V - Z
WVSS-I
The Water Vapor Sensing System (WVSS) was used for obtaining weather observations which was a revolution in that it used a convenient platform for a variety of environmental parameters. History: The FAA interest in the WVSS program is to support the Aviation Weather Research Program. Researchers will use the data to improve algorithms for the detection, analysis and prediction of mesoscale weather phenomena and activity which affects the aviation industry: ceiling and visibility, precipitation type and amount, thunderstorms, microbursts, and icing. Two other areas where water vapor has a secondary role are in convective turbulence (where water vapor gradients affect atmospheric stability) and in flight-level winds where the synoptic scale wind patterns are often significantly disrupted by mesoscale outflow (where water vapor plays a role in mesoscale development and decay). Program: Results of the first phase of evaluation of the WVSS-I, which uses a thin-film capacitor to measure relative humidity, are available from this web site under the document entitled "Water Vapor Profiles from Commercial Aircraft". THese results include evaluation of measurements from the six UPS B-757 aircraft that had the Allied Signal avionics and the WVSS-I software. Results from six months of data (July 1, 1999 to December 31, 1999) from these WVSS-I equipped aircraft reveal that the commercial aircraft profiles are competitive with radiosondes on ascent/descent and superior to radiosondes in the upper troposphere. Comparisons of opportunity with radiosondes (when equipped aircraft made ascent/descents near a radiosonde site) also revealed the capability of depicting the moist absolutely unstable layers (MAULs) describe by Bryan and Fritsch (2000). See the document above for the detailed reference. A number of examples of MAULs including those seen in radiosondes are shown in this document. Statistics on the occurrence of MAULs (in the commercial aircraft data and in radiosondes) are also provided. Data from the second phase of the WVSS-I evaluation (after the new Teledyne avionics equipment had replaced the Allied Signal avionics is available to users as describe below. One of the limitations of the WVSS-I technology ( the Vaisala thin-film capacitor technology which was an improved version over that same sensor used in Vaisala radiosondes) is the "aging" of the sensor over time. This aging eventually results in a dry-bias over time -- requiring that the sensor be replaced and then recalibrated. Note that such aging takes place while the unit sits on a shelf (although presumably at a slower pace than when it is in operation on the aircraft). THere is considerable variability in this aging process -- this being one of the major reasons for abandoning this sensor for the WVSS-II sensor described below. As the WVSS-I Program is experimental (not operational) with a limited budget, these sensors were not immediately replaced when they should have been -- instead, part of the evaluation process was to see how long the sensors would last. Thus, Table 1 below shows those time periods when the 30 UPS aircraft were installed, working, and did not exhibit the obvious dry-bias. This table will be maintained on a monthly basis through calendar year 2002. The green-filled boxes are the months with good data. For more information, visit the homepage of the Water Vapor Sensing System Program at "http://www.ofps.ucar.edu/wvss/" [Summary provided by JOSS]