A21E-01 08:00h
Climate Change Science Program Synthesis and Assessment Reports: Aerosol Properties and Their Impacts on Climate
The Climate Change Science Program (CCSP) is developing and extending its research activities to support policymaking and adaptive management. The program includes a set of "Synthesis and Assessment Products," active participation in international assessments such as those of the Intergovernmental Panel on Climate Change, improvements in modeling and other resources to facilitate comparison of response options, and development, with users, of tools to support adaptive management and planning. These efforts are building on substantial ongoing efforts of agencies and departments participating in the CCSP. One of the products focuses on aerosol properties and their impact on climate. The very complex mixture of aerosol types and their spatial distributions provide diverse warming and cooling influences on climate, and impact the formation of both water droplets and ice crystals in clouds. Our poor understanding of aerosol properties and distributions results in large uncertainties about the net impact of aerosols on climate and impairs our ability to project climate changes. The product will be produced in two phases: Phase-I aims for a few explicit and focused scientific "review nuggets" in the near term that would be not only stand alone as CCSP-facilitated products, but that would also be useful input to community-wide activities like the IPCC and Phase-II that would connect and focus the new (2006/7) level of community-wide understanding of climate change (and aerosol-climate inclusively) to explicit decision-support information and tools. In this light, we have embarked on Phase I of a synthesis product entitled, "Aerosol properties and their impacts on climate," which addresses Goal 2, "Improve quantification of the forces bringing about changes in the Earth's climate and related systems," under the Strategic Plan of the CCSP. We present here the status of this first phase of work, which is focused on new assessment and synthesis information stimulated by the CCSP. Three topics have been selected as foci and will be reported on here: (1) dependence of radiative forcing by tropospheric aerosols on aerosol composition in the North Atlantic, North Pacific, and North Indian Ocean based on in-situ observations, (2) a review of measurement-based understanding of aerosol radiative forcing and aerosol sources derived from the analysis of remote-sensing observations, and (3) a model intercomparison study to quantify the uncertainties associated with indirect aerosol forcing. This work will feed in to the next IPCC assessment and serve as an input to Phase II of the product, which will focus on issues of special concern to North America.
A21E-02 INVITED 08:10h
Review of Measurements of the Aerosol Global Direct Radiative Forcing
Of all the ways that anthropogenic aerosols are suspected of impacting the Earth's energy balance, direct shortwave forcing during clear-sky conditions (DF) is by far the most straightforward to measure and understand. So, how well do we know this quantity? Model-based estimates of DF have treated individual anthropogenic components, but this approach is not practical for global satellite observations, which inherently sense the total aerosol and cannot readily detect chemical composition. In conjunction with members of the "A-Train" science teams, an observational strategy is being developed that attacks the DF problem in terms of three observable parameters: mid-visible aerosol optical depth (AOD), fine-mode fraction of optical depth (FMF) - which is taken as a proxy for anthropogenic fraction - and radiative forcing efficiency per unit optical depth (RFE). This talk will assess knowledge of each parameter. AOD is rather well known, with a long history of observations by multiple satellites and an extensive validation program in place. FMF (deduced from the wavelength dependence of AOD) is routinely reported by satellites, but these data have unknown accuracy, due to the absence of a validation program, and show discontinuities at land/ocean boundaries indicative of artifacts. Many investigators have combined AOD measurements with satellite measurements of broadband flux to estimate RFE over the oceans. These studies suffer from limited coverage (a few percent of the ocean) and a lack of collocated, sub-orbital measurements sufficient to diagnose the causes of RFE variation (such as differences in ambient relative humidity or aerosol single scattering albedo). Covariation among these three parameters has not yet been assessed. Summarizing, the lower limit on uncertainty in DF is about a factor of three and the upper limit is unknown. New and enhanced satellite sensors offer the potential for greatly reduced uncertainty in the near future. However, achieving this will require that the synthesis and integration of observations be given much greater priority than at present.
http://www.atmos.washington.edu/~cheeka/DAFC/DAFC.html
A21E-03 08:30h
Estimate of the aerosol anthropogenic component and forcing from satellite data
Satellite measurements of aerosol do not contain information on the chemical composition needed to resolve anthropogenic vs. natural aerosol components. Besides, the same chemical species can have natural and anthropogenic origins. However the ability of the new satellite instruments (MODIS, MISR, POLDER) to distinguish fine from coarse aerosols over the oceans, can be used as a signature of the presence of anthropogenic component and used to measure the fraction of the aerosol originating from anthropogenic activity with an uncertainty of 10 percent for aerosol optical thickness larger than 0.1. We develop the methods and investigated it using model calculations (GOCART) and satellite data (MODIS). Preliminary application to 2 years of global MODIS data shows that 0.20±0.08 of the aerosol optical thickness and radiative effect has anthropogenic origin. The resultant aerosol forcing over cloud free oceans is -1.3±0.6 W/m2, larger than model simulations. Further research until the presentation will probably modify these values.
A21E-04 08:45h
A Synergy Analysis of Global Aerosols and Assessment of Direct Radiative Effect
Aerosol-cloud-radiation interactions constitute the largest uncertainty in quantifying the anthropogenic impact on the earth's energy budget and climate. To reduce these uncertainties, an integration of satellite remote sensing, numerical modeling, and ground-based measurement is desired, because individual methods have both advantages and limitations and are complementary to each other. Recent satellite sensors such as MODIS and MISR provide an unprecedented opportunity of characterizing aerosol optical depths on nearly global scale. These sensors also measure the spectral variation and anisotropy of land surface reflection, which are required for assessing the aerosol direct effect. On the other hand, current satellite sensors can not adequately characterize the whole complexity of global aerosol system. As such a synergy of satellite retrievals and model simulations such as GOCART would enhance the capability of satellite remote sensing. The synergy should be evaluated and constrained with ground-truth data from AERONET measurements. In this study we examine the variability of aerosol optical depth from MODIS and MISR retrievals and GOCART simulations using statistical analysis tools, and compare them with AERONET measurements to obtain error parameters. An optimal interpolation analysis is then performed to merge MODIS and MISR retrievals with GOCART simulations to maximize the utility of aerosol products. This integration derives an annual cycle of global aerosol optical depth that has higher correlation and lower bias and matches better with AERONET measurements. The annual average optical depth in 60S-60N is 0.162~0.167 for MODIS and GOCART integration, 0.148~0.169 for MISR and GOCART integration, and 0.151~0.157 for MODIS (over ocean), MISR (over land) and GOCART integration, depending on error parameters. The aerosol direct effect is also assessed using the so-generated aerosol optical depth in conjunction with GOCART simulations of single-scattering albedo and asymmetry factor and the MODIS-retrieved surface albedo. On global average and under clear sky, the aerosol cooling at the surface is about a factor of 2 more than that at the top-of-atmosphere.
A21E-05 09:00h
Internally Consistent MODIS Estimate of Aerosol Clear-Sky Radiative Effect Over the Global Oceans
Modern satellite remote sensing, and in particular the MODerate resolution Imaging Spectroradiometer (MODIS), offers a measurement-based pathway to estimate global aerosol radiative effects and aerosol radiative forcing. Over the oceans, MODIS retrieves the total aerosol optical thickness, but also reports which combination of the 9 different aerosol models was used to obtain the retrieval. Each of the 9 models is characterized by a size distribution and complex refractive index, which through Mie calculations correspond to a unique set of single scattering albedo, assymetry parameter and spectral extinction for each model. The combination of these sets of optical parameters weighted by the optical thickness attributed to each model in the retrieval produces the best fit to the observed radiances at the top of the atmosphere. Thus the MODIS ocean aerosol retrieval provides us with (1) An observed distribution of global aerosol loading, and (2) An internally-consistent, observed, distribution of aerosol optical models that when used in combination will best represent the radiances at the top of the atmosphere. We use these two observed global distributions to initialize the column climate model by Chou and Suarez to calculate the aerosol radiative effect at top of the atmosphere and the radiative efficiency of the aerosols over the global oceans. We apply the analysis to 3 years of MODIS retrievals from the Terra satellite and produce global and regional, seasonally varying, estimates of aerosol radiative effect over the clear-sky oceans.
A21E-06 09:12h
Global Top-of-Atmosphere Direct Radiative Effect of Aerosols over Ocean from Merged CERES and MODIS Observations for 2000-2003
The direct effect of aerosols is defined as the difference between radiative fluxes in the absence and presence of aerosols. In this study, the direct radiative effect of aerosols is estimated for 46 months (March, 2000 to December, 2003) of merged CERES and MODIS Terra global measurements over ocean. Unlike previous studies, this analysis includes the contribution from clear regions in both cloud-free and partly cloudy CERES fields-of-view (FOVs). In partly cloudy conditions, the direct effect is derived by applying narrow-to-broadband radiance regressions to MODIS pixels identified as clear according to the Ignatov-Stowe aerosol retrieval algorithm. The narrow-to-broadband regression is a multi-channel regression that uses clear MODIS and CERES radiances. Radiative fluxes are determined from the radiances using angular distribution models developed from CERES. The global and regional 46-month time series of the aerosol direct effect will be presented to highlight its seasonal and year-to-year variation over roughly four years. To investigate the influence of cloud screening, the direct effect derived using clear regions identified with the Ignatov-Stowe approach will be compared with that using clear scenes from the MOD04 aerosol product for selected months.
A21E-07 09:24h
Modeling study of aerosol effects on climate system under comparisons with satellite retrievals
To estimate aerosol effects on climate change more reliably by modeling studies, it is essential to compare with a variety of measured aerosol and cloud parameters in detail. The global aerosol radiation-transport model, SPRINTARS, simulate three-dimensional distributions and direct/indirect effects of main tropospheric aerosols, i.e., black carbon, organic carbon, sulfate, soil dust, and sea salt, coupled with a general circulation model. In this study, distributions of aerosol and cloud parameters simulated by SPRINTARS are compared with satellite retrievals of MODIS and AVHRR in order to understand their uncertainties and to improve the present model. Under these comparisons the aerosol direct and indirect radiative forcings are estimated and the feedback mechanism of the hydrological cycle is analyzed.
A21E-08 09:36h
Aerosol direct radiative forcing over oceans from merged MODIS/CERES analysis
Using 10 months of Single Scanner Footprint (SSF) data that combines the multi-spectral Moderate Resolution Imaging Spectroradiometer (MODIS) cloud and aerosol products with the Clouds and the Earth's Radiant Energy System (CERES) Top-of-Atmosphere (TOA) broadband shortwave and longwave fluxes, we provided the observational estimates of the instantaneous cloud-free shortwave aerosol radiative forcing (SWARF) over the global oceans. Different from our previous research, we corrected for both the sample biases and the diurnal cycle of SWARF. We also accounted for the variation in the CERES TOA radiation fields due to changes in aerosol optical properties by using aerosol Angular Distribution Models that are constructed as functions of MODIS aerosol optical depth, the ratio of small mode to total aerosol optical depth, and Special Sensor Microwave Imager (SSM/I) wind speed. We estimated the instantaneous TOA SWARF from Terra overpass time to be -6.4 +- 2.6 Wm-2, and is -5.3 +- 1.7 Wm-2 over cloud-free oceans after accounting for the sample bias and diurnal averaging. Our study is a measurement-based assessment of cloud-free aerosol radiative forcing and could be used as a validation tool for numerical modeling studies.
A21E-09 09:48h
AEROSOL RADIATIVE FORCING A MEASUREMENT BASED APPROACH
Atmospheric aerosol introduces one of the largest uncertainties when quantifying the anthropogenic impact on the Earth-Atmosphere-System. Aerosol (climate) forcing estimates are usually based on simulations with global models. With increasing detail on atmospheric properties (via in-situ samples or remote sensing from ground or space) now, sufficient measurements are available to explore a measurement based approach (when quantifying the aerosol impact on climate). Based on a data synergy of ground statistics (AERONET: aerosol composition) and remote sensing from space (MODIS/MISR: aerosol concentration and distribution, MODIS: solar surface albedo, ISCCP: cloud cover/ vertical distribution), for the year 2000/2001 global monthly simulated fields for clear-sky and all-sky forcing are presented, where clear-sky estimates are compared to CERES data. In addition forcing detail forcing on location (atmosphere versus surface) and anthropogenic contribution (by considering only sub-micron aerosol sizes) is provided.