THE ROLE OF SHALLOW CONVECTION IN THE WATER AND ENERGY CYCLES OF THE ATMOSPHERE K. von Salzen , Norman A. McFarlane, and Michael Lazare Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, Victoria, British Columbia, V8W 2Y2, Canada 1. INTRODUCTION Shallow convection plays an important role in the global water and energy cycles of the atmosphere. Considerable fluxes of moisture at the top of the subtropical planetary boundary layer (PBL) are associated with updrafts in shallow cumulus clouds and compensating fluxes of dryer free tropospheric air in the vicinity of shallow cumuli (e.g. Riehl et al. 1951; Nitta and Esbensen 1974). It has been hypothesized that the release of latent heat through re-condensation of water vapour detrained from shallow cumulus clouds leads to important dynamical feedbacks between shallow and deep convection in the tropics (e.g. Johnson and Lin 1997). The main goal of this study is to determine the impacts of shallow convection on global water and energy cycles, with emphasis on the effects of shallow convection on clouds and radiation. A version of the fourth generation atmospheric general circulation model (hereafter referred to as AGCM4) that is currently under development at the Canadian Centre for Climate Modelling and Analysis (CCCma) is used in this study. A new parameterization of the bulk effects of transient shallow cumulus clouds (von Salzen and McFarlane 2002) is used to represent the effects of shallow convection in AGCM4. 2. MODEL DESCRIPTION 2.1 General The model version used in this study is based on CCCma AGCM3 (McFarlane et al., manuscript in preparation for Atmos.-Ocean). The spectral representation currently used in AGCM4 corresponds to a 47 wave triangularly truncated (T47) spherical harmonic expansion. The vertical region is spanned by 35 layers from the surface to about 1 hPa. The cloud microphysics scheme of Lohmann and Roeckner (1996), which accounts for cloud water and cloud ice as separate prognostic variables, is used. The scheme has been modified to include the semi- _________________________________________ Corresponding author’s address: Knut von Salzen, Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, University of Victoria, Box 1700 STN CSC, Victoria, BC, V8W 2Y2, Canada; E-Mail: knut.vonsalzen@ec.gc.ca. empirical cloudiness parameterization of Xu and Randall (1996) and the parameterizations of autoconversion and accretion by Khairoutdinov and Kogan (2000). An interactive sulphur cycle is included in the model based on the approach by Lohmann et al. (1999). In the model, sulphate aerosols affect clear-sky radiative transfer directly and indirectly via the first and second indirect effect in stratiform clouds. Radiation calculations in AGCM4 are based on the correlated k-distribution method for gaseous transmission (Li and Barker 2002). The radiative effects of cloud overlap, cloud infrared scattering, and cloud subgrid-scale variability (Barker 1996; Li and Barker 2002) are also included in the model. 2.2 Deep Convection The cumulus parameterization of Zhang and McFarlane (1995) is used to represent the effects of deep convection (hereafter denoted by ZM) in the model. The ZM-parameterization is a bulk mass flux scheme which includes a representation of convective scale motions. It is designed to account for the effects of penetrative convection and downdrafts produced by evaporation of rain. As a modification of the original approach, the ZM-parameterization in AGCM4 is applied only to cumulus cloud ensembles with maximum cloud top heights above the ambient freezing level as predicted by the parameterization. This effectively limits the application of the parameterization to cumulonimbus and cumulus congestus types of clouds. The closure condition for deep convection is that the convective available potential energy (CAPE) is consumed at an exponential rate by the cumulus clouds. 2.3 Shallow Convection The effects of shallow convection are parameterized following von Salzen and McFarlane (2002). In the parameterization, parcels of air are lifted from the PBL into the layer above the PBL. Shallow cumulus clouds are formed once the parcels reach the level of free convection (LFC), at which the parcels become positively buoyant. Above the LFC, the parcels are modified by entrainment of environmental air into the ascending top of the cloud and also by organized entrainment at the lateral boundaries of the cloud. The