Gravity wave spectra in the lower stratosphere diagnosed from project loon balloon trajectories

Abstract

Project Loon has been launching superpressure balloons since January 2013 to provide worldwide Internet coverage. These balloons typically fly between 18 and 21 km and provide measurements of winds and pressure fluctuations in the lower stratosphere. We divide 1560 Loon flights into 3405 two‐day segments for gravity wave analysis. We derive the kinetic energy spectrum from the horizontal balloon motion and estimate the temperature perturbation spectrum (proportional to the potential energy spectrum) from the pressure variations. We fit the temperature (and kinetic energy) data to the functional form T′2 = T′o2[ω/ωο)α, where ω is the wave frequency, ωο is daily frequency, T′o is the base temperature amplitude, and α is the spectral slope. Both the kinetic energy and temperature spectra show −1.9 ± 0.2 power‐law dependence in the intrinsic frequency window 3–50 cycles/day. The temperature spectrum slope is weakly anticorrelated with the base temperature amplitude. We also find that the wave base temperature distribution is highly skewed. The tropical modal temperature is 0.77 K. The highest amplitude waves occur over the mountainous regions, the tropics, and the high southern latitudes. Temperature amplitudes show little height variation over our 18–21 km domain. Our results are consistent with other limited superpressure balloon analyses. The modal temperature is higher than the temperature currently used in high‐frequency gravity wave parameterizations.

Convective Hydration of the Upper Troposphere and Lower Stratosphere

“…Here we use 0.15 K. We also use the Loon gravity wave-altitude-varying T o values described above. For both gravity wave schemes, α is set to À2 (equation (1); see section 2.2) as has been observed by both aircraft and balloon measurements (Bacmeister et al, 1996;Hertzog & Vial, 2001;Podglajen et al, 2016;Schoeberl et al, 2017).…”

“…Schoeberl et al () used constant‐altitude high‐frequency gravity wave temperature perturbations ( T o ) ranging between 0 and 0.35 K. Here we use 0.15 K. We also use the Loon gravity wave‐altitude‐varying T o values described above. For both gravity wave schemes, α is set to −2 (equation ; see section 2.2) as has been observed by both aircraft and balloon measurements (Bacmeister et al, ; Hertzog & Vial, ; Podglajen et al, ; Schoeberl et al, ).…”

The Impact of Mesoscale Gravity Waves on Homogeneous Ice Nucleation in Cirrus Clouds

“…We refer to cirrus as upper tropospheric ice clouds that form in situ, for example, in frontal systems. Superpressure balloon (SPB) measurements at altitudes of 18–21 km directly link mesoscale vertical wind speed and the associated temperature variability to gravity waves and quantified spectral properties (Podglajen et al, ; Schoeberl et al, ). Since these airborne measurement platforms are advected by the wind field, properties of fluctuations derived from them are highly useful for Lagrangian cloud studies.…”

A Stochastic Representation of Temperature Fluctuations Induced by Mesoscale Gravity Waves

“…Analyses of wind and pressure fluctuations measured on more than 1,500 SPBs floating mostly between 18 and 21 km in the Southern Hemisphere LS are described in Schoeberl et al. (). These measurements extend the above SPB campaigns to include the Southern Hemisphere jet region.…”

“…Gravity waves (GW) are an important factor influencing meteorological processes in the stably stratified upper troposphere (UT) and lower stratosphere (LS) on the mesoscale (Craig & Selz, ). Balloon, aircraft, and radar measurements link vertical air motion variability to GW activity (Bacmeister et al., ; ; Ecklund et al., ; Hoyle et al., ; Li et al., ; Podglajen et al., ; Murphy & Gary, ; Schoeberl et al., ). Measurements show that MTF are widespread globally and occur frequently, even in conditions not directly affected by convective and mountain wave activity or by the jet stream, introducing the notion of background conditions (Gary, ).…”