Duration: 2009 (1 year)

In this study the variation of the wind drag coefficient during storm wind conditions is analysed. A model system, based on the MIKE software by DHI, is implemented covering the entire Baltic Sea. A three-dimensional flow model is coupled sequentially with a spectral wave model. The wind shear stresses are determined iteratively by the wave model and applied at the free surface of the flow model.

The basic concept for the iterative procedure follows the “wind over waves coupling theory” proposed by Janssen (1989, 1991). Comparisons of modelled and computed water levels for a storm in February 2002 using different formulations of wind drag coefficients prove the adequacy of the iterative procedure.

Simulations covering about 30 extreme storm scenarios are carried out, leading to approximately 200 million wind drag coefficients. The results reveal a high variability of wind drag coefficients for wind speed classes up to around 30m/s. For wind speeds above 30m/s a distinction between decreasing and increasing wind drag coefficients most probably associated to wave breaking in deep water is observed.  The main advantages of the application of the proposed method in storm surge models are therefore the independence of wind speed related empirical formulations with limited validity ranges and the consideration of local and temporal effects such as fetch length and wave age.

Research division:


  • Prof. Dr. R. Mayerle


  • G. Bruss