[1] A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper [Gong et al., 2002].

The Mackenzie River is the largest North American source of freshwater for the Arctic Ocean. This basin is subjected to wide fluctuations in its climate and it is currently experiencing a pronounced warming trend. As a major Canadian contribution to the Global Energy and Water Cycle Experiment (GEWEX), the Mackenzie GEWEX Study (MAGS) is focusing on understanding and modeling the fluxes and reservoirs governing the flow of water and energy into and through the climate system of the Mackenzie River Basin. MAGS necessarily involves research into many atmospheric, land surface, and hydrological issues associated with cold climate systems. The overall objectives and scope of MAGS will be presented in this article.

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As a result of recent satellite-based observation programs, knowledge of the radiation budget at the top of the atmosphere has improved substantially. In comparison, there has been little improvement in knowledge of the radiation budgets at the surface and in the atmosphere. Based on a simple parameterization, global climatologies of the solar radiation budget at the surface and in the atmosphere are developed from 5 years of Earth Radiation Budget Experiment data and European Centre for Medium Range Weather Forecasts humidity data. Both data sets have global coverage on 2.5 ø x 2.5 ø grids. Global distributions of the solar radiation budget at the surface give maximum seasonal values of the net solar radiation for the subtropical oceans of more than 300 W m -2. The maximum seasonal absorption in the atmosphere is about 120 W m -2. Shortwave cloud forcing at the surface and in the atmosphere is also derived. Clouds reduce the seasonally averaged surface net solar radiation by up to 175 W m -2, whereas they increase seasonal net solar radiation in the atmosphere by less than 15 W m -2. The globally and annually averaged net solar radiation budgets in the atmosphere and at surface are 83 and 157 W m -2, respectively. Expressed as percentages of the solar radiation incident at the top of the atmosphere, these values correspond to a globally and annually averaged absorption in the atmosphere and at the surface of 24.3% and 46.0%, respectively, and a planetary albedo of 29.7%. 1. Paper number 93JD00003. 0148-0227/93/93 JD-00003 $05.00 major challenge in relating quantities measured from space to the quantities at the surface. Since the 1980s there has been great interest in exploiting the potential of such relationships. One of the goals of the World Climate Research Program is to derive the global surface radiation budget (SRB) from satellite measurements with a accuracy of 10 W m -2 for monthly mean fluxes averaged over regions of 250 x 250 km 2 [Suttles and Ohring, 1986]. There are still major difficulties in determining the longwave component of the surface radiation budget from space, but considerable progress has been made in determining the shortwave surface radiation budget (SSRB). Until recently, the majority of techniques derived the insolation at the surface, which is one component of the SSRB [Schmetz, 1989]. Global climatologies of surface insolation have been developed by Raschke et al. [1987] and Pinker and Laszlo [1992] from International Satellite Cloud Climatology Project (ISCCP) reduced data sets. To obtain the SSRB from insolation requires knowledge of surface albedo. This was the approach adopted by Raschke and Preuss [1979], who developed the first global measurements of solar radiation budgets in the atmosphere and at the surface from 1 month of NIMBUS 3 measurements. The accumulation of the errors in the estimation of the insolation and albedo limits the usefulness of this approach. In an attempt to find a direct relation between the SSRB and TOA fluxes, Weare [1989] looked for correlations bet...

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