Stephen D. Eckermann

An overview of the past, present and future of gravity‐wave drag parametrization for numerical climate and weather prediction models

Abstract An overview of the parametrization of gravity ‐wave drag in numerical ‐weather prediction and climate simulation models is presented. The focus is primarily on understanding the current status of gravity wave drag parametrization as a step towards the new parametrizations that will be needed for the next generation of atmospheric models. Both the early history and latest developments in the field are discussed. Parametrizations developed specifically for orographic and convective sources of gravity waves are described separately, as are newer parametrizations that collectively treat a spectrum of gravity wave motions. The differences in issues in and approaches for the parametrization of the lower and upper atmospheres are highlighted. Various emerging issues are also discussed, such as explicitly resolved gravity waves and gravity wave drag in models, and a range of unparametrized gravity wave processes that may need attention for the next generation of gravity wave drag parametrizations in models.

A Three-Dimensional Nonhydrostatic Ray-Tracing Model for Gravity Waves: Formulation and Preliminary Results for the Middle Atmosphere

Abstract The WKB ray-tracing formalism is extended to accommodate internal gravity waves of all frequencies in a rotating, stratified, and compressible three-dimensional atmosphere. This includes the derivation of equations governing the dispersion and refraction of the ray paths, a realistic wave amplitude equation that takes into account both radiative and turbulent damping effects, and extensions of previous wave saturation schemes to accommodate dynamical and convective instabilities along generally slanted axes. These equations have been numerically coded into a global ray-tracing model that the authors have applied to the three-dimensional CIRA 1986 reference atmosphere model in a series of preliminary experiments to investigate the impact of the newly incorporated features on synthesized wave fields in the middle atmosphere. Three main points emerge from these experiments. First, there is a striking reduction in the high-frequency cutoff with decreasing horizontal wavenumber due to a more complete ...

Stratospheric horizontal wavenumber spectra of winds, potential temperature, and atmospheric tracers observed by high‐altitude aircraft

Abstract : Horizontal wavenumber power spectra of vertical and horizontal wind velocities, potential temperatures, and ozone and N(2)O mixing ratios, as measured in the mid-stratosphere during 73 ER-2 flights (altitude approx. 20km) are presented. The velocity and potential temperature spectra in the 100 to 1-km wavelength range deviate significantly from the uniform -5/3 power law expected for the inverse energy-cascade regime of two-dimensional turbulence and also for inertial-range, three-dimensional turbulence. Instead, steeper spectra approximately consistent with a -3 power law are observed at horizontal scales smaller than 3 km for all velocity components as well as potential temperature. Shallower spectra are observed at scales longer than 6 km. For horizontal velocity and potential temperature the spectral indices at longer scales are between -1.5 and -2.0. For vertical velocity the spectrum at longer scales become flat. It is argued that the observed velocity and potential temperature spectra are consistent with gravity waves. At smaller scales, the shapes are also superficially consistent with a Lumley-Shur-Weinstock buoyant subrange of turbulence and/or nonlinear gravity waves. Contemporaneous spectra of ozone and N(sub 2)O mixing ratio in the 100 to 1-km wavelength range do conform to an approximately uniform -5/3 power law. It is argued that this may reflect interactions between gravity wave air-parcel displacements and laminar or filamentary structures in the trace gas mixing ratio field produced by enstropy-cascading two-dimensional turbulence.

Global Gravity Wave Variances from Aura MLS: Characteristics and Interpretation

Abstract The gravity wave (GW)–resolving capabilities of 118-GHz saturated thermal radiances acquired throughout the stratosphere by the Microwave Limb Sounder (MLS) on the Aura satellite are investigated and initial results presented. Because the saturated (optically thick) radiances resolve GW perturbations from a given altitude at different horizontal locations, variances are evaluated at 12 pressure altitudes between ∼21 and 51 km using the 40 saturated radiances found at the bottom of each limb scan. Forward modeling simulations show that these variances are controlled mostly by GWs with vertical wavelengths λz > 5 km and horizontal along-track wavelengths of λy ∼ 100–200 km. The tilted cigar-shaped three-dimensional weighting functions yield highly selective responses to GWs of high intrinsic frequency that propagate toward the instrument. The latter property is used to infer the net meridional component of GW propagation by differencing the variances acquired from ascending (A) and descending (D) o...

Upper Atmosphere Research Satellite (UARS) MLS observation of mountain waves over the Andes

Abstract : Stratospheric air temperature (radiance) fluctuations over the Andes observed by the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) are presented. The MLS radiance variances show strong annual variability over the Andes mountains in South America, which is closely correlated to the background wind conditions associated with mountain wave generation and propagation. The variances are significantly larger in southern hemispheric winter when the winds in the troposphere and stratosphere over the Andes are both westerly and mountain wave critical levels (zerowind lines) are absent. The annual variation of MLS radiance variance agrees well with data from radiosondes and output from the Naval Research Laboratory (NRL) Mountain Wave Forecast Model (MWFM) over the same region and period. The amplitude of the radiance variances seems to correlate well with the intensity of surface wind at upstream positions westward of the Andes, which is also related to the meridional temperature gradient in the region. Horizontal scale analysis suggests that mountain waves over the Andes might have two preferential horizontal wavelengths at 110 and 400 km.

Widespread Solid Particle Formation by Mountain Waves in the Arctic Stratosphere

Observations of polar stratospheric clouds (PSCs) by lidar show that the clouds often contain solid particles, which are most likely composed of nitric acid hydrates. However, laboratory experiments indicate that such hydrate particles are not easily formed under Arctic synoptic scale conditions, suggesting that solid PSC particles should be rather rare. Here we show results from a model study indicating that mountain-induced mesoscale temperature perturbations may be an important source of nitric acid hydrate particles in the Arctic. Multiple Arctic vortex trajectories were combined with a global mountain wave forecast model to calculate the potential for solid particle formation during December and January 1994/1995. The mountain wave model was used to calculate adiabatic cooling over several thousand ridge elements. Nitric acid hydrate particles were assumed to form in the mountain waves according to several microphysical mechanisms, and were then advected using polar vortex-filling synoptic trajectories to generate maps of solid particle occurrence. The calculations show that mountain waves may be a significant source of PSCs containing solid particles that are observed on the synoptic scale. In particular, the east coast of Greenland, the Norwegian mountains, and the Urals are found to be solid particle sources, with the PSCs often predicted to survive several thousand kilometers downstream.

Momentum flux estimates for South Georgia Island mountain waves in the stratosphere observed via satellite

Abstract : We show high-resolution satellite observations of mountain wave events in the stratosphere above South Georgia Island in the remote southern Atlantic Ocean and compute the wave momentum fluxes for these events. The fluxes are large, and they imply important drag forces on the circulation. Small island orography is generally neglected in mountain wave parameterizations used in global climate models because limited model resolution treats the grid cell containing the island as ocean rather than land. Our results show that satellite observations can be used to quantitatively constrain mountain wave momentum fluxes, and they suggest that mountain waves from island topography may be an important missing source of drag on the atmospheric circulation.

Intraseasonal wind variability in the equatorial mesosphere and lower thermosphere: long-term observations from the central Pacific

Abstract Analysis of intraseasonal (10–100 days) oscillations in the equatorial mesosphere and lower thermosphere (MLT) is presented, based on over five years of velocity data acquired by a radar system at Christmas Island (2 °N, 157 °W), in the central Pacific. Strong peaks in the zonal winds are found at periods of ~60 days, ~35–40 days, and ~22–25 days. These peaks, as well as the mean annual variations of the activity within the various period ranges, are similar to 30–60 day and 20–25 day oscillations that occur in the equatorial troposphere. Weaker (but nonetheless clear) periodicities are also found in the meridional winds at ~60 days and ~35 days. A strong quasi-60-day variation is detected in gravity-wave variances, with much weaker signals at ~40 days and ~25 days. Strong variations in diurnal tidal amplitudes are observed with periods of ~60 days, ~40 days, and ~25 days. These observations lead us to propose the following explanation for the observed intraseasonal variability of the equatorial MLT region. Intraseasonal cycles in tropical tropospheric convection produce intraseasonal variations in the intensity of gravity waves and nonmigrating diurnal tides impinging upon the mesosphere. This accounts for the intraseasonal peaks we observe in gravity-wave and tidal activity. This intraseaonally modulated wave activity induces similar periodicities in the wave-induced driving of the zonal MLT flow, which in turn forces the observed intraseasonal peaks in the zonal MLT winds. If this explanation is valid, these observations provide an unusually clear example of the driving of MLT flow patterns by waves emanating from tropospheric systems, and highlight the importance of convectively generated waves in understanding the dynamics of the equatorial middle atmosphere.

The Deep Propagating Gravity Wave Experiment (DEEPWAVE): An Airborne and Ground-Based Exploration of Gravity Wave Propagation and Effects from Their Sources throughout the Lower and Middle Atmosphere

AbstractThe Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropson...

Hodographic analysis of gravity waves: Relationships among Stokes parameters, rotary spectra and cross-spectral methods

The elliptical rotation with height of horizontal velocities produced by gravity waves provides considerable information about the wave field. Methods of statistically characterizing velocity ellipses from data currently fall into three main categories: (1) hodographic analyses, (2) cross-spectral analyses, and (3) rotary spectral analyses. The three methods have some intuitive similarities, yet precise interrelationships among them are presently unclear. The three techniques are interrelated here using the so-called “Stokes parameters” of the wave field, which initially provide a concise description of the hodographic analysis method (1). On Fourier transforming the Stokes parameters, standard formulae employed in rotary-spectral and cross-spectral analysis methods can then be expressed in terms of the resulting “Stokes-parameter spectra.” The results highlight some drawbacks in the use of cross-coherence spectra between velocity components to verify the existence of a coherent wave motion. A more robust measure is suggested, based on the “degree of polarization” of classical hodograph-based Stokes-parameter analysis, which can be generalized to provide an analogous spectral measure for evaluation in rotary-spectral and cross-spectral analyses.