The early history of the use of real-time Aircraft Communications and Reporting System (ACARS) for transmitting winds and temperatures is described in Fleming (1996). The water vapor addition to ACARS began as a Federal Aviation Administration (FAA) grant to the author in 1991.
The proper measurement of atmospheric water vapor has not been easy. Satellites do a poor job with accuracy and have very poor vertical resolution in the lower troposphere where proper observations are really required. Radiosonde measurements of water vapor have a checkered history and are still of poor quality (see references in various documents available from this Home Page). Significant improvement in accuracy, precision, and space/time coverage can be obtained from the use of commercial aircraft equipped with a proper sensor. Two orders of magnitude improvement in the number of vertical profiles of winds, temperature, and water vapor - over and above the number of such profiles from radiosondes can be achieved at modest costs as described below.
Figure 1 shows a map of the continental United States (CONUS) with blue
dots indicating radiosonde sites where sensors on balloons are launched twice
per day. Considering the airports used by the major and regional air carriers
(the red plus signs in Fig. 1) one can see a possible solution for a national
mesoscale upper air system. The addition of a water vapor sensor on commercial
aircraft (measurements of winds and temperature already exist on the aircraft)
leads to the ability to communicate real-time profiles (during ascent and
descent) of the three key atmospheric variables.
Only 2400 of the 5000+ aircraft flying each day need provide ascent/descent profiles from the airports shown in Fig. 1. The average for the major carriers is 4 takeoffs and 4 landings for each aircraft/day (for the regionals the number is 6/day). These numbers imply an average of 10 profiles per day per aircraft. Thus, if 2400 aircraft have the water vapor sensor then there would be 24,000 profiles of winds, temperature, and water vapor generated each day. At the major hubs, the number of potential profiles is so many that they would be reduced to lower communication congestion and costs. The total number of daily aircraft profiles would be reduced to approximately 21,600. When these profiles are average over the 450 the average number is 48 per day per site. The horizontal resolution increase over the radiosonde network is greater than a factor of 4 and the average time resolution increase is an additional factor of 24 - the actual increase would be approximately 21,600/150 = 144.
The radiosonde system provides 150 profiles per day at a recurring cost of approximately $250 for each profile. The capital cost (including installation) of the WVSS-II (discussed below) is $18,000 each, but it has a twenty-year lifetime. When the costs are distributed over 20 years and maintenance and communication costs are included over that period, the total is $70,000. The number of aircraft profiles over that period is
This program was not intended to replace existing radiosondes, but rather to provide a new observing tool to support mesoscale research and understanding. Such data, properly cared for and archived can support a range of atmospheric science application areas.