Gravity Wave Momentum Fluxes Estimated From Project Loon Balloon Data

We present estimates of gravity wave momentum fluxes calculated from Project Loon superpressure balloon data collected between 2013 and 2021. In total, we analyzed more than 5,000 days of data from balloon flights in the lower stratosphere, flights often over regions or during times of the year without any previous in-situ observations of gravity waves. Maps of mean momentum fluxes show significant regional variability; we analyze that variability using the statistics of the momentum flux probability distributions for six regions: the Southern Ocean, the Indian Ocean, and the tropical and extratropical Pacific and Atlantic Oceans. The probability distributions are all approximately log-normal, and using their geometric means and geometric standard deviations we statistically explain the sign and magnitude of regional mean and 99th percentile zonal momentum fluxes and regional momentum flux intermittencies. We study the dependence of the zonal momentum flux on the background zonal wind and argue that the increase of the momentum flux with the wind speed over the Southern Ocean is likely due to a varying combination of both wave sources and filtering. Finally, we show that as the magnitude of the momentum flux increases, the fractional contributions by high-frequency waves increases, waves which need to be parameterized in large-scale models of the atmosphere. In particular, the near-universality of the log-normal momentum flux probability distribution, and the relation of its statistical moments to the mean momentum flux and intermittency, offer useful checks when evaluating parameterized or resolved gravity waves in models. Atmospheric gravity waves flux momentum away from their sources, depositing it as drag when they dissipate. Global climate models cannot resolve the entire gravity wave spectrum, so they must parameterize this drag by making assumptions about the gravity wave field developed from wave theory and validated by observations. We present new estimates of gravity wave momentum fluxes observed by Project Loon balloons, which flew on surfaces of constant density in the lower stratosphere and could detect gravity waves using their GPS sensors. The balloons detected significant regional and temporal variability in the momentum fluxes, which we explain by analyzing the statistics of their probability distributions. We also find that the momentum flux varied with the background wind over the Southern Ocean in ways that point to the competing effects of changing wave sources and filtering of the waves by the background flow, and that high-frequency waves were important contributors to large values of the momentum flux. These new estimates of the momentum flux come from many regions of the stratosphere that have never been sampled by neutral-density balloons before and provide observational constraints for the development and validation of the representation of gravity waves in models. We estimate and analyze atmospheric gravity wave momentum fluxes from Project Loon balloon data collected in the lower stratosphere We relate regional momentum flux variability to the statistics of the approximately log-normal flux probability distributions Our results can be used as observational constraints when developing and validating gravity wave parameterizations