Transition from geostrophic turbulence to inertia–gravity waves in the atmospheric energy spectrum
Midlatitude fluctuations of the atmospheric winds on scales of thousands of kilometers, the most energetic of such fluctuations, are strongly constrained by the Earth's rotation and the atmosphere's stratification. As a result of these constraints, the flow is quasi-2D and energy is trapped at large scales—nonlinear turbulent interactions transfer energy to larger scales, but not to smaller scales. Aircraft observations of wind and temperature near the tropopause indicate that fluctuations at horizontal scales smaller than about 500 km are more energetic than expected from these quasi-2D dynamics. We present an analysis of the observations that indicates that these smaller-scale motions are due to approximately linear inertia–gravity waves, contrary to recent claims that these scales are strongly turbulent. Specifically, the aircraft velocity and temperature measurements are separated into two components: one due to the quasi-2D dynamics and one due to linear inertia–gravity waves. Quasi-2D dynamics dominate at scales larger than 500 km; inertia–gravity waves dominate at scales smaller than 500 km.
Additional Information© 2014 National Academy of Sciences. Freely available online through the PNAS open access option. Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 17, 2014 (received for review June 14, 2014). Published ahead of print November 17, 2014. We thank the principal investigators of the MOZAIC project, the European Commission for supporting the MOZAIC project, ETHER [Centre National d'Etudes Spatiales (CNES)–CNRS/Institut National des Sciences de l'Univers (INSU)] for hosting the database, and the participating airliners for transporting the instrumentation free of charge. J.C. and R.F. thank Glenn Flierl for offering useful feedback during the preparation of the manuscript and acknowledge financial support under Grants ONR-N-00014-09-1-0458 and NSF-CMG-1024198. O.B. gratefully acknowledges financial support under Grants NSF-CMG-1024180, NSF-DMS-1312159, and NSF-DMS-1009213. Author contributions: J.C. designed research; J.C., R.F., and O.B. performed research; J.C. analyzed data; and J.C., R.F., and O.B. wrote the paper. The authors declare no conflict of interest. This Direct Submission article had a prearranged editor. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1410772111/-/DCSupplemental.
Published - PNAS-2014-Callies-17033-8.pdf
Supplemental Material - pnas.201410772SI.pdf