Water vapor measurements from commercial aircraft: progress and plans

After considerable delays due to contract problems, union intervention, strikes, and technical uncertainties, the WVSS is now working on five United Parcel Service (UPS) aircraft and producing about 1000 wind, temperature, and water vapor reports per day. The area of coverage includes the GCIP region, most of the continental United States, the Gulf of Mexico and flights into Mexico.

UPS has recently switched avionics vendors ( Teledyne Controls – also the vendor of choice for American Airlines) and new water vapor software for these avionics boxes should be ready in the fall of 1999. This will allow the completion of installation of another 60 WVSS units and will add another 15,000 reports per day of water vapor information.

The accuracy of the WVSS is comparable to radiosondes in the lower troposphere ( the main FAA and GCIP interests), better than radiosondes in wet conditions ( through clouds and precipitation), and significantly better than radiosondes in the upper troposphere where radiosondes are quite poor. The reason for the better performance in the upper troposphere is the Mach number effect ( dynamic heating in the aircraft probe due to the speed of the aircraft) which provides significantly better sensor response times than experienced by radiosondes at these levels. However, this same Mach number effect leads to a large random error (at flight level only ) for this relative humidity sensor which is part of this first generation WVSS.

The FAA has continued to fund this WVSS program and will carry it through until a subsequent procurement of an operational system with the National Weather Service. The NOAA Office of Global Programs has funded the initial start on a second generation system that involves a diode laser as the sensor for water vapor information. This system improves the accuracy of information at all levels, but especially at the flight levels of the upper troposphere and stratosphere where the water vapor information can be obtained to one part per million by volume ( 1 ppmv).

A description of both of these WVSS units will be provided along with a display of how they fit into a composite atmospheric observing system for water vapor.