WARM-CORE VORTEX AMPLIFICATION OVER LAND

A convectively generated mesoscale vortex that was instrumental in ini tiating and organizing five successive mesoscale convective systems ov er a period of three days is documented. Two of these convective syste ms were especially intense and resulted in widespread heavy rain with localized flooding. Based upon radar and satellite data, the detectabl e size of the vortex became much larger following the strong convectiv e developments, nearly tripling its initial diameter over its three-da y life cycle. During nighttime, when convection typically intensified within the vortex, movement of the system tended to slow. Following di ssipation of the convection in the morning, the daytime movement accel erated. 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 e ssentially colocated and the system was nearly axisymmetric in the ver tical. Over the three-day life cycle of the system, the strength of th e vortex, as measured by the magnitude of the midlevel potential vorti city maximum, steadily increased. At low levels, isentropic surfaces s loped 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, ascen ded 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 conve ctive system. A conceptual model describing the redevelopment mechanis m is presented. Analyses of the large-scale environment of the vortex show that it formed and persisted in a deep and broad zone of southwes terly flow just upstream of a synoptic-scale ridge. At tropopause leve ls, a large anticyclone covered the region. Potential buoyant energy i n the vortex environment typically ranged from about 1000 J kg-1 at 12 00 UTC to 1900 J kg-1 at 0000 UTC. Extreme values were as large as 350 0 J kg-1. Except for a low-level jet, wind speed and vertical wind she ar were relatively small throughout the troposphere, especially in the vortex-bearing layer (700-300 mb) where shear values were only about 0.8 X 10(-3) s-1. The deep midlevel layer of weak shear provided a fav orable environment for the formation and persistence of the nearly axi symmetric vertical disturbance. Since the vortex formed and grew over land, this study demonstrates that warm-core mesovortex genesis and am plification do not require heat and moisture fluxes from a tropical ma rine surface. Evidently, ambient CAPE is sufficient for vortex formati on and limited growth. However, since the vortex growth primarily occu rred in the middle troposphere, and since anticyclonic outflow was usu ally present at the surface, marine surface fluxes may be necessary fo r transformation of such convectively generated vortices into surface- based tropical disturbances.