A case study of a continental mesoscale convective vortex that developed attributes of an incipient tropical disturbance

Continental mesoscale convective vortices (MCVs) are warm core systems in the middle and lower troposphere that have been documented to develop in the stratiform region of mesoscale convective systems. A cyclonic vorticity maximum is characteristically found in mid-levels in association with a diabatic heating maximum. An anticyclonic vorticity maximum can be found in near the surface and is regulated by the strength of the near-surface cold pool. In a small subset of continental MCVs, the mid-level cyclonic circulation is observed to build downward toward the surface in conjunction with the regeneration of organized deep moist convection within the cyclonic circulation of the MCV. It has also been postulated that low-level cyclonic circulation growth beneath the mid-level MCV can grow in situ, in the presence of deep moist convection, through vorticity generation by convergence and stretching. To the extent that the cyclonic circulation of the MCV is balanced, a sea-level pressure minimum can develop below the MCV. These processes may occur more readily when the underlying surface is a warm ocean (sea-surface temperature > 26 C) rather than land.

During the period 5-14 June 2003, a subtropical jet (STJ) was positioned from just south of Hawaii east-northeastward to the southwest United States (US), then northeastward to the North Atlantic. The STJ provided a freeway for embedded upper-level disturbances to propagate from the eastern Pacific to the south-central US. These disturbances enhanced convective development as they crossed the Rockies and encountered unstable air. At 0000 UTC 10 June 2003 (00Z/10), and upper-level disturbance embedded in the STJ enhanced convective development as it crossed the Rockies and encountered unstable air in eastern New Mexico and western Texas. A mid-level cyclonic disturbance formed in the trailing stratiform region of this convection and moved northeast toward Oklahoma. This mid-level cyclonic disturbance developed into a tropospheric deep MCV with an evident surface circulation by 12Z/11 over northern Arkansas and southern Missouri. The uniqueness of this MCV lies in its longevity, as it lasted several diurnal heating cycles, its penetration to the surface and its transition into a frontal cyclone as its low-level circulation interacted with existing baroclinicity over the Great Lakes by 00Z/13.

The purpose of this presentation is to document the structure and life cycle of a long-lived continental MCV from its incipient mid-level development, its increase in intensity and penetration downward to the surface as it acquired some characteristics of a tropical disturbance, and its final transformation into baroclinic cyclone by a very unconventional pathway. Emphasis will be placed on determining the intermediate stage of the MCV's life cycle when it possessed some attributes of a tropical disturbance, prompting a comparison of warm season continental MCVs with oceanic MCVs over warm sea surface temperatures in the early stages of tropical cyclogenesis.