Radiometric correction of observations from microwave humidity sounders

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.

An Uncertainty Quantified Fundamental Climate Data Record for Microwave Humidity Sounders

“…That means an offset was determined [2], a slope and an off-set were determined [3], or an offset and a non-linearity were determined [4] (for the MSU (Microwave Sounding Unit) instrument) to reduce the bias in brightness temperature. A recent attempt used interpolation to produce look-up tables with calibration coefficients that change from month to month in order to reconcile the AMSU-B and MHS sensors [5]. Chung et al [6] also inter-calibrated AMSU-B and MHS instruments using a zonal-average method for adjusting inter-satellite differences as in [7].…”

Onboard Radio Frequency Interference as the Origin of Inter-Satellite Biases for Microwave Humidity Sounders

“…In this regard, our analysis shows a very important result, first, because it reveals the origin of the inter-satellite biases that prevents the construction of a stable, consistent long time series. Recent attempts to produce a consistent long time series by using interpolation as an intercalibration method still results in biases of, for example, up to 4 K for channel 5 of NOAA-16 [5]. Having shown the impact of RFI and having provided a dedicated correction, we can improve the consistency of NOAA-16 and NOAA-19 with NOAA-18.…”

“…So far, a consistent correction of these biases based on a recalibration has not been possible because the origin of these biases was unknown. Instead, lookup tables with empirical calibration coefficients that change from month to month are employed as a makeshift correction method [5]. There have been further bias correction efforts (e.g., References [4,6]) for other instruments.…”