Christopher Geach

Resonance lidars targeting the resonance line of potassium at 770 nm have been employed over the past 5 decades to yield numerous insights into the dynamics of the middle and upper atmosphere. It has long been recognized that these measurements can be severely affected by nonlinear behavior in the scattering process, that is, saturation effects, particularly when high pulse energies, narrow beam divergences, and narrow laser linewidths are employed. The measurements by the potassium resonance lidars developed at the Leibniz Institute of Atmospheric Physics are in this respect unique—these are the only systems for which saturation effects have been detected. However, the observed degree of saturation is inconsistent with theoretical estimations thus far. A new method of calculating the degree of saturation, previously applied only to the line of sodium, is described in a companion paper (Geach & Kaifler, 2026, https://doi.org/10.1029/2026JA035269 ); here, we apply the method to the resonance line of potassium. We show that the observed degree of saturation, approximately 70%, is consistent with the model; we show that a spectral signature of saturation effects, predicted by the model, is consistent with the observations; we present the saturation‐induced biases on temperature and wind measurements; and we discuss the implications of saturation on ideal instrument design.

Atmospheric resonance lidars targeting metallic species in the mesosphere and lower thermosphere have yielded insights into the dynamics of the middle and upper atmosphere for over 50 years. Recently, the technique has been extended to the metastable helium layer, which reaches up to 1,000 km or above. It has long been recognized that high energy concentrations in the laser beam can lead to nonlinear behavior during the scattering process, a phenomenon known as saturation. Nonetheless, commonly used approaches to calculating the severity of saturation effects are not accurate for modern resonance lidar systems. Here, we present an approach to calculating saturation for sodium, iron, and helium resonance lidars and discuss the biases in wind and temperature that arise from neglecting saturation. The saturation calculators presented here are publicly available and can be adapted to estimate saturation for current or future resonance lidar systems.

Airglow originating from metastable helium He(S) at 1,083 nm has been used to study the upper thermosphere since its discovery in 1959, yielding insights into, for example, solar EUV intensities and thermospheric photoelectron densities. For over 6 decades, these measurements were made passively, relying on solar illumination of the He(S) layer to produce a detectable signal, resulting in a column integration of the He(S) layer. Recently, the first height‐resolved measurements of He(S) density were made by fluorescence lidar, opening a new window for studying the upper thermosphere. We report on a series of 51 measurements by this instrument spanning an entire winter season and extending to an altitude of 1,000 km, revealing a broad He(S) layer that peaks at higher altitudes than previously expected.

Airglow originating from metastable helium He(2^3S) at 1083 nm has been used to study the upper thermosphere since its discovery in 1959, yielding insights into e.g. solar EUV intensities and thermospheric photoelectron densities. However, passive airglow measurements are fundamentally limited, because they represent a vertical integration of the entire He(2^3S) layer, spanning 100s of km, and because they rely on solar illumination of the He(2^3S) layer, so temporal changes are difficult to distinguish from vertical variations. Recently, the first height-resolved measurements of He(2^3S) density were made by fluorescence lidar, opening a new window for studying the upper thermosphere. We report on a series of 51 measurements by this instrument spanning an entire winter season and extending to an altitude of 1000 km, revealing a broad He(2^3S) layer that extends to, and peaks at, higher altitudes than previously expected.

The Polar Mesospheric Cloud (PMC) Turbulence experiment performed optical imaging and Rayleigh lidar PMC profiling during a 6‐day flight in July 2018. A mosaic of seven imagers provided sensitivity to spatial scales from ∼20 m to 100 km at a ∼2‐s cadence. Lidar backscatter measurements provided PMC brightness profiles and enabled definition of vertical displacements of larger‐scale gravity waves (GWs) and smaller‐scale instabilities of various types. These measurements captured an interval of strong, widespread Kelvin‐Helmholtz instabilities (KHI) occurring over northeastern Canada on July 12, 2018 during a period of significant GW activity. This paper addresses the evolution of the KHI field and the characteristics and roles of secondary instabilities within the KHI. Results include the imaging of secondary KHI in the middle atmosphere and multiple examples of KHI “tube and knot” (T&K) dynamics where two or more KH billows interact. Such dynamics have been identified clearly only once in the atmosphere previously. Results reveal that KHI T&K arise earlier and evolve more quickly than secondary instabilities of uniform KH billows. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD035834 reveals that they also induce significantly larger energy dissipation rates than secondary instabilities of individual KH billows. The expected widespread occurrence of KHI T&K events may have important implications for enhanced turbulence and mixing influencing atmospheric structure and variability.

Polar mesospheric cloud (PMC) imaging and lidar profiling performed aboard the 5.9‐day PMC Turbo balloon flight from Sweden to northern Canada in July 2018 revealed a wide variety of gravity wave (GW) and instability events occurring nearly continuously at approximately 82 km. We describe one event exhibiting GW breaking and associated vortex rings driven by apparent convective instability. Using PMC Turbo imaging with spatial and temporal resolution of 20 m and 2 s, respectively, we quantify the GW horizontal wavelength, propagation direction, and apparent phase speed. We identify vortex rings with diameters of 2–5 km and horizontal spacing comparable to their size. Lidar data show GW vertical displacements of ±0.3 km. From the data, we find a GW intrinsic frequency and vertical wavelength of 0.009 ± 0.003 rad s⁻¹ and 9 ± 4 km, respectively. We show that these values are consistent with the predictions of numerical simulations of idealized GW breaking. We estimate the momentum deposition rate per unit mass during this event to be 0.04 ± 0.02 m s⁻² and show that this value is consistent with the observed GW. Comparison to simulation gives a mean energy dissipation rate for this event of 0.05–0.4 W kg⁻¹, which is consistent with other reported in situ measurements at the Arctic summer mesopause.

The Polar Mesospheric Cloud Turbulence (PMC Turbo) instrument consists of a balloon‐borne platform which hosts seven cameras and a Rayleigh lidar. During a 6‐day flight in July 2018, the cameras captured images of Polar Mesospheric Clouds (PMCs) with a sensitivity to spatial scales from ~20 m to 100 km at a ~2‐s cadence and a full field of view (FOV) of hundreds of kilometers. We developed software optimized for imaging of PMCs, controlling multiple independent cameras, compressing and storing images, and for choosing telemetry communication channels. We give an overview of the PMC Turbo design focusing on the flight software and telemetry functions. We describe the performance of the system during its first flight in July 2018.

Polar mesospheric cloud (PMC) imaging and lidar profiling performed aboard the 5.9 day PMC Turbo balloon flight from Sweden to northern Canada in July 2018 revealed a wide variety of gravity wave (GW) and instability events occurring nearly continuously at approximately 82 km. We describe one event exhibiting GW breaking and associated vortex rings driven by apparent convective instability. Using PMC Turbo imaging with spatial and temporal resolution of 20 m and 2 s, respectively, we quantify the GW horizontal wavelength, propagation direction, and apparent phase speed, and we identify vortex rings with diameters of 3-5 km and horizontal spacing of ~5 km. Lidar data show GW vertical displacements of ±0.3 km. From the data, we find a GW intrinsic frequency and vertical wavelength of 0.009 ± 0.003 rad s-1 and 9 ± 4 km, respectively. We show that these values are consistent with the predictions of numerical simulations of idealized GW breaking. We estimate the momentum deposition rate per unit mass during this event to be 0.04 ± 0.02 m s-2 and show that this value is consistent with the observed GW. Comparison to simulation gives a mean energy dissipation rate for this event of 0.05-0.4 W kg-1, which is consistent with other reported in-situ measurements at the Arctic summer mesopause.