The Experts below are selected from a list of 96 Experts worldwide ranked by ideXlab platform
Yk J Cho - One of the best experts on this subject based on the ideXlab platform.
-
the vertical structure of jupiter s equatorial zonal wind above the cloud deck derived using mesoscale gravity waves
arXiv: Earth and Planetary Astrophysics, 2013Co-Authors: Chris Watkins, Yk J ChoAbstract:Data from the Galileo Probe, collected during its descent into Jupiter's atmosphere, is used to obtain a vertical profile of the zonal wind from $\mathbf{\sim 0.5}$ bar (upper troposphere) to $\mathbf{\sim 0.1\, \mu{bar}}$ (lower thermosphere) at the probe entry site. This is accomplished by constructing a map of gravity wave Lomb-Scargle periodograms as a function of altitude. The profile obtained from the map indicates that the wind speed above the visible cloud deck increases with height to $\mathbf{\sim 150}$ m\,s$\mathbf{^{-1}}$ and then levels off at this value over a broad altitude range. The location of the Turbopause, as a region of wide wave spectrum, is also identified from the map. In addition, a cross-equatorial oscillation of a jet, which has previously been linked to the quasi-quadrennial oscillation in the stratosphere, is suggested by the profile.
-
the vertical structure of jupiter s equatorial zonal wind above the cloud deck derived using mesoscale gravity waves
Geophysical Research Letters, 2013Co-Authors: Chris Watkins, Yk J ChoAbstract:[1] Data from the Galileo Probe, collected during its descent into Jupiter's atmosphere, is used to obtain a vertical profile of the zonal wind from ~0.5 bar (upper troposphere) to ~0.1 µbar (lower thermosphere) at the probe entry site. This is accomplished by constructing a map of gravity wave Lomb-Scargle periodograms as a function of altitude. The profile obtained from the map indicates that the wind speed above the visible cloud deck increases with height to ~150 m s-1 and then levels off at this value over a broad altitude range. The location of the Turbopause, as a region of wide wave spectrum, is also identified from the map. In addition, a cross-equatorial oscillation of a jet, which has previously been linked to the QQO in the stratosphere, is suggested by the profile.
Satonori Nozawa - One of the best experts on this subject based on the ideXlab platform.
-
change in Turbopause altitude at 52 and 70 n
Atmospheric Chemistry and Physics, 2016Co-Authors: C M Hall, C E Meek, A H Manson, Silje E Holmen, Satonori NozawaAbstract:Abstract. The Turbopause is the demarcation between atmospheric mixing by turbulence (below) and molecular diffusion (above). When studying concentrations of trace species in the atmosphere, and particularly long-term change, it may be important to understand processes present, together with their temporal evolution that may be responsible for redistribution of atmospheric constituents. The general region of transition between turbulent and molecular mixing coincides with the base of the ionosphere, the lower region in which molecular oxygen is dissociated, and, at high latitude in summer, the coldest part of the whole atmosphere. This study updates previous reports of Turbopause altitude, extending the time series by half a decade, and thus shedding new light on the nature of change over solar-cycle timescales. Assuming there is no trend in temperature, at 70° N there is evidence for a summer trend of ∼ 1.6 km decade−1, but for winter and at 52° N there is no significant evidence for change at all. If the temperature at 90 km is estimated using meteor trail data, it is possible to estimate a cooling rate, which, if applied to the Turbopause altitude estimation, fails to alter the trend significantly irrespective of season. The observed increase in Turbopause height supports a hypothesis of corresponding negative trends in atomic oxygen density, [O]. This supports independent studies of atomic oxygen density, [O], using mid-latitude time series dating from 1975, which show negative trends since 2002.
-
Turbopause determination climatology and climatic trends using medium frequency radars at 52 n and 70 n
Journal of Geophysical Research, 2008Co-Authors: C M Hall, C E Meek, A H Manson, Satonori NozawaAbstract:[1] We explore methods for identifying the Turbopause in its various guises using results from the medium frequency (MF) radars at Saskatoon (52°N) and Tromso (70°N) and using data from 1999 to 2007 inclusive. The classical radar Turbopause, identified as the altitude at which molecular viscosity inhibits turbulence, is determined and exhibits clear annual variation at both latitudes, occurring slightly lower down at 52°N as compared with 70°N. Second, we determine the altitude at which the zonal wind in the mesosphere undergoes a zero crossing as a possible height regime for gravity wave breaking. Third, we then investigate the altitude at which the mean square velocity fluctuation undergoes a transition from height independence to a steep increase with height. This altitude has recently been proposed as a “wave” Turbopause, and our method and determination are in good agreement with those of other authors. This wave Turbopause lies somewhat under the classical radar Turbopause and exhibits annual and semiannual variations at 70°N and 52°N, respectively. The relationships between these different transition altitudes are discussed, and we conclude that the recently defined wave Turbopause may represent a useful parameterization of mesospheric dynamics for future research. Examining the classical radar Turbopause time series we identify a trend in the 70°N results but none in those from 52°N. The possible reasons for the high-latitude finding are then discussed in the context of determinations of other features of the middle atmosphere and lower thermosphere. We conclude that mesospheric cooling in the underlying mesosphere is a probable cause.
C M Hall - One of the best experts on this subject based on the ideXlab platform.
-
change in Turbopause altitude at 52 and 70 n
Atmospheric Chemistry and Physics, 2016Co-Authors: C M Hall, C E Meek, A H Manson, Silje E Holmen, Satonori NozawaAbstract:Abstract. The Turbopause is the demarcation between atmospheric mixing by turbulence (below) and molecular diffusion (above). When studying concentrations of trace species in the atmosphere, and particularly long-term change, it may be important to understand processes present, together with their temporal evolution that may be responsible for redistribution of atmospheric constituents. The general region of transition between turbulent and molecular mixing coincides with the base of the ionosphere, the lower region in which molecular oxygen is dissociated, and, at high latitude in summer, the coldest part of the whole atmosphere. This study updates previous reports of Turbopause altitude, extending the time series by half a decade, and thus shedding new light on the nature of change over solar-cycle timescales. Assuming there is no trend in temperature, at 70° N there is evidence for a summer trend of ∼ 1.6 km decade−1, but for winter and at 52° N there is no significant evidence for change at all. If the temperature at 90 km is estimated using meteor trail data, it is possible to estimate a cooling rate, which, if applied to the Turbopause altitude estimation, fails to alter the trend significantly irrespective of season. The observed increase in Turbopause height supports a hypothesis of corresponding negative trends in atomic oxygen density, [O]. This supports independent studies of atomic oxygen density, [O], using mid-latitude time series dating from 1975, which show negative trends since 2002.
-
Turbopause determination climatology and climatic trends using medium frequency radars at 52 n and 70 n
Journal of Geophysical Research, 2008Co-Authors: C M Hall, C E Meek, A H Manson, Satonori NozawaAbstract:[1] We explore methods for identifying the Turbopause in its various guises using results from the medium frequency (MF) radars at Saskatoon (52°N) and Tromso (70°N) and using data from 1999 to 2007 inclusive. The classical radar Turbopause, identified as the altitude at which molecular viscosity inhibits turbulence, is determined and exhibits clear annual variation at both latitudes, occurring slightly lower down at 52°N as compared with 70°N. Second, we determine the altitude at which the zonal wind in the mesosphere undergoes a zero crossing as a possible height regime for gravity wave breaking. Third, we then investigate the altitude at which the mean square velocity fluctuation undergoes a transition from height independence to a steep increase with height. This altitude has recently been proposed as a “wave” Turbopause, and our method and determination are in good agreement with those of other authors. This wave Turbopause lies somewhat under the classical radar Turbopause and exhibits annual and semiannual variations at 70°N and 52°N, respectively. The relationships between these different transition altitudes are discussed, and we conclude that the recently defined wave Turbopause may represent a useful parameterization of mesospheric dynamics for future research. Examining the classical radar Turbopause time series we identify a trend in the 70°N results but none in those from 52°N. The possible reasons for the high-latitude finding are then discussed in the context of determinations of other features of the middle atmosphere and lower thermosphere. We conclude that mesospheric cooling in the underlying mesosphere is a probable cause.
-
seasonal variation of the Turbopause one year of turbulence investigation at 69 n by the joint university of tromso university of saskatchewan mf radar
Journal of Geophysical Research, 1998Co-Authors: C M Hall, A H Manson, C E MeekAbstract:Following upgrades to the University of Tromso/University of Saskatchewan MF radar, located in northern Norway at 69°N, 19°E, we have been able to complete a full calendar year of estimates of mesospheric turbulent intensity. The results represent the first such study using continuous measurements from this region, and temporal and height variations are satisfyingly in accordance with expectation. Since in the past there has been a degree of disagreement as to absolute intensities, we briefly compare our results with some from totally independent methods. The resulting dissipation rates, to be regarded as maxima for the turbulent dissipation, are used to identify an upper limit to the Turbopause. The Arctic Turbopause appears to exhibit an annual variation, being lower in the summer, in agreement with Danilov et al. [1979] but refuting Blum and Schuchard [1978].
-
measurements of the arctic Turbopause
Annales Geophysicae, 1998Co-Authors: C M Hall, A H Manson, C E MeekAbstract:We use estimates of turbulent intensity determined by the joint University of Saskatchewan / University of Tromso MF radar near Tromso, Norway (69°N 19°E) to estimate the height of the Turbopause as a function of season. Here we present the deduced trend in the Turbopause during the transition from winter to early summer. Since the radar operates continually, progressively more information is becoming available which will eventually show seasonal and inter-annual variation; however, we deem these preliminary results to be already of interest to the community.
D Offermann - One of the best experts on this subject based on the ideXlab platform.
-
relative intensities of middle atmosphere waves
Journal of Geophysical Research, 2009Co-Authors: D Offermann, O Gusev, M Donner, J M Forbes, M E Hagan, M G Mlynczak, J Oberheide, Peter Preusse, Hauke SchmidtAbstract:[1] Climatologies of gravity waves, quasi-stationary planetary waves, and tides are compared in the upper stratosphere, mesosphere, and lower thermosphere. Temperature standard deviations from zonal means are used as proxies for wave activity. The sum of the waves is compared to directly measured total temperature fluctuations. The resulting difference is used as a proxy for traveling planetary waves. A preliminary climatology for these waves is proposed. A ranking of the four wave types in terms of their impact on the total wave state of the atmosphere is achieved, which is dependent on altitude and latitude. At extratropical latitudes, gravity waves mostly play a major role. Traveling planetary waves are found to play a secondary role. Quasi-stationary planetary waves and tides yield a lesser contribution there. Vertical profiles of total temperature fluctuations show a sharp vertical gradient change (“kink” or “bend”) in the mesosphere. This is interpreted in terms of a change of wave damping, and the concept of a “wave Turbopause” is suggested. The altitude of this wave Turbopause is found to be mostly determined by the relative intensities of gravity waves and planetary waves. The Turbopause is further analyzed, including earlier mass spectrometer data. It is found that the wave Turbopause and the mass spectrometer Turbopause occur rather close together. The Turbopause forms a layer about 8 km thick, and the data suggest an additional 3 km mixing layer on top.
-
the wave Turbopause
Journal of Atmospheric and Solar-Terrestrial Physics, 2007Co-Authors: D Offermann, O Gusev, J Oberheide, Hauke Schmidt, M Jarisch, K U Grossmann, James M Russell, M G MlynczakAbstract:Abstract The “wave Turbopause” is defined as the mesospheric altitude level where the temperature fluctuation field indicates a substantial increase in wave amplitudes in the vertical direction. The Turbopause altitude is analyzed on the basis of four years of SABER data (2002–2005, Version 1.06). Substantial seasonal and latitudinal variations are found, with some interannual variability also present. Seasonal changes are annual at high latitudes, semi-annual at low latitudes, and a mixture of both at middle latitudes. Southern hemisphere data are similar as in the North if shifted by half a year. Latitudinal variations show a minimum in the tropics and two relative maxima at middle latitudes. The “wave Turbopause” is found near to zero-wind lines or low-wind zones (zonal wind). It is compared to rocket and other measurements, and interesting similarities are obtained. The wave Turbopause can also be found in the HAMMONIA GCM. A preliminary analysis shows results similar to those of the SABER measurements.
-
global wave activity from upper stratosphere to lower thermosphere a new Turbopause concept
Journal of Atmospheric and Solar-Terrestrial Physics, 2006Co-Authors: D Offermann, J Oberheide, M Jarisch, James M Russell, O A Gusev, Ingo Wohltmann, M G MlynczakAbstract:Abstract Global temperature measurements are available from CRISTA (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere) and from SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) up to 110 km . Standard deviation from zonal mean temperature is used as a wave activity indicator (proxy). Altitude/latitude plots of these standard deviations σ or variances σ 2 show a structure that is dependent on the season. There is also substantial zonal asymmetry. Vertical cuts through the σ -field show a remarkable transition between 90 and 100 km : linear fit curves above 100 km have a gradient similar to the amplitude increase of freely (upward) propagating waves. The corresponding gradients below 90 km are much flatter and thus indicate considerable wave damping. The intersection of the two fit curves is dubbed the “wave-Turbopause” here, and is believed to be near the turbulent or transport Turbopause. This wave-Turbopause is found in the vicinity of 90 – 95 km for CRISTA-1, CRISTA-2, and SABER. It is compared to the corresponding cold point mesopause and to the isolines of estimated potential vorticities to show similarities with the tropopause. The height of the wave-Turbopause depends on latitude. It also has considerable seasonal variation, which is very different at high and low latitudes.
Alexander S Medvedev - One of the best experts on this subject based on the ideXlab platform.
-
influence of parameterized small scale gravity waves on the migrating diurnal tide in earth s thermosphere
Journal of Geophysical Research, 2017Co-Authors: Erdal Yigit, Alexander S MedvedevAbstract:Effects of subgrid-scale gravity waves (GWs) on the diurnal migrating tides are investigated from the mesosphere to the upper thermosphere for September equinox conditions, using a general circulation model coupled with the extended spectral nonlinear GW parameterization of Yigit et al. [2008].. Simulations with GW effects cut-off above the Turbopause and included in the entire thermosphere have been conducted. GWs appreciably impact the mean circulation and cool the thermosphere down by up to 12-18%. GWs significantly affect the winds modulated by the diurnal migrating tide, in particular in the low-latitude mesosphere and lower thermosphere and in the high-latitude thermosphere. These effects depend on the mutual correlation of the diurnal phases of the GW forcing and tides: GWs can either enhance or reduce the tidal amplitude. In the low-latitude MLT, the correlation between the direction of the deposited GW momentum and the tidal phase is positive due to propagation of a broad spectrum of GW harmonics through the alternating winds. In the Northern Hemisphere high-latitude thermosphere, GWs act against the tide due to an anti-correlation of tidal wind and GW momentum, while in the Southern high-latitudes they weakly enhance the tidal amplitude via a combination of a partial correlation of phases and GW-induced changes of the circulation. The variable nature of GW effects on the thermal tide can be captured in GCMs provided that a GW parameterization (1) considers a broad spectrum of harmonics, (2) properly describes their propagation, and (3) correctly accounts for the physics of wave breaking/saturation.
-
gravity waves in the thermosphere during a sudden stratospheric warming
Geophysical Research Letters, 2012Co-Authors: Erdal Yigit, Alexander S MedvedevAbstract:[1] We examine for the first time the propagation of gravity waves (GWs) of lower atmospheric origin to the thermosphere above the Turbopause during a sudden stratospheric warming (SSW). The study is performed with the Coupled Middle Atmosphere-Thermosphere general circulation model and the implemented spectral GW parameterization of Yigit et al. (2008). Simulations reveal a strong modulation by SSWs of GW activity, momentum deposition rates, and the circulation feedbacks at heights up to the upper thermosphere (∼270 km). Wave-induced root mean square wind fluctuations increase by a factor of three during the warming above the Turbopause. This occurs mainly due to a reduction of filtering eastward traveling harmonics by the weaker stratospheric jet. Compared to nominal conditions, these GW harmonics propagate to higher altitudes and have a larger impact on the mean flow in the thermosphere, when they are dissipated. The evolution of stratospheric and mesospheric winds during an SSW life-cycle creates a robust and distinctive response in GW activity and mean fields in the thermosphere above the Turbopause up to 300 km.
-
heating and cooling of the thermosphere by internal gravity waves
Geophysical Research Letters, 2009Co-Authors: Erdal Yigit, Alexander S MedvedevAbstract:For the first time, estimates of heating and cooling in the upper thermosphere due to dissipating and breaking gravity waves ( GWs) of tropospheric origin have been obtained with a comprehensive general circulation model ( GCM). A GW parameterization specifically designed for thermospheric heights has been implemented in the CMAT2 GCM covering altitudes from the tropopause to the F-2 region, and simulations for the June solstice have been performed. They reveal that the net thermal effect of GWs above the Turbopause is cooling. The largest ( up to -170 K d(-1) in a zonally and temporally averaged sense) cooling takes place in the high latitudes of both hemispheres near 210 km. The instantaneous values of heating and cooling rates are highly variable, and reach up to 500 and -3000 K d(-1) in the F-2 region, respectively. Inclusion of the GW thermal effects reduces the simulated model temperatures by up to 200 K over the summer pole and by 100 to 170 K at other latitudes near 210 km.
