Spatiotemporal development of irreversible mixing in midlatitude baroclinic wave life cycles: Morphology, energetics, and nonisentropic mixing activity

Abstract

The significance of nonisentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic-scale baroclinic wave life cycles along the subtropical jet stream, using a nonhydrostatic, anelastic, mesoscale model subject to a free-slip surface boundary condition. A variety of morphological features of mixing is identified such as a mesoscale columnar vortex associated with the onset of frontal fracture, episodic overturnings along the surface fronts, “wrinkling” of the tropopause, and injection of tropospheric air into the stratosphere. The evolution of the degree of nonisentropic irreversible mixing is first analyzed by computing the change of the “base” component of potential energy that cannot be converted into kinetic energy. The structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity. This activity is found to be primarily longitudinal, extending from the surface to the lower stratosphere, to form a three-dimensional spiral in the synoptic-scale cyclone and along the fronts. However, an exceptional region also exists along the warm front, where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small-scale turbulence. In all simulations, the net transfer of mass and heat across the tropopause is from troposphere to stratosphere. The maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere. The same climatological choice leads to a minimum net irreversible mixing, which occurs primarily at Earth’s surface.