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

Abstract

ABSTRACT:A new analysis of the neutral heat transfer coefficient C HN on data fromÖstergarnsholm is presented, which is primarily based on a limited set of measurements with the very accurate MIUU (Meteorological Institute of the University of Uppsala) instrument, but with additional information from an extensive set of measurements with Solent sonic R2. Sonic data are, however, used with great caution, since for wind speed U above 10 m s −1 , a strongly windspeed-dependent correction is shown to be required. This error is roughly proportional to (U − 10) for sea-air temperature differences less than 4-5 K. For a larger temperature difference, no correction appears to be necessary in the wind speed range 10-15 m s −1 .We infer from our data that for conditions when unstable and near-neutral conditions prevail, measurements of the sea surface -air temperature difference are accurate to within 0.1 K at our site. This means that data for a range of relatively small temperature differences (0.5-1.5 K) which were often rejected in previous studies could be retained. It is observed that a rapid increase of C H and C HN occurs in that range.For wind speed above 10 m s −1 , C HN is observed to increase rapidly with U 10 . 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 fromÖstergarnsholm, it was speculated that observed high C HN values could be due to spray. Calculations with a spray model showed, however, conclusively that for wind speeds less than 14 m s −1 , the spray effect on the sensible heat flux is expected to be small. The high C HN values must instead be due to dynamic effects.It is demonstrated that when the Obukhov length L is less than about −150 m a regime with very specific characteristics ensues. This regime is dominated by surface-layer scale eddies, which cause Monin-Obukhov relations for the exchange of sensible heat to break down. The characteristics of this surface-layer regime are treated in detail in the companion paper.The rise of C HN with wind speed is shown to be closely related to a corresponding increase of z 0T with roughness Reynolds number for winds above 10 m s −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 This means that during those conditions, traditional surface renewal theory for heat is no longer valid.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 regime with −L > 150 m.

Air‐Sea Turbulent Fluxes From a Wave‐Following Platform During Six Experiments at Sea

“…This may partly support the underestimation found above, because the bulk estimates of C HV do not explicitly account for wave age. Nevertheless, this parallel with the work by Smedman et al (2007) would need to be further investigated with new data, because C H and C HV are not the same quantities.…”

“…According to our data, C H or C HV do not increase with with wind speed, over a 2-8 m/s range, which was also observed by other authors such as Smedman et al (2007). It is even rather the opposite; that is, C HV slightly decreases with wind speed.…”

“…The values found compare reasonably well to the bulk ones, but they are underestimated by 0.1 − 0.2 × 10 −3 . Interestingly, Smedman et al (2007), Figure 17) find that the mean value of C H (0.91 × 10 −3 ) is smaller by 0.1 − 0.18 × 10 −3 in swell conditions than for younger seas (1.01 − 1.09 × 10 −3 ). This may partly support the underestimation found above, because the bulk estimates of C HV do not explicitly account for wave age.…”

“…This contrasts with earlier findings of Makin and Mastenbroek (1996) who suggested that the Stanton number was rather independent of wind speed in the range 4-10 m/s. Larsen et al (2004) and more recently Smedman et al (2007) reached a similar conclusion. Specifically, the later authors found that in unstable conditions, C H was independent of wind speed, if the wind speed was below 9 m/s and if the mean sea minus air temperature difference was larger than 3°.…”

Mapping Regional Turbulent Heat Fluxes via Assimilation of MODIS Land Surface Temperature Data into an Ensemble Kalman Smoother Framework

“…While there is almost no vegetation in the north of HRB and LAI is invariant, the corresponding retrieved C HN values vary in different monthly modeling periods. This is because C HN depends not only on the vegetation phenology (LAI) but also to a lesser extent on the wind speed, friction velocity, and solar elevation (Duynkerke, 1992;Smedman et al, 2007).…”