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  • Author or Editor: Jielun Sun x
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L. T. Mahrt
and
Jielun Sun

Abstract

The exchange coefficients for area-averaged surface fluxes can become anomalously large when the large-scale flow is weak and significant fluxes of heat and moisture are driven by mesoscale motions within the averaging or subgrid area. To prevent this erratic behavior of the exchange coefficient, the “subgrid velocity scale” must be included to account for generation of turbulent fluxes by subgrid mesoscale motions. This velocity scale is obtained by spatially averaging the local time-averaged velocity used in the bulk aerodynamic relationship. The subgrid velocity scale is distinct from the free convection velocity scale included in the bulk aerodynamic relationship to represent transport induced by convectively driven boundary-layer-scale eddies (Godfrey and Beljaars; Beljaars).The formulation of Godfrey and Beljaars is derived by time averaging the velocity scale of the bulk aerodynamic relationship.

The behavior of the subgrid velocity scale is explored using data from five different field programs. Ubiquitous “nameless” mesoscale motions of unknown origin are found in all of the datasets. The addition of the subgrid velocity scale reduces the dependence of the exchange coefficients on grid size. Based on the data analysis, the subgrid velocity scale increases with grid size and contains a contribution due to surface heterogeneity.

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Dean Vickers
,
L. Mahrt
,
Jielun Sun
, and
Tim Crawford

Abstract

The horizontal and vertical structure of the mean flow and turbulent fluxes are examined using aircraft observations taken near a barrier island on the east coast of the United States during offshore flow periods. The spatial structure is strongly influenced by the surface roughness and surface temperature discontinuities at the coast. With offshore flow of warm air over cool water, the sea surface momentum flux is large near the coast and decreases rapidly with increasing offshore distance or travel time. The decrease is attributed to advection and decay of turbulence from land. The rate of decrease is dependent on the characteristic timescale of the eddies in the upstream land-based boundary layer that are advected over the ocean. As a consequence, the air–sea momentum exchange near the coast is influenced by upstream conditions and similarity theory is not adequate to predict the flux.

The vertical structure reveals an elevated layer of downward momentum flux and turbulence energy maxima over the ocean. This increase in the momentum flux with height contributes to acceleration of the low-level mean wind. In the momentum budget, the vertical advection term, vertical flux divergence term, and the horizontal pressure gradient term are all of comparable magnitude and all act to balance large horizontal advection. An interpolation technique is applied to the aircraft data to develop fetch–height cross sections of the mean flow and momentum flux that are suitable for future verification of numerical models.

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