The Experts below are selected from a list of 252 Experts worldwide ranked by ideXlab platform
William R Boos - One of the best experts on this subject based on the ideXlab platform.
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monsoon depression amplification by moist Barotropic Instability in a vertically sheared environment
Quarterly Journal of the Royal Meteorological Society, 2019Co-Authors: Michael Diaz, William R BoosAbstract:Author(s): Diaz, M; Boos, WR | Abstract: This study makes the case that monsoon depressions over South Asia can form from a variant of moist Barotropic Instability. Using an idealized numerical framework in which the atmosphere is partitioned into a basic state and a perturbation, we simulate vortices resembling monsoon depressions that draw energy from the meridional shear of the monsoon trough and amplify when they interact with precipitating ascent. The influence of the basic state vertical shear on the vortex induces upward velocity, which couples precipitation with a Rossby-wave-like mode arising from dry Barotropic growth, allowing the vortex to intensify. Sensitivity experiments reveal that both the sheared basic state and latent heating are necessary to achieve positive growth rates and that this process requires a sufficiently large initial perturbation. Trajectory analyses suggest that the combined flow of the vortex and the large-scale monsoon transport diabatically generated potential vorticity from southwest of the vortex into the vortex center, thus enabling growth. In contrast with tropical cyclones, this mechanism does not require a feedback between surface wind speed and surface heat and moisture fluxes, though this feedback does ultimately result in a slightly stronger vortex.
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monsoon depression amplification by moist Barotropic Instability in a vertically sheared environment
Quarterly Journal of the Royal Meteorological Society, 2019Co-Authors: Michael Diaz, William R BoosAbstract:Author(s): Diaz, M; Boos, WR | Abstract: © 2019 Royal Meteorological Society This study makes the case that monsoon depressions over South Asia can form from a variant of moist Barotropic Instability. Using an idealized numerical framework in which the atmosphere is partitioned into a basic state and a perturbation, we simulate vortices resembling monsoon depressions that draw energy from the meridional shear of the monsoon trough and amplify when they interact with precipitating ascent. The influence of the basic state vertical shear on the vortex induces upward velocity, which couples precipitation with a Rossby-wave-like mode arising from dry Barotropic growth, allowing the vortex to intensify. Sensitivity experiments reveal that both the sheared basic state and latent heating are necessary to achieve positive growth rates and that this process requires a sufficiently large initial perturbation. Trajectory analyses suggest that the combined flow of the vortex and the large-scale monsoon transport diabatically generated potential vorticity from southwest of the vortex into the vortex center, thus enabling growth. In contrast with tropical cyclones, this mechanism does not require a feedback between surface wind speed and surface heat and moisture fluxes, though this feedback does ultimately result in a slightly stronger vortex.
W A Norton - One of the best experts on this subject based on the ideXlab platform.
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amplification of the 2 day wave from mutual interaction of global rossby gravity and local modes in the summer mesosphere
Journal of Geophysical Research, 2007Co-Authors: W A Norton, Maisa RojasAbstract:[1] The 2-day wave is a planetary-scale wave recurrently observed in the summer mesosphere. It is predominantly zonal wave number 3 (though wave numbers 2 and 4 are also observed) and has a period of approximately 2 days. It is generally accepted that the 2-day wave has characteristics of both global-scale Rossby-gravity normal mode and baroclinic/Barotropic Instability. Here we examine how local Instability amplifies the global Rossby-gravity mode. We perform a two-dimensional Instability calculation to calculate unstable modes. The fastest growing modes have wave number 3, a 35-hour Rossby mode localized in the summer hemisphere at high latitudes, and a Rossby-gravity mode with period 42 hours with similar characteristics to the observed 2-day wave. The Rossby-gravity mode was examined as the background state was changed from an isothermal atmosphere to realistic conditions. The wave was slowly Doppler shifted toward shorter periods and its spatial structure became distorted. Approaching realistic conditions the mode becomes weakly unstable. Near this point a localized Rossby mode also emerges which exists on the reversed potential vorticity gradients of the summer jet. This has the necessary characteristics to phase lock with the Rossby-gravity mode. Hence we propose that the 2-day wave amplifies from the interaction of global-scale Rossby-gravity and local modes. A nonlinear model was used to examine the unstable modes as they grow to finite amplitude. It was found that even though the 35-hour Rossby wave had the largest growth rates, it did not reach large amplitude. It was the slower growing 42-hour Rossby-gravity mode which dominated the long time evolution.
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sensitivity of mesospheric mean flow planetary waves and tides to strength of gravity wave drag
Journal of Geophysical Research, 1999Co-Authors: W A Norton, John ThuburnAbstract:A global circulation model which extends from the surface to 125 km is used to study how the strength of gravity wave drag affects the dynamics of the mesosphere. The strength of gravity wave drag has a strong influence on the zonal mean state of the mesosphere, in particular the magnitude and variability of the summer mesopause temperature and the shear on the top of the mesospheric jets. This change in the zonal mean state strongly affects the susceptibility of the mesosphere to baroclinic and/or Barotropic Instability and hence the formation of the 2-day wave. The 2-day wave, in turn, interacts nonlinearly with the diurnal tide, producing secondary waves and a reduction in amplitude of the diurnal tide. Previous studies with quasi-linear mechanistic tidal models have captured some semiannual variation in tidal amplitude through direct interactions between tides and gravity waves. Our fully nonlinear global circulation model results support an alternative explanation in terms of interactions between planetary waves and tides.
Inna Polichtchouk - One of the best experts on this subject based on the ideXlab platform.
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equatorial superrotation in held and suarez like flows with weak equator to pole surface temperature gradient
Quarterly Journal of the Royal Meteorological Society, 2016Co-Authors: Inna PolichtchoukAbstract:Equatorial superrotation under zonally symmetric thermal forcing is investigated in a set-up close to that of the classic Held and Suarez set-up. In contrast to the behaviour in the classic set-up, a transition to equatorial superrotation occurs when the equator-to-pole surface equilibrium entropy gradient is weakened. Two factors contribute to this transition: (i) the reduction of breaking Rossby waves from the midlatitudes that decelerate the equatorial flow and (ii) the presence of Barotropic Instability in the equatorial region, providing stirring to accelerate the equatorial flow. In the latter, Kelvin waves excited by Instability near the Equator generate and maintain the superrotation. However, the superrotation is unphysically enhanced if simulations are underresolved and/or overdissipated.
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equatorial superrotation in held suarez like flows with weak equator to pole surface temperature gradient
arXiv: Atmospheric and Oceanic Physics, 2016Co-Authors: Inna Polichtchouk, James Y K ChoAbstract:Equatorial superrotation under zonally-symmetric thermal forcing is investigated in a setup close to that of the classic Held & Suarez (1994) setup. In contrast to the behaviour in the classic setup, a transition to equatorial superrotation occurs when the equator-to-pole surface equilibrium entropy gradient is weakened. Two factors contribute to this transition: 1) the reduction of breaking Rossby waves from the mid-latitude that decelerate the equatorial flow and 2) the presence of Barotropic Instability in the equatorial region, providing stirring to accelerate the equatorial flow. In the latter, Kelvin waves excited by Instability near the equator generate and maintain the superrotation. However, the superrotation is unphysically enhanced if simulations are under-resolved and/or over-dissipated.
S K Mishra - One of the best experts on this subject based on the ideXlab platform.
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a primitive equation Barotropic Instability study of the monsoon onset vortex 1979
Quarterly Journal of the Royal Meteorological Society, 2007Co-Authors: S K Mishra, M D Patwardhan, L GeorgeAbstract:Il s'agit du tourbillon precurseur qui se forme devant le courant arrivant de mousson et qui a quelquefois l'apparence d'un tourbillon cyclonique a 700 mb. Un analyse de la stabilite barotrope, dans le cadre des equations primitives, du profil du vent zonal a 700 mb sur la mer d'Oman, a ete effectuee pour chaque jour situe entre le 10 et le 14 juin 1979, au cours de la phase d'installation de la mousson. On a trouve que l'ecoulement devenait progressivement instable. Le mode le plus instable, dans les equations primitives, presente un temps de pliage de constante e, de 3,1 jours, une longueur d'onde de 3000 km et une v de phase portant a l'ouest, de 1,9 ms −1 . Il s'avere que l'effet non geotrophique reduit le taux de croissance et change la structure du mode instable. On note que lorsque le mode instable normalise est superpose a l'etat fondamental, alors le champ total est proche des observations. Calcul de l'energetique du mode et identification des processus physiques responsables de la propagation de l'onde vers l'ouest
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genesis of the northeast brazil upper tropospheric cyclonic vortex a primitive equation Barotropic Instability study
Journal of the Atmospheric Sciences, 2007Co-Authors: S K Mishra, Brahmananda V Rao, Sergio H FranchitoAbstract:Abstract The primitive equation Barotropic unstable linear normal modes are computed using an eigenvalue approach for daily latitudinal profiles of zonal flow in the upper-tropospheric layer of 100–350 hPa before and after formation of cyclonic vortices during January 1993 and November 2001 off the coast of northeast Brazil. The wave kinetic energy equation for u- and υ-motion is presented. Equations are derived to isolate the contribution of divergence and other dynamical processes in the movement and growth of unstable modes. Numerical accuracy and physical nature of unstable modes are tested. In a short span of 2–3 days, prior to formation of vortices, a progressive and a sharp intensification of the basic flow shear zone and its Barotropic Instability are seen with time. The horizontal structure, momentum transport, and zonal and meridional scales of the most unstable normalized wave are obtained and compared with the vortex extracted from the 200-hPa observed winds using a bandpass smoother. A close ...
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nonlinear Barotropic Instability of upper tropospheric tropical easterly jet on the sphere
Journal of the Atmospheric Sciences, 1993Co-Authors: S K MishraAbstract:Abstract The nonlinear evolution of perturbation superimposed on the Barotropic, unstable observed mean easterly jet at 100 hPa is studied over the sphere. The nondivergent Barotropic nonlinear global spectral model with rhomboidal truncation at zonal wavenumber 21 is integrated for 120 days for initial random and linear unstable perturbations. The model includes a Rayleigh friction and restoring mechanism for zonal wind to its initial distribution. Time variations of eddy and zonal kinetic energy, zonal-wave and wave-wave interactions, and eddy and zonal kinetic energy dissipations are examined. The growth of perturbation begins with exponential increase in its kinetic energy for a short period, followed by a linear increase. The perturbations undergo oscillations before approaching a steady state. During the initial exponential phase, the nonlinear interactions further destabilize the jet, and this effect is more pronounced for the random initial perturbation. It is found that oscillations in kinetic en...
Maisa Rojas - One of the best experts on this subject based on the ideXlab platform.
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amplification of the 2 day wave from mutual interaction of global rossby gravity and local modes in the summer mesosphere
Journal of Geophysical Research, 2007Co-Authors: W A Norton, Maisa RojasAbstract:[1] The 2-day wave is a planetary-scale wave recurrently observed in the summer mesosphere. It is predominantly zonal wave number 3 (though wave numbers 2 and 4 are also observed) and has a period of approximately 2 days. It is generally accepted that the 2-day wave has characteristics of both global-scale Rossby-gravity normal mode and baroclinic/Barotropic Instability. Here we examine how local Instability amplifies the global Rossby-gravity mode. We perform a two-dimensional Instability calculation to calculate unstable modes. The fastest growing modes have wave number 3, a 35-hour Rossby mode localized in the summer hemisphere at high latitudes, and a Rossby-gravity mode with period 42 hours with similar characteristics to the observed 2-day wave. The Rossby-gravity mode was examined as the background state was changed from an isothermal atmosphere to realistic conditions. The wave was slowly Doppler shifted toward shorter periods and its spatial structure became distorted. Approaching realistic conditions the mode becomes weakly unstable. Near this point a localized Rossby mode also emerges which exists on the reversed potential vorticity gradients of the summer jet. This has the necessary characteristics to phase lock with the Rossby-gravity mode. Hence we propose that the 2-day wave amplifies from the interaction of global-scale Rossby-gravity and local modes. A nonlinear model was used to examine the unstable modes as they grow to finite amplitude. It was found that even though the 35-hour Rossby wave had the largest growth rates, it did not reach large amplitude. It was the slower growing 42-hour Rossby-gravity mode which dominated the long time evolution.