Cold pools as the “snow plows” of convective triggering

 

The evaporation of precipitation in the sub-cloud layer of deep convective clouds cools the boundary layer, leading to negatively buoyant cold pools (CPs) that spread along the surface. New convective cells occur at increased likelihood near the gust fronts of such CPs – as was shown by observations and high-resolution simulations alike. However, pinning down whether this is due to single gust fronts or collisions of multiple ones is much less settled. Conceptual models show that this very distinction, single vs. multi-CP collisions, may be key to the character of the resulting spatial organization of deep convection, and related extreme precipitation events.

In our research group at the Niels Bohr Institute, we address these questions by using idealized large-eddy simulations (LES). The LES runs show that the maximum vertical velocities resulting from two colliding CPs are up to a factor three times higher than at an undisturbed CP front and extend higher into the atmosphere. We assume that the likelihood of triggering convection scales with the height and strength of the updrafts, generated at the gust front of the CP.

We are further interested in the extent to which such cold pool collisions are responsible for the increase in precipitation intensity during convectively active periods. To this end, we study the energy transfer in the feedback loop between ‘first generation’ precipitation events to CP(s) and ‘second generation’ precipitation events. Some preliminary results suggest that a peripheral rotating torus – often referred to as a vortex ring – is a crucial component of a cold pool due to its key role in the dynamic interaction of and between