For the first time, a formal comparison is made between gravity wave momentum fluxes in models and those derived from observations. Although gravity waves occur over a wide range of spatial and ...temporal scales, the focus of this paper is on scales that are being parameterized in present climate models, sub-1000-km scales. Only observational methods that permit derivation of gravity wave momentum fluxes over large geographical areas are discussed, and these are from satellite temperature measurements, constant-density long-duration balloons, and high-vertical-resolution radiosonde data. The models discussed include two high-resolution models in which gravity waves are explicitly modeled, Kanto and the Community Atmosphere Model, version 5 (CAM5), and three climate models containing gravity wave parameterizations, MAECHAM5, Hadley Centre Global Environmental Model 3 (HadGEM3), and the Goddard Institute for Space Studies (GISS) model. Measurements generally show similar flux magnitudes as in models, except that the fluxes derived from satellite measurements fall off more rapidly with height. This is likely due to limitations on the observable range of wavelengths, although other factors may contribute. When one accounts for this more rapid fall off, the geographical distribution of the fluxes from observations and models compare reasonably well, except for certain features that depend on the specification of the nonorographic gravity wave source functions in the climate models. For instance, both the observed fluxes and those in the high-resolution models are very small at summer high latitudes, but this is not the case for some of the climate models. This comparison between gravity wave fluxes from climate models, high-resolution models, and fluxes derived from observations indicates that such efforts offer a promising path toward improving specifications of gravity wave sources in climate models.
Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from ...global observations of gravity waves are sparse. For the period 2002–2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and a proxy for absolute gravity wave drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology; data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes; and data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere the SABER absolute gravity wave drag proxy correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75–85 km, the SABER absolute gravity wave drag proxy correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: all reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2, this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models; therefore, a more realistic representation is needed.
Atmospheric gravity waves (GWs) are essential for the dynamics of the middle atmosphere. Recent studies have shown that these waves are also important for the thermosphere/ionosphere (T/I) system. ...Via vertical coupling, GWs can significantly influence the mean state of the T/I system. However, the penetration of GWs into the T/I system is not fully understood in modeling as well as observations. In the current study, we analyze the correlation between GW momentum fluxes observed in the middle atmosphere (30–90 km) and GW-induced perturbations in the T/I. In the middle atmosphere, GW momentum fluxes are derived from temperature observations of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument. In the T/I, GW-induced perturbations are derived from neutral density measured by instruments on the Gravity field and Ocean Circulation Explorer (GOCE) and CHAllenging Minisatellite Payload (CHAMP) satellites. We find generally positive correlations between horizontal distributions at low altitudes (i.e., below 90 km) and horizontal distributions of GW-induced density fluctuations in the T/I (at 200 km and above). Two coupling mechanisms are likely responsible for these positive correlations: (1) fast GWs generated in the troposphere and lower stratosphere can propagate directly to the T/I and (2) primary GWs with their origins in the lower atmosphere dissipate while propagating upwards and generate secondary GWs, which then penetrate up to the T/I and maintain the spatial patterns of GW distributions in the lower atmosphere. The mountain-wave related hotspot over the Andes and Antarctic Peninsula is found clearly in observations of all instruments used in our analysis. Latitude–longitude variations in the summer midlatitudes are also found in observations of all instruments. These variations and strong positive correlations in the summer midlatitudes suggest that GWs with origins related to convection also propagate up to the T/I. Different processes which likely influence the vertical coupling are GW dissipation, possible generation of secondary GWs, and horizontal propagation of GWs. Limitations of the observations as well as of our research approach are discussed. Keywords. Ionosphere (ionosphere–atmosphere interactions)
Gravity wave (GW) parametrizations for general circulation models (GCMs) restrict the propagation of GWs to the vertical direction. The influence of this vertical‐only propagation assumption on the ...distribution of GW drag (GWD) has not yet been investigated. Thus, we present results of two global GW ray tracing simulations, one with full three‐dimensional propagation (GWO) and a second one with vertical‐only propagation (GWV) of GWs for January and July 2008. The Gravity wave Regional Or Global RAy Tracer (GROGRAT) was used to perform these simulations with a global homogeneous and isotropic launch distribution. Both simulations, GWO and GWV, are analyzed with respect to GWD in the zonal and meridional direction. The location of zonal GWD maxima changes. GWO shows in comparison to GWV a poleward shift of zonal GWD in both seasons with increased GWD at the summer stratopause. The meridional GWD is much stronger in the GWO case, spatially correlated with the zonal drag, and is generally poleward directed. These features in zonal and meridional drag are consistent with a general prevalence of poleward propagation of GWs. Additional simulations suggest that this is due to the Coriolis effect as well as wind filtering around the tropopause, allowing more GWs to propagate into the middle atmosphere. We infer how GWs of different horizontal wavelengths and phase speeds cause the main differences in GWD in the middle atmosphere. A simple test for GCMs is proposed to assess the effects of the altered meridional drag on the general circulation and the interaction with planetary waves.
Key Points
Meridional GW drag found to be poleward directed regardless of the seasonZonal GW drag found poleward shifted and increased at the summer stratopauseEasy‐to‐implement test for GCM modeling proposed
Gravity waves are one of the main drivers of atmospheric dynamics. The
spatial resolution of most global atmospheric models, however, is too coarse
to properly resolve the small scales of gravity ...waves, which range from tens
to a few thousand kilometers horizontally, and from below 1 km to tens of
kilometers vertically. Gravity wave source processes involve even smaller
scales. Therefore, general circulation models (GCMs) and chemistry climate
models (CCMs) usually parametrize the effect of gravity waves on the global
circulation. These parametrizations are very simplified. For this reason,
comparisons with global observations of gravity waves are needed for an
improvement of parametrizations and an alleviation of model biases. We present a gravity wave climatology based on atmospheric infrared limb
emissions observed by satellite (GRACILE). GRACILE is a global data set of
gravity wave distributions observed in the stratosphere and the mesosphere by
the infrared limb sounding satellite instruments High Resolution Dynamics
Limb Sounder (HIRDLS) and Sounding of the Atmosphere using Broadband Emission
Radiometry (SABER). Typical distributions (zonal averages and global maps) of
gravity wave vertical wavelengths and along-track horizontal wavenumbers are
provided, as well as gravity wave temperature variances, potential energies
and absolute momentum fluxes. This global data set captures the typical
seasonal variations of these parameters, as well as their spatial variations.
The GRACILE data set is suitable for scientific studies, and it can serve for
comparison with other instruments (ground-based, airborne, or other satellite
instruments) and for comparison with gravity wave distributions, both
resolved and parametrized, in GCMs and CCMs. The GRACILE data set is
available as supplementary data at
https://doi.org/10.1594/PANGAEA.879658.
Atmospheric gravity waves (GWs) are an important coupling mechanism in the middle atmosphere. For instance, they provide a large part of the driving of long-period atmospheric oscillations such as ...the Quasi-Biennial Oscillation (QBO) and the semiannual oscillation (SAO) and are in turn modulated. They also induce the wind reversal in the mesosphere–lower thermosphere region (MLT) and the residual mean circulation at these altitudes. In this study, the variations in monthly zonal mean gravity wave square temperature amplitudes (GWSTAs) and, for the first time, absolute gravity wave momentum flux (GWMF) on different timescales such as the annual, semiannual, terannual and quasi-biennial variations are investigated by spectrally analyzing SABER observations from 2002 to 2015. Latitude–altitude cross sections of spectral amplitudes and phases of GWSTA and absolute GWMF in the stratosphere and mesosphere are presented and physically interpreted. It is shown that the time series of GWSTA and GWMF at a certain altitude and latitude results from the complex interplay of GW sources, propagation through and filtering in lower altitudes, oblique propagation superposing GWs from different source locations, and, finally, the modulation of the GW spectrum by the winds at a considered altitude and latitude. The strongest component is the annual variation, dominated in the summer hemisphere by subtropical convective sources and in the winter hemisphere by polar vortex dynamics. At heights of the wind reversal, a 180∘ phase shift also occurs, which is at different altitudes for GWSTA and GWMF. In the intermediate latitudes a semiannual variation (SAV) is found. Dedicated GW modeling is used to investigate the nature of this SAV, which is a different phenomenon from the tropical SAO also seen in the data. In the tropics a stratospheric and a mesospheric QBO are found, which are, as expected, in antiphase. Indication for a QBO influence is also found at higher latitudes. In previous studies a terannual variation (TAV) was identified. In the current study we explain its origin. In particular the observed patterns for the shorter periods, SAV and TAV, can only be explained by poleward propagation of GWs from the lower-stratosphere subtropics into the midlatitude and high-latitude mesosphere. In this way, critical wind filtering in the lowermost stratosphere is avoided and this oblique propagation is hence likely an important factor for MLT dynamics.
Temperature data obtained by the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) are analyzed for gravity waves (GWs). Amplitude, phase and vertical wavelength are ...determined from detrended temperature height profiles. The retrieved phases are utilized to estimate the horizontal wavelengths. At 25 km altitude an equatorial maximum of horizontal wavelength with a decrease toward mid and high latitudes is found. Simultaneous estimates of both horizontal and vertical wavelengths and temperature amplitudes allow the direct calculation of GW momentum flux (MF) from satellite observations for the first time. However, histograms of horizontal wavelength distributions indicate severe undersampling which prevents the retrieval of the propagation directions of the waves, and suggests our MF estimates may be too low, particularly at the high latitudes. Therefore an empirical aliasing correction has been applied. A world map of MF at 25 km altitude shows high variability and pronounced source regions and deviates in structure from a map of GW variances at the same altitude. Results from the Warner and McIntyre GW parameterization scheme (three‐part model) show better agreement with CRISTA MF estimates than with CRISTA squared GW amplitudes. Best agreement is found for low model launch levels. Large error ranges of the estimated MF values obtained in this paper could be substantially reduced by improved horizontal sampling in future satellite missions.
We use theory and global ray modeling to investigate how the potential of gravity waves to transport momentum flux globally from the lower atmosphere into the mesosphere and lower thermosphere (MLT) ...varies with horizontal wavelength and ground‐based phase speed. Ray modeling is performed using the Gravity Wave Regional or Global Ray Tracer (GROGRAT) interfaced to realistic three‐dimensional global winds and temperatures from 0 to 100 km altitude, specified by fusing analysis fields at lower altitudes to GCM results higher up. We focus on gravity waves in the short 10‐ to 50‐km horizontal wavelength range that are unresolved by global models and, according to theory, can transport appreciable momentum flux into the MLT. Ray results for different seasons reproduce some of the limits derived from simple wave theory: that horizontal wavelengths shorter than 10 km tend to be removed by vertical reflection or evanescence at the source and slower phase speeds are more prone to critical level removal, leading to a preference for waves with longer horizontal wavelengths and faster ground‐based phase speeds to reach the MLT. These findings are compared to the wavelength scales currently resolved by satellite limb and nadir sounders, highlighting wavelength ranges currently measured and those currently unresolved. A road map is developed for how current and future satellite measurements can be combined to measure the full space‐time spectrum of gravity waves relevant to eddy flux deposition and momentum forcing of the global MLT. In particular, recommendations for new satellite measurement strategies that fill current measurement gaps are provided.
Convection as one dominant source of atmospheric gravity waves (GWs) has been the focus of investigation over recent years. However, its spatial and temporal forcing scales are not well known. In ...this work we address this open issue by a systematic verification of free parameters of the Yonsei convective GW source scheme based on observations from the High Resolution Dynamics Limb Sounder (HIRDLS). The instrument can only see a limited portion of the gravity wave spectrum due to visibility effects and observation geometry. To allow for a meaningful comparison of simulated GWs to observations, a comprehensive filter, which mimics the instrument limitations, is applied to the simulated waves. By this approach, only long horizontal-scale convective GWs are addressed. Results show that spectrum, distribution of momentum flux, and zonal mean forcing of long horizontal-scale convective GWs can be successfully simulated by the superposition of three or four combinations of parameter sets reproducing the observed GW spectrum. These selected parameter sets are different for northern and southern summer. Although long horizontal-scale waves are only part of the full spectrum of convective GWs, the momentum flux of these waves is found to be significant and relevant for the driving of the QBO (quasi-biennial oscillation). The zonal momentum balance is considered in vertical cross sections of GW momentum flux (GWMF) and GW drag (GWD). Global maps of the horizontal distribution of GWMF are considered and consistency between simulated results and HIRDLS observations is found. The latitude dependence of the zonal phase speed spectrum of GWMF and its change with altitude is discussed.
PDF
