P 5 . 13 Cloud Detection from MISR using a Band - Differenced Angular Signature

Cloud detection has historically been prone to more uncertainty in the polar regions than other regions of the earth. Low-level clouds and snow have similar radiation properties in visible and near-infrared channels, especially in nadir views. Since satellites have historically used nadir views for cloud detection, large errors in cloud detection for snow and ice covered regions have been typical. However, through the use of the Multi-angle Imaging SpectroRadiometer (MISR) onboard EOS Terra, coincident nadir and non-nadir visible/near-IR radiation measurements are now a possibility, which has the potential to circumvent the errors that have plagued previous satellite measurements. MISR provides multi-angle coverage of the earth at nine specific angles: 0°, ±26.1°, ±45.6°, ±60°, and ±70.5°. This is done using 9 separate cameras, four of which view in the forward direction with respect to the satellite orbital track, one which views at nadir, and four which view in the aft direction. Each camera has four spectral channels: the blue band, the green band, the red band, and the near-infrared band (443 nm, 555 nm, 670 nm, and 865 nm respectively). MISR operates in a push broom fashion, with a cross-track resolution of 275 m, and a swath width of approximately 360 km (For more information on the specifications of MISR, see Diner et. al., 1998). The ability of MISR to make multi-angle measurements of the same scene creates new opportunities for cloud detection in the polar regions. While clouds, snow and ice may all appear to be approximately the same in nadir views, in non-nadir views, the radiative scattering properties have a much larger contrast. This led Di Girolamo and Davies (1994) to develop the Band-Differenced Angular Signature (BDAS), which will be shown to be effective at cloud detection for the polar regions. The BDAS technique utilizes both the capability of MISR to measure at multiple angles and the capability of MISR to measure at multiple wavelengths. The BDAS takes the difference between the 443 nm and the 865 nm for two cameras: the camera at 60 degrees measuring forward scattered radiation, and the camera at 70.5 degrees measuring forward scattered radiation. Once the wavelengths have been differenced, the 70.5 degree banddifferenced result is subtracted from the 60 degree band-differenced result, to create the BDAS.