An Old and Improved Bulk Algorithm for Air-Sea Fluxes: COARE2.6a

In 1993, Chris Fairall, Frank Bradley and David Rogers began development of a bulk air-sea flux algorithm for use by the Coupled Ocean-Atmosphere Response Experiment (COARE) community. The model was originally based on the 1979 parameterization of 1979 Liu, Katsaros and Businger (LKB), but it took account of the light wind, strongly convective conditions over tropical oceans. The approach was to make the model more generally applicable by incorporating more rigorous treatment of physical processes. After a lengthy development procedure, the version 2.5b bulk algorithm "package" was made available in November 1993 and appeared in print in early 1996. Modifications to the basic LKB code were made for wind roughness length, Monin-Obukhov profile functions for strong convection, low-wind "gustiness", cool skin physics, daytime near-surface warming based on a simplified version of the Price, Weller and Pinkel ocean mixing model, and sensible heat due to rainfall. In January of 2000, version 2.6a was released. The empirical constants in the convective portion of the profile functions have been changed for improved matching to direct profile observations. The Kansas stable profile functions have been replaced by those from Beljaars and Holtslag, which is a better fit to new profile data taken over the Arctic ice cap in the SHEBA project. A fixed value of the Charnock parameter (=0.011) has been replaced by a formulation with a simple wind-speed dependence above 12 m/s based results from recent field programs. The LKB scalar roughness relationship has been replaced with a much simpler one that fits both the COARE, several additional ETL flux data sets, and HEXMAX data base. The stability iteration loop has been reduced from 20 to 3 using bulk Richardson number parameterization for an improved first guess. The latent heat flux has been reformulated in terms of mixing ratio, q, instead of water vapor density, Q, to eliminate the need for a Webb correction. In this paper we will describe the basis of these improvements in more detail.