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Shrinivas Moorthi and Max J. Suarez

Abstract

A simple implementation of the Arakawa and Schubert (1974) cumulus parameterization is presented. The major simplification made is to “relax”the state toward equilibrium each time the parameterization is invoked, rather than requiring that the final state be balanced, as in the original Arakawa-Schubert implementation. This relaxed Arakawa-Schubert (RAS) scheme is evaluated in off-line tests using the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) Phase III data. The results show that RAS is equivalent to the standard implementation of Arakawa-Schubert but is more economical and simpler to code. RAS also avoids the ill-posed problem that occurs in Arakawa-Schubert as a result of having to solve for a balanced state.

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Akio Arakawa and Max J. Suarez

Abstract

A vertical finite-difference scheme for the primitive equations in sigma coordinates is obtained by requiring that the discrete equations retain some important properties of the continuous equations. A family of schemes is derived whose members conserve total energy, maintain an integral constraint on the vertically integrated pressure gradient force, have a local differencing of the hydrostatic equation, and give exact forms of the hydrostatic equation and the pressure gradient force for particular atmospheres. The proposed scheme is a member of this family that in addition conserves the global mass integral of the potential temperature under abiabatic processes.

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Paul S. Schopf and Max J. Suarez

Abstract

A reexamination of the coupled delayed-action oscillator model of Suarez and Schopf for the El Nino/Southern Oscillation (ENSO) phenomenon is made, by deriving it using a parameterized atmosphere and explicit linear ocean wave dynamics. The derivation attempts to clarify the role of boundary reflections, damping, and scale sensitivity in determining the characteristic timescale of the model. Making the assumption that SST anomalies are related to thermocline perturbations in the central to eastern part of the basin, and that wind anomalies are related to SST anomalies, ocean wave dynamics are invoked to solve for the relationship between wind anomalies and the relevant thermocline displacement.

A perturbation to SST causes wind anomalies which drive Kelvin waves eastward, thereby increasing the SST perturbation. The wind perturbations also generate Rossby waves in the ocean, which propagate westward, eventually reflecting from the western boundary as Kelvin waves. The thermocline displacements of these waves have the opposite sense to those of the directly driven Kelvin waves and form the basis for the delayed, negative feedback in the delayed-action oscillator. By solving the wave dynamics explicitly, we are able to conclude that: 1) The delayed action oscilator—in its simplest form with no eastern boundary reflection and a single Rossby wave reflected from the western boundary—forms the basic oscillator mechanism. 2) Very little of the Rossby wave energy propagating to the western boundary needs to be reflected into the Kelvin wave in order for the system to oscillate (as little as 20% in some cases). 3) The zonal extent of the wind field response to SST anomalies has almost no influence on the solutions. 4) Broadening of the meridional shape of the winds, which inparts more of the Rossby wave energy to higher meridional modes has the net effect of lengthening the delay without a large impact on the amplitude of the returning signal. 5) The role of the eastern boundary is relatively unimportant.

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Julio T. Bacmeister and Max J. Suarez

Abstract

A detailed examination of the atmospheric momentum budget over the equatorial Pacific and its relation to oceanic wind stresses is undertaken using the results of a 20-yr (1979–99), forced-SST, AGCM experiment. The results show that free-tropospheric pressure gradients play a significant role in forcing boundary layer flow in the model. In particular, the time-mean and interannual variability of wind stress at the surface is found to be dominated by forcing from the free troposphere. The NCEP reanalyses from 1979–99 are also examined and a similar result is found, although the relative importance of this free-tropospheric forcing is somewhat higher in the model.

The seasonal cycle of free-tropospheric forcing in the model is found to be substantially stronger in the model than in the reanalysis, and that has a clearly negative impact on the simulated seasonal cycle of surface wind stresses. In the model, these free-tropospheric pressure gradients are not balanced by turbulent stresses or other dissipative forces. Rather, the momentum budget analysis shows that they are balanced by advective momentum tendencies, with vertical advection of momentum in the descending branch of the Walker circulation playing an important role.

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Randal D. Koster and Max J. Suarez

Abstract

Observed monthly precipitation anomalies are standardized across midlatitude land, and ergodicity is invoked to combine the spatially distributed data into probability density functions (pdfs) of precipitation conditioned on the strength of earlier anomalies. The conditional pdfs, though broad and overlapping, are indeed distinct at a high (99.9%) level of confidence. This implies a nonzero degree of predictability for midlatitude precipitation, even at 3-month leads. This behavior is reproduced by an AGCM only when land–atmosphere feedback in the model is enabled.

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David A. Randall and Max J. Suarez

Abstract

Result obtained with a mixed layer model are used to study the dynamics of stratomulus formation and dissipation in subtropical marine stratocumulus cloud regimes. The model used allows entrainment to be driven by shear as well as buoyancy, and includes a very crude parameterization of the partial blackness of thin cloud layers. Model results show that for some values of the large-scale divergence there are three equilibrium mixed layer structures, two of which are stable. One of the stable equilibria is cloudy, deep, and buoyancy-driven, while the other is clear, shallow, and shear-driven. It is found that as a result of hysteresis effects a transient increase in the large-scale divergence can produce a long-lasting break in the clouds.

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Edward Colón, James Lindesay, and Max J. Suarez

Abstract

The intent of this study is to measure the total contribution of individual wind- and moist flux–induced feedback mechanisms on the generation and maintenance of simulated Madden–Julian oscillations (MJOs). This task is accomplished through the use of an idealized GCM that is also employed to examine the impact of sea surface temperatures on the robustness of the MJO signal. Among the mechanisms investigated are Conditional Instability of the Second Kind (CISK), Wind Induced Surface Heat Exchange (WISHE), and the feedback due to specific humidity differences between the surface and the boundary layer. A series of numerical experiments was conducted in which one or more of these feedbacks were suppressed. The resulting structure and periodicity of the simulated mode was analyzed using eigenvector and spectral analysis methods. The relative phases of surface wind, turbulent flux, and boundary layer moisture anomaly fields were then contrasted. It was found that the model produced a far stronger signal when the turbulent surface flux contributions were unsuppressed. However, the inclusion of turbulent surface fluxes yielded simulated modes possessing unrealistic frequencies of 16.79–28.83 cycles yr−1 compared with values in other studies ranging from 6.08 to 12.16 cycles yr−1 (30–60 days). Overall, the modes appeared to be most sensitive to the model sea surface temperatures and the availability of moisture modulated by wind-driven feedbacks.

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Max J. Suarez and Dean G. Duffy

Abstract

When sufficiently large zonally asymmetric tropical heating is introduced in a two-level model of global atmospheric flow, its general circulation becomes strongly superrotating. The nature of the superrotating solutions is studied by examining momentum and heat budgets for a range of values of thermal forcing. Changes in the transport of zonal momentum by transient eddies appear to play the key role in the transition to superrotation. The dramatic bifurcation of the solutions of this model may help explain the maintenance and variability of the zonal mean flow in the tropics.

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Isacc M. Held and Max J. Suarez

Abstract

A useful but as yet under-utilized tool for climatic studies is an atmospheric model in which the time evolution of large-scale eddies is resolved explicitly, but in a relatively simple dynamical framework. One such model is described in detail in this study–a two-level primitive equation model on a sphere with variable static stability, finite-differenced in the meridional direction but Fourier analyzed and then very severely truncated in the zonal direction. Two versions of the model-moist and dry–are developed, the maintenance of the model's static stability being markedly different in the two versions.

Statistically steady states are obtained for a variety of spectral truncations For both versions of the model in order to determine the fewest zonal wavenumbers one can retain and still obtain a reasonable zonally averaged circulation. Including only one wave, of wavelength typical of strongly unstable waves in mid-latitudes, results in a circulation with a subpolar jet as well as a subtropical jet in the zonal wind. The addition of a longer wave (i.e., the addition of wavenumber 3 to wavenumber 6) results in the destruction of the subpolar jet.No further dramatic changes in the zonally averaged flow occur as more waves are added to the system.

Features of the model's dynamics which might limit its utility are emphasized, notably the dependence of the strength of the Hadley cell on the details of the convective adjustment scheme. We find, however, that the total energy transported by the Hadley cell is insensitive to such details.

Climatic sensitivity experiments with thee models will be described in forthcoming papers.

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Isaac M. Held and Max J. Suarez

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.

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