Convective and rotational instabilities in moist air

Severe weather events are well known to be strongly associated with heavy cloud systems. Hydrodynamic description of instability responsible for these phenomena is obvious to take explicitly into account thermodynamics of moist air. The present work is meant to investigate the basic hydrodynamic motions in moist air in the framework of the thermodynamic approach with the latent heat release of phase transformations in moist atmosphere. The usual hydrodynamic phenomena known to take place in dry air, such as sound wave propagation and convection, prove to be affected by the latent heat release, and the threshold of the convective instability in the presence of water phase transformations is observed to decrease. In the presence of rotation, however, the water phase transformations appear to give rise to a new instability with the mechanism not associated with archimedean forces, realizing a positive feedback loop between vertical pressure drop and vertical gas flux with intensive enough Coriolis force present. This instability can be responsible for severe weather events, such as tornadoes and tropical cyclones.