Costs and global impacts of black carbon abatement strategies

Abatement of particulate matter has traditionally been driven by health concerns rather than its role in global warming. Here we assess future abatement strategies in terms of how much they reduce the climate impact of black carbon (BC) and organic carbon (OC) from contained combustion. We develop global scenarios which take into account regional differences in climate impact, costs of abatement and ability to pay, as well as both the direct and indirect (snow-albedo) climate impact of BC and OC. To represent the climate impact, we estimate consistent region-specific values of direct and indirect global warming potential (GWP) and global temperature potential (GTP). The indirect GWP has been estimated using a physical approach and includes the effect of change in albedo from BC deposited on snow. The indirect GWP is highest in the Middle East followed by Russia, Europe and North America, while the total GWP is highest in the Middle East, Africa and South Asia. We conclude that prioritizing emission reductions in Asia represents the most cost-efficient global abatement strategy for BC because Asia is (1) responsible for a large share of total emissions, (2) has lower abatement costs compared to Europe and North America and (3) has large health cobenefits from reduced PM₁₀ emissions.     

The SAMUM-1 experiment over Southern Morocco: overview and introduction

In May/June 2006, the largest mineral dust experiment to date (Saharan Mineral Dust Experiment, SAMUM-1) was conducted in Southern Morocco. The aim was to characterize dust particles near the world's largest mineral dust source, and to quantify dust-related radiative effects. At one of the two ground-based measurement sites dust particle size distribution, optical, hygroscopic, chemical and structural particle characteristics were measured. One research aircraft mainly measured solar spectral irradiances and surface albedo. The other aircraft provided in situ physical aerosol measurements and samples and lidar profiles through the dust layers. Three ground-based lidars were operated at the second ground-based measurement site. They determined optical dust properties, particle shape and temporal development of dust layers. Columnar, ground-based sun photometer measurements complemented the lidar data. Additionally a station in Évora, Portugal monitored dust outbreaks from the North African source region to the Iberian Peninsula during SAMUM-1.
Volumetric and columnar closure exercises utilized these detailed measurements of dust characteristics together with optical and radiative transfer models. Concurrent developments of a mesoscale dust transport model were validated with the experimental data. The paper gives an overview over rationale and design of SAMUM-1, introduces and highlights the subsequent reports on experimental and modelling results.     

Regional Saharan dust modelling during the SAMUM 2006 campaign

The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006 SAMUM field campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations. However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust source placement by the model. While simulated winds and dust distributions are well matched for dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.     

Introduction to special section on 'East Asia Climate and Environment'

Activities within the collaborative project East Asia Climate and Environment focus on the impact on the composition of chemically active greenhouse compounds from the rapidly growing emissions in Asia. Estimates of emissions (past and future) are discussed in light of the demand for energy in the different sectors. The impact includes regional scale contributions through short-lived climate compounds like particles and ozone, while global scale contributions are demonstrated through changes in oxidation capacity affecting compounds like CH₄. One key issue is the important and increasing contribution from China to atmospheric chemical changes.     

Air pollution and precipitation suppression over SE Australia: critical review of evidence presented by Rosenfeld (2000) and Rosenfeld (2006)

A hypothesis that air pollution in the form of small particles has caused a secular decrease in precipitation over South East (SE) Australia was advanced by Rosenfeld (2000) , who concluded that the hypothesis was proven. This paper critically reviews the experimental design used in that paper and subsequent work presented in Rosenfeld (2006). It is shown that the meteorological conditions on the single day studied were inappropriate for testing the hypothesis, as was the synoptic weather pattern. Moreover, the study failed to consider prior published work in Australia and readily available independent data such as rainfall patterns that call into question the approach used. We conclude that the experimental design was incapable of addressing the hypothesis, which therefore remains unproven.     

Running GCM physics and dynamics on different grids: algorithm and tests

An 'alternate grid' (GridAlt) technique is presented, which allows the different components of a general circulation model's governing equations to be computed on distinct grids chosen for that component or process. In the implementation presented here, the tendencies of state variables from the physical parametrizations are computed on a vertical grid with very fine resolution near the surface, whereas the remaining terms in the equations of motion are computed using an Eta coordinate with coarser vertical resolution.
Results from a suite of aquaplanet experiments show that much of the benefit of increased vertical resolution in the whole model can be realized by enhancing the vertical resolution of the 'physics grid' using GridAlt. The benefit is realized in the fields which are computed directly in the physical parametrizations, and in the vertical structure of the relative humidity and mass streamfunction. Results from a suite of realistically configured simulations demonstrated an impact of GridAlt that was similar to its impact in the simplified simulations, as well as an improved response to El Niño Southern Oscillation forcing. It is concluded that the present implementation of GridAlt offers a practical way to allow GCMs to better capture the near-surface structure of the atmosphere.     

Is asymptotic non-divergence of the large-scale tropical atmosphere consistent with equatorial wave theories?

Observations suggest that the large-scale tropical atmospheric circulations, associated with intraseasonal variabilties, are dominated more by the vorticity than the divergence. The present paper examines the consistency of the above observations with linear equatorial wave theories. Both free and forced linear waves are considered. The free equatorial waves are classified into two major categories: (1) the Rossby waves, strongly dominated by vorticity and (2) the inertial-gravity waves, relatively dominated by the divergence. Both the Kelvin and the mixed-Rossby gravity waves are intermediate of these two major categories.
In the forced case, the wave response is predominantly inertial-gravity wave-like for periods less than 5 d, thus predominantly divergent. On the other hand, for forcing with the longer periods, the wave response closely following free Rossby-wave structures, asymptotically approaches to a non-divergent state. The asymptotic tendency for non-divergence is found to be much stronger than observed. The difference is so stark that, notably, the tropical intraseasonal variability cannot be consistent with linear equatorial waves theories.     

State of mixing, shape factor, number size distribution, and hygroscopic growth of the Saharan anthropogenic and mineral dust aerosol at Tinfou, Morocco

The Saharan Mineral Dust Experiment (SAMUM) was conducted in May and June 2006 in Tinfou, Morocco. A H-TDMA system and a H-DMA-APS system were used to obtain hygroscopic properties of mineral dust particles at 85% RH. Dynamic shape factors of 1.11, 1.19 and 1.25 were determined for the volume equivalent diameters 720, 840 and 960 nm, respectively.
During a dust event, the hydrophobic number fraction of 250 and 350 nm particles increased significantly from 30 and 65% to 53 and 75%, respectively, indicating that mineral dust particles can be as small as 200 nm in diameter. Log-normal functions for mineral dust number size distributions were obtained from total particle number size distributions and fractions of hydrophobic particles. The geometric mean diameter for Saharan dust particles was 715 nm during the dust event and 570 nm for the Saharan background aerosol.
Measurements of hygroscopic growth showed that the Saharan aerosol consists of an anthropogenic fraction (predominantly non natural sulphate and carbonaceous particles) and of mineral dust particles. Hygroscopic growth and hysteresis curve measurements of the 'more' hygroscopic particle fraction indicated ammonium sulphate as a main component of the anthropogenic aerosol. Particles larger than 720 nm in diameter were completely hydrophobic meaning that mineral dust particles are not hygroscopic.     

Role of the metric in forecast error growth: how chaotic is the weather?

The atmosphere is often cited as an archetypal example of a chaotic system, where prediction is limited by the model's sensitivity to initial conditions. Experiments have indeed shown that forecast errors, as measured in 500 hPa heights, can double in 1.5 d or less. Recent work, however, has shown that, when errors are measured in total energy, model error is the primary contributor to forecast inaccuracy. In this paper we attempt to reconcile these apparently conflicting sets of results by examining the role of the chosen metric. Using a simple medium-dimensional model for illustration, it is found that the metric has a strong effect, not just on apparent error growth, but on the perceived causes of error. If an insufficiently global metric is used, then it may appear that error is due to sensitivity to initial condition, when in fact it is caused by sensitivity to error in the other variables. If the goal is to diagnose the causes of error, a sufficiently global metric must be used. The simple model is used to predict the internal rate of growth of the ECMWF operational model, and preliminary results compared. It is found that both 500 hPa and total energy results are consistent with high model error and a relatively low internal rate of growth. Experiments are suggested to further verify the results for weather models.