Stratospheric horizontal wavenumber spectra of winds, potential temperature, and atmospheric tracers observed by high‐altitude aircraft

Abstract

Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N2O mixing ratios, as measured in the mid‐stratosphere during 73 ER‐2 flights (altitude ≈ 20km) are presented. The velocity and potential temperature spectra in the 100 to 1‐km wavelength range deviate significantly from the uniform −5/3 power law expected for the inverse energy‐cascade regime of two‐dimensional turbulence and also for inertial‐range, three‐dimensional turbulence. Instead, steeper spectra approximately consistent with a −3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between −1.5 and −2.0. For vertical velocity the spectrum at longer scales becomes flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley‐Shur‐Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N2O mixing ratio in the 100 to 1‐km wavelength range do conform to an approximately uniform −5/3 power law. It is argued that this may reflect interactions between gravity wave air‐parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstrophy‐cascading two‐dimensional turbulence.