326 results on '"Held, Isaac M."'
Search Results
2. The Sensitivity of Superrotation to the Latitude of Baroclinic Forcing in a Terrestrial Dry Dynamical Core
- Author
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Zurita-Gotor, Pablo, primary, Anaya-Benlliure, Álvaro, additional, and Held, Isaac M., additional
- Published
- 2022
- Full Text
- View/download PDF
3. Planetary Waves and Their Interaction with Smaller Scales
- Author
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Held, Isaac M., Shapiro, Melvyn A., editor, and Grønås, Sigbjørn, editor
- Published
- 1999
- Full Text
- View/download PDF
4. Nonlinear generation of long waves and the reversal of eddy momentum fluxes in a two-layer quasi-geostrophic model
- Author
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Hsieh, Tsung-Lin, primary, Chang, Chiung-Yin, additional, Held, Isaac M., additional, and Zurita-Gotor, Pablo, additional
- Published
- 2021
- Full Text
- View/download PDF
5. Westward-Propagating Rossby Modes in Idealized GCMs
- Author
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Zurita-Gotor, Pablo, primary and Held, Isaac M., additional
- Published
- 2021
- Full Text
- View/download PDF
6. The tropical response to extratropical thermal forcing in an idealized GCM: the importance of radiative feedbacks and convective parameterization
- Author
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Kang, Sarah M., Frierson, Dargan M.W., and Held, Isaac M.
- Subjects
Geophysics -- Environmental aspects ,Atmospheric physics -- Research ,Precipitation (Meteorology) -- Environmental aspects ,Earth sciences ,Science and technology - Abstract
The response of tropical precipitation to extratropical thermal forcing is reexamined using an idealized moist atmospheric GCM that has no water vapor or cloud feedbacks, simplifying the analysis while retaining the aquaplanet configuration coupled to a slab ocean from the authors' previous study. As in earlier studies, tropical precipitation in response to high-latitude forcing is skewed toward the warmed hemisphere. Comparisons with a comprehensive GCM in an identical aquaplanet, mixed-layer framework reveal that the tropical responses tend to be much larger in the comprehensive GCM as a result of positive cloud and water vapor feedbacks that amplify the imposed extratropical thermal forcing. The magnitude of the tropical precipitation response in the idealized model is sensitive to convection scheme parameters. This sensitivity as well as the tropical precipitation response can be understood from a simple theory with two ingredients: the changes in poleward energy fluxes are predicted using a one-dimensional energy balance model and a measure of the 'total gross moist stability' [[DELTA]m, which is defined as the total (mean plus eddy) atmospheric energy transport per unit mass transport] of the model tropics converts the energy flux change into a mass flux and a moisture flux change. The idealized model produces a low level of compensation of about 25% between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics regardless of the convection scheme parameter. Because Geophysical Fluid Dynamics Laboratory Atmospheric Model 2 (AM2) with prescribed clouds and water vapor exhibits a similarly low level of compensation, it is argued that roughly 25% of the compensation is dynamically controlled through eddy energy fluxes. The sensitivity of the tropical response to the convection scheme in the idealized model results from different values of [DELTA]m: smaller [DELTA]m leads to larger tropical precipitation changes for the same response in the energy transport.
- Published
- 2009
7. Horizontally homogeneous rotating radiative-convective equilibria at GCM resolution
- Author
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Held, Isaac M. and Zhao, Ming
- Subjects
Convection (Meteorology) -- Observations ,Dynamic meteorology -- Research ,Cloud physics -- Research ,Tropical cyclones -- Observations ,Clouds -- Dynamics ,Clouds -- Research ,Earth sciences ,Science and technology - Abstract
Rotating radiative-convective equilibrium, using the column physics and resolution of GCMs, is proposed as a useful framework for studying the tropical storm-like vortices produced by global models. These equilibria are illustrated using the column physics and dynamics of a version of the GFDL Atmospheric Model 2 (AM2) at resolutions of 220, 110, and 55 km in a large 2 x [10.sup.4] km square horizontally homogeneous domain with fixed sea surface temperature and uniform Coriolis parameter. The large domain allows a number of tropical storms to exist simultaneously. Once equilibrium is attained, storms often persist for hundreds of days. The number of storms decreases as sea surface temperatures increase, while the average intensity increases. As the background rotation is decreased, the number of storms also decreases. At these resolutions and with this parameterization of convection, a dense collection of tropical storms is always the end state of moist convection in the cases examined.
- Published
- 2008
8. Simulation of the recent multidecadal increase of Atlantic hurricane activity using an 18-km-grid regional model
- Author
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Knutson, Thomas R., Sirutis, Joseph J., Garner, Stephen T., Held, Isaac M., and Tuleya, Robert E.
- Subjects
Atlantic Ocean -- Environmental aspects ,Atlantic Ocean -- Models ,Tropical cyclones -- Models ,Simulation methods -- Usage ,Meteorology, Maritime -- Research ,Ocean-atmosphere interaction -- Models ,Business ,Earth sciences - Abstract
In this study, a new modeling framework for simulating Atlantic hurricane activity is introduced. The model is an 18-km-grid nonhydrostatic regional model, run over observed specified SSTs and nudged toward observed time-varying large-scale atmospheric conditions (Atlantic domain wavenumbers 0-2) derived from the National Centers for Environmental Prediction (NCEP) reanalyses. Using this 'perfect large-scale model' approach for 27 recent August-October seasons (1980-2006), it is found that the model successfully reproduces the observed multidecadal increase in numbers of Atlantic hurricanes and several other tropical cyclone (TC) indices over this period. The correlation of simulated versus observed hurricane activity by year varies from 0.87 for basin-wide hurricane counts to 0.41 for U.S. landfalling hurricanes. For tropical storm count, accumulated cyclone energy, and TC power dissipation indices the correlation is ~0.75, for major hurricanes the correlation is 0.69, and for U.S. landfalling tropical storms, the correlation is 0.57. The model occasionally simulates hurricanes intensities of up to category 4 (~942 mb) in terms of central pressure, although the surface winds (< 47 m [s.sup-1]) do not exceed category-2 intensity. On interannual time scales, the model reproduces the observed ENSO-Atlantic hurricane covariation reasonably well. Some notable aspects of the highly contrasting 2005 and 2006 seasons are well reproduced, although the simulated activity during the 2006 core season was excessive. The authors conclude that the model appears to be a useful tool for exploring mechanisms of hurricane variability in the Atlantic (e.g., shear versus potential intensity contributions). The model may be capable of making useful simulations/projections of pre-1980 or twentieth-century Atlantic hurricane activity. However, the reliability of these projections will depend on obtaining reliable large-scale atmospheric and SST conditions from sources external to the model.
- Published
- 2007
9. Sensitivity of the latitude of the surface Westerlies to surface friction
- Author
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Chen, Gang, Held, Isaac M., and Robinson, Walter A.
- Subjects
Air jets -- Speed ,Friction -- Observations ,Eddies -- Observations ,Atmospheric circulation -- Research ,Earth sciences ,Science and technology - Abstract
The sensitivity to surface friction of the latitude of the surface westerlies and the associated eddy-driven midlatitude jet is studied in an idealized dry GCM. The westerlies move poleward as the fiction is reduced in strength. An increase in the eastward phase speed of midlatitude eddies is implicated as playing a central role in this shift. This shift in latitude is mainly determined by changes in the friction on the zonal mean flow rather than the friction on the eddies. If the friction on the zonal mean is reduced instantaneously, the response reveals two distinctive adjustment time scales. In the fast adjustment over the first 10-20 days, there is an increase in the barotropic component of zonal winds and a substantial decrease in the eddy kinetic energy; the shift in the surface westerlies and jet latitude occurs in a slower adjustment. The space time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies. The view is supported by simulating the upper-tropospheric dynamics in a stochastically stirred nonlinear shallow water model.
- Published
- 2007
10. A gray-radiation aquaplanet moist GCM. Part II: energy transports in altered climates
- Author
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Frierson, Dargan M.W., Held, Isaac M., and Zurita-Gotor, Pablo
- Subjects
Atmospheric electricity -- Research ,Energy transfer (Ecology) -- Research ,Atmospheric radiation -- Research ,Earth sciences ,Science and technology - Abstract
A simplified moist general circulation model is used to study changes in the meridional transport of moist static energy by the atmosphere as the water vapor content is increased. The key assumptions of the model are gray radiation, with water vapor and other constituents having no effect on radiative transfer, and mixed layer aquaplanet boundary conditions, implying that the atmospheric meridional energy transport balances the net radiation at the top of the atmosphere. These simplifications allow the authors to isolate the effect of moisture on energy transports by baroclinic eddies in a relatively simple setting. The authors investigate the partition of moist static energy transport in the model into dry static energy and latent energy transports as water vapor concentrations are increased, by varying a constant in the Clausius--Clapeyron relation. The increase in the poleward moisture flux is rather precisely compensated by a reduction in the dry static energy flux. These results are interpreted with diffusive energy balance models (EBMs). The simplest of these is an analytic model that has the property of exact invariance of total energy flux as the moisture content is changed, but the assumptions underlying this model are not accurately satisfied by the GCM. A more complex EBM that includes expressions for the diffusivity, length scale, velocity scale, and latitude of maximum baroclinic eddy activity provides a better fit to the GCM's behavior.
- Published
- 2007
11. Mechanisms of the Great Plains low-level jet as simulated in an AGCM
- Author
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Jiang, Xianan, Lau, Ngar-Cheung, Held, Isaac M., and Ploshay, Jeffrey J.
- Subjects
North America -- Environmental aspects ,Atmosphere -- Models ,Latitude -- Models ,Earth -- Atmosphere ,Earth -- Models ,Earth sciences ,Science and technology - Abstract
A model diagnosis has been performed on the nocturnal Great Plains low-level jet (LLJ), which is one of the key elements of the warm season regional climate over North America. The horizontal-vertical structure, diurnal phase, and amplitude of the LLJ are well simulated by an atmospheric general circulation model (AGCM), thus justifying a reevaluation of the physical mechanisms for the formation of the LLJ based on output from this model. A diagnosis of the AGCM data confirms that two planetary boundary layer (PBL) processes, the diurnal oscillation of the pressure gradient force and of vertical diffusion, are of comparable importance in regulating the inertial oscillation of the winds, which leads to the occurrence of maximum LLJ strength during nighttime. These two processes are highlighted in the theories for the LLJ proposed by Holton (1967) and Blackadar (1957). A simple model is constructed in order to study the relative roles of these two mechanisms. This model incorporates the diurnal variation of the pressure gradient force and vertical diffusion coefficients as obtained from the AGCM simulation. The results reveal that the observed diurnal phase and amplitude of the LLJ can be attributed to the combination of these two mechanisms. The LLJ generated by either Holton's or Blackadar's mechanism alone is characterized by an unrealistic meridional phase shift and weaker amplitude. It is also shown that the diurnal phase of the LLJ exhibits vertical variations in the PBL. more clearly at higher latitudes, with the upper PBL wind attaining a southerly peak several hours earlier than the lower PBL. The simple model demonstrates that this phase tilt is due mainly to sequential triggering of the inertial oscillation from upper to lower PBL when surface cooling commences after sunset. At lower latitudes, due to the change of orientation of prevailing mean wind vectors and the longer inertial period, the inertial oscillation in the lower PBL tends to be interrupted by strong vertical mixing in the following day, whereas in the upper PBL, the inertial oscillation can proceed in a low-friction environment for a relatively longer duration. Thus, the vertical phase tilt initiated at sunset is less evident at lower latitudes.
- Published
- 2007
12. Dynamic radiative--convective equilibria using GCM column physics
- Author
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Held, Isaac M., Zhao, Ming, and Wyman, Bruce
- Subjects
Geophysical prediction -- Models ,Geophysical prediction -- Research ,Atmospheric circulation -- Models ,Atmospheric circulation -- Research ,Earth sciences ,Science and technology - Abstract
The behavior of a GCM column physics package in a nonrotating, doubly periodic, homogeneous setting with prescribed SSTs is examined. This radiative--convective framework is proposed as a useful tool for studying some of the interactions between convection and larger-scale dynamics and the effects of differing modeling assumptions on convective organization and cloud feedbacks. For the column physics utilized here, from the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 model, many of the properties of the homogeneous, nonrotating model are closely tied to the fraction of precipitation that is large-scale, rather than convective. Significant large-scale precipitation appears above a critical temperature and then increases with further increases in temperature. The amount of large-scale precipitation is a function of horizontal resolution and can also be controlled by modifying the convection scheme, as is illustrated here by modifying assumptions concerning entrainment into convective plumes. Significant similarities are found between the behavior of the homogeneous model and that of the Tropics of the parent GCM when ocean temperatures are increased and when the convection scheme is modified.
- Published
- 2007
13. A gray-radiation aquaplanet moist GCM. Part I: static stability and eddy scale
- Author
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Frierson, Dargan M.W., Held, Isaac M., and Zurita-Gotor, Pablo
- Subjects
Atmospheric circulation -- Observations ,Earth sciences ,Science and technology - Abstract
In this paper, a simplified moist general circulation model is developed and used to study changes in the atmospheric general circulation as the water vapor content of the atmosphere is altered, The key elements of the model physics are gray radiative transfer, in which water vapor and other constituents have no effect on radiative fluxes, a simple diffusive boundary layer with prognostic depth, and a mixed layer aquaplanet surface boundary condition. This GCM can be integrated stably without a convection parameterization, with large-scale condensation only, and this study focuses on this simplest version of the model. These simplifications provide a useful framework in which to focus on the interplay between latent heat release and large-scale dynamics. In this paper, the authors study the role of moisture in determining the tropospheric static stability and midlalitude eddy scale. In a companion paper, the effects of moisture on energy transports by baroclinic eddies are discussed. The authors vary a parameter in the Clausius--Clapeyron relation to control the amount of water in the atmosphere, and consider circulations ranging from the dry limit to 10 times a control value. The typical length scale of midlatitnde eddies is found to be remarkably insensitive to the amount of moisture in the atmosphere in this model. The Rhines scale evaluated at the latitude of the maximum eddy kinetic energy fits the model results for the eddy scale well. Moist convection is important in determining the extratropical lapse rate, and the dry stability is significantly increased with increased moisture content.
- Published
- 2006
14. The Parameter Dependence of Eddy Heat Flux in a Homogeneous Quasigeostrophic Two-Layer Model on a β Plane with Quadratic Friction
- Author
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Chang, Chiung-Yin, primary and Held, Isaac M., additional
- Published
- 2021
- Full Text
- View/download PDF
15. the gap between simulation and understanding in climate modeling
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Held, Isaac M.
- Subjects
Climate -- Research ,Simulation methods -- Analysis ,Business ,Earth sciences - Abstract
The problem of creating truly convincing numerical simulations of our Earth's climate will remain a challenge for the next generation of climate scientists. Hopefully, the ever increasing power of computers will make this task somewhat less frustrating than it is at present. But, increasing computational power also raises issues as to how we would like to see climate modeling and the study of climate dynamics evolve in the twenty-first century. One of the key issues we will need to address is the widening gap between simulation and understanding.
- Published
- 2005
16. Abrupt transition to strong superrotation in an axisymmetric model of the upper troposphere
- Author
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Shell, Karen M. and Held, Isaac M.
- Subjects
Atmosphere -- Research ,Earth sciences ,Science and technology - Abstract
Abrupt transitions to strongly superrotating states have been found in some idealized models of the troposphere. These transitions are thought to be caused by feedbacks between the eddy momentum flux convergence in low latitudes and the strength of the equatorial flow. The behavior of an axisymmetric shallow-water model with an applied tropical torque is studied here to determine if an abrupt transition can be realized without eddy feedbacks. The upper-tropospheric layer is relaxed to a radiative equilibrium thickness, exchanging mass and thus momentum with the nonmoving lower layer. For low values of the applied torque, the circulation is earthlike; however, for larger values, an abrupt transition to a strongly superrotating state can occur. In some cases, the system remains superrotating as the torque is subsequently decreased. A simple analytical model is used to better understand the system. The bifurcation is caused by a feedback between the applied torque and the strength of the Hadley cell. As the torque increases, the strength of the cell decreases, reducing the damping caused by momentum transfer from the lower layer.
- Published
- 2004
17. Boundary effects in potential vorticity dynamics
- Author
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Schneider, Tapio, Held, Isaac M., and Garner, Stephen T.
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Planetary boundary layer -- Environmental aspects ,Vortex-motion -- Environmental aspects ,Earth sciences ,Science and technology - Abstract
Many aspects of geophysical flows can be described compactly in terms of potential vorticity dynamics. Since potential temperature can fluctuate at boundaries, however, the boundary conditions for potential vorticity dynamics are inhomogeneous, which complicates considerations of potential vorticity dynamics when boundary effects are dynamically significant. A formulation of potential vorticity dynamics is presented that encompasses boundary effects. It is shown that, for arbitrary flows, the generalization of the potential vorticity concept to a sum of the conventional interior potential vorticity and a singular surface potential vorticity allows one to replace the inhomogeneous boundary conditions for potential vorticity dynamics by simpler homogeneous boundary conditions (of constant potential temperature). Functional forms of the surface potential vorticity are derived from field equations in which the potential vorticity and a potential vorticity flux appear as sources of flow quantities in the same way in which an electric charge and an electric current appear as sources of fields in electrodynamics. For the generalized potential vorticity of flows that need be neither balanced nor hydrostatic and that can be influenced by diabatic processes and friction, a conservation law holds that is similar to the conservation law for the conventional interior potential vorticity. The conservation law for generalized potential vorticity contains, in the quasigeostrophic limit, the well-known dual relationship between fluctuations of potential temperature at boundaries and fluctuations of potential vorticity in the interior of quasigeostrophic flows. A nongeostrophic effect described by the conservation law is the induction of generalized potential vorticity by baroclinicity at boundaries, an effect that plays a role, for example, in mesoscale flows past topographic obstacles. Based on the generalized potential vorticity concept, a theory is outlined of how a wake with lee vortices can form in weakly dissipative flows past a mountain. Theoretical considerations and an analysis of a simulation show that a wake with lee vortices can form by separation of a generalized potential vorticity sheet from the mountain surface, similar to the separation of a friction-induced vorticity sheet from an obstacle, except that the generalized potential vorticity sheet can be induced by baroclinicity at the surface.
- Published
- 2003
18. Entropy budget of an atmosphere in radiative-convective equilibrium. Part II: latent heat transport and moist processes
- Author
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Pauluis, Olivier and Held, Isaac M.
- Subjects
Atmospheric research -- Analysis ,Atmospheric thermodynamics -- Research ,Carnot heat engine ,Earth sciences ,Science and technology - Abstract
In moist convection, atmospheric motions transport water vapor from the earth's surface to the regions where condensation occurs. This transport is associated with three other aspects of convection: the latent heat transport, the expansion work performed by water vapor, and the irreversible entropy production due to diffusion of water vapor and phase changes. An analysis of the thermodynamic transformations of atmospheric water yields what is referred to as the entropy budget of the water substance, providing a quantitative relationship between these three aspects of moist convection. The water vapor transport can be viewed as an imperfect heat engine that produces less mechanical work than the corresponding Carnot cycle because of diffusion of water vapor and irreversible phase changes. The entropy budget of the water substance provides an alternative method of determining the irreversible entropy production due to phase changes and diffusion of water vapor. This method has the advantage that it does not require explicit knowledge of the relative humidity or of the molecular flux of water vapor for the estimation of the entropy production. Scaling arguments show that the expansion work of water vapor accounts for a small fraction of the work that would be produced in the absence of irreversible moist processes. It is also shown that diffusion of water vapor and irreversible phase changes can be interpreted as the irreversible counterpart to the continuous dehumidification resulting from condensation and precipitation. This leads to a description of moist convection where it acts more as an atmospheric dehumidifier than as a heat engine.
- Published
- 2002
19. Entropy budget of an atmosphere in radiative-convective equilibrium. Part I: maximum work and frictional dissipation
- Author
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Pauluis, Olivier and Held, Isaac M.
- Subjects
Atmospheric research -- Analysis ,Atmospheric thermodynamics -- Research ,Earth sciences ,Science and technology - Abstract
The entropy budget of an atmosphere in radiative-convective equilibrium is analyzed here. The differential heating of the atmosphere, resulting from surface heat fluxes and tropospheric radiative cooling, corresponds to a net entropy sink. In statistical equilibrium, this entropy sink is balanced by the entropy production due to various irreversible processes such as frictional dissipation, diffusion of heat, diffusion of water vapor, and irreversible phase changes. Determining the relative contribution of each individual irreversible process to the entropy budget can provide important information on the behavior of convection. The entropy budget of numerical simulations with a cloud ensemble model is discussed. In these simulations, it is found that the dominant irreversible entropy source is associated with irreversible phase changes and diffusion of water vapor. In addition, a large fraction of the frictional dissipation results from falling precipitation, and turbulent dissipation accounts for only a small fraction of the entropy production. This behavior is directly related to the fact that the convective heat transport is mostly due to the latent heat transport. In such cases, moist convection acts more as an atmospheric dehumidifier than as a heat engine. The amount of work available to accelerate convective updrafts and downdrafts is much smaller than predicted by studies that assume that moist convection behaves mostly as a perfect heat engine.
- Published
- 2002
20. Hypohydrostatic Simulation of a Quasi-Steady Baroclinic Cyclone
- Author
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Hsieh, Tsung-Lin, primary, Garner, Stephen T., additional, and Held, Isaac M., additional
- Published
- 2020
- Full Text
- View/download PDF
21. Overtransmission of Rossby Waves at a Lower-Layer Critical Latitude in the Two-Layer Model
- Author
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Gliatto, Matthew T., primary and Held, Isaac M., additional
- Published
- 2020
- Full Text
- View/download PDF
22. Frictional Dissipation in a Precipitating Atmosphere
- Author
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Pauluis, oliver, Balaji, V., and Held, Isaac M.
- Subjects
Meteorological research -- Analysis ,Friction -- Analysis ,Energy dissipation -- Analysis ,Tropics -- Natural history ,Turbulence -- Analysis ,Earth sciences ,Science and technology - Abstract
The frictional dissipation in the shear zone surrounding falling hydrometeors is estimated to be 2-4 W m(super -2) in the Tropics. A numerical model of radiative-convective equilibrium with resolved three-dimensional moist convection confirms this estimate and shows that the precipitation-related dissipation is much larger than the dissipation associated with the turbulent energy cascade from the convective scale. Equivalently, the work performed by moist convection is used primarily to lift water rather than generate kinetic energy of the convective airflow. This fact complicates attempts to use the entropy budget to derive convective velocity scales.
- Published
- 2000
23. A Linear Stochastic Model of a GCM's Midlatitude Storm Tracks
- Author
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Zhang, Yunqing and Held, Isaac M.
- Subjects
Stochastic systems -- Models ,Storms -- Models ,Dynamic meteorology -- Models ,Eddies -- Models ,Earth sciences ,Science and technology - Abstract
A linear stochastic model is used to simulate the midlatitude storm tracks produced by an atmospheric GCM. A series of six perpetual insolation/SST GCM experiments are first performed for each month. These experiments capture the 'midwinter suppression' of the Pacific storm track in a particularly clean way. The stochastic model is constructed by linearizing the GCM about its January climatology and finding damping and stirring parameters that best reproduce that model's eddy statistics. The model is tested by examining its ability to simulate other GCM integrations when the basic state is changed to the mean flow of those models, while keeping the stirring and damping unchanged. The stochastic model shows an impressive ability to simulate a variety of eddy statistics. It captures the midwinter suppression of the Pacific storm track qualitatively and is also capable of simulating storm track responses to E1 Nino. The model results are sensitive to the manner in which the model is stirred. Best results for eddy variances and fluxes are obtained by stirring the temperature and vorticity at low levels. However, a better simulation of the spatial structure of the dominant wave train as defined by covariance maps is obtained by stirring the temperature equation only, and at all levels.
- Published
- 1999
24. Nonlinear Generation of Long Waves and the Reversal of Eddy Momentum Fluxes in a Two-Layer Quasigeostrophic Model.
- Author
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Hsieh, Tsung-Lin, Chang, Chiung-Yin, Held, Isaac M., and Zurita-Gotor, Pablo
- Subjects
EDDY flux ,BAROCLINICITY ,EDDIES ,JET planes ,BAROCLINIC models ,WATER waves ,NONLINEAR waves ,ROSSBY waves - Abstract
Although classical theories of midlatitude momentum fluxes focus on the wave–mean flow interaction, wave–wave interactions may be important for generating long waves. It is shown in this study that this nonlinear generation has implications for eddy momentum fluxes in some regimes. Using a two-layer quasigeostrophic model of a baroclinic jet on a β plane, statistically steady states are explored in which the vertically integrated eddy momentum flux is divergent at the center of the jet, rather than convergent as in Earthlike climates. One moves toward this less familiar climate from more Earthlike settings by reducing either β, frictional drag, or the width of the baroclinic zone, or by increasing the upper bound of resolvable wavelengths by lengthening the zonal channel. Even in Earthlike settings, long waves diverge momentum from the jet, but they are too weak to compete with short unstable waves that converge momentum. We argue that long waves are generated by breaking of short unstable waves near their critical latitudes, where long waves converge momentum while diverging momentum at the center of the jet. Quasi-linear models with no wave–wave interaction can qualitatively capture the Earthlike regime but not the regime with momentum flux divergence at the center of the jet, because the nonlinear wave breaking and long-wave generation processes are missing. Therefore, a more comprehensive theory of atmospheric eddy momentum fluxes should take into account the nonlinear dynamics of long waves. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
25. Dynamics of barotropic storm tracks
- Author
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Swanson, Kyle L., Kushner, Paul J., and Held, Isaac M.
- Subjects
Eddies -- Analysis ,Storm surges -- Analysis ,Earth sciences ,Science and technology - Abstract
Longitudinal variations in the upper-tropospheric time-mean flow strongly modulate the structure and amplitude of upper-tropospheric eddies. This barotropic modulation is studied using simple models of wave propagation through zonally varying basic states that consist of contours separating regions of uniform barotropic potential vorticity. Such basic states represent in a simple manner the potential vorticity distribution in the upper troposphere. Predictions of the effect of basic-state zonal variations on the amplitude and spatial structure of eddies and their associated particle displacements are made using conservation of wave action or, equivalently, the linearized 'pseudoenergy' wave activity. The predictions are confirmed using WKB theory and linear numerical calculations. The interaction of finite-amplitude disturbances with the basic flow is also analyzed numerically using nonlinear contour-dynamical simulations. It is found that breaking nonlinear contour waves undergo irreversible amplitude attenuation, scale lengthening, and frequency lowering upon passing through a region of weak basic-state flow.
- Published
- 1997
26. The diffusive approximation for eddy fluxes in baroclinically unstable jets
- Author
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Pavan, Valentina and Held, Isaac M.
- Subjects
Eddies -- Models ,Jets -- Models ,Air flow -- Models ,Earth sciences ,Science and technology - Abstract
A series of statistically steady states for baroclinically unstable jets in a two-layer quasigeostrophic model is examined, in order to evaluate diffusive approximations to the eddy potential vorticity or heat fluxes. The flow is forced by thermal relaxation to an unstable 'radiative equilibrium' temperature gradient. The statistically steady states are studied as a function of the width of the radiative equilibrium jet. A local diffusive 'theory' for the eddy fluxes is obtained from integrations of a homogeneous, doubly periodic model with prescribed environmental potential vorticity gradients. The flux-gradient relationship generated by the homogeneous model predicts the magnitude and shape of the eddy fluxes in the unstable jet flows remarkably well, as long as the jet is not too narrow. These results confirm the relevance of diffusive closures for eddy potential vorticity and heat fluxes in such flows. For narrow jets that produce eddy fluxes with a half-width of one to two radii of deformation, this local theory underpredicts the fluxes.
- Published
- 1996
27. A scaling theory for horizontally homogeneous, baroclinically unstable flow on a beta plane
- Author
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Held, Isaac M. and Larichev, Vitaly D.
- Subjects
Eddies -- Research ,Beta testing -- Models ,Earth sciences ,Science and technology - Abstract
The scaling argument developed by the authors in a previous work for eddy amplitudes and fluxes in a horizontally homogeneous, two-layer model on an f plane is extended to a [Beta] plane. In terms of the nondimensional number [Xi] = U/([Beta][[Lambda].sup.2]), where [Lambda] is the deformation radius and U is the mean thermal wind, the result for the rms eddy velocity V, the characteristic wavenumber of the energy-containing eddies and of the eddy-driven jets [k.sub.j], and the magnitude of the eddy diffusivity for potential vorticity D, in the limit [Xi] [much greater than] 1, are as follows: V/U[approximately equal to] [Xi], [k.sub.j][Lambda] [approximately equal to] [[Xi].sup.-1], D/(U[Lambda]) [approximately equal to] [[Xi].sup.2]. Numerical simulations provide qualitative support for this scaling but suggest that it underestimates the sensitivity of these eddy statistics to the value of [Xi]. A generalization that is applicable to continuous stratification is suggested that leads to the estimates V [approximately equal to]s [([Beta][T.sup.2]).sup.-1], [k.sub.j] [approximately equal to] [Beta]T, D [approximately equal to] [([[Beta].sup.2][T.sup.3]).sup.-1], where T is a timescale determined by the environment; in particular, it equals [Lambda][U.sup.-1] in the two-layer model and N[(f[[Delta].sub.z]U).sup.-1] in a continuous flow with uniform shear and stratification. This same scaling has also been suggested as relevant to a continuously stratified fluid in the opposite limit, [Xi] [much less than] 1. Therefore, the authors suggest that it may be of general relevance in planetary atmospheres and in the oceans.
- Published
- 1996
28. A proposal for the intercomparison of the dynamical cores of atmospheric general circulation models
- Author
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Held, Isaac M. and Suarez, Max J.
- Subjects
Atmospheric research -- Models ,Business ,Earth sciences - Abstract
A benchmark calculation is proposed for evaluating the dynamical cores of atmospheric general circulation models independently of the physical parameterizations. The test focuses on the long-term statistical properties of a fully developed general circulation; thus, it is particularly appropriate for intercomparing the dynamics used in climate models. To illustrate the use of this benchmark, two very different atmospheric dynamical cores - one spectral, one finite difference - are compared. It is found that the long-term statistics produced by the two models are very similar. Selected results from these calculations are presented to initiate the intercomparison.
- Published
- 1994
29. Radiative-convective equilibrium with explicit two-dimensional moist convection
- Author
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Held, Isaac M., Hemler, Richard S., and Ramaswamy, V.
- Subjects
Convection (Meteorology) -- Models ,Atmospheric radiation -- Models ,Clouds -- Dynamics ,Earth sciences ,Science and technology - Abstract
Radiative-convective statistical equilibria are obtained using a two-dimensional model in which radiative transfer is interactive with the predicted moisture and cloud fields. The domain is periodic in x, with a width of 640 km, and extends from the ground to 26 km. The lower boundary is a fixed-temperature water-saturated surface. The model produces a temperature profile resembling the mean profile observed in the tropics. A number of integrations of several months' duration are described in this preliminary examination of the model's qualitative behavior. The model generates a QBO-like oscillation in the x-averaged winds with an apparent period of approximately 60 days. This oscillation extends into the troposphere and influences the convective organization. In order to avoid the associated large vertical wind shears, calculations are also performed in which the x-averaged winds are constrained to vanish. The convection then evolves into a pattern in which rain falls only within a small part of the domain. The moisture field appears to provide the memory that localizes the convection. If the vertical shears are fixed at a modest nonzero value, this localization is avoided. Comparing calculations with surface temperatures of 25 degrees and 30 degrees C, the planetary albedo is found to decrease with increasing temperature, primarily due to a reduction in low-level cloudiness.
- Published
- 1993
30. Baroclinic wave packets in models and observations
- Author
-
Lee, Sukyoung and Held, Isaac M.
- Subjects
Southern Hemisphere -- Natural history ,Wave packets -- Research ,Earth sciences ,Science and technology - Abstract
Coherent baroclinic wave packets are present in the Southern Hemisphere, most clearly in the summer season. These coherent packets are also found in a hierarchy of models of nonlinear baroclinic instability - a two-layer quasigeostrophic (QG) model on a beta-plane, a two-level primitive equation (PE) model, and a general circulation model. The flows are chaotic, but the packet itself can remain remarkably coherent, despite the complex evolution of the flow within the packet. In both QG and PE models, the packets become more robust as the supercriticality of the flow is reduced. In both models and the observations, the packets move with a group velocity that is greater than the phase speed of the individual disturbances, so that these disturbances exhibit downstream development. The structure of the baroclinic waves in the packet as a function of longitude resembles the life cycles of sinusoidal baroclinic waves as a function of time. More than one packet can exist in the domain at the same time. In the QG model, the number of packets increases in a systematic way as the length of the channel increases.
- Published
- 1993
31. Large-scale dynamics and global warming
- Author
-
Held, Isaac M.
- Subjects
Global warming -- Research ,Ocean-atmosphere interaction -- Research ,Climatic changes -- Research ,Business ,Earth sciences - Abstract
Predictions of future climate change raise a variety of issues in large-scale atmospheric and oceanic dynamics. Several of these are reviewed in this essay, including the sensitivity of the circulation of the Atlantic Ocean to increasing freshwater input at high latitudes; the possibility of greenhouse cooling in the southern oceans; the sensitivity of monsoonal circulations to differential warming of the two hemispheres; the response of midlatitude storms to changing temperature gradients and increasing water vapor in the atmosphere; and the possible importance of positive feedback between the mean winds and eddy-induced heating in the polar stratosphere.
- Published
- 1993
32. Nonlinear equilibration of two-dimensional Eady waves: a new perspective
- Author
-
Garner, Stephen T., Nakamura, Noboru, and Held, Isaac M.
- Subjects
Eddies -- Analysis ,Waves -- Analysis ,Atmospheric diffusion -- Analysis ,Earth sciences ,Science and technology - Abstract
The equilibration of two-dimensional baroclinic waves differs fundamentally from equilibration in three dimensions because two-dimensional eddies cannot develop meridional temperature or velocity structure. It was shown in an earlier paper that frontogenesis together with diffusive mixing in a two-dimensional Eady wave brings positive potential vorticity (PV) anomalies deep into the atmosphere from both boundaries and allows the disturbance to settle into a steady state without meridional gradients. Here we depart from the earlier explanation of this equilibration and associate the PV intrusions with essentially the same kind of vortex 'roll-up' that characterizes the evolution of barotropic shear layers. To avoid subgrid turbulence parameterizations and computational diffusion, the analogy is developed using Eady's generalized baroclinic instability problem. Eady's generalized model has two semi-infinite regions of large PV surrounding a layer of relatively small PV. Without boundaries, frontal collapse, or strong diffusion the model still produces equilibrated states, with structure similar to the vortex streets that emerge from unstable barotropic shear layers. The similarity is greatest when the baroclinic development is viewed in isentropic coordinates. The contrast between the present equilibrated solutions, which exhibit no vertical tilt, and Blumen's diffusive frontogenesis model, which allows the wave to retain its phase tilt, is briefly discussed.
- Published
- 1992
33. Quasigeostrophic turbulence in a three-layer model: effects of vertical structure in the mean shear
- Author
-
Held, Isaac M. and O'Brien, Enda
- Subjects
Winds -- Research ,Atmospheric turbulence -- Research ,Wind shear -- Research ,Earth sciences ,Science and technology - Abstract
A three-layer, horizontally homogeneous, quasigeostrophic model is selected as one of the simplest environments in which to study the sensitivity of baroclinic eddy fluxes in the atmosphere to the vertical structure of the basic-state temperature gradients or vertical wind shears. Eddy statistics obtained from the model are interpreted in terms of linear theory and a modified 'baroclinic adjustment' hypothesis. Both linear theory and the baroclinic adjustment construction are found to provide useful predictions for the vertical structure of the eddy potential vorticity flux. For equal values of the mean vertical shear, eddy fluxes and energies are greater when the shear is concentrated at lower levels (d squared U/dz squared < 0) than when the shear is concentrated at higher levels (d squared U/dz squared > 0). Eddy fluxes are more sensitive to lower- than to upper-level mean temperature gradients. This relative sensitivity is a function of gamma = f squared lambda/(beta-N squared H), where lambda is the mean vertical shear and H is the depth of the fluid. It is enhanced as gamma is reduced, as the unstable modes become shallower, until the eddies become almost completely insensitive to the strength of the upper-layer wind for gamma < 0.5.
- Published
- 1992
34. The stationary response to large-scale orography in a general circulation model and a linear model
- Author
-
Cook, Kerry H. and Held, Isaac M.
- Subjects
Weather, Influence of mountains on -- Research ,Standing waves -- Research ,Mountains -- Research ,Atmospheric circulation -- Research ,Earth sciences ,Science and technology - Abstract
Stationary waves generated over orography in a linear model (GCM) are compared to examine how the atmosphere's response is established for small mountains and how linear theory breaks down over large orographic features. Both models have nine vertical levels and are low-resolution (R 15) spectral models. The linear model solves the stationary linear primitive equations. The GCM's control integration uses zonally uniform and hemispherically symmetric boundary conditions, with a global swamp surface. Five experiments are performed by perturbing the GCM with Gaussian mountains of various heights introduced in midlatitudes. The stationary wave model is linearized about zonal mean fields from the GCM climatology. The linear model's response to a Gaussian mountain at 45 [degrees] N latitude is dominated by a single wave train radiating toward the southeast. For mountain heights between 0.7 and 2 km, the GCM's stationary waves are similar to the linear model response to orography, although amplitudes increase less rapidly than linearly with mountain height. For larger mountains, closed isentropes and distinctly nonlinear flow occur along the surface of the mountain and a large poleward-radiating wave train develops. The development of closed isentropes, and the breakdown of linear theory, can be predicted whenever the slope of the surface exceeds the slope of the isentropes in the unperturbed (no mountain) basic state., Atmospheric response for small mountains and breakdown of linear theory over large orographic features were examined. The stationary waves produced over orography in a linear model and a general circulation model (GCM) were compared. Both linear model and GCM were low-resolution spectral models and possessed nine vertical levels. GCM's stationary waves were found to be similar to that of linear models in mountain heights between 0.7 and 2 km, but its responses were found to be dependent on the mountain heights.
- Published
- 1992
35. Effects of the Mean Flow on Martian Transient Eddy Activity: Studies with an Idealized General Circulation Model
- Author
-
Mooring, Todd A., primary, Held, Isaac M., additional, and Wilson, R. John, additional
- Published
- 2019
- Full Text
- View/download PDF
36. The Control of Surface Friction on the Scales of Baroclinic Eddies in a Homogeneous Quasigeostrophic Two-Layer Model
- Author
-
Chang, Chiung-Yin, primary and Held, Isaac M., additional
- Published
- 2019
- Full Text
- View/download PDF
37. 100 Years of Progress in Understanding the General Circulation of the Atmosphere
- Author
-
Held, Isaac M., primary
- Published
- 2019
- Full Text
- View/download PDF
38. The Finite-Amplitude Evolution of Mixed Kelvin–Rossby Wave Instability and Equatorial Superrotation in a Shallow-Water Model and an Idealized GCM
- Author
-
Ministerio de Economía y Competitividad (España), National Science Foundation (US), Zurita-Gotor, Pablo, Held, Isaac M., Ministerio de Economía y Competitividad (España), National Science Foundation (US), Zurita-Gotor, Pablo, and Held, Isaac M.
- Abstract
An instability involving the resonant interaction of a Rossby wave and a Kelvin wave has been proposed to drive equatorial superrotation in planetary atmospheres with a substantially smaller radius or a smaller rotation rate than Earth, that is, with a large thermal Rossby number. To pursue this idea, this paper investigates the equilibration mechanism of Kelvin–Rossby instability by simulating the unforced initial-value problem in a shallow-water model and in a multilevel primitive equation model. Although the instability produces equatorward momentum fluxes in both models, only the multilevel model is found to superrotate. It is argued that the shortcoming of the shallow-water model is due to its difficulty in representing Kelvin wave breaking and dissipation, which is crucial for accelerating the flow in the tropics. In the absence of dissipation, the zonal momentum fluxed into the tropics is contained in the eddy contribution to the mass-weighted zonal wind rather than the zonal-mean zonal flow itself. In the shallow-water model, the zonal-mean zonal flow is only changed by the eddy potential vorticity flux, which is very small in our flow in the tropics and can only decelerate the flow in the absence of external vorticity stirring.
- Published
- 2018
39. The Finite-Amplitude Evolution of Mixed Kelvin–Rossby Wave Instability and Equatorial Superrotation in a Shallow-Water Model and an Idealized GCM
- Author
-
Zurita-Gotor, Pablo, primary and Held, Isaac M., additional
- Published
- 2018
- Full Text
- View/download PDF
40. The Role of the Water Vapor Feedback in the ITCZ Response to Hemispherically Asymmetric Forcings
- Author
-
Clark, Spencer K., primary, Ming, Yi, additional, Held, Isaac M., additional, and Phillipps, Peter J., additional
- Published
- 2018
- Full Text
- View/download PDF
41. Modeling Water Vapor and Clouds as Passive Tracers in an Idealized GCM
- Author
-
Ming, Yi, primary and Held, Isaac M., additional
- Published
- 2018
- Full Text
- View/download PDF
42. Lower-Tropospheric Eddy Momentum Fluxes in Idealized Models and Reanalysis Data
- Author
-
Lutsko, Nicholas J., primary, Held, Isaac M., additional, Zurita-Gotor, Pablo, additional, and O’Rourke, Amanda K., additional
- Published
- 2017
- Full Text
- View/download PDF
43. Lower-Tropospheric Eddy Momentum Fluxes in Idealized Models and Reanalysis Data
- Author
-
National Science Foundation (US), Princeton University, Lutsko, Nicholas J., Held, Isaac M., Zurita-Gotor, Pablo, O’Rourke, Amanda K., National Science Foundation (US), Princeton University, Lutsko, Nicholas J., Held, Isaac M., Zurita-Gotor, Pablo, and O’Rourke, Amanda K.
- Abstract
In Earth’s atmosphere eddy momentum fluxes (EMFs) are largest in the upper troposphere, but EMFs in the lower troposphere, although modest in amplitude, have an intriguing structure. To document this structure, the EMFs in the lower tropospheres of a two-layer quasigeostrophic model, a primitive equation model, and the Southern Hemisphere of a reanalysis dataset are investigated. The lower-tropospheric EMFs are very similar in the cores of the jets in both models and the reanalysis data, with EMF divergence (opposing the upper-tropospheric convergence) due to relatively long waves with slow eastward phase speeds and EMF divergence (as in the upper troposphere) due to shorter waves with faster eastward phase speeds. As the two-layer model is able to capture the EMF divergence by long waves, a qualitative picture of the underlying dynamics is proposed that relies on the negative potential vorticity gradient in the lower layer of the model. Eddies excited by baroclinic instability mix efficiently through a wide region in the lower layer, centered on the latitude of maximum westerlies and encompassing the lower-layer critical latitudes. Near these critical latitudes, the mixing is enhanced, resulting in increased EMF convergence, with compensating EMF divergence in the center of the jet. The EMF convergence at faster phase speeds is due to deep eddies that propagate on the upper-tropospheric potential vorticity gradient.
- Published
- 2017
44. A Moist Static Energy Budget–Based Analysis of the Sahel Rainfall Response to Uniform Oceanic Warming
- Author
-
Hill, Spencer A., primary, Ming, Yi, additional, Held, Isaac M., additional, and Zhao, Ming, additional
- Published
- 2017
- Full Text
- View/download PDF
45. Tropical Cyclones in Rotating Radiative–Convective Equilibrium with Coupled SST
- Author
-
Zhou, Wenyu, primary, Held, Isaac M., additional, and Garner, Stephen T., additional
- Published
- 2017
- Full Text
- View/download PDF
46. The Partitioning of the Poleward Energy Transport between the Tropical Ocean and Atmosphere
- Author
-
HELD, ISAAC M.
- Subjects
Ocean-atmosphere interaction -- Environmental aspects ,Fluid dynamics -- Research ,Atmospheric electricity -- Measurement ,Atmospheric pressure -- Research ,Earth sciences ,Science and technology - Abstract
The mass transport in the shallow, wind-driven, overturning cells in the tropical oceans is constrained to be close to the mass transport in the atmospheric Hadley cell, assuming that zonally integrated wind stresses on land are relatively small. Therefore, the ratio of the poleward energy transport in low latitudes in the two media is determined by the ratio of the atmospheric gross static stability to that of the ocean. A qualitative discussion of the gross stability of each medium suggests that the resulting ratio of oceanic to atmospheric energy transport, averaged over the Hadley cell, is roughly equal to the ratio of the heat capacity of water to that of air at constant pressure, multiplied by the ratio of the moist- to the dry-adiabatic lapse rates near the surface. The ratio of oceanic to atmospheric energy transport should be larger than this value near the equator and smaller than this value near the poleward boundary of the Hadley cell.
- Published
- 2001
47. Corrigendum
- Author
-
Hill, Spencer A., primary, Ming, Yi, additional, and Held, Isaac M., additional
- Published
- 2016
- Full Text
- View/download PDF
48. The Response of an Idealized Atmosphere to Orographic Forcing: Zonal versus Meridional Propagation
- Author
-
Lutsko, Nicholas J., primary and Held, Isaac M., additional
- Published
- 2016
- Full Text
- View/download PDF
49. Applying the Fluctuation–Dissipation Theorem to a Two-Layer Model of Quasigeostrophic Turbulence
- Author
-
Lutsko, Nicholas J., Held, Isaac M., Zurita-Gotor, Pablo, Lutsko, Nicholas J., Held, Isaac M., and Zurita-Gotor, Pablo
- Abstract
The fluctuation–dissipation theorem (FDT) provides a means of calculating the response of a dynamical system to a small force by constructing a linear operator that depends only on data from the internal variability of the unperturbed system. Here the FDT is used to estimate the response of a two-layer quasigeostrophic model to two zonally symmetric torques, both barotropic, with the same sign of the forcing in the two layers, and baroclinic, with opposite sign forcing in the two layers. The supercriticality of the model is also varied to test how the FDT fares, as this parameter is varied. To perform the FDT calculations the data are decomposed onto empirical orthogonal functions (EOFs) and only those EOFs that are well resolved are retained in the FDT calculations. In the barotropic case good qualitative estimates are obtained for all values of the supercriticality, though the FDT consistently overestimates the response, perhaps because of significant non-Gaussian behavior present in the model. Nevertheless, this adds to the evidence that the annular-mode time scale plays an important role in determining the response of the midlatitudes to small perturbations. The baroclinic case is more challenging for the FDT. However, by constructing different bases with which to calculate the EOFs, it is shown that the issue in this case is that the baroclinic variability is poorly sampled, not that the FDT fails. The strategies developed in order to generate these estimates may be applicable to situations in which the FDT is applied to larger systems.
- Published
- 2015
50. Applying the Fluctuation–Dissipation Theorem to a Two-Layer Model of Quasigeostrophic Turbulence
- Author
-
Lutsko, Nicholas J., primary, Held, Isaac M., additional, and Zurita-Gotor, Pablo, additional
- Published
- 2015
- Full Text
- View/download PDF
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