Lifting by convergence lines

The lifting depth of a convergence line in an unstratified boundary layer b eneath a stably stratified atmosphere is examined with both analytical and numerical models. Cases are considered with and without Row in the layer ab ove the convergence line. Three different stability profiles above the boun dary layer are also considered: an inversion, continuous stratification, an d a combination of the two. For the case in which there is no how above the convergence line, analytica l solutions are obtained for the Lifting depth for the three different stab ility profiles. Simulations of the Row with a nonlinear, nonhydrostatic mod el show good agreement with these analytical predictions. The presence of flow in the upper layer increases the complexity of the pro blem due to the presence of gravity waves in the steady-state solution. For an atmosphere with just an inversion, the analytical model predicts that, for hydrostatic Row, the depth of lifting is independent of the upper-level Row; while for nonhydrostatic conditions the lifting first increases as th e upper-level Row increases. but then reaches a maximum and subsequently de creases. For an atmosphere with continuous stratification in the upper laye r, the depth of lifting decreases with increasing upper-level flow for both hydrostatic and nonhydrostatic conditions. For the case of both an inversi on and continuous stratification, a condition is found when the damping eff ect of the continuous stratification approximately balances the amplifying nonhydrostatic effects. The numerical simulations show reasonable agreement for an atmosphere containing only an inversion; however, for the case of c ontinuous stratification, shearing instabilities develop along the interfac e at the top of the boundary layer that make it difficult to compare with t he analytical predictions. These instabilities are reduced by the presence of an inversion at the top of the convergence line, and in the combined cas e of continuous stratification and an inversion, there is again reasonable agreement with the analytical predictions.