Cross Calibration between ATMS and AMSU-A

The cross-calibrated measurements from Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit-A (AMSU-A) on board different NOAA polar-orbiting satellites have been extensively used for detecting atmospheric temperature trend during the last several decades. MSU/AMSU-A were recently replaced by the Advanced Technology Microwave Sounder (ATMS) with the launch of Suomi National Polar-orbiting Partnership (SNPP) satellite. ATMS inherited most of the sounding channels from its predecessor AMSU. It is important to extend this satellite microwave climate data record by ATMS observations. However, ATMS has many channel characteristics that are different from AMSU-A, especially field-of-views (FOVs) the sizes and distributions. In this study, the Backus-Gilbert method was firstly used for optimally remapping the ATMS FOVs to AMSU-A like FOVs. Differences in ATMS brightness temperatures introduced by remapping are illustrated using Hurricane Sandy as an example. Secondly, the simultaneous nadir overpass (SNO) method is used for deriving a linear, temperature dependent offset for each pair of ATMS and AMSU-A channels NOAA-15, -18, -19, and MetOp-A satellites. The brightness temperature range covered by a 1.4 year SNO data-matching period from January 1 2012 to March 31 2013 is shown to be reasonably large to encompass the global brightness temperature range derived from a global double differencing method. Thirdly, the SNO derived offsets are used for cross calibration between ATMS resample data and NOAA-18 AMSU-A data. Temporal evolutions of brightness temperatures after the cross calibrations suggests that the ATMS resample data is well merged with AMSU-A data from NOAA-15, -18, -19 and MetOp-A satellites. The global ocean-averaged inter-satellite biases for the pentad dataset are significantly reduced (by more than an order of magnitude) after cross calibration.