Evaluation of GPS Dropwindsonde Data

Principal Investigator: James L. Franklin, HRD/AOML/ERL

Objective: To evaluate the performance of the new GPS dropwindsonde developed by NCAR for use on the NOAA Gulfstream IV-SP jet aircraft.

Rationale: Until 1996, the NOAA WP-3D aircraft were equipped with omega dropwindsondes (ODWs) that were designed in the 1970s and relied on the Omega navigational network for windfinding . The Omega network is being phased out, with funding scheduled to terminate in September 1997. NOAA has recently commissioned a new jet aircraft for hurricane reconnaissance, the Gulfstream IV-SP (G-IV), that will begin operational missions in 1997. The primary mission of the G-IV will be to release dropwindsondes in the hurricane environment to improve operational forecasts of hurricane track. Previous research missions suggest that operational numerical guidance should improve by as much as 30% as a result of these data (Burpee et al. 1996). With the anticipated demise of the Omega network, a new design of dropwindsonde has been developed by the National Center for Atmospheric Research (NCAR). The new sonde uses the Global Positioning System (GPS) rather than Omega for windfinding. The thermodynamic sensors and data telemetry on this dropwindsonde are also new.

Method: Field testing of the GPS dropwindsonde from one of the NOAA WP-3D aircraft began in August 1996, with installation and testing on the G-IV in September. Data from the new dropwindsondes have been compared to flight-level data from the G-IV and the WP-3D aircraft, simultaneous releases of ODWs, nearby rawinsondes, and meteorological buoys.

Accomplishment: Through the end of November, 85 GPS sondes had been released from the G-IV and the P-3. About half of the total releases occurred during HRD's annual Hurricane Field Program. Several biases were noted in the thermodynamic data from the earliest drops, including a cold temperature bias of 1-2 degrees and a dry humidity bias of about 10%. Sonde splash pressures also consistently appeared to be low by 1-2 mb. NCAR was able to determine that the pressure bias, as well as much of the temperature bias, was a dynamic effect caused by the fast fall speed of the sonde (about 12 m/s at the surface) that could be corrected in software. The humidity bias is believed to be caused by contamination of the sensor as a result of outgassing from the resin-impregnated cardboard tube that forms the body of the sonde. This problem has not yet been fully resolved. Early problems with GPS firmware have also been resolved, producing wind accuracies that are almost always better than 0.5 m/s. At the time of this writing, typical measurement accuracies are believed to be about 0.5 mb, 0.5 C, 5-10%, and 0.3 m/s for pressure, temperature, relative humidity, and wind, respectively. The most significant remaining problem is a 15-20% rate of failure of parachute deployment, which results in a complete loss of wind data throughout the descent. NCAR is currently working on modifications to the parachute.

Even with the unresolved engineering problems, typical soundings show much more detail and greater accuracy than earlier instruments could provide. Only the humidity measurements are significantly outside design specifications. The instruments are now adequate to carry out the synoptic surveillance mission, and there is every reason to expect further improvement as remaining engineering problems are resolved. From a research perspective, the GPS sondes will present the scientific community with new opportunities to study such key problems as: hurricane boundary layer dynamics and thermodynamics, eye thermal structure, and the kinematics of the upper tropospheric eddies and outflow channels.

Reference:

Burpee, R. W., J. L. Franklin, S. J. Lord, and S. D. Aberson, 1996: The impact of omega dropwindsonde observations on operational hurricane track forecast models. Bull. Amer. Meteor. Soc., 77, 925--933.