A re-evaluation of the role of subsidence in valley and basin warming

The role of thermally driven upslope flows and associated compensating subsidence in the diurnal warming of valley and basin atmospheres is re-investigated. Based on theoretical analysis and numerical modeling it is shown under which conditions the redistribution of heat by the slope flow circulation becomes a significant factor in the valley/basin temperature evolution.

If a slope flow is close to equilibrium (a near balance between diabatic heating and along-slope advection), it can be shown that the warming due to subsidence is equivalent to a warming by direct horizontal heat input, e.g. by detrainment of overheated air from the slope towards the interior of the valley. In the real atmosphere, slope flows are close to the equilibrium state if the terrain is steep and homogenous, and the atmospheric stratification strong. If these conditions are not met, valley warming will be significantly altered by the presence of the upslope/subsidence overturning circulation.

Results of a simple numerical model of horizontal heat input and dry-adiabatic adjustment, with and without organized overturning motion, are compared with observations of inversion breakup taken during the Meteor Crater field campaign (METCRAX) conducted in Arizona in 2006. It is investigated to what extent a sequence of observed temperature profiles allows to separate the contributions from subsidence and quasi-horizontal heat input. The applicability of the METCRAX results to larger-scale basins is discussed with reference to non-dimensional atmospheric and topographical basin parameters.