Simulating the Transport of Aerosols with GEOS-5
William Putman and Arlindo da Silva February 2013
Aerosols play an important role in both weather and climate. They are transported around the globe far from their source regions, interacting with weather systems, scattering and absorbing solar and terrestrial radiation, and modifying cloud micro- and macro-physical properties. They are recognized as one of the most important forcing agents in the climate system (Forster et al. 2007). Aerosols can change cloud cover by heating the atmosphere; they can reduce snow and ice albedo by the deposition of absorbing aerosols and subsequent heating and melting of snow and ice (e.g., Yasunari et al. 2011). Several missions in the Decadal Survey portfolio (e.g., ACE, Geo-CAPE), as well as the Cloud-Aerosol Transport System (CATS) mission, which will fly on the International Space Station (ISS), aim to get new information on aerosol distributions and properties.
Fully coupled atmosphere-aerosol models that are now available can account for many feedback mechanisms between aerosol concentrations and meteorological variables. For example, the GMAO has conducted a high resolution, two-year-long simulation with GEOS-5, using aerosol processes derived from the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART; Colarco et al., 2009) model. GOCART models the emissions, mixing, chemistry, and deposition of some of the key aerosol types found in the troposphere, like sulfates, dust, and black carbon.
The simulation, at a nominal 10km horizontal resolution with 72 vertical levels to 0.01 hPa, covers the period from May 2005 to May 2007. Such simulations help evaluate the performance of the high-resolution model in comparisons with satellite observations, and can be used to look at climate impacts on extreme weather events such as tropical storms or snowstorms. The simulation can also serve as a "nature run" that forms the basis for Observing System Simulation Experiments (OSSEs; Errico et al., 2013). For this, the high-resolution model simulation must capture the variations that are important to simulating the observations and that are targeted for the particular application. In addition to providing a realistic representation of the meteorology, with GOCART the simulation also provides information on global aerosol distributions, aerosol vertical profiles, and aerosol optical thicknesses.
The simulation was initialized from analyses generated with the GEOS-5 atmospheric data assimilation system and associated aerosol data assimilation. The emissions of dust and sea salt are dynamic, depending on the overlying surface winds; black carbon emissions from biomass burning are time-dependent and are retrieved from MODIS observations. Other emissions are from continually out-gassing volcanoes and from known explosive volcanic eruptions. Anthropogenic emissions are specified from standard inventories.
The animations here represent aerosol optical thickness from a portion of this high resolution GEOS-5 simulation, from September 2006 to April 2007. Major tropospheric aerosol types are emitted into the atmosphere and transported by winds, turbulence, and deep convection across the globe. Dust is represented with orange to red colors, sea salt with blue, organic and black carbon with green to yellow to white, and sulfates with ash brown to white. The yellow and red dots on the land surface indicate the locations of wildfires and human-initiated burning that have been detected by the MODIS instrument aboard the NASA Terra and Aqua satellites.
In the beginning of these animations, fires burning over South America and Africa can be seen emitting large amounts of black carbon into the atmosphere. At the same time, dust from the Sahara and the Middle East is picked up by winds and transported west, where it becomes wrapped up in two tropical cyclones over the Atlantic in early to mid September. Sulfur emissions from Europe, Asia, and North America are also pulled into the flow and advected eastward and poleward, and are occasionally pulled into cyclones. Mount Nyiragongo, in the Democratic Republic of the Congo, continuously erupts throughout the animations. The Tibetan Plateau is apparent as an obstacle to the westerly winds that have swept across the Gobi desert in Asia and picked up dust.
As time progresses in the simulation, biomass burning in South America begins to subside, while fires in Indonesia intensify and emit large amounts of black carbon into the atmosphere in October and November. At the same time, several strong cyclones in the western Pacific can be seen lifting sea salt aerosols and entraining dust, sulfates, and black carbon. In December, biomass burning in Africa appears to intensify, while fires in southeastern Australia ignite and emit black carbon that mixes with dust from the Australian deserts. These aerosols then get drawn into the westerlies of the southern hemisphere, which travel around the southern mid-to-high latitudes mostly undisturbed by land, but which interact with the polar easterlies to form large cyclones that can easily be seen in the distribution of sea salt aerosols. In North America, winter frontal systems can be periodically seen spinning up over the center of the continent and pushing sulfate emissions as well as advected dust, black carbon, and sea salt aerosols away from the landmass into the Atlantic and the Gulf of Mexico.
In January 2007, the simulation captures the eruption of the Karthala volcano on Grand Comore Island off Africa's eastern coast. This eruption began on January 12 and lasted for a few days. The sulfate aerosols disperse, mixing with the emissions from biomass burning in central Africa, and are pulled in two directions by opposing wind belts. By February, sulfate emissions from Mount Karthala appear to have subsided, but dust and black carbon continue to drift off the African continent and make their way to the Americas or to Europe. Later in February, biomass burning in Thailand and neighboring countries increases. Black carbon emissions quickly mix with dust from the Middle East and the Gobi and with sulfates from industry in China. These aerosols are then transported eastward, are pulled into mid-latitude cyclones that churn up sea salt, and eventually cross the Pacific to reach North America.
The GMAO is currently working on improving this simulation, with updated model physics, revised emission inventories, and with the incorporation of CO and CO2 tracers for wider applicability to upcoming NASA observing missions.
Since the initial release of Animation 1 in May of 2012, it and other results of the first GEOS-5 nature run have appeared in several thousand news or science articles, blogs, and social network postings. See a selection of media links about this work.
Colarco, P., A. da Silva, M. Chin and T. Diehl, 2009: Online simulations of global aerosol distributions in the NASA GEOS-4 model and comparisons to satellite and ground-based aerosol optical depth. J. Geophys. Res., 115, D14207, doi:10.1029/2009JD012820.
Errico, R. M., R. Yang, N. Privé, K.-S. Tai, R. Todling, M. Sienkiewicz, and J. Guo, 2012: Development and validation of observing system simulation experiments at the Global Modeling and Assimilation Office. Q. J. R. Meteorol. Soc., doi:10.1002/qj.2027.
Forster, P., and coauthors, 2007: Changes in atmospheric constituents and in radiative forcing, in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., Cambridge University Press, New York.
Yasunari, T. J., R. D. Koster, K.-M. Lau, T. Aoki, Y. C. Sud, T. Yamazaki, H. Motoyoshi, and Y. Kodama, 2011: Influence of dust and black carbon on the snow albedo in the NASA Goddard Earth Observing System version 5 land surface model. J. Geophys. Res., 116, D02210, doi:10.1029/2010JD014861.