ON THE DETERMINATION OF THE NEUTRAL DRAG COEFFICIENT IN THE CONVECTIVE BOUNDARY-LAYER

Based on the idea that free convection can be considered as a particul ar case of forced convection, where the gusts driven by the large-scal e eddies are scaled with the Deardorff convective velocity scale, a ne w formulation for the neutral drag coefficient, C-Dn, in the convectiv e boundary layer (CBL) is derived. It is shown that (i) a concept of C -Dn can still be used under strongly unstable conditions including a p ure free-convection regime even when no logarithmic portion in the vel ocity profile exists; (ii) gustiness corrections must be applied for r ational calculations of C-Dn; and (iii) the stratification Psi functio n used in the derivation of C-Dn should satisfy the theoretical free-c onvection limit. The new formulation is compared with the traditional relationship for C-Dn, and data collected over the sea (during the Tro pical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experi ment (TOGA COARE) and the San Clemente Ocean Probing Experiment (SCOPE )) and over land (during the BOREX-95 experiment) are used to illustra te the difference between the new and traditional formulations. Compar ed to the new approach, the traditional formulation strongly overestim ates C-Dn and z(o) in the CBL for mean wind speed less than about 2 m s(-1). The new approach also clarifies several contradictory results f rom earlier works. Some aspects related to an alternate definition of the neutral drag coefficient and the wind speed and the stress averagi ng procedure are considered.