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Abstract
Radiometers operating at millimeter wavelengths show enhanced sensitivity to low water vapor and liquid contents relative to conventional microwave radiometers that operate below 30 GHz (1 cm). This sensitivity makes the higher frequency radiometers particularly appealing for accurate observations in the extremely dry and cold conditions typical of the Arctic. For this reason, the Center for Environmental Research (CET) of University of Colorado (CU) designed a 25-channel instrument, called the Ground-based Scanning Radiometer (GSR), spanning the millimeter- and submillimeter- wave spectrum (from 50 to 380 GHz). The set of frequencies was selected for the simultaneous retrieval of atmospheric temperature and humidity profiles, precipitable water vapor, cloud liquid path, and cloud depolarization ratio. The GSR was deployed during the Water Vapor Intensive Operational Period (WVIOP, March-April 2004) and the Radiative Heating in Underexplored Bands Campaign (RHUBC, February-March 2007) both held at the Atmospheric Radiation Measurement (ARM) Program’s site in Barrow, Alaska. The GSR joined the resident ARM instrumentation, including a dual channel Microwave Radiometer (MWR) and a 12-channel Microwave Radiometer Profiler; additionally, two other millimeterwave radiometers were deployed by ARM during RHUBC, the 4-channel GVR and the 15-channel MP183. In particular, the GSR, GVR, and MP183 show several channels located around the strong 183 GHz water vapor band, which may be very important for ground-based water vapor measurements in very dry conditions. In this presentation, we compare independent observations near 183 GHz and we quantify the overall agreement obtained during the experiment. Then, we discuss the retrieval technique applied to GSR data for obtaining temperature and humidity profiles, based on optimal estimation method initialized with forecasts from a Numerical Weather Prediction model. Examples of GSR-based retrievals will be presented and compared with ARM operational products.