Assessment of intercalibration methods for satellite microwave humidity sounders

Abstract

[1] Three methods for intercalibrating humidity sounding channels are compared to assess their merits and demerits. The methods use the following: (1) natural targets (Antarctica and tropical oceans), (2) zonal average brightness temperatures, and (3) simultaneous nadir overpasses (SNOs). Advanced Microwave Sounding Unit-B instruments onboard the polar-orbiting NOAA 15 and NOAA 16 satellites are used as examples. Antarctica is shown to be useful for identifying some of the instrument problems but less promising for intercalibrating humidity sounders due to the large diurnal variations there. Owing to smaller diurnal cycles over tropical oceans, these are found to be a good target for estimating intersatellite biases. Estimated biases are more resistant to diurnal differences when data from ascending and descending passes are combined. Biases estimated from zonal-averaged brightness temperatures show large seasonal and latitude dependence which could have resulted from diurnal cycle aliasing and scene-radiance dependence of the biases. This method may not be the best for channels with significant surface contributions. We have also tested the impact of clouds on the estimated biases and found that it is not significant, at least for tropical ocean estimates. Biases estimated from SNOs are the least influenced by diurnal cycle aliasing and cloud impacts. However, SNOs cover only relatively small part of the dynamic range of observed brightness temperatures.

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

“…Ways to compare measurements by different satellite instruments include the simultaneous nadir overpasses technique (limiting the comparisons to the highest latitudes, John et al, 2012) and the use of "natural targets" that have very little variability or the averaging over a lot of scenes (John et al, 2013a). Another technique is the double difference technique that uses NWP fields input to a RTM as a transfer function between two radiometers that have few common overpasses.…”

Answers to comments

“…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.…”

Evaluating the Diurnal Cycle of Upper Tropospheric Humidity in Two Different Climate Models Using Satellite Observations

“…The amplitudes a 1 and a 2 are 0.06% RH and 0.11% RH respectively. However, combining the ascending and descending orbits to deduce the inter-satellite biases can almost completely eliminate the 24 h amplitude (a 1 ) of the diurnal cycle with a very small remnant diurnal amplitude (a 2 ) [36].…”