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- Author or Editor: H. L. Tanaka x
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Abstract
In this study, a numerical simulation of the Arctic Oscillation (AO) is conducted using a simple barotropic model that considers the barotropic–baroclinic interactions as the external forcing. The model is referred to as a barotropic S model since the external forcing is obtained statistically from the long-term historical data, solving an inverse problem. The barotropic S model has been integrated for 51 years under a perpetual January condition and the dominant empirical orthogonal function (EOF) modes in the model have been analyzed. The results are compared with the EOF analysis of the barotropic component of the real atmosphere based on the daily NCEP–NCAR reanalysis for 50 yr from 1950 to 1999.
According to the result, the first EOF of the model atmosphere appears to be the AO similar to the observation. The annular structure of the AO and the two centers of action at Pacific and Atlantic are simulated nicely by the barotropic S model. Therefore, the atmospheric low-frequency variabilities have been captured satisfactorily even by the simple barotropic model.
The EOF analysis is further conducted to the external forcing of the barotropic S model. The structure of the dominant forcing shows the characteristics of synoptic-scale disturbances of zonal wavenumber 6 along the Pacific storm track. The forcing is induced by the barotropic–baroclinic interactions associated with baroclinic instability.
The result suggests that the AO can be understood as the natural variability of the barotropic component of the atmosphere induced by the inherent barotropic dynamics, which is forced by the barotropic–baroclinic interactions. The fluctuating upscale energy cascade from planetary waves and synoptic disturbances to the zonal motion plays the key role for the excitation of the AO.
Abstract
In this study, a numerical simulation of the Arctic Oscillation (AO) is conducted using a simple barotropic model that considers the barotropic–baroclinic interactions as the external forcing. The model is referred to as a barotropic S model since the external forcing is obtained statistically from the long-term historical data, solving an inverse problem. The barotropic S model has been integrated for 51 years under a perpetual January condition and the dominant empirical orthogonal function (EOF) modes in the model have been analyzed. The results are compared with the EOF analysis of the barotropic component of the real atmosphere based on the daily NCEP–NCAR reanalysis for 50 yr from 1950 to 1999.
According to the result, the first EOF of the model atmosphere appears to be the AO similar to the observation. The annular structure of the AO and the two centers of action at Pacific and Atlantic are simulated nicely by the barotropic S model. Therefore, the atmospheric low-frequency variabilities have been captured satisfactorily even by the simple barotropic model.
The EOF analysis is further conducted to the external forcing of the barotropic S model. The structure of the dominant forcing shows the characteristics of synoptic-scale disturbances of zonal wavenumber 6 along the Pacific storm track. The forcing is induced by the barotropic–baroclinic interactions associated with baroclinic instability.
The result suggests that the AO can be understood as the natural variability of the barotropic component of the atmosphere induced by the inherent barotropic dynamics, which is forced by the barotropic–baroclinic interactions. The fluctuating upscale energy cascade from planetary waves and synoptic disturbances to the zonal motion plays the key role for the excitation of the AO.
Abstract
In this study, nonlinear numerical simulations of amplification of low-frequency planetary waves and concurrent blocking formations were performed. The simulations are conducted by a barotropic spectral model derived from three-dimensional spectral primitive equations with a basis of vertical structure functions and Hough harmonics. The model is truncated to include only barotropic Rossby components of the atmosphere with simple physics of biharmonic diffusion, topographic forcing, baroclinic instability, and zonal surface stress. These four physical processes are found to be sufficient to produce a realistic and persistent dipole blocking with a sharp transition from zonal to meridional flows on a sphere.
Analyzing energetics of blocking formations in the model, we showed an amplification of the meridional dipole mode was confirmed by means of the upscale energy cascade from synoptic disturbances under an environment of persistently amplified wavenumber 2. When the persistent wavenumber 2 exists, synoptic disturbances contribute to amplify the dipole mode of wavenumber 1. In contrast, when the persistent wavenumber 2 is absent, synoptic disturbances contribute to accelerate zonal flow with enhanced wave-mean flow interactions, and wavenumber 1 is not amplified. Therefore, we find that the persistent wavenumber 2 plays a catalytic role in drawing synoptic wave energy and feeding wavenumber 1. The topographic forcing in amplifying wavenumber 2 appears to be necessary for the blocking system in the model, although it is not the main energy source for the system.
Abstract
In this study, nonlinear numerical simulations of amplification of low-frequency planetary waves and concurrent blocking formations were performed. The simulations are conducted by a barotropic spectral model derived from three-dimensional spectral primitive equations with a basis of vertical structure functions and Hough harmonics. The model is truncated to include only barotropic Rossby components of the atmosphere with simple physics of biharmonic diffusion, topographic forcing, baroclinic instability, and zonal surface stress. These four physical processes are found to be sufficient to produce a realistic and persistent dipole blocking with a sharp transition from zonal to meridional flows on a sphere.
Analyzing energetics of blocking formations in the model, we showed an amplification of the meridional dipole mode was confirmed by means of the upscale energy cascade from synoptic disturbances under an environment of persistently amplified wavenumber 2. When the persistent wavenumber 2 exists, synoptic disturbances contribute to amplify the dipole mode of wavenumber 1. In contrast, when the persistent wavenumber 2 is absent, synoptic disturbances contribute to accelerate zonal flow with enhanced wave-mean flow interactions, and wavenumber 1 is not amplified. Therefore, we find that the persistent wavenumber 2 plays a catalytic role in drawing synoptic wave energy and feeding wavenumber 1. The topographic forcing in amplifying wavenumber 2 appears to be necessary for the blocking system in the model, although it is not the main energy source for the system.
Abstract
In this study, baroclinic instability of the northern winter atmosphere is investigated in the context of the dynamical interpretation of the Arctic oscillation. The unstable solutions, obtained by a method of 3D normal mode expansion, are compared for observed zonal basic states with strong and weak polar vortices in reference to the Arctic oscillation index.
As a result of the eigenvalue problem of the linear stability analysis, a characteristic unstable solution is obtained that dominates in high latitudes when the polar vortex is strong. The mode is called a monopole Charney mode M 1, which is similar to an ordinary Charney mode M C in midlatitudes. In order to understand the origin of the M 1 mode, a hypothetical zonal basic state that has only the polar jet with no subtropical jet is analyzed. It is found that the M 1 mode in high latitudes is excited by the baroclinicity associated with the polar vortex. The M 1 mode in high latitudes is dynamically the same Charney mode as M C but is excited by the baroclinicity of the polar jet instead of the subtropical jet.
As the M C mode intensifies the subtropical jet by the eddy momentum transfer, the M 1 mode transfers eddy momentum to high latitudes to intensify the polar jet. Since M 1 mode appears during the strong polar jet and feeds the westerly momentum to the polar jet, there is a positive feedback between the M 1 mode and the polar vortex. This positive feedback would produce a persistent strong polar jet, which may in tern result in the occurrence of the annular mode of the Arctic oscillation.
Abstract
In this study, baroclinic instability of the northern winter atmosphere is investigated in the context of the dynamical interpretation of the Arctic oscillation. The unstable solutions, obtained by a method of 3D normal mode expansion, are compared for observed zonal basic states with strong and weak polar vortices in reference to the Arctic oscillation index.
As a result of the eigenvalue problem of the linear stability analysis, a characteristic unstable solution is obtained that dominates in high latitudes when the polar vortex is strong. The mode is called a monopole Charney mode M 1, which is similar to an ordinary Charney mode M C in midlatitudes. In order to understand the origin of the M 1 mode, a hypothetical zonal basic state that has only the polar jet with no subtropical jet is analyzed. It is found that the M 1 mode in high latitudes is excited by the baroclinicity associated with the polar vortex. The M 1 mode in high latitudes is dynamically the same Charney mode as M C but is excited by the baroclinicity of the polar jet instead of the subtropical jet.
As the M C mode intensifies the subtropical jet by the eddy momentum transfer, the M 1 mode transfers eddy momentum to high latitudes to intensify the polar jet. Since M 1 mode appears during the strong polar jet and feeds the westerly momentum to the polar jet, there is a positive feedback between the M 1 mode and the polar vortex. This positive feedback would produce a persistent strong polar jet, which may in tern result in the occurrence of the annular mode of the Arctic oscillation.
Abstract
Three-dimensional normal mode functions are applied to the analysis of the energetics of the general circulation during the FGGE year. The GFDL FGGE data are used for the computation of both the normal mode energetics and the standard spectral energetics.
The normal mode energetics of the global circulation are presented in a barotropic and baroclinic decomposition for the zonal mean and eddy energies for the stationary and transient components of the flow. The energy generated in the zonal mean baroclinic component is first transformed to the eddy baroclinic component through the process of atmospheric baroclinic instability. It is then further transformed to eddy and zonal mean barotropic components by the nonlinear up-scale cascade of kinetic energy. The zonal mean kinetic energy thus maintains its barotropic structure by the activities of baroclinic waves. The time series of energy variables during the FGGE Northern Hemisphere winter clearly indicates a sequence of energy transformations from the zonal baroclinic component via the synoptic-scale baroclinic component, to the planetary-scale barotropic component.
Comparison of the normal mode energetics with the standard spectral energetics in the zonal wavenumber domain indicates a general consistency of both schemes in the spectral energy transformations.
Abstract
Three-dimensional normal mode functions are applied to the analysis of the energetics of the general circulation during the FGGE year. The GFDL FGGE data are used for the computation of both the normal mode energetics and the standard spectral energetics.
The normal mode energetics of the global circulation are presented in a barotropic and baroclinic decomposition for the zonal mean and eddy energies for the stationary and transient components of the flow. The energy generated in the zonal mean baroclinic component is first transformed to the eddy baroclinic component through the process of atmospheric baroclinic instability. It is then further transformed to eddy and zonal mean barotropic components by the nonlinear up-scale cascade of kinetic energy. The zonal mean kinetic energy thus maintains its barotropic structure by the activities of baroclinic waves. The time series of energy variables during the FGGE Northern Hemisphere winter clearly indicates a sequence of energy transformations from the zonal baroclinic component via the synoptic-scale baroclinic component, to the planetary-scale barotropic component.
Comparison of the normal mode energetics with the standard spectral energetics in the zonal wavenumber domain indicates a general consistency of both schemes in the spectral energy transformations.
Abstract
Observed atmospheric energy peaks in a three-dimensional (3-D) spectral domain are compared with energy peaks predicted by the theory of atmospheric baroclinic instability. The 3-D scale index for global-scale atmospheric motions is represented by the eigenfrequencies of 3-D normal mode functions on a sphere, based on the fact that the eigenfrequencies of Rossby modes are related to the 3-D scale of the waves through the intrinsic wave dispersion relation.
When the observed atmospheric energy level is expressed as a function of the eigenfrequencies, a distinct spectral peak appears in the intermediate value of the eigenfrequencies of Rossby modes. The energy spectrum of atmospheric barotropic components clearly separates a −5/3 power law in the high-frequency range, relative to the energy peak, and a 3 power law in the low-frequency range. The peak may describe a certain energy source for large-scale atmospheric motions. For zonal wavenumber 6, we find that the observed spectral peak coincides with the peak predicted from atmospheric baroclinic instability; the energy peak can be produced by baroclinic instability. For zonal wavenumber 2, we also find that the observed special peak coincides with that predicted from low-frequency baroclinic instability on a sphere. The results suggest that the low-frequency unstable modes of zonal wavenumber 2 contribute a substantial fraction of the observed spectral peak in a manner similar to zonal wavenumber 6.
Abstract
Observed atmospheric energy peaks in a three-dimensional (3-D) spectral domain are compared with energy peaks predicted by the theory of atmospheric baroclinic instability. The 3-D scale index for global-scale atmospheric motions is represented by the eigenfrequencies of 3-D normal mode functions on a sphere, based on the fact that the eigenfrequencies of Rossby modes are related to the 3-D scale of the waves through the intrinsic wave dispersion relation.
When the observed atmospheric energy level is expressed as a function of the eigenfrequencies, a distinct spectral peak appears in the intermediate value of the eigenfrequencies of Rossby modes. The energy spectrum of atmospheric barotropic components clearly separates a −5/3 power law in the high-frequency range, relative to the energy peak, and a 3 power law in the low-frequency range. The peak may describe a certain energy source for large-scale atmospheric motions. For zonal wavenumber 6, we find that the observed spectral peak coincides with the peak predicted from atmospheric baroclinic instability; the energy peak can be produced by baroclinic instability. For zonal wavenumber 2, we also find that the observed special peak coincides with that predicted from low-frequency baroclinic instability on a sphere. The results suggest that the low-frequency unstable modes of zonal wavenumber 2 contribute a substantial fraction of the observed spectral peak in a manner similar to zonal wavenumber 6.
Abstract
As an alternative to the finite difference method, we explore the use of the spectral method with normalmodes as the basis functions for discretizing dependent variables in the vertical direction in order to obtainnumerical solutions to time dependent atmospheric equations. The normal modes are free solutions to the timedependent perturbation equations linearized around the atmosphere at rest. To demonstrate the feasibility ofnormal mode representation in the spectral vertical discretization, the vertical normal mode expansion is appliedto the quasi-geostrophic potential vorticity equation to investigate the traditional baroclinic instability of Charneyand Green types on a zonal flow with a constant vertical shear. The convergence of the numerical solutions isexamined in detail in relation to the spectral resolution of expansion functions.We then extend the method of vertical normal mode expansion to solve the problem of baroclinic instabilityon the sphere. Two aspects are different from the earlier example. One is use of the primitive equations insteadof the quasi-geostrophic system and the other is application of normal mode expansions in the horizontal, aswell as vertical direction. First, we derive the evolution equations for the spectral coefficients of truncated seriesin three-dimensional normal mode functions by application of the Galerkin procedure to the global primitiveequations linearized around a basic zonal flow with vertical and meridional shear. Then, an eigenvalue-eigenfunction problem is solved to investigate the stability of perturbation motions superimposed on the 30' jetexamined earlier by Simmons, Hoskins and Frederiksen. From these two examples, it is concluded that thenormal mode spectral method is a viable numerical technique for discretizing model variables in the vertical.
Abstract
As an alternative to the finite difference method, we explore the use of the spectral method with normalmodes as the basis functions for discretizing dependent variables in the vertical direction in order to obtainnumerical solutions to time dependent atmospheric equations. The normal modes are free solutions to the timedependent perturbation equations linearized around the atmosphere at rest. To demonstrate the feasibility ofnormal mode representation in the spectral vertical discretization, the vertical normal mode expansion is appliedto the quasi-geostrophic potential vorticity equation to investigate the traditional baroclinic instability of Charneyand Green types on a zonal flow with a constant vertical shear. The convergence of the numerical solutions isexamined in detail in relation to the spectral resolution of expansion functions.We then extend the method of vertical normal mode expansion to solve the problem of baroclinic instabilityon the sphere. Two aspects are different from the earlier example. One is use of the primitive equations insteadof the quasi-geostrophic system and the other is application of normal mode expansions in the horizontal, aswell as vertical direction. First, we derive the evolution equations for the spectral coefficients of truncated seriesin three-dimensional normal mode functions by application of the Galerkin procedure to the global primitiveequations linearized around a basic zonal flow with vertical and meridional shear. Then, an eigenvalue-eigenfunction problem is solved to investigate the stability of perturbation motions superimposed on the 30' jetexamined earlier by Simmons, Hoskins and Frederiksen. From these two examples, it is concluded that thenormal mode spectral method is a viable numerical technique for discretizing model variables in the vertical.
Abstract
In this study, we carried out a quantitative heat budget analysis of the polar troposphere in and around Alaska for the winter of 1988/89. The winter was characterized by drastic temperature variations. The surface minimum temperatures in the interior of Alaska were lower than −40°C during two weeks in late January. This cold period was followed by extremely warm weather during February, especially in northern Alaska.
The results of the heat budget analysis for the three-month average (December to February) show that the heat energy of the continental air mass in and around Alaska is maintained by a balance of warm advection, adiabatic warming, and radiative cooling. The analysis of the long-term variation during this period shows that the severe cold in late January was caused by an anomalous reduction in warm advection of sensible heat. The abnormally warm weather in February was caused by enhanced warm advection associated with a blocking formation over the North Pacific. We find that strong adiabatic warming due to large-scale descending motions occurs as a precursor to the blocking formation and the warm advection. It is suggested that the unusual temperature variation during this winter was caused by the enhanced, low-frequency variation of the 1arge-scale flow. The local radiative cooling acts to reduce the rapid temperature variation due to advection change.
Abstract
In this study, we carried out a quantitative heat budget analysis of the polar troposphere in and around Alaska for the winter of 1988/89. The winter was characterized by drastic temperature variations. The surface minimum temperatures in the interior of Alaska were lower than −40°C during two weeks in late January. This cold period was followed by extremely warm weather during February, especially in northern Alaska.
The results of the heat budget analysis for the three-month average (December to February) show that the heat energy of the continental air mass in and around Alaska is maintained by a balance of warm advection, adiabatic warming, and radiative cooling. The analysis of the long-term variation during this period shows that the severe cold in late January was caused by an anomalous reduction in warm advection of sensible heat. The abnormally warm weather in February was caused by enhanced warm advection associated with a blocking formation over the North Pacific. We find that strong adiabatic warming due to large-scale descending motions occurs as a precursor to the blocking formation and the warm advection. It is suggested that the unusual temperature variation during this winter was caused by the enhanced, low-frequency variation of the 1arge-scale flow. The local radiative cooling acts to reduce the rapid temperature variation due to advection change.
Abstract
Atmospheric blocking occurred over the Rocky Mountains at 1200 UTC 15 December 2005. The operational medium-range ensemble forecasts of the Canadian Meteorological Center (CMC), the Japan Meteorological Agency (JMA), and the National Centers for Environmental Prediction (NCEP), as initialized at 1200 UTC 10 December 2005, showed remarkable differences regarding this event. All of the NCEP members failed to predict the correct location of the blocking, whereas almost all of the JMA members and most of the CMC members were successful in predicting the correct location. The present study investigated the factors that caused NCEP to incorrectly predict the blocking location, based on an ensemble-based sensitivity analysis and the JMA global spectral model (GSM) multianalysis ensemble forecasts with NCEP, regionally amplified NCEP, and globally amplified NCEP analyses.
A sensitive area for the blocking formation was detected over the central North Pacific. In this area, the NCEP control analysis experienced problems in the handling of a cutoff cyclone, and the NCEP initial perturbations were ineffective in reducing uncertainties in the NCEP control analysis. The JMA GSM multianalysis ensemble forecasts revealed that regional amplification of initial perturbations over the sensitive area could lead to further improvements in forecasts over the blocking region without degradation of forecasts over the Northern Hemisphere (NH), whereas the global amplification of initial perturbations could lead to improved forecasts over the blocking region and degraded forecasts over the NH. This finding may suggest that excessive amplification of initial perturbations over nonsensitive areas is undesirable, and that case-dependent rescaling of initial perturbations may be of value compared with climatology-based rescaling, which is widely used in current operational ensemble prediction systems.
Abstract
Atmospheric blocking occurred over the Rocky Mountains at 1200 UTC 15 December 2005. The operational medium-range ensemble forecasts of the Canadian Meteorological Center (CMC), the Japan Meteorological Agency (JMA), and the National Centers for Environmental Prediction (NCEP), as initialized at 1200 UTC 10 December 2005, showed remarkable differences regarding this event. All of the NCEP members failed to predict the correct location of the blocking, whereas almost all of the JMA members and most of the CMC members were successful in predicting the correct location. The present study investigated the factors that caused NCEP to incorrectly predict the blocking location, based on an ensemble-based sensitivity analysis and the JMA global spectral model (GSM) multianalysis ensemble forecasts with NCEP, regionally amplified NCEP, and globally amplified NCEP analyses.
A sensitive area for the blocking formation was detected over the central North Pacific. In this area, the NCEP control analysis experienced problems in the handling of a cutoff cyclone, and the NCEP initial perturbations were ineffective in reducing uncertainties in the NCEP control analysis. The JMA GSM multianalysis ensemble forecasts revealed that regional amplification of initial perturbations over the sensitive area could lead to further improvements in forecasts over the blocking region without degradation of forecasts over the Northern Hemisphere (NH), whereas the global amplification of initial perturbations could lead to improved forecasts over the blocking region and degraded forecasts over the NH. This finding may suggest that excessive amplification of initial perturbations over nonsensitive areas is undesirable, and that case-dependent rescaling of initial perturbations may be of value compared with climatology-based rescaling, which is widely used in current operational ensemble prediction systems.
Synoptic images of the global cloud field have been created from infrared measurements taken aboard four geostationary and two polar-orbiting platforms simultaneously observing the earth. A series of spatial and temporal interpolations together with data reliability criteria are used to composite data from the individual satellites into synoptic images of the global cloud pattern. The composite Global Cloud Imagery (GCI) have a horizontal resolution of about half a degree and a temporal resolution of 3 h, providing an unprecedented view of the earth's cloud field. Each composite image represents a nearly instantaneous snapshot of the global cloud pattern. Collectively, the composite imagery resolve, on a global basis, most of the variability associated with organized convection, including several harmonics of the diurnal cycle.
The dense and 3-dimensional nature of the GCI make them a formidable volume of information to treat in a practical and efficient manner. To facilitate analysis of global cloud behavior, the GCI has been constructed with certain homogeneous properties. In addition to synoptic coverage of the globe, data are spaced uniformly in longitude, latitude, and time, and contain no data voids. An interactive Image Analysis System (IAS) has been developed to investigate the space-time behavior of global cloud activity. In the IAS, data, hardware, and software are integrated into a single system capable of providing a variety of space-time covariance analyses. Because of its customized architecture and the homogeneous properties of the GCI, the IAS can perform such analyses on the 3-dimensional data with interactive speed. Statistical properties of cloud variability are presented along with other preliminary results derived from the GCI.
Synoptic images of the global cloud field have been created from infrared measurements taken aboard four geostationary and two polar-orbiting platforms simultaneously observing the earth. A series of spatial and temporal interpolations together with data reliability criteria are used to composite data from the individual satellites into synoptic images of the global cloud pattern. The composite Global Cloud Imagery (GCI) have a horizontal resolution of about half a degree and a temporal resolution of 3 h, providing an unprecedented view of the earth's cloud field. Each composite image represents a nearly instantaneous snapshot of the global cloud pattern. Collectively, the composite imagery resolve, on a global basis, most of the variability associated with organized convection, including several harmonics of the diurnal cycle.
The dense and 3-dimensional nature of the GCI make them a formidable volume of information to treat in a practical and efficient manner. To facilitate analysis of global cloud behavior, the GCI has been constructed with certain homogeneous properties. In addition to synoptic coverage of the globe, data are spaced uniformly in longitude, latitude, and time, and contain no data voids. An interactive Image Analysis System (IAS) has been developed to investigate the space-time behavior of global cloud activity. In the IAS, data, hardware, and software are integrated into a single system capable of providing a variety of space-time covariance analyses. Because of its customized architecture and the homogeneous properties of the GCI, the IAS can perform such analyses on the 3-dimensional data with interactive speed. Statistical properties of cloud variability are presented along with other preliminary results derived from the GCI.
Abstract
Four numerical simulations of the global atmosphere for January 1979 are analyzed to study the formation of blocking in terms of Northern Hemisphere energetics. The Goddard Laboratory for Atmospheres (GLA) 4° × 5° latitude-longitude grid general circulation model (GCM) and 2° × 2.5° grid GCM are employed with the GLA and Geophysical Fluid Dynamics Laboratory (GFDL) initial datasets.
The difficulty in simulating a realistic blocking due to inadequate wave–wave interaction can be attributed in part to inadequate grid resolution. Among four simulations, the simulations by the high resolution GCM produce realistically strong blockings with compatible spectral energetics as in the observed blocking episodes. The latitude–height cross sections of the energy variables of wavenumber 1 is presented to describe the dipole structure of blockings. Blocking development is also examined in time series of barotropic and baroclinic components of energy and associated conversions.
Abstract
Four numerical simulations of the global atmosphere for January 1979 are analyzed to study the formation of blocking in terms of Northern Hemisphere energetics. The Goddard Laboratory for Atmospheres (GLA) 4° × 5° latitude-longitude grid general circulation model (GCM) and 2° × 2.5° grid GCM are employed with the GLA and Geophysical Fluid Dynamics Laboratory (GFDL) initial datasets.
The difficulty in simulating a realistic blocking due to inadequate wave–wave interaction can be attributed in part to inadequate grid resolution. Among four simulations, the simulations by the high resolution GCM produce realistically strong blockings with compatible spectral energetics as in the observed blocking episodes. The latitude–height cross sections of the energy variables of wavenumber 1 is presented to describe the dipole structure of blockings. Blocking development is also examined in time series of barotropic and baroclinic components of energy and associated conversions.
