Wind-Tunnel Simulation of Stable Atmospheric Boundary Layers with an Overlying Inversion

Abstract

Four cases of an overlying inversion imposed on a stable boundary layer are investigated, extending the earlier work of Hancock and Hayden (Boundary-Layer Meteorol 168:29-57, 2018), where no inversion was imposed. The inversion is imposed to one or other of two depths within the layer: midway or deep. Four cases of changed surface condition are also investigated, and it is seen that the surface and imposed conditions behave independently. A change of imposed inversion condition leaves the bottom 1/3 of the layer almost completely unaffected; a change of the surface condition leaves the top 2/3 unaffected. Comparisons are made against two sets of local-scaling systems over the full height of the boundary layer. Both show some influence of the inversion condition. The surface heat flux and the reduction in surface shear stress, and hence the ratio of the boundary-layer height to surface Obukhov length, are determined by the temperature difference across the surface layer (not the whole layer), bringing all cases together in single correlations as functions of a surface-layer bulk Richardson number.

Wind-Tunnel Simulation of Approximately Horizontally Homogeneous Stable Atmospheric Boundary Layers

“…2, where the stresses tend to increase with the streamwise distance X, rather than decrease, but the prime consideration here is the behaviour in the stable simulation cases. As observed in Hancock and Hayden (2020), the stresses in the centre of the layer decrease with the imposition of an inversion, more so for the deep inversion. Although there is some change in the profiles of shear stress and heat flux, the surface values vary little with either Fig.…”

“…The working-section inlet-temperature profiles Θ IN (z) for cases 2-4 are shown in Fig. 1, where Θ 0 is the surface temperature for X ≥ 5 m. Note that case 2 corresponds to the 'final case' example given in Hancock and Hayden (2018) and cases 3-4 correspond to cases 3 and 5 in Hancock and Hayden (2020).…”

“…For heights z ≳ h∕3 the temperature profiles exhibit little variation with streamwise fetch, but for z ≲ h∕3 there is a clear reduction of temperature with increasing fetch at constant z. Hancock and Hayden (2020) found that imposing an inversion has, in fact, no more than a minor effect on the flow for z ≲ h∕3 . Most striking was no change in the heat-flux profiles.…”

“…In that study, the temperature above the boundary layer was constant (as also in Caughey et al 1979). In the second investigation, Hancock and Hayden (2020) give results for the effect of imposing an overlying inversion compared with the case of no inversion (as in the 2018 paper). These, too, compare favourably with the scaling frameworks of Nieuwstadt (1984) and Sorbjan (2010).…”

“…The streamwise development of two cases of a stable boundary layer with an overlying inversion is investigated, with baseline cases of no inversion and neutral flow. Given that Hancock and Hayden (2020) found that the depth or 'penetration' to which the inversion is imposed has a much greater effect than the strength of the inversion (of 20 K m −1 and 40 K m −1 ), only one inversion gradient is employed here (of 20 K m −1 ). The only other systematic study of the effect of an overlying inversion, as far as we are aware, is that of Ohya and Uchida (2003).…”