Warm Core Vortex Amplification over Land

A convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented. Two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding. Based upon radar and satellite data, the detectable size of the vortex became much larger following the strong convective developments, nearly tripling its initial diameter over its three-day life cycle. During nighttime, when convection typically intensified within the vortex, movement of the system tended to slow. Following dissipation of the convection in the morning, the daytime movement accelerated. Cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well-defined minimum directly above. The vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical. Over the three-day life cycle of the system, the strength of the vortex, as measured by the magnitude of the midlevel potential vorticity maximum, steadily increased. At low levels, isentropic surfaces sloped upward from the rear of the potential vorticity anomaly into the vortex center so that relatively fast-moving low-level southwesterly flow, which was overtaking the slow-moving vortex from the rear, ascended as it approached the vortex center. Computations of the magnitude and duration of the ascent indicate that the lifting was sufficient to initiate new convection only if parcels realized the maximum possible ascent by flowing into the innermost region of the vortex circulation. In support of this interpretation, satellite observations show that new convection repeatedly developed near the vortex center instead of along well-defined surface outflow boundaries that encircled the convective system. A conceptual model describing the redevelopment mechanism is presented. Analyses of the large-scale environment of the vortex show that it formed and persisted in a deep and broad zone of southwesterly flow just upstream of a synoptic-scale ridge. At tropopause levels, a large anticyclone covered the region. Potential buoyant energy in the vortex environment typically ranged from about 1000 J kg−1 at 1200 UTC to 1900 J kg−1 at 0000 UTC. Extreme values were as large as 3500 J kg−1. Except for a low-level jet, wind speed and vertical wind shear were relatively small throughout the troposphere, especially in the vortex-bearing layer (700–300 mb) where shear values were only about 0.8 × 10−3 s−1. The deep midlevel layer of weak shear provided a favorable environment for the formation and persistence of the nearly axisymmetric vertical disturbance. Since the vortex formed and grew over land, this study demonstrates that warm-core mesovortex genesis and amplification do not require heat and moisture fluxes from a tropical marine surface. Evidently, ambient CAPE is sufficient for vortex formation and limited growth. However, since the vortex growth primarily occurred in the middle troposphere, and since anticyclonic outflow was usually present at the surface, marine surface fluxes may be necessary for transformation of such convectively generated vortices into surface-based tropical disturbances.