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This section originally contains an animated flash presentation, Click here to switch to the full version.
Introduction |
What does it mean to use MODIS retrievals consistently? |
Estimating Aerosol Radiative Effect |
Results
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Aerosols exert an important effect on the Earth's radiative balance by directly absorbing and scattering incoming sunlight. Traditionally, chemical transport and general circulation models enjoyed a monopoly on estimating the role of aerosols in the Earth's climate.
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Observations of aerosols from ground-based, airborne or satellite instruments have been used only to validate these models. The prevailing strategy dictates that measurements improve models, and then models, not measurements, answer climate questions.
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However, results of model simulations of aerosol effect on the distribution of solar radiation vary considerably because aerosols vary considerably. Each type of aerosol affects sunlight differently by absorbing and scattering the light in varying amounts, in different directions and at different wavelengths. Simulating the wide variety of aerosol effects in a model remains a difficult task. The cumulative effect of these differences in aerosol properties can result in vastly different estimates of global change, and in fact be the difference between whether the planet is warming or cooling.
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In this study Remer and Kaufman present an observationally based estimate of the aerosol radiative effect in clear-sky situations over the oceans. They bypass much of the guesswork in assigning aerosol optical properties to different types of aerosols by using a consistent set of information gleaned from the satellite observations. The satellite sensor that they use is the MODerate resolution Imaging Spectroradiometer (MODIS). If used consistently MODIS will provide highly accurate estimates of the reflected sunlight by the particles. An example of MODIS-measured aerosol radiative effect for four seasons is shown in the figure.
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The animated image originally accompanying this page contains 3 different images showing the seasonal change of the aerosol radiation effect at the top of the atmosphere as captured from the Terra satelite.
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The figure on this page illustrates the MODIS procedure for retrieving aerosol optical thickness over ocean. MODIS measures the spectral reflectance in 6 solar reflectance channels ranging from 0.55 μm to 2.130 μm. The procedure then must find the right combination of aerosol properties that produced these reflectances. To simplify the problem, the procedure limits the potential aerosol properties to 9 well-defined sets of aerosol characteristics that we will call aerosol modes. Each mode has unique physical and optical characteristics that through radiative transfer calculations define a unique spectral reflectance at the top of the atmosphere. The procedure finds two modes, one with smaller particles and one with larger particles, that when combined, result in the best match to the real reflectances measured by MODIS.
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There are three characteristics that define the aerosols' optical properties: the amount of light that the particles deflect from the incident beam, the amount of light absorbed and the particles' size. Remer and Kaufman acknowledge that MODIS retrievals of size and absorption are less accurate than retrievals of total light extinction. However, they point out that the complete set: extinction, size and absorption chosen by MODIS must be the best fit to the reflectances at the top of the atmosphere (See figure). Using the consistent set chosen by the retrieval, rather than mixing and matching different properties from different sources, insures that the set of aerosol characteristics will be the best match to the measured reflectances at top of atmosphere.
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The second image in this page is of the distribution of the optical depth among the different modes
observed from the Terra satellite for three such sections and one section from the Aqua
satellite are shown here.
These examples demonstrate two points. The first is that the global distribution
of aerosol optical properties is more complex than simply the distribution of aerosol optical thickness,
or even the distribution of fine mode fraction.
The second point is that differences between Terra and Aqua demonstrate the sensitivity
of the retrieval algorithm to small perturbations in instrument calibration and software.
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The aerosol effect is the difference in radiative flux at top of the atmosphere between an atmosphere with aerosol and an atmosphere with no aerosol. Radiative flux is not the same as satellite-observed reflectances. Reflectances are measured at a specific angle and wavelength. Flux is integrated over all angles and for the entire solar spectrum.
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Remer and Kaufman use a radiative transfer model (CLIRAD-SW) to model the radiative flux. Model inputs are based on the 9 MODIS aerosol modes. The model is run both with aerosol optical thickness (a measure of light extinction) and for an atmosphere with no aerosol. The difference in these two runs is the radiative effect. The satellite observes the scene, once per day, providing instantaneous aerosol conditions. Values of aerosol effect based on instantaneous conditions are normalized to provide the 24 hour average. The figure shows calculated 24-hour aerosol radiative effect for a variety of aerosol loadings for each of the 9 aerosol modes of the MODIS retrieval.
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Each MODIS observation provides two chosen modes, the relative weight between them and the aerosol optical thickness. Thus, all that Remer and Kaufman had to do was use the calculations represented by the figure with the MODIS retrievals of modes and aerosol optical thickness to provide the aerosol effect. The aerosol effect was calculated over the global oceans in 13 sections using MODIS data that span the time from September 2001 to October 2004.
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The image on this page shows the daily averaged aerosol radiative effect for a 12 hour day with the solar zenith angle equal to 0 at noon, a variety of aerosol optical thicknesses and the nine modes of the MODIS aerosol retrieval over ocean.
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The results include estimates of monthly mean direct aerosol radiative effect over the oceans, globally and in 13 regional sections, for both the Terra and Aqua satellites.
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The figure shows seasonal values in each of the regions. The largest aerosol radiative effect occurs in the northern Indian ocean in the summertime when desert dust is transported from the Arabian peninsula. The northern hemisphere, in general, experiences much stronger aerosol effect than the southern hemisphere, even in the Fall when southern biomass burning increases the effect somewhat. The least aerosol effect occurs in the southern tropical Pacific, where there are few nearby sources of aerosol.
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Global values of total clear-sky aerosol shortwave radiative effect over the oceans in cloud free conditions are found to be -4.7 ± 0.3 Wm-2 to -5.2 ± 0.3 Wm-2 . The global values of aerosol optical thickness and radiative effect are remarkably consistent from season to season and year to year. These numbers closely resemble other observationally-based estimates of the same quantity. Individual regions show greater variability, spatially, seasonally and annually.
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In summary, the MODIS analysis of the aerosol effect on the radiative fluxes adds a new measurement perspective to a climate change problem dominated so far by models.
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The figure on this page displays the seasonal numerical values of aerosol optical thickness
and aerosol radiative effect at the top of the atmosphere from the Terra satellite.
The four numbers in each latitude-longitude section represents a seasonal mean for that
section from all available monthly data, devided to 13 different zones.
Introduction |
What does it mean to use MODIS retrievals consistently? |
Estimating Aerosol Radiative Effect |
Results
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