Ceiling and Visibility Research and Development
Figure 1. Conceptual view of the relationship between the NCV system (red) and the NWS forecast preparation process (blue). NCV analysis and forecast grids flow to NWS as initial fields for the TAF and GFA forecast process. Following forecaster input, modified grids populate the National Digital Forecast Database. Derived warning information flows to end users (green).
Adverse ceiling and visibility (C&V) conditions create serious flight safety hazards for general aviation, as well as costly reductions in traffic flow efficiency for commercial operations. On a yearly basis, C&V hazards result in 30-40 fixed-wing general aviation accidents, ~65-75 deaths and losses of ~$150-160M. Reduced C&V conditions are second only to convective weather as a cause of U.S. flight delays. RAL research funded by the FAA Aviation Weather Research Program (AWRP) directly addresses both safety and efficiency concerns through the development of improved C&V weather information and decision support systems. While the National C&V (NCV) effort is focused on enroute safety in the nation's airspace, the Terminal Ceiling and Visibility (TCV) Product Development Team works to improve safety and efficiency in the airport terminal area, developing methods and automated systems to provide forecasts to help reduce flight delays at major, high-traffic air terminals in the northeast U.S.
FY06 Accomplishments
The NCV real-time analysis grids for ceiling (Fig. 1), visibility and flight category have been made available as experimental products used by the National Weather Service Eastern Region aviation forecast modernization process through the year. They are shown to be accurate for local conditions and have good reliability in representing the most probable conditions between reporting sites. These results encourage further development and use of NCV gridded ceiling and visibility products. High-resolution terrain data for both CONUS and Alaska domains have been added to the NCV product, providing realistic representation of terrain obscuration, a major hazard under low C&V conditions. In addition to its analysis product, the NCV team has also developed, and is now testing, a forecast product. The product's data mining forecast method was found to frequently outperform other input forecast types for forecasts out to 2-4 hours, adding skill to the overall system.
Figure 2. Evolution of ambient moisture conditions for an atmospheric layer into which mm-size droplets are falling. Condensation on colder-than-ambient droplets falling through pseudo-adiabatic conditions causes supersaturation to decrease with time (circles). Evaporation of warmer-than-ambient droplets falling through a temperature inversion causes supersaturation to increase with time (triangles). An isothermal layer (squares) induces no evaporation or condensation and thus maintains steady saturation
Research efforts within TCV have focused on how fog and low cloud ceiling events occur in association with precipitation. A detailed microphysical numerical model has been developed to investigate the evolution of the state of raindrops falling into atmospheric layers of contrasting temperature profiles. Results confirm the role of warm raindrops evaporating into the colder air of inversions as a fog-inducing mechanism. Supersaturated conditions are obtained in the lower levels of inversions through the influx of water vapor from the warm evaporating drops, whereas a dehydration effect is observed when cold drops fall into a pseudo-adiabatic layer (Fig. 2). Insights into one of the numerous factors leading to fog formation were obtained from these numerical experiments.
FY07 Plans:
While there is significant uncertainty in funding available for FY07, core plans include effort to support the May 2007 NCV CONUS analysis product decision process for operational status and continue to advance NCV’s agile selection forecast methodology with a November 2007 goal for experimental status. With several seasons’ data in hand, the operation of the Brookhaven site for field studies of ceiling and visibility will be suspended and instrumentation removed. Terminal C&V statistical work will center on evaluation of options for operational application.