Millimeter-wave radiometric observations of the troposphere: a comparison of measurements and calculations based on radiosonde and Raman lidar

A comparison of clear-air brightness temperatures is performed between radiometric measurements and atmospheric radiative transfer calculations. The measurements were made using the NASA Goddard Space Flight Center´s Millimeter-wave Imaging Radiometer (MIR) in a series of airborne and ground-based atmospheric experiments at six millimeter-wave frequencies: 89; 150; 183.3±1, ±3, ±7; and 220 GHz. With the inclusion of the 220 GHz channel, these measurements are the first passive observations of the atmosphere made simultaneously at the six frequencies. The MIR was operated concurrently with supporting meteorological instruments (radiosonde and Raman lidar) to construct a paired set of both spatially and temporally coincident calibrated brightness temperatures and atmospheric profile parameters. Calculated brightness temperatures based on the measured atmospheric profile parameters were obtained using a numerical radiative transfer model. Incremental water-vapor weighting functions were used to study the impact of radiosonde hygrometer errors on the radiative transfer calculations. The aircraft-based brightness temperature comparisons are generally within 3 K for the channels sensitive to the lower atmospheric levels (89, 150, 183.3±7, and 220 GHz), but show discrepancies of up to 11 K for the opaque channels (183.3±1 and ±3 GHz) caused primarily by radiosonde bias. The ground-based calculations are similarly found to be sensitive to hygrometer errors in the lower atmosphere. Ground-based comparisons between MIR observations and lidar-based calculations are typically within ±6 K