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Critical re-evaluation of the bulk transfer coefficient for sensible heat over the ocean during unstable and neutral conditions
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (meteorologi)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Air and Water Science.
2007 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 622, 227-250 p.Article in journal (Refereed) Published
Abstract [en]

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 (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 wind-speed-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 CH and CHN occurs in that range. For wind speed above 10 m s-1, CHN is 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 m s-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 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 CHN with wind speed is shown to be closely related to a corresponding increase of ZOTwith 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.

Place, publisher, year, edition, pages
2007. Vol. 133, no 622, 227-250 p.
Keyword [en]
Atlantic Ocean, North Atlantic, Northeast Atlantic, Europe, Scandinavia, Baltic Sea, Sweden, Wave breaking, Wind velocity, Wind effect, air-sea interface, Observation data, Heat transfer coefficient, Sensible heat, Turbulent transfer, heat transfer, Atmospheric boundary layer, Marine atmosphere
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:uu:diva-96142DOI: 10.1002/qj.6ISI: 000244847500018OAI: oai:DiVA.org:uu-96142DiVA: diva2:170618
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2013-04-22Bibliographically approved
In thesis
1. Fluxes of Sensible and Latent Heat and Carbon Dioxide in the Marine Atmospheric Boundary Layer
Open this publication in new window or tab >>Fluxes of Sensible and Latent Heat and Carbon Dioxide in the Marine Atmospheric Boundary Layer
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Oceans cover about 70% of the earth’s surface. They are the largest source of the atmospheric water vapour and act as enormous heat reservoirs. Thus in order to predict the future weather and climate it is of great importance to understand the processes governing the exchange of water vapour and heat between the ocean and atmosphere. This exchange is to a large extent mediated by turbulent eddies. Current numerical climate and weather forecast models are unable to resolve the turbulence, which means that the turbulent exchange needs to be simplified by using parameterizations.

Tower based measurements at the Östergarnsholm Island in the Baltic Sea have been used to study the air-sea turbulent exchange of latent and sensible heat and the heat flux parameterizations. Although the measurements are made at an island, data obtained at this site is shown to represent open ocean conditions during most situations for winds coming from the east-south sector. It is found that during conditions with small air-sea temperature differences and wind speeds above 10 m s-1, the structure of the turbulence is re-organized. Drier and colder air from aloft is transported to the surface by detached eddies, which considerably enhance the turbulent heat fluxes. The fluxes where observed to be much larger than predicted by current state-of-the-art parameterizations. The turbulence regime during these conditions is termed the Unstable Very Close to Neutral Regime, the UVCN-regime.

The global increase of the latent and sensible heat fluxes due to the UVCN-regime is calculated to 2.4 W m-2 and 0.8 W m-2 respectively. This is comparable to the current increase of the radiative forcing due to anthropogenic emissions of greenhouse gases, reported in Intergovernmental Panel on Climate Change fourth assessment report (IPCC AR4). Thus the UVCN-effect could have a significant influence when predicting the future weather and climate.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 44 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 334
Meteorology, Sensible heat flux, Latent heat flux, Carbon dioxide, Turbulent exchange, Global heat fluxes, Marine boundary layer, Air-sea interaction, Meteorologi
urn:nbn:se:uu:diva-8184 (URN)978-91-554-6954-2 (ISBN)
Public defence
2007-09-28, Axel Hamergssalen, Geocentrum, Villavägen 16, Uppsala, 10:00
Available from: 2007-09-04 Created: 2007-09-04Bibliographically approved

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