Observing Sea Ice

Given the important role of sea ice in the Earth system and in Arctic operations, there is a need for regular and reliable sea ice observations. The following section summarizes how sea ice observations historically started as in-situ observations from boats and vessels, and gives an overview of today’s in-situ and remote sensing (rs) methods.

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1.2.1 In-Situ Observations

The first known observations of sea ice date back to the traveler Pythias of Massilia in the time between 350 to 320 B.C. While no direct records by Pythias exist, other authors have documented his journey towards the North and his reports of the frozen sea [53, 54]. The earliest confirmed first-hand records of sea ice appear in 825 A.D., written by Irish monks who encountered sea ice (theMare Concretum) during voyages to Iceland [55]. Further descriptions of sea ice are found in the literature continuously throughout the following centuries. However, the goal of the mariners at the time was never to study the ice, but to avoid it [55,56]. First scientific papers discussing physical properties of sea ice and different sea ice conditions were published in the 1870s [55]. By that time, the expansion of ocean trade routes had led to increased interest in shorter connections between Europe and the orient, in particular the Northwest and Northeast Passages (Figure 1.4). First successful crossings of the Arctic Ocean along these two routes were accomplished by Baron Adolf Erik Nordenskiöld in 1879 and Roald Amundsen in 1906, respectively [55].

It was not before the second half of the 20th century that there was a marked increase in sea ice research and in-situ observations. Today, there are multiple scientific expeditions by different nations each year. The typical and most basic in-situ measurements are regularicewatch observations from vessels [57, 58]. Other common in-situ observations on the ice are measurements of ice thickness and roughness, temperature and salinity profiles, as well as thickness and properties of snow cover on the sea ice [2]. Although not strictly in-situ, airborne ice thickness measurements by electromagnetic (em) induction systems are sometimes also referred to as in-situ data when compared with satelliters imagery [59, 60].

Despite the increasing scientific interest and large number of expeditions, in-situ obser-vations of sea ice are still sparse and can only cover a small fraction of the polar regions.

Furthermore, they are biased towards summer conditions, as the Arctic is more easily accessed in the summer compared to the colder and darker polar winter. Recent efforts that are trying to address this issue include the Norwegian N-ICE2015 expedition [61], or the Multidisciplinary drifting Observatory for the Study of Arctic Climate (mosaic) expedition (autumn 2019-autumn 2020), which aims at obtaining more in-situ data from the central Arctic during wintertime [62].

The fact that observations and descriptions of sea ice started as visual in-situ observations by mariners has shaped and defined our understanding and definition of different sea ice types today. Sea ice types are traditionally classified by their visual appearance and by the ice thickness, which is a critical parameter for ice-going vessels. The implications of this for sea ice classification from imaging radar data are discussed in Chapter 4.

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1.2.2 Remote Sensing Observations

Since the late 1970s, spacebornershas revolutionized the world of sea ice observations, making regular large-scale and Arctic- and Antarctic-wide monitoring possible. Today, different sensors on multiple satellite platforms provide a vast number of observations that are frequently repeated [63]. The measurements are carried out over a wide range of theemspectrum, at visible, infrared, and microwave wavelengths, using both active and passive sensors. Active sensors generate their own signal and are independent of a natural radiation source. Passive sensors rely either on solar illumination (optical sensors) or on radiation that is naturally emitted from the Earth surface and atmosphere (passive microwave radiometry).

Synthetic aperture radar is an active system that is widely used in sea ice monitoring today.

It can achieve a high spatial resolution by utilizing the coherent nature of the transmitted radar pulse [64]. Furthermore, the data acquisition is independent of sunlight and cloud conditions. Since synthetic aperture radar (sar) is the main data source used in this dissertation, it is introduced and discussed in detail in a separate chapter (Chapter 3).

Optical images are often used as complementary data to radar images. Also in this thesis, overlappingsarand optical images are used for the definition of ice types, the selection of training regions, and the validation of results (Paper-2, Chapter 4). However, opticalrs in the polar regions is generally limited because of the darkness during polar winter and frequent cloud conditions in the summer.

As the Earth atmosphere is essentially regarded as transparent for wavelengths above 3 cm [65], microwaversis generally very little influenced by clouds. Besidessar, methods for microwaversinclude radar scatterometers and passive microwave (pm) radiometers [63].

pmradiometry became available for sea ice observations with the start of the Nimbus-7 satellite carrying the Scanning Multichannel Microwave Radiometer (smmr) in 1978. The smmrand its successors form the basis of the sea ice extent time series. The examples of sicin Figure 1.1 and Figure 1.2 are obtained using data frompmsensors.pmradiometers measure radiation that is naturally emitted from the Earth surface and atmosphere [66].

Power and wavelength of the radiation is controlled by the surface temperature𝑇 _and the emissivity 𝜖. This allows to generally distinguish open water surfaces from sea ice surfaces in thepmdata. The spatial resolution of apmradiometer depends on the size of the reflector in the antenna and the frequency of the radiation. Higher frequencies result in finer resolution (89 GHz,∼3 km), but are more sensitive to atmospheric disturbances.

Lower frequencies are less affected by the atmosphere, but result in coarser resolution (6 GHz,∼40 km). Most retrieval algorithms combine different frequency and polarization channels and use empirically derived formulas to estimatesic[67, 68]. The typically used channels have spatial resolutions between 5 and 15 km, resulting insicproducts with a km-scale grid-spacing. Sea ice extent can also be derived from radar scatterometers [63,69,70].

A comparison of the different methods is given inMeier and Stroeve (2008)[71].

sic and sea ice area are the primary sea ice products obtained from pm radiometry