How Are Vorticity Rivers in Supercell Storms Produced and Are They Important Vorticity Sources for Tornadoes?

Abstract A vorticity river is defined as a near-ground vertical vorticity band often extending from the north side of a supercell toward an incipient or mature tornado. Three successive vorticity rivers in a numerical simulation of a supercell tornado are found after the tornado has fully formed. Air parcels that enter the vorticity rivers first ascend in the inflow region of the supercell south of the forward-flank convergence boundary (FFCB) while moving northwestward. Upon approaching the downdraft region north of the FFCB, the air parcels start to descend and turn westward and then southward toward the tornadic region, converting the crosswise vorticity that the parcels initially carry into the streamwise vorticity. The horizontal streamwise vorticity of the parcels is enhanced by horizontal stretching when the flow accelerates toward the tornadic region. Near-ground positive vertical vorticity develops via upward tilting of horizontal streamwise vorticity as the air parcels descend along a gradually decreasing slope. As the parcels enter the region of strong surface convergence generated by downdrafts of varying intensity on both sides of the vorticity river, their vertical vorticity is further intensified by upward tilting of horizontal vorticity and vertical stretching. The calculated influxes of horizontal and vertical vorticity into the tornadic region show that the contribution to the tornado vorticity by the vorticity rivers is minimal, given their narrow width and shallow depth. Backward trajectory analysis shows that only about 7% of total parcels entering the tornado originate from vorticity rivers, suggesting that their direct contribution to tornado intensification or maintenance is insignificant. Significance Statement A vorticity river is a narrow band of vertical vorticity extending from the north side of simulated supercell storms southward toward an intensifying or mature tornado. The source of vorticity in the vorticity river and its contribution to tornado formation and maintenance are examined based on a tornado simulation. Trajectory analyses show that the vertical vorticity in vorticity rivers is generated first by upward tilting of horizontal vorticity and then intensified rapidly by near-ground vertical stretching due to along-river convergence. Budget calculations of vorticity into the tornadic region show that the contribution of the narrow and shallow vorticity rivers to the intensification and maintenance of the tornado is limited. Only a very small fraction of air parcels entering the tornado passes through the rivers. The primary source of both vertical and horizontal vorticity inflow into the tornadic region comes from the occlusion zone on the northeast side of the tornado.