scholarly journals Meridional Momentum Flux and Superrotation in the Multiscale IPESD MJO Model

2007 ◽  
Vol 64 (5) ◽  
pp. 1636-1651 ◽  
Author(s):  
Joseph A. Biello ◽  
Andrew J. Majda ◽  
Mitchell W. Moncrieff
Keyword(s):  
Velocity Field ◽  
Momentum Flux ◽  
Madden Julian Oscillation ◽  
Synoptic Scale ◽  
Multiscale Models ◽  
Planetary Scale ◽  
Westerly Winds ◽  
Momentum Fluxes ◽  
Convergent Flow ◽  
Balance Dynamics

Abstract The derivation of the meridional momentum flux arising from a multiscale horizontal velocity field in the intraseasonal, planetary, equatorial synoptic-scale dynamics (IPESD) multiscale models of the equatorial troposphere is presented. It is shown that, because of the balance dynamics on the synoptic scales, the synoptic-scale component of the meridional momentum flux convergence must always vanish at the equator. Plausible Madden–Julian oscillation (MJO) models are presented along with their planetary-scale meridional momentum fluxes. These models are driven by synoptic-scale heating fluctuations that have vertical and meridional tilts. Irrespective of the sign of the synoptic-scale meridional momentum flux (direction of the tilts) in each of the four MJO examples, the zonal and vertical mean meridional momentum flux convergence from the planetary scales always drives westerly winds near the equator: this is the superrotation characteristic of actual MJOs. The concluding discussion demonstrates that equatorial superrotation occurs when the planetary flow due to the vertical upscale momentum flux from synoptic scales reinforces the horizontally convergent flow due to planetary-scale mean heating.

scholarly journals A Multiscale Asymptotic Theory of Extratropical Wave–Mean Flow Interaction

2018 ◽  
Vol 75 (6) ◽  
pp. 1833-1852 ◽  
Author(s):  
Lina Boljka ◽  
Theodore G. Shepherd
Keyword(s):  
Asymptotic Methods ◽  
Wave Activity ◽  
Heat Fluxes ◽  
Synoptic Scale ◽  
Mean Flow ◽  
Planetary Scale ◽  
Momentum Budget ◽  
Momentum Fluxes ◽  
Diabatic Processes ◽  
Adiabatic Term

Abstract Multiscale asymptotic methods are used to derive wave activity equations for planetary- and synoptic-scale eddies and their interactions with a zonal mean flow. The eddies are assumed to be of small amplitude, and the synoptic-scale zonal and meridional length scales are taken to be equal. Under these assumptions, the zonal-mean and planetary-scale dynamics are planetary geostrophic (i.e., dominated by vortex stretching), and the interaction between planetary- and synoptic-scale eddies occurs only through the zonal mean flow or through diabatic processes. Planetary-scale heat fluxes are shown to enter the angular momentum budget through meridional mass redistribution. After averaging over synoptic length and time scales, momentum fluxes disappear from the synoptic-scale wave activity equation while synoptic-scale heat fluxes disappear from the baroclinicity equation, leaving planetary-scale heat fluxes as the only adiabatic term coupling the baroclinic and barotropic components of the zonal mean flow. In the special case of weak planetary waves, the decoupling between the baroclinic and barotropic parts of the flow is complete with momentum fluxes driving the barotropic zonal mean flow, heat fluxes driving the wave activity, and diabatic processes driving baroclinicity. These results help explain the apparent decoupling between the baroclinic and barotropic components of flow variability recently identified in observations and may provide a means of better understanding the link between thermodynamic and dynamic aspects of climate variability and change.

scholarly journals Intraseasonal to interannual variability of Kelvin wave momentum fluxes as derived from high-resolution radiosonde data

2017 ◽  
Vol 17 (14) ◽  
pp. 8971-8986
Author(s):  
Jeremiah P. Sjoberg ◽  
Thomas Birner ◽  
Richard H. Johnson
Keyword(s):  
Time Series ◽  
Momentum Flux ◽  
Radiosonde Data ◽  
Vertical Resolution ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Daily Time Series ◽  
Momentum Fluxes ◽  
Daily Time ◽  
Wave Momentum

Abstract. Observational estimates of Kelvin wave momentum fluxes in the tropical lower stratosphere remain challenging. Here we extend a method based on linear wave theory to estimate daily time series of these momentum fluxes from high-resolution radiosonde data. Daily time series are produced for sounding sites operated by the US Department of Energy (DOE) and from the recent Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign. Our momentum flux estimates are found to be robust to different data sources and processing and in quantitative agreement with estimates from prior studies. Testing the sensitivity to vertical resolution, our estimated momentum fluxes are found to be most sensitive to vertical resolution greater than 1 km, largely due to overestimation of the vertical wavelength. Climatological analysis is performed over a selected 11-year span of data from DOE Atmospheric Radiation Measurement (ARM) radiosonde sites. Analyses of this 11-year span of data reveal the expected seasonal cycle of momentum flux maxima in boreal winter and minima in boreal summer, and variability associated with the quasi-biennial oscillation of maxima during easterly phase and minima during westerly phase. Comparison between periods with active convection that is either strongly or weakly associated with the Madden–Julian Oscillation (MJO) suggests that the MJO provides a nontrivial increase in the lowermost stratospheric momentum fluxes.

scholarly journals Intraseasonal to interannual variability of Kelvin wave momentum fluxes as derived from high-resolution radiosonde data

2017 ◽  
Author(s):  
Jeremiah P. Sjoberg ◽  
Thomas Birner ◽  
Richard H. Johnson
Keyword(s):  
Time Series ◽  
High Resolution ◽  
Momentum Flux ◽  
Radiosonde Data ◽  
Boreal Summer ◽  
Vertical Resolution ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Momentum Fluxes ◽  
Wave Momentum

Abstract. Observational estimates of Kelvin wave momentum fluxes in the tropical lower stratosphere remains challenging. Here we extend a method based on linear wave theory to estimate time series of these momentum fluxes from high-resolution radiosonde data. Testing the sensitivity to vertical resolution, our estimated momentum fluxes are found to be most sensitive to vertical resolution greater than 1 km, largely due to overestimation of the vertical wavelength. Estimates of momentum fluxes derived from reanalyses and coarse-resolution satellite data are notably larger. Daily time series are produced for sounding sites operated by the U.S. Department of Energy (DOE) and from the recent Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign. Our momentum flux estimates are found to be robust to different data sources and processing, and in quantitative agreement with estimates from prior studies. Climatological analysis is performed over the selected 11 year span of data from the ARM sites. Analyses for the available 11-year span of data reveal the expected seasonal cycle of momentum flux maxima in boreal winter and minima in boreal summer and variability associated with the quasi-biennial oscillation (QBO) of maxima during easterly phase and minima during westerly phase. Analysis of Madden-Julian Oscillation (MJO) active periods suggests that the MJO provides a nontrivial increase in lowermost stratospheric momentum fluxes, though statistical significance is not found due to the small number of events observed in the available time series.

Mountain-Wave Momentum Flux in an Evolving Synoptic-Scale Flow

2005 ◽  
Vol 62 (9) ◽  
pp. 3213-3231 ◽  
Author(s):  
Chih-Chieh Chen ◽  
Dale R. Durran ◽  
Gregory J. Hakim
Keyword(s):  
Numerical Simulations ◽  
Momentum Flux ◽  
Nonlinearity Parameter ◽  
Synoptic Scale ◽  
Incident Flow ◽  
Mountain Wave ◽  
Pressure Drag ◽  
Momentum Fluxes ◽  
Wave Induced ◽  
Wave Momentum

Abstract The evolution of mountain-wave-induced momentum flux is examined through idealized numerical simulations during the passage of a time-evolving synoptic-scale flow over an isolated 3D mountain of height h. The dynamically consistent synoptic-scale flow U accelerates and decelerates with a period of 50 h; the maximum wind arrives over the mountain at 25 h. The synoptic-scale static stability N is constant, so the time dependence of the nonlinearity parameter, ɛ(t) = Nh/U(t), is symmetric about a minimum value at 25 h. The evolution of the vertical profile of momentum flux shows substantial asymmetry about the midpoint of the cycle even though the nonlinearity parameter is symmetric. Larger downward momentum fluxes are found during the accelerating phase, and the largest momentum fluxes occur in the mid- and upper troposphere before the maximum background flow arrives at the mountain. For a period of roughly 15 h, this vertical distribution of momentum flux accelerates the lower-tropospheric zonal-mean winds due to low-level momentum flux convergence. Conservation of wave action and Wentzel–Kramers–Brillouin (WKB) ray tracing are used to reconstruct the time–altitude dependence of the mountain-wave momentum flux in a semianalytic procedure that is completely independent of the full numerical simulations. For quasi-linear cases, the reconstructions show good agreement with the numerical simulations, implying that the basic asymmetry obtained in the full numerical simulations may be interpreted using WKB theory. These results demonstrate that even slow variations in the mean flow, with a time scale of 2 days, play a dominant role in regulating the vertical profile of mountain-wave-induced momentum flux. The time evolution of cross-mountain pressure drag is also examined in this study. For almost-linear cases, the pressure drag is well predicted under steady-state linear theory by using the instantaneous incident flow. Nevertheless, for mountains high enough to preserve a moderate degree of nonlinearity when the synoptic-scale incident flow is strongest, the evolution of cross-mountain pressure drag is no longer symmetric about the time of maximum wind. A higher drag state is found when the cross-mountain flow is accelerating. These results suggest that the local character of the topographically induced disturbance cannot be solely determined by the instantaneous value of the nonlinearity parameter ɛ.

scholarly journals Planetary Scale Selection of the Madden–Julian Oscillation*

2009 ◽  
Vol 66 (8) ◽  
pp. 2429-2443 ◽  
Author(s):  
Tim Li ◽  
Chunhua Zhou
Keyword(s):  
Boundary Layer ◽  
Rossby Wave ◽  
Initial Perturbation ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Scale Selection ◽  
Planetary Scale ◽  
Zonal Wavenumber ◽  
Nonlinear Heating ◽  
Selection Of

Abstract Numerical experiments with a 2.5-layer and a 2-level model are conducted to examine the mechanism for the planetary scale selection of the Madden–Julian oscillation (MJO). The strategy here is to examine the evolution of an initial perturbation that has a form of the equatorial Kelvin wave at zonal wavenumbers of 1 to 15. In the presence of a frictional boundary layer, the most unstable mode prefers a short wavelength under a linear heating; but with a nonlinear heating, the zonal wavenumber 1 grows fastest. This differs significantly from a model without the boundary layer, in which neither linear nor nonlinear heating leads to the long wave selection. Thus, the numerical simulations point out the crucial importance of the combined effect of the nonlinear heating and the frictional boundary layer in the MJO planetary scale selection. The cause of this scale selection under the nonlinear heating is attributed to the distinctive phase speeds between the dry Kelvin wave and the wet Kelvin–Rossby wave couplet. The faster dry Kelvin wave triggered by a convective branch may catch up and suppress another convective branch, which travels ahead of it at the phase speed of the wet Kelvin–Rossby wave couplet if the distance between the two neighboring convective branches is smaller than a critical distance (about 16 000 km). The interference between the dry Kelvin wave and the wet Kelvin–Rossby wave couplet eventually dissipates and “filters out” shorter wavelength perturbations, leading to a longwave selection. The boundary layer plays an important role in destabilizing the MJO through frictional moisture convergences and in retaining the in-phase zonal wind–pressure structure.

scholarly journals Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part I: A Simple Model of North Atlantic Oscillations

2007 ◽  
Vol 64 (1) ◽  
pp. 3-28 ◽  
Author(s):  
Dehai Luo ◽  
Anthony R. Lupo ◽  
Han Wan
Keyword(s):  
Life Cycle ◽  
Theoretical Model ◽  
North Atlantic ◽  
Low Frequency ◽  
Positive Phase ◽  
Negative Phase ◽  
Synoptic Scale ◽  
Planetary Scale ◽  
The North ◽  
The North Atlantic Oscillation

Abstract A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positive-phase NAO favors the occurrence of European blocking events.

scholarly journals A new approach to momentum flux determinations using SKiYMET meteor radars

2005 ◽  
Vol 23 (7) ◽  
pp. 2433-2439 ◽  
Author(s):  
W. K. Hocking
Keyword(s):  
New Mexico ◽  
Momentum Flux ◽  
Middle Atmosphere ◽  
Beam Forming ◽  
New Approach ◽  
Meteorology And Atmospheric Dynamics ◽  
Momentum Fluxes ◽  
Waves And Tides ◽  
Resolute Bay

Abstract. The current primary radar method for determination of atmospheric momentum fluxes relies on multiple beam studies, usually using oppositely directed coplanar beams. Generally VHF and MF radars are used, and meteor radars have never been successfully employed. In this paper we introduce a new procedure that can be used for determination of gravity wave fluxes down to time scales of 2-3h, using the SKiYMET meteor radars. The method avoids the need for beam forming, and allows simultaneous determination of the three components of the wind averaged over the radar volume, as well as the variance and flux components , where refers to the fluctuating eastward wind, refers to the fluctuating northward wind, and refers to the fluctuating vertical wind. Data from radars in New Mexico and Resolute Bay are used to illustrate the data quality, and demonstrate theoretically expected seasonal forcing. Keywords. Meteorology and atmospheric dynamics (Middle atmosphere dynamics; Waves and tides; Climatology)

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