Akiko Higurashi

Tropospheric Aerosol Optical Thickness from the GOCART Model and Comparisons with Satellite and Sun Photometer Measurements

The Georgia Institute of Technology‐Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model is used to simulate the aerosol optical thickness t for major types of tropospheric aerosols including sulfate, dust, organic carbon (OC), black carbon (BC), and sea salt. The GOCART model uses a dust emission algorithm that quantifies the dust source as a function of the degree of topographic depression, and a biomass burning emission source that includes seasonal and interannual variability based on satellite observations. Results presented here show that on global average, dust aerosol has the highest t at 500 nm (0.051), followed by sulfate (0.040), sea salt (0.027), OC (0.017), and BC (0.007). There are large geographical and seasonal variations of t, controlled mainly by emission, transport, and hygroscopic properties of aerosols. The model calculated total ts at 500 nm have been compared with the satellite retrieval products from the Total Ozone Mapping Spectrometer (TOMS) over both land and ocean and from the Advanced Very High Resolution Radiometer (AVHRR) over the ocean. The model reproduces most of the prominent features in the satellite data, with an overall agreement within a factor of 2 over the aerosol source areas and outflow regions. While there are clear differences among the satellite products, a major discrepancy between the model and the satellite data is that the model shows a stronger variation of t from source to remote regions. Quantitative comparison of model and satellite data is still difficult, due to the large uncertainties involved in deriving the t values by both the model and satellite retrieval, and by the inconsistency in physical and optical parameters used between the model and the satellite retrieval. The comparison of monthly averaged model results with the sun photometer network [Aerosol Robotics Network (AERONET)] measurements shows that the model reproduces the seasonal variations at most of the sites, especially the places where biomass burning or dust aerosol dominates.

Global three‐dimensional simulation of aerosol optical thickness distribution of various origins

A global three-dimensional model that can treat transportation of various species of aerosols in the atmosphere is developed using a framework of an atmospheric general circulation model (AGCM). Main aerosols in the troposphere, i.e., soil dust, carbonaceous (organic and black carbon), sulfate, and sea-salt aerosols, are introduced into this model. Prior to the model calculations the meteorological parameters are calculated by the AGCM with the nudging technique using reanalysis data. To evaluate aerosol effects on the climate system and to compare simulated results with observations, the optical thickness and Angstrom exponent are also calculated taking into account the size distribution and composition. The model results are validated by both measured surface aerosol concentrations and retrieved aerosol optical parameters from National Oceanic and Atmospheric Administration/Advanced Very High Resolution Radiometer. A general agreement is found between the simulated result and the observation globally and seasonally. One of the significant results is that the simulated relative contribution of anthropogenic carbonaceous aerosols to the total optical thickness is comparable to that of sulfate aerosols at midlatitudes of the Northern Hemisphere, which agrees with recent observations. This result leads to a conclusion that the radiative effect evaluation of aerosols on the climate system is necessary to be modified because optical properties of carbonaceous aerosols are different from those of sulfate aerosols. The other finding is that the seasonal shift off the west coast of North Africa observed by satellites, i.e., the latitude of the maximum optical thickness moves seasonally, is also reproduced in consideration of a mixed state of soil dust and carbonaceous aerosols.

A possible correlation between satellite-derived cloud and aerosol microphysical parameters

The column aerosol particle number and low cloud microphysical parameters derived from AVHRR remote sensing are compared over ocean for four months in 1990. There is a positive correlation between cloud optical thickness and aerosol number concentration, whereas the effective particle radius has a negative correlation with aerosol number. The cloud liquid water path (LWP), on the other hand, tends to be constant with no large dependence on aerosol number. This result contrasts with results from recent model simulations which imply that there is a strong positive feedback between LWP and aerosol number concentration. Estimates for indirect forcing over oceans derived from the satellite data/model comparison range from −0.7 to −1.7 Wm−2.

Overview of the Atmospheric Brown Cloud East Asian Regional Experiment 2005 and a study of the aerosol direct radiative forcing in east Asia

2005 which is smaller in magnitude than in the APMEX region, mainly because of large cloud fraction in this region (0.70 at Gosan versus 0.51 at Hanimadhoo in the ISCCP total cloud fraction). We suggest there may be an underestimation of the forcing due to overestimation of the simulated cloudiness and aerosol scale height. On the other hand, the possible error in the simulated surface albedo may cause an overestimation of the magnitude of the forcing over the land area. We also propose simple formulae for shortwave radiative forcing to understand the role of aerosol parameters and surface condition to determine the aerosol forcing. Such simple formulae are useful to check the consistency among the observed quantities.

Development of a two-channel aerosol retrieval algorithm on a global scale using NOAA AVHRR

Abstract This study proposes a two-channel satellite remote sensing algorithm for retrieving the aerosol optical thickness and the Angstrom exponent, which is an index for the aerosol size distribution. An efficient lookup table method is adopted in this algorithm to generate spectral radiances in channels 1 and 2 of National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) over ocean areas. Ten-day composite maps of the aerosol optical thickness and the Angstrom exponent have been obtained from AVHRR global area coverage data in January and July of 1988. Aerosol optical thickness maps show that the major aerosol sources are located off the west coast of northern and southern Africa, and the Arabian Peninsula. The most important contributor is soil-derived particles from the Sahara Desert that cross the Atlantic Ocean. The authors’ optical thickness values tend to be larger than values given by the NOAA operational algorithm. A 10-day composite map of Angstrom...

Significance of direct and indirect radiative forcings of aerosols in the East China Sea region

� 8W /m 2 at the top of atmosphere (TOA) and � 10 to � 23 W/m 2 at Earth’s surface of Gosan (33.28N, 127.17E) and Amami-Oshima (28.15N, 129.30E) sites, though there is a large regional difference caused by changes in the aerosol optical thickness and single scattering albedo. The cloud forcing is estimated as � 20 to � 40 W/m 2 , so that the aerosol direct forcing can be comparable to the cloud radiative forcing at surface. However, the estimate of the aerosol direct forcing thus obtained strongly depends on the estimation method of the aerosol properties, especially on the single scattering albedo, generating a method difference about 40%. The radiative forcing of the aerosol indirect effect is roughly estimated from satellite method and SPRINTARS model as � 1t o� 3W /m 2 at both TOA and surface. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 1610 Global Change: Atmosphere (0315, 0325); 9320 Information Related to Geographic Region: Asia;

A use of two‐channel radiances for an aerosol characterization from space

We report new satellite retrievals of an aerosol particle size index, Angstrom exponent, along with aerosol optical thickness, obtained from a two channel algorithm for NOAA/AVHRR and ADEOS/OCTS satellite-borne radiometers. The results show characteristic features of the global aerosol size index on global scale. Small aerosol particles were detected in fairly large-scale areas around mid-latitude industrial regions and tropical biomass burning regions, whereas large aerosol particles were detected over most of the ocean area and over subtropical arid areas.

A Study of Global Aerosol Optical Climatology with Two-Channel AVHRR Remote Sensing

Global distributions of the aerosol optical thickness and Angstrom exponent are estimated from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer channel-1 and -2 radiances for four months in 1990. Global distributions of those Angstrom parameters are consistent with present knowledge on the distributions of desert-derived, biomass-burning, and anthropogenic pollutant aerosols obtained by ground- based and aircraft measurements. Especially, it is found that thin anthropogenic aerosols can be identified with large Angstrom exponent values around the east coast of North America, Europe, and eastern Asia. Satellite- retrieved values of Angstrom parameters are further compared with measured spectral optical thickness obtained by the National Aeronautics and Space Administration Aerosol Robotic Network sky radiometer network.

Modeling study of long‐range transport of Asian dust and anthropogenic aerosols from East Asia

[1] A three-dimensional aerosol transport-radiation model, SPRINTARS, successfully simulates the long-range transport of the large-scale Asian dust storms from East Asia to North America which crossed the North Pacific Ocean during the springtime of 2001 and 2002. It is found from the calculated dust optical thickness that 10 to 20% of the Asian dust around Japan reached North America. The simulation also reveals the importance of the contribution of anthropogenic aerosols, which are carbonaceous and sulfate aerosols emitted from the industrialized areas in East Asia, to air turbidity during the dust storms. The contribution of the anthropogenic aerosol to the total optical thickness is simulated to be of a comparable order to that of the Asian dust, which is consistent with the observed values of the particle size index from the satellite and ground-based sun/ sky photometry. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0365 Atmospheric Composition and Structure: Troposphere— composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 3359 Meteorology and Atmospheric Dynamics: Radiative processes. Citation: Takemura, T., I. Uno, T. Nakajima, A. Higurashi, and I. Sano, Modeling study of long-range transport of Asian dust and anthropogenic aerosols from East Asia, Geophys. Res. Lett., 29 (24), 2158, doi:10.1029/2002GL016251, 2002.

Intercomparison of Satellite Retrieved Aerosol Optical Depth over the Ocean

For an 8-month period aerosol optical depth (AOD) is compared, derived over global oceans with five different retrieval algorithms applied to four satellite instruments flown on board three satellite platforms. The Advanced Very High Resolution Radiometer (AVHRR) was flown on board NOAA-14, the Ocean Color and Temperature Scanner (OCTS) and the Polarization and Directionality of the Earth’s Reflectances (POLDER) on board the Advanced Earth Observing Satellite(ADEOS), and the Total Ozone Mapping Spectrometer (TOMS) on board the Earth Probe satellites. The aerosol data are presented on the same format and converted to the same wavelength in the comparison and can therefore be a useful tool in validation of global aerosol models, in particular models that can be driven with meteorological data for the November 1996 to June 1997 period studied here. Large uncertainties in the global mean AOD are found. There is at least a factor of 2 difference between the AOD from the retrievals. The largest uncertainties are found in the Southern Hemisphere, and the smallest differences mostly near the continents in the Northern Hemisphere. The largest relative differences are probably caused by differences in cloud screening.