Abstract A new semi‐empirical turbulence parameterization is presented. Key features of the scheme include representation of turbulent diffusivities in terms of the turbulent kinetic energy that is determined by solving a quasi‐equilibrium form of the equation representing the turbulent kinetic energy budget. The new parameterization is innovative in the treatment of turbulent transfer in stably stratified conditions and the representation of nonlocal contributions to the vertical transport of heat, moisture, and scalar prognostic variables in convectively active boundary layers. A key element in the modeling of turbulence in stably stratified conditions is the formulation of the turbulent Prandtl number based on the results of recently published theoretical, modeling, and observational studies of stratified turbulence in the atmospheric boundary layer. The new parameterization has been implemented in the CanAM4 single column model. Its performance in comparison with that of the operational CanAM4 turbulence parameterization is documented in terms of selected results from case studies for clear‐sky conditions based on meteorological observations from the KNMI‐mast at Cabauw, Netherlands, and the Second Dynamics and Chemistry of Marine Stratocumulus case study of stratocumulus‐topped marine boundary layers. The performance of the new and operational schemes is qualitatively similar in clear‐sky conditions in both convective and stable boundary layer regimes. However, they perform differently for the extended simulations for the Second Dynamics and Chemistry of Marine Stratocumulus case study. The new scheme maintains an elevated stratocumulus layer throughout a 30‐hr simulation, but peak liquid water contents are larger than large eddy simulations.

It was shown by Craig and Cohen that fluctuations of cumulus clouds under homogeneous large-scale forcing satisfy the Gibbs canonical ensemble in a strict radiative-convective equilibrium (RCE). In the limit of random noninteracting convective cells, an analytical expression for the distribution function of total mass flux over a region of given size was derived.The authors examine the consistency of the Gibbs canonical ensemble as a representation for the mass flux fluctuations when the large-scale forcing is time dependent. A cloud-resolving simulation (CRM) with interactive radiation, fixed imposed surface temperature, and diurnally varying solar forcing to mimic the diurnal cycle over the tropical ocean is used.As a necessary condition for the existence of a state of quasi-equilibrium, the time-scale separation between convective processes and forcing is studied. Detailed evaluation of time scales of convective adjustment and memory in a three-month run confirms the hypothesis of time-scale separation.The Craig and Cohen theory, in a varying range of heights between the cloud base up to the level of neutral buoyancy (LNB), is tested. It is shown that, although the theory is capable of reproducing the qualitative features of the variability, systematic deviations are detected. By quantifying the spatial distribution of the clouds, the authors suggest that deviations are associated with clustering effects.

The Canadian Centre for Climate Modelling and Analysis third generation atmospheric general circulation model (AGCM3) is described. The discussion summarizes the details of the complete physics package emphasizing the changes made relative to the second generation version of the model. AGCM3 is the underlying model for applications which include the IPCC fourth assessment, coupled atmosphere-ocean seasonal forecasting, the first generation of the CCCma earth system model (CanESM1), and middle-atmosphere chemistry-climate modelling (CCM). Here we shall focus on issues related to an upwardly extended version of AGCM3, the Canadian Middle-Atmosphere Model (CMAM). The CCM version of CMAM participated in the 2006 WMO/UNEP Scientific Assessment of Ozone Depletion and issues concerning its climate such as the impact of gravity-wave drag, the modelling of a spontaneous QBO, and the seasonality of the breakdown of the Southern Hemisphere polar vortex are discussed here.

Quantifying the combined effects of ozone depletion and changes in other greenhouse gases and radiatively and chemically active atmospheric constituents is of great importance to human welfare. Achieving this goal requires evaluating the impact of stratospheric ozone depletion and its recovery on the tropospheric climate as well as elucidating the effects of climate change on the evolution of ozone itself. This, in turn, requires the understanding and quantification of the long-term sensitivity of the climate system to significant perturbations in the radiation budget of the atmosphere associated with human activities. Detecting and quantifying such effects also requires a quantitative understanding of the role of natural events, such as volcanoes and solar variability, on the composition and evolution of the atmosphere and ultimately of the effects of such events on the climate change signal throughout the active atmosphere and at the surface.With the advent of climate modelling as a key tool for studying and predicting the evolution of the climate system, the linked concepts of radiative forcing and climate sensitivity have come into wide use as a means to understand and quantify key aspects of modelling results. These concepts are reviewed and their relevance to understanding and quantifying the radiative impact of ozone and ozone depleting substances on the radiative forcing of the climate system are discussed.Recent observational, theoretical, and modelling studies have revealed many new features of stratosphere-troposphere coupling that are relevant to understanding the role of stratospheric processes in climate variability and change. Aspects of these studies are reviewed briefly.

The sensitivity of the atmospheric radiation budget to ignoring small ice particles (D <= 100 mu m) in parameterization of the mean effective size of ice particles was investigated by using the Canadian Centre for Climate Modelling and Analysis (CCCma) third-generation general atmospheric circulation model (AGCM3). The results indicate that small ice particles play two crucial roles in the radiative transfer that influence the simulated climate. First, they inhibit the IR radiation from escaping to space and, second, they enhance the scattering of solar radiation. On average, these two effects tend to partially cancel each other out. However, based on AGCM simulations, the small ice crystals make clouds more opaque to IR radiation. Generally, 5-yr seasonally averaged GCM results suggest that the strongest anomalies in outgoing longwave radiation (OLR) are found in the Tropics, reaching 15 to 25 W m(-2) in areas where cold high cirrus anvil clouds are prevalent. The global average change in net cloud radiative forcing was 2.4 W m(-2) in June-August (JJA) and 1.7 W m(-2) in December-February (DJF). The change in globally averaged 5-yr mean cloud forcing was close to 1.9 W m(-2). When the small particles were included, the globally averaged 5-yr mean precipitation decreased by about 8%, but cloudiness increased only slightly (by 2%). The 5-yr averaged global mean surface (screen) temperature also increased slightly (about 0.2 degrees C) when the small ice particles were included.

P. J. Kushner, University of Toronto, Canada (paul.kushner@utoronto.ca) J. Austin, NOAA GFDL, USA (John.Austin@noaa.gov) M. P. Baldwin, NWRA, USA (mark@nwra.com) N. Butchart, UK Met Office, UK (neal.butchart@metoffice.gov.uk) M. A. Giorgetta, MPI for Meteorology, Germany (marco.giorgetta@zmaw.de) P. H. Haynes, DAMTP, University of Cambridge, UK (P.H.Haynes@damtp.cam.ac.uk) E. Manzini, CMCC/INGV, Italy (manzini@bo.ingv.it) N. A. McFarlane, SPARC IPO, Canada (norm.mcfarlane@ec.gc.ca) A. O’Neill, University of Reading, UK (alan@met.reading.ac.uk) J. Perlwitz, University of Colorado, USA (judith.perlwitz@noaa.gov) L. M. Polvani, APAM and EESC, Columbia University, USA (lmp@columbia.edu) W. A. Robinson, NSF, USA (robinson@atmos.uiuc.edu) F. Sassi, NCAR, USA (sassi@ucar.edu) J. F. Scinocca, CCCma, Canada (john.scinocca@ec.gc.ca) T. G. Shepherd, University of Toronto, Canada (tgs@atmosp.physics.utoronto.ca)

The extended Canadian Middle Atmosphere Model is used to investigate the large‐scale dynamics of the mesosphere and lower thermosphere (MLT). It is shown that the 4‐day wave is substantially amplified in southern polar winter in the presence of instabilities arising from strong vertical shears in the MLT zonal mean zonal winds brought about by parameterized nonorographic gravity wave drag. A weaker 4‐day wave in northern polar winter is attributed to the weaker wind shears that result from weaker parameterized wave drag. The 2‐day wave also exhibits a strong dependence on zonal wind shears, in agreement with previous modeling studies. In the equatorial upper mesosphere, the migrating diurnal tide provides most of the resolved westward wave forcing, which varies semiannually in conjunction with the tide itself; resolved forcing by eastward traveling disturbances is dominated by smaller scales. Nonmigrating tides and other planetary‐scale waves play only a minor role in the zonal mean zonal momentum budget in the tropics at these heights. Resolved waves are shown to play a significant role in the zonal mean meridional momentum budget in the MLT, impacting significantly on gradient wind balance. Balance fails at low latitudes as a result of a strong Reynolds stress associated with the migrating diurnal tide, an effect which is most pronounced at equinox when the tide is strongest. Resolved and parameterized waves account for most of the imbalance at higher latitudes in summer. This results in the gradient wind underestimating the actual eastward wind reversal by up to 40%.

The Canadian Middle Atmosphère Modelling (MAM) project is a collaboration between thé Atmospheric Environment Service (AES) of Environment Canada and several Canadian universities. Its goal is thé development of a comprehensive General Circulation Model of thé troposphere-stratosphere-mesosphere System, starting from thé AES/CCCma third-génération atmospheric General Circulation Model. This paper describes thé basic features of thé first-generation Canadian MAM and some aspects ofits radiative-dynamical climatology. Standard first-order mean diagnostics are presentedfor monthly means and for thé annual cycle ofzonal-mean winds and températures. The mean méridional circulation is examined, and comparison is made between thé steady diabatic, downward controlled, and residual streamfunctions. It isfound thaï downward contrat holds quite well in thé monthly mean through most of thé middle atmosphère, even during equinoctal periods. The relative rôles of différent drag processes in determining thé mean downwelling over thé wintertime polar middle stratosphère is examined, and thé vertical structure of thé drag is quantified. RÉSUMÉ Le projet canadien de Modélisation de l'Atmosphère Moyenne (MAM) est une collaboration entre le Service de l'Environnement Atmosphérique (SEA) d'Environnement Canada et certaines universités canadiennes. Le but est de développer un modèle de circulation générale pour le domaine troposphère-stratosphère-mésosphère à partir du modèle canadien de circulation générale (SEA/CCCtna) de troisième génération. Cet article décrit les caractéristiques de base du MAM canadien de première génération et quelques aspects de sa climatologie radiative et dynamique. Les diagnostics standards du premier ordre sont présentés pour les moyennes mensuelles et le cycle annuel des vents zonaux moyens et des températures. La circulation méridionale moyenne du modèle est examinée et comparée avec la circulation diabolique stationnaire, résiduelle, et celle établie à partir des principes du contrôle vers le bas (downward control). Pour les moyennes mensuelles, la circulation du contrôle vers le bas correspond assez bien aux résultats du modèle dans l'ensemble de l'atmosphère moyenne et ce, même durant les êquinoxes. Le rôle relatif des différents ATMOSPHERE-OCEAN 35 (3) 1997, 293-331 0705-5900/97/0000-0293$ 1.25/0 © Canadian Meteorological and Océanographie Society 294 / S.R. Beagley et al. mécanismes de résistance qui déterminent la subsidence au cours de l'hiver dans les régions polaire, est examiné et la structure verticale de ces processus est quantifiée.

A global, three-dimensional climate model, developed by coupling the CCCma second-generation atmospheric general circulation model (GCM2) to a version of the GFDL modular ocean model (MOM1), forms the basis for extended simulations of past, current and projected future climate. The spin-up and coupling procedures are described, as is the resulting climate based on a 200 year model simulation with constant atmospheric composition and external forcing. The simulated climate is systematically compared to available observations in terms of mean climate quantities and their spatial patterns, temporal variability, and regional behavior. Such comparison demonstrates a generally successful reproduction of the broad features of mean climate quantities, albeit with local discrepancies. Variability is generally well-simulated over land, but somewhat underestimated in the tropical ocean and the extratropical storm-track regions. The modelled climate state shows only small trends, indicating a reasonable level of balance at the surface, which is achieved in part by the use of heat and freshwater flux adjustments. The control simulation provides a basis against which to compare simulated climate change due to historical and projected greenhouse gas and aerosol forcing as described in companion publications.

The Doppler spread parameterization is used to represent the effects of a background spectrum of non-orographic gravity waves in the Canadian Middle Atmosphere Model. Two versions of the Doppler spread parameterization are used, differing only in the range of the vertical wave number part of the input spectrum. Results from seasonal simulations made with combinations of Doppler spread and orographic gravity wave drag parameterizations are discussed and compared with those made with the basic version of the model which contains an orographic gravity wave drag parameterization but no explicit parameterization for non-orographic waves. Results obtained indicate that the Doppler spread parameterization brings about a number of improvements in simulations of the zonal mean structure of the middle atmosphere including closing of the mesospheric jets and reversal of the mean zonal winds above the mesopause in middle and high latitudes.

A new surface-flux parameterization is presented and its impact on climate simulations with the Canadian Centre for Climate Modelling and Analysis (CCCMA) general circulation model (GCM) is discussed. The parameterization is based on the Monin-Obukhov similarity theory using well established flux-profile relationships for the unstable conditions. However, recently proposed new relationships are used in stable conditions. The new formulation allows different roughness lengths for heat and momentum, and gives transfer coefficients that are in agreement with Monin-Obukhov similarity theory. It also includes a parameterization for the free-convective boundary layer, which often occurs over warm surfaces within light winds. In circumstances where the surface layer is not neutrally stratified the proposed flux parameterization yields surface transfer coefficients that are different from those resulting from the standard surface flux formulation used in the GCM. The most marked effects of implementing the new formulation in the GCM are found over land and adjacent oceanic regions in winter where significant differences are found in the surface heat and moisture fluxes and surface temperatures.

Simulation of regional climate by limited-area model coupled with global low-resolution model is becoming a standard approach to achieve high-resolution climate projections at a computationally affordable cost. A regional climate model (RCM) based on a state-of-the-art numerical formulation is under development at the Université du Québec à Montréal. A brief description of this Canadian RCM (CRCM) is given. The characteristics of its dynamical formulation and the processes involved in its physical parameterisation are described. Results of a short simulation show that CRCM develops fine-scale features of climatological interest.

The Canadian Climate Center (CCC) GCM has been modified to allow its use for studies in atmospheric chemistry. The initial experiments reported here have been run to test and allow sensitivity studies of the new transport module. The impact of different types of parameterization for the convective mixing have been studied based on the large scale evolution of Rn-222 and Pb-210. Preliminary results have shown that the use of a scheme, which mixes unstable columns over a very short time scale, produces a global distribution of lead that agrees in some aspects with observations. The local impact of different mixing schemes on a short lived tracer like the radon is very important.