Theoretical and numerical studies of organized convective lines

Abstract

An extensive study of organized convection is carried out through an analytical approach and numerical modeling, with specific emphasis on the dynamics of two-dimensional convective lines. A hierarchical generalization of dynamic models is made by adding more realism and by introducing a distinct overturning-type jump updraft. The basic form of the momentum flux profile displays a striking similarity in the models. Shear in the jump inflow and baroclinic vorticity generation only alter the detailed shape of the momentum flux, but they have significant influences on selecting convective regimes. Based on the 22 and 23 June soundings during COPT81 experiment, a series of simulations are conducted with a two-dimensional version of the non-hydrostatic cloud model. Two distinct mechanisms associated with the generation of convective cells are identified: the cold outflow forcing from decaying cells and the initiation by gravity waves. The role of ice microphysics depends on environmental conditions. For the case of strong convective instability, ice impact is important to the system-scale structure but not essential to the convective-scale dynamics. On the other hand, in the situation of weak convective instability, ice effect is crucial to reproduce the observed storm. There is a profound universality in the vertical flux of line-normal momentum in the simulations. The archetypal model is capable of replicating the shape of the line-normal momentum flux reasonably but underestimates the amplitude. The application of the buoyant-type model considerably improves the momentum-flux prediction. The eddy transport is almost independent of the shear in both the initial and domain-averaged wind profiles. In contrast, in the line-parallel direction the eddy transport behaves largely in a down-gradient sense. The Schneider-Lindzentype formulation fails to represent the line-normal momentum flux but predicts the along-line momentum flux well. An idealized model is constructed to examine the density current in non-conservative fluids and sheared environments. It is shown that the incorporation of a stable stratification enhances the depth and propagation of density currents; the opposite behavior is found in buoyant fluids. In a sheared environment the density current deepens and moves faster if the shear changes from negative to positive.