The task of developing the GPS Dropsonde was especially challenging. The specifications required, among other things, that the new sonde be capable of being launched from high-altitude, fast, jet aircraft, and that the system be able to acquire data from four sondes in the air at the same time. The requirements demanded measurements of high accuracy, precision, and resolution under extreme environmental conditions of shock, vibration, and cold. The sonde had to be small, extremely light, and, most importantly, cheap, since it is expendable and is used in large quantities.
In its first field application, the FASTEX program (see below), the new GPS Dropsonde demonstrated that it offers the atmospheric science community unprecedented accuracy and resolution in vertical profiles of wind and PTH (pressure, temperature, and humidity). Measurements are obtained at intervals of 0.5 s, which corresponds to an average vertical resolution of 7 m (even better at lower altitudes close to the surface). This high resolution compares with the 60-s (LORAN) and up to 240-s (Omega) resolutions of the older LD2 sonde. Furthermore, the new system allows highly detailed measurements all the way down through the boundary layer to the surface.
The use of the new GPS Dropsonde in NOAA’s hurricane research and reconnaissance operations has dramatically improved observing capabilities. The old NOAA Omega dropwindsonde system had serious limitations (low vertical resolution, no winds in the boundary layer, and poor performance in bad weather) which restricted the use of the system to providing information only on the large-scale synoptic environment of the storm. The new GPS Dropsonde not only solves these problems, but for the first time allows measurements in the highly turbulent eyewall regions of hurricanes. Drops during August-September 1997 into the eyewalls of Hurricanes Guillermo and Erika resulted in detailed wind and PTH profiles all the way to the surface. Maximum wind speeds of almost 150 knots were measured. These new capabilities are having a major impact on NOAA’s hurricane research and reconnaissance programs, and are expected to lead to more accurate hurricane advisories and warnings.
This international program was by far ATD’s most challenging
field deployment during the year. FASTEX was focused on studying the
mesoscale structure of winter cyclones developing over the North
Atlantic ocean and the relationship between cyclone intensification
and upstream precursors embedded in the large-scale flow. ATD
supported both the airborne and surface portions of FASTEX observing
operations, and provided the computing network and communications at
the main control center in Shannon, Ireland. The newly developed NCAR
GPS Dropsonde was used for the first time on two NOAA aircraft (the
G-IV and a WP-3D) as well as on an NCAR-leased Lear 36 aircraft. A
total of 750 sondes were dropped during the experiment, with a
data-capture rate of over 90% being achieved by the end of the
program. The high-resolution, highly mobile vertical soundings from
the airborne GPS Dropsondes allowed PIs to study target areas where
small analysis errors were expected to most significantly affect
predictions, or where cyclones were in early development
stages.
ATD also operated an Integrated Sounding System (ISS) on board each of two research ships. The two ISS systems provided wind, temperature, and humidity profiles and surface meteorological measurements to document atmospheric boundary-layer structure and to help characterize surface fluxes on both sides of fronts at the surface.
The NCAR/NSF Electra aircraft, equipped with the ELDORA Doppler radar and a variety of microphysical and other sensors, was used by a group of NCAR, university, and French PIs to map mesoscale structures of mature or deepening cyclone systems. The rapid-scanning abilities of ELDORA helped document the turbulent ascent and descent of air in shallow rain showers associated with cold and warm frontal systems.
Also flown on the Electra during FASTEX was the Weather Avoidance Radar Data System (WARDS), newly developed by ATD. The WARDS surveillance display allowed investigators to see convective bands ahead of the aircraft, and thus helped them to design and refine flight patterns for optimal ELDORA data collection.
ATD is continuing to make refinements to the radar and its software to improve future data quality. A limited set of S-Pol antenna pattern measurements was collected during the PROWS field experiment. The radar location at the Eastlake site was a few miles south of the Boulder Atmospheric Observatory (BAO) tower. A coherent source was placed on top of the BAO tower and the antenna was scanned for recording azimuth-elevation radiation patterns. Both vertical and horizontal polarization patterns were analyzed, and the differential reflectivity (ZDR) bias and minimum linear depolarization return (LDR) were determined. Based on this analysis, the ZDR bias is 0.4 dB and the minimum detectable LDR is -27 dB.
A Bistatic Receiver system for S-Pol was jointly developed by ATD and the University of Oklahoma. This new system consists of three receiving antennas, radio data-transmission equipment, and real-time data-display software. The system provides the equivalent of real-time multiple-Doppler radar measurements of winds, using only a single radar as a signal source. The Bistatic system can be operated on any S-Pol field project. It is a much more economical way to obtain two- or three-dimensional wind fields than by using multiple radars.
Three additional PAM-III (Flux-PAM) surface meteorological stations were constructed in early 1997, allowing the deployment of a network of six PAM-III stations to support the CASES field program in April – May 1997. A particular highlight of this field program was the concurrent instrumentation of two additional CASES field sites, using components of the ASTER facility, to also measure fluxes of ozone and carbon dioxide and to meet the investigators' request for archival of high-rate turbulence data for detailed post-project analysis. In preparation for the simultaneous deployment of these formerly separate facilities, the PAM and ASTER data communications, display, and archival software were completely integrated so that the two are now essentially a single, unified field facility. Over the past year, testing and development has continued to refine and quantify the performance of standard meteorological as well as flux-measuring sensors. Particular effort has been devoted to the measurement of water-vapor fluxes. This has involved continued evaluation and development of the bandpass covariance technique, as well as the prototype development of a low-cost, fast-response, infrared-absorption hygrometer.
Tunable Diode Laser (TDL) Hygrometer: The quality of TDL measurements of water vapor at near-infrared wavelengths has recently improved through the availability of advanced lasers. NASA's Jet Propulsion Laboratory (JPL) has developed a highly sensitive TDL hygrometer for measurement of the low water-vapor concentrations found in the stratosphere. ATD and JPL are collaborating on the development of a similar instrument for the measurement of the much higher water-vapor concentrations found in the boundary layer and the lower troposphere. Flight tests of the new system are planned for FY 1998.
UV Hygrometer (UVH): Recent laboratory tests conducted by ATD in FY 1997 succeeded in verifying the calibration of the UVH down to the 100-ppm level. The sensor was also tested on the NASA DC-8 aircraft during the SUCCESS program. Comparisons with the cryogenic frost-point hygrometer during these flights showed very good agreement during periods when the UVH was unaffected by in-cloud sensor wetting.
DIAL Water-Vapor Profiling Lidar. During the last year, the development of system design concepts was initiated on this new, relatively inexpensive, USWRP-supported lidar system for the remote profiling of water vapor. When completed, the system will be an important addition to ATD profiling capabilities. Initial laboratory testing of the system is planned for early FY 1999.
Current planning indicates that a state-of-the-art, mid-sized, high-performance jet aircraft would provide these capabilities. The aircraft would be modified to support an array of advanced instrumentation, science stations, and communications technologies. Both ATD and the university community would be heavily involved in developing the new instrumentation.
This initiative will likely compete for funding from NSF’s Major Research Equipment (MRE) budget in the FY 1999-2000 time frame. If the necessary funding commitment is obtained, acquisition, modification, and instrumentation of the new aircraft would take approximately four years. Activities in FY 1998 will include more detailed specification of the scientific requirements, analyses of alternatives and costs, and refinement of the PDP.
