A Relationship between the Bowen Ratio and Sea–Air Temperature Difference under Unstable Conditions at Sea

Abstract

At the air?sea interface, estimates of evaporation or latent heat flux and the Monin?Obukhov stability parameter require the measurements of dewpoint (Tdew) or wet-bulb temperature, which are not routinely available as compared to those of air (Tair) and sea surface temperature (Tsea). On the basis of thermodynamic considerations, this paper first postulates that the quantity of (qsea ? qair) for the difference in specific humidity between the sea surface and its overlying air is related to the quantity of (Tsea ? Tair). Using hourly measurements of all three temperatures, that is, Tsea, Tair, and Tdew from a buoy in the Gulf of Mexico under a severe cold air outbreak, a linear correlation between (qsea ? qair) and (Tsea ? Tair) does exist with a compelling high correlation coefficient, r, of 0.98 between these two quantities. Second, based on this Clausius-Clapeyron effect, the Bowen ratio B is proposed to relate to the quantity of (Tsea ? Tair) only such that B = a(Tsea ? Tair)b. Using all data for these three temperatures available from four stations in the Gulf from 1993 through 1997 reveal that for deepwater a varies from 0.077 to 0.078, b from 0.67 to 0.71, and r from 0.85 to 0.89. Similar equations for the nearshore region are also provided. Limited datasets from the open ocean also support this generic relationship between B and the quantity of (Tsea ? Tair).