Search Results

You are looking at 1 - 10 of 73 items for

  • Author or Editor: Max Suarez x
  • All content x
Clear All Modify Search
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.

Full access
Siegfried D. Schubert and Max Suarez

Abstract

Average predictability and error growth are studied in a realistic two-level general circulation model of the atmosphere via a series of Monte Carlo experiments for fixed external forcing (perpetual winter in the Northern Hemisphere). For realistic initial errors, the dependence of the limit of dynamic predictability on total wave number is similar to that found for the ECMWF model for 1980/81 winter conditions, with the lowest wavenumbers showing significant skill for forecast ranges of more than 1 month. For very small amplitude error (1.2 m rms height at 500 mb) distributed according to the climate spectrum, the total error growth is superexponential, reaching a maximum growth rate (2-day doubling time) in about 1 week.

A simple empirical model of error variance involving two broad wavenumber bands (large scales: n < 10 and small scales: 10 ≤ n ≤ 15), provides an excellent fit of the GCM's error growth behavior. The interpretation of the empirical model, based on an analogy with the stochastic dynamic equations developed by Epstein, suggests that the initial rapid increase in the growth rate of errors in the large scales is primarily due to interactions with the small-scale error. These interactions have preferred geographical locations associated with the position of the climate mean jet streams. However, the error growth of the small scales is large unaffected by the presence of the large-scale error. The initial strong growth rate (2-day doubling time) of the small scales is attributed to the model's high level of eddy activity.

Full access
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.

Full access
Randal D. Koster and Max J. Suarez

Abstract

Water balance considerations at the soil surface lead to an equation that relates the autocorrelation of soil moisture in climate models to 1) seasonality in the statistics of the atmospheric forcing, 2) the variation of evaporation with soil moisture, 3) the variation of runoff with soil moisture, and 4) correlation between the atmospheric forcing and antecedent soil moisture, as perhaps induced by land–atmosphere feedback. Geographical variations in the relative strengths of these factors, which can be established through analysis of model diagnostics, lead to geographical variations in simulated soil moisture memory. The use of the equation to characterize controls on soil moisture memory is demonstrated with data from the modeling system of the National Aeronautics and Space Administration Seasonal-to-Interannual Prediction Project.

Full access
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.

Full access
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.

Full access
Yehui Chang, Siegfried Schubert, and Max Suarez

Abstract

This study examines the cause of the extreme snowstorm activity along the U.S. East Coast during the winter of 2009/10 with a focus on the role of sea surface temperature (SST) anomalies. The study employs the Goddard Earth Observing System, version 5 (GEOS-5) atmospheric general circulation model (AGCM) run at high resolution and forced with specified observed or idealized SST. Comparisons are made with the winter of 1999/2000, a period that is characterized by SST anomalies that are largely of opposite sign.

When forced with observed SSTs, the AGCM response consists of a band of enhanced storminess extending from the central subtropical North Pacific, across the southern United States, across the North Atlantic, and across southern Eurasia, with reduced storminess to the north of these regions. Positive precipitation and cold temperature anomalies occur over the eastern United States, reflecting a propensity for enhanced snowstorm activity. Additional idealized SST experiments show that the anomalies over the United States are, to a large extent, driven by the ENSO-related Pacific SST. The North Atlantic SSTs contribute to the cooler temperatures along the East Coast of the United States, while the Indian Ocean SSTs act primarily to warm the central part of the country.

It is further shown that the observed upper-tropospheric height anomalies have a large noise (unforced) component over the Northern Hemisphere, represented over the North Atlantic by a North Atlantic Oscillation (NAO)-like structure. The signal-to-noise ratios of the temperature and precipitation fields nevertheless indicate a potential for predicting the unusual storm activity along the U.S. East Coast several months in advance.

Full access
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.

Full access
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.

Full access
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.

Full access