Critical re-evaluation of the bulk transfer coefficient for sensible heat over the ocean during unstable and neutral conditions

A new analysis of the neutral heat transfer coefficient CHN on data from Östergarnsholm is presented, which is primarily based on a limited set of measurements with the very accurate MIUU-instrument, but with additional information from an extensive set of measurements with Solent sonic R2. Sonic data are, however, used with great caution, because for wind speed above 10 ms-1, a strongly wind-speed-dependent correction is shown to be required. It was demonstrated that for conditions when unstable and near-neutral conditions prevail, measurements of the sea surface – air temperature difference were accurate to within 0.1 K. This means that data for a range of relatively small temperature differences (0.5 – 1.5 K) which are often rejected in previous studies could be retained. It was observed that a rapid increase of CH and CHN occurs in that range. For wind speed above 10 ms-1, CHN was observed to increase rapidly with U10. During those conditions, the wave field at the site is known to have characteristics very similar to those in deep-sea conditions. In a previous analysis of data from Östergarnsholm, it was speculated that observed high CHN-values could be due to spray. Calculations with a spray-model showed, however, conclusively that for wind speeds less than 14 ms-1, the spray effect on the sensible heat flux is expected to be small. The high CHN-values must instead be due to dynamic effects. It was demonstrated that when the Obukhov length is less than about -150 m a ‘forced convection regime', with very specific characteristics ensues. This regime is dominated by surface-layer scale eddies, which cause MO-relations for the exchange of sensible heat to break down. The characteristics of this surface-layer regime will be outlined briefly. The rise of CHN with wind speed was shown to be closely related to a corresponding increase of z0T with roughness Reynolds number for winds above 10 ms-1. This means that during those conditions, traditional surface renewal theory for heat is no longer valid. It is suggested that this, in turn, is a result of increasing importance of wave-breaking with increasing wind and with a possible link to processes in near-surface atmospheric layers in the ‘forced convection regime'.