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Re-evaluation of the bulk exchange coefficient for humidity at sea during unstable and neutral conditions
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Air and Water Science. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (AWEP)
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. (AWEP)
2008 (English)In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 38, no 1, 257-272 p.Article in journal (Refereed) Published
Abstract [en]

Processes influencing the air-sea exchange of humidity during unstable and neutral stratification were studied using tower measurements from the island of Ostergarnsholm in the Baltic Sea. For small air-sea temperature differences, the neutral exchange coefficient for humidity C-EN was found to increase with increasing wind speed, attaining a value of approximately 1.8 x 10(-3) at 13 m s(-1). The high C-EN values were observed during situations when the characteristics of the turbulence structure differed from what would be expected from traditional theory. Results from spectral analysis point to a situation in which the vertical transport of humidity is dominated by smaller-scale eddies. Quadrant analysis showed that these eddies enhance the humidity flux by bringing down drier air from layers aloft. These findings are consistent with recent analyses of the neutral boundary layer in which a change of turbulence regime has been observed. The conclusion is made that this dynamic effect accounts for the observed increase in C-EN. Here, C-EN was calculated using a wave-dependent normalized wind gradient, which had the effect of reducing the value by about 10% during swell relative to calculations using a non-wave-dependent normalized wind gradient.

Place, publisher, year, edition, pages
2008. Vol. 38, no 1, 257-272 p.
Keyword [en]
Air–sea interaction, Humidity, Turbulence, Boundary layer, Eddies
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-96143DOI: 10.1175/2007JPO3754.1ISI: 000252772600016OAI: oai:DiVA.org:uu-96143DiVA: diva2:170619
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2013-04-29Bibliographically 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 334
Keyword
Meteorology, Sensible heat flux, Latent heat flux, Carbon dioxide, Turbulent exchange, Global heat fluxes, Marine boundary layer, Air-sea interaction, Meteorologi
Identifiers
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
Opponent
Supervisors
Available from: 2007-09-04 Created: 2007-09-04Bibliographically approved

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Sahlée, ErikSmedman, Ann-SofiHögström, UlfRutgersson, Anna

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