Canadian Middle Atmosphere Model Research Articles

Stratospheric and tropospheric NO 2 observed by SCIAMACHY: first results

Observations from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) onboard Envisat are used to quantifying the vertical distribution of stratospheric nitrogen dioxide (11–41 km) from limb scattering geometry and the tropospheric column from limb–nadir matching. SCIAMACHY is validated with spatially and temporally coincident observations from Optical Spectrograph and Infrared Imager System (OSIRIS) and from Halogen Occultation Experiment (HALOE). Comparison with the Canadian Middle Atmosphere Model provides a further check of consistency. Errors in pointing are detected and corrected using the recently developed `spectral knee' technique. An instrumental artifact, presumably a tangent-height-dependent wavelength drift, is causing fine spectral structure but can be taken into account as a pseudo-absorber in the least squares fitting. Extending the fitting window to longer wavelengths than those currently employed by other optical satellite-borne NO 2 sensors allows for the retrieval to penetrate the lower stratosphere. This extended fitting window is used to retrieve lower stratospheric NO 2 in the denoxified Antarctic polar vortex. Finally, tropospheric NO 2 columns, retrieved from limb–nadir matching, are presented.

Correlations of long‐lived chemical species in a middle atmosphere general circulation model

Correlations between various chemical species simulated by the Canadian Middle Atmosphere Model, a general circulation model with fully interactive chemistry, are considered in order to investigate the general conditions under which compact correlations can be expected to form. At the same time, the analysis serves to validate the model. The results are compared to previous work on this subject, both from theoretical studies and from atmospheric measurements made from space and from aircraft. The results highlight the importance of having a data set with good spatial coverage when working with correlations and provide a background against which the compactness of correlations obtained from atmospheric measurements can be confirmed. It is shown that for long‐lived species, distinct correlations are found in the model in the tropics, the extratropics, and the Antarctic winter vortex. Under these conditions, sparse sampling such as arises from occultation instruments is nevertheless suitable to define a chemical correlation within each region even from a single day of measurements, provided a sufficient range of mixing ratio values is sampled. In practice, this means a large vertical extent, though the requirements are less stringent at more poleward latitudes.

Planetary-Wave-Induced Transport in the Stratosphere

In the stratosphere, chemical tracers are drawn systematically from the equator to the pole. This observed Brewer–Dobson circulation is driven by wave drag, which in the stratosphere arises mainly from the breaking and dissipation of planetary-scale Rossby waves. While the overall sense of the circulation follows from fundamental physical principles, a quantitative theoretical understanding of the connection between wave drag and Lagrangian transport is limited to linear, small-amplitude waves. However, planetary waves in the stratosphere generally grow to a large amplitude and break in a strongly nonlinear fashion. This paper addresses the connection between stratospheric wave drag and Lagrangian transport in the presence of strong nonlinearity, using a mechanistic three-dimensional primitive equations model together with offline particle advection. Attention is deliberately focused on a weak forcing regime, such that sudden warmings do not occur and a quasi-steady state is reached, in order to examine this question in the cleanest possible context. Wave drag is directly linked to the transformed Eulerian mean (TEM) circulation, which is often used as a surrogate for mean Lagrangian motion. The results show that the correspondence between the TEM and mean Lagrangian velocities is quantitatively excellent in regions of linear, nonbreaking waves (i.e., outside the surf zone), where streamlines are not closed. Within the surf zone, where streamlines are closed and meridional particle displacements are large, the agreement between the vertical components of the two velocity fields is still remarkably good, especially wherever particle paths are coherent so that diabatic dispersion is minimized. However, in this region the meridional mean Lagrangian velocity bears little relation to the meridional TEM velocity, and reflects more the kinematics of mixing within and across the edges of the surf zone. The results from the mechanistic model are compared with those from the Canadian Middle Atmosphere Model to test the robustness of the conclusions.

An Accurate Spectral Nonorographic Gravity Wave Drag Parameterization for General Circulation Models

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.

Thermal effects of saturating gravity waves in the atmosphere

Breaking/saturating gravity waves (GWs) not only exert drag on the mean flow due to their momentum deposition but also affect the background thermally because of the associated energy flux divergence. We present a rigorous derivation of terms describing the thermal effects of GWs on the mean flow, based on the corresponding energy cycle for wave/mean flow interactions. The combined effect of saturating GWs is to produce both differential heating and cooling by inducing a downward wave heat flux, and an irreversible conversion of wave energy into heat. The former effect can also be represented as thermal diffusion acting on the mean potential temperature gradient. This rigorous theory for the thermal exchange between waves and the mean flow can be closed once the mechanism for GW dissipation is parameterized. To illustrate the procedure, we employ our recent nonlinear theory of GW spectra to derive expressions for the wave‐induced heating rates. This yields a parameterization of the thermal effects of GWs, which is suitable for use in general circulation models and requires the source GW spectrum as the only tunable parameter. We present results of numerical calculations of wave heating terms for typical wind and temperature profiles as well as simulations with the full‐scale Canadian Middle Atmosphere Model (CMAM).

Seasonal variations of the semi-diurnal and diurnal tides in the MLT: multi-year MF radar observations from 2–70° N, modelled tides (GSWM, CMAM)

Abstract. In an earlier paper (Manson et al., 1999a) tidal data (1990–1997) from six Medium Frequency Radars (MFR) were compared with the Global Scale Wave Model (GSWM, original 1995 version). The radars are located between the equator and high northern latitudes: Christmas Island (2° N), Hawaii (22° N), Urbana (40° N), London (43° N), Saskatoon (52° N) and Tromsø (70° N). Common harmonic analysis was applied, to ensure consistency of amplitudes and phases in the 75–95 km height range. For the diurnal tide, seasonal agreements between observations and model were excellent while for the semi-diurnal tide the seasonal transitions between clear solstitial states were less well captured by the model. Here the data set is increased by the addition of two locations in the Pacific-North American sector: Yamagawa 31° N, and Wakkanai 45° N. The GSWM model has undergone two additional developments (1998, 2000) to include an improved gravity wave (GW) stress parameterization, background winds from UARS systems and monthly tidal forcing for better characterization of seasonal change. The other model, the Canadian Middle Atmosphere Model (CMAM) which is a General Circulation Model, provides internally generated forcing (due to ozone and water vapour) for the tides. The two GSWM versions show distinct differences, with the 2000 version being either closer to, or further away from, the observations than the original 1995 version. CMAM provides results dependent upon the GW parameterization scheme inserted, but one of the schemes provides very useful tides, especially for the semi-diurnal component.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

Extended Canadian Middle Atmosphere Model: Zonal‐mean climatology and physical parameterizations

This paper describes the energetics and zonal‐mean state of the upward extension of the Canadian Middle Atmosphere Model, which extends from the ground to ∼210 km. The model includes realistic parameterizations of the major physical processes from the ground up to the lower thermosphere and exhibits a broad spectrum of geophysical variability. The rationale for the extended model is to examine the nature of the physical and dynamical processes in the mesosphere/lower thermosphere (MLT) region without the artificial effects of an imposed sponge layer which can modify the circulation in an unrealistic manner. The zonal‐mean distributions of temperature and zonal wind are found to be in reasonable agreement with observations in most parts of the model domain below ∼150 km. Analysis of the global‐average energy and momentum budgets reveals a balance between solar extreme ultraviolet heating and molecular diffusion and a thermally direct viscous meridional circulation above 130 km, with the viscosity coming from molecular diffusion and ion drag. Below 70 km, radiative equilibrium prevails in the global mean. In the MLT region between ∼70 and 120 km, many processes contribute to the global energy budget. At solstice, there is a thermally indirect meridional circulation driven mainly by parameterized nonorographic gravity‐wave drag. This circulation provides a net global cooling of up to 25 K d−1.

A comparison of mesospheric temperatures from the Canadian Middle Atmosphere Model and HALOE observations: Zonal mean and signature of the solar diurnal tide

Currently climate models are being extended to the middle atmosphere in order to investigate climate sensitivity in that region. This paper presents a comparison between the Canadian Middle Atmosphere Model (CMAM) climatology and 9 years of HALOE temperature measurements for the middle atmosphere. The analysis shows an overall agreement throughout the entire middle atmosphere with discrepancies within 10 K and less in various areas for both equinox and solstice conditions. For historical purposes, the analysis also includes a comparison with the CIRA‐86 reference model. In this case, the results indicate a significant CIRA‐86 warm anomaly in the upper mesosphere over the tropics which is particularly pronounced during equinox. Further analysis of the HALOE sunrise and sunset occultation measurements with the CMAM has revealed the signature of diurnal tides.

The Seasonal Variation of the Propagating Diurnal Tide in the Mesosphere and Lower Thermosphere. Part II: The Role of Tidal Heating and Zonal Mean Winds

A linear mechanistic tidal model is used to understand the mechanisms responsible for the seasonal variation of the propagating diurnal tide in the mesosphere and lower thermosphere simulated in the Canadian Middle Atmosphere Model (CMAM). The linear model uses a spectral approach to represent the horizontal structure of the tidal perturbations and employs dissipative processes that do not depend on season. By constraining the model with the zonal mean zonal winds, zonal mean temperatures, and tidal heating from the CMAM, the relative role of each of these terms is assessed. The linear model is able to reproduce all of the important tidal features found in the CMAM, in particular the semiannual amplitude variation in the lower thermosphere at low latitudes that is seen in observations. From this analysis the effects of both heating and mean winds are found to be responsible for the seasonal variation of the tidal amplitude, while variations in the tidal phase are attributed solely to changes in the mean winds. The strong sensitivity of the tide to the mean winds is the novel result of this study. This sensitivity is attributed to latitudinal shears in the zonal mean easterlies in the summer mesosphere. Although these shears occur on an annual basis, their impact on tidal amplitudes in the lower thermosphere is semiannual as a result of the 6-month shift in seasons between the two hemispheres. Simulations using observational datasets from the Committee on Space Research (COSPAR) International Reference Atmosphere (CIRA) and the High Resolution Doppler Imager (HRDI) reveal significant differences in the resulting tidal structure from that obtained using the CMAM winds, and point to possible deficiencies in these datasets.

The Seasonal Variation of the Propagating Diurnal Tide in the Mesosphere and Lower Thermosphere. Part I: The Role of Gravity Waves and Planetary Waves

Abstract The seasonal variation of the propagating diurnal tide in the mesosphere and lower thermosphere is examined using results from a 2-yr simulation of the extended version of the Canadian Middle Atmosphere Model (CMAM). The CMAM is shown to be able to reproduce not only the observed semiannual amplitude variation of the tide in the lower thermosphere but also more subtle features such as amplitude maxima that are stronger in March/April than in September/October, a 4- to 6-h shift in phase between winter and summer in the Northern Hemisphere, and a weaker seasonal variation of phase in the Southern Hemisphere. Part I of this two-part series of papers investigates the importance of two of the mechanisms that have been proposed to explain the observed variation of tidal amplitude, namely, 1) interactions with small-scale gravity waves and 2) interactions with planetary-scale waves like the quasi–2-day wave. Analysis of the tidal momentum and thermodynamic budgets shows that the direct effects of param...

The Effect of Back-Reflection in the Parameterization of Non-Orographic Gravity-Wave Drag.

An efficient method of solution is developed for the Warner and McIntyre parameterization of the drag associated with nonhydrostatic non-orographic inertia-gravity waves. The scheme is sufficiently fast as to enable, for the first time, fully interactive multi-year climate simulations that include the effects of rotation and nonhydrostatic wave dynamics in the parameterization of non-orographic gravity wave drag. It is found that the new scheme alleviates much of the middle-atmosphere wind biases that occur in the Canadian Middle Atmosphere Model when either of its two operational hydrostatic non-orographic gravity wave drag parameterizations are used. The addition of the nonhydrostatic process of back-reflection has an important impact on the amount momentum flux launched into the stratosphere by the parameterization scheme. This quantity is a free parameter in the problem and it is specified here to be independent of time and geographic location. However, due to back-reflection, the net momentum flux that actually enters the stratosphere undergoes a systematic seasonal and latitudinal variation which is a consequence of the seasonal and latitudinal variation of the winds and temperatures in the middle atmosphere. This strong influence results in a characteristic latitudinal distribution for the net momentum flux entering the stratosphere in winter and summer. During these seasons, mid- to high-latitude launch momentum flux that is directed oppositely to the mesospheric jet can be reduced by as much as 75% due to back-reflection. The momentum flux launched into the stratosphere at tropical latitudes, however, is relatively unaffected by back-reflection. This has important implications for the forcing of tropical oscillations such as the semi-annual oscillation and the quasi-biennial oscillation in general circulation models.

Issues in Stratosphere-troposphere Coupling.

An overview is given of current issues concerning the coupling between the stratosphere and troposphere. The tropopause region, more generally the upper troposphere/lower stratosphere, is the region of direct contact where exchange of material takes place. Dynamical coupling through angular momentum transfer by waves occurs nonlocally, and provides a generally negative torque on the stratosphere which drives an equator-to-pole circulation (i.e., towards the Earth’s axis of rotation). This wave-driven circulation is the principal mechanism for intraseasonal and interannual variability in the extratropical stratosphere. Although such variability is generally dynamical in origin, there are important chemical and radiative feedbacks. The location of the tropopause has implications for radiative forcing of climate, through its effect on the distribution of relatively short-lived greenhouse gases (ozone and water vapour). Some outstanding puzzles in our current understanding are identified. Attention is focused on possible climate sensitivities, and how these may be tested and constrained. Results from the Canadian Middle Atmosphere Model (CMAM), a fully interactive radiative-chemical-dynamical general circulation model, are used to illustrate some of the points.

Arctic polar vortex variability in the Canadian middle atmosphere model

Arctic polar vortex variability in the Canadian Middle Atmosphere Model (CMAM) is documented. Two integrations of the CMAM are compared to assess the effects of parametrized non‐orographic gravity waves on the frequency and timing of stratospheric sudden warmings (SSWs), as well as on the potential for polar stratospheric cloud (PSC) formation. The first integration does not include parametrized non‐orographic gravity waves while the second integration does. The first integration captures the basic features of the observed northern hemisphere (NH) winter circulation although the polar night jet (PNJ) is significantly too weak and poleward shifted. Together with the weaker than observed PNJ are SSWs that occur too often and too early, resulting in an unrealistically low early to mid‐winter potential for PSC formation. Whereas, by design, the inclusion of parametrized non‐orographic gravity waves in the second integration significantly improves the southern hemisphere (SH) circulation, the strength of the NH PNJ is further reduced relative to the first integration, and the SSW frequency and PSC formation potential further degraded. Finally, the synoptic evolution and wavebreaking signature of the simulated and observed SSW events are compared through the application of a composite high resolution contour advection algorithm.

Gravity wave activity and dynamical effects in the middle atmosphere (60–90km): observations from an MF/MLT radar network, and results from the Canadian Middle Atmosphere Model (CMAM)

It has become increasingly clear that Gravity Waves (GW) have an essential and often dominant role in the dynamics of the Middle Atmosphere. This leads to them having strong impacts upon the thermal structure and the distribution of atmospheric constituents. However, the radar observations of GW have been limited in their latitudinal extent during the past decade, and although satellite observations are now significantly contributing, global-seasonal climatologies of important characteristics are still inadequate. With regard to models, the inclusion of GW-drag effects has been problematic. Usually no seasonal or latitudinal variation in the subgrid-scale GW-drag parameterization scheme is included, and varieties of parameterization schemes have been used. Although these often make conflicting assumptions, they generally produce similarly acceptable end-products, e.g. zonal-mean zonal wind fields. In this paper, we report upon the beginnings of a substantial program, using observations from a network of MF radars (North America, Pacific and Europe), and data from the Canadian Middle Atmosphere Model (CMAM). This model allows the tidal and planetary wave fields to be assessed, characteristics and climatologies of which are well known from the MF Radars. Here we focus upon the tides. There are useful similarities in the observed and modeled background wind and wave fields, and strong indications that the two non-orographic GW-drag parameterization schemes (Hines; Medvedev–Klaassen) have significant and differing effects upon the dynamics of the modeled atmosphere. It is shown that this comparison process is valuable in the evaluation, and potentially the optimization, of parameterization schemes.

Realistic semiannual oscillation simulated in a middle atmosphere general circulation model

Although several general circulation modelling studies have presented simulations containing a tropical middle atmosphere semiannual oscillation (6 month period), none so far have been able to reproduce the observed amplitude and vertical structure extending into the mesosphere. Here we present simulations with the Canadian Middle Atmosphere Model which utilize a nonlinear broad‐spectrum gravity‐wave parameterization [Medvedev and Klaassen, 1995, 2000] and exhibit a semiannual oscillation in good agreement with observations.

Ozone climatology using interactive chemistry: Results from the Canadian Middle Atmosphere Model

The climatology of ozone produced by the Canadian Middle Atmosphere Model (CMAM) is presented. This three‐dimensional global model incorporates the radiative feedbacks of ozone and water vapor calculated on‐line with a photochemical module. This module includes a comprehensive gas‐phase reaction set and a limited set of heterogeneous reactions to account for processes occurring on background sulphate aerosols. While transport is global, photochemistry is solved from about 400 hPa to the top of the model at ∼95 km. This approach provides a complete and comprehensive representation of transport, emission, and photochemistry of various constituents from the surface to the mesopause region. A comparison of model results with observations indicates that the ozone distribution and variability are in agreement with observations throughout most of the model domain. Column ozone annual variation is represented to within 5–10% of the observations except in the Southern Hemisphere for springtime high latitudes. The vertical ozone distribution is generally well represented by the model up to the mesopause region. Nevertheless, in the upper stratosphere, the model generally underestimates the amount of ozone as well as the latitudinal tilting of ozone isopleths at high latitude. Ozone variability is analyzed and compared with measurements. The comparison shows that the phase and amplitude of the seasonal variation as well as shorter timescale variations are well represented by the model at various latitudes and heights. Finally, the impact of incorporating ozone radiative feedback on the model climatology is isolated. It is found that the incorporation of ozone radiative feedback results in a cooling of ∼8 K in the summer stratopause region, which corrects a warm bias that results when climatological ozone is used.

The Extended Canadian Middle Atmosphere Model

First results from the extended Canadian middle atmosphere model are presented. The model extends from the surface to the middle thermosphere and includes relevant physical parameterizations. To simulate unresolved gravity waves a non‐orographic gravity wave drag parameterization has also been implemented. Results from a two‐year simulation are discussed and are shown to be in good qualitative agreement with upper atmosphere research satellite wind observations. The realistic features which are reproduced include the reversal of the mesospheric jets and the semi‐annual variation of the migrating diurnal tide in the low latitude mesosphere.

On the nature of large‐scale mixing in the stratosphere and mesosphere

Studies of tracer transport in the stratosphere have shown that adiabatic quasi‐horizontal tracer evolution is controlled primarily by the large‐scale low‐frequency component of the flow. This behavior is consistent with the concept of chaotic advection, wherein the Eulerian velocity field is spatially coherent and temporally quasi‐regular on timescales over which the Lagrangian evolution is chaotic. In this study, winds from a middle atmosphere general circulation model (the Canadian Middle Atmosphere Model) are used to compare and contrast the nature of tracer evolution in the stratosphere and mesosphere. It is found that the concept of chaotic advection is relevant in the stratosphere but not in the mesosphere. The explanation for this behavior is the increased strength of gravity wave activity in the mesosphere as compared with the stratosphere, which leads to shallower kinetic energy spectra on synoptic scales and a much shorter Eulerian correlation time. The shallower kinetic energy spectra imply that tracer evolution in the mesosphere is spectrally local, in contrast with the spectrally nonlocal regime that prevails in the stratosphere. This means that tracer advection calculations in the mesosphere are controlled primarily by the gravity wave spectrum and are intrinsically resolution dependent.

An introduction to stratospheric chemistry: Survey article

This paper is an expansion of the lectures presented by the authors at the Canadian Middle Atmosphere Model (CMAM) summer school, 25–29 August 1997. It attempts to fulfill several goals as did the curriculum for the summer school. Firstly, it has a large tutorial component allowing newcomers a rapid introduction to the field, secondly it surveys the field of stratospheric chemistry, and thirdly it attempts to bring the reader to the forefront of some of the problem areas in stratospheric chemistry. As such, it includes much background information such as the magnitude of rate coefficients, photolysis rates and their calculation and time constants. The intended audience is individuals about to commence study or research in the field of atmospheric chemistry or established atmospheric scientists in other areas wishing to expand their knowledge of this topic. The basic chemistry of the ozone layer is described with an emphasis on the catalytic cycles and the equilibria between “reservoir” and “active” species. To this end, a compendium of the important aspects of heterogeneous chemistry is also provided. The application of box (0‐dimensional) and more elaborate (1‐, 2‐ and 3‐dimensional) models to compare and validate with measurements is discussed. Also, factors affecting the present and future trends in ozone concentration, both at mid‐latitudes and in the polar region, are presented.

On the importance of gravity waves in the middle atmosphere and their parameterization in general circulation models

Abstract This tutorial paper discusses the problem of parameterizing unresolved gravity waves in general circulation models (GCMs) of the middle atmosphere. For readers who are unfamiliar with middle atmosphere dynamics a review of the basic dynamics of both the large-scale circulation and internal gravity waves is presented. A fairly detailed and physically-based description is given of several gravity wave drag (GWD) schemes that are currently employed in middle atmosphere GCMs. These include the parameterizations of McFarlane (1987) , Medvedev and Klaassen (1995) , and Hines, 1997a , Hines, 1997b , which are used in the Canadian Middle Atmosphere Model, as well as the parameterization of Fritts and Lu (1993) , which is used in the TIME-GCM. Results from a mechanistic model and the two above mentioned GCMs are presented and discussed. This paper is not intended as a review of all GWD parameterizations nor is it meant as a quantitative comparison of the schemes that have been chosen.