A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective-scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub-cloud layer, acts to remove CAPE at an exponential rate with a specified adjustment time scale. The performance of the scheme and its sensitivity to choices of disposable parameters is illustrated by presenting results from a series of idealized single-column model tests. These tests demonstrate that the scheme permits establishment of a quasi-equilibrium between large-scale forcing and convective response. However, it is also shown that the strength of convective downdrafts is an important factor in determining the nature of the equilibrium state. Relatively strong down-drafts give rise to an unsteady irregularly fluctuating state characterized by alternate periods of deep and shallow convection. The effect of using the scheme for GCM climate simulations is illustrated by presenting ATMOSPHERE-OCEAN 33 (3) 1995, 407^46 0705-5900/95/0000-0407501.25/0 © Canadian Meteorological and Oceanographic Society 408 / G.J. Zhang and N.A. McFarlane selected results of a multi-year simulation carried oui with thé Canadian Climate Centre GCM using thé new parameterization (thé CONV simulation). Comparison of thèse results with thosefor a climate simulation mode with thé standard model (thé CONTROL simulation, as documented by McFarlane et al., 1992) reveals thé importance of other parameterized processes in determining thé ultimate effect of introducing thé new convective scheme. The radiative response to changes in thé cloudiness régime is particularly important in this regard. RÉSUMÉ On présente un schéma simplifié de paramétrisation des Cumulus, utilisable dans les MCG. La paramétrisation est basée sur un concept de panache semblable à celui proposé par Arakawa et Schubert (1974). Toutefois, il utilise trois hypothèses qui simplifient significativement la formulation et la mise en oeuvre du schéma. On suppose qu 'un ensemble de courants ascendants à l'échelle convective et les courants ascendants saturés associés sont possibles lorsque l'atmosphère est localement conditionnellement instable dans la basse troposphère. L'ensemble des courants ascendants ne comprend cependant que les panaches qui ont suffi...

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In the companion to the present paper, the soil model associated with CLASS (Canadian Land Surface Scheme) was outlined. In this paper, the accompanying vegetation model is described. This model includes physically based treatment of energy and moisture fluxes from the canopy as well as radiation and precipitation cascades through it, and incorporates explicit thermal separation of the vegetation from the underlying ground. Seasonal variations of canopy parameters are accounted for. The morphological characteristics of the 'composite canopy' associated with each grid square are calculated as weighted averages over the vegetation types present. Each grid square is divided into a maximum of four separate subareas: bare soil, snow-covered, vegetation-covered, and snow-and-vegetation covered.

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