Controls on Draft Size and Cold-pool Characteristics

Trapp, R. J., University of Illinois at Urbana-Champaign

Cloud Processes

Convective Processes

Marion G and R Trapp. 2019. "The Dynamical Coupling of Convective Updrafts, Downdrafts, and Cold Pools in Simulated Supercell Thunderstorms." Journal of Geophysical Research: Atmospheres, 124(2), 10.1029/2018JD029055.

Science

In thunderstorms, wider updraft cores result in wider downdraft cores, which subsequently lead to the development of deeper and stronger cold pools.

Impact

This work highlights the importance of including the effects of the environmental winds in "parameterized" representations of convective clouds and storms within weather and climate models. It also reveals that processes internal to convective clouds have a strong "two-way" coupling instead of the "one-way" coupling typically considered.  

Summary

Much attention has been focused recently on convectively generated cold pools, especially given  their documented roles in the initiation, inhibition, and organization of deep moist convection. Our group has been particularly interested in how the salient cold-pool characteristics relate back to the characteristics of the convective storm and, in turn, to the undisturbed environment.

A set of idealized numerical simulations of highly organized convective storms shows strong support of our hypothesis that wider updraft cores result in wider downdraft cores, which subsequently lead to the development of deeper and stronger cold pools. These inter-relationships are highly sensitive to the environmental buoyancy and vertical wind shear, with large convective available potential energy, strong wind shear, and a deep mixed layer ultimately contributing to the largest drafts and thus the deepest (and strongest) cold pools. Diagnoses of the forcings of vertical accelerations are used to understand the convective dynamics underlying these sensitivities. We find that draft width (and therefore cold-pool depth) is influenced early in the storm life cycle by the buoyancy forcing and especially the linear dynamic forcing: this owes to a well-known interaction between the newly initiated updraft and the environmental shear. Draft width later in the storm life cycle is controlled mostly by the nonlinear dynamic forcing -- specifically, that due to low pressure induced by rotation about vertical and horizontal axes.

These results, overall, have implications for the development (or improvement) of cold-pool parameterizations, as well as of convective parameterizations in general.  Some follow-on work – which includes the use of data collected during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign – is now showing that these inter-relationships are also critical in promoting the upscale growth of discrete convective entites into mesoscale convective systems.