PV-θ view of diabatic–dynamical interaction in the general circulation

Abstract

This paper studies the question how the zonal mean potential vorticity (PV) distribution in potential temperature (θ) coordinates is established in the atmosphere by the interaction of diabatic processes (cross-isentropic transport of mass) with adiabatic dynamical processes (isentropic transport of mass and potential vorticity substance). As an aid in dissecting this interaction, a simplified model of the general circulation is constructed, which contains parametrisations of radiative transfer, wave drag and water cycle. This model reproduces the following four observed features of the atmosphere below 10 hPa: (1) a permanently present eastward subtropical jet, which in winter is separated from an eastward stratospheric jet by a zone (referred to as the ‘surf zone’), between θ=380 K and θ=550 K, where planetary wave drag reduces PV over the polar cap; (2) a stratospheric zonal wind reversal in spring or beginning of summer; (3) a tropical cold layer at 100 hPa, and (4) a realistic distribution of zonal mean cross-isentropic flow. The strength of the cross-isentropic flow depends on wave drag, latent heat release and the thermal inertia of both the atmosphere and the earth's surface. Of special interest is the layer between θ=315 K and θ=370 K (the ‘Middleworld’), which lies in the troposphere in the tropics and in the stratosphere in the extratropics. Mass converges diabatically into this layer in the deep tropics, mainly due to latent heat release, and diverges out of this layer elsewhere due to radiation flux divergence. Meridional isentropic vorticity flux divergence in the tropical Middleworld, associated with the upper branch of the Hadley circulation, creates a region in the subtropics, at θ=350 K and adjacent isentropic levels, with a marked isentropic meridional PV-gradient, forming the isentropic dynamical tropopause.

• potential vorticity
• thermal inertia
• diabatic circulation
• zonal wind jets
• tropopause
• surf zone
• planetary wave drag
• radiation
• water cycle

To access the supplementary material to this article, please see Supplementary files under Article Tools online.

To access the supplementary material to this article, please see Supplementary files under Article Tools online.

10. Acknowledgements

I wish to thank the three reviewers for very useful comments and criticism, Paul Berrisford for providing the ERA-40 diabatic heating data, Koen Manders, Niels Zweers and Roos de Wit for help in developing and debugging the numerical model and for stimulating scientific input in the early stages of this research project, Yvonne Hinssen and Theo Opsteegh for very useful discussions on potential vorticity inversion, Bruce Denby for his large contribution in developing the computer graphics, the students of my course on climate and the water cycle for inspiration and useful suggestions on incorporating and validating the parametrisations associated with the water cycle, Marcel Portanger for advice and help on computer problems and, finally, all my colleagues at IMAU for allowing me to work on such a large and time-consuming project.

Notes

To access the supplementary material to this article, please see Supplementary files under Article Tools online.