Background Wind Profile Research Articles

Identification of spectrum of secondary acoustic-gravity waves in the middle and upper atmosphere in a high-resolution numerical model

Considerable attention has recently been paid to the study of so-called “secondary” acoustic-gravity waves (AGWs) that arise as a result of instability and nonlinear interactions of “primary” wave modes propagating from atmospheric sources, among themselves, and with the mean flow. In this paper, for the first time, the horizontal spatial spectra of primary and secondary AGWs are separated at fixed altitude levels in the middle and upper atmosphere at different time moments, which are simulated using a three-dimensional nonlinear high-resolution model AtmoSym. It is shown that in a short time after switching on the plane wave source at the lower boundary of the model, the spectrum consists of a peak corresponding to primary AGW and quasi-white noise generated by random atmospheric disturbances and the numerical model. Later, secondary peaks appear in the spectra at horizontal wave numbers, which are multiples of the wave numbers of primary AGW. The proposed separation of the spectra of primary and secondary AGWs makes it possible to estimate the relative contribution of secondary AGW at different altitudes, at different times, and with different stability of background temperature and wind profiles in the atmosphere.

Выделение спектра вторичных акустико-гравитационных волн в средней и верхней атмосфере в численной модели высокого разрешения

Considerable attention has recently been paid to the study of so-called “secondary” acoustic-gravity waves (AGWs) that arise as a result of instability and nonlinear interactions of “primary” wave modes propagating from atmospheric sources, among themselves, and with the mean flow. In this paper, for the first time, the horizontal spatial spectra of primary and secondary AGWs are separated at fixed altitude levels in the middle and upper atmosphere at different time moments, which are simulated using a three-dimensional nonlinear high-resolution model AtmoSym. It is shown that in a short time after switching on the plane wave source at the lower boundary of the model, the spectrum consists of a peak corresponding to primary AGW and quasi-white noise generated by random atmospheric disturbances and the numerical model. Later, secondary peaks appear in the spectra at horizontal wave numbers, which are multiples of the wave numbers of primary AGW. The proposed separation of the spectra of primary and secondary AGWs makes it possible to estimate the relative contribution of secondary AGW at different altitudes, at different times, and with different stability of background temperature and wind profiles in the atmosphere.

Comparison of gravity wave propagation directions observed by mesospheric airglow imaging at three different latitudes using the M-transform

Abstract. We developed user-friendly software based on Matsuda et al.'s (2014) 3D-FFT method (Matsuda-transform, M-transform) for airglow imaging data analysis as a function of Interactive Data Language (IDL). Users can customize the range of wave parameters to process when executing the program. The input for this function is a 3-D array of a time series of a 2-D airglow image in geographical coordinates. We applied this new function to mesospheric airglow imaging data with slightly different observation parameters obtained for the period of April–May at three different latitudes: Syowa Station, the Antarctic (69∘ S, 40∘ E); Shigaraki, Japan (35∘ N, 136∘ E); and Tomohon, Indonesia (1∘ N, 122∘ E). The day-to-day variation of the phase velocity spectrum at the Syowa Station is smaller and the propagation direction is mainly westward. In Shigaraki, the day-to-day variation of the horizontal propagation direction is larger than that at the Syowa Station; the variation in Tomohon is even larger. In Tomohon, the variation of the nightly power spectrum magnitude is remarkable, which indicates the intermittency of atmospheric gravity waves (AGWs). The average nightly spectrum obtained from April–May shows that the dominant propagation is westward with a phase speed <50 m s−1 at the Syowa Station and east-southeastward with a phase speed of up to ∼80 m s−1 in Shigaraki. The day-to-day variation in Tomohon is too strong to discuss average characteristics; however, a phase speed of up to ∼100 m s−1 and faster is observed. The corresponding background wind profiles derived from MERRA-2 indicate that wind filtering plays a significant role in filtering out waves that propagate eastward at the Syowa Station. On the other hand, the background wind is not strong enough to filter out relatively high-speed AGWs in Shigaraki and Tomohon and the dominant propagation direction is likely related to the distribution and characteristics of the source region, at least in April and May.

Tsunami‐driven gravity waves in the presence of vertically varying background and tidal wind structures

AbstractMany characteristics of tsunami‐driven gravity waves (TDGWs) enable them to easily propagate into the thermosphere and ionosphere with appreciable amplitudes capable of producing detectable perturbations in electron densities and total electron content. The impact of vertically varying background and tidal wind structures on TDGW propagation is investigated with a series of idealized background wind profiles to assess the relative importance of wave reflection, critical‐level approach, and dissipation. These numerical simulations employ a 2‐D nonlinear anelastic finite‐volume neutral atmosphere model which accounts for effects accompanying vertical gravity wave (GW) propagation such as amplitude growth with altitude. The GWs are excited by an idealized tsunami forcing with a 50 cm sea surface displacement, a 400 km horizontal wavelength, and a phase speed of 200 ms−1 consistent with previous studies of the tsunami generated by the 26 December 2004 Sumatra earthquake. Results indicate that rather than partial reflection and trapping, the dominant process governing TDGW propagation to thermospheric altitudes is refraction to larger and smaller vertical scales, resulting in respectively larger and smaller vertical group velocities and respectively reduced and increased viscous dissipation. Under all considered background wind profiles, TDGWs were able to attain ionospheric altitudes with appreciable amplitudes. Finally, evidence of nonlinear effects is observed and the conditions leading to their formation is discussed.

Quality and Impact of Indian Doppler Weather Radar Wind Profiles: A Diagnostic Study

In the tropics, efficient weather forecasts require high-quality vertical profiles of winds to overcome improper coupling of mass and wind fields and balance relationships in the region. The India Meteorological Department (IMD) operates the network of Doppler Weather Radar (DWR) in microwave frequencies (S-band or C-band) at various locations in India. The National Centre for Medium Range Weather Forecasting (NCMRWF) receives the volume velocity processing (VVP) wind profiles from all DWRs through the Global Telecommunication System (GTS) network in near real time. The radar VVP wind is a mean horizontal wind derived at different heights from radial velocities suitable for numerical weather prediction applications. Three numerical experiments, CNTL (without VVP winds), 3DVAR and HYBRID with the assimilation of VVP winds by means of 3-dimensional variational (3dvar) and hybrid data assimilation systems were conducted using the NCMRWF Global Forecast System (NGFS) model. This study had two objectives: (1) quality assessment of VVP winds and (2) investigation of the impact of VVP wind profiles on NGFS model forecast. The quality of VVP wind profiles was assessed against the NGFS model background and radiosonde wind profiles. The absolute values of zonal and meridional wind observation minus background (O−B) increased with the pressure for all DWRs. All radars exhibited the accepted (rejected) ratio as a decreasing (increasing) function of pressure. The resemblance between the zonal and meridional O−B statistics for 3DVAR and HYBRID experiments is apparently remarkable. The accepted VVP winds and radiosonde winds in both experiments (3DVAR and HYBRID) were consistent. The correlation coefficient (R) was higher at Patna (Patiala) for zonal (meridional) winds in the 3DVAR experiment and at Patna (Jaipur) in the HYBRID experiment. At Chennai, the R value was lower in both the experiments for both wind components. However, because of the assimilation of VVP winds by means of 3dvar, the root-mean-square error (RMSE) of zonal and meridional winds improved by approximately 2–3% up to the day 5 forecast in the analysis performed below 700 hPa. Further improvement in RMSE by approximately 5% was observed in both the wind components because of the hybrid data assimilation. The zonal and meridional wind RMSEs in the HYBRID (3DVAR) experiment improved by ≈5 (3)% compared with the 3DVAR (CNTL) experiment.

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

AbstractNew measurements at the ALOMAR observatory in northern Norway (69°N, 16°E) using the Weber sodium lidar and the Advanced Mesospheric Temperature Mapper (AMTM) allow for a comprehensive investigation of a gravity wave (GW) event on 22 and 23 January 2012 and the complex and varying propagation environment in which the GW was observed. These observational techniques provide insight into the altitude ranges over which a GW may be evanescent or propagating and enable a clear distinction in specific cases. Weber sodium lidar measurements provide estimates of background temperature, wind, and stability profiles at altitudes from ~78 to 105 km. Detailed AMTM temperature maps of GWs in the OH emission layer together with lidar measurements quantify estimates of the observed and intrinsic GW parameters centered near 87 km. Lidar measurements of sodium densities also allow more precise identification of GW phase structures extending over a broad altitude range. We find for this particular event that the extent of evanescent regions versus regions allowing GW propagation can vary largely over a period of hours and significantly change the range of altitudes over which a GW can propagate.

The Evolution of Convective Storms Initiated by an Isolated Mountain Ridge

Abstract The evolution of convective storms over the Black Hills, an isolated mountain ridge in South Dakota and Wyoming and a regional convection hotspot, is investigated using a 10-yr observational climatology and quasi-idealized numerical simulations. Radar-observed diurnally forced mountain-convection events are classified according to their maximum cell-track length and duration, which are quantified using an automated cell-tracking algorithm. Environmental conditions during these events are obtained from operational radiosonde and model-analysis data. These data suggest that mountain-forced convective cells generally struggle to survive in the convectively inhibited flow downwind of the Black Hills. Those cells that do survive downwind prefer environments with strong bulk vertical shear over the 0–6-km layer, which favors organized multicellular or supercellular convection. Under slightly weaker shear, the cells tend to dissipate rapidly as they propagate downwind. Relatively weak winds aloft, when coupled with low-level winds aligned with the long terrain axis, support longer-lived, quasi-stationary cells with flash-flooding potential. The weak winds favor slow cell propagation while the along-ridge flow limits the negative feedbacks of storm outflow on the elevated convergence over the ridge, allowing convection to repeatedly initiate in the same location. The storm evolution is relatively insensitive to the background thermodynamic profile, provided that sufficient moist instability exists to support deep convection. Convection-permitting numerical simulations reinforce that changes in the background wind profile alone can explain the observed variations in cell evolution. They also suggest that the longevity of convective cells downwind of the ridge is sensitive to terrain-induced modifications to the vertical wind shear.

Gravity wave propagation in a nonisothermal atmosphere with height varying background wind

We derive a gravity wave propagation equation for a compressible and non‐isothermal atmosphere with a variable background wind profile. Impact of all the gradient terms on the vertical wavenumber depends only on the intrinsic horizontal phase velocity and the background atmosphere. For the background wind variation, any one of the linear first order derivative, second order derivative and the square of the first order derivative terms can be the dominant term under different conditions. For temperature variation, only the linear first order derivative is important for waves having a slow intrinsic horizontal phase velocity. Our equation indicates that the effect of wind shear on the vertical wavenumber is opposite to that predicted by the Taylor‐Goldstein equation, which assumes an incompressible fluid. We also derive an expression for the amplitude of the vertical wind perturbation.

Gravity waves above a convective boundary layer: A comparison between wind‐profiler observations and numerical simulations

AbstractWind‐profiler observations and three‐dimensional numerical modelling are used to investigate gravity waves generated over convectively active boundary layers on 12 and 13 May 2001 at Lindenberg, Germany. The results illustrate the importance of changes in the background wind and stability profile in influencing the vertical propagation of the gravity waves. The resultant changes in vertical propagation cause the dominance of relatively long‐wavelength, long‐period waves at high levels and the dominance of relatively short‐wavelength, short‐period waves near the top of the convective boundary layer. There are important differences between the two days in the wave‐propagation characteristics, due to subtle changes in the background flow conditions. Copyright © 2007 Royal Meteorological Society

Analysis of a jet stream induced gravity wave associated with an observed ice cloud over Greenland

Abstract. A polar stratospheric ice cloud (PSC type II) was observed by airborne lidar above Greenland on 14 January 2000. It was the unique observation of an ice cloud over Greenland during the SOLVE/THESEO 2000 campaign. Mesoscale simulations with the hydrostatic HRM model are presented which, in contrast to global analyses, are capable to produce a vertically propagating gravity wave that induces the low temperatures at the level of the PSC afforded for the ice formation. The simulated minimum temperature is ~8 K below the driving analyses and ~4.5 K below the frost point, exactly coinciding with the location of the observed ice cloud. Despite the high elevations of the Greenland orography the simulated gravity wave is not a mountain wave. Analyses of the horizontal wind divergence, of the background wind profiles, of backward gravity wave ray-tracing trajectories, of HRM experiments with reduced Greenland topography and of several diagnostics near the tropopause level provide evidence that the wave is emitted from an intense, rapidly evolving, anticyclonically curved jet stream. The precise physical process responsible for the wave emission could not be identified definitely, but geostrophic adjustment and shear instability are likely candidates. In order to evaluate the potential frequency of such non-orographic polar stratospheric cloud events, the non-linear balance equation diagnostic is performed for the winter 1999/2000. It indicates that ice-PSCs are only occasionally generated by gravity waves emanating from spontaneous adjustment.

Analysis of a temperature inversion event in the lower mesosphere

Rayleigh lidar measurements of stratospheric and lower mesospheric temperatures obtained at the Urbana Atmospheric Observatory (40.1°N, 88.1°W) on 17/18 November 1997 revealed large temperature inversions at altitudes between 55 to 65 km. Prior to and during a large increase (by up to about 50K) in the amplitude of the mesosphere inversion layer (MIL), a clear and persistent vertical wave structure between 30 and 65 km was observed. The wave has a vertical wavelength of about 12 km and an apparent period of about 12 hours. However, the intrinsic characteristics of the wave are uncertain due to the lack of information regarding the background wind profile and the relative direction of the wave propagation vector with respect to the background wind vector. Two different cases, corresponding to small and large background wind speeds projected onto the horizontal direction of wave propagation, are studied numerically to represent two scenarios with different intrinsic wave characteristics devised to explain the development of the MIL event observed. When the projected background wind is small, the wave is likely to be an inertial‐gravity wave. It is shown that the breaking of such a wave does not produce the large heating rate observed. However, the numerical modeling shows that such an inertial‐gravity wave can modulate the stability of a separate internal gravity wave, and the breaking of this internal gravity wave produces a heating rate similar to the observed rate. When the projected background wind is large, however, the observed wave could be an internal gravity wave with a large intrinsic phase speed. The analysis shows that the breaking of this wave can generate a large heating rate and a MIL that is similar to the observed event. We close with a discussion of the observational implications of these two scenarios. Possible wave sources are also discussed, and it appears that the observed MIL event might be related to a developing frontal system in the tropopause region observed just prior to the onset of the observed event.

Turbulence statistics of a Kelvin–Helmholtz billow event observed in the night-time boundary layer during the Cooperative Atmosphere–Surface Exchange Study field program

Abstract An apparent shear flow instability occurred in the stably stratified night-time boundary layer on 6 October 1999 over the Cooperative Atmosphere–Surface Exchange Study (CASES-99) site in southeast Kansas. This instability promoted a train of billows which appeared to be in different stages of evolution. Data were collected by sonic anemometers and a high-frequency thermocouple array distributed on a 60 m tower at the site, and a high resolution Doppler lidar (HRDL), situated close to the tower. Data from these instruments were used to analyze the characteristics of the instability and the billow event. The instability occurred in a layer characterized by a minimum Richardson number Ri∼0.13, and where an inflection in the background wind profile was also documented. The billows, which translated over the site for approximately 30 min, were approximately L∼320 m in length and, after billow evolution they were contained in a layer depth H∼30 m. Their maximum amplitude, determined by HRDL data, occurred at a height of 56 m. Billow overturns, responsible for mixing of heat and momentum, and high-frequency intermittent turbulence produce kurtosis values above the Gaussian value of 3, particularly in the lower part of the active layer.

A Numerical Study of Three-Dimensional Gravity Waves Triggered by Deep Tropical Convection and Their Role in the Dynamics of the QBO

Abstract A 3D mesoscale model is used to study the structure of convectively triggered gravity waves in the Tropics and their role in the dynamics of the middle atmosphere. Simulations with three stratospheric background zonal wind cases are examined. In the first case the background wind profile is constant; the other two are representative of the easterly and westerly phases of the quasi-biennial oscillation (QBO). Spectral analysis is used to link the structure of the triggered gravity waves to the dominant vertical wavelength of the latent heating within the convection. In the QBO–wind shear cases, upward propagating gravity waves are damped as they approach their critical layer. The signature of critical-layer absorption is clearly visible in the profiles of vertical momentum-flux divergence. In the simulations with open boundary conditions, the response to vertical momentum-flux divergence takes the form of large dynamic pressure differences between the east and west boundaries together with acceler...

A Study of Landscape-Generated Deep Moist Convection

Abstract A two-dimensional version of a cloud-resolving model was used to study the generation of deep moist convection over heterogeneous landscapes. Alternating patches of dry and wet soil were simulated for various profiles of background wind. Results suggested a significant, systematic impact of patch length and background wind on moist convection. Rainfall occurred most intensely along sea-breeze-like fronts, which formed at patch boundaries. Total accumulated rainfall—as the average over simulations with the same patch size but with different background wind profiles—was largest for a patch length of 128 km. This patch length was similar in size to a local radius of deformation (ro = HN/ω). The deposition of rainfall generated a much different distribution of soil moisture after one day of model simulation. This new distribution, however, was far from equilibrium, as the landscape still consisted of a number of wet and dry soil patches. The cloud structure of moist convection was also examined using...

Interpretations of observed climatological patterns in stratospheric gravity wave variance

Observational analyses of gravity waves in the stratosphere have revealed various climatological patterns in gravity wave activity. Seasonal, geographical, and vertical variations have all been observed. In this work, a linear model of gravity wave propagation is applied to investigate the underlying causes of some of the observed patterns. A collection of monochromatic gravity waves that represent a broad spectrum of wavenumbers and frequencies is input at 6‐km altitude in the model. Propagation of the waves through realistic background atmospheric wind and stability fields is treated with linear ray theory and a simple saturation condition to limit amplitudes to stable values. The wave spectrum at the 6‐km source height is specified to be constant at all latitudes, longitudes, and times, so the variability that appears at higher altitudes is due entirely to background atmosphere variations. Before the model results are compared to the observations, the spectrum of waves is filtered in a way that mimics the limitations of each of the observation techniques. The filtering is described in terms of vertical wavelength and is referred to as the “observational filter.” In a vertically varying background wind, gravity waves are Doppler‐shifted in intrinsic frequency and refracted to different vertical wavelengths as they propagate vertically through the atmosphere. The observational filter and the wave refraction effects can thus couple in interesting ways that have not been explicitly considered in previous work. The model shows that this coupling can give rise to geographical, seasonal, and vertical variations in gravity wave observations without any variations in the spectrum or amplitude of gravity wave sources in the troposphere. Thus careful consideration of both the background wind profile and observational filter can greatly affect the interpretation of the observed climatological patterns in gravity wave activity.

Sudden narrow temperature‐inversion‐layer formation in ALOHA‐93 as a critical‐layer‐interaction phenomenon

A sudden and dramatic mesospheric heating event was observed over Haleakala, Maui, on October 21, 1993 (day 294), during the ALOHA‐93 Campaign. It consisted of a persistent, narrow temperature‐inversion layer about 3–4 km wide near 87 km altitude with a peak temperature rise approaching 40 k, lasting for about 3.5 hours. Owing to the large number of ground‐based instruments recording the event, it is possible to attempt to seek out a physical explanation for this temperature rise. There is powerful evidence to suggest that the temperature‐inversion layer is associated with energy deposition, direct and indirect, resulting from gravity wave/critical layer interaction. Indeed, lidar wind profiles and mesospheric wave structures simultaneously observed with CCD imagers reveal the presence of a critical layer at the appropriate altitude. The data also show a narrow unstable background wind profile over a 2–3 km thickness in the immediate vicinity of the critical layer. We will show quantitatively that there is sufficient energy available from the dissipation of the observed gravity wave and from the observed instability in the background winds to account for the heating. After the critical layer has disappeared, the temperature rise subsides, and the background wind again becomes stable at all height levels of interest.

Measurements of gravity wave activity within and around the Arctic stratospheric vortex

Lidar measurements of gravity wave activity have been conducted at Eureka in the High Arctic since 1993. The waves are detected by the fluctuations they induce in temperature. It has been found that the amount of wave energy in the upper stratosphere is related to the position of the stratospheric polar vortex. In each of the four winters reported here, the wave activity was a maximum within the westerly jet at the edge of the vortex, a minimum inside the vortex near its centre and intermediate outside the vortex. The spectra of wave induced fluctuations show that it is at the longest resolved vertical wavelengths (8 to 15 km) that wave energy is being influenced by the background meteorological conditions. These findings are interpreted in terms of the Doppler shifting and critical level filtering that is imposed by the background wind profile.

Observations of the two‐day wave in WINDII data during January, 1993

Analysis of variations in the zonal and meridional wind observed by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) from January 19 to January 31, 1993 reveal a strong quasi two‐day wave (QTDW) signature. These observations were taken during the time of peak amplitude for this QTDW event [Wu et al., 1993] and extend from 41°N to 71°S. Daytime and nighttime results are presented for the altitude ranges 90 to 150 km and 90 to 102 km respectively. The QTDW is found to occur with significant amplitude well into the winter hemisphere and lower thermosphere. Its latitudinal structure below 103 km is found to be similar to that calculated by Hagan et al. [1993] using the background wind profile of Portniagin and Solov'yeva [1992]. The peak amplitude occurs at 94 km with a magnitude of about 50 m/s for the meridional wind and 30 m/s for the zonal wind. Mean profiles from the summer midlatitudes show a secondary maximum in amplitude near 115 km and a node at 130 km. Above 130 km the wave appears to be evanescent.

A simulated spectrum of convectively generated gravity waves: Propagation from the tropopause to the mesopause and effects on the middle atmosphere

This work evaluates the interaction of a simulated spectrum of convectively generated gravity waves with realistic middle atmosphere mean winds. The wave spectrum is derived from the nonlinear convection model described by Alexander et al. [1995] that simulated a two‐dimensional midlatitude squall line. This spectrum becomes input to a linear ray tracing model for evaluation of wave propagation as a function of height through climatological background wind and buoyancy frequency profiles. The energy defined by the spectrum as a function of wavenumber and frequency is distributed spatially and temporally into wave packets for the purpose of estimating wave amplitudes at the lower boundary of the ray tracing model. A wavelet analysis provides an estimate of these wave packet widths in space and time. Without this redistribution of energies into wave packets the Fourier analysis alone inaccurately assumes the energy is evenly distributed throughout the storm model domain. The growth with height of wave amplitudes is derived from wave action flux conservation coupled to a convective instability saturation condition. Mean flow accelerations and wave energy dissipation profiles are derived from this analysis and compared to parameterized estimates of gravity wave forcing, providing a measure of the importance of the storm source to global gravity wave forcing. The results suggest that a single large convective storm system like the simulated squall line could provide a significant fraction of the zonal mean gravity wave forcing at some levels, particularly in the mesosphere. The vertical distributions of mean flow acceleration and energy dissipation do not much resemble the parameterized profiles in form because of the peculiarities of the spectral properties of the waves from the storm source. The ray tracing model developed herein provides a tool for examining the role of convectively generated waves in middle atmosphere physics.

Effect of background winds on vertical wavenumber spectra of atmospheric gravity waves

Observations of quasi‐invariant m−3 power spectra of various atmospheric perturbations at large vertical wavenumbers m have received considerable theoretical attention. Yet some other observations, most notably in the stratosphere, have revealed significant departures from this shape and much reduced spectral densities at large m. Here it is argued that these changes arise when a spectrum of gravity waves encounter mean wind changes which cause intrinsic horizontal phase speeds to increase. Concomitant changes in vertical wavenumbers, amplitudes, and shear variance can produce changes in spectral character qualitatively similar to observations. The two important dynamical processes governing the spectral response are refraction of vertical wavenumbers and conservation of Eliassen‐Palm flux for nondissipating waves, processes already encapsulated in both single‐wave and spectral parameterizations of gravity‐wave processes. Retrospective application of these ideas to various observations in the literature leads to successful “prediction” (based on the background wind profile) of either quasi‐invariant m−3 spectra or attenuated spectra at large m. However, while explaining their occurrence, the precise changes in spectral shape predicted by simple models of wind‐shifted spectra often differ from those observed.