Drag Coefficient

The drag coefficient C_D is the dimensionless factor in the bulk formula for surface stress, tau = rho_air C_D U^2, that relates wind stress to the square of the wind speed measured at 10 meters. Over the ocean C_D is about 1.0 to 2.5 times 10^-3 and rises with wind speed because the sea surface roughens, then may level off or fall at hurricane-force winds. It sets the momentum input that drives the wind-driven circulation, Ekman transport, and surface waves. Charnock’s relation parameterizes the roughness length from the friction velocity.