What Controls Timing of Deep Convective Onset in the Tropics? (Hint: Much more than only an environmental thermodynamic profile.)

On long time scales and/or large spatial scales, precipitation rate in the Tropics has long been known to be related through a power law to tropospheric water vapor. At the root of this relationship is the impact of environmental water vapor on the dilution of buoyancy inside the deep convective updrafts where condensation occurs. Moist environments tend to support deep convection because when updrafts entrain the moist environmental air, their buoyancy is not as diluted as that of an identical updraft in a dry environment. This simple logic has ramifications for deep convective onset on the scale of an individual cloud at hundreds of meters up to onset of the Madden-Julian Oscillation, which coincides with the deepening and growth of numerous convective elements spread out over thousands of kilometers. Also, the idea of “recharging” the atmosphere—from diurnal to weekly time scales—is partially rooted in the idea that the atmosphere becomes gradually moister over time and therefore gradually more supportive of deep convection. Observations, however, indicate that a moist environment is a necessary but not itself sufficient condition for deep convective growth.

Recent literature has stressed the important of updraft size on cloud deepening. The fundamental hypothesis is that updrafts are more shielded from dilution by unsaturated non-cloudy air—and therefore more likely to continue ascending—as their width increases. In this presentation, results are shown from an LES simulation of a population of tropical marine convection that support several conclusions: 1) Deepening updrafts are wider and less diluted than non-deepening updrafts, 2) There exists a “critical” size to which an updraft must widen to before it can deepen (akin to a critical environmental humidity or buoyancy), and 3) This “critical” size probably depends on the environmental humidity, becoming smaller as humidity increases. A Lagrangian analysis of parcel trajectories run online during LES integration revealed properties of fluid before it entered deepening cloudy updrafts. For example, compared to parcels that topped out in non-deepening shallow updrafts, parcels that ultimately ascended in-cloud to the upper troposphere started out in wider boundary layer updraft eddies below cloud base. Additionally, the parcels that ultimately ascended in deep convection were located in areas of larger sub-cloud layer convergence (or less divergence) compared to parcels that entered cloud but did not ascend. The documented correlation between sub-cloud layer convergence and eddy size drives a new unconfirmed hypothesis that boundary layer convergence magnitude can impact the size of eddies that ultimately grow into clouds. If true, then this could point to a mechanism in which environmental humidity sets the threshold for minimum updraft size required for deep convection while boundary layer convergence controls whether that updraft size is achieved.