A52A-01 INVITED
Global trends in visibility: implications for dust sources
There is a large uncertainty in the relative roles of human land use, climate change and carbon dioxide fertilization in changing desert dust source strength over the past 100 years, and the overall sign of human impacts on dust is not known. We used visibility data from meteorological stations in dusty regions to assess the anthropogenic impact on long term trends in desert dust emissions. Visibility data are available at thousands of stations globally from 1900 to the present, but we focused on 359 stations with more than 30 years of data in regions where mineral aerosols play a dominant role in visibility observations. We evaluated the 1974 to 2003 time period because most of these stations have reliable records only during this time. We first evaluated the visibility data against AERONET aerosol optical depth data, and found that only in dusty regions are the two moderately correlated. Correlation coefficients between visibility derived variables and AERONET optical depths indicate a moderate correlation (~0.47), consistent with capturing about 20% of the variability in optical depths. Two visibility derived variables appear to compare the best with AERONET observations: the fraction of observations with visibility less than 5km (VIS5) and the surface extinction (EXT). Regional trends show that in many dusty places, VIS5 and EXT are statistically significantly correlated with the palmer drought severity index (based on precipitation and temperature) or surface wind speeds, consistent with dust temporal variability being largely driven by meteorology. This is especially true for North African and Chinese dust sources, but less true in the Middle East, Australia or South America, where there are not consistent patterns in the correlations. Climate indices such as El Nino or the North Atlantic Oscillation are not correlated with visibility derived variables in this analysis. There are few stations where visibility measures are correlated with cultivation or grazing estimates on a temporal basis, although this may be a function of the very coarse temporal resolution of the land use datasets. On the other hand, spatial analysis of the visibility data suggests that natural topographic lows are not correlated with visibility, but land use is correlated at a moderate level. This analysis is consistent with land use being important in some regions, but meteorology driving interannual variability during 1974-2003.
A52A-02 INVITED
Satellite Monitoring of Long-Range Transport of Asian Dust Storms from Sources to Sinks
Among the many components that contribute to air pollution, airborne mineral dust plays an important role due to its biogeochemical impact on the ecosystem and its radiative-forcing effect on the climate system. In East Asia, dust storms frequently accompany the cold and dry air masses that occur as part of spring-time cold front systems. China's capital, Beijing, and other large cities are on the primary pathway of these dust storm plumes, and their passage over such popu-lation centers causes flight delays, pushes grit through windows and doors, and forces people indoors. Furthermore, during the spring these anthropogenic and natural air pollutants, once generated over the source regions, can be transported out of the boundary layer into the free troposphere and can travel thousands of kilometers across the Pacific into the United States and beyond. In this paper, we will demonstrate the capability of a new satellite algorithm to retrieve aerosol optical thickness and single scattering albedo over bright-reflecting surfaces such as urban areas and deserts. Such retrievals have been dif-ficult to perform using previously available algorithms that use wavelengths from the mid-visible to the near IR because they have trouble separating the aerosol signal from the contribution due to the bright surface reflectance. The new algorithm, called Deep Blue, utilizes blue-wavelength measurements from instruments such as SeaWiFS and MODIS to infer the properties of aerosols, since the surface reflectance over land in the blue part of the spectrum is much lower than for longer wavelength channels. Deep Blue algorithm has recently been integrated into the MODIS processing stream and began to provide aerosol products over land as part of the opera-tional MYD04 products. In this talk, we will show the comparisons of the MODIS Deep Blue products with data from AERONET sunphotometers on a global ba-sis. The results indicate reasonable agreements between these two. These new satellite products will allow scientists to determine quantitatively the aerosol properties near sources and their evolution along transport pathway using high spatial resolution measurements from SeaWiFS and MODIS-like instruments. We will also utilize the multiyear satellite measurements from MODIS and SeaWiFS to investigate the interannual variability of source strength, pathway, and radia-tive forcing associated with these dust outbreaks in East Asia.
A52A-03
Airborne Dust Modified the North American Climate During the 1930's Dust Bowl
In the 1930's Dust Bowl, drought in Mid-Western North America, in conjunction with wide-scale planting of drought-vulnerable crops, resulted in massive dust storms. The presence of dust in the atmosphere may have directly altered the energy budget of North America by the scattering and absorption of radiation and thus may have acted as a feedback to the regional drought conditions. Through a climate modeling sensitivity study of North American climate investigating the impact of airborne dust during the 1930's (using a regional model, RegCM3), we find that areas with moderate to high dust-loading have reduced surface temperatures (~1K) and reduced evapotranspiration (~0.5 mm/day). We also find spatially-coherent, statistically significant changes in precipitation patterns over eastern North America during Spring, Summer, and Fall: areas gain and lose as much as 2 mm/day of precipitation. We are working on a more detailed analysis to determine the causal relationship(s) between airborne dust and precipitation patterns; we hypothesize that the spatially non-uniform change in the energy budget, caused by dust loading, modifies regional dynamics and indirectly modifies precipitation patterns.
A52A-04
On the Seasonal and Annual Variability of Saharan Mineral Dust in the Caribbean
Dust plumes originating in North Africa span the tropical Atlantic, depositing their load over large expanses, including the Caribbean and Gulf of Mexico. Mineral dust aerosols are known to play an important role in the climate system, affect cloud properties and impact many oceanic biogeochemical cycles. The seasonal and annual export of mineral dust is highly variable and controlled by large scale circulation patterns. Thus it is important to identify the key constituents of the large scale circulation that control dust export if we are to understand the impacts of mineral dust. In a new approach, we ascribe changes in the large scale circulation to the movement and strength of major "Centers of Action." We compare the atmospheric variability to variability found in TOMS/NIMBUS-7 and TOMS/Earth Probe satellite aerosol data over the Caribbean Basin. We find an increasing trend in the quantity of mineral dust over the Caribbean during the 1980s, with elevated dust levels extending into the 21st century. Furthermore, the yearly duration of the time in which dust is present in the Caribbean has been drastically increasing. We show that both increased emissions and changes in transport patterns contribute to the observed trend. By use of the "Centers of Action" approach we identify the longitudinal displacement and the pressure of the Hawaiian High and the longitudinal displacement of the Azores High as key players, with the Hawaiian High being especially important for intense dust events in the Caribbean.
A52A-05 INVITED
Interactions between Dust Aerosols and Ecosystems in the Global Oceans
Iron is an important micro-nutrient for phytoplankton in the oceans. Iron is brought to the surface layers of the ocean, where most of the phytoplankton live, from the deep ocean, the coasts, and, most importantly, by soil dust aerosols. Since phytoplankton represent the base of the food web, dust aerosols play a role in ocean ecosystem structure, function, and variability. Since the distribution of dust aerosols, and deposition of iron from them, is variable in time and space, so is the importance of dust aerosols for ocean ecosystems. Using a coupled three-dimensional biogeochemical model of the global oceans, we investigated the relationships between dust aerosols and ocean ecosystems. By varying the amount of iron deposition deriving from dust, we can quantify the local, regional, and global effects on ocean phytoplankton, ecosystems, and primary production. In the most extreme experiment, all dust aerosol deposition was ceased for 25 years. Reduction in dissolved iron concentrations was greater than 60 per cent. Phytoplankton populations nearly disappeared in the central ocean gyres. A 10 per cent decline was observed in the North Pacific. But surprisingly, global chlorophyll concentrations did not change, and global primary production declined by only about 3 per cent. Losses incurred by reduced dissolved iron were compensated by increased nitrate, enhancing nitrogen- limited ecosystems at the expense of iron-limited ecosystems. These results show the importance of regional variability in the relationship between dust aerosols and ocean ecosystems.
A52A-06
Combustion Iron Distribution and Deposition
Iron is hypothesized to be an important micronutrient for ocean biota, thus modulating carbon dioxide uptake by the ocean biological pump. Studies have assumed that atmospheric deposition of iron to the open ocean is predominantly from mineral aerosols. For the first time, we model the source, transport and deposition of iron from combustion sources. Iron is produced in small quantities during fossil fuel burning, incinerator use, and biomass burning. The sources of combustion iron are concentrated in the industrialized regions and biomass burning regions, largely in the tropics. Model results suggest that combustion iron can represent up to 50 percent of the total iron deposited, but over open ocean regions is usually less than 5 percent of the total iron, with the highest values (less than 30 percent) close to the East Asian continent in the North Pacific. For ocean biogeochemistry the bioavailability of the iron is important, and this is often estimated by the fraction which is soluble (Fe(II)). Previous studies have argued that atmospheric processing of the relatively insoluble Fe(III) occurs to make it more soluble (Fe(II)). Modeled estimates of soluble iron amounts based solely on atmospheric processing as simulated here cannot match the variability in daily averaged in situ concentration measurements in Korea, which is located close to both combustion and dust sources. The best match to the observations is that there is substantial direct emissions of soluble iron from combustion processes. If we assume observed soluble Fe/black carbon (BC) ratios in Korea are representative of the whole globe, we obtain the result that deposition of soluble iron from combustion contribute 20-100 percent of the soluble iron deposition over many ocean regions. This implies that more work should be done refining the emissions and deposition of combustion sources of soluble iron globally.
A52A-07
Effects from Multiple Species of Insoluble Aerosol Particles on the Glaciation and Precipitation Production of Deep Convective Clouds in a Tropical Atlantic Hurricane
During July 2005, the Tropical Cloud Systems and Processes (TCSP) experiment of NASA occurred in Costa Rica. The general mission of TCSP was to investigate the formation and evolution of tropical cyclones. Improved understanding of the mechanisms and effects of ice nucleation in deep convective clouds in a given tropical cyclone has been a key goal of the TCSP program because such clouds produce much of the ice for its upper- level cirrus outflow. An empirical parametrisation of heterogeneous ice nucleation has been formulated for application in cloud and large-scale atmospheric models. It represents dependencies on predicted mass concentrations for multiple chemical species of ice nucleus (IN) aerosols. The scheme includes condensation-, immersion- and contact- freezing modes, in addition to vapour deposition, as mechanisms for heterogeneous nucleation. These species of IN include mineral dust, black carbon and biogenic particles. The scheme represents the suppression of heterogeneous nucleation at low humidities and warm subzero temperatures seen in several laboratory studies. Simulations with a microphysical model of deep convection observed in TCSP are presented. Impacts of various chemical species of IN on the mechanisms for glaciation of deep convective clouds and of their outflow to cirrus are shown. Modification of precipitation production by altered IN loadings, as seen in desert dust epsisodes near the tropical Atlantic, is discussed.
A52A-08
A Trend in the Northward Transport of Saharan Dust and Its Links to the Trend in North Atlantic Tropical Cyclone Energy
During late spring and early fall, Saharan dust is transported toward the tropical North Atlantic Ocean by African easterly waves. The warm and dry anomalies associated with the Saharan dust, the Saharan air layer, can suppress the tropical cyclone (TC) activity over the tropical North Atlantic Ocean. On the other hand, it is well known that increasing sea surface temperatures (SSTs) over the tropical North Atlantic Ocean is responsible for the increasing tropical cyclone intensity in the last two decades. In this study, we analyze two 27-year simulations (MATCH and GEOS-4) of dust transport over the tropical North Atlantic Ocean using, respectively, NCEP reanalysis and GMAO assimilated winds and temperatures for 1979-2005. We evaluate the trends in the transport of Saharan dust together with the trends in surface temperatures and TC energy over the North Atlantic Ocean. We find that the increasing trend in TC energy has a spatial pattern similar to the trend in covective available potential energy (CAPE), which is further related to both increasing trends in sea surface temperatures and in northward transport of Saharan dust over the North Atlantic Ocean.