On the nature of drag coefficient over a tropical coastal station

Abstract

The turbulent flow over a coastal region is investigated to study the drag coefficient ($C_D$) behavior during on-shore and off-shore winds. The analysis of turbulent data over 2 years is carried out to examine the dependence of $C_D$ on mean wind speed ($\overline{U}$) and stability parameter ($\zeta$). The drag coefficient is found to show a parabolic dependence with wind speed for on-shore flows and a slightly linear trend for off-shore flows for neutral and weakly unstable cases. Only for strongly unstable and stable cases ($\zeta >0$), high values of $C_D$ are observed for low wind speed ($\overline{U}<2\text {ms}^{-1}$). The likely cause for high values of $C_D$ during low wind speed is attributed to an increase in turbulent intensity caused due to the presence of coherent structures. On further analysis of $C_D$ with $\zeta$, it is found that under stable conditions ($\zeta >0$), $C_D$ shows a systematic decrease with increasing $\zeta$. On the contrary, for unstable cases ($\zeta <0$), the values of $C_D$ peaks around $\zeta \approx -0.13$, before decreasing with increasing $-\zeta$.