Lee E. Branscome

Life Cycles of Moist Baroclinic Eddies

Abstract The interaction between moisture and baroclinic eddies was examined through eddy life-cycle experiments using a global, primitive equation model. How condensation affects the structural evolution of eddies, their fluxes of heat, moisture, and momentum, and their subsequent interaction with the zonal average state was examined. Initial states corresponded to climatological winter and summer zonal average states. For most experiments the perturbation had a fundamental zonal wavenumber 7, representing an appropriate scale for transient eddies that reach substantial amplitudes in the atmosphere. Additional experiments used fundamental wavenumber 4, 10, or 14. The waves vertical motion produced midtropospheric supersaturation whose heating further amplified the vertical motion. Consequently, the largest effects of condensation were associated with vertical transports. Compared to corresponding dry experiments, intensified vertical motions increased the maximum kinetic energy attained by the wave, but...

A Parameterization of Transient Eddy Heat Flux on a Beta-Plane

Abstract A parameterization of transient eddy heat flux is developed which incorporates baroclinic wave behavior in a continuously stratified fluid on a β-plane. The meridional and vertical heat transports are more sensitive to forced changes in the mean state than suggested by earlier parameterizations. This strong response is analogous to the rapid flux variation in a two-level model near neutral stability. A scheme for applying the parameterization to observed mean zonal flow is suggested and computed fluxes are similar in magnitude and structure to observed eddy heat flux. The flux modeling is consistent with the stronger dependence on meridional temperature gradient and large seasonal variation of the vertical eddy scale observed in lower latitudes.

Effect of Surface Fluxes on the Nonlinear Development of Baroclinic Waves

Abstract The nonlinear development of baroclinically unstable waves in the presence of surface friction and heat flux is studied, using a global primitive equation model. The experiments use zonal wavenumber 3.7 or 12 and a variety of initial conditions, mostly representative of observed initial states. Other initial states consist of solidbody rotation with vertical shear of the zonal wind. In addition to comparisons of inviscid and dissipative experiments, the effect of linear and nonlinear drag formulations is compared. Starting from a small-amplitude perturbation in the temperature field, a modal structure emerges and grows exponentially for a few days. Unstable waves assume a structure that reduces frictional energy IOU when surface drag is present, but they still retain a normal mode character during a period of rapid growth. As the wave grows in amplitude, the ratio of upper-level to low-level eddy kinetic energy increases substantially in the presence of nonlinear surface drag. In the absence of s...

Mean Flow Adjustment during Life Cycles of Baroclinic Waves

Abstract We use a global, primitive equation model to study the evolution of waves growing in a zonal mean state that is initially baroclinically unstable. The waves produce changes in the zonal mean state that we compare with changes predicted by baroclinic adjustment theories We examine mean state adjustment by representative zonal wavenumbers 3, 7 or 12. In the absence of surface processes, as the wave grows to its maximum amplitude, it reduces the zonal mean states potential vorticity gradient through the lower troposphere, in accord with adjustment theories. Over the latitudes with largest wave amplitude, changes in the static stability and the zonal winds vertical shear contribute about equally to the potential vorticity gradient adjustment. However, during the last day of a waves growth, momentum fluxes strengthen the barotropic component of the zonal wind and the potential vorticity gradient in the middle troposphere, changes that are not anticipated by adjustment theory. The static stability a...

Diabatically Forced, Nearly Inviscid Eddy Regimes

Abstract We use a scaling analysis and numerical solutions of a quasigeostrophic two-level model on a β plane to explore what kind of eddy regimes can occur when the eddies are forced solely by differential diabatic heating, and friction is negligible outside a boundary layer near the lower surface. The eddy regimes all have a convergent eddy momentum flux and a Ferrel cell near the center of the channel flow, and a downgradient eddy heat flux that exceeds the upgradient heat flux by the Ferrel cell. Quantitatively the regimes can be characterized by R, the ratio of the heat transport by the Ferrel cell to the heat transport by the eddies. This ratio depends on a nondimensional parameter, δ, which is proportional to the characteristic relaxation time associated with the diabatic heating and to the convergence of the eddy momentum flux. If δ → ∞, then R → 1, the divergence of the Eliassen-Palm flux approaches zero, and the temperature structure approaches equilibrium; that is, the motions have very little ...

The Charney Baroclinic Stability Problem: Approximate Solutions and Modal Structures

Abstract The classic Charney baroclinic stability problem is examined through perturbation techniques in the short-wave limit, near the first neutral curve separating Charney and Green modes, and near the second neutral curve separating long and short Green modes. This method provides simple analytical expressions for the vertical structure of the growing waves and the dependence of phase speeds and growth rates on mean flow parameters. The rapidly growing Charney modes have horizontal and vertical scales which crucially depend on the β-parameter. Structures of heat and potential vorticity fluxes are also represented by approximate solutions and their dependence on wavenumber is examined.

Tropical-extratropical interactions on intraseasonal time scales in a global spectral model

Abstract Observational studies have revealed some coherent extratropical patterns associated with the tropical Madden–Julian (MJ) wave. This study is an attempt to clarify and constrain the interpretation of these patterns by investigating tropical–extratropical interactions on intraseasonal time scales in a global spectral model (GSM). Forcing representative of northern winter is used. A simple heating-only cumulus parameterization scheme is included to generate the MJ wave. The wave period in the model falls within the 30–60 day range observed and has a structure consistent with observations. Various statistical techniques including compositing, empirical orthogonal function (EOF) analysis, and singular value decomposition (SVD) have been used to identify those extratropical patterns associated with the tropical MJ wave. Under zonally symmetric external conditions (no topography) the MJ wave maintains a highly regular amplitude and phase speed. Nevertheless, there is no statistically significant coheren...

Sensitivity of transient eddies to climate change in the CCC general circulation model

Abstract We use eddy life‐cycle simulations to evaluate the response of atmospheric transient eddies to a global warming caused by CO2 doubling in the CCC general circulation model. In simulations using Northern Hemisphere winter conditions, transient waves attain larger kinetic energy and encompass a wider range of latitudes in the warmer climate. This behaviour contrasts with a previous investigation that used output from the NCAR and GFDL models. Our analysis indicates two primary factors for the difference between model responses: (1) a smaller change in the mid‐latitude temperature gradient in the CCC model, which allows (2) increased atmospheric water vapour in mid‐latitudes to catalyze a more rapidly evolving life‐cycle.

The effects of large-scale topography on the circulation in low-order models

Abstract The effects of topography are examined in a class of low-order quasi-geostrophic models on a midlatitude β-plane. In the absence of topography the models are capable of producing qualitatively realistic zonal-mean circulations. The maintenance of the zonally symmetric and asymmetric circulations are examined with different spectral truncations and topographic configurations. The response to an isolated mountain peak is the most thoroughly investigated. When the model is run without wave–wave interactions, the time-mean wave pattern forced by the isolated mountain is a superposition of waves which are either in phase or 180° out of phase with the mountain. When they are included, transient wave-wave interactions alter the mean zonal flow, which leads to a substantial modification of the time-mean wave. Specifically, the amplitude of the longest planetary wave in the model is enhanced as that wave is pushed closer to resonance by the change in the midlevel zonal flow. A phase shift relative to the ...