Effective Buoyancy, Inertial Pressure, and the Mechanical Generation of Boundary-Layer Mass-Flux by Cold Pools
Unresolved questions about the dynamics of convective clouds can be lumped into two broad categories, namely, how do these clouds get created, and how do they evolve once created? For the first process, there are two possibilities: (1) Pools of warm, humid air at the surface launch off the surface under the force of their own buoyancy. (2) Pools of warm, humid air are forced off the surface by other, colder pools of air that collide with them.
Which process dominates? To find out, researchers derived a decomposition of forces that cleanly separates between these two effects: effective buoyancy (driven by buoyancy alone) and the inertial acceleration (driven by the motion of the fluid alone). Solving for these two terms requires solving a Poisson equation, which was done in the context of high-resolution, large-eddy simulations of deep convection. The results are unambiguous: air parcels are launched off the surface by the forcing from colder pools that collide with them, not by the force of their own buoyancy. This finding is a critical piece of input for convective parameterizations in global climate models.
Jeevanjee, N., and D. M. Romps. 2015. “Effective Buoyancy, Inertial Pressure, and the Mechanical Generation of Boundary-Layer Mass Flux by Cold Pools,” Journal of the Atmospheric Sciences 72, 3199–3213. DOI: 10.1175/JAS-D-14-0349.1.