ISAR

Sea surface temperature (SST) is one of the most appropriate and important climate parameters: a widespread increase is an indicator of global warming and modifications of the geographical distribution of SST are an extremely sensitive indicator of climate change. There is high demand for accurate, reliable, high spatial and temporal resolution SST measurements for the parameterisation of ocean-atmosphere heat, momentum and gas (SST is therefore critical to understanding the processes controlling the global carbon dioxide budget) fluxes, for detailed diagnostic and process orientated studies to better understand the behavior of the climate system, as model boundary conditions, for assimilation into climate models and, for the rigorous validation of climate model output. These demands require complete confidence in the content, interpretation, accuracy, reliability and continuity of observational SST data; criteria that can only be fulfilled by the successful implementation of an on-going data product validation strategy.

ISAR mounted on the SPIRIT bridge port rail. The GPS (left antenna) provides location data and the Iridium SBD (right antenna) is used for ship-to-shore data telemetry every half hour. The AMF2 IRT radiometers are located to the left of the ISAR.

The Infrared Sea surface Temperature Autonomous Radiometer (ISAR) system has been developed to collect high quality in situ observations of the radiative sea surface (skin) temperature, SSST, that can be used to validate the SSST observations made by several satellite systems (ENVSAT, MODIS, AQUA). The ISAR is carefully calibrated to NIST standards and one of this IOP is to ensure that the satellite retrievals of SSST have characterized uncertainties and that the validation procedure provides traceability to NIST standards and therefore can be legitimately considered a Climate Data Record. Traceability to NIST standards is retained by careful maintenance of the laboratory calibration equipment at Univ. of Miami and University of Washington, and periodic recalibration of the reference thermometers.

The ISAR is a compact (570 x 220 mm) precision, self-calibrating, infrared radiometer capable of measuring in situ SSST accurate to ±0.1 C rmse that has been developed in collaboration with scientists at the National Oceanography Centre, Southampton (NOCS) UK, Brookhaven National Laboratory (BNL) USA, and European Commission, Italy. The ISAR uses two precision calibration black body (BB) cavities to maintain the radiance calibration of a solid-state infrared detector having a spectral window of 9.6-11.5μm. All ISAR target views are made using a single route optical path via a protective scan drum arrangement that may be programmed to any target view to positioned over a range of 160°. In this way, views of the atmosphere can be made at regular intervals providing the necessary measurements required to correct sea view radiance data for sky radiance reflection due to the non-unity value of sea water emissivity.

The ISAR system on SPIRIT has an external GPS and a pitch and roll sensor. These provide important information for estimating the precise SSST. In addition it incorporates an Iridium short-burst data (SBD) modem so SSST measurements can be transmitted to shore on a regular (half hour) schedule. During the cruises, SSST data are given quality assurance by comparisons with SST maps and with co-located data buoys.

A trackline showing SSST for MAGIC Leg04. Day points are shown as year day from 292 to 306. The typical temperature field from 16 C to 27 C is apparent. The cold tongue of water near the California coast is indicative of the prevalent coastal upwelling there.

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