VARIABILITY OF SURFACE-ROUGHNESS AND TURBULENCE INTENSITIES AT A COASTAL SITE IN INDIA

Surface-layer features with different prevailing wind directions for two distinct seasons (Southwest Monsoon and Northeast Monsoon) on the west coast of India are studied using data obtained from tower-based sensors at a site located about 500 m from the coast. Only daytime runs have been used for the present analysis. The surface boundary-layer fluxes have been estimated using the eddy correlation method. The surface roughness z(0) obtained using the stability-corrected wind profiles (Paulson, 1970) has been found to be low for the Southwest monsoon season. For the other season, z(0) is relatively high. The drag coefficient C-D varies with height in the NE monsoon season but not in the season with low z(0). This aspect is reflected in the wind profiles for the two seasons and is discussed in detail, The scaling behaviour of friction velocity u* and the turbulence intensity of longitudinal, lateral and vertical winds (sigma(u), sigma (v), and sigma(w), respectively) are further examined to study their dependence on fetch. Our study shows that for the non-dimensional case, sigma(u)/u* and sigma(v)/u* do not show any surface roughness dependence in either season. On the other hand, for sigma(w)/u* for the season with low z(0), the values are seen to agree well with that of Panofsky er al. (1977) for homogeneous terrain whereas for the other season with high z(0), the results seem to conform more to the values observed by Smedman and Hogstrom (1983) for coastal terrain. The results are discussed in the light of observations by other investigators.