High Latitude Sea Surface Skin Temperatures Derived from Saildrone Infrared Measurements

Infrared (IR) radiometers on polar-orbiting satellites offer the best way to derive skin sea surface temperature (SSTskin) in the Arctic. However, it is particularly difficult for satellite instruments to establish high-accuracy SSTskin datasets at high latitudes, compared with other areas of the global ocean. To better assess errors in the satellite SSTskin retrievals, accurate in situ measurements are required as validation data. Considering that the use of subsurface SST taken by buoys as the validation datasets introduces inadequacies due to the cool skin effect and diurnal warming, IR radiometers mounted on ships or other platforms to measure the SSTskin have been recognized as providing appropriate, accurate surface measurements for the validation procedures. Here, we introduce SSTskin derived from a simple IR radiometric system carried on Saildrone uncrewed surface vehicles (USVs), which are powered by wind and solar. From 15th May to 11th October 2019, two Saildrones funded by the National Aeronautics and Space Administration (NASA), SD-1036 and SD-1037, were equipped with IR radiometers in a "unicorn" structure on the deck for the determination of the SSTskin. We present an algorithm to derive SSTskin from the downward- and upward-looking radiometers and estimate the main contributions to inaccuracy of the SSTskin. After stringent quality control of data and eliminating measurements influenced by sea ice and precipitation, and restricting the acceptable tilt angle of the Saildrone USV, and based on line-by-line radiative transfer model (LBLRTM) simulations, SSTskin can be derived to an accuracy of 0.12 K. The error budget of the derived SSTskin is developed and the largest component comes from the instrumental uncertainties assuming the viewing geometry is adequately determined. Thus, Saildrones equipped with these sensors provide sufficiently accurate SSTskin retrievals for studying the physics of the thermal skin effect in conjunction with accurate sub-surface thermometer measurements, and for validating satellite-retrieved SSTskin at high latitudes.