Effects of Diabatic Heating on the Ageostrophic Circulation of an Upper Tropospheric Jet Streak

Abstract

The interaction between the mass circulations within a mesoscale convective complex (MCC) and the entrance region of an upper tropospheric polar jet streak is examined to investigate mechanisms responsible for linking these two scales of motion. During NASA's fourth Atmospheric Variability Experiment (AVE IV), maximum wind speeds within a jet streak increased nearly 15 m s−1 over three to six hours as the jet streak propagated eastward over the Great Lakes region. Severe convection located on the rear anticyclonic flank of the jet streak within the direct circulation of the entrance region also intensified and increased in areal extent. The results analyzed within isentropic coordinates establish that latent heating in the MCC modified the direct mass circulation in the jet streak entrance region through the forcing of diabatic components of ageostrophic motion. The net isallobaric ageostrophic component in the entrance region, determined through the gradients of differential heating and mass flux, exceeded 4 m s−1. The mass divergence in the upper troposphere was due to the slight excess of the diabatic isallobaric mode over the opposing adiabatic mode, while mass convergence in the lower troposphere was due to the slight excess of the adiabatic isallobaric mode over the diabatic mode. The intensity of the other diabatically forced ageostrophic component, induced through vertical advection of momentum in a sheared environment, ranged from 5 to 10 m s−1 in the middle and upper troposphere of the jet's entrance region. Over much of the convective region, both the total isallobaric and the inertial diabatic ageostrophic components were directed from the anticyclonic to the cyclonic side of the jet streak at jet streak level in the same sense as pre-existing ageostrophic motion in the upper branch of the jet's direct mass circulation. This diabatically forced ageostrophic motion directed along the pressure gradient of the larger scale resulted in additional generation of kinetic energy which ultimately produced stronger winds in the jet streak downstream. A comparison between actual and geostrophic momentum forms for ageostrophic motion revealed discrepancies of 20 m s−1 that were mainly due to differences in the horizontal fields of inertial advective ageostrophic motion. This expected result points out that the rapid evolution of ageostrophic motion within the shorter time scales of MCCs limits the applicability of geostrophic momentum theory in prescribing the structure of ageostrophic motion.