Fetch limited drag coefficients

Measurements made at a tower located 2 lan off the coast of Denmark in shallow water during the Rise Air Sea Experiment (RASEX) are analyzed to investigate the behaviour of the drag coefficient in the coastal zone. For a given wind speed, the drag coefficient is larger during conditions of short fetch (2-5 km) off-shore flow with younger growing waves than it is for longer fetch (15-25 km) on-shore flow. For the strongest on-shore winds, wave breaking enhances the drag coefficient. Variation of the neutral drag coefficient in RASEX is dominated by variation of wave age, frequency bandwidth of the wave spectra and wind speed. The frequency bandwidth is proportional to the broadness of the wave height spectra and is largest during conditions of light wind speeds. Using the RASEX data, simple models of the drag coefficient and roughness length are developed in terms of wind speed, wave age and bandwidth. An off-shore flow model of the drag coefficient in terms of nondimensional fetch is developed for situations when the wave state is not known.