Evaluation of Moderate-resolution Imaging Spectroradiometer (MODIS) shortwave infrared bands for optimum nighttime fog detection

A bi-spectral technique for nighttime detection of potentially hazardous low clouds and fog based on shortwave Infrared (SWIR) data near 4.0 µm and longwave infrared (LWIR) data near 11 µm obtained from meteorological satellites has been in use for nearly two decades. The availability of 1 km resolution IR data from the Moderate-resolution Imaging Spectroradiometer (MODIS) instrument on the experimental NASA Terra and Aqua spacecraft (launched in 1999 and 2002, respectively) provides the opportunity to evaluate a number of SWIR wavelengths simultaneously to determine their relative effectiveness in detecting low clouds at night.

A comparison of four SWIR bands on MODIS for nighttime fog detection was made for a case of extensive fog and stratus in the Salt Lake Basin on December 5, 2004. The bi-spectral “fog product” was obtained from the brightness temperature difference (BTD) of each SWIR band from LWIR band 31 (11.0 µm), using the same contrast and background brightness settings for each image. A qualitative image comparison was then made, using the same cloud/no cloud threshold for all images. The shortest wavelength (band 20 centered near 3.7 µm) was determined to provide the best detection (based on contrast of the cloud edge) and lowest false detection (caused by certain soil types and instrument noise). The 3.9 µm channel (Band 21) was a close second in quality, while the longest wavelength (4.5 µm) produced the worst detection, perhaps due to its proximity to a CO2 absorption region. Results from one or two additional cases will also be shown.

The Advanced Very High Resolution Radiometer (AVHRR), currently on NOAA polar-orbiting satellites, and the Visible Infrared Imager/Radiometer Suite (VIIRS) instruments planned for National Polar-orbiting Operational Environmental Satellite System (NPOESS) beginning in 2008 use a 3.7 µm SWIR band, and thus provide the best possible fog nighttime detection. A minor tuning of future Geostationary Operational Environmental Satellite (GOES) Advanced Baseline Imager (ABI) wavelengths (proposed for 3.9 µm) may lead to some improvements in fog detection, although effects on other products (e.g. wildfire detection) must be also be considered. Evaluations of optimal shortwave bands for fog detection could lead to improved safety and efficiency for the aviation industry, as well as surface transportation.