MODIS

What is MODIS?

MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days, acquiring data in 36 spectral bands, or groups of wavelengths. Two bands are imaged at a nominal resolution of 250 m at nadir, with five bands at 500 m and the remaining 29 bands at 1,000 m. A ±55-degree scanning pattern at the EOS orbit of 705 km achieves a 2,330-km swath and provides global coverage every one to two days.    

Terra (source: http://terra.nasa.gov/About/earth_am1.jpg) Table showing all of the MODIS bands

MODIS products relevant to aerosol research

Below is a list of some of the aerosol data products currently (2002) derived from MODIS

Optical Depth over Land at 0.47, 0.55 and 0.66 µm Optical Depth over Ocean at 7 bands (0.47, 0.55, 0.66, 0.87, 1.24, 1.64 and 2.13 µm) Mass Concentration over Land and Ocean Effective Particle Radius over Ocean Cloud condensation Nuclei over Ocean Assymmetry Factor over Ocean Scattering Angle Backscattering Ratio Ratio of Optical Depth of Small Mode vs. Effective Optical Depth at 0.55 µm over Ocean Angstrom Exponent over Land

2. Dust imagery

It is possible to detect dust (but not to derive its optical thickness or particle concentration) in MODIS images using the thermal infrared channels. Dust particles are large enough to interfere with the transmission of these longer wavelengths, thus there is a decrease of earth-emitted thermal infrared reaching the satellite when dust is present.

  At left is an example MODIS image showing dust from the Saharan desert moving westward into the Atlantic Ocean. Source: http://www.esipfed.org/images/earth/west_africa_dust.jpg

    3. Volcanic Ash imagery

MODIS is able to image the ash plumes of volcanos using thermal infrared channels. The best view of volcanic ash is created by subtracting the brightness of one thermal infrared wavelength (12µm) from another (11µm). Example of volcanic ash detection: the Cleveland Volcano  

4. Direct Detection of Fires

The MODIS fire and thermal anomalies product provides eight-day or monthly summaries of the day and night occurrence of fire and a summary of the number of fires in strength classes. For example, the MOD14A2 product is a gridded 1 km composite of the most-confident fire pixel detected in each grid cell over the eight-day composite interval. Fire detection is performed using the 3.9 and 11 µm channel brightness temperatures. The strategy is based on absolute detection of the fire, the fire must be strong enough relative to the background to account for variability of the surface temperature and reflection by sunlight.

The image shown at left is an example of the MODIS/Terra Thermal Anomalies/Fire 8-day level-3 global 1-kilometer gridded product (MOD14A2). The image was produced along the coast of Angola and Namibia and has been pseudo-colored for illustrative purposes: Red = Fire Blue = water White = cloud Green = non-fire land pixels Black = missing data Yellow = unknown   source: http://edcdaac.usgs.gov/modis/mod14a2.html    

How MODIS measures the Optical Depth of Aerosols: the Algorithms