What does it mean to use MODIS retrievals consistently?
Click on the numbers to see different modes
Illustration of the MODIS aerosol retrieval over land.
Each mode (bottom) defines a spectral curve of reflectances at top of atmosphere (top).
The reflectances from one fine mode and one coarse mode are combined to produce the total LUT spectral curve (black curve in top panel).
Different modes and relative strengths of each mode are attempted during the inversion until there is a good fit between the total LUT curve and the observed reflectances.
The figure to the right 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.
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.
Examples 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.
