Microwave radiometers deployed by the U. S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program provide crucial data for a wide range of research applications. The accuracy and stability of these instruments also makes them ideal for improving climate data records: to detect and correct discontinuities in the long-term climate records, to validate and calibrate the climate data, to characterize errors in the climate records, and to plan for the future Global Climate Observing System (GCOS) Reference Upper-Air network. This paper presents an overview of these capabilities with examples from ARM data.

Two-channel microwave radiometers (MWR) operating at 23.8 and 31.4 GHz are deployed at each of eleven ARM Climate Research Facility (ACRF) field sites in the U. S. Southern Great Plains (SGP), Tropical Western Pacific (TWP), North Slope of Alaska (NSA), and with the ARM Mobile Facility in Niamey, Niger for the purpose of retrieving precipitable water vapor (PWV) and liquid water path (LWP). At these locations PWV ranges from as low as 1 mm (1 kg/m2) at the NSA to 70 mm or more in the TWP; LWP can exceed 2 mm at many sites. The MWR accommodates this wide dynamic range for all non-precipitating conditions with a root-mean-square error of about 0.4 mm for PWV and 0.02 mm (20 g/m2) for LWP. The calibration of the MWR is continuously and autonomously monitored and updated to maintain accuracy. Comparisons of collocated MWRs will be presented. Site-specific linear statistical retrievals are used operationally; more sophisticated retrievals are applied in post-processing the data.

Because PWV is an integral measure, derived from both the relative humidity and temperature profiles of the radiosonde, it is a particularly useful reference quantity. Comparison of PWV measured by the MWR with PWV from radiosondes reveals dry biases and diurnal trends as well as general calibration variability in the radiosondes. To correct the bias and reduce the variability ARM scales the relative humidity measurements from the radiosondes to produce agreement with the PWV measured by the MWR. Comparisons of infrared spectral radiances calculated using these scaled radiosondes with high spectral resolution measurements exhibit dramatically reduced bias and variability. This ability to detect and correct errors in the radiosondes measurements will be critical for detecting climate change.

The MWR has also been used for a variety of ground- and satellite-based remote sensor retrieval development and validation studies, including precipitable water vapor and slant water vapor retrievals using the Global Positioning System (GPS). The MWR can provide a valuable comparison for GPS-derived zenith wet delay and PWV values, e.g., for evaluating improved mapping functions and detecting errors due, for example, to multi-path contributions.

For precipitable water vapor amounts less than 4 mm, which commonly occur in cold, dry Arctic conditions, the 0.4 mm root-mean-square error of the MWR precipitable water vapor measurement is problematic. To obtain increased sensitivity under these conditions, a new G-band water vapor radiometer (GVR) operating at 183.31±1, ±3, ±7, and ±14 GHz is deployed at the NSA Barrow site. The GVR offers a valuable reference for radiosonde and GPS water vapor measurements at Arctic locations that are expected to be particularly sensitive to climate change.