Determining the AMSR-E SST Footprint from Co-Located MODIS SSTs

Abstract

This study was undertaken to derive and analyze the Advanced Microwave Scanning 1 Radiometer EOS (AMSR-E) sea surface temperature (SST) footprint associated with the Remote 2 Sensing Systems (RSS) Level-2 (L2) product. The footprint, in this case, is characterized by the 3 weight attributed to each 4 × 4 km square contributing to the SST value of a given AMSR-E pixel. 4 High-resolution L2 SST fields obtained from the MODerate-resolution Imaging Spectroradiometer 5 (MODIS), carried on the same spacecraft as AMSR-E, are used as the sub-resolution “ground truth“ 6 from which the AMSR-E footprint is determined. Mathematically, the approach is equivalent to 7 a linear inversion problem, and its solution is pursued by means of a constrained least square 8 approximation based on the bootstrap sampling procedure. The method yielded an elliptic-like 9 Gaussian kernel with an aspect ratio ≈ 1.58, very close to the AMSR-E 6.93 GHz channel aspect 10 ratio, ≈ 1.7. (The 6.93 GHz channel is the primary spectral frequency used to determine SST.) The 11 semi-major axis of the estimated footprint is found to be aligned with the instantaneous field-of-view 12 of the sensor as expected from the geometric characteristics of AMSR-E. Footprints were also analyzed 13 year-by-year and as a function of latitude and found to be stable – no dependence on latitude or on 14 time. Precise knowledge of the footprint is central for any satellite-derived product characterization 15 and, in particular, for efforts to deconvolve the heavily oversampled AMSR-E SST fields and for studies 16 devoted to product validation and comparison. A preliminarly analysis suggests that use of the 17 derived footprint will reduce the variance between AMSR-E and MODIS fields compared to the results 18 obtained ignoring the shape and size of the footprint as has been the practice in such comparisons 19 to date. 20