O the Interaction of Baroclinic Instability and Cumulus Convection in Polar Air Stream Cyclones.

A linearized stability analysis of some idealized and observed atmospheric jet flows has been performed. The semigeostrophic approximation was used and a cumulus parameterization based on low-level moisture convergence included. Calculations using idealized basic states showed that the nature of the instability, i.e. whether it is dominated by baroclinic instability or by release of latent heat through conditional instability of the second kind, is determined by the constant of proportionality between heating and convergence. For disturbances dominated by baroclinic processes, the effects of diabatic heating are equivalent to a reduction in static stability. It is possible for latent heat to be the dominant source of disturbance energy even when the structure is that of a baroclinic wave, although in this case the mode will have significant amplitude only in the region where heat is released. The effects of heating were found to be greater in regions where the background static stability was small. Thus the low vertical stability found on the cold-air side of the jets where comma clouds typically form gives an environment where heating is especially effective in producing small scale disturbances. Calculations of the instabilities of two observed jets showed similar behavior. The wavelength and structure of the modes agreed well with those of the comma clouds which were observed to form.