Water Vapor Sensing System

Project Description
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.


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).


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

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

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]