Monitoring scan asymmetry of microwave humidity sounding channels using simultaneous all angle collocations (SAACs)

Abstract

[1] Simultaneous all angle collocations (SAACs) of microwave humidity sounders (AMSU-B and MHS) on-board polar orbiting satellites are used to estimate scan-dependent biases. This method has distinct advantages over previous methods, such as that the estimated scan-dependent biases are not influenced by diurnal differences between the edges of the scan and the biases can be estimated for both sides of the scan. We find the results are robust in the sense that biases estimated for one satellite pair can be reproduced by double differencing biases of these satellites with a third satellite. Channel 1 of these instruments shows the least bias for all satellites. Channel 2 has biases greater than 5 K, thus needs to be corrected. Channel 3 has biases of about 2 K and more and they are time varying for some of the satellites. Channel 4 has the largest bias which is about 15 K when the data are averaged for 5 years, but biases of individual months can be as large as 30 K. Channel 5 also has large and time varying biases for two of the AMSU-Bs. NOAA-15 (N15) channels are found to be affected the most, mainly due to radio frequency interference (RFI) from onboard data transmitters. Channel 4 of N15 shows the largest and time varying biases, so data of this channel should only be used with caution for climate applications. The two MHS instruments show the best agreement for all channels. Our estimates may be used to correct for scan-dependent biases of these instruments, or at least used as a guideline for excluding channels with large scan asymmetries from scientific analyses.

A review of sources of systematic errors and uncertainties in observations and simulations at 183 GHz

“…Far from nadir, channels sounding deep into the troposphere, located on the wings of the 183.31 GHz line, might be more affected by antenna issues than higher peaking channels, located in the center of the line, due to radiation measured by the side lobes. Such asymmetries were found for AMSU-B and MHS (Buehler et al, 2005b;John et al, 2013b), but so far the monitoring of SAPHIR has not shown any scan asymmetry. For ATMS, comparisons of temperature data records (TDRs, calibrated antenna temperatures) and sensor data records (SDRs, BT after further applying beam efficiency and scan-position-dependent bias corrections) for the 183 GHz channels show the same behavior meaning that the TDR to SRD conversion is not responsible for the bias, although it seems to introduce some dependence on the viewing angle that warrants further investigation.…”

The CM SAF ATOVS data record: overview of methodology and evaluation of total column water and profiles of tropospheric humidity

“…Consequently, potential non-linear effects as a function of scene brightness temperature are not considered. It has also been shown that there might be scan asymmetry in the AMSU-B brightness temperatures (Buehler et al, 2005;John et al, 2013), which has not been accounted for here.…”

CloudSat-constrained cloud ice water path and cloud top height retrievals from MHS 157 and 183.3 GHz radiances

“…For example, there was a radio-frequency interference (RFI) problem in CH#3 and CH#4 of AMSU-B (Atkinson, 2001;Buehler et al, 2005), and gain variations/degredations are found in CH#3-CH#5 of AMSU-B on NOAA-16 and NOAA-17 (John et al, 2013). MHS exhibits smaller scandependent biases than AMSU-B, but suspicious behaviors have been reported for CH#3 on NOAA-18 and NOAA-19 and Metop-A (John et al, 2013). The MHS instruments on NOAA-18 and Metop-A have so far shown the best overall radiometric calibration for all five channels.…”