Effects Of Changing Surface Heat Flux On Atmospheric Boundary-Layer Flow Over Flat Terrain

Abstract

We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u à when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t h less than about h/(10u à ). Standard Monin-Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h/|L MO | over the time scale t MO~| L MO |/(10u à ) (where L MO and t MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h/ |L MO | () 1), over the time scale t MO . The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A lowlevel wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.

Turbulence Spectra for Boundary-Layer Winds in Tropical Cyclones: A Conceptual Framework and Field Measurements at Coastlines

“…The mean flow is also affected by the strong convective turbulence changing the vertical gradient of the turbulent shear stress (i.e. dτ/dz > 0, where τ is the shear stress) that amplifies the jet flow in the lower boundary layer (Owinoh et al 2005). Thus in the lower part of the convective layer, where z ≈ z max , either the mean azimuthal velocity U reaches a single maximum with peak value U max , (where dU/dz = 0), or a secondary maximum occurs depending on these two processes.…”

“…(5) sub-ranges V and VI are not significant in the outer-vortex region where w * u * , thus l s ≈ u 8 * / g 5 (γ /ρ) 3 1/2 decreases and the processes associated with droplets are confined to the Charnock distance α c w 2 * g −1 . Note that the low-level jet may still exist in the outer vortex regions associated with interaction between the buoyant convection and the mean swirl (Owinoh et al 2005).…”

“…Since U also decreases with radius r over a distance of order R ew , the mean flow structure in the lower part of the tropical cyclone is significantly different from that of the unidirectional atmospheric boundary layer over a flat surface. Rapid distortion theory shows how an unsteady shear flow moving over an area of strong convection leads to a low-level jet in the mean flow, which is caused by the very strong convective turbulence dominating the shear turbulence near the surface (Owinoh et al 2005).…”

Coriolis effects on wind jets and cloudiness along coasts

“…They are dynamically quite distinct from low-level, thermal-boundary layer (TBL) jets driven by variations in the surface heat flux with time or spatially in the wind direction (Owinoh et al 2004). Examples of the latter are the well-known nocturnal jet (e.g.…”

“…turning of the wind with height (Bonner 1968). Computations, theory and recent laboratory experiment (Owinoh et al 2004) show that the basic driving force is a change in the turbulent shear stress caused by changes in stability (or Richardson number Ri associated with changing surface heat flux). (Note Ri is the square of the ratio of buoyancy frequency to the wind shear (dU/dz).)…”