Vertical Divergence of the Atmospheric Momentum Flux near the Sea Surface at a Coastal Site

L. Mahrt aNorthwest Research Associates, Corvallis, Oregon

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Erik Nilsson bDepartment of Earth Sciences, Uppsala University, Uppsala, Sweden

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Anna Rutgersson bDepartment of Earth Sciences, Uppsala University, Uppsala, Sweden

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Heidi Pettersson cFinnish Meteorological Institute, Helsinki, Finland

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Abstract

Motivated by previous studies, we examine the underestimation of the sea surface stress due to the stress divergence between the surface and the atmospheric observational level. We analyze flux measurements collected over a 6-yr period at a coastal tower in the Baltic Sea encompassing a wide range of fetch values. Results are posed in terms of the vertical divergence of the stress scaled by the stress at the lowest observational level. The magnitude of this relative stress divergence increases with increasing stability and decreases with increasing instability, possibly partly due to the impact of stability on the boundary layer depth. The magnitude of the relative stress divergence increases modestly with decreasing wave age. The divergence of the heat flux is not well correlated with the divergence of the momentum flux evidently due to the greater influence of advection on the temperature. Needed improvement of the conceptual framework and needed additional measurements are noted.

Significance Statement

Flux measurements over the sea are typically made at 10 m above the surface. However, the vertical flux divergence measured between two levels suggests that the variation of the surface fluxes and the 10-m fluxes may be significant such that the observed flux at 10 m can seriously underestimate the surface fluxes. This underestimation is documented from long-term flux measurements from a tower in the coastal zone of the Baltic.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: L. Mahrt, mahrt@nwra.com

Abstract

Motivated by previous studies, we examine the underestimation of the sea surface stress due to the stress divergence between the surface and the atmospheric observational level. We analyze flux measurements collected over a 6-yr period at a coastal tower in the Baltic Sea encompassing a wide range of fetch values. Results are posed in terms of the vertical divergence of the stress scaled by the stress at the lowest observational level. The magnitude of this relative stress divergence increases with increasing stability and decreases with increasing instability, possibly partly due to the impact of stability on the boundary layer depth. The magnitude of the relative stress divergence increases modestly with decreasing wave age. The divergence of the heat flux is not well correlated with the divergence of the momentum flux evidently due to the greater influence of advection on the temperature. Needed improvement of the conceptual framework and needed additional measurements are noted.

Significance Statement

Flux measurements over the sea are typically made at 10 m above the surface. However, the vertical flux divergence measured between two levels suggests that the variation of the surface fluxes and the 10-m fluxes may be significant such that the observed flux at 10 m can seriously underestimate the surface fluxes. This underestimation is documented from long-term flux measurements from a tower in the coastal zone of the Baltic.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: L. Mahrt, mahrt@nwra.com
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