A new effective static stability for eddies in a moist atmosphere

Water vapor directly affects atmospheric circulations through the release of latent heat. But it is difficult to include latent heat release in dynamical theories because of the associated nonlinearity (precipitation generally occurs where there is upward motion). A new effective static stability is derived that fundamentally captures the effect of latent heat release on moist eddy circulations. It differs from the usual dry static stability by an additive term that depends on temperature and a parameter measuring the up-down asymmetry of vertical velocity statistics. Latent heat release reduces the effective static stability experienced by eddies, but cannot reduce it to zero so long as there are non-precipitating regions of the eddies.

Applications of the effective static stability are illustrated using the seasonal cycle in reanalysis and climate-change simulations with an idealized GCM. The effective static stability is shown to help account for changes in the thermal stratification of the extratropical troposphere, the extent of the Hadley cells, the intensity of extratropical transient eddies, and the extratropical eddy length.