The Mesoscale Forcing of a Midlatitude Upper-Tropospheric Jet Streak by a Simulated Convective System. Part I: Mass Circulation and Ageostrophic Processes

Abstract

The mutual forcing of a midlatitude upper-tropospheric jet streak by organized mesoscale adiabatic and diabatic processes within a simulated convective system (SCS) is investigated. Using isentropic diagnostics, results from a three-dimensional numerical simulation of an SCS are examined to study the isallobaric flow field, modes of dominant ageostrophic motion, and stability changes in relation to the mutual interdependence of adiabatic processes and latent heat release. Isentropic analysis affords an explicit isolation of a component of isallobaric flow associated with diabatic processes within the SCS. Prior to convective development within the simulation, atmospheric destabilization occurs through adiabatic ageostrophic mass adjustment and low-level convergence in association with the preexisting synoptic-scale upper-tropospheric jet streak. The SCS develops in a baroclinic zone and quickly initiates a vigorous mass circulation. By the mature stage, a pronounced vertical couplet of low-level convergence and upper-level mass divergence is established, linked by intense midtroposphoric diabatic heating. Significant divergence persists aloft for several hours subsequent to SCS decay. The dominant mode of ageostrophic motion within which the low-level mass convergence develops is the adiabatic isallobaric component, while the mass divergence aloft develops principally through the diabatic isallobaric component. Both components are intrinsically linked to the convectively forced vertical mass transport. The inertial diabatic ageostropiiic component is largest near the level of maximum heating and is responsible for the development of inertial instability to the north of the SCS, resulting in this quadrant being preferred for outflow. The inertial advective component, the dominant term that produces the new downstream wind maximum, rapidly develops north of the SCS and through mutual adjustment creates the baroclinic support for the new jet streak. The results establish the synergistic relationship between the synoptic and mesoscale ageostrophic flow in the organization and maintenance of the SCS, as well as in the subsequent three-dimensional modification of the environmental flow field. The findings reinforce the need for operational models to accurately and explicitly simulate the effects of organized midlatitude convection on the larger-scale environment.