"Characterisation of the bi-directional reflectance of bare sea ice surfaces for the inter-calibration and validation of satellite remote sensing products" (by Maxim Lamare)

In February 2016, the Santander Travel Award allowed me to spend two weeks doing field measurements at the Sea Ice Environmental Research Facility (SERF) located at the University of Manitoba, Winnipeg, Canada. SERF is an outdoor experimental facility that generates artificial sea ice under controlled conditions. The main feature of the facility is an outdoor seawater pool (20m x 10m x 2.5 m) equipped with a removable sliding roof. In the winter the seawater freezes in the pool, forming an artificial sea ice cover on which measurements can be undertaken. My proposal was selected, and I was assigned a section of the pool to perform measurements for 2 weeks.

SERF with the removable roof on at the start of the campaign.

My research focuses on the calibration of optical satellite sensors using snow and sea ice. At Royal Holloway, I use a combination of modelling and laboratory studies to understand and reduce biases between satellite sensors. Indeed, although satellites are ideal tools to observe expansive and inaccessible areas such as polar regions, the use of multiple platforms for repeat coverage or multi-temporal studies requires a good knowledge of sensor to sensor biases to allow for accurate data comparison. Sea ice has been established by the Committee on Earth Observing Satellites (CEOS) as a natural target serving as an international reference standard. Therefore, the characterisation of the reflectance of sea ice is necessary. Furthermore, satellites observe the surface of the Earth with different viewing angles, depending on the orbit and sensor characteristics. Therefore, I perform multi-angular measurements of reflectance to match the satellites' viewing angles.

I am currently recording the multi-angular reflectance of artificial sea ice in a small tank at Royal Holloway. However, the work is carried out using artificial illumination on a small target surface. The field campaign was an excellent way of comparing and validating the work carried out at Royal Holloway, by measuring sea ice in natural illumination conditions whilst maintaining a controlled and safe environment.

To undertake multi-angular measurements of the bare sea ice at SERF, I used a Gonio Radiometric Spectrometer System (GRASS). The instrument is designed to record quasi-simultaneous, multi-angle, hyperspectral measurements of the Earth's surface reflectance. To do so, 16 signal collectors attached to a rotating hemispherical frame aim at the same target area. The collectors are connected by fibre optic cables to a spectroradiometer operating from 400 nm to 1700 nm. The spectral signatures obtained over the azimuth angles (0 - 360º) and zenith angles (0 - 60º) were collated to generate hemispherical directional reflectance factor (HDRF) plots that will be directly compared with the model outputs and laboratory work at RHUL.

The GRASS instrument deployed on the artificial sea ice at SERF.

After setting up the instrument and conducting initial tests, measurements were performed over the 2 weeks. The meteorological conditions were not ideal during the first week of the campaign, with overcast days and snowfall. Indeed, having to rely on a clear sky for natural illumination, the measurements were constrained to sunny days and I was reduced to taking ancillary measurements (Chlorophyl content below the ice that can affect the reflectance, thickness and temperature…). However the weather settled during the second week, allowing me to perform multiple daily acquisitions, capturing a variety of surface conditions: from bare smooth sea ice to snow covered sea ice.

Rotating the instrument to align the signal collectors with the sun.

The two weeks spent in Winnipeg were a fantastic experience. I came away with novel data albeit the difficult weather conditions. A first analysis of the data shows good agreement between the results from SERF and the modelling, which is very promising. I also interacted with researchers from different backgrounds, the sea ice community being large at the University of Manitoba.

An underwater picture of the sea ice: not a common point of view.

The fieldwork campaign, which would not have been possible without the Santander Travel Award, enabled me to validate my work at RHUL using an international research facility, and was a great opportunity for my academic career.