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

This study was undertaken to derive and analyze the advanced microwave scanning radiometer-Earth observing satellite (EOS) (AMSR-E) sea surface temperature (SST) footprint associated with the remote sensing systems (RSS) level-2 (L2) product. The footprint, in this case, is characterized by the weight attributed to each <inline-formula> <math display="inline"> <semantics> <mrow> <mn>4</mn> <mo>&#215;</mo> <mn>4</mn> </mrow> </semantics> </math> </inline-formula> km square contributing to the SST value of a given (AMSR-E) pixel. High-resolution L2 SST fields obtained from the moderate-resolution imaging spectroradiometer (MODIS), carried on the same spacecraft as AMSR-E, are used as the sub-resolution &#8220;ground truth&#8222; from which the AMSR-E footprint is determined. Mathematically, the approach is equivalent to a linear inversion problem, and its solution is pursued by means of a constrained least square approximation based on the bootstrap sampling procedure. The method yielded an elliptic-like Gaussian kernel with an aspect ratio &#8776;1.58, very close to the AMSR-E <inline-formula> <math display="inline"> <semantics> <mrow> <mn>6.93</mn> <mspace width="0.166667em"></mspace> <mi>GHz</mi> </mrow> </semantics> </math> </inline-formula> channel aspect ratio, &#8776;1.74. (The <inline-formula> <math display="inline"> <semantics> <mrow> <mn>6.93</mn> <mspace width="0.166667em"></mspace> <mi>GHz</mi> </mrow> </semantics> </math> </inline-formula> channel is the primary spectral frequency used to determine SST.) The semi-major axis of the estimated footprint is found to be aligned with the instantaneous field-of-view of the sensor as expected from the geometric characteristics of AMSR-E. Footprints were also analyzed year-by-year and as a function of latitude and found to be stable&#8212;no dependence on latitude or on time. Precise knowledge of the footprint is central for any satellite-derived product characterization and, in particular, for efforts to deconvolve the heavily oversampled AMSR-E SST fields and for studies devoted to product validation and comparison. A preliminary analysis suggests that use of the derived footprint will reduce the variance between AMSR-E and MODIS fields compared to the results obtained ignoring the shape and size of the footprint as has been the practice in such comparisons to date.

Abstract licence: CC BY

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