ESTIMATION OF DRAG COEFFICIENT OVER THE WESTERN DESERT SECTOR OF THE INDIAN-SUMMER MONSOON TROUGH

In the estimation of momentum fluxes over land surfaces by the bulk ae rodynamic method, no unique value of the drag coefficient (C-D) is fou nd in the literature. The drag coefficient is generally estimated from special observations at different parts of the world. In this study a n attempt is made to estimate drag coefficient over the western desert sector of India using data sets of Monsoon Trough Boundary Layer Expe riment (MONTBLEX) during the summer monsoon season of 1990. For this p urpose, the fast and slow response data sets obtained simultaneously f rom a 30 m high micro-meteorological tower at Jodhpur are used. All th e observations used in this study are confined to a wind speed regime of 2.5-9.0 ms(-1). A comparison of momentum fluxes computed by eddy co rrelation (direct estimation) with profile and bulk aerodynamic (C-D = 3.9 x 10(-3), Garratt, 1977) methods revealed that though the nature of variation of the fluxes by all these methods is almost similar, bot h the indirect methods give an under-estimated value of the fluxes. Th e drag coefficient is estimated as a function of wind speed and surfac e stability by a multiple regression approach. An average value of the estimated drag coefficient is found to be of the order of 5.43 x 10(- 3). The estimated value of C-D is validated with a set of independent observations and found to be quite satisfactory. The recomputed moment um fluxes by bulk aerodynamic method using the estimated drag coeffici ent are in close agreement with the directly estimated fluxes.