Henry G. Leighton

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

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

Estimation of SW Flux Absorbed at the Surface from TOA Reflected Flux

Abstract Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earths climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known. Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends stro...

Global climatologies of solar radiation budgets at the surface and in the atmosphere from 5 years of ERBE data

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° × 2.5° 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%.

The Mackenzie GEWEX Study: The Water and Energy Cycles of a Major North American River Basin

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.

A New Parameterization for the Determination of Solar Flux Absorbed at the Surface from Satellite Measurements

Abstract An earlier parameterization that relates the outgoing solar flux at the top of the atmosphere to the flux absorbed at the surface is modified and extended to allow for variations in atmospheric properties that were not considered in the original parameterization. Changes to the parameterization have also been introduced as a result of better treatment of water vapor absorption in the detailed radiative transfer calculations. Corrections are introduced that account for the height of the surface (surface pressure), ozone amount, aerosol type and amount, and cloud height and cloud type, which is characterized by an effective cloud droplet radius. Finally, the results of applying the parameterization to Earth Radiation Budget Satellite measurements are compared with the measurements of the net solar flux measured from two instrumented towers.

A Three-Dimensional Cloud Chemistry Model

Abstract A cloud chemistry model is formulated in term of continuity equations for chemical species in the aqueous and aqueous phases within the cloud. The model includes scavenging of SO2, HNO3, HN3, H2O3, and sulphate aerosol particles. Calculations have been performed within the framework of a three-dimensional convective cloud model. The results are compared with aircraft measurements of cloud water chemistry.

Tropospheric sulfur cycle in the Canadian general circulation model

Emission, transport, chemistry, and scavenging of the gaseous sulfur species dimethyl sulfide and sulfur dioxide (SO 2 ) and sulfate aerosols are calculated on-line with the meteorology in the general circulation model (GCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA). Additionally, prognostic equations for cloud water and cloud ice have been introduced. The sensitivity of this sulfur cycle to differences in GCM physics and dynamics has been studied by comparing the results to those obtained with the ECHAM GCM which has a very similar sulfur cycle and cloud scheme, but a different turbulent diffusion and convection scheme. The differences in the global mean burdens of SO 2 and sulfate are less than 2%. Simulated surface SO 2 concentrations with CCCMA in winter as well as the seasonal cycle are in better agreement with observations at several sites than those simulated with ECHAM because of stronger boundary layer mixing in CCCMA. The simulated surface SO 4 2- with CCCMA, however, is often higher than observed and in ECHAM. Additionally, sensitivity experiments showed that the global sulfur budgets are most sensitive to changes in the cloud cover parameterization and less sensitive to changes in pH calculation and oxidation of SO 2 in convective clouds. The results of the sensitivity experiments give evidence for the importance of all of these effects on the geographical and vertical distribution of sulfur and cloud liquid water.

Narrowband to Broadband Conversion with Spatially Autocorrelated Reflectance Measurements

Abstract A new technique for estimating broadband reflectance from Advanced Very High-Resolution Radiometer (AVHRR) narrowband reflectances in channel 1 and 2 is developed. The data used are simultaneous and coincident narrowband and broadband measurements made by the AVHRR and Earth Radiation Budget Experiment (ERBE) radiometers aboard NOAA-9 during four days in July 1985 in the region north of 60°N. The limitations and inefficiency of classical regressional methods when applied to datasets with high spatial auto-correlation, which is often the case for remotely sensed data, are discussed. A statistical variable, Morans I, is introduced, which is specifically designed for testing against a null hypothesis of spatial independence. On the basis of Morans I and a correlogram analysis of the spatial autocorrelation of measured reflectances, the data are sampled to provide a spatially independent dataset. In addition to sampling, the data are also screened with respect to spatial homogeneity. Both scene-dep...

Sensitivity of sulphate aerosol size distributions and CCN concentrations over North America to SO x emissions and H2O2 concentrations

To assess the influence of aerosols on climate, the Northern Aerosol Regional Climate Model (NARCM) is currently being developed. NARCM includes size-segregated aerosols as prognostic and interactive constituents. In this paper, the model is being applied to sulphate aerosol over North America during time periods in July and December 1994. The results give evidence for considerable regional and seasonal variations in sulphate aerosol size distributions over North America. Comparisons of the results with different observations yield a reasonably good agreement in terms of meteorological and physicochemical parameters. Some of the differences in sulphate concentrations and wet deposition rates can be attributed to differences in cloud amounts and precipitation between model results and observations. Indirect tests of the simulated aerosol mass mean diameters are also encouraging. Additional simulations for hypothetical decreases in anthropogenic sulphur emissions and increases in hydrogen peroxide (H2O2) background concentrations are performed for the same time periods to study the responses of concentration, size distribution, and wet deposition of sulphate aerosol to these changes. Also, responses of cloud condensation nuclei (CCN) number concentrations are investigated. The simulation results show that sulphate aerosol concentrations respond almost linearly in both time periods to decreases in sulphur emissions but that CCN number concentrations respond nonlinearly due to decreases in sulphate mass mean diameters. Especially for the December period, increases in hydrogen peroxide background concentrations lead to increases in CCN number concentrations at critical diameters larger than about 0.07 μm. These results lead to the hypothesis that increased in-cloud oxidation in convective clouds due to future increases in oxidant concentrations may produce larger CCN which eventually can be easily activated in subsequently forming stratiform clouds.

Energy and water cycles in a high-latitude, north-flowing river system: Summary of results from the Mackenzie GEWEX Study-phase I

Abstract The MacKenzie Global Energy and Water Cycle Experiment (GEWEX) Study, Phase 1, seeks to improve understanding of energy and water cycling in the Mackenzie River basin (MRB) and to initiate and test atmospheric, hydrologic, and coupled models that will project the sensitivity of these cycles to climate change and to human activities. Major findings from the study are outlined in this paper. Absorbed solar radiation is a primary driving force of energy and water, and shows dramatic temporal and spatial variability. Cloud amounts feature large diurnal, seasonal, and interannual fluctuations. Seasonality in moisture inputs and outputs is pronounced. Winter in the northern MRB features deep thermal inversions. Snow hydrological processes are very significant in this high-latitude environment and are being successfully modeled for various landscapes. Runoff processes are distinctive in the major terrain units, which is important to overall water cycling. Lakes and wetlands compose much of MRB and are p...