Using Project Loon Superpressure Balloon Observations to Investigate the Inertial Peak in the Intrinsic Wind Spectrum in the Midlatitude Stratosphere. Issue 15 (13th August 2019)

• Record Type: Journal Article
• Authors: Conway, J. P.; Bodeker, G. E.; Waugh, D. W.; Murphy, D. J.; Cameron, C.; Lewis, J.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 15(2019)
• Page Start: 8594
• Page End: 8604
• Publication Date: 2019-08-13
• Subjects: stratospheric winds -- super‐pressure balloons -- inertia‐gravity waves -- balloon trajectories; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2018JD030195
• Languages: English
• ISSNs: 2169-897X

Abstract

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. Plain Language Summary: Knowledge of winds in the stratosphere is vital for understanding Earth's climate, and much effort is being devoted toward producing a better simulation of stratospheric wind variations in weather and climate models. Unfortunately, direct observations of winds in the stratosphere are sparse, especially over the oceans in the Southern Hemisphere midlatitudes. In this study, we use the movement of Project Loon balloons to diagnose variations in horizontal winds in the stratosphere between 18‐ and 21‐km altitude. The Project Loon data provide a much larger suite of data than was previously available. Of interest are variations in wind speed caused by the interaction of atmospheric waves with effects caused by the rotation of the Earth. The data from Project Loon allow us to characterize in detail these wind speed variations and show that fluctuations in wind speed with periods around a day dominate in the midlatitude stratosphere. The variations in wind speed caused by these waves present a challenge to forecasts of balloon trajectories as well as an accurate representation of transport and mixing within the stratosphere. Key Points: Project Loon balloons provide direct observation of the dominant peak in the intrinsic wind spectrum at the local inertial frequency Signatures of inertial wave activity are observed in most balloon segments and are stronger in weaker zonal wind speeds and during summer Near‐inertial anticyclonic oscillations are clearly associated with the peak in the kinetic energy spectrum at the local inertial frequency