Processes Coupling Tropical Deep Convection to the Large-Scale Environment in a Plume Buoyancy-Based Framework

Abstract:

Observations have shown that tropical convection is influenced by fluctuations in temperature and moisture in the lower free-troposphere (LFT, 600--850 hPa), as well as moist enthalpy (ME) fluctuations beneath the 850 hPa level, referred to as the deep boundary layer (DBL, 850--1000 hPa). A framework is developed that consolidates these three quantities within the context of the buoyancy of an entraining plume. A ``plume buoyancy equation" is derived based on a relaxed version of the weak-temperature gradient approximation. It unifies the processes that have been theorized to lead to convective coupling in convectively-coupled waves such as surface fluxes, moisture advection and adiabatic lifting. Analysis of this equation reveals that vertical moist static energy (MSE) advection by convection and adiabatic compression from gravity waves eliminate convective instability on a timescale of 2-4 hours, consistent with the concept of convective quasi-equilibrium. Under convectively-suppressed conditions, DBL sources of ME are largely balanced by vertical MSE advection at the top of the DBL. In spite of this balance, a small positive buoyancy tendency is observed, suggesting a slow buoyancy recharge timescale of several days. Scale analysis of buoyancy tendency for synoptic-scale (horizontal scale ~ 1000 km) tropical motion systems indicates that these systems modulate buoyancy at a comparable rate to the buoyancy recharge timescale. Because the plume buoyancy equation can explain convective coupling from both temperature and moisture fluctuations, it can be used as an alternative to the commonly-used moist static energy budget, which underestimates the importance of small LFT temperature fluctuations in convection.