Convective lifecycles and convective self-amplification via below-threshhold gross moist stability

During the growth phase of tropical deep convection, the moisture convergence associated with the convection is typically larger than the precipitation, leading to moistening and further enhancements in deep convection. Conversely, during the decaying phase of deep convection, precipitation is usually larger than moisture convergence and hence the atmosphere dries and is less conducive to deep convection. We examine how a version of the gross moist stability (GMS) and the concept of moisture-convection feedbacks can be used to describe and quantify this aspect of convective lifecycles.

The GMS, a concept originated by Neelin and Held in 1987, has been used to describe the relationship between large-scale forcing and tropical moist convection in simple models and observations (the latter more recently). More specifically, GMS can be defined as "the ratio of the vertically integrated horizontal divergence of some intensive quantity conserved in moist adiabatic processes and a measure of the strength of moist convection per unit area" (Raymond 2009). We utilize a version of this definition, the approximation that tropospheric temperatures are relatively constant once the diurnal cycle is averaged over and a moisture-precipitation relationship.

Using these approximations, we find that when the GMS is less (greater) than a threshold value, convection is unstable (stable), causing further convective enhancement and precipitation increases (decreases). We use TOGA-COARE field campaign observations to test this idea and examine the evolution of the amount of precipitation and the vertical profile of winds compared to the GMS and threshold GMS. We define the different stages of the lifecycle of moist convection using the GMS.