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 z one. For a given wind speed, the drag coefficient is larger during con ditions of short fetch (2-5 km) off-shore flow with younger growing wa ves than it is for longer fetch (15-25 km) on-shore flow. For the stro ngest on-shore winds, wave breaking enhances the drag coefficient. Var iation of the neutral drag coefficient in RASEX is dominated by variat ion of wave age, frequency bandwidth of the wave spectra and wind spee d. The frequency bandwidth is proportional to the broadness of the wav e height spectra and is largest during conditions of light wind speeds . Using the RASEX data, simple models of the drag coefficient and roug hness length are developed in terms of wind speed, wave age and bandwi dth. An off-shore flow model of the drag coefficient in terms of nondi mensional fetch is developed for situations when the wave state is not known.