Transition regimes in valley airflows

Li, J.G.; Atkinson, B.W.. 1999 Transition regimes in valley airflows. Boundary-Layer Meteorology, 91 (3). 385-411.

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A three-dimensional, non-hydrostatic model was used to examine the dynamical characteristics of morning and evening transition periods in the atmosphere over four idealised valleys. The simulations provided detailed structure over full diurnal cycles of the valley-wind system. An essentially two-dimensional simulation (Case 1) clearly showed valley-side slope flows, driven by pressure gradients and modulated by vertical diffusion and Coriolis effects. The rotation of the wind was clockwise on both valley sides, contrary to most observations in nature. Three-dimensional simulations (Cases 2–4) rectified this feature and that for Case 4 satisfactorily modelled the valley-plain wind system throughout the diurnal cycle. Three types of transition were identified with the aid of different tools: hodographs; space-time evolution of the wind fields; and the evolution of the forcing terms in the momentum and temperature equations. Whichever type or Case was considered, the evening transition was longer than the morning one and the along-valley transition followed the along-slope one. In Cases 1 and 4 the evening transition started up to 2 h before sunset and the morning transition started up to 2.5 h after sunrise. In the three-dimensional cases the evening transition began at about 1700 and ended at about 2400, starting at the bottom of the valley and propagating up both valley sides, but at different speeds. It also started at the ground and propagated vertically. The morning transition began at about 0900 and ended at about 1100, also starting at the bottom of the valley and propagating both vertically and up the valley sides, albeit with different regimes on the two sides. The along-valley transition lagged that on the slopes by about 1.5 h. In Case 1 the forcing terms were dominated by the pressure gradient and the vertical diffusion, with the Coriolis effects introducing an along-valley component to the slope flows. The three dimensional cases were more complex, with not only the addition of the effects of advection and horizontal diffusion but also more temporal variation of more of the forcings than in Case 1.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 0006-8314
Additional Keywords: Boundary layer, Slope airflow, Numerical model, Transition, Valley airflow
Date made live: 05 May 2004 +0 (UTC)

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