Abstract
The accuracy and precision of satellite sea surface temperature (SST) products in nearshore coastal waters are not well known, owing to a lack of in-situ data available for validation. It has been suggested that recreational watersports enthusiasts, who immerse themselves in nearshore coastal waters, be used as a platform to improve sampling and fill this gap. One tool that has been used worldwide by surfers is the Smartfin, which contains a temperature sensor integrated into a surfboard fin. If tools such as the Smartfin are to be considered for satellite validation work, they must be carefully evaluated against state-of-the-art techniques to quantify data quality. In this study, we developed a Simple Oceanographic floating Device (SOD), designed to float on the ocean surface, and deployed it during the 28th Atlantic Meridional Transect (AMT28) research cruise (September and October 2018). We attached a Smartfin to the underside of the SOD, which measured temperature at a depth of ∼0.1 m, in a manner consistent with how it collects data on a surfboard. Additional temperature sensors (an iButton and a TidbiT v2), shaded and positioned a depth of ∼1 m, were also attached to the SOD at some of the stations. Four laboratory comparisons of the SOD sensors (Smartfin, iButton and TidbiT v2) with an accurate temperature probe (±0.0043 K over a range of 273.15 to 323.15 K) were also conducted during the AMT28 voyage, over a temperature range of 290–309 K in a recirculating water bath. Mean differences (δ), referenced to the temperature probe, were removed from the iButton (δ=0.292 K) and a TidbiT v2 sensors (δ=0.089 K), but not from the Smartfin, as it was found to be in excellent agreement with the temperature probe (δ=0.005 K). The SOD was deployed for 20 min periods at 62 stations (predawn and noon) spanning 100 degrees latitude and a gradient in SST of 19 K. Simultaneous measurements of skin SST were collected using an Infrared Sea surface temperature Autonomous Radiometer (ISAR), a state-of-the-art instrument used for satellite validation. Additionally, we extracted simultaneous SST measurements, collected at slightly different depths, from an underway conductivity, temperature and depth (CTD) system. Over all 62 stations, the mean difference (δ) and mean absolute difference (ϵ) between Smartfin and the underway CTD were −0.01 and 0.06 K respectively (similar results obtained from comparisons between Smartfin and iButton and Smartfin and TidbiT v2), and the δ and ϵ between Smartfin and ISAR were 0.09 and 0.12 K respectively. In both comparisons, statistics varied between noon and predawn stations, with differences related to environmental variability (wind speed and sea-air temperature differences) and depth of sampling. Our results add confidence to the use of Smartfin as a citizen science tool for evaluating satellite SST data, and data collected using the SOD and ISAR were shown to be useful for quantifying near-surface temperature gradients.
Highlights
The Global Climate Observing System classifies sea surface temperature (SST) as an Essential Climate Variable [1,2]
We exploited an opportunity on the 28th Atlantic Meridional Transect cruise to compare Smartfin data with continuous SST measurements collected using an Infrared SST Autonomous Radiometer (ISAR) and underway conductivity, temperature and depth (CTD) system, over 100 degrees of latitude (∼11,000 km) and a 19 K range in SST, as part of the European Space Agency (ESA) AMT4OceanSatFlux project
We exploited an opportunity on the 28th Atlantic Meridional Transect (AMT28) research cruise to compare SST data collected from a Smartfin with an Infrared Sea surface temperature Autonomous Radiometer (ISAR) measuring SST skin, and state-of-the-art oceanographic instrumentation used on research cruises for measuring SST at ∼5 m depth
Summary
The Global Climate Observing System classifies sea surface temperature (SST) as an Essential Climate Variable [1,2]. Temperature gradients in the very upper ocean are known to occur [18,19] and need to be corrected for when comparing SST measurements collected at slightly different depths To avoid these issues, well-calibrated ship-based infrared radiometers have been developed for satellite validation purposes, and are considered the standard tools for collecting Fiducial Reference Measurements (FRM) of SST [20], with SI traceability that follow the guidelines outlined by the Quality Assurance framework for Earth Observation. Differences between sensors were quantified, and analyzed in the context of environmental variability and depth of sampling, to assess the Smartfin as a tool for satellite SST validation, acknowledging the need to repeat this type of work in the future in nearshore coastal waters
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