Effective stratification for the dry and moist isentropic circulations

Pauluis at al. (2008) have recently shown that the mean circulations on dry and moist isentropes differ significantly in the midlatitudes. In particular, the total mass transport on moist isentropes is approximately twice as large as that on dry isentropes. The additional mass transport on moist isentropes is associated with a low-level flow of warm, moist subtropical air that rises into the upper troposphere within the stormtracks. This paper analyzes the relationship between isentropic mass transport and the entropy transport. Our analysis indicates that, while the dry stratification is larger than the moist stratification in the tropical regions, the two stratifications are approximately equal in the midlatitudes stormtracks.

For a direct circulation, like the Hadley circulation, with air flowing toward the equator at low level and toward the poles at high level, the effective stratification is directly tied to the vertical variations of the dry and moist entropy. In this case, the stratification for moist entropy can be written as the sum of stratification for dry entropy minus and a contribution from the latent heat content. As water vapor decreases with height, the vertical stratification for moist entropy will be smaller than that for dry entropy when the circulation corresponds to a direct overturning cell.

In contrast, when the circulation is dominated by turbulent mixing either on horizontal surfaces or potential temperature surfaces, the effective stratification should be closely related to the horizontal fluctuations of dry and moist entropy. As the horizontal variations of latent heat are larger than the horizontal variations of temperatures in the midlatitudes, one expects the moist stratification should be larger than the dry stratification.

The fact that the effective stratifications for dry and moist entropy are approximately equal indicates that the midlatitudes circulations can neither be viewed as a direct overturning cell nor as a flow dominate by horizontal mixing. Rather, it is argued here that the equivalence between the dry and moist stratification indicates that the warm moist air parcels moving from the subtropics to the stormtracks are close to be convectively unstable and ascent into the upper troposphere once they become saturated.