ETL Deploys New Ground-Based Radiometer for Polar Profiling

March 25, 2004

Contact: Ed Westwater

Water vapor, cloud liquid, and cloud ice all play critical roles within climate feedback mechanisms, and especially in the fragile polar environments. However, their measurement in the cold and largely dry Arctic and Antarctic have been particularly difficult. Current techniques using radiosondes, conventional microwave radiometers, and cloud radars are either too infrequent or too insensitive to provide the information necessary to accurately close the radiation budget at the Earth's poles. As a result, polar climate feedback and climate change processes remain poorly understood. In order to begin to develop a precise and long-term record of polar water vapor and cloud parameters for the NOAA Study of Environmental Arctic Change (SEARCH) program, the Microwave Systems Development Division, ETL, has developed and successfully demonstrated a prototype Ground Scanning Radiometer (GSR) that provides unprecedented sensitivity to water vapor and clouds, enhanced calibration accuracy, and environmental robustness suitable for long term monitoring of the polar atmosphere.

Proposed originally by ETL in 1998, the GSR concept uses narrow-beam millimeter-wave and submillimeter-wave radiometers at several key bands (55, 89, 183, 340, and 380 GHz) housed within a purged drum to scan the sky from horizon to horizon. The 183 GHz channels, in particular, provide over an order of magnitude increase in vapor sensitivity. The radiometers are calibrated periodically by viewing two unique calibration targets, each of which was the object of extensive design effort. These "best practice" calibration targets are currently being considered for use by NIST for purposes of microwave radiometer calibration traceability. The configuration permits nearly all-weather operation without interference by hydrometeors. The 89 and 340 GHz channels are dual-polarized and are expected to provide enhanced sensitivity to cloud phase.

The first data from the GSR have been observed as part of a joint ETL and U.S. Department of Energy (DoE) experiment being conducted at the DoE Atmospheric Radiation Measurement (ARM) Programs's "Great White" field site near Barrow, Alaska from March 9 to April 9, 2004, by ETL personnel. Preliminary analyses show the data to be highly accurate. The instrument operation was suitable to long term deployment as part of the SEARCH program.