Facilitating the Use of the MODIS Aqua L2 SST Dataset

Short Abstract

The MODIS Aqua L2 SST dataset obtained from the PO-DAAC for the period 2002-2021 is being modified to improve its usability. To begin with the L2  field is being regridded to correct for the bow-tie effect. The bow-tie effect gives rise to an irregular spacing of along-track pixels with the effect increasing away from nadir and, for MODIS, resulting in a non-monotonic ordering of the pixels at distances of about 400 pixels from nadir. This makes estimates of the SST gradient problematic at best. Furthermore, the data are provided in 5 minute granules, resulting in seams every 2030 scan lines. To avoid these seams, the regridded granules are combined into complete orbits starting at 75°S. The starting location minimizes ocean areas exposed to orbit-to-orbit seams.

In addition to geometric modifications, pixels mistakenly flagged as bad are unmasked. Such pixels are found in high-gradient regions and dynamic regions where the retrieved values differ by more than 2°C from that of the reference field. To address the high gradient problem, the mask is divided into objects and each object is examined to determine whether or not the associated SST values are reasonable. To address the reference temperature issue the min/max of SSTs in 5°x5° regions were determined from the complete masked dataset and retrieved values >10% below the minimum and <10% above the maximum were retained.