% for wind speeds &gt; 3ms<SUP>−1</SUP>, and cannot be explained by the different roughness lengths at the two sites