Based on understanding of physical processes represented by the Monin-Obukhov (MO) bulk formula for momentum, physical representation of drag coefficients will be discussed. We will first discuss contribution of the most energetic large non-local coherent eddies to turbulent momentum transfer. Based on field observations, we then demonstrate that the neutral relationship between momentum transfer and wind speed from the MO bulk formula represents a special neutral condition, which is independent of wind speed. When the surface layer is stratified, either unstably or stably, a neutral surface layer can be created through mechanically generated turbulent mixing if winds are strong. Consequently the neutral relationship between momentum transfer and wind speed under strong winds is different from the one described by the MO bulk formula. Due to the correlation between wind speed and the surface-layer stratification, the traditionally defined drag coefficient is only independent of wind speed under the special MO neutral condition and varies with stratification and wind speed when the surface layer is stratified. As a result, if the surface drag coefficient is used to represent impacts of the unique surface on the turbulent momentum transfer, it needs to be estimated by using neutral observations only. The traditionally defined drag coefficient can be used to calculate the surface drag coefficient only when the neutral surface layer satisfies the neutral MO bulk formula. However it cannot be used to calculate the surface drag coefficient when the neutral stratification is achieved by mechanically generated turbulent mixing through strong wind shear. This is because when the surface is either heated or cooled, weak wind speeds at a given height are always associated with non-zero atmospheric stratification.
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