A32B-01 INVITED 10:20h
Aerosols, Clouds and Global Dimming
Several studies have reported that there was a large reduction of surface solar radiation worldwide during the 1950s to 1980s, as large as 5 to 10 W.m-2 and a partial recovery of this dimming during the following decades. If this was indeed the case on a global scale, then this dimming trend from the 1950s to the end of the twentieth century must have played a large role in the observed surface temperature trends. Given the uncertainties in surface and satellite solar radiation measurements, we need independent determination of the magnitude and causes of the dimming trends. In this study we explore if trends in radiative forcing due to aerosols, particularly soot and dust, can account for the observed dimming. We include both the direct and the indirect effect of aerosols as well as the observed trends in cloudiness. Satellite based Aerosol data from MODIS, CERES and AVHRR are combined with surface based observations (AERONET and GEBA), field observations (INDOEX, ACE-Asia) and aerosol assimilation models (GOCART) to estimate the global reduction of solar radiation from aerosols and clouds over selected land and oceanic regions of the globe for the last several decades. Our focus is on Asia, N. America, Indian and Pacific oceans where data are available from aircraft, surface and satellites. For the regions investigated in this study, the surface dimming can range from -5 to -20 W.m-2 for the entire twentieth century, while the trends for particular decadal periods depend on the assumed emission histories for aerosols, particularly black carbon. Long range transport of aerosols from populated regions to the marine environment, several thousand kilometers away, is the major factor contributing to the global nature of the dimming. While, the present data and models are not of sufficient accuracy to confirm the observed trends in global dimming, this study clearly shows that global dimming is a major factor in the observed temperature trends. For example, we show that over S. Asia and Indian Ocean, it is nearly impossible to explain the observed trends (for 1950 to 2000) in temperatures and rainfall without accounting for the effects of global dimming.
A32B-02 10:44h
Measuring the Unmeasurable: Why Measurements Alone Cannot Quantify Aerosol Radiative Forcing of Climate Change
Present estimates of radiative forcing of climate change by aerosols, based mainly on modeled aerosol loading and properties, vary greatly. This situation has given rise to suggestions that aerosol forcing be measured rather than modeled. In principle, ae rosol direct forcing might be determined from satellite measurements of irradiance or radiance. However irradiance measurements are problematic because of the need for extensive homogeneous cloud-free scenes; radiance measurements, albeit with much less s tringent spatial homogeneity requirements, rely on radiance-to-flux conversion with attendant need for aerosol phase function. Both approaches require accurate surface albedo, including spectral bi-directional reflectance for the radiance approach. Alte rn atively surface-based measurement of direct and diffuse downwelling surface irradiance relative to that of a Rayleigh sky yield instantaneous aerosol forcing of surface irradiance to a few watts per square meter, but transferring this surface forcing t o t op-of-atmosphere forcing requires aerosol single scattering albedo and asymmetry parameter. Restriction of measurements to cloud-free situations, would limit measurements to situations of low relative humidity, a concern given the great increase in fo rcin g by hygroscopic aerosols with increasing RH. Generalization from times and locations of measurement requires knowledge of the geographical and vertical distribution of aerosol extensive and intensive optical properties, including humidity dependence. Sim ilar considerations apply to direct measurement of aerosol indirect forcing due to enhancement of cloud reflectance and/or persistence. Determination of aerosol forcing of climate change over the industrial period requires attribution of aerosol inf luence s to natural vs. anthropogenic aerosol as a function of secular time. All these considerations speak to the need for understanding and model-based representation of aerosol amount, composition, and microphysical properties and their dependence on e mission s. Central to developing such understanding and model-based representation are measurements of the properties and evolution of aerosols and characterization of the cloud drop activation process and its relation to controlling variables. These cons ideratio ns suggest that estimates of aerosol forcing will remain strongly model-dependent, with attendant need for rigorous evaluation of the performance of model components that describe aerosol loading and properties and radiative forcing and cloud modification by aerosols of well specified properties.
A32B-03 10:56h
The Long-Term (1965-2003) Relationship Between Dust Concentrations in the Trade Winds at Barbados and Tropical Cyclone and Hurricane Activity in the Atlantic and Caribbean
Recent studies show that the development of tropical cyclones (TCs) can be strongly impacted by interactions with the Saharan Air Layer (SAL) [Dunion, J.P., and C.S. Velden, The Impact of the Saharan Air Layer on Atlantic Tropical Cyclone Activity. Bull. Amer. Meteor. Soc., 85 353-365 (2004)]. The SAL is a layer of hot, dry, dust-laden air that is often present over large areas of the tropical and sub-tropical North Atlantic and Caribbean Sea during the summer and fall. In the presence of the SAL tropical waves and disturbances often do not strengthen as predicted by models and TCs tend to weaken. A number of mechanisms have been proposed to explain these effects, some of them involving dust-related processes. If dust does indeed play a role in modulating TC intensity, then changes in climate that affect dust generation and transport over the Atlantic could have an impact on the occurrence and properties of TCs. Large changes in dust transport have indeed occurred over this region as shown by aerosol measurements made on Barbados in the West Indies since 1965. [Prospero, J. M., Lamb, P. J., African Droughts and Dust Transport to the Caribbean: Climate Change Implications. Science 302: 1024 1027, 2003]. Large interannual-decadal changes in dust concentrations in the trade winds are highly anti-correlated with rainfall in the Sahel-Soudano region of North Africa, a region that includes many major dust sources. If African dust carried in the SAL is playing a role in modulating TC activity, then we might expect to see a relationship between dust transport and various measures of TC activity. In this presentation we report on the relationships between Barbados dust concentrations and a number of measures of TC activity including the number of named storms, the number of hurricanes, the number of major hurricanes and the Accumulated Cyclone Energy (ACE) index, which accounts for the collective intensity and duration of Atlantic tropical storms and hurricanes during a given hurricane season. Preliminary results suggest that TC activity was indeed lower during years when dust concentrations were high but the relationship is not a clear one.
A32B-04 11:08h
Climatology of Aerosol Radiative Properties Over the Oceans
Evaluation of the direct radiative forcing by atmospheric aerosols requires knowledge of both the quantity and the radiative properties of the particles. Quantity is generally described by aerosol optical depth, which can be measured globally from space. Satellite sensors are not well suited for measuring the needed radiative properties of the particles, in particular, the relative amounts of scattering versus absorption and the angular distribution of the scattered light. The single-scattering albedo, defined as the ratio of aerosol scattering coefficient to extinction coefficient, describes the absorptive nature of the particles. The angular scattering properties are described by two different properties, depending on the measurement method: the asymmetry parameter is the average cosine of the scattering angle, and the hemispheric backscatter fraction is the fraction of the light that is scattered back towards the light source. This paper presents the results of a review of measurements of these radiative properties from ground stations, ships, and aircraft over the oceans. Most of the publications come from short-term, intensive field campaigns, with a fairly limited number of observation days. In addition to the survey of 52 published papers, 19 of which reported measurements of the needed radiative properties, two databases of long-term, ground-based measurements of aerosol radiative properties were analyzed to describe seasonal variations at the few oceanic sites where long-term measurements are available. The AERONET database contains results from 9 island or coastal stations with multi-year observations of column-averaged asymmetry parameter, although these sites rarely encounter conditions with sufficient aerosol loadings for determination of single-scattering albedo. The CMDL database contains results from four island or coastal stations with multi-year, in-situ observations at the surface of hemispheric backscatter fraction and single-scattering albedo. The AERONET and CMDL databases contribute a total of nearly 10,000 observation-days towards the climatology, overwhelming the statistical contribution from the roughly 900 observation-days of intensive field campaigns. Intensive field campaigns represent a much larger geographic area than the few locations in the long-term databases. Together, the two types of data sets provide the basis for a measurement-based evaluation of direct radiative forcing. In addition to presenting the climatological averages of the aerosol radiative properties, the paper will discuss the limitations of the current data sets in terms of their spatial and temporal coverage and the adequacy of the parameters that are measured.
A32B-05 11:20h
Direct Observations of the Aerosols Effects on Terrestrial Carbon and Water Cycles
We present multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon and water cycle. The daytime growing season (summer) Co2 and water vapor flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO2 and water vapor exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture. The water vapor - aerosol feedback is a more complex function of leaf area index and the surface soil moisture, and needs to be considered in future assessment studies. Results from ongoing modeling studies to study the influence of aerosol - clouds within the climate system including the biophysical response will also be presented.
http://www4.ncsu.edu/~dsniyogi/
A32B-06 11:32h
Evaluation of Global Aerosol Direct Radiative Forcing Using the AERONET Data
The AERONET is a world-wide sun/sky radiometer measurement network and has several years of observation data at more than 300 locations. The size distribution and complex refractive indices are retrieved from the sky radiance measurement, which can be utilized for calculating almost all the optical variables by Mie theory. Among those variable, the optical thickness, single scattering albedo and asymmetry factor are used for evaluating the aerosol direct radiative forcings (ADRFs) in short wavelength regime. The Column Radiative Model from the NCAR CCM is used to estimate ADRF at local or in regional scale. The ADRF of the Asian dusts and non-dust aerosols are compared during the ACE-Asia IOP as a case study. The regional mean aerosol forcings are also investigated, the ADRF in Asia regions and Sahara Desert is turned out to be relatively large, which is due to the big optical thickness of dust aerosols. The Asian dusts influence a lot in Spring while the Saharan Dust influence all the year. The aerosol's influences on the solar irradiation at the other regions, where the AERONET sites exist, are examined extensively, and compared with a 3D aerosol transport model (SPRINTARS)'s calculation and satellite (CERES) measurement. Moreover, the categorization of the aerosol types is tried out of AERONET data, which have been collected from 1993 to 2003. Six types of aerosol types can be identified - mineral dust, biomass burning aerosol, tropospheric background, urban, marine, and anthropogenic carbon. Using the aerosol categorization various aerosol optical properties are investigated, and the dominant aerosols or the mixed status of several aerosol types are examined in several regions.
http://air.snu.ac.kr
A32B-07 11:44h
Radiation Closure Studies for Clear-Sky Conditions During the ARM 2003 Aerosol Intensive Observation Period
During May 2003 the ARM aerosol community held a month-long aerosol intensive observation period where many, and often redundant, aerosol physical, chemical, and optical properties were measured. One of the many goals of the study was to assess the agreement between down-welling solar (shortwave) irradiance measurements and solar models. In this study we test four models for total column aerosol loads ranging between 0.06 to 0.36 optical depths at 500 nm whenever we are certain that the sky is completely free of clouds. The four models tested include MODTRAN, RAPRAD, SBDART, and SMARTS2. The inputs to the models are carefully screened, paying particular attention to the aerosol single scattering albedo and the asymmetry parameters, and are identical for each model. Likewise, the extraterrestrial spectral irradiance used as an input is identical for each model. Most of the presentation discusses broadband results, but we will touch on spectral behavior as time allows.
A32B-08 11:56h
Raman Lidar Measurements of Aerosol Profiles Over the Southern Great Plains
Profiles of aerosol backscattering and extinction, water vapor mixing ratio, and relative humidity have been routinely measured around-the-clock during the last several years by a turnkey, automated Raman lidar system located at the Dept. of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Climate Research Facility (CRF). These Raman lidar aerosol extinction profiles are used to evaluate aerosol extinction profiles and aerosol optical thickness (AOT) simulated by the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) and Interaction with Chemistry and Aerosols (INCA) global aerosol models' simulations for the year 2000. The GOCART and INCA model average aerosol extinction profiles show good agreement with the Raman lidar profiles for altitudes above about 2 km; below 2 km the average model profiles are significantly (30-50%) lower than the Raman lidar profiles. The vertical variability in average aerosol extinction profiles simulated by these models is less than the variability in the corresponding Raman lidar profiles. Planetary Boundary Layer (PBL) heights derived from the Raman lidar water vapor and aerosol profiles acquired between January 2000 and December 2002 were used to evaluate the amount of aerosol optical thickness (AOT) within and above the PBL. A significant fraction ($>$25%) of the AOT was typically located above the PBL. Raman lidar measurements of the aerosol extinction/backscatter ratio acquired during the May 2003 Aerosol Intensive Operations Period (IOP) are used in conjunction with airborne remote and in situ aerosol measurements to estimate aerosol refractive index and single scatter albedo. Comparisons of these and other results derived from the Raman lidar measurements acquired during this IOP with measurements from other remote and in situ sensors are presented.
A32B-09 12:08h
A First Look at How Climatic Variation in the Southern Hemisphere Impacts Aerosol and Trace Gas Production From Biomass Burning
A conceptual model of how climatic changes in the Southern Hemisphere impact aerosol and trace gas production associated with Biomass Burning emissions from South America, Southern Africa and Australia is presented. The conceptual model makes use of daily meteorological processes that influence the emission and transport of Southern Hemisphere aerosols and trace gases observed during a series of large-scale intensive atmospheric chemistry and remote sensing validation activities. Observations of atmospheric transports associated with aerosol and trace gases from biomass burning and industrial emissions used in this paper originate from the Southern African Fire-Atmosphere Research Initiative (SAFARI-92) and its follow on activity, the Southern African Regional Science Initiative (SAFARI 2000). Results produced during additional campaigns, most notably, the Brazilian-led Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) will also be utilized. The creation of this empirical model uses an approach that is able to extrapolate data obtained during intensive field campaigns to the regional and inter-hemispherical scale using remotely sensed data. Emphasis will be placed on up-scaling the knowledge gained during focused campaigns to create a conceptual model of the linkages and processes of the physical and biological system on a hemispherical scale. Human induced alterations to the coupled land-atmosphere exchanges will also be discussed conceptually as a potentially important driver of the system.