Factors Controlling the Gross Moist Stability of Tropical Convection

The gross moist stability, a concept invented by Isaac Held and David Neelin, is instrumental in the dynamics of tropical weather systems. For instance, one hypothesis about the Madden-Julian oscillation is that it is driven by the existence of small or even negative gross moist stability in certain limited regions of the disturbance. Evidence is accumulating that tropical cyclone formation is similarly aided by small gross moist stability within the developing system. Low gross moist stability is important because it is proportional to the ratio of the net moist entropy source (surface fluxes minus radiative cooling) to the latent heating rate. Thus, smaller gross moist stability corresponds to greater heating per unit entropy forcing.

Analysis of observations from recent field programs on tropical cyclogenesis has added greatly to our understanding of the factors controlling gross moist stability. Two factors are of particular importance, the saturation fraction, or the ratio of tropospheric precipitable water to saturated tropospheric precipitable water, and the moist convective instability of the troposphere. The latter is the more important factor and is represented in our work by an “instability index” which is equal to the difference between the saturated moist entropy in the 1-3 km layer and the 5-7 km layer. Zero instability index is consistent with a moist adiabatic profile while positive values represent moist convective (not conditional) instability. Our somewhat counter-intuitive key result is that smaller instability index results in more bottom-heavy convective mass flux profiles and lower values of the gross moist stability.

What promotes smaller instability index? Our tropical cyclogenesis work indicates that the primary factor in lowering the instability index is the existence of a layer of strong positive potential vorticity at middle levels. This configuration is well known to produce a warm anomaly above the potential vorticity layer and a cool anomaly below, corresponding to a lower than normal value of the instability index. The potential vorticity distribution is itself affected by the convection, resulting in a complex, two-way interaction between convection and larger scale tropical disturbances. For instance, a mid-level potential vorticity anomaly could be produced by mid-level convergence associated with the top-heavy convective mass fluxes typical of weakly disturbed tropical conditions. The potential vorticity anomaly could then in turn produce bottom-heavy convection with maximum convergence near the surface, resulting in low-level spinup and possible tropical cyclone formation.

These results better define the dynamics of the moisture mode instability, which only occurs when the gross moist stability is small or negative.