Proposed Improvements to the Nighttime GOES Fog Product to Provide Ceiling and Visibility Information

The temperature difference image between the short wave (3.9 micron) and long wave (10.7 micron) infrared (IR) imager channels on the Geostationary Operational Environmental Satellites (GOES), also referred to as the “fog product,” has become an important tool for the analysis of fog and low clouds at night. One weakness of the product, however, is that it cannot easily distinguish between water clouds that cause very low ceilings and visibilities at the surface, versus higher-based stratus, stratocumulus, and altostratus that do not represent significant hazards to aviation. Empirical rules have been developed to help forecasters make such assessments from the satellite imagery. For example, when the GOES fog product image detects a low cloud, fog is usually present when the following characteristics are observed: enhanced brightness, smooth texture, slow growth or movement, and well defined cloud edges. The difficulty with these rules is that some expertise is required to successfully diagnose the fog image without data from surface stations, which is sometimes not available for the area of interest. A more quantitative product is desired that would provide an estimate of the probability of very low ceilings and visibilities that create hazardous flight conditions, especially for non-instrument rated pilots.

As a result of the National Weather Service’s Pacific Coastal Forecast Systems workshop held in July, 1998 in Silver Spring, Maryland, there was a proposal to produce an experimental, enhanced GOES fog product that would provide probabilities of certain critical ceilings and visibilities for aviation users along the Pacific Coast. Data has been collected during the summer of 1997 across the United States, including some coastal areas, that indicates some correlation between the difference in temperature between GOES longwave IR and surface observations, and ceilings and visibilities, at locations where the two channel difference product shows that low clouds exist. The feasibility of such an approach will be investigated, leading to a prototype, experimental product. The product format will likely consist of the GOES fog product in its current image format, overlain by contours, or a color enhancement that indicate probabilities, for example, of Instrument Flight Rule (IFR) conditions, characterized by cloud ceilings <1000 feet, and visibilities < 3 miles.