Upscale Feedbacks During the Mesoscale Predictability Experiment

One of the two main objectives of the Mesoscale Predictability Experiment (MPEX) was to pursue fundamental scientific questions of convective storm–environmental feedbacks and predictability. Specifically, we proposed to: 1) quantify the observed environmental modifications and upscale feedbacks from deep convection, and relate these back to the characteristics of the convection; 2) evaluate model simulations of upscale feedbacks from deep convection with MPEX observations; and 3) explore the predictability of convectively disturbed atmospheres.

In support of these proposed research activities, 15 missions were completed by the ground-based radiosonde (or “upsonde”) teams during the MPEX field phase (15 May to 15 June 2013), with approximately 225 soundings collected. We selected four of these cases for in-depth analysis: the 19 May, 20 May, and 31 May tornadic supercells, and the 29 May squall line-bow echo. These cases featured particularly well-coordinated soundings of the near-storm environment, in addition to pre-convective environmental soundings. To facilitate interpretation of the sounding data, each case was simulated using CM1, a 3D idealized cloud model, initialized with relevant a pre-convective sounding.

The basic conclusion from the MPEX sounding analysis and complementary model simulations is that the most significant upscale feedback of the three supercell thunderstorms was in the form of a surface-based cold pool. This was manifested as a marked reduction of convective available potential energy (CAPE), and as a modification of deep-layer vertical wind shear. Thus, the strength, areal extent, and depth of the cold pool were critical to the upscale feedback, and highly influenced how rapidly the local atmosphere returned to an undisturbed state. Accordingly, such an influence of the squall line was much larger than that of the supercells, because the squall line had a more expansive cold pool.

Significant feedbacks aloft tended to be transient, moving with the storm rather than trailing behind in the wake or otherwise extending well beyond its confines. For example, the most significant modifications to vertical shear owed to storm-induced (e.g., mesocyclonic) enhancements of the mid-level winds. Similarly, mid-level lapse rates were modified mostly within updraft (and downdraft) regions, which fostered vigorous diabatic and adiabatic heating and cooling. The more expansive updraft of the squall line resulted in a lapse-rate reduction on a larger horizontal scale.