Expansion of roughness length parameterization in the ocean surface layer based on measured data

Charnock scheme is known as the most widely used method to calculate the flux exchange in the ocean surface layer. Due to the simplicity of the application and run with minimum meteorological data, it is one of the most popular schemes in the surface layer. Edson et al. (2013) introduced the method of variable coefficients for the Charnock relationship and used data collected from four oceanic field experiments for this purpose. They have introduced a linear regression equation among neutral wind speeds at 10 m (U10N) in range of 7 to 18 m/s with coefficients of Charnock relation. This proposed linear equation is considered by the investigators in the recent versions of 3.8 and 3.9 of the WRF model and is evaluated in some cases. This scheme is considered as the default for the calculation of surface fluxes. The applicability of this method only for U10N between 7 and 18 m/s is known as a disadvantage of this scheme. In this study, the aforementioned problem (U10N limitation) was considered, and neutral wind velocity at 10 m was fitted measured data of Edson in a range of 5 to 30 m/s by a second-order function. However, the average error in second-order fitness has slightly increased, but considering speeds of 18 to 30 m/s can cover a slight decrease in fitness accuracy. For U10N higher than 30 m/s, according to a large number of studies in the field of reducing or fixed drag coefficients at speeds above 30 m/s, the assumption of reducing the Charnock coefficient was studied for U10N more than 30 m/s. The Charnock coefficient was assumed at U10N higher than 30 m/s would decrease linearly with increasing wind speed and eventually reach zero at 90 m/s. Due to the lack of physical interpretation for zeroing the Charnock coefficient at 90 m/s, the maximum U10N of 80 m/s is considered, which is equal to 0.005 for the value of the Charnock coefficient. Modification of the Edson et al. (2013) scheme in this paper has led to a reduction in the growth rate of the drag coefficient. The main results of the present research are that even with zeroing the Charnock coefficient at 90 m/s and taking into account the maximum U10N at 80 m/s in the Edson et al. scheme, the reduction in drag coefficient at U10N more than 30 m/s cannot be created. Therefore, if future measurements suggest additional flux production at U10N higher than 30 m/s for the Edson et al. scheme, stronger strikes are needed to reduce roughness length, and the decreasing trend does not occur in the drag coefficient even with zeroing the Charnock coefficient. Therefore, it is seen that adding ocean spray effects to well-known schemas such as Charnock has many problems. In this study, although the main defects of the Edson et al. scheme (Quasi-exponential growth of the drag coefficient with increasing wind speed) have been resolved, more field measurements will be required for the scheme verification proposed in this paper.