Comparison of microwave radiometers and radiosondes during the Nauru-99 experiment

Previous experience, both in the Tropical Western Pacific and at the Atmospheric Radiation Measurement (ARM) Program´s Southern Great Plains Cloud And Radiation Testbed (CART) site indicated the need for adjustments to Vaisala Humicap RS80 humidity soundings. The need for such corrections was identified by measurements of water vapor by microwave radiometers and the subsequent adjustments by scaling of water vapor profiles. The scaled water vapor profiles, when inserted into the LBLRTM (Line By Line Radiative Transfer Model) have been in much better agreement with well-calibrated infrared radiance measurements by the Atmospheric Emitted Radiance Interferometer (AERI) operated at many ARM CART sites. During Nauru-99, a variety of ground- and ship-based instruments were available to test the quality of radiosonde (RAOB) soundings in the tropical environment around the island of Nauru. In particular, nearly simultaneous RAOB soundings from the Atmospheric Radiation and Cloud Station (ARCS-2) and from the R RONALD H. BROWN (RHB) were available. To track the age of radiosondes, the serial numbers of the Vaisala RAOBs were available for subsequent analysis. A variety of remote sensing and in situ measurements were also available. These instruments at ARCS-2 include the Radiometrics Corporation´s Microwave Radiometer (MWR) and a Vaisala ceilometer. Of special interest to the authors´ analysis was their independent calibration of the MWR that used a blackbody calibration target cooled by liquid nitrogen