On the parameterization of surface momentum transport via drag coefficient in low-wind conditions

The subgrid-scale surface momentum transport, which plays an important role in determining the exchange between the atmosphere and the underlying surface, is often parameterized in terms of the surface mean wind speed via drag coefficient (C-D), a parameter that needs to be determined externally often through the Monin-Obukhov Similarity (MOS) Theory. However, some characteristics of C-D derived from observations for overland conditions, particularly the substantial increase of C-D with a decrease in wind speed in low-wind conditions, cannot be explained by MOS. This issue is investigated using data collected by a portable meteorological tower. By analyzing the turbulent kinetic energy budget, a novel parameterization framework for momentum fluxes is proposed. The new parameterization not only appropriately describes the observed variation of C-D but also can be simplified to MOS with certain assumptions. Moreover, the effect of stability, which traditionally has to be determined empirically, can now be determined internally within the new framework.