The Experts below are selected from a list of 183 Experts worldwide ranked by ideXlab platform
Matthew C Wheeler - One of the best experts on this subject based on the ideXlab platform.
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some space time spectral analyses of tropical convection and planetary scale Waves
Journal of the Atmospheric Sciences, 2008Co-Authors: Harry H Hendon, Matthew C WheelerAbstract:Abstract Three aspects of space–time spectral analysis are explored for diagnosis of the organization of tropical convection by the Madden–Julian oscillation (MJO) and other Equatorial Wave modes: 1) definition of the background spectrum upon which spectral peaks are assessed, 2) alternate variance preserving display of the spectra, and 3) the space–time coherence spectrum. Here the background spectrum at each zonal Wavenumber is assumed to result from a red noise process. The associated decorrelation time for the red noise process for tropical convection is found to be half as long as for zonal wind, reflecting the different physical processes controlling each field. The significance of spectral peaks associated with Equatorial Wave modes for outgoing longWave radiation (OLR), which is a proxy for precipitating deep convection, and zonal winds that stand out above the red background spectrum is similar to that identified using a background spectrum resulting from ad hoc smoothing of the original spectrum...
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convectively coupled Equatorial Waves analysis of clouds and temperature in the Wavenumber frequency domain
Journal of the Atmospheric Sciences, 1999Co-Authors: Matthew C Wheeler, George N KiladisAbstract:A Wavenumber-frequency spectrum analysis is performed for all longitudes in the domain 158S‐158N using a long (;18 years) twice-daily record of satellite-observed outgoing longWave radiation (OLR), a good proxy for deep tropical convection. The broad nature of the spectrum is red in both zonal Wavenumber and frequency. By removing an estimated background spectrum, numerous statistically significant spectral peaks are isolated. Some of the peaks correspond quite well to the dispersion relations of the Equatorially trapped Wave modes of shallow water theory with implied equivalent depths in the range of 12‐50 m. Cross-spectrum analysis with the satellite-based microWave sounding unit deep-layer temperature data shows that these spectral peaks in the OLR are ‘‘coupled’’ with this dynamical field. The equivalent depths of the convectively coupled Waves are shallower than those typical of Equatorial Waves uncoupled with convection. Such a small equivalent depth is thought to be a result of the interaction between convection and the dynamics. The convectively coupled Equatorial Waves identified correspond to the Kelvin, n 5 1 Equatorial Rossby, mixed Rossby-gravity, n 5 0 eastward inertiogravity, n 5 1 westward inertio-gravity (WIG), and n 5 2 WIG Waves. Additionally, the Madden‐Julian oscillation and tropical depression-type disturbances are present in the OLR spectra. These latter two features are unlike the convectively coupled Equatorial Waves due to their location away from the Equatorial Wave dispersion curves in the Wavenumber-frequency domain. Extraction of the different convectively coupled disturbances in the time‐longitude domain is performed by filtering the OLR dataset for very specific zonal Wavenumbers and frequencies. The geographical distribution of the variance of these filtered data gives further evidence that some of the spectral peaks correspond to particular Equatorial Wave modes. The results have implications for the cumulus parameterization problem, for the excitation of Equatorial Waves in the lower stratosphere, and for extended-range forecasting in the Tropics.
Tiffany A Shaw - One of the best experts on this subject based on the ideXlab platform.
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on the role of planetary scale Waves in the abrupt seasonal transition of the northern hemisphere general circulation
Journal of the Atmospheric Sciences, 2014Co-Authors: Tiffany A ShawAbstract:The role of planetary-scale Waves in the abrupt seasonal transition of the Northern Hemisphere (NH) general circulation is studied. In reanalysis data, the winter-to-summer transition involves the growth of planetary-scale Wave latent heat and momentum transports in the region of monsoons and anticyclones that dominate over the zonal-mean transport beginning in midspring. The Wave-dominated regime coincides with an abrupt northward expansion of the cross-Equatorial circulation and reversal of the trade winds. In the upper troposphere, the transition coincides with the growth of cross-Equatorial planetary-scale Wave momentum transport and a poleward shift of subplanetary-scale Wave transport and jet stream. The dynamics of the seasonal transition are captured by idealized aquaplanet model simulations with a prescribed subtropical planetary-scale Wave sea surface temperature (SST) perturbation. The SST perturbation generates subtropical planetary-scale Wave streamfunction variance and transport in the lower and uppertroposphereconsistentwithquasigeostrophictheory.BeyondathresholdSST,atransitionofthezonalmean circulation occurs, which coincides with a localized reversal of absolute vorticity in the NH tropical upper troposphere. The transition is abrupt in the lower troposphere because of the quadratic dependence of the Wave transport on the SST perturbation and involves seasonal-time-scale feedbacks between the Wave and zonal-meanflow in theupper troposphere,including cross-Equatorial Wave propagation. Thezonal-mean vertical and meridional flows associated with the circulation response are in balance with the planetary-scale Wave momentumand latentheat meridionalflux divergences. The resultshighlightthe leading-order roleofmonsoon‐ anticyclone transport in the seasonal transition, including its impact on the meridional extent of the Hadley and Ferrel cells. They can also be used to explain why the transition is less abrupt in the Southern Hemisphere.
J. M. Russell Iii - One of the best experts on this subject based on the ideXlab platform.
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Equatorial Wave analysis from SABER and ECMWF temperatures
Atmospheric Chemistry and Physics Discussions, 2007Co-Authors: M. Ern, P. Preusse, M. Krebsbach, M. G. Mlynczak, J. M. Russell IiiAbstract:Equatorial planetary scale Wave modes such as Kelvin Waves or Rossby-gravity Waves are excited by convective processes in the troposphere. In this paper an analysis for these and other Equatorial Wave modes is carried out with special focus on the stratosphere using temperature data from the SABER instrument as well as ECMWF temperatures. Space-time spectra of symmetric and antisymmetric spectral power are derived to separate the different Equatorial Wave types and the contribution of gravity Waves is determined from the spectral background of the space-time spectra. Both gravity Waves and Equatorial planetary scale Wave modes are main drivers of the quasi-biennial oscillation (QBO) in the stratosphere. Temperature variances attributed to the different Wave types are calculated for the period from February 2002 until March 2006 and compared to previous findings. A comparison between SABER and ECMWF Wave analyses shows that in the lower stratosphere SABER and ECMWF spectra and temperature variances agree remarkably well while in the upper stratosphere ECMWF tends to overestimate Kelvin Wave components. Gravity Wave variances are partly reproduced by ECMWF but have a significant low-bias. A case study for the time period of the SCOUT-O3 tropical aircraft measurement campaign in Darwin/Australia (in November and December 2005) is performed and we find that in the lower stratosphere also the longitude-time distribution of the Kelvin Waves is correctly reproduced by ECMWF.
Peter Preusse - One of the best experts on this subject based on the ideXlab platform.
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Wave fluxes of Equatorial kelvin Waves and qbo zonal wind forcing derived from saber and ecmwf temperature space time spectra
Atmospheric Chemistry and Physics, 2009Co-Authors: Peter PreusseAbstract:Abstract. The quasi-biennial oscillation (QBO) of the zonal mean zonal wind is a dynamical phenomenon of the tropical middle atmosphere. Influences of the QBO can even be found at mid and high latitudes. It is widely accepted that the phase descent of alternating tropical easterlies and westerlies is driven by atmospheric Waves of both global scale (Equatorial Wave modes like Kelvin, Equatorial Rossby, Rossby-gravity, or inertia-gravity Waves), as well as mesoscale gravity Waves. However, the relative distribution of the different types of Waves to the forcing of the QBO winds is highly uncertain. This is the case because until recently there were no high resolution long-term global measurements in the stratosphere. In our study we estimate Kelvin Wave momentum flux and the contribution of zonal wind forcing by Kelvin Waves based on space-time spectra determined from both Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements as well as temperatures from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Peak values of total Kelvin Wave zonal wind forcing found are about 0.2 m/s/day. There is good agreement between SABER and ECMWF results. Altitude-time cross sections are shown and the results are compared to the total Wave forcing required to balance the background atmosphere. Sometimes Kelvin Wave forcing is sufficient to explain almost the whole total Wave forcing required for the momentum balance during the transition from QBO easterly to westerly winds. This is especially the case during the periods of strong westerly wind shear when the zonal wind is between −20 and 10 m/s at the equator in the altitude range 20 to 35 km. During other parts of the phases of strong westerly wind shear, however, the contribution of Kelvin Waves can be comparably low and the missing Wave forcing, which is often attributed to mesoscale gravity Waves or intermediate scale Waves, can be the by far dominant contribution of the QBO forcing. It is also found that seasonal variations of Kelvin Wave accelerations could play an important role for the maintenance of the QBO westerly wind jets in the lower stratosphere.
Brian J Hoskins - One of the best experts on this subject based on the ideXlab platform.
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linking african easterly Wave activity with Equatorial Waves and the influence of rossby Waves from the southern hemisphere
Journal of the Atmospheric Sciences, 2018Co-Authors: Guiying Yang, John Methven, Steven J Woolnough, Kevin I Hodges, Brian J HoskinsAbstract:AbstractA connection is found between African easterly Waves (AEWs), Equatorial westward-moving mixed Rossby–gravity (WMRG) Waves, and equivalent barotropic Rossby Waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of Equatorial Waves is calculated by projection of broadband-filtered ERA-Interim data onto a horizontal structure basis obtained from Equatorial Wave theory. Mechanisms enabling interaction between the Wave types are identified. AEWs are dominated by a vorticity Wave that tilts eastward below the African easterly jet and westward above: the tilt necessary for baroclinic Wave growth. However, a strong relationship is identified between amplifying vorticity centers within AEWs and Equatorial WMRG Waves. Although the Waves do not phase lock, positive vorticity centers amplify whenever the cross-Equatorial motion of the WMRG Wave lies at the same longitude in the upper troposphere (southward flow) and east of this in the lower troposphere (northward flow). Two mechanisms could ...
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convectively coupled Equatorial Waves a new methodology for identifying Wave structures in observational data
Journal of the Atmospheric Sciences, 2003Co-Authors: Guiying Yang, Brian J Hoskins, J M SlingoAbstract:Abstract Convectively coupled Equatorial Waves are fundamental components of the interaction between the physics and dynamics of the tropical atmosphere. A new methodology, which isolates individual Equatorial Wave modes, has been developed and applied to observational data. The methodology assumes that the horizontal structures given by Equatorial Wave theory can be used to project upper- and lower-tropospheric data onto Equatorial Wave modes. The dynamical fields are first separated into eastward- and westward-moving components with a specified domain of frequency–zonal Wavenumber. Each of the components for each field is then projected onto the different Equatorial modes using the y structures of these modes given by the theory. The latitudinal scale yo of the modes is predetermined by data to fit the Equatorial trapping in a suitable latitude belt y = ±Y. The extent to which the different dynamical fields are consistent with one another in their depiction of each Equatorial Wave structure determines t...