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Heat/mass transfer in the slightly unstable atmospheric surface layer
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)
2007 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, no 622, 37-51 p.Article in journal (Refereed) Published
Abstract [en]

In very slightly unstable conditions, when the Obukhov length is much greater than the surface layer depth, it is observed that the structure of the surface layer turbulence does not accord with standard similarity theory. In particular the efficiency of the turbulent exchange of sensible and latent heat is observed to be more strongly enhanced than is consistent with the standard model. Also the profiles of dissipation of turbulent kinetic energy and temperature fluctuation variance are found to depend on the structure of the whole boundary layer (i.e. are non-local), indicating that a large-scale transport process is at work. At the same time, co-spectral analysis shows how the large-scale eddy motions that determine the heat transport process near the surface are typically 1/5 of the surface layer depth. All these features are found to be similar in measurements at a marine site and at a flat land site, indicating that they are determined by the dynamics of the whole boundary layer rather than being simply dependent on the surface boundary conditions. We hypothesize that in slightly unstable conditions there is a bifurcation in the structure of the large-scale eddy motions. This is a transition from quasi-steady longitudinal roll structures to detached eddies. In the particular regime identified here, the unsteady component dominates the heat transport at the surface. The observed enhancement in the surface layer of the eddy diffusivity of heat is observed to be significantly greater than for momentum. This is shown to be consistent with observations and with a model calculation of the interaction between the surface layer and the descending detached eddies.

Place, publisher, year, edition, pages
2007. Vol. 133, no 622, 37-51 p.
Keyword [en]
turbulence energy, similarity theory, vortex shedding, bifurcation, turbulence structure, sensible heat, latent heat, heat mass transfer, air-sea interface, atmospheric boundary layer, marine atmosphere
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-96141DOI: 10.1002/qj.7ISI: 000244847500004OAI: oai:DiVA.org:uu-96141DiVA: diva2:170617
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.
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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Smedman, Ann-SofiHögström, UlfSahleé, Erik

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