Ceiling and Visibility
Low ceilings and reduced surface visibilities yield critical impacts across the spectrum of aviation activities. For the general aviation pilot, limited ceiling and/or visibility (C&V) present a significant safety hazard – one that must be carefully considered in making pre–flight go/no–go decisions, route plans, and in–flight IFR avoidance and escape decisions. For commercial aviation, advanced aircraft equipment and pilot experience enable safe operations in low C&V conditions, but traffic flow capacity at impacted terminals drops markedly. This reduction in capacity is a major cause of flight delays and cancellations, and can propagate 'upstream' from impacted terminals to feeder terminals where no C&V problem exists.
RAL's work toward C&V nowcasting and forecasting addresses both the safety and terminal capacity impacts outlined above.
National Ceiling and Visibility
CVA Real-time Analysis Product
RAL's Ceiling and Visibility Analysis (CVA) has passed FAA evaluation to qualify for operational use as a planning tool to enhance pilot situational awareness. CVA presents current observations at ˜1600 METAR sites across the continental U.S. (CONUS), along with estimates of expected conditions between observing points based on careful spatial interpolation. To remain current in rapidly changing conditions, the product is updated every 5 minutes, and is rendered on a 5 km grid. A second–generation CVA product will implement more rigorous probabilistic representation of the uncertainty in C&V conditions between observing points.
The product is available on the Experimental ADDS website, is coming to the Operational ADDS website during summer 2012, and is also used to populate RAL's experimental Helicopter Emergence Medical Services (HEMS) Tool.
CVF Probabilistic Forecast Product
Recognizing that probabilistic weather forecasts are critical to meeting the air traffic efficiency goals of NextGen, RAL is working under FAA sponsorship to develop a probabilistic 1–10 hr C&V forecast product (CVF). This probabilistic approach acknowledges the inherent uncertainty in the forecast process and seeks to quantify and convey that uncertainty directly to the user. Forecast probabilities can be intelligently interpreted by human decision–makers, and can be used quantitatively in decision support systems where a variety of inputs and consequences are considered. In mature form, CVF will forecast the probabilities for ceiling and visibility conditions ranging from fully obscured to clear
RAL's CVF development combines existing operational forecast resources through use of new techniques, and typically yields forecast skill that exceeds that of its input resources. For example, blending of LAMP forecast data with time–lagged ensembles derived from the Rapid Refresh Model (RAP) yields the improved IFR ceiling forecast skill shown for the Philadelphia International airport shown at the top of the figure. Similar results for forecasts of IFR visibility are shown at the bottom. Under NOAA funding, development is seeking to determine how CVF techniques might best enhance the tools available to operational forecasters today.
Figure: Rapid Refresh ensemble (blue diamonds), and NOAA’s LAMP (in red) for 5 hr forecasts of ceiling less than 1000 ft (top) and visibility less than 3 statute miles (bottom). The case shown is for Philadelphia International Airport over the period November 2010 through June 2011. Left: Critical success index vs. probability of detection. Right: Brier skill score. Realistic values of forecast latency are assumed here – nominally 2 hr for the Rapid Refresh, 1 hr for LAMP and 10 min for CVF. CVF, which utilizes both the Rapid Refresh and LAMP, is seen to achieve the highest skill scores of the group.
CVF development will include evaluation of the forecast skill increases attainable through use of time–lagged ensembles from the High Resolution Rapid Refresh and NAM models, as well as use of RAL–developed statistical forecasting methods. Conventional ensembles comprised of members from the NAM and Rapid Refresh will also be evaluated. The first–generation experimental CVF product will undergo evaluation by the National Weather Service starting in November, 2012.