Roll circulations in the convective region of a simulated squall line

Further insight is gained through a series of more idealized simulations wherein a specified MAUL is perturbed by analytic potential temperature perturbations. These simulations confirm classical studies based on linear analysis because the smallest perturbations grow fastest (with the exception of the very smallest scales that are affected by diffusion). The results also confirm that, with shear, updrafts oriented across the shear vector are inhibited by the shear.

An explanation for the 3-km roll spacing also emerges from these simulations. The argument focuses on the perturbations that exist in the cold pool underneath the MAUL; they induce pressure fields that extend upward into the overlying MAUL. The perturbations with large horizontal scale have pressure fields that extend farther vertically and with a greater amplitude, and thus are more effective at initiating motions in the overlying MAUL.

The convective scale that ultimately emerges within the MAUL is somewhere between two scales, whereby comparatively large scales are perturbed more strongly by perturbations in the cold pool, but the comparatively small scales grow faster.