On most expeditions, sea-ice and snow conditions are observed and recorded not just at permanent stations, but also along the expedition route, directly from on board ship. This includes parameters like snow and ice thickness, floe size and the distribution of melt ponds. Generally speaking, there are three types of vessel-based observations: bridge observations, systematic photography, and readings taken by remote sensing systems.
Vessel-based observations can e.g. help to validate remote sensing data. They can also offer additional information to support other fieldwork.
Snow and sea-ice conditions are visually checked at regular intervals along the ship’s route – for instance, hourly – from the bridge. These observations are made in keeping with standardised international protocols that, while differing in a few details, are essentially the same for the Arctic and Antarctic. These observations have been made since the beginning of systematic polar expeditions on research vessels and are stored in data archives dating back more than 30 years.
Once the ship has reached the sea-ice region, a member of the expedition spends ca. 15 minutes observing their surroundings and recording what they see in the logbook provided. Normally, three different ice classes are used, together with the following parameters: ice concentration, ice thickness, snow cover, snow thickness, ice deformation in concentration and height, melt ponds, and the percentage of sea ice containing biomass and/or sediment. In addition, the meteorological conditions and selected water properties at the time of observation, together with the place and time, are recorded. Further comments concern the ship’s bearing (speed, course, passage through ice) or observations of organisms on and near the sea ice. The data is usually rounded out with photographs taken from the bridge in all directions.
One disadvantage of this method: the subjective view of the observer, who constantly changes. For the majority of the information, each observer can only estimate and generally classify it. Conversely, one advantage of this method is that multiple observations can be made in the course of a given day or journey.
Cameras or video cameras are permanently mounted on a high point on the ship, e.g. the bridge or crow’s nest (an elevated observation point). These cameras systematically capture photographs and/or video sequences, a method that serves to document the voyage through the target region. To date, these images have primarily been made in the visible spectrum (normal photographs), which makes their quality heavily dependent on the weather and lighting conditions. The photographs are used for a variety of purposes; they are often used e.g. to document and assess surface characteristics and weather events. However, they can also be explicitly used for scientific studies and be processed as their own dataset. In the latter case, the photographs are e.g. projected in 3D, which allows different surface types to be classified and their distribution to be analysed. The quality of the dataset chiefly depends on the weather conditions and the ship’s movements. These systematically taken photographs are also a good supplement to bridge observations, as they document the intervals between human observations.
The sensors used on aeroplanes and satellites to monitor the sea ice can also be mounted on the decks of vessels so that they can observe the ocean’s surface while cruising or when stopped. Depending on the specific application, the sensors are mounted at different heights, positions and facing different directions. As a rule, they are connected to on-board recording systems capable of storing massive amounts of data.
In addition to directly assessing surface characteristics, these measurements are well-suited as reference and calibration values for sensors on other platforms (“ground truthing”). The quality and scope of the data greatly depend on the respective sensor, the season, and the ship’s movements.