Introduction to the Special Issue "The 2009 WE-Heraeus-Seminar on satellite remote sensing of aerosols: techniques, limitations, and validation"

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How well do we understand the Earth's radiation budget and the role of clouds? Selected results of the GEWEX radiation flux assessment

Multi-year average radiative flux maps of three satellite data-sets (CERES, ISSCP and GEWEX-SRB) are compared to each other and to typical values by global modeling (median values of results of 20 climate models of the 4th IPCC Assessment). Diversity assessments address radiative flux products and at the top of the atmosphere (TOA) and the surface, with particular attention to impacts by clouds. Involving both data from surface and TOA special attention is given to the vertical radiation flux divergence and on the infrared Greenhouse effect, which are rarely shown in literature. © 2013 AIP Publishing LLC

An assessment of radiation budget data provided by ISCCP and GEWEX-SRB

The projects ISCCP and GEWEX-SRB compute global data sets of radiation budget components at the top of the atmosphere and at the surface. Time series range from July 1983 to June 2001, and to October 1995, respectively. Comparing monthly averages over broader zones we find that the SRB underestimates the incident radiation at TOA by more than 2–5 Wm−2 over the tropics and up to 40 Wm−2 over polar regions. The ISCCP infrared radiation fluxes near the surface and at TOA, in particular over both polar zones, are higher than those of the SRB. Clouds in the ISCCP appear optically less effective than in the SRB. Interannual and month-to-month variations are observed indicating serious errors in ancillary data. Complete reprocessing is recommended. End products need validation within this large domain in space and time with correlated radiation budget measurements at TOA and at ground

Evaluation of black carbon estimations in global aerosol models

We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. These model-measurement intercomparisons include BC surface and aircraft concentrations, aerosol absorption optical depth (AAOD) from AERONET and Ozone Monitoring Instrument (OMI) retrievals and BC column estimations based on AERONET. In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retrievals, the models are generally biased low. The average ratio of model to retrieved AAOD is less than 0.7 in South American and 0.6 in African biomass burning regions; both of these regions lack surface concentration measurements. In Asia the average model to observed ratio is 0.6 for AAOD and 0.5 for BC surface concentrations. Compared with aircraft measurements over the Americas at latitudes between 0 and 50 N, the average model is a factor of 10 larger than observed, and most models exceed the measured BC standard deviation in the mid to upper troposphere. At higher latitudes the average model to aircraft BC is 0.6 and underestimates the observed BC loading in the lower and middle troposphere associated with springtime Arctic haze. Low model bias for AAOD but overestimation of surface and upper atmospheric BC concentrations at lower latitudes suggests that most models are underestimating BC absorption and should improve estimates of refractive index, particle size, and optical effects of BC coating. Retrieval uncertainties and/or differences with model diagnostic treatment may also contribute to the model-measurement disparity. Largest AeroCom model diversity occurred in northern Eurasia and the remote Arctic, regions influenced by anthropogenic sources. Changing emissions, aging, removal, or optical properties within a single model generated a smaller change in model predictions than the range represented by the full set of AeroCom models. Upper tropospheric concentrations of BC mass from the aircraft measurements are suggested to provide a unique new benchmark to test scavenging and vertical dispersion of BC in global models

How accurate did GCMs compute the insolation at TOA for AMIP-2?

Monthly averages of solar radiation reaching the Top of the Atmosphere (TOA) as simulated by 20 General Circulation Models (GCMs) during the period 1985–1988 are compared. They were part of submissions to AMIP-2 (Atmospheric Model Intercomparison Project). Monthly averages of ISCCP-FD (International Satellite Cloud Climatology Project – Flux Data) are considered as reference. Considerable discrepancies are found: Most models reproduce the prescribed Total Solar Irradiance (TSI) value within ±0.7 Wm−2. Monthly zonal averages disagree between ±2 to ±7 Wm−2, depending on latitude and season. The largest model diversity occurs near polar regions. Some models display a zonally symmetric insolation, while others and ISCCP show longitudinal deviations of the order of ±1 Wm−2. With such differences in meridional gradients impacts in multi-annual simulations cannot be excluded. Sensitivity studies are recommended

Initial fate of fine ash and sulfur from large volcanic eruptions

Large volcanic eruptions emit huge amounts of sulfur and fine ash into the stratosphere. These products cause an impact on radiative processes, temperature and wind patterns. In simulations with a General Circulation Model including detailed aerosol microphysics, the relation between the impact of sulfur and fine ash is determined for different eruption strengths and locations, one in the tropics and one in high Northern latitudes. Fine ash with effective radii between 1 μm and 15 μm has a lifetime of several days only. Nevertheless, the strong absorption of shortwave and long-wave radiation causes additional heating and cooling of ±20 K/day and impacts the evolution of the volcanic cloud. Depending on the location of the volcanic eruption, transport direction changes due to the presence of fine ash, vortices develop and temperature anomalies at ground increase. The results show substantial impact on the local scale but only minor impact on the evolution of sulfate in the stratosphere in the month after the simulated eruptions

Comparison of radiative energy flows in observational datasets and climate modeling

This study examines radiative flux distributions and local spread of values from three major observational datasets (CERES, ISCCP, and SRB) and compares them with results from climate modeling (CMIP3). Examinations of the spread and differences also differentiate among contributions from cloudy and clear-sky conditions. The spread among observational datasets is in large part caused by noncloud ancillary data. Average differences of at least 10 W m-2 each for clear-sky downward solar, upward solar, and upward infrared fluxes at the surface demonstrate via spatial difference patterns major differences in assumptions for atmospheric aerosol, solar surface albedo and surface temperature, and/or emittance in observational datasets. At the top of the atmosphere (TOA), observational datasets are less influenced by the ancillary data errors than at the surface. Comparisons of spatial radiative flux distributions at the TOA between observations and climate modeling indicate large deficiencies in the strength and distribution of model-simulated cloud radiative effects. Differences are largest for lower-altitude clouds over low-latitude oceans. Global modeling simulates stronger cloud radiative effects (CRE) by +30 W m-2 over trade wind cumulus regions, yet smaller CRE by about -30 W m-2 over (smaller in area) stratocumulus regions. At the surface, climate modeling simulates on average about 15 W m-2 smaller radiative net flux imbalances, as if climate modeling underestimates latent heat release (and precipitation). Relative to observational datasets, simulated surface net fluxes are particularly lower over oceanic trade wind regions (where global modeling tends to overestimate the radiative impact of clouds). Still, with the uncertainty in noncloud ancillary data, observational data do not establish a reliable reference. © 2016 American Meteorological Society

Comparisons of downwelling radiation to model predictions based on groundbased measurements during FIRE 1991

Surface radiation measurements and simultaneous ground-based measurements of the atmosphere during the FIRE'91 cirrus field experiment provided an opportunity to identify crucial measurements and parameterization deficiencies in current cloud-radiation models. Comparisons between measured and calculated broadband surface fluxes with only a small data subset already reveal these needs: accurate humidity and aerosol vertical profiles for clear cases, accurate vertical extinction profiles and dimensions for clouds, and understanding of the (solar) scattering properties of cirrus