Using Project Loon Superpressure Balloon Observations to Investigate the Inertial Peak in the Intrinsic Wind Spectrum in the Midlatitude Stratosphere

Abstract

Location information from superpressure balloons flown by Project Loon provide an unprecedented opportunity to analyze wind fields in the midlatitude stratosphere. Horizontal velocity spectra from the balloons' quasi-intrinsic frame of reference show clear evidence of a persistent peak in the intrinsic wind spectrum around the inertial frequency. In the Southern Hemisphere midlatitudes, peak-to-peak amplitudes of horizontal velocity perturbations (on the order of 20 m/s) are larger than those seen in previous superpressure balloon campaigns in polar regions and similar to those observed in vertical soundings in the midlatitudes. A rotary spectral analysis shows that near-circular anticyclonic rotation of horizontal wind perturbations around the inertial frequency dominate at most times and locations. The strongest anticyclonic rotation is more common in balloon flight segments with weak zonal winds and during the austral summer. Flight segments with strong eastward zonal velocities during austral winter and spring are more likely to have mixed cyclonic and anticyclonic power around the inertial frequency. These results confirm previous model and radiosonde observations of the peak in horizontal kinetic energy at the inertial frequency and demonstrate they are associated with increased anticyclonic wave power indicative of near-inertial oscillations or inertia-gravity waves. Flight segments with mixed cyclonic and anticyclonic power around the inertial frequency display a continuum of wave power from planetary to gravity wave scales. These results help explain the divergence of actual and modeled balloon trajectories in previous studies and provide a baseline against which reanalysis or meteorological model realizations of the intrinsic velocity field can be assessed.