Antecedent rainfall, dry air aloft, and calm surface winds contributed to the development of widespread, dense fog across the Southeast during the early morning hours of November 24, 2010. As of 1400 UTC, dense fog advisories remained in effect for portions of Louisiana, Mississippi, Alabama, Georgia, Florida, Eastern Tennessee and Eastern Kentucky. Currently, SPoRT provides a “fog product” from MODIS that identifies potential fog pixels with an 11-3.9 micron brightness temperature difference, comparable to a technique available from GOES imagery. Below are two examples of the current MODIS fog product for this event, acquired from the NASA Terra (0400 UTC) and Aqua (0800 UTC) satellites:
One limitation of a single channel difference is that it omits other valuable information that can be provided by additional channels on MODIS. Additional information can be incorporated through the use of “RGB” products, or color composites of red, green, and blue color values that are derived from either three single bands, three pairs of channel differences, or even three completely different satellite-derived products. SPoRT has developed RGB capabilities for both AWIPS and NAWIPS displays used by the National Weather Service Weather Forecast Offices and NOAA/NWS National Centers. Although these displays are limited in the number of unique colors they can display (254, and 95, respectively), RGB products can provide additional detail beyond a simple channel difference.
Below are the same two MODIS passes, demonstrating an RGB product developed by EUMETSAT to identify the microphysical characteristics of clouds for nighttime detection. In this product, red colors are obtained from the 12.0-10.8 brightness temperature difference, the green color is obtained from the 11-3.9 difference (the current SPoRT fog product), and the blue colors are based upon the brightness temperature at 10.8 microns.
Note that in these NAWIPS images, fog reports in the Southeast are co-located with shades of light blue. Although the coverage area of these light blue shades is comparable to the yellows within the standard SPoRT product from MODIS, the RGB product better discriminates fog in the Southeast (blues) from low clouds streaming off of the Great Lakes (yellow-greens), whereas the SPoRT single channel difference depicts both regions as the same range of yellows. Therefore, adding additional spectral information in the form of colors provides some additional information. This type of color enhancement differs from simpler color table modifications, where brightness temperatures or differences are denoted with varying color — in the RGB concept, colors are a direct response to spectral characteristics.
SPoRT plans to continue development of RGB capabilities for NAWIPS, AWIPS, and AWIPS II, with eventual transition of products to forecasters for evaluation. These types of RGB products are currently available from METEOSAT-9 (EUMETSAT) and will be available as a future GOES-R capability. Demonstrating RGB capabilities from MODIS serves as a proxy for future operational instruments available over CONUS.