Abstract
The effect of surface heating on the flow past an isolated obstacle is examined with the aid of a nonlinear numerical model. These simulations extend the results of Part I, which considered the adiabatic, stratified flow around the obstacle. When the obstacle is heated, substantial low-level shear develops in the lee as the flow converges at low levels and diverges above. A linear model is developed to explain some of the details of this shear pattern. In this model, vertical shear is produced by differential heating and removed by mixing.
Some of the small-scale circulations that develop in the convective boundary layer are then discussed. A thermal instability predominates in the lowest levels of the boundary layer with its axis aligned along the low-level shear vector. Higher in the boundary layer, a transverse mode appears and breaks the thermal instability into three-dimensional maxima. The transverse nature of this mode, the existence of an inflection point, and the low Richardson number suggest that this mode is a shearing instability.
The convergence/vorticity zone in the lee of the obstacle (described in Part I) is then examined in detail. Several small-scale vortices develop along this zone at points where the thermal instabilities intersect. Observational studies have indicated that these boundary layer vortices often spawn tornadoes. It is shown that the vertical vorticity in these circulations is due to stretching of the preexisting vorticity along the convergence zone.
The small-scale circulations in the boundary layer force a gravity wave response (with λ∼10 km) in the stratified atmosphere above. The vertical velocity in these waves exceeds 1 m s−1 in certain regions of the flow. A model is developed to explain how the boundary layer eddies with horizontal scales of ∼2–4 km can force a 10 km wave response above. This model depends on the fact that the vertical group velocity is inversely proportional to the horizontal wavelength as well as on a feedback process in which the gravity waves modulate the boundary layer eddies.
