Ceiling and Visibility Forecasting

Aircraft landing in low ceiling and visibility conditions. (Photo: Eric Dunetz)

Terminal restrictions associated with low cloud ceiling and visibility conditions reduce operating capacity at major airports within the National Airspace System (NAS).  FAA Traffic Flow Managers must work within these terminal capacity restraints, particularly with regard to lowered aircraft acceptance rates at major airports that serve as hubs for commercial airlines. When scheduled arrival demand exceeds arrival runway capacity, traffic managers typically hold upstream aircraft on the ground to throttle down the rate of incoming traffic to minimize the risk of excessive airborne holding. Efficient implementation of delay strategies requires accurate prediction of onset and cessation of restricted capacity.

Ceiling and Visibility Prototype Forecast System at SFO Airport

MIT Lincoln Laboratory has been involved in efforts aimed at improved ceiling and visibility prediction at key U.S. terminals.  Most notably, a prototype system was developed to forecast the time of summer stratus cloud dissipation at San Francisco International Airport (SFO) (see Figure 1).

C&V airport impactsNumber of airport impacts based on low ceiling and/or visibility (C&V) conditions at major U.S. airports. Of the top 10 delay airports, SFO is the most heavily impacted by C&V conditions because of the marine stratus cloud deck.

During the warm season, stratus clouds typically form and dissipate in the approach zone on a daily cycle, forming overnight and clearing several hours after sunrise.  Persistence of the stratus in the approach zone during the morning hours prevents dual approaches to SFO’s closely spaced parallel runways. Traffic management decisions are based on expectations of whether the stratus will restrict operating capacity during the heavy arrival period from mid-morning to late afternoon.

The prototype forecast system gathers information from a network of sensors in the San Francisco Bay region and generates a prediction of approach zone clearing, which is updated at regular intervals throughout the morning. This prototype system was delivered to the National Weather Service Forecast Office in Monterey that continues to operate the system as part of their aviation forecast support to the FAA. A follow-on product is being developed which uses the system forecasts to provide specific Ground Delay Program (GDP) parameter selection guidance for air traffic managers responsible for determining the start/end time of delay programs and the scope of aircraft impacted.

Ceiling and Visibility Forecast Research in the Northeast

Ceiling and visibility forecast research has also been conducted for major Northeast U.S. airports whose Instrument Meteorological Conditions (IMC) exposure tends to be associated with large-scale extra-tropical weather systems.  Unlike SFO, their attendant cloud shields and reduced visibility conditions are more transient in nature. In this regard, research was focused on a forecast technique to monitor and track the aviation-impacting conditions associated with these systems. Slideshow 1 shows the sequence of steps in generating ceiling and visibility forecasts at high resolution over a large domain, and then converting them to terminal-specific forecasts.

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