Storm-relative flow and its relationship to low-level vorticity in simulated storms

Using convective storm simulations from an eight-dimensional parameter space study, we examine the relationship between low-level vorticity production and the orientation of storm-relative winds. It is found that when the angle between the storm-relative low-level (e.g., near-surface) and anvil-level (e.g., 10-14 km) wind is near 180 degrees, storm inflow tends to be roughly parallel to the edge of the storm's main precipitation shield. For this case, baroclinically generated horizontal vorticity that is induced along the forward flank outflow boundary is purely streamwise, and can then be ingested by the updraft, allowing an increase in low-level vertical vorticity to occur. When this angle is greater than 180 degrees, storm-relative inflow trajectories emanate from the direction of the precipitation footprint and thus from rain-cooled air. In this scenario, the storm effectively undercuts itself and low-level rotation is inhibited. When the angle is less than 180 degrees, which implies a storm motion that deviates far off the hodograph, low-level rotation is also inhibited. The findings are robust even when an estimate is used in lieu of the actual storm motion vector, and are also relatively insensitive to the choice of anvil layer. The results point to key storm--environment relationships that help regulate the potential strength of a storm's low-level mesocyclone.