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At last count, almost one-third of the ATD staff are slated to spend at least part of next winter in the tropical western Pacific. Lest you get envious, we hasten to add that most of these personnel will be on remote, primitive islands (see Figure 1), or in small, tossing ships, spending long hours manning various observing systems.
Even a larger fraction of ATD have been working feverishly over the last few months (actually years, in some cases) to develop new observing systems and to ready existing ones for the massive international field study called Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE, hereinafter abbreviated to COARE). Over 700 scientists, technical and logistical specialists, and students from some sixteen countries are expected to participate in the four-month Intensive Observing Period (IOP) starting November 1. U.S. participation in the field phase includes scientists from over 30 universities.
COARE is designed to study and understand atmospheric and oceanic processes associated with an immense pool of warm (>28 C) water in a continental-U.S.-sized region along the equator just north and east of Papua New Guinea (see Figure 2). For reasons not yet understood, the ocean water in this area is warmer than that in other tropical regions. This is important because atmospheric circulations are driven by the tremendous amounts of solar energy absorbed by the tropical oceans.
Processes connected with the warm pool can have a dramatic influence on short-term climate variations all over the world. For example, during El Niño conditions this warm pool extends eastward toward the west coast of South America and may also be associated with droughts in Australia, India and Africa, floods in South America, and severe winter storms in the United States.
The COARE field experiment will be controlled and coordinated from an operations, forecast, and communications center in Townsville, Australia. An aircraft operations group consisting of three large turboprop aircraft (the NCAR Electra and two NOAA P-3s) and their scientific and supporting staff will be located at Honiara, Guadalcanal, Solomon Islands, a site made famous by World War II. Deployment of the three Doppler-radar-equipped aircraft at this location will shorten their flight times to the center of the primary measurements area. This area, called the Intensive Flux Array (IFA), covers a roughly 700 km x 700 km expanse of ocean straddling and just south of the equator, and will contain some 12 oceanographic moorings as well as 11 research vessels. Four other aircraft, from NASA, the U.K., and Flinders University, will also participate during portions of the field study.
Instrumented buoys, moorings, and oceanographic research ships will supply measurements of temperature, salinity, currents, and other variables within the upper layers of the ocean. Two of the research ships will also serve as platforms for stabilized Doppler radars to be operated under the supervision of Colorado State University. ATD is furnishing experienced radar technicians and engineers to help operate these systems.
The research aircraft and surface-based instrumentation will seek to describe the atmosphere and to measure fluxes from the warm ocean surface. A main focus will be on convective systems, since these storms modify the fluxes from the ocean surface through changes in wind, temperature, and humidity. The large amounts of rain from these systems also influence the temperature and salinity in the upper ocean.
Obtaining measurements of these diverse processes over such a large, sparsely populated, almost landless region has made experiment logistics extremely difficult. To address this problem, ATD will debut two major new observing tools: the airborne Electra Doppler Radar (ELDORA) and the Integrated Sounding System (ISS), both of which were developed jointly with other organizations. In addition, ATD's Research Aviation Facility (RAF) has developed a new fast-response hygrometer for use on each of the three turboprop aircraft.
ELDORA has now been mounted in the Electra aircraft (Figure 3) for extensive flight testing prior to going to COARE. The French National Center for Telecommunications Studies has worked with ATD for the last five years in designing and building ELDORA (which in French is called ASTRAIA - Analyze Stereoscopique par Radar à l'Impulsion Aeroporte). Major improvements to earlier single-beam systems that are embodied in ELDORA include improved accuracy in radial velocity and reflectivity measurements, higher spatial density of observations, and a new dual-beam scanning technique that significantly improves the safety and speed of collecting observations near intense storms.
The X-band (3.2-cm wavelength) radar design includes two complete radar systems, one scanning forward and the other aft. This fore-aft dual-beam arrangement allows the radar to collect dual-Doppler information simultaneously above, below, and on both sides of the flight track while the aircraft flies along straight flight paths.
To obtain additional independent samples for higher sampling accuracy, ELDORA makes use of a complex transmitted waveform in three frequencies and a factor of 2 to 4 over-sampling in range. The resultant factor of 6 to 12 improvement in the number of independent samples is designed to compensate for the high scan rate. The system includes real-time polarimetric and planar displays. Basic data recording is on ExaByte high-density tape cartridges and includes velocity, spectral width, and reflectivity.
The mobility of the Electra-borne ELDORA is likely to be the key to its success in COARE. The system will be able to take measurements with 0.3- to 1.0-km spatial resolution out to a range of 30 to 60 km. Electra flights during the program will be of about 7.5 hours duration. At typical flight speeds of 8 km/min, over 3000 km range can be expected. In contrast, a ship collecting data at sea would take about a week to cross the same amount of territory, while a land-based system can scan only about 100 km distance.
In COARE, for the first time, three airborne Doppler radars (ELDORA and the two Doppler radars on board the NOAA P-3s) will collect coordinated data sets. A novel data collection strategy will use two of the turboprops, flying parallel tracks 50 km apart, to gather near-simultaneous Doppler data within convective storms situated between them. These coordinated Doppler data will allow more accurate estimates of the vertical fluxes of heat, momentum, and moisture to be calculated. The third turboprop will map larger-scale motions with its Doppler radar, as well as collect complementary in situ boundary layer data to evaluate the effects of the convective-scale transports on the sea-air flux exchanges.
The ISS was developed jointly by ATD s Surface and Sounding Systems Facility (SSSF) and NOAA s Aeronomy Laboratory. The ISS combines several state-of-the-art in situ and remote sensors into a single transportable unit. The instrumentation for each ISS deployed in COARE includes a 915-MHz wind profiler combined with a radio-acoustic sounding system (RASS), an Omega navigational-aid-based atmospheric sounding system, and a ground station measuring wind speed and direction, pressure, rainfall, temperature, humidity, and various parameters in the radiation budget. A network of six ISSs is being deployed by NCAR and NOAA personnel prior to the 1 November start date for the COARE IOP.
The first ISS installation took place in late April on Manus Island. Manus, located some 1,000 km west of the center of the IFA, is one of the key COARE equatorial ISS sites because of its location in the western portion of the COARE domain where westerly wind bursts, a key phenomenon to be studied in the program, may first develop. Air-sea interactions are maximized during such events.
The second ISS was installed in mid-July on Kapingamarangi Atoll, located almost astride the equator near the center of the IFA (see Figure 4). Eight NCAR and NOAA personnel endured a rough, six-day boat ride from Cairns, Australia, to get there, as there is no airport on the atoll. Since there also are no tourist accommodations on Kapingamarangi, the installation crew had to bring and erect not only modular buildings for the electronic equipment and diesel electric generators, but also a shelter (complete with food) for the people who will be operating the system from now until the end of June 1993. In addition, they set up a 60-ft tower for the meteorological sensors and two smaller towers for radiation sensors and antennas. An INMARSAT satellite-link telephone was also installed to provide voice and data communications to Townsville and Boulder. Several of the 400 inhabitants of the island were hired to assist in the construction work.
The next ISS system will be installed on the island of Nauru in August 1992, with the Kavieng station following in early October. Two ISS systems will be installed on Chinese research vessels at ports in China in October. A big difference with the shipboard systems is that the Doppler wind profiler antenna must be mounted on a stable platform to remove the ship s motion and keep the phased-array, electronically beam-steered antenna horizontal at all times.
Just a few days after being installed, both the Manus and Kapingamarangi stations began sending high-quality data back to Boulder via GOES satellite link. Getting these new ISS data from this data-sparse region is, in itself, an exciting breakthrough. During the period 17-23 June, the ISS system at Manus Island may have observed a westerly wind burst, which cause intense air-sea interactions as mentioned earlier. The westerly wind burst observed by the Manus ISS may prove to be particularly interesting since the event happened near the time when the ocean temperatures changed dramatically as an El Niño pattern rapidly dissipated. The profiler data taken in this westerly burst show that the westerlies developed to a height of 3.5 km (see Figure 5). Prior to the time period shown, the low-level winds gradually shifted from light easterly to northerly over a period of several days. A general increase in the magnitude of the easterlies aloft was also noted. The sounding data from the ISS showed an increased likelihood of deep convection, which resulted in several episodes of precipitation that were measured by the ISS ground station.
Although ATD staff are straining under the time pressure as they face the tough observing challenges of COARE, we sense that they are excited, as we are, to have an opportunity to contribute to a momentous field study that is sure to furnish a landmark data set for climate studies for years to come.
David Parsons is a Scientist III holding a joint appointment in ATD's Surface and Sounding Systems Facility (SSSF) and the MMM Division. He is also the leader of the ISS development team. Hal Cole is an Engineer V and serves as Deputy Manager of SSSF. Peter Hildebrand is a Scientist III and serves as Manager of ATD's Remote Sensing Facility (RSF).