The primary finding is that vertical alignment of mesoscale horizontal circulations in the lower and middle troposphere appears necessary for genesis to occur. A lack of vertical alignment creates system-relative flow that generally adds to the relative flow contributed by the environmental vertical wind shear. We separate the environmental shear from the total shear using a vorticity and divergence surgery technique that removes the contribution of the vorticity and divergence attributed to the storm. In general, the environmental shear is responsible for the misalignment, but misalignment contributes additional system relative flow.
Gaston featured extensive dry air surrounding an initially moist core. The misalignment in Gaston allowed extremely dry air in the environment to invade the middle troposphere above the lower-tropospheric circulation and severely limited the area influenced by deep moist convection, thus providing little chance of maintaining or rebuilding the vortex in sheared flow. By contrast, Karl and Matthew developed in a moister environment overall, with moisture increasing with time in the middle and upper troposphere. For Karl, deep convection was initially organized away from the lower-tropospheric circulation center, creating a misalignment of the vortex. The vortex gradually re-aligned over several days and genesis followed this realignment within roughly one day. Matthew experienced weaker shear, was vertically aligned through most of its early evolution and developed more rapidly than Karl.
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