Structure and Maintenance of Potential Vorticity in the Midlatitude Atmosphere

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9627040 Nakamura This project involves numerical and theoretical investigations of the mean structure of the potential vorticity (PV) in the earth's midlatitude atmosphere. Of particular interest are (1) how the nonconservative processes (friction, radiation, latent heating, etc.) interact with advective processes to maintain the highly inhomogeneous PV structures such as those found at the tropopause and at the edge of the stratospheric polar vortex and (2) how the values and gradients of the tropospheric PV are maintained. A new diagnostic technique to define the mean PV profile, based on the Lagrangian mass coordinate and recently developed by the principal investigator, will be employed. In a radical departure from the conventional zonal- and time-mean approaches, the method characterizes the isentropic transport as redistribution of mass between PV contours for which nonconservative processes such as diffusion are essential. Chaotic advection is viewed as the stretching of the PV contours, affecting the available interface for microscale diffusion. This Lagrangian diagnostic appears to identify strong PV gradients much better than conventional methods. Within this framework the baroclinic adjustment and the height of the tropopause will be investigated in terms of the mass budget of the troposphere, considering both the stratosphere-troposphere mass exchange and the mass sink/sources (heat fluxes) at the ground, with stirring due to baroclinic eddies regulating these processes. Using a two-dimensional model, the PI will first simulate a statistical steady state of the baroclinic atmosphere and examine the mechanism of tropopause formation and maintenance. The analysis will be extended to three dimensions, based on the high resolution simulations with the GFDL's SKYHI global circulation model. The same approach will be used to investigate the maintenance of the stratospheric polar vortex edge except that breaking Rossby waves will replace baroclinic waves as the stirr ing agent. These studies are directed at finding unifying theories of various aspects of the midlatitude general circulation. ***