Mass and Angular Momentum Diagnostics of Cyclones within Numerically Simulated Linear Baroclinic Waves

Abstract

The mass and angular momentum budgets of extratropical cyclones within three numerically simulated adiabatic linear baroclinic waves are investigated through quasi-Lagrangian diagnostics. The waves analyzed, wavenumbers 5, 11 and 17, were chosen to represent a range of baroclinically active horizontal scales observed in the atmosphere within which cyclonic circulations develop. Results are compared with diagnostics of an observed atmospheric cyclone in order to examine the extent to which linear baroclinic instability theory explains extratropical cyclone development in the real atmosphere. Diagnostics are computed in both isobaric and isentropic coordinates. In the isobaric framework, inward mass and angular momentum transport occurs in the lower troposphere of the linear cyclones while the transport is outward in the upper troposphere Upward transport of these quantities throughout the troposphere redistributes these quantities vertically. Within isobaric coordinates the mass and angular momentum budgets of the cyclones within linear waves resemble results from actual cyclones. Analysis of these linear model cyclones in isentropic coordinates shows that the mass and angular momentum transport is inward in upper isentropic layers and outward below. Angular momentum is transferred from upper to lower isentrople layers by pressure and inertial torques within the baroclinic structure. The vertical distribution of transport process and torques within linear baroclinic wave is in general agreement with the results occurring during the dry-baroclinic phase of development in a 1971 Alberta cyclone but differs markedly from the distribution observed during the moist-baroclinic phase. The major dissimilarity between the development of the model cyclones and leeside dry-baroclinic cyclogenesis is the existence of a much stronger eddy component of horizontal angular momentum transport in the model cyclones. A strong eddy transport is required for the cyclones to simultaneously deepen and spin-up over the flat terrain in the linear model, while such a transfer is not necessary for development over sloped topography.