Bibliography - Michael Herzog

1. Guo, Huan, Joyce Penner, Michael Herzog, and Shang-Ping Xie, 2007: Investigation of the first and second aerosol indirect effects using data from the May 2003 Intensive Operational Period at the Southern Great Plains. Journal of Geophysical Research, 112, D15206, DOI:10.1029/2006JD007173.
   Abstract

   The Active Tracer High-Resolution Atmospheric Model is used to examine the aerosol indirect effect (AIE) for a spring continental stratus cloud on the basis of data collected during the 17 May 2003 Aerosol Intensive Operation Period (AIOP) at the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains site. Model results for our base case, which uses observed aerosol concentrations, agree reasonably well with the available observations, giving confidence that the basic model is reasonable. Sensitivity tests are performed to explore the response of the clouds to changes in the aerosol number concentration and surface fluxes. During the major part of the simulation, from 0630 through 1400 local standard time (LST), an increase in the aerosol number concentration (N_a ) results in a decrease of the mean cloud droplet size and an increase of the cloud liquid water path (LWP) until aerosol number concentration levels reach 1200 cm^-3. Further increases in aerosol concentration do not increase the liquid water path because the depletion of cloud water by precipitation is negligible above this number concentration. After 1400 LST, the liquid water path decreases when aerosols increase as long as N _a < 600 cm^-3 and remains unchanged for higher aerosol concentrations. The decrease of LWP is associated with the evaporative cooling below cloud base which leads to more condensation of water vapor, a result that is consistent with afternoon satellite observations of the response of continental clouds to increases in droplet concentrations. A sensitivity test with a stronger surface latent flux increases both the cloud geometrical thickness and cloud water content. On the other hand, a sensitivity test with a stronger surface sensible heat flux leads to a higher cloud base and a shallower and drier cloud. The response of the cloud geometrical thickness to changes in surface sensible heat flux dominates that of the cloud water content. The cloud fraction is also reduced at the end of the simulation time period. Because the surface heat fluxes will likely change when aerosol and droplet number concentrations change, these sensitivity tests show that a fully coupled simulation with a land surface model will be needed to fully assess the response of the cloud to changing aerosol concentrations. Nevertheless, since the thermodynamic boundary layer profiles do not change significantly when aerosol concentrations are changed, our results for changing aerosol concentrations are qualitatively correct.

2. Guo, Huan, Joyce Penner, Michael Herzog, and Hanna Pawlowska, January 2007: Examination of the aerosol indirect effect under contrasting environments during the ACE-2 experiment. Atmospheric Chemistry and Physics, 7(2), DOI:10.5194/acp-7-535-2007.
   Abstract

   The Active Tracer High-resolution Atmospheric Model (ATHAM) has been adopted to examine the aerosol indirect effect in contrasting clean and polluted cloudy boundary layers during the Second Aerosol Characterization Experiment (ACE-2). Model results are in good agreement with available in-situ observations, which provides confidence in the results of ATHAM. Sensitivity tests have been conducted to examine the response of the cloud fraction (CF), cloud liquid water path (LWP), and cloud optical depth (COD) to changes in aerosols in the clean and polluted cases. It is shown for two cases that CF and LWP would decrease or remain nearly constant with an increase in aerosols, a result which shows that the second aerosol indirect effect is positive or negligibly small in these cases. Further investigation indicates that the background meteorological conditions play a critical role in the response of CF and LWP to aerosols. When large-scale subsidence is weak as in the clean case, the dry overlying air above the cloud is more efficiently entrained into the cloud, and in so doing, removes cloud water more efficiently, and results in lower CF and LWP when aerosol burden increases. However, when the large-scale subsidence is strong as in the polluted case, the growth of the cloud top is suppressed and the entrainment drying makes no significant difference when aerosol burden increases. Therefore, the CF and LWP remain nearly constant. In both the clean and polluted cases, the COD tends to increase with aerosols, and the total aerosol indirect effect (AIE) is negative even when the CF and LWP decrease with an increase in aerosols. Therefore, the first AIE dominates the response of the cloud to aerosols.

3. Rotstayn, L D., Wenju Cai, Martin R Dix, G D Farquhar, Y Feng, Paul Ginoux, Michael Herzog, Akihiko Ito, Joyce Penner, M L Roderick, and Minghuai Wang, 2007: Have Australian rainfall and cloudiness increased due to the remote effects of Asian anthropogenic aerosols? Journal of Geophysical Research, 112(D09202), DOI:10.1029/2006JD007712.
   Abstract

   There is ample evidence that anthropogenic aerosols have important effects on climate in the Northern Hemisphere but little such evidence in the Southern Hemisphere. Observations of Australian rainfall and cloudiness since 1950 show increases over much of the continent. We show that including anthropogenic aerosol changes in 20th century simulations of a global climate model gives increasing rainfall and cloudiness over Australia during 1951–1996, whereas omitting this forcing gives decreasing rainfall and cloudiness. The pattern of increasing rainfall when aerosols are included is strongest over northwestern Australia, in agreement with the observed trends. The strong impact of aerosols is primarily due to the massive Asian aerosol haze, as confirmed by a sensitivity test in which only Asian anthropogenic aerosols are included. The Asian haze alters the meridional temperature and pressure gradients over the tropical Indian Ocean, thereby increasing the tendency of monsoonal winds to flow toward Australia. Anthropogenic aerosols also make the simulated pattern of surface-temperature change in the tropical Pacific more like La Niña, since they induce a cooling of the surface waters in the extratropical North Pacific, which are then transported to the tropical eastern Pacific via the deep ocean. Transient climate model simulations forced only by increased greenhouse gases have generally not reproduced the observed rainfall increase over northwestern and central Australia. Our results suggest that a possible reason for this failure was the omission of forcing by Asian aerosols. Further research is essential to more accurately quantify the role of Asian aerosols in forcing Australian climate change.

4. Jablonowski, Christiane, Michael Herzog, Joyce Penner, R C Oehmke, Q F Stout, B van Leer, and K G Powell, 2006: Block-structured adaptive grids on the sphere: Advection experiments. Monthly Weather Review, 134(12), DOI:10.1175/MWR3223.1.
   Abstract

   A spherical 2D adaptive mesh refinement (AMR) technique is applied to the so-called Lin–Rood advection algorithm, which is built upon a conservative and oscillation-free finite-volume discretization in flux form. The AMR design is based on two modules: a block-structured data layout and a spherical AMR grid library for parallel computer architectures. The latter defines and manages the adaptive blocks in spherical geometry, provides user interfaces for interpolation routines, and supports the communication and load-balancing aspects for parallel applications. The adaptive grid simulations are guided by user-defined adaptation criteria. Both statically and dynamically adaptive setups that start from a regular block-structured latitude–longitude grid are supported. All blocks are logically rectangular, self-similar, and independent data units that are split into four in the event of refinement requests, thereby doubling the horizontal resolution. Grid coarsenings reverse this refinement principle. Refinement and coarsening levels are constrained so that there is a uniform 2:1 mesh ratio at all fine–coarse-grid interfaces. The adaptive advection model is tested using three standard advection tests with increasing complexity. These include the transport of a cosine bell around the sphere, the advection of a slotted cylinder, and a smooth deformational flow that describes the roll-up of two vortices. The latter two examples exhibit very sharp edges and gradients that challenge not only the numerical scheme but also the AMR approach. The adaptive simulations show that all features of interest are reliably detected and tracked with high-resolution grids. These are steered by either a threshold- or gradient-based adaptation criterion that depends on the characteristics of the advected tracer field. The additional resolution clearly helps preserve the shape and amplitude of the transported tracer while saving computing resources in comparison to uniform-grid model runs.

5. Kinne, S, M Schulz, C Textor, S Guibert, Yves Balkanski, Susanne E Bauer, Paul Ginoux, Michael Herzog, and Larry W Horowitz, et al., 2006: An AeroCom initial assessment – optical properties in aerosol component modules of global models. Atmospheric Chemistry and Physics, 6, 1815-1834.
   Abstract PDF

   The AeroCom exercise diagnoses multi-component aerosol modules in global modeling. In an initial assessment simulated global distributions for mass and mid-visible aerosol optical thickness (aot) were compared among 20 different modules. Model diversity was also explored in the context of previous comparisons. For the component combined aot general agreement has improved for the annual global mean. At 0.11 to 0.14, simulated aot values are at the lower end of global averages suggested by remote sensing from ground (AERONET ca. 0.135) and space (satellite composite ca. 0.15). More detailed comparisons, however, reveal that larger differences in regional distribution and significant differences in compositional mixture remain. Of particular concern are large model diversities for contributions by dust and carbonaceous aerosol, because they lead to significant uncertainty in aerosol absorption (aab). Since aot and aab, both, influence the aerosol impact on the radiative energy-balance, the aerosol (direct) forcing uncertainty in modeling is larger than differences in aot might suggest. New diagnostic approaches are proposed to trace model differences in terms of aerosol processing and transport: These include the prescription of common input (e.g. amount, size and injection of aerosol component emissions) and the use of observational capabilities from ground (e.g. measurements networks) or space (e.g. correlations between aerosol and clouds).

6. Kinne, S, Ü Lohmann, J Feichter, M Schulz, C Timmreck, S Ghan, R Easter, Mian Chin, Paul Ginoux, T Takemura, I Tegen, D Koch, Michael Herzog, Joyce Penner, G Pitari, B Holben, T Eck, A Smirnov, O Dubovik, I Slutsker, D Tanré, O Torres, M Mishchenko, I V Geogdzhayev, D A Chu, and Y J Kaufman, 2003: Monthly averages of aerosol properties: a global comparison among models, satellite data, and AERONET ground data. Journal of Geophysical Research, 108(D20), 4634, DOI:10.1029/2001JD001253.
   Abstract

   New aerosol modules of global (circulation and chemical transport) models are evaluated. These new modules distinguish among at least five aerosol components: sulfate, organic carbon, black carbon, sea salt, and dust. Monthly and regionally averaged predictions for aerosol mass and aerosol optical depth are compared. Differences among models are significant for all aerosol types. The largest differences were found near expected source regions of biomass burning (carbon) and dust. Assumptions for the permitted water uptake also contribute to optical depth differences (of sulfate, organic carbon, and sea salt) at higher latitudes. The decline of mass or optical depth away from recognized sources reveals strong differences in aerosol transport or removal among models. These differences are also a function of altitude, as transport biases of dust do not always extend to other aerosol types. Ratios of optical depth and mass demonstrate large differences in the mass extinction efficiency, even for hydrophobic aerosol. This suggests that efforts of good mass simulations could be wasted or that conversions are misused to cover for poor mass simulations. In an attempt to provide an absolute measure for model skill, simulated total optical depths (when adding contributions from all five aerosol types) are compared to measurements from ground and space. Comparisons to the Aerosol Robotic Network (AERONET) suggest a source strength underestimate in many models, most frequently for (subtropical) tropical biomass or dust. Comparisons to the combined best of Moderate-Resolution Imaging Spectroradiometer (MODIS) and Total Ozone Mapping Spectrometer (TOMS) indicate that away from sources, model simulations are usually smaller. Particularly large are discrepancies over tropical oceans and oceans of the Southern Hemisphere, raising issues on the treatment of sea salt in models. Totals for mass or optical depth in many models are defined by the absence or dominance of only one aerosol component. With appropriate corrections to that component (e.g., to removal, to source strength, or to seasonality) a much better model performance can be expected. Still, many important modeling issues remain inconclusive as the combined result of poor coordination (different emissions and meteorology), insufficient model output (vertical distributions, water uptake by aerosol type), and unresolved measurement issues (retrieval assumptions and temporal or spatial sampling biases).

7. Penner, Joyce, S Zhang, Mian Chin, C C Chuang, J Feichter, Y Feng, I V Geogdzhayev, Paul Ginoux, Michael Herzog, and Brian J Soden, et al., 2002: A comparison of model- and satellite-derived aerosol optical depth and reflectivity. Journal of the Atmospheric Sciences, 59(3), 441-460.
   Abstract PDF

   The determination of an accurate quantitative understanding of the role of tropospheric aerosols in the earth's radiation budget is extremely important because forcing by anthropogenic aerosols presently represents one of the most uncertain aspects of climate models. Here the authors present a systematic comparison of three different analyses of satellite-retrieved aerosol optical depth based on the Advanced Very High Resolution Radiometer (AVHRR)-measured radiances with optical depths derived from six different models. Also compared are the model-derived clear-sky reflected shortwave radiation with satellite-measured reflectivities derived from the Earth Radiation Budget Experiment (ERBE) satellite. The three different satellite-derived optical depths differ by between -0.10 and 0.07 optical depth units in comparison to the average of the three analyses depending on latitude and month, but the general features of the retrievals are similar. The models differ by between -0.09 and +0.16 optical depth units from the average of the models. Differences between the average of the models and the average of the satellite analyses range over -0.11 to +0.05 optical depth units. These differences are significant since the annual average clear-sky radiative forcing associated with the difference between the average of the models and the average of the satellite analyses ranges between -3.9 and 0.7 W m-2 depending on latitude and is -1.7 W m-2 on a global average annual basis. Variations in the source strengths of dimethylsulfide-derived aerosols and sea salt aerosols can explain differences between the models, and between the models and satellite retrievals of up to 0.2 optical depth units. The comparison of model-generated reflected shortwave radiation and ERBE-measured shortwave radiation is similar in character as a function of latitude to the analysis of modeled and satellite-retrieved optical depths, but the differences between the modeled clear-sky reflected flux and the ERBE clear-sky reflected flux is generally larger than that inferred from the difference between the models and the AVHRR optical depths, especially at high latitudes. The difference between the mean of the models and the ERBE-analyzed clear-sky flux is 1.6 W m-2. The overall comparison indicates that the model-generated aerosol optical depth is systematically lower than that inferred from measurements between the latitudes of 10° and 30°S. It is not likely that the shortfall is due to small values of the sea salt optical depth because increases in this component would create modeled optical depths that are larger than those from satellites in the region north of 30°N and near 50°S. Instead, the source strengths for DMS and biomass aerosols in the models may be too low. Firm conclusions, however, will require better retrieval procedures for the satellites, including better cloud screening procedures, further improvement of the model's treatment of aerosol transport and removal, and a better determination of aerosol source strengths.

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