Abstract
Abstract. The Advanced Microwave Sounding Unit-B (AMSU-B) and Microwave Humidity Sounder (MHS) are total power microwave radiometers operating at frequencies near the water vapor absorption line at 183 GHz. The measurements of these instruments are crucial for deriving a variety of climate and hydrological products such as water vapor, precipitation, and ice cloud parameters. However, these measurements are subject to several errors that can be classified into radiometric and geometric errors. The aim of this study is to quantify and correct the radiometric errors in these observations through intercalibration. Since the bias in the calibration of microwave instruments changes with scene temperature, a two-point intercalibration correction scheme was developed based on averages of measurements over the tropical oceans and nighttime polar regions. The intercalibration coefficients were calculated on a monthly basis using measurements averaged over each specified region and each orbit, then interpolated to estimate the daily coefficients. Since AMSU-B and MHS channels operate at different frequencies and polarizations, the measurements from the two instruments were not intercalibrated. Because of the negligible diurnal cycle of both temperature and humidity fields over the tropical oceans, the satellites with the most stable time series of brightness temperatures over the tropical oceans (NOAA-17 for AMSU-B and NOAA-18 for MHS) were selected as the reference satellites and other similar instruments were intercalibrated with respect to the reference instrument. The results show that channels 1, 3, 4, and 5 of AMSU-B on board NOAA-16 and channels 1 and 4 of AMSU-B on board NOAA-15 show a large drift over the period of operation. The MHS measurements from instruments on board NOAA-18, NOAA-19, and MetOp-A are generally consistent with each other. Because of the lack of reference measurements, radiometric correction of microwave instruments remain a challenge, as the intercalibration of these instruments largely depends on the stability of the reference instrument.
Highlights
Measurements from microwave instruments on board spaceborne platforms operating near the water vapor absorption line at 183 GHz are one of the main sources of observations for tropospheric water vapor, total precipitable water vapor, and cloud ice water path (Ferraro et al, 2005)
Because of the negligible diurnal cycle of both temperature and humidity fields over the tropical oceans, the satellites with the most stable time series of brightness temperatures over the tropical oceans (NOAA-17 for Advanced Microwave Sounding Unit-B (AMSU-B) and NOAA-18 for Microwave Humidity Sounder (MHS)) were selected as the reference satellites and other similar instruments were intercalibrated with respect to the reference instrument
The results show that channels 1, 3, 4, and 5 of AMSU-B on board NOAA-16 and channels 1 and 4 of AMSU-B on board NOAA-15 show a large drift over the period of operation
Summary
Measurements from microwave instruments on board spaceborne platforms operating near the water vapor absorption line at 183 GHz are one of the main sources of observations for tropospheric water vapor, total precipitable water vapor, and cloud ice water path (Ferraro et al, 2005). These data are increasingly assimilated into NWP models for the purpose of improving weather forecasting or atmospheric reanalyses (Rienecker et al, 2011). Geometric errors are related to a shift in the Earth location of measurements and are introduced by sources such as timing error, instrument mounting errors, and errors in instrument modeling and geolocation algorithms (Moradi et al, 2013a). Moradi et al (2013a) investigated the geolocation errors in these instruments using the difference between ascending and descending observations along the coastlines and reported several errors including more than 1 degree antenna pointing error in AMSU-A on board NOAA-15, about 1 degree pointing error in AMSU-A2 on board NOAA-18, Published by Copernicus Publications on behalf of the European Geosciences Union
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