LAUSR

The evolution of an idealized tropical-cyclone-like vortex forced by a prescribed distribution of diabatic heating rate and near-surface frictional force is studied using a recently developed, prognostic, axisymmetric balance model. Starting with a prescribed tangential wind field, a series of calculations is carried out in which the strength of the diabatic heating rate is varied while keeping the formulation for the frictional force the same. When the strength of the heating rate falls below a certain value, the secondary circulation it generates is no longer able to oppose the tendency of the boundary layer to produce a layer of outflow in the lower troposphere, above the boundary layer. In these circumstances, this net outflow advects absolute momentum surfaces outwards above the boundary layer and the tangential velocity there declines. In the balance model, and presumably in reality, intensification of the tangential velocity requires the diabatic heating rate to be strong enough to ventilate the inflow in the frictional boundary layer, thereby averting frictionally induced outflow just above the boundary layer. In reality, the strength and location of diabatic heating will be strongly influenced by the boundary-layer dynamics and thermodynamics, a coupling not present in the current model formulation.