Temperature measurements by vertically staring ground-based Rayleigh lidars are often used to detect middle atmospheric gravity waves. In time-height diagrams of temperature perturbations, stationary mountain waves are identifiable by horizontal phase lines. Vertically tilted phase lines, on the other hand, indicate that the wave source or the propagation conditions are transient. Idealized numerical simulations illustrate that and how a wave source moving in the direction of the mean wind entails upward-tilted phase lines. The inclination angle depends on the horizontal wavelength and the wave source’s propagation speed. On this basis, the goal is to identify and characterize transient non-orographic gravity waves (NOGWs), e.g., from propagating upper-level jet/front systems, in virtual and actual Rayleigh lidar measurements. Compositions of selected atmospheric variables from a meteorological forecast or reanalysis are thoughtfully combined to associate NOGWs with processes in the troposphere and stratosphere. For a virtual observation over the Southern Ocean, upward-tilted phase lines indeed dominate the time-height diagram during the passage of an upper-level trough. The example also emphasizes that temporal filtering of temperature measurements is appropriate for NOGWs, especially in the presence of a strong polar night jet that implies large vertical wavelengths. During two selected observational periods of the COmpact Rayleigh Autonomous Lidar (CORAL) in the lee of the southern Andes, upward-tilted phase lines are mainly associated with mountain waves and transient background wind conditions. One nighttime measurement by CORAL coincides with the passage of an upper-level trough, but large-amplitude mountain waves superpose the small-amplitude NOGWs in the middle atmosphere.

Many chemical processes depend non-linearly on temperature. Gravity-wave-induced temperature perturbations have been previously shown to affect atmospheric chemistry, but accounting for this process in chemistry-climate models has been a challenge because many gravity waves have scales smaller than the typical model resolution. Here, we present a method to account for subgrid-scale orographic gravity-wave-induced temperature perturbations on the global scale for the Whole Atmosphere Community Climate Model (WACCM). The method consists of deriving the temperature perturbation amplitude $\hat{T}$ consistent with the model’s subgrid-scale gravity wave parametrization, and imposing $\hat{T}$ as a sinusiodal temperature perturbation in the model’s chemistry solver. Because of limitations in the gravity wave parameterization, scaling factors may be necessary to maintain a realistic wave amplitude. We explore scaling factors between 0.6 and 1 based on comparisons to altitude-dependent $\hat{T}$ distributions in two observational datasets. We probe the impact on the chemistry from the grid-point to global scales, and show that the parametrization is able to represent mountain wave events as reported by previous literature. The gravity waves for example lead to increased surface area densities of stratospheric aerosols. This in turn increases chlorine activation, with impacts on the associated chemical composition. We obtain large local changes in some chemical species (e.g., active chlorine, NOx, N2O5) which are likely to be important for comparisons to airborne or satellite observations, but find that the changes to ozone loss are more modest. This approach enables the chemistry-climate modeling community to account for subgrid-scale gravity wave temperature perturbations in a consistent way.

Jet streams are important sources of non-orographic internal gravity waves and clear air turbulence (CAT). We analyze non-orographic gravity waves and CAT during a merging of the polar front jet stream (PFJ) with the subtropical jet stream (STJ) above the southern Atlantic. Thereby, we use a novel combination of airborne observations covering the meso-scale and turbulent scale in combination with high-resolution deterministic short-term forecasts. Coherent phase fronts stretching along a highly sheared tropopause fold are found in the ECMWF IFS (integrated forecast system) forecasts. During the merging event, the PFJ reverses its direction from antiparallel to parallel with respect to the STJ, going along with strong wind shear and horizontal deformation. Temperature perturbations in limb-imaging and lidar observations onboard the research aircraft HALO in the framework of the SouthTRAC campaign show remarkable agreement with the IFS data. Ten hours earlier, the IFS data show a new “X-shaped” phase line pattern emanating from the sheared tropopause fold. The analysis of tendencies in the IFS wind components shows that these gravity waves are excited by a local body force as the PFJ impinges the STJ. In situ observations of temperature and wind components at 100 Hz confirm upward propagation of the probed portion of the gravity waves. They furthermore reveal embedded episodes of light-to-moderate CAT, Kelvin Helmholtz waves, and indications for partial wave reflection. Patches of low gradient Richardson numbers in the IFS data coincide with episodes where CAT was observed, suggesting that this event was well accessible to turbulence forecasting.

Long-term high-resolution temperature data of the Compact Rayleigh Autonomous Lidar (CORAL) is used to evaluate temperature and gravity wave (GW) activity in ECMWF Integrated Forecasting System (IFS) over R\’io Grande (53.79$^{\circ}$S, 67.75$^{\circ}$W), which is a hot spot of stratospheric GWs in winter. Seasonal and altitudinal variations of the temperature differences between the IFS and lidar are studied for 2018 with a uniform IFS version. Moreover, interannual variations are considered taking into account updated IFS versions. We find monthly mean temperature differences $<2$~K at 20-40~km altitude. At 45-55~km, the differences are smaller than 4~K during summer. The largest differences are found during winter (4~K in May 2018 and -10~K in August 2018, July 2019 and 2020). The width of the difference distribution (15th/85th percentiles), the root mean square error, and maximum differences between instantaneous individual profiles are also larger during winter ($>\pm10$~K) and increase with altitude. We relate this seasonal variability to middle atmosphere GW activity. In the upper stratosphere and lower mesosphere, the observed temperature differences result from both GW amplitude and phase differences. The IFS captures the seasonal cycle of GW potential energy ($E_p$) well, but underestimates $E_p$ in the middle atmosphere. Experimental IFS simulations without damping by the model sponge for May and August 2018 show an increase in the monthly mean $E_p$ above 45~km from only $\approx10$~\% of the $E_p$ derived from the lidar measurements to 26~\% and 42~\%, respectively. GWs not resolved in the IFS are likely explaining the remaining underestimation of the $E_p$.

7 4 Servicio Meteorológico Nacional, Buenos Aires, Argentina 8 Key Points: 9 • moderate to severe clear air turbulence over the Drake Passage is analyzed from 10 unique 10 Hz flight level in situ observations during a research flight as part of 11 the SOUTHTRAC campaign in 2019 12 • the main source of the turbulence is the enhanced flow deformation and shear 13 in the vicinity of a tropopause fold generated by an eastward propagating polar 14 low 15 • both spectral as well as structure function methods are applied to quantify the 16 eddy-dissipation rates, the intermittency, and the flow anisotropy during the 17 CAT event 18 Corresponding author: Paola Rodriguez Imazio, [email protected] 19 An aircraft turbulence encounter over the Drake Passage is investigated by com-20 bining unique high-frequency flight level data, vertical profiles of a near-simultaneous 21 radiosonde profile and numerical results from global and regional numerical weather 22 prediction (NWP) models. Meteorological analysis reveals an intense polar low prop-23 agating from the Bellinghausen Sea toward the Drake Passage. A small and deep 24 stratospheric intrusion formed a tropopause fold that promoted strong upper-level fron-25 togenesis and enhanced shear and horizontal deformation of the upper tropospheric 26 and lower stratospheric (UTLS) airflow. In this region, the Basic HALO Measure-27 ment and Sensor System (BAHAMAS) aboard the HALO research aircraft flying at 28 FL450 detected large peak-to-peak variations in all meteorological parameters. The 29 computed EDR values (cubic root of the eddy dissipation rate) indicate moderate to 30 severe turbulence. The location of this turbulence encounter was well-predicted by the 31 CAT indices derived from the NWP results. The enhanced CAT indices emphasize 32 the large shear and horizontal deformation of the airflow as the cause of the turbu-33 lence. Horizontal and vertical energy spectra calculated from the 10 Hz BAHAMAS 34 data show a well-defined energy cascade towards small scales with Kolmogorov scal-35 ing. Maximum EDR values of about 0.5 derived both from the spectra and structure 36 functions for the wind speed agree quantitatively very well. In addition, the structure 37 functions support the detection of turbulent atmospheric conditions with signatures of 38 flow anisotropy generated by enhanced thermal stratification in the UTLS. The scales 39 involved are between the buoyancy length scale L B ≈ 1500 m and the Ozmidov scale 40 L O ≈ 111 m. 41 Plain Language Summary 42 This case study documents an aircraft encounter with clear-air turbulence (CAT) 43 that occurred over the remote Drake Passage. Exceptional atmospheric conditions are 44 known to exist there as low pressure systems regularly pass through the passage. Dur-45 ing a research flight conducted as part of the SOUTHTRAC campaign in November 46 2019, moderate to severe turbulence was detected using flight-level observations of 47 wind and temperature with a frequency of 10 Hz. The vertical distribution of atmo-48 spheric variables is provided by a high-resolution radiosonde profile launched from the 49 Argentinian research station Marambio. In addition, the results of global and regional 50 numerical weather prediction models are used to provide the meteorological context 51 and to derive indices commonly used to predict CAT for aviation. Two different anal-52 ysis techniques are used to quantify the eddy dissipation rates associated with the 53 CAT event. Both methods show that the event exhibits the characteristic feature of 54 the bursty and intermittent nature of turbulence in the upper troposphere and lower 55 stratosphere. Flow anisotropy is large outside the mixing regions of the CAT patch. 56

Gravity waves (GWs) generated by orographic forcing, also known as mountain waves (MWs) have been studied for decades. First measured in the troposphere, then in the stratosphere, they were only imaged at mesospheric altitude in 2008. Their characteristics have been investigated during several recent observation campaigns, but many questions remain concerning their impacts on the upper atmosphere, and the effects of the background environment on their deep propagation. An Advanced Mesospheric Temperature Mapper (AMTM) and the Southern Argentina Agile MEteor Radar (SAAMER) have been operated simultaneously during the Austral winter 2018 from Rio Grande, Argentina (53.8°S). This site is located near the tip of South America, in the lee of the Andes Mountains, a region considered the largest MW hotspot on Earth. New AMTM image data obtained during a 6-month period show almost 100 occurrences of MW signatures penetrating into the upper mesosphere. They are visible ~30% of time at the height of the winter season (mid-May to mid-July). Their intermittency is highly correlated with the zonal wind controlled by the semi-diurnal tide, revealing the direct effect of the atmospheric background on MW penetration into the Mesosphere Lower Thermosphere (MLT, altitude 80-100 km). Measurements of their momentum fluxes (MF) were determined to reach very large values (average ~250 m/s), providing strong evidence of the importance and impacts of small-scale gravity waves at mesospheric altitudes.