Aviation Weather Forecasts Beyond Today
Aviation Weather Forecasts Beyond Today
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Wednesday, 7 January 2015: 10:45 AM
129A (Phoenix Convention Center - West and North Buildings)
Aviation weather forecast information has traditionally been focused on today's weather. Airport weather forecasts (TAFs) are for 24-30 hours into the future. The aviation weather hazards forecasts (AIRMETs, SIGMETs, CCFP, Area Forecasts, and related guidance products) generated by the Aviation Weather Center (AWC) are typically for periods of 12 hours or less into the future. While the current products are critical for air traffic control and safe flying today, there is a lack of information available for flight planning beyond today. This longer range planning is especially critical for General Aviation (GA). If a GA pilot does a cross country flight today, will they be able to return tomorrow? Should the GA pilot rent an airplane for flying this weekend, or will the weather be unfavorable for flying? There is a need for aviation forecasts beyond today. Other segments of weather forecasting have been steadily increasing the forecast times of their products. Public weather forecast products now provide forecasts of precipitation, temperature, winds, and clouds out to a week into the future. These increases in forecast times have been made possible because of improvements in the numerical forecast models. While the models have been developed primarily to support public weather forecasts, the model grids can be adopted to provide aviation forecasts. For instance, the models need to generate clouds for the radiation budget calculations needed for temperature forecasts. The models have grids of cloud tops, cloud bases, surface pressure, and fractional cloud cover. The cloud bases in pressure coordinates can be converted into heights above ground using the Hypsometric equation. Using the North American Model (NAM) forecast grids, a suite of aviation weather forecasts have been generated with forecast times every 3 hours out to 84 hours into the future. The available products include ceilings, visibility, cloud tops, mountain obscuration, boundary layer turbulence, boundary layer depth, wind gusts, low level wind shear, thunderstorms, simulated radar, density altitude, sea level pressure, icing base height, and winds aloft. These products are publically available at http://fltwx.db.erau.edu/aviationfcst.php. The aviation weather displays at the above web site are somewhat different than traditional weather forecast displays. Model and observational data have traditionally been displayed as graphics. Satellite and radar data have traditionally been displayed as images. All of these data fields are originally just numbers. The display techniques are used to help people make sense of the numbers by putting them into a picture format. The big difference between the graphical displays and image displays has been the original data resolution. Graphical display techniques are typically used for data sets on the order of 100 or so points in each direction. Image display techniques are typically used for data sets on the order of 1000 or so points in each direction. Weather forecast models have traditionally been displayed with graphical techniques. However, the size of the model data sets has been increasing, and is starting to approach the resolutions of traditional image data sets. For instance the National Weather Service (NWS) North American Model (NAM) output grid has 428 rows by 614 columns. The traditional method of contouring a grid to make a picture is to draw lines of equal (iso) values. The interpolation between grid points for the isoline allows the resultant picture to have a higher spatial resolution than the original gridded data. The traditional method of making an image is to transform each data pixel into an image pixel. Most computer display systems have 8 bit displays, so the data value is scaled to fit within the 0 to 256 brightness range. For contoured graphics, the area between contours can be colored the same color as the contour. Most computer displays support polygon objects, so the filled contour can be presented as polygons to the screen display. Typically filled graphics will support around 32 different colors of graphics. Converting model data into images rather than graphics allows for the user to see a wider range of structure in the data as well as being able to display fields with widely varying values. Graphical displays are typically stored in .GIF format files, while images are frequently stored as .JPG files. JPG files use data compression, so the file sizes are typically ¼ to ½ the size of GIF files. Hence using images rather than graphics allow for faster downloads of multiple images for looping displays. Consider the following example of the density altitude correction (difference between the runway altitude and the density altitude) image for July 17, 2014 at 18Z. Density altitude is the altitude relative to the standard atmosphere conditions (ISA). "Density altitude" can also be considered to be the pressure altitude adjusted for non-standard temperature. The display is the number of feet to be added to or subtracted from the airport altitude to equal the density altitude above sea level. Note the surface heating in the southeastern US and the western US have resulted in density altitudes several thousand feet above the surface altitude. When the original model grid was displayed as a contoured graphic, there were so many lines that one could not understand either the large scale process, or the small scale local processes. Making the model data into images allows for better understandings of the meteorological processes going on. The use of image display technology for model data shows considerable promise for better understanding small scale structures resolved by the higher resolution models of today. The use of these forecast images for providing aviation weather guidance products should enhance the ability of the General Aviation community to better plan for flight operations.
Supplementary URL: http://fltwx.db.erau.edu/aviationfcst.php