Abstract
A long-lived mesoscale convectively-generated vortex (MCV) associated with a mesoscale convective complex (MCC) is documented. The MCV, with a Rossby number of approximately 0.5, is investigated as a feature intrinsic to the organization of the MCC. On 6–7 July 1982 a particularly large and intense MCC developed in a region of high convective available potential energy (CAPE) but weak vertical wind shear (bulk Richardson number ∼150), and weak advection of temperature and vorticity. Convection initially was organized in a narrow line with elements propagating relative to the mean environmental flow. These elements subsequently developed a large semicircular area of stratiform precipitation and a surface mesolow to the rear. Heavy rain fell over a broad area; amounts as great as 10.9 cm accompanied by flooding were reported in central Oklahoma. As the large semicircular rain area dissipated, a three layered structure became evident: a large upper tropospheric anticyclone, a rain cooled mesoscale high pressure system at the surface with a trailing mesoscale low, and a middle level cyclonic vortex. The midlevel vortex is clearly identifiable in visible satellite imagery and in radar observations. The upper level anticyclone was initially unstable and dissipated rapidly. The midlevel cyclone, however, became inertially stable and persisted with little change for over two days in a relatively benign synoptic regime of low wind speed, weak shear and low CAPE. During this period, the MCV exhibited a deep column of convergence, positive relative vorticity and mesoscale saturated ascent with a layer of weak divergence aloft. The MCV was also accompanied by local showers and depressed afternoon surface temperatures.
The combination of high CAPE and weak vertical shear in the pre-MCC environment was conducive to the rapid formation of the large trailing stratiform cloud region in the middle and upper troposphere. Transformation of the environment from dry midlevels with high CAPE to moist midlevels and low CAPE resulted in a virtual warming that hydrostatically corresponded to about a 5–15 m height fall at midlevels. In the weak-gradient barotropic environment, perturbations of this magnitude were comparable to the height depression of the cyclonic disturbance. Following the establishment of inertial stability, the vortex was advected eastward with little change in relative vorticity for over 36 h. The presence of a surface-based cyclonic circulation during the period of inertial stability suggests that the dynamical forcing from differential vorticity advection may have been enhanced, particularly in the daytime, by frictional convergence. The combination of vorticity advection, frictional convergence and boundary layer warming was sufficient to initiate a new cycle of deep convection near the center of the vortex.
