The Value of SAR-in Altimetry for Gravity Prediction in Coastal Regions
DTU Space, Denmark
Cryosat-2 offers the first ever possibility to perform coastal altimetric studies using SAR-Interferometry as well as SAR altimetry. With this technological leap forward Cryosat-2 is now able to observe sea level in very small water bodies and also to provide coastal sea level very close to the shore.
We perform an investigation into the retrieval of marine gravity in several of the fjords in eastern Greenland. Among the fjords are the Scoresbysund Fjord which is the largest and deepest fjord in the World. In the marginal zone of Greenland the SAR-in is mainly used because of the huge topographic changes as Cryosat-2 is designed to map the margins of the ice-sheet.
For retrieval of marine gravity and also mean sea surfaces, the main important parameter is the spatial density of the sea level data. With only a few points in the fjords from Envisat, the new retracked Cryosat-2 SARin data offers a huge step forward in terms of data quality and data availability. We employ data from the first 5 years (summers) of Cryosat-2 to quantify the improvement that be achieved.
By comparing the data with and without the off-nadir correction we can furthermore study the improvement that can be expected from the SAR-in w.r.t. SAR data from gravity field retrieval in complex coastal regions
Sea Level Trends, Variability and Processes Around the Australian coast
1University of Tasmania, Australia; 2CSIRO, Hobart, Australia
There have been a number of recent improvements in coastal altimetry that offer the potential for improved understanding of coastal oceanographic processes, and their dynamic link between the coastal and open ocean. These processes affect our inference of sea level variability and trends as observed at coastal tide gauges and with offshore satellite altimetry.
Here, we investigate sea level trends and variability around the Australian coast using standard release TOPEX, Jason-1 and OSTM/Jason-2 data. We implement a novel multivariate noise analysis to assess the contribution of climate-mode variability to the data observed in the open ocean, as well as at coastal tide gauge sites. In an attempt to assess the nature of differences observed in a coastally retracked altimeter dataset, we use the Adaptive Leading-Edge Sub-waveform retracker (ALES) dataset spanning the OSTM/Jason-2 mission, together with the improved GPD+ wet tropospheric correction. We apply a variant of the noise analysis technique applied to the open ocean data to quantify differences in trend and variability between the tide gauge and coastal altimeter data points at various distances from the coast. Processes affecting these differences are discussed.
Coastal Sea Level from CryoSat-2 SARIn Altimetry in Norway
1Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, Ås, Norway; 2DTU Space, Technical University of Denmark, Kgs. Lyngby, Denmark
Conventional altimeters determine the sea surface height (SSH) with an accuracy of a few centimeters over the open ocean. However, in coastal areas the noise is seriously increased from numerous effects, which degrade the quality. The Norwegian coast adds further complications to the use of satellite altimetry, due to its very complicated coastlines with many islands, mountains, and deep, narrow fjords. The European Space Agency (ESA) CryoSat-2 (CS2) satellite carries a synthetic aperture interferometric radar altimeter (SIRAL) which is able to observe sea level closer to the coast than conventional altimeters, without degradation. In this work, we investigate the potential of CS2 data to provide improved observations in the Norwegian coastal zone.
Initially we evaluate the performance of SAR altimetry by comparing CS2 SARIn observations with 22 tide gauges, and evaluate the two major geophysical corrections applied to CS2 data for the determination of the SSH. We demonstrate that we can significantly improve the comparison with tide-gauge observations if we substitute the standard CS2 geophysical correction for the ocean tide and dynamic atmosphere corrections with local corrections.
Secondly, we compare CS2 with conventional altimetry at the Stavanger tide-gauge, revealing an improvement of ~2-3 cm.
Monitoring Storm Surges using Satellite Altimetry
Fisheries and Oceans Canada, Canada
Storm surges are the main factor that causes coastal flooding, resulting in catastrophic damage to properties and loss of life in coastal communities. Thus it is important to enhance our capabilities of observing and forecasting storm surges for mitigating damage and loss. In this talk we provide examples of storm surges observed by nadir satellite altimetry, during Hurricane Sandy, Igor, and other cyclone events. The satellite results are evaluated against tide-gauge data. The storm surges are discussed for dynamic mechanisms. We also discuss the potential of a wide-swath altimetry mission to be launched in 2021, the Surface Water and Ocean Topography (SWOT), for observing storm surges.
The Importance of Sentinel-3 for Extending the Arctic Sea Level Record
DTU Space, Denmark
Seasonal ice cover in the Arctic Ocean causes severe limitations on the use of altimetry and tide gauge data for sea level studies. In order to overcome this issue we reprocessed conventionel altimetry data with editing tailored to Arctic conditions, hereby more than doubling the amount of altimetry in the Arctic Ocean recovering up to 10 times the amount of data in regions like the Beaufort Gyre region compared with conventional datasets. With recent data from the Cryosat-2 SAR altimetry the time-series now runs from 1991-2015 a total of nearly 25 years.
We here present a new multi-decade altimetric dataset looking at the importance of the recent released Sentinel-3 datasets for extending this. Sentinel-3 SAR altimetry is particularly important in order to study in and out flow of the Arctic Freshwater in the future but also for continuing the sea level monitoring and studies of long term changes.
Out sea level record exhibit a mean sea level trend of 2.1±1.3 mm/year (without Glacial Isostatic Adjustment correction) since 1991 covering the Arctic Ocean between 66°N and 82°N with significant higher trend in the Beaufort Gyre region showing an increase in sea level up to 2011.
Mass Redistribution from Satellite Altimetry
Institute of geodesy and geophysics, Chinese Academy of Sciences, China, People's Republic of
The gravity field changes in the oceans can be derived from the mass redistribution, which is the major driving forces of geodetic variations. Much works on combining GRACE, satellite altimetry and steric changes from oceanographic observations, have present large-scale mass redistribution in the oceans. Due to the limitation of the GRACE filter and steric model, less works on small-scale mass redistribution, which is key and important to explain some regional geophysic phenomena. In this study, we only focus on the gravity change from satellite altimeters, and those small-scale mass redistribution. Analysis on the errors of the marine gravity anomaly changes havs been discussed, also its potential impact on the final explain has been estimated. Further research on the derived gravity anomaly change and the progress of Chinese satellite altimetry for improving data resolution are introduced.
Assessment of a Coastal Altimetry Data Product in the Indonesian Coastal Waters
1Department of Marine Science and Technology, Faculty of Fisheries and Marine Science, Bogor Agricultural University, Indonesia; 2Consiglio Nazionale delle Ricerche, Istituto di Biofisica, Area Ricerca CNR San Cataldo, 56127 Pisa, Italy; 3Colorado Center for Astrodynamics Research, Colorado University, Boulder, CO 80309-0431, USA; 4Center for Remote Sensing and Ocean Sciences, Udayana University, Bali, Indonesia
Indonesia is the largest archipelagic nation in the world, as around 70 percent of its total territory is water, and it has 17,480 islands. Its coastline is some 92 thousand kilometers long, making it the second longest after Canada. Therefore, coastal monitoring of sea level variability at all spatial and temporal scales is very important around Indonesian coastal waters. Satellite altimeter data and related applications for coastal studies are still developing in Indonesia. In this study we assess a satellite altimeter data product available from the Centre of Topography of the Oceans and the Hydrosphere (CTOH). The focus in on along track Envisat data collected over the Indonesian coastal region during 2002 to 2010. We compute the root mean square (RMS) of the Sea Level Anomaly (SLA) and percentage of valid CTOH Envisat data for 184 tracks over the Indonesian coastal waters. The average percentage of valid data on the first footprint from the coastline in shallow water such as the Java Sea (50%) is lower than the coastal deep sea (90%) such as eastern Indian Ocean. The RMS of data near the coast is higher than open seas. The SLA variability on annual time scales clearly shows that the SLA is negative during July to September and is positive during December to February in the coastal region. The highest amplitude of SLA occurred during Indian Ocean Dipole (IOD) positive phase in 2006. The fluctuation of SLA shows that the seasonal and interannual variability is affected by monsoons and global climate such as IOD. Sea level trends along track during the Envisat record shows that the Indonesian coastal waters region has experienced rising sea levels at rates more than the global mean. Trends in the region over this time period are positive and approach values greater than 5 mm yr−1 in some tracks.
Assimilation of Blended Altimetry and Tide Gauge Observations in a North Sea – Baltic Sea Hydrodynamic Model for Storm Surge Forecasting
1Danish Meteorological Institute, Denmark; 2Department of Earth System Science, University of California, Irvine, USA; 3European Space Agency/ESTEC, Noordwijk, Netherlands
One of the main challenges of assimilation of coastal altimetry is that the data frequency does not match the short time scales of the coastal ocean. We have addressed this issue by combining altimetry and tide gauge observations in a statistical model of sea level. The statistical model provides hourly sea level in each point along charted altimetry tracks and is routinely interpolated to a 2D field of near real time sea level, independent of numerical weather prediction models and hydrodynamical models. In this study we investigate the benefit of assimilating the statistical model into our hydrodynamical storm surge modelling system, allowing frequent adjustments of modelled sea level inaccuracies. Results show improved overall performance of the model, especially in the semi-enclosed Baltic Sea, giving improved preconditioning for forecasting of storm surges. RMS improvements range from 6% to 34%. Combined with near-real-time verification of sea level forecasts during storm surge situations, this has the potential for improving storm surge warnings, saving life and property.
Coastal Altimetry in Support of NASA's Oceans Melting Greenland (OMG) Project
1University of Colorado, United States of America; 2JPL/NASA, United States of America
Recent increases in ice discharge from marine-terminating glaciers on Greenland’s margins appears to have coincided with a warming of the oceans in these same regions. This discovery has led to the understanding that ocean heat content is playing a major role in the mass balance of Greenland’s marine-terminating glaciers and their subsequent contribution to global sea level rise. The spatial and temporal variability of the warmer water reaching Greenland’s marine-terminating glaciers and fjords is highly uncertain due to limited in situ and remote sensing measurements. Accurate coastal altimetry measurements could allow for the determination of ocean heat content changes both on Greenland’s continental shelf and in fjords. Using altimetry in coastal Greenland, however, presents numerous challenges due to floating ice, wind forced sea surface height changes, poorly resolved tide models, and a lack of tide gauges for measurement validation. Increased interest in Greenland’s coastal processes from the scientific community though makes it important to try and recover potentially valuable signals from the available altimetry. Here we present a case study from a fjord leading to one of Greenland’s rapidly retreating tidewater glaciers. Using data from Cryosat-2 and SARAL/AltiKa, we attempt characterize measurement and correction uncertainties to better understand whether an ocean forced signal can be recovered. This signal is compared to limited in situ temperature profiles taken by NASA’s Oceans Melting Greenland mission. This case study is meant to serve as a ‘proof of concept’ and investigate whether costal altimetry should be further explored as a means to characterize ocean forcing at Greenland’s marine terminating glaciers.