Numerical Simulation of the Meso-β Scale Structure and Evolution of the 1977 Johnstown Flood. Part II: Inertially Stable Warm-Core Vortex and the Mesoscale Convective Complex

Abstract

A mesoscale warm-core vortex associated with the mesoscale convective complex (MCC) that produced the 1977 Johnstown flood is examined using a three-dimensional nested-grid model simulation of the flood episode. In the simulation, the vortex plays a key role in determining the evolution of the MCC, a squall line, and the distribution of heavy precipitation. The vortex has a space scale of 100?200 km in diameter and a time scale of more than 18 hours. Its low pressure center extends from the midtroposphere down to the surface, and its maximum vorticity occurs between 850 and 700 mb. A pool of cool moist downdraft air develops in the surface to the 850 mb layer beneath the warm core, while a cold dome forms in the vicinity of the tropopause above the warm core. Following forcing from repeated deep convection prior to model initial time, the vortex is initiated by mesoscale ascent associated with a traveling meso-α scale wave. Genesis takes place in a nearly saturated, slightly conditionally unstable environment with weak horizontal deformation and vertical shear. The vortex is then energetically supported primarily by latent heat release from stratiform (resolvable-scale) cloud condensation in the low- to midtroposphere. In the decaying stage, the vortex is maintained by inertial stability. The evolution of the warm-core mesovortex appears to depend upon the concurrent development of deep convection and the mesoscale flow structure. In particular, moist downdrafts play an important role in controlling the strength of the vortex and the amount of resolvable-scale rainfall. Associated with the mesovortex, an intense vertical circulation with strong low-level convergence and upper-level divergence develops. In addition, a strong cyclonic circulation extends to 300 mb where a changeover to anticyclonic circulation occurs. It is found that equivalent potential temperature and the horizontal momentum are nearly uniformly distributed in the immediate environment of the vortex. The resultant weak horizontal deformation provides an important energy-preserving mechanism for the maintenance of the warm-core structure while inertial stability of large tangential winds helps the longevity of the vortex circulation. At upper levels, a mesohigh with strong anticyclonic outflow develops above the vortex. The mesohigh behaves like an ?obstacle,? forcing the horizontal environmental wind flow around it. To the northeast of the upper level mesohigh, a northwesterly jet streak develops between the strong anticyclonic outflow and a baroclinic zone farther north. The results suggest that successful prediction of the evolution of mesoscale convective weather systems not only hinges upon the convective parameterization, but also depends upon the model's ability to reproduce the timing and location of resolvable-scale condensation. The resolvable-scale phase changes, and associated latent heat release, strongly affect the mesoscale circulation and contributes about 30% to 40% of the total precipitation from the mesoscale convective systems.