Characteristics of Gravity Waves from Convection and Implications for Their Parameterization in Global Circulation Models

A parameterization of gravity wave (GW) drag, suitable for implementation into general circulation models (GCMs) extending into the thermosphere is presented. Unlike existing schemes, the parameterization systematically accounts for wave dissipation in the upper atmosphere due to molecular viscosity, thermal conduction, ion friction, and radiative damping in the form of the Newtonian cooling. This is in addition to using the commonly employed breaking/saturation schemes, based on either linear Hodges‐Lindzen instability criteria or its nonlinear extension to multiple‐harmonic spectra. The scheme was evaluated in a series of tests of increasing complexity. In the thermosphere, the simulations suggest that the dissipation competes with the instability caused by amplitude growth, and can seriously alter GW propagation and the associated wave drag. Above the mesopause the GW drag is generally created by harmonics with fast horizontal phase velocities, which under favorable conditions can propagate into the F2 layer. The effects of thermospheric dissipation are more complex than a simple exponential decay of GW fluxes above certain levels. We examine the sensitivity of the GW drag profiles to the variations of the source spectra typically employed in GCMs. These results suggest that GWs can provide strong coupling between the meteorological events in the lower atmosphere and the circulation well above the middle atmosphere.

Abstract. 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.

The stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere‐troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid‐scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON‐NWP.

Tropical convection is inherently multiscalar, involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters. In addition to being a crucial component of the atmospheric water cycle and the global heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to the momentum transport. The first resides entirely in the troposphere and is due to ascent, descent, and organized circulations associated with precipitating convective systems. The second resides in the troposphere, stratosphere, and farther aloft and is caused by vertically propagating gravity waves. Both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models; yet in existing parameterizations, these two processes are treated independently. This paper examines the relationship between the convective momentum transport and convectively generated gravity wave momentum flux by utilizing idealized simulations of multiscale tropical convection in different wind shear conditions. The simulations produce convective systems with a variety of regimes of convective organization and therefore different convective momentum transport properties and gravity wave spectra. A number of important connections are identified, including a consistency in the sign of the momentum transports in the lower troposphere and stratosphere that is linked to the generation of gravity waves by tilted convective structures. These results elucidate important relationships between the convective momentum transport and the gravity wave momentum flux that will be useful for interlinking their parameterization in the future.

Abstract. Nearly all general circulation models significantly fail to reproduce the observed behaviour of the southern wintertime polar vortex. It has been suggested that these biases result from an underestimation of gravity wave drag on the atmosphere at latitudes near 60° S, especially around the "hot spot" of intense gravity wave fluxes above the mountainous Southern Andes and Antarctic peninsula. Here, we use Global Positioning System radio occultation (GPS-RO) data from the COSMIC satellite constellation to determine the properties of gravity waves in the hot spot and beyond. We show considerable southward propagation to latitudes near 60° S of waves apparently generated over the southern Andes. We propose that this propagation may account for much of the wave drag missing from the models. Furthermore, there is a long leeward region of increased gravity wave energy that sweeps eastwards from the mountains over the Southern Ocean. Despite its striking nature, the source of this region has historically proved difficult to determine. Our observations suggest that this region includes both waves generated locally and orographic waves advected downwind from the hot spot. We describe and use a new wavelet-based analysis technique for the quantitative identification of individual waves from COSMIC temperature profiles. This analysis reveals different geographical regimes of wave amplitude and short-timescale variability in the wave field over the Southern Ocean. Finally, we use the increased numbers of closely spaced pairs of profiles from the deployment phase of the COSMIC constellation in 2006 to make estimates of gravity wave horizontal wavelengths. We show that, given sufficient observations, GPS-RO can produce physically reasonable estimates of stratospheric gravity wave momentum flux in the hot spot that are consistent with measurements made by other techniques. We discuss our results in the context of previous satellite and modelling studies and explain how they advance our understanding of the nature and origins of waves in the southern stratosphere.

Abstract. To investigate the effects of the gravity wave (GW) drag on the general circulation in the thermosphere, a nonlinear GW parameterization that estimates the GW drag in the whole-atmosphere system is implemented in a whole-atmosphere general circulation model (GCM). Comparing the simulation results obtained with the whole-atmosphere scheme with the ones obtained with a conventional linear scheme, we study the GW effects on the thermospheric dynamics for solstice conditions. The GW drag significantly decelerates the mean zonal wind in the thermosphere. The GWs attenuate the migrating semidiurnal solar-tide (SW2) amplitude in the lower thermosphere and modify the latitudinal structure of the SW2 above a 150 km height. The SW2 simulated by the GCM based on the nonlinear whole-atmosphere scheme agrees well with the observed SW2. The GW drag in the lower thermosphere has zonal wavenumber 2 and semidiurnal variation, while the GW drag above a 150 km height is enhanced in high latitude. The GW drag in the thermosphere is a significant dynamical factor and plays an important role in the momentum budget of the thermosphere. Therefore, a GW parameterization accounting for thermospheric processes is essential for coarse-grid whole-atmosphere GCMs in order to more realistically simulate the atmosphere–ionosphere system.

Internal gravity waves (GWs) are ubiquitous in the atmosphere, making significant contributions to the mesoscale motions. Since the majority of their spectrum is unresolved in global circulation models, their effects need to be parameterized. In recent decades GWs have been increasingly studied in high-resolution simulations, which, unlike direct observations, allow us to explore full spatiotemporal variations of the resolved wave field. In our study we analyze and refine a traditional method for GW analysis in a high-resolution simulation on a regional domain around the Drake Passage. We show that GW momentum drag estimates based on the Gaussian high-pass filter method applied to separate GW perturbations from the background are sensitive to the choice of a cutoff parameter. The impact of the cutoff parameter is higher for horizontal fluxes of horizontal momentum, which indicates higher sensitivity for horizontally propagating waves. Two modified methods, which choose the parameter value from spectral information, are proposed. The dynamically determined cutoff is mostly higher than the traditional cutoff values around 500 km, leading to larger GW fluxes and drag, and varies with time and altitude. The differences between the traditional and the modified methods are especially pronounced during events with significant drag contributions from horizontal momentum fluxes. Significance Statement In this study, we highlight that the analysis of gravity wave activity from high-resolution datasets is a complex task with a pronounced sensitivity to the methodology, and we propose modified versions of a classical statistical gravity wave detection method enhanced by the spectral information. Although no optimal methodology exists to date, we show that the modified methods improve the accuracy of the gravity wave activity estimates, especially when oblique propagation plays a role.

Momentum deposition by orographically generated atmospheric gravity waves has been incorporated into a general circulation model of the troposphere and lower stratosphere by McFarlane, using a parameterization of real terrain and the waves that terrain would generate. Similar modeling will no doubt follow for extension to greater heights, where the effects of momentum deposition are expected to be greater. MeFarlane's parameterization treated an upward flux of momentum directed into a single azimuth (namely, that opposed to the surface wind) and tacitly assumed a Boussinesq approximation and horizontally isotropic terrain. These limitations are probed here and, to some extent, removed. The analysis is generalized from the Boussinesq to the “incompressible” approximation, with consequences that can be important in some circumstances. It is found appropriate to represent the momentum flux from isotropic terrain by two azimuths rather than one, these two being at angles of 32.5° or more from the single azimuth previously chosen. On the other hand, it is found appropriate to represent the momentum flux from anisotropic terrain by a single azimuth, that opposing the wind component normal to the long axis of the terrain, the transition from “isotropic” to “anisotropic” occurring at a length/breadth ratio of about two. Anomalous effects introduced by plateaus are also examined, and arguments are presented for removing them somewhat arbitrarily because of associated blocking.

The austral stratospheric final warming date is often predicted with substantial delay in several climate models. This systematic error is generally attributed to insufficient parameterized gravity wave (GW) drag in the stratosphere around 60°S. A simulation with a general circulation model [Laboratoire de Météorologie Dynamique zoom model (LMDZ)] with a much less pronounced bias is used to analyze the contribution of the different types of waves to the dynamics of the final warming. For this purpose, the resolved and unresolved wave forcing of the middle atmosphere during the austral spring are examined in LMDZ and reanalysis data, and a good agreement is found between the two datasets. The role of parameterized orographic and nonorographic GWs in LMDZ is further examined, and it is found that orographic and nonorographic GWs contribute evenly to the GW forcing in the stratosphere, unlike in other climate models, where orographic GWs are the main contributor. This result is shown to be in good agreement with GW-resolving operational analysis products. It is demonstrated that the significant contribution of the nonorographic GWs is due to highly intermittent momentum fluxes produced by the source-related parameterizations used in LMDZ, in qualitative agreement with recent observations. This yields sporadic high-amplitude GWs that break in the stratosphere and force the circulation at lower altitudes than more homogeneously distributed nonorographic GW parameterizations do.

By using a general circulation model that contains the region from the ground surface to the upper thermosphere, characteristics of gravity waves in the mesosphere and thermosphere are examined. At 100 km height, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength λx ≈ 2000 km), 40(λx ≈ 1000 km) and 80 (λx ≈ 500 km) are 6 h, 3 h and 1.5–2 h, respectively. For the individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force. This means that gravity waves with larger horizontal phase velocity (larger vertical wavelength) can penetrate into the lower thermosphere. The vertical energy flux due to gravity waves indicates that upward propagation from the lower atmosphere to the thermosphere is dominant. Fluctuations of the horizontal wind associated with gravity waves and its relation to the ionospheric variation are discussed. Short‐period gravity waves into the thermosphere induce day‐to‐day variations of the zonal wind in the upper thermosphere. This result suggests that day‐to‐day variability of the zonal wind caused by gravity waves is expected to contribute to the ionospheric variability.

Wind measurements from continuous meteor radar observations during June 2004–May 2007 are used to estimate the gravity wave momentum fluxes in the Mesosphere Lower Thermosphere (MLT) region over Trivandrum (8.5°N, 76.9°E), a low‐latitude station in India. The radial velocity variances in the 82–98 km height region, which are mainly caused by gravity waves, measured by the meteor radar are used to determine the gravity wave momentum fluxes. Using a novel method proposed by Hocking (2005), altitude profiles of momentum fluxes of short period (less than 2–3 h) gravity waves are estimated for three continuous years. Seasonal variation in the gravity wave momentum fluxes showed semi annual variation with equinoctial maximum and solstitial minimum. By using estimated gravity wave momentum fluxes, an attempt is made to quantify their contribution in driving the Mesospheric Semi Annual Oscillation (MSAO). The mean flow acceleration estimated from the divergence of gravity wave momentum fluxes is compared with the observed mean flow acceleration computed from monthly mean zonal winds during six MSAO cycles over three years. This comparison reveled that the gravity wave contribution toward the westerly phase of MSAO varies from ∼20–60% while that toward the easterly phase varies from ∼30–70%. Variations are observed from cycle‐to‐cycle in the gravity wave forcing toward both phases of MSAO. The significance of the present study lies in estimating the gravity wave momentum fluxes in the MLT region and quantifying their contribution toward the generation of MSAO over the low‐latitude for the first time.

Gravity wave momentum fluxes in the MLT—Part II: Meteor radar investigations at high and midlatitudes in comparison with modeling studies

The vertical propagation and attenuation of mountain waves launched by New Zealand terrain during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) field campaign are investigated. New Zealand mountain waves were frequently attenuated in a lower-stratospheric weak wind layer between z = 15 and 25 km. This layer is termed a “valve layer,” as conditions within this layer (primarily minimum wind speed) control mountain wave momentum flux through it, analogous to a valve controlling mass flux through a pipe. This valve layer is a climatological feature in the wintertime midlatitude lower stratosphere above the subtropical jet. Mountain wave dynamics within this valve layer are studied using realistic Weather Research and Forecasting (WRF) Model simulations that were extensively validated against research aircraft, radiosonde, and satellite observations. Locally, wave attenuation is horizontally and vertically inhomogeneous, evidenced by numerous regions with wave-induced low Richardson numbers and potential vorticity generation. WRF-simulated gravity wave drag (GWD) is peaked in the valve layer, and momentum flux transmitted through this layer is well approximated by a cubic function of minimum ambient wind speed within it, consistent with linear saturation theory. Valve-layer GWD within the well-validated WRF simulations was 3–6 times larger than that parameterized within MERRA. Previous research suggests increasing parameterized orographic GWD (performed in MERRA2) decreases the stratospheric polar vortex strength by altering planetary wave propagation and drag. The results reported here suggest carefully increasing orographic GWD is warranted, which may help to ameliorate the common cold-pole problem in chemistry–climate models.

The instability of the Northern Hemisphere polar vortex is mainly caused by the breaking of planetary-scale (Rossby) waves (RWs). However, gravity waves (GWs) may also play an important role in polar vortex preconditioning before breakdown events. Moreover, GWs affect dynamics in the stratosphere by altering the upward propagation of RWs at short time scales and therefore indirectly influence polar vortex stability. Due to the coarse spatial resolution of global chemistry-climate models, current efforts in climate research rely on simulations where the majority of the GW spectrum is parameterized. In the present study, we apply a recently developed method for detecting strong orographic gravity wave (OGW) drag events in the lower stratosphere above the Himalayas. For this, we use a specified dynamics simulation of the chemistry-climate model CMAM (Canadian Middle Atmosphere Model) spanning the period 1979–2010. We show that strong OGW drag events above the Himalayas are associated with anomalously increased upward RW propagation in the stratosphere. This, in turn, is associated with an increase of the refractive index in the mid-latitude lower stratosphere, a region where the OGW drag dominates. Our results also illustrate that OGW strong events have the potential to alter ozone variability via changes of mixing in the surf zone and advection from lower latitudes. Altogether, we detail a preconditioning process of the polar vortex morphology by GWs above the Himalayas and how this relates to the proximity to polar vortex breakdown.

Background: UV occultations measured by Voyager 2 during its flyby of Uranus in 1986 detected a warm stratosphere and extremely hot thermosphere [1, 2], far in excess of solar irradiance [3, 4] or internal heating [5]. These measurements imply that Uranus has the coldest lower stratosphere and yet the hottest thermosphere of any Solar System planet [6] (see Figure 1, dashed line). Uranus has the weakest internal heat of any of the giant planets [5]. The fundamental lack of understanding about the energy balance of Uranus is an example of the “giant planet energy crisis” [7].Furthermore, the Voyager 2 UV occultation measurements and models were in stark tension with Earth-based stellar occultations observed between 1977 and 1996. In [8] and [9], we present the results of reprocessing 26 archival occultations using modern techniques. While stratospheric tensions have decreased, thermospheric tensions remain, calling into question the Voyager 2 findings. In [9], we present a new 1-D atmospheric model that successfully reproduces these results. Aims: We present a new representative temperature-pressure profile for the atmosphere of Uranus from reprocessed stellar occultation observations and 1-D atmospheric modeling [8, 9]. We discuss the energy inputs to the thermosphere, circulation necessary to explain the new profiles, and the significant, potential magnetosphere-ionosphere-thermosphere interactions at Uranus. We describe critical measurements from a likely Uranus Orbiter and Probe (UOP) mission as well as from Earth beforehand that would enable the study of the energy inputs to Uranus. Prior Results & Conclusions: We reprocessed 26 archival Uranus stellar occultations, which produced temperature-pressure profiles inconsistent with Voyager 2 UVS profiles [8, 9]. A 1-D atmospheric model comprised of radiative-convective and conduction models was developed for Uranus based on these reprocessed profiles. The model reproduces the profiles and suggests a heat sink exists in the lower thermosphere. Figure 1 shows the new reference temperature-pressure profile for Uranus in comparison to the Voyager 2 profile and profiles for other planets.We conclude that the mesopause of Uranus is likely significantly higher in altitude (~10-4 mbar) than suggested from Voyager 2 profiles (~1 mbar), consistent with the mesopause levels of the other giant planets [9, 10]. We find that the stratosphere of Uranus likely contains a nearly isothermal region, again, consistent with those of the other giant planets [9, 10]. Last, we find that the required thermospheric energy flux is tens of times the solar EUV flux, underscoring the energy crisis and motivating detailed study of energy sources in the Uranian system. Ongoing Work: Measurements of Jupiter and Saturn have identified an energy crisis similar to that of Uranus. The emerging solution involves gravity waves activity that facilitates heating of the thermospheres by inducing Rayleigh drag in the polar regions. This breaks down the Coriolis force and enables meridional transport of strong auroral [11, 12] heating. While it is likely that Uranus has significant gravity wave activity [13, 14], it is possible that the auroral inputs to Uranus occur around the entire planet and are not limited to the poles [15, 16]. Much work is underway to better understand the shape and behavior of the magnetosphere of Uranus. Gravity waves may also provide the dynamical transport needed to create the heat sink observed in our new 1-D model.Therefore, additional observations of the prevalence and properties of gravity waves are critical to extending recent modeling work from Jupiter and Saturn to Uranus. While UOP may make many such observations in the 2050s, we outline Earth-based observations that can be made prior. Upcoming Stellar Occultations: In [17], we predicted Uranus and Neptune occultations 2025 – 2035 that could be observed from the ground and/or from low-Earth orbit. Additional predictions can be found in [18]. The most promising Uranus events occur in 2025, 2031, and 2032. We will describe how we intend to observe the 2025 occultation from ground-based and airborne assets, as well as other upcoming observing campaigns. We will present simulated results for temperature profiles of Uranus as well as gravity wave detections. The Shadow Chaser: In [17], we outlined the case for a low-Earth orbit small satellite to observe stellar occultations to better constrain the temperature and density profiles of the upper atmosphere of Uranus and to detect gravity waves. This mission concept, called the Shadow Chaser, is being explored at NASA Langley Research Center. By observing above the atmosphere, the Shadow Chaser could observe during the day and would not be impacted by scintillation, greatly increasing signal-to-noise. We will present simulations showing the Uranus profiles that would result from the Shadow Chaser observing the 2031 Uranus event, during which Uranus will occult a 4th magnitude K-band star.