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John F. Scinocca

Abstract

An accurate method of solution is developed for the Warner and McIntyre parameterization of nonorographic gravity wave drag for the case of hydrostatic dynamics in the absence of rotation. The new scheme is sufficiently fast that it is suitable for operational use in a general circulation model. Multiyear climate runs of the Canadian Middle Atmosphere Model are performed using the new scheme, and these are compared against equivalent climate runs using the full Warner and McIntyre scheme, which includes nonhydrostatic and rotational wave dynamics. The results indicate that the new operational scheme closely reproduces the seasonal distribution of time- and zonal-mean zonal winds, and the seasonal evolution of lower stratospheric temperatures that are obtained when the full Warner and McIntyre scheme is used. In addition, these quantities are shown to compare favorably with observations.

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Charles McLandress
and
John F. Scinocca

Abstract

A comparison is undertaken of the response of a general circulation model (GCM) to the nonorographic gravity wave drag parameterizations of Hines, Warner and McIntyre, and Alexander and Dunkerton. The analysis is restricted to a comparison of each parameterization’s nonlinear dissipation mechanism since, in principle, this is the only component that differs between the schemes. This is achieved by developing a new, more general parameterization that can represent each of these dissipation mechanisms, while keeping all other aspects of the problem identical.

The GCM simulations reveal differences in the climatological response to the three dissipation mechanisms. These differences are documented for both tropopause and surface launch elevations of the parameterized waves. The simulations also reveal systematic differences in the height at which momentum is deposited. This behavior is investigated further in a set of experiments designed to reduce these systematic differences, while leaving the details of the dissipation mechanisms unaltered. These sensitivity experiments demonstrate that it is possible to obtain nearly identical responses from all three mechanisms, which indicates that the GCM response is largely insensitive to the precise details of the dissipation mechanisms. This finding is supported by an additional experiment in which the nonlinear dissipation mechanisms are turned off and critical-level filtering is left to act as the only source of dissipation. In this experiment, critical-level filtering effectively replaces the nonlinear dissipation mechanism, producing a nearly identical response.

The results of this study suggest that climate modeling efforts would potentially benefit more from the refinement of other aspects of the parameterization problem, such as the properties of the launch spectrum, than they have benefited from the refinement of dissipation mechanisms.

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Michael Sigmond
and
John F. Scinocca

Abstract

Employing a comprehensive atmospheric general circulation model, the authors have shown in a previous study that the time-mean Northern Hemisphere (NH) winter circulation response to a CO2 doubling perturbation depends significantly on parameterized orographic gravity wave drag (OGWD) parameter settings, which are essentially related to the strength of OGWD. A possible implication is that aspects of the greenhouse gas–induced circulation response could depend directly on the formulation and internal parameters settings of the OGWD scheme. Such a result would further heighten the importance of OGWD parameterizations for climate studies and have far-reaching implications for modeled projections of future climate change.

In this study the causal relationship between OGWD and changes in time-mean NH wintertime circulation response to CO2 doubling is investigated. This is accomplished by introducing a methodology that allows one to hold the OGWD forcing fixed to its 1 × CO2 value when CO2 is doubled. Employing this methodology for perturbation experiments with different strengths of OGWD, the authors find that the changes in OGWD forcing due to CO2 doubling have essentially no impact on the time-mean zonal-mean zonal wind response. The primary conclusion is that the OGWD influence is limited to its impact on the 1 × CO2 basic-state climatology, which defines the propagation characteristics of resolved waves. Different strengths of OGWD result in control basic states with different refractive properties for the resolved waves. It is shown that the action of resolved waves, as well as their sensitivity to such differences in the control climatology, explains essentially all of the NH wintertime circulation sensitivity identified here and in a previous study. Implications for climate change projections and climate-model development are discussed.

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Alexander Hasha
,
Oliver Bühler
, and
John Scinocca

Abstract

Operational gravity wave parameterization schemes in GCMs are columnar; that is, they are based on a ray-tracing model for gravity wave propagation that neglects horizontal propagation as well as refraction by horizontally inhomogeneous basic flows. Despite the enormous conceptual and numerical simplifications that these approximations provide, it has never been clearly established whether horizontal propagation and refraction are indeed negligible for atmospheric climate dynamics. In this study, a three-dimensional ray-tracing scheme for internal gravity waves that allows wave refraction and horizontal propagation in spherical geometry is formulated. Various issues to do with three-dimensional wave dynamics and wave–mean interactions are discussed, and then the scheme is applied to offline computations using GCM data and launch spectra provided by an operational columnar gravity wave parameterization scheme for topographic waves. This allows for side-by-side testing and evaluation of momentum fluxes in the new scheme against those of the parameterization scheme. In particular, the wave-induced vertical flux of angular momentum is computed and compared with the predictions of the columnar parameterization scheme. Consistent with a scaling argument, significant changes in the angular momentum flux due to three-dimensional refraction and horizontal propagation are confined to waves near the inertial frequency.

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Oliver Bühler
,
Alexander Hasha
, and
John Scinocca
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John F. Scinocca
and
Bruce R. Sutherland

Abstract

A new effect related to the evaluation of momentum deposition in conventional parameterizations of orographic gravity wave drag (GWD) is considered. The effect takes the form of an adjustment to the basic-state wind about which steady-state wave solutions are constructed. The adjustment is conservative and follows from wave–mean flow theory associated with wave transience at the leading edge of the wave train, which sets up the steady solution assumed in such parameterizations. This has been referred to as “self-acceleration” and it is shown to induce a systematic lowering of the elevation of momentum deposition, which depends quadratically on the amplitude of the wave. An expression for the leading-order impact of self-acceleration is derived in terms of a reduction of the critical inverse Froude number Fc , which determines the onset of wave breaking for upwardly propagating waves in orographic GWD schemes. In such schemes Fc is a central tuning parameter and typical values are generally smaller than anticipated from conventional wave theory. Here it is suggested that self-acceleration may provide some of the explanation for why such small values of Fc are required. The impact of Fc on present-day climate is illustrated by simulations of the Canadian Middle Atmosphere Model.

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John F. Scinocca
and
Norman A. McFarlane

Abstract

This paper investigates the temporal properties of tropical precipitation in the Canadian Centre for Climate Modelling and Analysis (CCCma) third-generation atmospheric general circulation model (AGCM3). AGCM3 employs the penetrative mass-flux (PMF) scheme of Zhang and McFarlane (ZM) for the parameterization of deep cumulus convection. It is found that the temporal variability of the ZM scheme is sensitive to a number of its internal parameters as well to the use of a prognostic, rather than diagnostic, closure condition for the cloud-base mass flux. Sensitivity experiments suggest that the ZM scheme can produce realistic amounts of variability when compared to direct radar observations of deep cumulus convection in the Tropics.

A central finding of this study is that the resolved large-scale stratiform precipitation (LSP) in the model can participate in the modeling of deep latent heating and so compete with the ZM scheme in the Tropics. In modeling deep latent heating the LSP is found to mimic the behavior of a moist-convective adjustment scheme. In AGCM3 it is found that typical parameter settings of the ZM scheme place it in a regime in which the temporal variability of tropical precipitation is dominated by this behavior of the LSP, while the temporal mean is dominated by the ZM scheme. In such circumstances it is the LSP, and not the ZM scheme, that provides the primary source of resolved tropical Kelvin and mixed Rossby–gravity waves in the GCM. Such competition between LSP and the parameterization of deep convection appears to be active in other modeling studies. Consequently, it has the potential to complicate efforts to understand the nature of resolved tropical waves in GCMs and their role in the forcing of the quasi-biennial and semiannual oscillations.

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Andrew Orr
,
Peter Bechtold
,
John Scinocca
,
Manfred Ern
, and
Marta Janiskova

Abstract

In model cycle 35r3 (Cy35r3) of the ECMWF Integrated Forecast System (IFS), the momentum deposition from small-scale nonorographic gravity waves is parameterized by the Scinocca scheme, which uses hydrostatic nonrotational wave dynamics to describe the vertical evolution of a broad, constant, and isotropic spectrum of gravity waves emanating from the troposphere. The Cy35r3 middle atmosphere climate shows the following: (i) an improved representation of the zonal-mean circulation and temperature structure; (ii) a realistic parameterized gravity wave drag; (iii) a reasonable stationary planetary wave structure and stationary wave driving in July and an underestimate of the generation of stationary wave activity in the troposphere and stationary wave driving in January; (iv) an improved representation of the tropical variability of the stratospheric circulation, although the westerly phase of the semiannual oscillation is missing; and (v) a realistic horizontal distribution of momentum flux in the stratosphere. By contrast, the middle atmosphere climate is much too close to radiative equilibrium when the Scinocca scheme is replaced by Rayleigh friction, which was the standard method of parameterizing the effects of nonorographic gravity waves in the IFS prior to Cy35r3. Finally, there is a reduction in Cy35r3 short-range high-resolution forecast error in the upper stratosphere.

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Annelize van Niekerk
,
John F. Scinocca
, and
Theodore G. Shepherd

Abstract

The parameterization of orographic drag processes in atmospheric models remains uncertain because of a lack of observational and theoretical constraints on their formulation and free parameters. While previous studies have demonstrated that parameterized orographic drag acting near the surface has a significant impact on the atmospheric circulation, this work follows a more systematic approach to investigate its impacts on the large-scale circulation and the circulation response to climate change. A set of experiments with a comprehensive atmospheric general circulation model is used to ascertain the range of climatological circulations that may arise from parameter uncertainty. It is found that the Northern Hemisphere (NH) wintertime stationary wave field is strongly damped over the North Pacific (NP) and amplified over the North Atlantic (NA) as a result of increased low-level parameterized orographic drag, both of which are shown to be conducive to higher-latitude westerlies. A comparison with the stationary wave field presented in other studies suggests that the too-zonal NA jet and equatorward NP jet biases that are prevalent in climate models may be at least partly due to their representation of orographic drag. The amplitude of the stationary wave response to climate change across the experiments is shown to scale with the magnitude of low-level parameterized orographic drag through its influence on the present-day climatological stationary wave amplitudes over different sectors of the NH, which is consistent with linear stationary wave theory. This work highlights the importance of fidelity in a model’s basic state for regional climate change projections.

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James A. Anstey
,
Theodore G. Shepherd
, and
John F. Scinocca

Abstract

The interannual variability of the stratospheric polar vortex during winter in both hemispheres is observed to correlate strongly with the phase of the quasi-biennial oscillation (QBO) in tropical stratospheric winds. It follows that the lack of a spontaneously generated QBO in most atmospheric general circulation models (AGCMs) adversely affects the nature of polar variability in such models. This study examines QBO–vortex coupling in an AGCM in which a QBO is spontaneously induced by resolved and parameterized waves.

The QBO–vortex coupling in the AGCM compares favorably to that seen in reanalysis data [from the 40-yr ECMWF Re-Analysis (ERA-40)], provided that careful attention is given to the definition of QBO phase. A phase angle representation of the QBO is employed that is based on the two leading empirical orthogonal functions of equatorial zonal wind vertical profiles. This yields a QBO phase that serves as a proxy for the vertical structure of equatorial winds over the whole depth of the stratosphere and thus provides a means of subsampling the data to select QBO phases with similar vertical profiles of equatorial zonal wind. Using this subsampling, it is found that the QBO phase that induces the strongest polar vortex response in early winter differs from that which induces the strongest late-winter vortex response. This is true in both hemispheres and for both the AGCM and ERA-40.

It follows that the strength and timing of QBO influence on the vortex may be affected by the partial seasonal synchronization of QBO phase transitions that occurs both in observations and in the model. This provides a mechanism by which changes in the strength of QBO–vortex correlations may exhibit variability on decadal time scales. In the model, such behavior occurs in the absence of external forcings or interannual variations in sea surface temperatures.

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