Near-real-time Arctic sea ice thickness and volume from CryoSat-2

Timely observations of sea ice thickness help us to understand the Arctic climate, and have the potential to support seasonal forecasts and operational activities in the polar regions. Although it is possible to calculate Arctic sea ice thickness using measurements acquired by CryoSat-2, the latency of the final release data set is typically 1 month due to the time required to determine precise satellite orbits. We use a new fast-delivery CryoSat-2 data set based on preliminary orbits to compute Arctic sea ice thickness in near real time (NRT), and analyse this data for one sea ice growth season from October 2014 to April 2015. We show that this NRT sea-ice-thickness product is of comparable accuracy to that produced using the final release CryoSat-2 data, with a mean thickness difference of 0.9 cm, demonstrating that the satellite orbit is not a critical factor in determining sea ice freeboard. In addition, the CryoSat-2 fast-delivery product also provides measurements of Arctic sea ice thickness within 3 days of acquisition by the satellite, and a measurement is delivered, on average, within 14, 7 and 6 km of each location in the Arctic every 2, 14 and 28 days respectively. The CryoSat-2 NRT sea-ice-thickness data set provides an additional constraint for short-term and seasonal predictions of changes in the Arctic ice cover and could support industries such as tourism and transport through assimilation in operational models

New observations of Arctic sea ice from satellite radar altimetry

Satellite observations of Arctic sea ice have observed a decline in extent for all months since 1979. The decline is coincident with abrupt global and Arctic warming over the last 30 years. Over this 30-year period the mean Arctic temperature has increased at almost twice the global average rate – a phenomena known as Arctic amplification. It is crucial to observe and understand changes in Arctic sea ice, as it is a major element of the Earth’s climate system. The sea ice cover acts to regulate solar absorption, ocean- atmosphere heat exchange, and freshwater and brine input into the Arctic Ocean and subpolar North Atlantic. The subsequent changes in the regional heat and freshwater budgets impact on patterns of atmospheric and oceanic circulation across the Arctic and at lower latitudes. These in turn impact on global weather patterns. To fully understand the global impacts of changes in the Arctic sea ice cover, long-term and accurate observations of the ice pack as a whole are required. However, it has previously been difficult to quantify trends in sea ice volume because detailed thickness observations have been lacking. The European Space Agency’s (ESA’s) CryoSat-2 satellite was launched in April 2010 and now provides unparalleled coverage of the Arctic Ocean up to 88°N. CryoSat-2 data have been used in this study to provide the first estimates of sea ice thickness and volume across the entire Northern Hemisphere. Using five years of CryoSat-2 measurements a 14% reduction in sea ice volume was observed between autumn 2010 and 2012, in keeping with the long-term decline in extent. However, 33% and 25% more ice were observed in autumn 2013 and 2014, respectively, relative to the 2010–2012 seasonal mean, which offset earlier losses. The increase was caused by the retention of thick sea ice northwest of Greenland during 2013 which, in turn, was associated with a 5% drop in the number of days on which melting occurred. This coincides with conditions more typical of the late 1990s. In contrast, springtime Arctic sea ice volume has remained stable. The sharp increase in sea ice volume after just one cool summer demonstrates the ability of Arctic sea ice to respond rapidly to a changing environment. Since April 2015, ESA have provided fast delivery CryoSat-2 data, which are based on preliminary orbits. The fast delivery data have been used to produce near real time (NRT) estimates of Arctic sea ice thickness and volume. This study finds that the NRT dataset provides a measurement within 14, 7 and 6 km of each location in the Arctic every 2, 14 and 28 days respectively. NRT sea ice thickness and volume data provide a new resource and opportunity for the developers of short-term sea ice forecast models. These models can provide information such as sea ice location, drift and thickness to operational users. Currently the utility of the NRT data for model and operational use is limited by a lack of availability in summer months. The expansion of sea ice thickness observations in to the melt season will form the basis of future work

Estimating Arctic sea ice thickness and volume using CryoSat-2 radar altimeter data

Arctic sea ice is a major element of the Earth’s climate system. It acts to regulate regional heat and freshwater budgets and subsequent atmospheric and oceanic circulation across the Arctic and at lower latitudes. Satellites have observed a decline in Arctic sea ice extent for all months since 1979. However, to fully understand how changes in the Arctic sea ice cover impact on our global weather and climate, long-term and accurate observations of its thickness distribution are also required. Such observations were made possible with the launch of the European Space Agency’s (ESA’s) CryoSat-2 satellite in April 2010, which provides unparalleled coverage of the Arctic Ocean up to 88°N. Here we provide an end-to-end, comprehensive description of the data processing steps employed to estimate Northern Hemisphere sea ice thickness and subsequent volume using CryoSat-2 radar altimeter data and complementary observations. This is a sea ice processor that has been under constant development at the Centre for Polar Observation and Modelling (CPOM) since the early 1990s. We show that there is no significant bias in our satellite sea ice thickness retrievals when compared with independent measurements. We also provide a detailed analysis of the uncertainties associated with our sea ice thickness and volume estimates by considering the independent sources of error in the retrieval. Each month, the main contributors to the uncertainty are snow depth and snow density, which suggests that a crucial next step in Arctic sea ice research is to develop improved estimates of snow loading. In this paper we apply our theory and methods solely to CryoSat-2 data in the Northern Hemisphere. However, they may act as a guide to developing a sea ice processing system for satellite radar altimeter data over the Southern Hemisphere, and from other Polar orbiting missions

Extending the Arctic Sea Ice Freeboard and Sea Level Record with the Sentinel-3 Radar Altimeters

In February 2016 and April 2018 the European Space Agency launched the Sentinel-3A and 3B satellites respectively, as part of the European Commission’s multi-satellite Copernicus Programme. Here we process Sentinel-3A waveform data to estimate Arctic sea level anomaly and radar freeboard from November 2017 to April 2018. We compare our results to those from the CryoSat-2 satellite, and find an intermission bias on sea-level anomaly of 2 cm. We also find a mean radar freeboard difference of 1 cm, which we attribute to the use of empirical retrackers to retrieve lead and floe elevations. Ahead of Sentinel-3B waveform data being made available, we use orbit files to estimate the improvement in sampling resolution afforded by the addition of Sentinel-3A and 3B data to the CryoSat-2 dataset. By combining data from the three satellites, grid resolution or time-sampling can be almost tripled compared with using CryoSat-2 data alone

Radar observations of Arctic ice

To many observers, the Arctic is synonymous with snow and ice. For example, the Arctic Ocean spans just over 14 million km2 – an area larger than that of Europe – which is variably covered in frozen ocean water, or sea ice, throughout the year.1 The Arctic Ocean is almost completely surrounded by land, which can often be covered in snow, permafrost (frozen soil, rock or sediment) or land ice. Observing how different forms of ice in the Arctic are changing, and understanding how they have evolved in the past, is crucial. Radar technology provides us with a tool to do this and allows us to visualise the glacial environment beneath the ice surface. This chapter provides an overview of modern radar-based observation methods and describes how measurements from them have contributed to a scientific understanding of ice with an emphasis on Arcticness