Mechanistic General Circulation Model Research Articles

Gravity waves generated by the Hunga Tonga–Hunga Ha′apai volcanic eruption and their global propagation in the mesosphere/lower thermosphere observed by meteor radars and modeled with the High-Altitude general Mechanistic Circulation Model

Abstract. The Hunga Tonga–Hunga Ha′apai volcano erupted on 15 January 2022, launching Lamb waves and gravity waves into the atmosphere. In this study, we present results using 13 globally distributed meteor radars and identify the volcanogenic gravity waves in the mesospheric/lower thermospheric winds. Leveraging the High-Altitude Mechanistic general Circulation Model (HIAMCM), we compare the global propagation of these gravity waves. We observed an eastward-propagating gravity wave packet with an observed phase speed of 240 ± 5.7 m s−1 and a westward-propagating gravity wave with an observed phase speed of 166.5 ± 6.4 m s−1. We identified these waves in HIAMCM and obtained very good agreement of the observed phase speeds of 239.5 ± 4.3 and 162.2 ± 6.1 m s−1 for the eastward the westward waves, respectively. Considering that HIAMCM perturbations in the mesosphere/lower thermosphere were the result of the secondary waves generated by the dissipation of the primary gravity waves from the volcanic eruption, this affirms the importance of higher-order wave generation. Furthermore, based on meteor radar observations of the gravity wave propagation around the globe, we estimate the eruption time to be within 6 min of the nominal value of 15 January 2022 04:15 UTC, and we localized the volcanic eruption to be within 78 km relative to the World Geodetic System 84 coordinates of the volcano, confirming our estimates to be realistic.

Numerical Simulations of Metallic Ion Density Perturbations in Sporadic E Layers Caused by Gravity Waves

AbstractTidal signatures in sporadic E (Es) layer have been confirmed by observations and simulations. However, the effect of gravity waves (GWs) on the Es layer formation process has not yet been fully understood. In this paper, the modulation of Es layers by GWs is examined through numerical simulations, in which a physics‐based model of Es layer is forced by neutral winds from the High Altitude Mechanistic General Circulation Model that can resolve GWs with horizontal wavelengths longer than 156 km (λh > 156 km). Comparison of the simulation results with and without the GWs (1,350 km > λh > 156 km) forcing reveals that the inclusion of GWs leads to short‐period (1.2–3 hr) density perturbations in Es layers, which are also seen in ground‐based ionosonde observations. At a given time, the metallic ion density at altitudes between 120 and 150 km can either increase (by up to ∼+600%) or reduce (by up to −90%) in response to GW forcing. The relative density perturbations are smaller (by up to 60%) between 90 and 120 km altitude. It is also found that the GW effect on the metallic ion density relates to the longitude, which is mostly explained by the geographical distribution of GWs activity in the mesosphere and lower thermosphere region. The longitudinal variation of the background geomagnetic field plays only a secondary role.

Primary and Secondary Gravity Waves and Large‐Scale Wind Changes Generated by the Tonga Volcanic Eruption on 15 January 2022: Modeling and Comparison With ICON‐MIGHTI Winds

AbstractWe simulate the primary and secondary atmospheric gravity waves (GWs) excited by the upward movement of air generated by the Hunga Tonga‐Hunga Ha'apai (hereafter “Tonga”) volcanic eruption on 15 January 2022. The Model for gravity wavE SOurce, Ray trAcing and reConstruction (MESORAC) is used to calculate the primary GWs and the local body forces/heatings generated where they dissipate. We add these forces/heatings to the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) to determine the secondary GWs and large‐scale wind changes that result. We find that a wide range of medium to large‐scale secondary GWs with concentric ring structure are created having horizontal wind amplitudes of u′, v′ ∼ 100–200 m/s, ground‐based periods of τr ∼ 20 min to 7 hr, horizontal phase speeds of cH ∼ 100–600 m/s, and horizontal wavelengths of λH ∼ 400–7,500 km. The fastest secondary GWs with cH ∼ 500–600 m/s are large‐scale GWs with λH ∼ 3,000–7,500 km and τr ∼ 1.5–7 hr. They reach the antipode over Africa ∼9 hr after creation. Large‐scale temporally and spatially varying wind changes of ∼80–120 m/s are created where the secondary GWs dissipate. We analyze the Tonga waves measured by the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) on the National Aeronautics and Space Administration Ionospheric Connection Explorer (ICON), and find that the observed GWs were medium to large‐scale with cH ∼ 100–600 m/s and λH ∼ 800–7,500 km, in good agreement with the simulated secondary GWs. We also find good agreement between ICON‐MIGHTI and HIAMCM for the timing, amplitudes, locations, and wavelengths of the Tonga waves, provided we increase the GW amplitudes by ∼2 and sample them ∼30 min later than ICON.

Secondary Gravity Waves From the Stratospheric Polar Vortex Over ALOMAR Observatory on 12–14 January 2016: Observations and Modeling

AbstractWe analyze the gravity waves (GWs) observed by a Rayleigh lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) (16.08°E, 69.38°N) in Norway at z ∼ 20–85 km on 12–14 January 2016. These GWs propagate upward and downward away from zknee = 57 and 64 km at a horizontally‐displaced location with periods τr ∼ 5–10 hr and vertical wavelengths λz ∼ 9–20 km. Because the hodographs are distorted, we introduce an alternative method to determine the GW parameters. We find that these GWs are medium to large‐scale, and propagate north/northwestward with intrinsic horizontal phase speeds of ∼35–65 m/s. Since the GW parameters are similar above and below zknee, these are secondary GWs created by local body forces (LBFs) south/southeast of ALOMAR. We use the nudged HIAMCM (HIgh Altitude Mechanistic general Circulation Model) to model these events. Remarkably, the model reproduces similar GW structures over ALOMAR, with zknee = 58 and 66 km. The event #1 GWs are created by a LBF at ∼35°E, ∼60°N, and z ∼ 58 km. This LBF is created by the breaking and dissipation of primary GWs generated and amplified by the imbalance of the polar night jet below the wind maximum; the primary GWs for this event are created at z ∼ 25–35 km at 49–53°N. We also find that the HIAMCM GWs agree well with those observed by the Atmospheric InfraRed Sounder (AIRS) satellite, and that those AIRS GWs south and north of ∼50°N over Europe are mainly mountain waves and GWs from the polar vortex, respectively.

Simulation Study of the 15 January 2022 Tonga Event: Development of Super Equatorial Plasma Bubbles

AbstractWe present high‐resolution simulation results of the response of the ionosphere/plasmasphere system to the 15 January 2022 Tonga volcanic eruption. We use the coupled Sami3 is Also a Model of the Ionosphere ionosphere/plasmasphere model and the HIgh Altitude Mechanistic general Circulation Model whole atmosphere model with primary atmospheric gravity wave effects from the Model for gravity wavE SOurces, Ray trAcing and reConstruction model. We find that the Tonga eruption produced a “super” equatorial plasma bubble (EPB) extending ∼30° in longitude and up to 500 km in altitude with a density depletion of 3 orders of magnitude. We also found a “train” of EPBs developed and extended over the longitude range 150°–200° and that two EPBs reached altitudes over 4,000 km. The primary cause of this behavior is the significant modification of the zonal neutral wind caused by the atmospheric disturbance associated with the eruption, and the subsequent modification of the dynamo electric field.

Multi‐Step Vertical Coupling During the January 2017 Sudden Stratospheric Warming

AbstractThis study analyzes a simulation of the Arctic winter 2016–2017 with focus on multi‐step vertical coupling (MSVC) by primary, secondary, and higher‐order gravity waves (GWs). We employ the HIgh Altitude Mechanistic general Circulation Model with nudging of the large scales to MERRA‐2 reanalysis. Simulation results confirm the well‐known effects from primary GWs in the winter middle atmosphere regarding strong westward GW drag and a warm winter polar stratopause during the strong‐vortex period in late December 2016, as well as weak eastward GW drag and mesospheric cooling during the sudden stratospheric warming (SSW) in late January and early February 2017. Since the amplitudes of the primary GWs that dissipate in the middle atmosphere are weaker for a reversed or weakened polar vortex, the theory for secondary GW generation predicts reduced MSVC in this case. This is confirmed by strongly reduced secondary and higher‐order GW amplitudes during the SSW and the weak‐vortex period in February. The wintertime higher‐order GWs show partial concentric ring structures above their sources in the lower thermosphere. We find that mostly those higher‐order GWs propagate to higher altitudes that have horizontal propagation directions against the tidal winds. The simulated GWs at 300 km height and observed perturbations of total electron content over Europe and North America during selected days of low geomagnetic activity show very good agreement regarding (a) the wave characteristics and (b) the reduction of amplitudes during the SSW and early February as compared to late December.

A High‐Resolution Whole‐Atmosphere Model With Resolved Gravity Waves and Specified Large‐Scale Dynamics in the Troposphere and Stratosphere

AbstractWe 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.

Explicit Global Simulation of Gravity Waves in the Thermosphere

AbstractWe present a new version of the high‐resolution Kühlungsborn Mechanistic general Circulation Model (KMCM) extended to z ∼ 450 km. This model is called HIAMCM (HI Altitude Mechanistic general Circulation Model) and explicitly simulates gravity waves (GWs) down to horizontal wavelengths of λh ∼ 165 km. We find predominant tertiary GWs in the winter thermosphere at middle/high latitudes. These GWs typically have horizontal wavelengths λh ∼ 300–1,100 km, ground‐based periods ∼ 25–90 min, and intrinsic horizontal phase speeds cIh ∼ 250–350 m s−1. Above z ∼ 200 km, the predominant GW horizontal propagation directions are roughly against the background winds from the diurnal tide; the GWs propagate mainly poleward at midnight, eastward at 6 local time (LT), equatorward at noon, and westward at 18 LT. Wintertime GWs at z ∼ 300 km having 165 km ≤ λh ≤ 330 km create a large hot spot over the Southern Andes/Antarctic Peninsula that agrees well with quiet time satellite measurements. Due to cancelation effects, the time‐averaged zonal mean Eliassen‐Palm flux divergence from the resolved GWs in the thermosphere is negligible compared to that of the tides and compared to the zonal component of the time‐averaged zonal mean ion drag. We also find that the thermospheric GWs dissipate mainly from macroturbulent diffusion and, above z ∼ 200 km, from molecular diffusion, whereas the tides dissipate mainly from ion drag. The averaged dissipative heating in the thermosphere due to tides is much stronger than that due to GWs.

Gravity wave mixing effects on the OH*-layer

Based on an advanced numerical model for excited hydroxyl (OH*) we simulate the effects of gravity waves (GWs) on the OH*-layer in the upper mesosphere. The OH* model takes into account (1) production by the reaction of atomic hydrogen (H) with ozone (O3), (2) deactivation by atomic oxygen (O), molecular oxygen (O2), and molecular nitrogen (N2), (3) spontaneous emission, and (4) loss due to chemical reaction with O. This OH* model is part of a chemistry-transport model (CTM) which is driven by the high-resolution dynamics from the KMCM (Kühlungsborn Mechanistic general Circulation Model) which simulates mid-frequency GWs and their effects on the mean flow in the MLT explicitly. We find that the maximum number density and the height of the OH*-layer peak are strongly determined by the distribution of atomic oxygen and by the temperature. As a results, there are two ways how GWs influence the OH*-layer: (1) through the instantaneous modulation by O and T on short time scales (a few hours), and (2) through vertical mixing of O (days to weeks). The instantaneous variations of the OH*-layer peak altitude due to GWs amount to 5–10 km. Such variations would introduce significant biases in the GW parameters derived from airglow when assuming a constant pressure level of the emission height. Performing a sensitivity experiment we find that on average, the vertical mixing by GWs moves the OH*-layer down by ~2 to 7 km and increases its number density by more than 50%. This effect is strongest at middle and high latitudes during winter where secondary GWs generated in the stratopause region account for large GW amplitudes.

Numerical Modeling of the Excitation, Propagation, and Dissipation of Primary and Secondary Gravity Waves during Wintertime at McMurdo Station in the Antarctic

AbstractWe analyze the results of the gravity wave (GW)‐resolving, high‐resolution Kühlungsborn Mechanistic general Circulation Model in July at McMurdo Station (166.69°E and 77.84°S), where strong downslope eastward winds create strong mountain wave (MW) events. These MWs have horizontal wavelengths of λH≃230 km, propagate to z∼ 40–60 km, and can have upward phases in time if the eastward wind accelerates in time. Additionally, inertia‐GWs (IGWs) with λH∼ 500–800 km and ground‐based periods of τr∼ 5–6 hr are generated in the troposphere from unbalanced, large‐scale flow. The density‐scaled GW amplitudes are ∼10 times smaller at z∼ 80–100 km than at z < 50 km because of severe wave dissipation. “Fishbone” structures are seen at z∼ 30–60 km with upward (downward) phases in time below (above) the “knee” at zknee. We horizontally filter the perturbations to isolate the GWs in a fishbone structure for a particular MW event. We find that these GWs have strikingly similar parameters below and above zknee=46 km, with ground‐based horizontal phase speeds of cH∼ 40–60 m/s, τr∼ 9–10 hr, λH∼ 1,600–2,050 km, vertical wavelengths of λz∼ 18–25 km, and azimuths of Υ= 145° –151° east of north. We show that these are secondary GWs excited by a body force at zknee created by MW dissipation approximately 400 km northwest of McMurdo 2.5 hr earlier and that the secondary GW scales and propagation directions are consistent with this force. Importantly, we show that most of the GWs at z > 70 km are secondary GWs not primary GWs from the troposphere.

Scale-Invariant Formulation of Momentum Diffusion for High-Resolution Atmospheric Circulation Models

A new version of the dynamic Smagorinsky model is presented that applies for nonisotropic momentum diffusion in high-resolution atmospheric circulation models. While the horizontal mixing length is computed in accordance with scale invariance in the mesoscale regime of the horizontal energy cascade, the associated dynamic vertical mixing length (DVML) is based on a recently developed scale invariance criterion and represents an application of the scaling laws of stratified macroturbulence. The DVML is validated in high-resolution simulations with the Kühlungsborn mechanistic general circulation model, using triangular spectral truncation at wavenumber 330 and a vertical level spacing of about 200 m in the upper troposphere. For a proper choice of the test filter, the model simulates a realistic horizontal kinetic energy spectrum in the troposphere along with a realistic intensity of the Lorenz energy cycle. This result is obtained without any hyperdiffusion, and it depends only little on whether the vertical mixing length is prescribed or set to the DVML. The globally averaged Smagorinsky parameter is about c S ≅ 0.53. The latitude–height cross sections show that c S maximizes in regions of strong mesoscale kinetic energy.

Numerical simulation of wave interactions during sudden stratospheric warming

Parameterizations of normal atmospheric modes (NAMs) and orographic gravity waves (OGWs) are implemented into the mechanistic general circulation model of the middle and upper atmosphere (MUA). Numerical experiments of sudden stratospheric warming (SSW) events are performed for climatological conditions typical for January and February using meteorological reanalysis data from the UK MET Office in the MUA model averaged over the years 1992–2011 with the easterly phase of quasi-biennial oscillation (QBO). The simulation shows that an increase in the OGW amplitudes occurs at altitudes higher than 30 km in the Northern Hemisphere after SSW. The OGW amplitudes have maximums at altitudes of about 50 km over the North American and European mountain systems before and during SSW, as well as over the Himalayas after SSW. At high latitudes of the Northern Hemisphere, significant (up to 50–70%) variations in the amplitudes of stationary planetary waves (SPWs) are observed during and after the SSW. Westward travelling NAMs have local amplitude maximums not only in the Northern Hemisphere, but also in the Southern Hemisphere, where there are waveguides for the propagation of these modes. Calculated variations of SPW and NAM amplitudes correspond to changes in the mean temperature and wind fields, as well as the Eliassen-Palm flux and atmospheric refractive index for the planetary waves, during SSW. Including OGW thermal and dynamical effects leads to an increase in amplitude (by 30–70%) of almost all SPWs before and during SSW and to a decrease (up to 20–100%) after the SSW at middle and high latitudes of the Northern Hemisphere.

How Does Interhemispheric Coupling Contribute to Cool Down the Summer Polar Mesosphere?

Abstract Interhemispheric coupling is commonly associated with events of high planetary wave activity in the winter stratosphere triggering a heating of the polar mesopause region in the opposite hemisphere. Here, a more fundamental role that this mechanism plays in the absence of planetary wave variability is highlighted. This study focuses directly on the mesospheric part of the coupling chain, which is induced by the gravity wave drag in the winter mesosphere. To investigate the effect that the winter residual flow has on the summertime high-latitude upwelling, the Kühlungsborn Mechanistic General Circulation Model (KMCM) is used to compare a control simulation to runs where the parameterized gravity waves are removed from the winter hemisphere. The model response in the summer mesosphere reveals that the winter mesospheric residual circulation fosters a net (and substantial) cooling of the summer polar mesopause. These results offer an extension of the current view of interhemispheric coupling: from a mode of internal variability to a constant, gravity wave–driven phenomenon that is modulated by planetary wave activity.

Nucleation of mesospheric cloud particles: Sensitivities and limits

AbstractNucleation of mesospheric ice particles is thought to occur via heterogeneous nucleation on meteor smoke particles. However, several factors determining the nucleation rate are poorly known. To study the effect of uncertainties in the nucleation rate on cloud properties, we use the Community Aerosol and Radiation Model for Atmospheres and systematically vary the nucleation rate over ±10 orders of magnitude. In one set of simulations, the background state of the atmosphere is described by climatological conditions. In a second set, gravity wave‐perturbed profiles from the Kühlungsborn Mechanistic general Circulation Model (KMCM) are used with typical temperature (vertical wind) perturbations at the mesopause on the order of 9 K (0.45 m/s). The resulting noctilucent cloud (NLC) characteristics are compared to lidar and satellite measurements. Realistic NLCs compared to the lidar measurements can only be modeled if the nucleation rate is reduced by up to 3 orders of magnitude compared to standard assumptions. For the same cases, the simulated NLCs compare best to the satellite measurements if the nucleation rate is reduced by 2 orders of magnitude or more. Dynamical processes at the mesopause strongly influence the NLC development. In a gravity wave‐perturbed atmosphere, the ice particles have only limited time for nucleation and growth. The growth time is limited by the vertical wind, because the vertical wind determines the residence time of the ice particles in the supersaturated region. Since the vertical wind amplitudes reach 1.5 m/s in KMCM (compared to a mean upwelling of ∼4 cm/s in the climatology), the ice particles remain significantly smaller in a gravity wave‐perturbed atmosphere than in climatological background conditions.

Horizontal Momentum Diffusion in GCMs Using the Dynamic Smagorinsky Model

Abstract A dynamic version of Smagorinsky’s diffusion scheme is presented that is applicable for large-eddy simulations (LES) of the atmospheric dynamics. The approach is motivated (i) by the incompatibility of conventional hyperdiffusion schemes with the conservation laws, and (ii) because the conventional Smagorinsky model (which fulfills the conservation laws) does not maintain scale invariance, which is mandatory for a correct simulation of the macroturbulent kinetic energy spectrum. The authors derive a two-dimensional (horizontal) formulation of the dynamic Smagorinsky model (DSM) and present three solutions of the so-called Germano identity: the method of least squares, a solution without invariance of the Smagorinsky parameter, and a tensor-norm solution. The applicability of the tensor-norm approach is confirmed in simulations with the Kühlungsborn mechanistic general circulation model (KMCM). The standard spectral dynamical core of the model facilitates the implementation of the test filter procedure of the DSM. Various energy spectra simulated with the DSM and the conventional Smagorinsky scheme are presented. In particular, the results show that only the DSM allows for a reasonable spectrum at all scales. Latitude–height cross sections of zonal-mean fluid variables are given and show that the DSM preserves the main features of the atmospheric dynamics. The best ratio for the test-filter scale to the resolution scale is found to be 1.33, resulting in dynamically determined Smagorinsky parameters cS from 0.10 to 0.22 in the troposphere. This result is very similar to other values of cS found in previous three-dimensional applications of the DSM.

Testing linear gravity wave theory with simultaneous wind and temperature data from the mesosphere

Linear gravity wave (GW) theory is tested on the basis of simultaneous measurements of horizontal winds from a medium frequency (MF) radar at Juliusruh (54.6°N, 13.4°E) and temperatures from combined Potassium (K) and Rayleigh–Mie–Raman (RMR) lidars at Kühlungsborn (54.1°N, 11.8°E). The applicability of linear GW theory to mesospheric observations is far from obvious given the fact that typically a whole spectrum of waves is observed which may interact non-linearly. Before analyzing our experimental dataset for its fit to expectations from linear GW theory, the chosen methodology is tested with model data from the Kühlungsborn Mechanistic general Circulation Model (KMCM). This model is a mechanistic general circulation model with high spatial resolution such that waves with horizontal wavelengths in excess of ∼350km are explicitly resolved yielding a semi-realistic wave motion field. This may be considered as a suitable test-bed for defining and optimizing wave analysis approaches. This effort reveals that Stokes parameters analysis of filtered time series of GW-induced wind and temperature fluctuations in comparison to wave amplitudes directly retrieved from the filtered time series allows us to demonstrate the validity of polarization relations based on linear wave theory. Indeed, applying the same methodology to the observations yields similarly conclusive results thus giving evidence for the applicability of linear wave theory to mesospheric observations after appropriate filtering. These investigations are complemented by a comparison of kinetic and potential energy per unit mass for model and measured data. This reveals that the ratio of kinetic and potential energy also roughly follows expectations from linear wave theory.

Gravity Wave Mixing and Effective Diffusivity for Minor Chemical Constituents in the Mesosphere/Lower Thermosphere

The influence of gravity waves (GWs) on the distributions of minor chemical constituents in the mesosphere-lower thermosphere (MLT) is studied on the basis of the effective diffusivity concept. The mixing ratios of chemical species used for calculations of the effective diffusivity are obtained from numerical experiments with an off-line coupled model of the dynamics and chemistry abbreviated as KMCM-MECTM (Kuehlungsborn Mechanistic general Circulation Model—MEsospheric Chemistry-Transport Model). In our control simulation the MECTM is driven with the full dynamical fields from an annual cycle simulation with the KMCM, where mid-frequency GWs down to horizontal wavelengths of 350 km are resolved and their wave-mean flow interaction is self-consistently induced by an advanced turbulence model. A perturbation simulation with the MECTM is defined by eliminating all meso-scale variations with horizontal wavelengths shorter than 1000 km from the dynamical fields by means of spectral filtering before running the MECTM.

Wave mixing effects on minor chemical constituents in the MLT region: Results from a global CTM driven by high-resolution dynamics

[1] The dynamical fields of a high-resolution mechanistic general circulation model (GCM) with resolved gravity waves (GWs) are used to drive a chemistry-transport model (CTM) of the mesosphere-lower thermosphere (MLT). Both models have been used in previous studies and are coupled in off-line mode in order to study the explicit effects of GWs on the transport and photochemistry of minor constituents of the MLT. This is done on the basis of a sensitivity experiment. In our control simulation the CTM is driven with the full dynamical fields from an annual cycle simulated with the GCM, where mid-frequency GWs down to horizontal wavelengths of 350 km are resolved and their wave-mean flow interaction is self-consistently induced by an advanced turbulence model. A perturbation simulation with the CTM is defined by eliminating all meso-scale variations with horizontal wavelengths shorter than 1000 km from the dynamical fields by means of spectral filtering before running the CTM. The response of the CTM to GWs perturbations reveals strong effects on the trace-gas concentrations. In particular, minor chemical constituents with large photochemical life-time are strongly affected by vertical wave mixing, while constituents with short life-time reflect the dependence of their reaction rates on meso-scale temperature perturbations and on the changed distributions of long-lived constituents. The mean model response is strongest around the mesopause. We present detailed discussions of the simulated gravity-wave mixing effects on the photochemistry and highlight the consequences for our understanding of the general circulation of the MLT.

An idealized radiative transfer scheme for use in a mechanistic general circulation model from the surface up to the mesopause region

A new and numerically efficient method to compute radiative flux densities and heating rates in a general atmospheric circulation model is presented. Our method accommodates the fundamental differences between the troposphere and middle atmosphere in the long-wave regime within a single parameterization that extends continuously from the surface up to the mesopause region and takes the deviations from the gray limit and from the local thermodynamic equilibrium into account. For this purpose, frequency-averaged Eddington-type transfer equations are derived for four broad absorber bands. The frequency variation inside each band is parameterized by application of the Elsasser band model extended by a slowly varying envelope function. This yields additional transfer equations for the perturbation amplitudes that are solved numerically along with the mean transfer equations. Deviations from local thermodynamic equilibrium are included in terms of isotropic scattering, calculating the single scattering albedo from the two-level model for each band. Solar radiative flux densities are computed for four energetically defined bands using the simple Beer–Bougert–Lambert relation for absorption within the atmosphere. The new scheme is implemented in a mechanistic general circulation model from the surface up to the mesopause region. A test simulation with prescribed concentrations of the radiatively active constituents shows quite reasonable results. In particular, since we take the full surface energy budget into account by means of a swamp ocean, and since the internal dynamics and turbulent diffusion of the model are formulated in accordance with the conservation laws, an equilibrated climatological radiation budget is obtained both at the top of the atmosphere and at the surface.

Seasonal variation of mesospheric waves at northern middle and high latitudes

The seasonal variation of the wave activity in the mesosphere/lower thermosphere is investigated using wind measurements with meteor and MF radars at Juliusruh (55°N, 13°E) and Andenes (69°N, 16°E), as well as on the basis of the simulated annual cycle using a gravity-wave resolving mechanistic general circulation model. For the observations, proxies for the activity of gravity waves (GWs) and waves with longer periods are computed from wind variances for defined bandwidths. Our corresponding proxy for the simulated GWs is the non-rotational kinetic energy due to the resolved mesoscales. Both observational and computational results show the strongest GW energy during winter and a secondary maximum during summer. Additional observational analysis of short-period GWs yields a more pronounced summer maximum. The semi-annual variation is consistent with the selective filtering of westward and eastward GWs by the mean zonal wind. The latitudinal dependence during summer is characterized by stronger GW energy between 65 and 85 km at middle latitudes than at polar latitudes, and a corresponding upward shift of the wind reversal towards the pole which is also reflected by the simulated GW drag. Also the observed oscillations with periods from 2 to 4 days show a latitudinal dependence and a clear seasonal cycle which is related to the mean zonal wind shear.