The Denver Cyclone. Part II: Interaction with the Convective Boundary Layer

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 su...