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Abstract. The Advanced Microwave Sounding Unit-B (AMSU-B) and Microwave HumiditySounder (MHS) are total power microwave radiometers operating at frequenciesnear the water vapor absorption line at 183 GHz. The measurements of theseinstruments are crucial for deriving a variety of climate and hydrologicalproducts such as water vapor, precipitation, and ice cloud parameters.However, these measurements are subject to several errors that can beclassified into radiometric and geometric errors. The aim of this study is toquantify and correct the radiometric errors in these observations throughintercalibration. Since the bias in the calibration of microwave instrumentschanges with scene temperature, a two-point intercalibration correctionscheme was developed based on averages of measurements over the tropicaloceans and nighttime polar regions. The intercalibration coefficients werecalculated on a monthly basis using measurements averaged over each specifiedregion and each orbit, then interpolated to estimate the daily coefficients.Since AMSU-B and MHS channels operate at different frequencies andpolarizations, the measurements from the two instruments were notintercalibrated. Because of the negligible diurnal cycle of both temperatureand humidity fields over the tropical oceans, the satellites with the moststable time series of brightness temperatures over the tropical oceans(NOAA-17 for AMSU-B and NOAA-18 for MHS) wereselected as the referencesatellites and other similar instruments were intercalibrated with respect tothe reference instrument. The results show that channels 1, 3, 4, and 5 ofAMSU-B on board NOAA-16 and channels 1 and 4 of AMSU-B on board NOAA-15 showa large drift over the period of operation. The MHS measurements frominstruments on board NOAA-18, NOAA-19, and MetOp-A are generally consistentwith each other. Because of the lack of reference measurements, radiometriccorrection of microwave instruments remain a challenge, as theintercalibration of these instruments largely depends on the stability of thereference instrument.

Radiometric correction of observations from microwave humidity sounders