Deployment of a high dynamic range and autonomous ocean-light instrument (OptiCAL) in the Arctic Ocean from July to December 2019 and corresponding surface irradiance (EPAR), solar and lunar zenith angles modelled with the HEIMDALL radiative transfer model.

The OptiCAL (Optical Chain And Logger) is an autonomous ice-tethered observatory (ITO) for measuring the underwater light climate. It comprises of a spar buoy (containing Iridium and GPS antenna, logger/controller, Iridium modem and batteries) and a chain with “nodes” at certain depths. Each node carries three upward and three downward looking light sensors. For each direction two sensors have a gel filter, one blue and one orange while the third sensor is unfiltered. See Schartmüller et al. (submitted for publication) for a detailed description of the instrument and the calibration.
In 2019 an OptiCAL equipped with 7 nodes (1m, 2m, 3m, 5m, 6m, 9m, 14m depth) was deployed by the Norwegian research vessel RV Kronprins Haakon in the Arctic Ocean. It drifted from the deployment site at 84.64°N 28.72°E on the 13.07.2019 to 82.65°N 12.54°E on the 14.12.2019 when contact was lost. The OptiCAL was deployed as part of the Arctic ABC Development project together with with other ITOs on an ice floe, including a SIMBA (Snow Ice Mass Balance Apparatus) unit (SAMS-Enterprise, 2023), an automatic weather station (AWS) and an Acoustic Zooplankton Fish Profiler (AZFP) with a XEOS GPS beacon.

For comparison three different surface irradiance datasets, solar and lunar zenith angles were modelled using the HEIMDALL radiative transfer model (Connan-McGinty et al., 2022). One for constant clear sky, one for constant cloud covered skies, and one with varying local cloud conditions predicted by ERA5 cloud data (Hersbach et al., 2018).

OptiCAL data raw data are archived as 'counts' from the sensors and are in units of millihertz [mHz]. Calibrated OptiCAL data using the calibration factor from Schartmüller et al. are archived in the unit [µmol m-2 s-1]. Modelled surface irradiance is also provided in the unit [µmol m-2 s-1].

The work was supported by three grants from the Norwegian Research Council: Arctic ABC (project no 244319), Arctic ABC Development (project no 245923), Deep Impact (project no 300333), and the Centre of Excellence AMOS (project no 223254).

References:

CONNAN-MCGINTY, S., BANAS, N. S., BERGE, J., COTTIER, F., GRANT, S., JOHNSEN, G., KOPEC, T. P., PORTER, M. & MCKEE, D. 2022. Midnight sun to Polar Night: A model of seasonal light in the Barents Sea. Journal of Advances in Modeling Earth Systems, 14, e2022MS003198.

HERSBACH, H., BELL, B., BERRISFORD, P., BIAVATI, G., HORÁNYI, A., MUÑOZ SABATER, J., NICOLAS, J., PEUBEY, C., RADU, R. & ROZUM, I. 2018. ERA5 hourly data on single levels from 1979 to present. Copernicus climate change service (c3s) climate data store (cds), 10.

SAMS-ENTERPRISE. 2023. Autonomous Snow and Ice Measurement [Online]. Available: https://www.sams-enterprise.com/services/autonomous-ice-measurement/ [Accessed 17.02.2023 2023].