Deployments of microwave and millimeterwave radiometers in the Arctic

Measurement of water vapor and cloud liquid during very cold (-20 to -40 Deg. C) and dry (precipitable water vapor < 5 mm) conditions is a very important, but difficult task. Starting in 1999, three radiometric experiments were conducted at the U. S. Department of Energypsilas Atmospheric Radiation Measurement (ARM) Programpsilas North Slope of Alaska (NSA) field site near Barrow, Alaska. Principal results from the first two experiments are summarized. Most recently, the Radiative Heating in Underexplored Bands Campaign (RHUBC) was conducted in February-March 2007. The millimeter- and submillimeter-wave channels of the ground-based scanning radiometer (GSR) are very sensitive to low water vapor and cloud contents and allow for accurate observations in the extremely dry and cold conditions typical of the Arctic. Moreover, window channels (e.g., at 90 and 340 GHz, both with horizontal and vertical polarizations) show a high sensitivity to Artic clouds over an extended range of liquid water path (LWP). For RHUBC, during conditions when the precipitable water vapor (PWV) was less than 2 mm, these radiometers were supplemented by frequent Vaisala RS92 radiosonde observations at the ARM site. In this paper, representative PWV and LWP retrievals from the GSR and several ARM instruments are compared. Based on comparisons with 87 RS92 RAOB launches, GSR PWV retrievals achieved almost unprecedented accuracy of 0.1 mm, or about 6 % of the mean PWV during the operating period.