Estimating the Drag Coefficient and Sea Surface Roughness Using a Morphometric Model

In the last decade, morphometric methods have been successfully used to estimate local-scale aerodynamic parameters from surface (roughness-element) geometry in the urban environment. By using routinely observed significant wave heights and average wave periods provided by the National Data Buoy Center dating back to 1970, a generalised representation of the local spatially and temporally averaged marine surface are produced. Through use of a simple morphometric model, the aerodynamic parameters of interest, the local-scale aerodynamic roughness length and zero-plane displacement height, are estimated for each available observation in the database. Corresponding observed wind speeds are used to create a drag coefficient-wind speed relationship. Comparison of the derived relationship to relations derived from dropsondes, angular momentum and upper ocean currents shows good agreement. The impact of applying the calculated aerodynamic parameters to correct buoy wind speed observations to 10 m height, with an emphasis on high-wind speed conditions, is investigated. The consequence of mean wind speeds being a combination of observations from both above and between waves is accounted for by a two-layer approach which models mean wind speeds above the waves using the logarithmic wind profile and an exponential profile between waves. Results show that wind observations corrected to 10 m height considering the effects of roughness length, displacement height and a buoy’s path can be much greater than those which only consider the height of the buoy’s anemometer.