Surface Heat Budget of the Arctic Ocean (SHEBA)
The Surface Heat Budget of the Arctic Ocean (SHEBA) project was
without any doubt one of the most challenging field projects ever
supported by ATD staff and instrumentation. In early fall of 1997,
SSSF engineers and technicians traveled to the SHEBA ice station about
300 miles north of Prudoe Bay to set up four of ATD's PAM -III
stations and a GLASS system for a thirteen month period. The SSSF
facilities were only a small part of a much larger array of
observational instruments set up within a 62-mile radius of the SHEBA
base station to measure a multitude of physical parameters for a full
annual cycle. While the GLASS measured the standard meteorological
measurements of wind, pressure, temperature, and humidity, the PAM-III
stations measured additional parameters such as turbulent fluxes of
momentum and heat, incoming and outgoing fluxes of long-wave and
short-wave radiation, and the surface heat flux at the snow/ice
boundary. The PAM-IIIs were heavily modified before going into the
field to withstand the harsh Arctic conditions. Electrical power was
produced with propane thermoelectric generators rather than solar
panels, and the electronics were housed with the generators to prevent
built-up of ice. GPS receivers and electronic compasses were used to
continuously monitor station location and orientation, as well as to
provide accurate time-keeping. The instruments had to be serviced
regularly by ATD staff, who traveled in pairs on snowmobiles, carried
rifles in case of polar bear encounters, and relied on survival suits,
life jackets, coast guard ships and helicopters once the ice sheet had
thinned during the summer months. The Canadian ice breaker "Les
Grosseilliers", which was deliberately frozen into the sea ice, served
as the base camp and sheltered approximately 50 scientists at the
SHEBA ice camp.
During May and July/August 1998, the NCAR C-130 and RAF and RSF staff
conducted joint SHEBA operations out of Fairbanks, Alaska. ATD's
Scanning Aerosol Backscatter Lidar (SABL), Airborne Imaging Microwave
Radiometer (AIMR) and Multichannel Radiometer (MCR) were part of the
aircraft's extensive instrumentation load. AIMR, a dual-channel (37
GHz and 90 GHz) dual-polarization microwave radiometer loaned to ATD
by Canada's AES, records the characteristics of surface sea ice. MCR
is a seven channel scanning radiometer originally built by NASA
Goddard used to map surface emissions in visible and infrared
portions of the electromagnetic spectrum. Both of these instruments
underwent through significant modifications and upgrades before
operations by ATD.
In the end, the observational component of this $19.5 million project,
jointly funded by the National Science Foundation, the Office of Naval
Research and various international organizations, was a full
success. The data collected will allow scientists to understand
interactive processes involving mass changes of the sea ice, storage
and retrieval of heat in the mixed layer of the ocean, and the
influence of clouds on the surface energy balance. In the long run,
results from SHEBA will help us better understand the role of high
latitudes in global climate and assist in predicting future climate
change and assessing the impact of global warming.
GPS Dropsondes Development
From a technology point of view, the development of the GPS dropsonde
system was one of ATD's main highlights this year. In just over two
years, the GPS dropsonde system has gone from a prototype developed
within ATD to one of the most important observing system in
atmospheric sciences. The GPS dropsonde system is now installed on 18
research aircraft within the U.S., Germany, and Canada and routinely
used in hurricane reconnaissance and other research missions. The
concept of the dropsonde seems to be simple: a sonde attached to a
parachute is dropped from an aircraft and measures temperature,
humidity, and pressure as it falls. From its position during descent,
derived from the sonde's communication with the global navigation
systems, winds are calculated. The actual development and deployment
however is quite difficult. A dropsonde needs to provide
laboratory-quality data in an inexpensive package that one can safely
toss out of an airplane moving at greater than 100 meters per second.
Faced with this challenge, ATD's dropsonde development team combined
new sensor and GPS technology from Vaisala with a completely new
structural, electronic, and transmission system of their own design.
In each and every application, the GPS dropsonde has provided
order-of-magnitude improvements in the quality of data and reliability
of operation over previous systems. The vertical resolution for
wind-speed measurements has increased nearly a hundredfold. As
Hurricane Guillermo raged across the Pacific in August 1997, the
sondes helped provide the first-ever high-resolution data on hurricane
eyewall structure. This spring, ATD helped equip nine Air Force
WC-130s that carry out routine reconnaissance flights for tracking
tropical cyclones.
ATD also improved the accuracy and range of the pressure, temperature,
and humidity sensors; upgraded the electronics; and built a completely
new in-flight processing system. Up to four sondes can now be launched
and tracked as closely as 20 seconds apart. The success of this
state-of-the-art instrument was widely noticed. Schematics and
descriptions of the dropsonde showed up on the science and weather
pages of newspapers and magazines across the country. NASA and NOAA
proudly showed the dropsonde during their press conferences, and
television stations ran video clips of weather officers launching the
dropsondes from USAF C130s. Hurricane forecasters evaluated landfall
models on the basis of how much dropsonde data their models
assimilated. The user community praised the unprecedented high
vertical resolution, excellent performance in bad weather and wind
measurements were they counted the most, in the eye of a hurricane and
near the surface.
Specifications for the GPS sonde were developed in collaboration with
NOAA and the German Aerospace Research Establishment, both of which
funded, with NCAR, the sonde's development. UCAR has licensed the
technology to Vaisala for sonde manufacture since 1996.
