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We analyze the gravity waves (GWs) from the ground to the thermosphere during 11–14 January 2016 using the nudged HI Altitude Mechanistic general Circulation Model. We find that the entrance, core and exit regions of the polar vortex jet are important for generating primary GWs and amplifying GWs from below. These primary GWs dissipate in the upper stratosphere/lower mesosphere and deposit momentum there; the atmosphere responds by generating secondary GWs. This process is repeated, resulting in medium to large‐scale higher‐order, thermospheric GWs. We find that the amplitudes of the secondary/higher‐order GWs from sources below the polar vortex jet are exponentially magnified. The higher‐order, thermospheric GWs have concentric ring, arc‐like and planar structures, and spread out latitudinally to 10 − 90°N. Those GWs with the largest amplitudes propagate against the background wind. Some of the higher‐order GWs generated over Europe propagate over the Arctic region then southward over the US to ∼15–20°N daily at ∼14 − 24 UT (∼9 − 16 LT) due to the favorable background wind. These GWs have horizontal wavelengths λH ∼ 200 − 2,200 km, horizontal phase speeds cH ∼ 165 − 260 m/s, and periods τr ∼ 0.3 − 2.4 hr. Such GWs could be misidentified as being generated by auroral activity. The large‐scale, higher‐order GWs are generated in the lower thermosphere and propagate southwestward daily across the northern mid‐thermosphere at ∼8–16 LT with λH ∼ 3,000 km and cH ∼ 650 m/s. We compare the simulated GWs with those observed by AIRS, VIIRS/DNB, lidar and meteor radars and find reasonable to good agreement. Thus the polar vortex jet is important for facilitating the global generation of medium to large‐scale, higher‐order thermospheric GWs via multi‐step vertical coupling. Plain Language Summary: Gravity waves (GWs) are perturbations in the Earth's atmosphere created by various processes. When a GW breaks, it imparts momentum to the atmosphere, which in turn can become unbalanced and generate secondary GWs. The same process happens at higher altitudes where the secondary GWs break, thereby generating higher‐order GWs. We simulate the primary, secondary and higher‐order GWs on 11–14 January 2016 using a GW‐resolving whole atmosphere model. We find that the entrance, core and exit regions of the jet that encircles the polar vortex generate primary GWs. In addition, the polar vortex jet magnifies the amplitudes of higher‐order GWs from sources below the jet, such as mountain waves. The resulting higher‐order GWs have horizontal wavelengths of hundreds to thousands of km with concentric ring, arc‐like and planar structures in the thermosphere. We compare the simulated GWs with AIRS, VIIRS/DNB, lidar, and meteor radar observations and find reasonable to good agreement. Key Points: The vertical shear of the horizontal wind in the entrance, core and exit regions of the polar vortex jet generate GWs in the stratosphereHigher‐order GWs generated over Europe and Asia propagate over the Arctic then southward over the CONUS at ∼9 − 16 LT due to favorable windsLarge‐scale GWs with λH ∼ 3,000 km and cH ∼ 650 m/s propagate southwestward across the northern thermosphere at ∼8–16 LT [ABSTRACT FROM AUTHOR]

The cloud imaging and particle size (CIPS) instrument onboard the Aeronomy of Ice in the Mesosphere satellite provides images of gravity waves (GWs) near the stratopause and lowermost mesosphere (altitudes of 50–55 km). GW identification is based on Rayleigh Albedo Anomaly (RAA) variances, which are derived from GW‐induced fluctuations in Rayleigh scattering at 265 nm. Based on 3 years of CIPS RAA variance data from 2019 to 2022, we report for the first time the seasonal distribution of GWs entering the mesosphere with high (7.5 km) horizontal resolution on a near‐global scale. Seasonally averaged GW variances clearly show spatial and temporal patterns of GW activity, mainly due to the seasonal variation of primary GW sources such as convection, the polar vortices and flow over mountains. Measurements of stratospheric GWs derived from Atmospheric InfraRed Sounder (AIRS) observations of 4.3 μm brightness temperature perturbations within the same 3‐year time range are compared to the CIPS results. The comparisons show that locations of GW hotspots are similar in the CIPS and AIRS observations. Variability in GW variances and the monthly changes in background zonal wind suggest a strong GW‐wind correlation. This study demonstrates the utility of the CIPS GW variance data set for statistical investigations of GWs in the lowermost mesosphere, as well as provides a reference for location/time selection for GW case studies. Key Points: Aeronomy of Ice in the Mesosphere cloud imaging and particle size (CIPS) provides first near‐global images of gravity waves at 50–55 km with high horizontal resolutionCIPS observes gravity wave hotspots generated by convection, topography, and the polar vortexLocations of gravity wave hotspots near the stratopause/lowermost mesosphere are similar to those in the mid‐stratosphere near 35 km [ABSTRACT FROM AUTHOR]

The Andes account for the largest source of orographic gravity waves (GWs) in the middle atmosphere. This results from persistent, strong zonal winds at the surface encountering the north‐south mountain chain, producing strong orographic lift, and resulting GWs. Here, we consider GWs in the stratosphere and mesosphere above the Andes as observed by the Cloud Imaging and Particle Size instrument, the Solar Occultation for Ice Experiment on the AIM satellite, and the Sounding of the Atmosphere using Broadband Emission Radiometry instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. GW variability is considered in the context of the location of the stratospheric wintertime westerly jet and planetary wave (PW) amplitude and phase. The occurrence of GWs in the middle and upper atmosphere depends not only on tropospheric sources, like the Andes, but also on the background winds through which they propagate. Results suggest that the propagation of GWs into the mesosphere is well correlated with winds throughout the middle and upper stratosphere. PWs cause the westerly jet to move over the Andes, resulting in an increase in GW amplitude throughout the middle and upper stratosphere. The evolution and variability of GWs are tied closely to the phase of the PW‐1 and are linked to PW‐2 when the PW amplitudes are sufficiently large. GW amplitude, as observed by all three data sets, increases in the upper stratosphere and lower mesosphere when the trough of the PW‐1 in the stratosphere is over the Andes. Plain Language Summary: In the Southern Hemisphere, strong wintertime winds continuously flow west to east at mid‐latitudes (40°–50°S). As these winds flow over surface features like the Andes Mountains, the air is forced upwards, creating waves in the atmosphere that propagate both horizontally and vertically. These gravity waves (GWs) transport energy from the surface to the upper atmosphere, where they break and deposit their momentum, causing either turbulent mixing, an acceleration of the winds, or the generation of additional waves. The energy deposited from GWs has a significant impact on the circulation in the upper atmosphere, so it is important to understand where they break and to characterize sources of variability. As GWs propagate vertically, they encounter horizontal wind shear associated with a band of strong winds in the middle atmosphere, causing them to turn toward the area of strong winds. The latitude where these strong winds occur can change significantly, driven by planetary‐scale waves. This work shows that the occurrence of GWs in the middle atmosphere, observed by several NASA instruments, strongly depends on the amplitude and orientation of the planetary scale waves and the resulting location of the middle atmospheric winds. Key Points: Gravity waves (GWs) at the stratopause show significant variability over the Andes associated with stratospheric winds and planetary wave (PW) phaseEnhanced GW activity follows the trough of the PW‐1 and strong stratospheric winds at mid‐southern latitudesGW activity is enhanced over the Andes when the PW trough and the polar night jet are over the Andes [ABSTRACT FROM AUTHOR]

The Hunga Tonga Hunga‐Ha'apai (HTHH) volcanic eruption on 15 January 2022 injected water vapor and SO2 into the stratosphere. Several months after the eruption, significantly stronger westerlies, and a weaker Brewer‐Dobson circulation developed in the stratosphere of the Southern Hemisphere and were accompanied by unprecedented temperature anomalies in the stratosphere and mesosphere. In August 2022, the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument observed record‐breaking temperature anomalies in the stratosphere and mesosphere that alternate signs with altitude. Ensemble simulations carried out with the Whole Atmosphere Community Climate Model (WACCM6) indicate that the strengthening of the stratospheric westerlies explains the mesospheric temperature changes. The stronger westerlies cause stronger westward gravity wave drag in the mesosphere. Although the enhanced gravity wave drag is partly balanced by a weakening of planetary wave forcing, the net result is an acceleration of the mesospheric mean meridional circulation. The stronger mesospheric circulation, in turn, plays a dominant role in driving the changes in mesospheric temperatures. This study highlights the impact of large volcanic eruptions on middle atmospheric dynamics and provides insight into their long‐term effects in the mesosphere. On the other hand, we could not discern a clear mechanism for the observed changes in stratospheric circulation. In fact, an examination of the WACCM ensemble reveals that not every member reproduces the large changes observed by SABER. We conclude that there is a stochastic component to the stratospheric response to the HTHH eruption. Plain Language Summary: This work studies the impact of the Hunga Tonga‐Hunga Ha'apai volcanic eruption, which took place on 15 January 2022, on the earth's mesosphere (55–80 km). The eruption injected water vapor and SO2 into the stratosphere, which was followed by changes in the wind patterns in the stratosphere (16–55 km). Concurrent with these changes, we observed unprecedented temperature changes in the mesosphere, with record high and low temperature anomalies in August that alternate signs with altitude. We used climate model simulations to show that the changes in stratospheric winds were ultimately responsible for these record‐breaking mesospheric temperatures. We found that the stronger winds in the stratosphere enhanced gravity wave breaking in the mesosphere, which led to changes in the circulation and thus the temperature. However, we could not find a clear mechanism for the changes observed in the stratosphere. Key Points: Sounding of the Atmosphere using Broadband Emission Radiometry observed unprecedented mesospheric temperature variations in the Southern Hemisphere in August 2022 after the Hunga Tonga Hunga‐Ha'apai eruptionWhole Atmosphere Community Climate Model simulations indicate that changes in the mesospheric temperature are due to a stronger mesospheric meridional circulationStronger stratospheric westerlies after eruption enhance westward gravity wave drag in the mesosphere, thus a stronger circulation [ABSTRACT FROM AUTHOR]

We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics. We utilize a spectral method that nudges only the large‐scale flow to MERRA‐2 reanalysis. The nudged HIAMCM simulates gravity waves (GWs) down to horizontal wavelengths of about 200 km from the troposphere to the thermosphere like the free‐running model, including the generation of secondary and tertiary GWs. Case studies show that the simulated large‐scale GWs are consistent with those in the reanalysis, while the medium‐scale GWs compare well with observations in the northern winter 2016 stratosphere from the Atmospheric InfraRed Sounder (AIRS). GWs having wavelengths larger than about 1,350 km can be described with the nonlinear balance equation. The GWs relevant in the stratosphere, however, have smaller scales and require a different approach. We propose that the GW amplification due to kinetic energy transfer from the large‐scale flow combined with GW potential energy flux convergence helps to identify the mesoscale GW sources due to spontaneous emission. The GW amplification is strongest in the region of maximum large‐scale vertical wind shear in the mid‐stratosphere. Maps of the time‐averaged stratospheric GW activity simulated by the HIAMCM and computed from AIRS satellite data show a persistent hot spot over Europe during January 2016. At about 40 km, the average GW amplitudes are maximum in the region of fastest large‐scale flow. We argue that refraction of GWs originating in the troposphere, as well as GWs from spontaneous emission in the stratosphere contribute to this effect. Plain Language Summary: We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics in the troposphere, stratosphere, and lower mesosphere. The HIAMCM is a spectral model with high spatial resolution (the shortest horizontal wavelength is about 156 km) and a model top at about 450 km. The nudging of the model to reanalysis is formulated in spectral space and restricted to the large‐scale flow. This ensures that gravity waves (GWs) are self‐consistently simulated over the whole altitude range like in the free‐running model. The simulated large‐scale GWs are quantitatively consistent with those in the reanalysis, while the medium‐scale GWs compare well with satellite observations in the northern winter 2016 stratosphere. We analyze a case of GW generation in the stratosphere from imbalance of the polar vortex. We propose that this process can be identified by the transfer of kinetic energy transfer from the large‐scale flow to the GWs combined with the GW potential energy flux convergence. The in‐situ generation of GWs in the stratosphere likely contributes to the averaged distribution of GW activity in the upper stratosphere, which is maximum around the edge of the polar vortex. Key Points: Nudging in spectral space allows for self‐consistent simulation of gravity waves up to the thermosphereSimulated gravity waves in the winter stratosphere agree well with satellite observationsSpontaneous emission of GWs in the winter stratosphere depends critically on vertical wind shear [ABSTRACT FROM AUTHOR]

We use the Specified Dynamics version of the Whole Atmosphere Community Climate Model Extended (SD-WACCMX) to model the descent of nitric oxide (NO) and other mesospheric tracers in the extended, elevated stratopause phase of the 2013 sudden stratospheric warming (SSW). The dynamics are specified with a high-altitude version of the Navy Global Environmental Model (NAVGEM-HA). Consistent with our earlier published results, we find that using a high-altitude meteorological analysis to nudge WACCMX allows for a realistic simulation of the descent of lower-thermospheric nitric oxide down to the lower mesosphere, near 60 km. This is important because these simulations only included auroral electrons and did not consider additional sources of NO from higher-energy particles that might directly produce ionization, and hence nitric oxide, below 80–85 km. This suggests that the so-called energetic particle precipitation indirect effect (EPP-IE) can be accurately simulated, at least in years of low geomagnetic activity, such as 2013, without the need for additional NO production, provided the meteorology is accurately constrained. Despite the general success of WACCMX in bringing upper-mesospheric NO down to 55–60 km, a detailed comparison of the WACCMX fields with the analyzed NAVGEM-HA H 2 O and satellite NO and H 2 O data from the Solar Occultation for Ice Experiment (SOFIE) and the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) reveals significant differences in the latitudinal and longitudinal distributions at lower altitudes. This stems from the tendency for WACCMX descent to maximize at sub-polar latitudes, and while such sub-polar descent is seen in the NAVGEM-HA analysis, it is more transient than in the WACCMX simulation. These differences are linked to differences in the transformed Eulerian mean (TEM) circulation between NAVGEM-HA and WACCMX, most likely arising from differences in how gravity wave forcing is represented. To attempt to compensate for the differing distributions of model vs. observed NO and to enable us to quantify the total amount of upper-atmospheric NO delivered to the stratopause region, we use potential vorticity and equivalent latitude coordinates. Preliminary results suggest both model and observations are generally consistent with NO totals in the range of 0.1–0.25 gigamoles (GM). [ABSTRACT FROM AUTHOR]

The energetic particle precipitation (EPP) indirect effect (IE) refers to the downward transport of reactive odd nitrogen (NOx = NO + NO2) produced by EPP (EPP‐NOx) from the polar winter mesosphere and lower thermosphere to the stratosphere where it can destroy ozone. Previous studies of the EPP IE examined NOx descent averaged over the polar region, but the work presented here considers longitudinal variations. We report that the January 2009 split Arctic vortex in the stratosphere left an imprint on the distribution of NO near the mesopause, and that the magnitude of EPP‐NOx descent in the upper mesosphere depends strongly on the planetary wave (PW) phase. We focus on an 11‐day case study in late January immediately following the 2009 sudden stratospheric warming during which regional‐scale Lagrangian coherent structures (LCSs) formed atop the strengthening mesospheric vortex. The LCSs emerged over the north Atlantic in the vicinity of the trough of a 10‐day westward traveling planetary wave. Over the next week, the LCSs acted to confine NO‐rich air to polar latitudes, effectively prolonging its lifetime as it descended into the top of the polar vortex. Both a whole atmosphere data assimilation model and satellite observations show that the PW trough remained coincident in space and time with the NO‐rich air as both migrated westward over the Canadian Arctic. Estimates of descent rates indicate five times stronger descent inside the PW trough compared to other longitudes. This case serves to set the stage for future climatological analysis of NO transport via LCSs. Plain Language Summary: Energetic particles from the sun and the magnetosphere impinge upon Earth's upper atmosphere and create reactive odd nitrogen (NOx) in the mesosphere and lower thermosphere. Descent in the winter polar vortex effectively transports this NOx down to the stratosphere where it can destroy ozone. State‐of‐the‐art models currently underestimate this vertical transport by a factor of 4. Previous studies have examined the NOx descent averaged over the entire polar region, but this study considers longitudinal variations. We examine a case study during late January 2009 and find a closed circulation coincident with the trough of a planetary wave over the north Atlantic at 90 km with shear zones inhibiting horizontal mixing to the north, east, and south. This circulation (1) contains elevated NOx, (2) is associated with five times stronger descent compared to other longitudes, and (3) is the natural upward continuation of the westward tilting polar vortex in the stratosphere and mesosphere. Thus, this meteorological feature near the mesopause provides a transport pathway for air to enter the top of the polar vortex. This is the first work to illustrate the zonally asymmetric nature of NOx descent in the polar winter upper mesosphere and couple it to the vortex below. Key Points: First demonstration of the impact of the split Arctic vortex on the geographic distribution of nitric oxide at the winter mesopauseFirst evidence that a Lagrangian coherent structure inhibits horizontal transport of nitric oxide at the polar winter mesopauseDescent of nitric oxide is five times stronger between 80 and 90 km in a westward traveling planetary wave trough compared to the ridge [ABSTRACT FROM AUTHOR]

Observations during 12 January 2016 revealed a series of events of significant gravity wave (GW) activity over Europe. Analysis of derived temperatures from the Atmospheric InfraRed Sounder (AIRS) provides insight into the sources of these GWs, and include a new observation of stratosphere polar night jet (PNJ) generated GWs. Mountain waves were present during this time as well over the French Alps and the Carpathian Mountains and had maximum temperature perturbations, T′, as large as 27 K over the French Alps. Further investigation of the mountain waves that demonstrated their presence in the stratosphere was determined not only by stratospheric conditions but also by strong winds in the troposphere and at the surface. GWs generated in the stratosphere by the PNJ had maximum T′ of 7 K. These observations demonstrate multiple sources of GWs during a dynamically active period and implicate the role of the PNJ in both the vertical propagation of GWs generated in the troposphere and the generation of GWs from the PNJ itself. Key Points: AIRS observations demonstrate the presence of gravity waves generated by the polar night jetDownward propagating gravity waves were observed at altitudes below the polar night jetThe polar night jet plays an important role in the growth of mountain waves in the stratosphere [ABSTRACT FROM AUTHOR]

This work evaluates the wintertime mesospheric polar vortices in the Whole Atmosphere Community Climate Model. Mesospheric altitudes are where high‐top models underestimate the concentration of nitrogen oxides produced by energetic particle precipitation. For this reason, we seek to determine the extent to which the observed mesospheric vortex size and frequency of occurrence is reproduced by the model. We compare 13 years (2005–2017) of "specified dynamics" Whole Atmosphere Community Climate Model (where meteorology in the troposphere and stratosphere is constrained by observations) to Sounding of the Atmosphere using Broadband Emission Radiometry and Mesospheric Limb Sounder observations. Near 75 km, the model reproduces observed winter mesospheric vortex frequency of occurrence rates and size reasonably well. The simulated Antarctic mesospheric vortex is displaced toward the Pacific longitude sector, and the Arctic mesospheric vortex is present 10–20% less often over the pole but ~5% more often in midlatitudes, particularly over the continents. These differences are attributed to larger planetary waves in the model. There are significant differences between the modeled and observed mean polar winter circulation near 90 km. Model‐measurement differences suggest that the simulated Antarctic mesospheric vortex does not extend to high enough altitudes, and the Arctic mesospheric vortex is too displaced from the pole. Despite these differences, the model accurately captures the evolution of structural changes to the mesospheric vortex following prolonged sudden stratospheric warmings. Thus, we conclude that model underestimates of nitrogen oxides produced by energetic particle precipitation after these warmings are not due to a misrepresentation of the mesospheric vortex strength and size below 80 km. Key Points: First evaluation of the mesospheric vortex in a high‐top general circulation model, the whole atmosphere community climate modelModel reproduces the observed mesospheric vortex reasonably well at 75 km but there are significant circulation differences near 90 kmBelow 80 km, the modeled Arctic vortex agrees with the observations following prolonged sudden stratospheric warmings [ABSTRACT FROM AUTHOR]

Rapp, Dörnbrack, and Preusse (2018, https://doi.org/10.1029/2018GL079142) uses radio occultation observations and meteorological data from the European Centre for Medium Range Weather Forecasting to quantify the effect of inertial instability on stratospheric temperature variability. A very interesting by‐product of their study is the take‐away message that inertial instability "pancake structures" may be misidentified as being caused by gravity waves. We place this result in the context of inertial instability research. We concur with Rapp et al.'s caution to researchers to first examine the large‐scale meteorological environment to rule out inertial instability when analyzing temperature fluctuations that appear to be gravity wave signals in vertical profile data. Plain Language Summary: Rapp, Dörnbrack, and Preusse (2018) uses satellite observations and meteorological data to quantify the effect of inertial instability on stratospheric temperature variability. A very interesting by‐product of their study is the take‐away message that inertial instability "pancake structures" may be misidentified as being caused by gravity waves. We place this result in the context of inertial instability research. We concur with Rapp et al.'s caution to researchers to first examine the large‐scale meteorological environment to rule out inertial instability when analyzing temperature fluctuations that appear to be gravity wave signals in vertical profile data. Key Points: This commentary on Rapp, Dörnbrack, and Preusse (2018) reinforces the significance of their results and provides historical contextInertial instability "pancake structures" can masquerade as gravity wave signaturesResearchers must examine the large‐scale meteorological situation before attributing temperature perturbations to gravity waves [ABSTRACT FROM AUTHOR]

Abstract: The polar vortices play a central role in vertically coupling the atmosphere from the ground to geospace by shaping the background wind field through which atmospheric waves propagate. This work extends the vertical range of previous polar vortex climatologies into the upper mesosphere. The mesospheric polar vortices are defined using the CO gradient method with Microwave Limb Sounder satellite data; the stratospheric polar vortices are defined using a stream function‐based algorithm with data from meteorological reanalyses. Strengths and weaknesses of the two vortex definitions are given, as well as recommendations for when, where, and why to use each definition. Midwinter mean vortex geometry in the mesosphere is funnel shaped in the Arctic, with a wide top and narrow bottom. The Antarctic mesospheric vortex tapers with height in early winter and broadens with height in late winter. The seasonal evolution of mesospheric vortex frequency of occurrence, size, and zonal symmetry in both hemispheres is presented. Unexpected behavior above 60 km includes late season vortex broadening in both hemispheres, especially following winters without sudden stratospheric warmings. Following extreme stratospheric disturbances the polar night jet in the mesosphere strengthens and shifts poleward, resulting in a mesospheric vortex that contracts. Overall, the mesospheric polar vortices are more similar between the two hemispheres than their stratospheric counterparts. The vortex climatology presented here serves as an observational benchmark to which the mesospheric polar vortices in high‐top climate models can be evaluated. [ABSTRACT FROM AUTHOR]

This study uses photochemical calculations along kinematic trajectories in conjunction with Limb Infrared Monitor of the Stratosphere (LIMS) observations to examine the changes in HNO3 and NO2 near 30 hPa in the region of the Aleutian High (AH) during the minor warming event of January 1979. An earlier analysis of Version 5 (V5) LIMS data indicated increases in HNO3 without a corresponding decrease in NO2 in that region and a wave-2 signature in the zonal distribution of HNO3, unlike the wave-1 signal in ozone and other tracers. Version 6 (V6) LIMS also shows an increase of HNO3 in that region, but NO2 is smaller than from V5. The focus here is to convey that both V6 HNO3 and NO2 are of better quality than from V5, as shown here by a re-examination of their mutual changes in the AH region. Photochemical model calculations initialized with LIMS V6 data show increases of about 2 ppbv in HNO3 over 10 days along trajectories terminating in the AH region on 28 January. Those increases are mainly a result of the nighttime heterogeneous conversion of N2O5 on background stratospheric sulfuric acid aerosols. Changes in the composition of the air parcels depend on the extent of exposure to sunlight and, hence, on the dynamically controlled history of the trajectories. Trajectories that begin in low latitudes and traverse to across the Pole in a short time lead to the low HNO3 in the region separating the anticyclone from the polar vortex, both of which contain higher HNO3. These findings help to explain the observed seasonal evolution and areal extent of both species. V6 HNO3 and NO2 are suitable, within their errors, for the validation of stratospheric chemistry/climate models. [ABSTRACT FROM AUTHOR]

The meteorological control of gravity wave activity through filtering by winds and generation by spontaneous adjustment of unbalanced flows is investigated. This investigation is based on a new analysis of Rayleigh LiDAR measurements of gravity wave activity in the upper stratosphere-lower mesosphere (USLM, 40-50 km) on 152 nights at Poker Flat Research Range (PFRR), Chatanika, Alaska (65° N, 147° W), over 13 years between 1998 and 2014. The LiDAR measurements resolve inertia-gravity waves with observed periods between 1 h and 4 h and vertical wavelengths between 2 km and 10 km. The meteorological conditions are defined by reanalysis data from the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The gravity wave activity shows large night-to-night variability, but a clear annual cycle with a maximum in winter, and systematic interannual variability associated with stratospheric sudden warming events. The USLM gravity wave activity is correlated with the MERRA winds and is controlled by the winds in the lower stratosphere through filtering by critical layer filtering. The USLM gravity wave activity is also correlated with MERRA unbalanced flow as characterized by the residual of the nonlinear balance equation. This correlation with unbalanced flow only appears when the wind conditions are taken into account, indicating that wind filtering is the primary control of the gravity wave activity. [ABSTRACT FROM AUTHOR]

The climate research community uses atmospheric reanalysis data sets to understand a wide range of processes and variability in the atmosphere, yet different reanalyses may give very different results for the same diagnostics. The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets using a variety of key diagnostics. The objectives of this project are to identify differences among reanalyses and understand their underlying causes, to provide guidance on appropriate usage of various reanalysis products in scientific studies, particularly those of relevance to SPARC, and to contribute to future improvements in the reanalysis products by establishing collaborative links between reanalysis centres and data users. The project focuses predominantly on differences among reanalyses, although studies that include operational analyses and studies comparing reanalyses with observations are also included when appropriate. The emphasis is on diagnostics of the upper troposphere, stratosphere, and lower mesosphere. This paper summarizes the motivation and goals of the S-RIP activity and extensively reviews key technical aspects of the reanalysis data sets that are the focus of this activity. The special issue "The SPARC Reanalysis Intercomparison Project (S-RIP)" in this journal serves to collect research with relevance to the S-RIP in preparation for the publication of the planned two (interim and full) S-RIP reports. [ABSTRACT FROM AUTHOR]

Measurements of turbulence and waves were made as part of the Mesosphere‐Lower Thermosphere Turbulence Experiment (MTeX) on the night of 25–26 January 2015 at Poker Flat Research Range, Chatanika, Alaska (65°N, 147°W). Rocket‐borne ionization gauge measurements revealed turbulence in the 70‐ to 88‐km altitude region with energy dissipation rates between 0.1 and 24 mW/kg with an average value of 2.6 mW/kg. The eddy diffusion coefficient varied between 0.3 and 134 m2/s with an average value of 10 m2/s. Turbulence was detected around mesospheric inversion layers (MILs) in both the topside and bottomside of the MILs. These low levels of turbulence were measured after a minor sudden stratospheric warming when the circulation continued to be disturbed by planetary waves and winds remained weak in the stratosphere and mesosphere. Ground‐based lidar measurements characterized the ensemble of inertia‐gravity waves and monochromatic gravity waves. The ensemble of inertia‐gravity waves had a specific potential energy of 0.8 J/kg over the 40‐ to 50‐km altitude region, one of the lowest values recorded at Chatanika. The turbulence measurements coincided with the overturning of a 2.5‐hr monochromatic gravity wave in a depth of 3 km at 85 km. The energy dissipation rates were estimated to be 3 mW/kg for the ensemble of waves and 18 mW/kg for the monochromatic wave. The MTeX observations reveal low levels of turbulence associated with low levels of gravity wave activity. In the light of other Arctic observations and model studies, these observations suggest that there may be reduced turbulence during disturbed winters. Plain Language Summary: Turbulence remains an outstanding challenge in understanding coupling, energetics, and dynamics of the atmosphere. However, turbulence is recognized as a critical component in our models of terrestrial and space weather. Obtaining routine and accurate measurements of turbulence continues to be a major challenge. We present new rocket‐borne measurements of turbulence in January 2015 at Poker Flat Research Range, Alaska. These rocket‐borne measurements were coordinated with a suite of ground‐based instruments. The rocket‐borne instruments captured the small‐scale structure of the turbulence. The ground‐based measurements documented the meteorological and space weather conditions. We find low levels of turbulence coinciding with a disturbed atmosphere where wave activity is reduced. These finding suggest that there may be systematically low levels of turbulence in the Arctic middle atmosphere, as the Arctic middle atmosphere is routinely disturbed in winter. Key Points: Low levels of turbulence in the upper mesosphere are observed in a disturbed Arctic atmosphere following an SSWLow levels of turbulence coincide with low levels of gravity wave activityTurbulence is found in both the topside and bottomside of MILs [ABSTRACT FROM AUTHOR]

We use data from the Aeronomy of Ice in the Mesosphere (AIM) explorer and from the NASA Modern Era Retrospective Analysis for Research and Applications (MERRA) stratospheric analysis to explore the variability in the onset of the Northern Hemisphere (NH) Polar Mesospheric Cloud (PMC) season. Consistent with recently published results, we show that the early onset of the NH PMC season in 2013 was accompanied by a warm springtime stratosphere; conversely, we show that the late onset in 2008 coincides with a very cold springtime stratosphere. Similar stratospheric temperature anomalies for 1997 and 2011 also are connected either directly, through observed temperatures, or indirectly, through an early PMC onset, to conditions near the mesopause. These 4 years, 2008, 1997, 2011, and 2013 represent the extremes of stratospheric springtime temperatures seen in the MERRA analysis and correspond to analogous extrema in planetary wave activity. The three years with enhanced planetary wave activity (1997, 2011 and 2013) are shown to coincide with the recently identified stratospheric Frozen In Anticyclone (FrIAC) phenomenon. FrIACs in 1997 and 2013 are associated with early PMC onsets; however, the dramatic FrIAC of 2011 is not. This may be because the 2011 FrIAC occurred too early in the spring. The link between NH PMC onset and stratospheric FrIAC occurrences represents a new mode of coupling between the stratosphere and mesosphere. Since FrIACs appear to be more frequent in recent years, we speculate that as a result, PMCs may occur earlier as well. Finally we compare the zonal mean zonal winds and observed gravity wave activity for the FrIACs of 2011 and 2013. We find no evidence that gravity wave activity was favored in 2013 relative to 2011, thus suggesting that direct forcing by planetary waves was the key mechanism in accelerating the cooling and moistening of the NH mesopause region in May of 2013. [ABSTRACT FROM AUTHOR]