Assessing IceBridge Snow Depth Retrievals over Arctic Sea Ice Using Estimates from Multiple Sources
1Jet Propulsion Laboratory, Pasadena, CA, United States of America; 2Goddard Space Flight Center/NASA, Greenbelt, MD, United States of America; 3University of Maryland, College Park, MD, United States of America; 4NOAA, College Park, United States of America; 5Environment and Climate Change Canada,Toronto, Canada; 6University of Kansas, Lawrence, United States of America; 7Cold Regions Research and Engineering Laboratory, Hanover, NH, United States of America; 8University of Colorado, Boulder, United States of America
The sensor suite of Operation IceBridge (OIB) includes an ultra-wideband snow radar that that can measure snow depth over sea ice by resolving the range location of the air–snow and snow–ice interfaces. Since 2009, this instrument has acquired data in annual OIB campaigns conducted during the Arctic and Antarctic springs. Progressive improvements in radar hardware and data processing methodologies have led to improved data quality for subsequent analysis to derive snow depth. Five algorithms now exist for snow depth retrieval, and they differ in the way that the air-snow and snow-ice interfaces are detected and localized in the radar returns, and in how the system limitations (e.g., noise, resolution, etc.) are accounted for. In 2014, the Snow Thickness On Sea Ice Working Group (STOSIWG) was formed and tasked with investigating how radar data quality affect snow depth retrievals and how retrievals from the various algorithms differ. The ultimate goal is to understand the limitations of the estimates and to produce a well-documented, long-term record that can be used for understanding broader changes in the Arctic climate system. Assessments of retrievals are based on comparisons with field measurements from multiple ground-based campaigns, including BROMEX and the field program at Eureka, Nunavut (sponsored by Environment and Climate Change Canada), the Warren et al. (1999) climatology and snowfall from ERA-Interim reanalysis. Here, we report on the current status and the results from the assessment of different retrieval approaches, and our plans for improvement of data products based on our current understanding informed by the inter-comparisons between approaches and by comparisons with field measurements.
Arctic Lead Detection using a Waveform Unmixing Algorithm from CryoSat-2 Data
Ulsan National Institute of Science of Technology, Korea, Republic of (South Korea)
Arctic areas consist of ice floes, leads, and polynyas. While leads and polynyas account for small parts in the Arctic Ocean, they play a key role in exchanging heat flux, moisture, and momentum between the atmosphere and ocean in wintertime because of their huge temperature difference. In this study, a linear waveform unmixing approach was proposed to detect lead fraction. CryoSat-2 waveforms for pure leads, sea ice, and ocean were used as end-members based on visual interpretation of MODIS images coincident with CryoSat-2 data. The unmixing model produced lead, sea ice, and ocean abundances and a threshold (> 0.7) was applied to make a binary classification between lead and sea ice. The unmixing model produced better results than the existing models in the literature, which are based on simple thresholding approaches. The results were also comparable with our previous research using machine learning based models (i.e., decision trees and random forest). A monthly lead fraction was calculated, dividing the number of detected leads by the total number of measurements. The lead fraction around Beaufort Sea and Fram strait was high due to the anti-cyclonic rotation of Beaufort Gyre and the outflows of sea ice to the Atlantic. The lead fraction maps produced in this study were matched well with monthly lead fraction maps in the literature. The areas with thin sea ice identified from our previous research correspond to the high lead fraction areas in the present study.
Tuning SAR Processing to Measure Sea Ice Freeboard
1CNRS, France; 2CNES; 3EUMETSAT; 4ESA-ESRIN
SAR altimetry has demonstrated its capabilities to measure the sea level with better performances than the LRM mode, as shown by sea surface height spectrum. Over sea-ice, several freeboard products computed from CryoSat-2 altimeter are already available, including within the official ESA Baseline-C GDR.
While these products provide relatively accurate estimates of seasonal and inter-annual sea-ice freeboard height, they are still too much imprecise to be integrated within climate models. Several studies have shown that the main uncertainty still comes from the freeboard measurement (for about 50%, the next one being related to the snow cover, followed by the barometer effect, the tides, the mean sea level, …).
Until now the studies to improve the heights measurements mainly concerned the level 2 processing related to the conversion of the waveforms to ranges over leads and floes:
- the waveform classification, with the objective to distinguish echoes originating from leads and from ice floes and to remove echoes that mixed-up both surfaces ;
- the waveform retrackers, with the objective to handle both the highly specular echoes over leads, and diffuse echoes over floes possibly cover by snow.
In this study we show that the previous L1 step, that consists in producing the synthetic Doppler echoes, may have a non-negligible impact on the freeboard retrieval depending on the chosen processing strategy.
Indeed, unlike for LRM altimetry, in SAR mode the raw echoes are downloaded and processed off-line, which offers a unique opportunity to master the chain process and to adapt it according to the type of the observed surfaces.
Several theoretical studies have proposed methodologies to improved the SAR focalization (exact-beam), to reduced the SAR side-lobe effects over specular surfaces (hamming), to interpolate waveforms to better handle very peaky waveforms (zero-padding), to account for the antenna patern, to increase the along-track sampling (eg, at 80hz) or even increase the along-track resolution (full-focused SAR).
The ESA GPOD service offers a web interface to configure and compute SAR data “à la carte” using the SARvatore processing chain (http://wiki.services.eoportal.org/tiki-index.php?page=GPOD+CryoSat-2+SARvatore+Software+Prototype+User+Manual). This interface allows to select a large set of SAR processing options: hamming filter, zero-padding, antenna compensation, exact-beam forming, single-look, 80hz.
Using this service, we have computed several datasets covering most of these configuration combinations. Each dataset is composed of all the tracks flying over the west part of the Arctic Ocean (Beaufort to Svalbard) from mid-March to mi-April 2015, i.e. during the 2015 Operation Ice Bridge (OIB) campaign.
These datasets have been converted into along-track freeboard using a same chain, and then gridded in stereo-polar projection maps of 10km resolution and compared with the OIB freeboard measurements. We present the various solutions and analyze the impact of each SAR processing option on the retrieved freeboard distribution compared with OIB.
We also show how some new parameters issued from the stacks and RIPs intermediate SAR product allow to better identify leads and floes, and could even help for the retracking step, avoiding side lobe and off-nadir effects.
Comparing Coincident IceBridge and CryoSat-2 Observations Over Sea Ice
1NASA/GSFC, USA; 2University of Maryland, USA; 3NASA/WFF, USA
In this study, we will compare sea surface elevation, sea ice surface elevation, and sea ice freeboard using 10 coincident CryoSat-2, ATM, and LVIS observations. We will apply identical ellipsoid, geoid model, tide models, and atmospheric corrections to CryoSat-2, ATM, and LVIS data. For CryoSat-2, we will use surface elevation generated based on the methods from four different groups (UCL, NASA/Goddard, AWI, and NASA/JPL). We will use surface elevation and sea ice freeboard in the OIB data product for ATM and the elevation and sea ice freeboard calculated through a Gaussian waveform fitting method for LVIS. The ATM and LVIS elevations and freeboards will be averaged at each CryoSat-2 footprint for comparison. The four different CryoSat-2 retrackers show quite significant mean differences in elevation. The mean elevations of ATM, LVIS, and Cryosat-2 are also quite different. We will investigate how these elevation differences propagate into the retrieval of sea ice freeboard. The results of this study are important for the mutual calibration/validation of the ATM, LVIS, and CryoSat-2 data products. The better understanding of ATM, LVIS, and Cryosat-2 results will also help to bridge the data gap between ICESat and ICESat-2.
Determing Arctic Freshwater Fluxes with Cryosat-2
1DTU space, Denmark; 2ENVEO, Austria; 3LEGOS, France; 4ESA ESRIN, Italy
in Support to Science Elementes ESA has initated a new CLIC initiative for the Arctic Ocean (Arctic +) on determining the Arctic Freshwater fluxes using Earth Observation data.
One of the project supported by this initiative is the ArcFlux works which started in september 2016 will be introduced in this presentation. This project aims to determining the largest component to the Arctic Freshwater budget, namely the contibution from large rivers, glaciers as well as in-out flow of freshwater through the ocean pathways. Here the importance of Cryosat-2 SAR altimetry for determing the arctic Freshwater fluxes will be presented and the first results will be shown
The Norwegian Coastal Current Observed by CryoSat2 SARIn Altimetry
1MNBU, Aas, Norway; 2DTU space, Denmark
The Norwegian Coastal Current (NCC) transports warm and relatively fresh water along the Norwegian coast and into the Barents Sea, with its origin in Baltic water entering Skagerrak. Along its way northward it is fed by additional freshwater discharge. The NCC is important for the regional marine ecosystem and contributes to the poleward transport of warm Atlantic Water, maintaining the relatively mild climate in northwest Europe.
Although satellite altimetry is a mature technique, globally observing the sea surface height with an accuracy of a few centimeters, numerous effects degrade the observations in the coastal zone. For example, the radar footprint is contaminated by land and bright targets, and the
range and geophysical corrections become difficult to model.
The rugged Norwegian coast presents a further challenge, and the NCC, at times only a few tens of kilometers wide, typically falls into a zone where conventional altimeters do not deliver reliable observations.The European Space Agency's CryoSat2 (CS2) satellite is the first to carry a SAR altimeter instead of the conventional pulselimited system, resulting in higher range precision and alongtrack resolution. This allows for tracking finer structures of the sea surface and get closer to the coast. We use CS2 low resolution and SARIn observations, for the period 20122015, along the Norwegian coast and determine a mean dynamic topography (CS2MDT) that is validated using tide gauges. In turn, geostrophic surface currents are derived from both the CS2MDT and the operational coastal numerical ocean model of MET Norway and compared. For the first time, the NCC is revealed by spacegeodetic techniques, giving confidence in the newgeneration SAR altimeters for coastal sealevel recovery.
GOCE and Cryosat-2 for Dynamical Coastal Topography and Tide Gauge Unification
1DTU space, Denmark; 2ESA ESTEC, Nordweijk, The Netherlands; 3University of Liverpool; 4University La Rochelle; 5University of Bonn; 6Universtiy of Bristol
ESA has recently released a study on the investigation and using ocean levelling as a novel approach to the study of height system unification across the oceans taking the recent development in geoid accuracy through GOCE data into account.
The suggested investigation involves the use of measurements and modelling to estimate Mean Dynamic Topography (MDT) of the ocean along a coastline, which contributes/requires reconciling altimetry, tide gauge and vertical land motion. Close to the coast the determination of the MDT is problematic due to i.e., the altimeter footprint, land motion or parameterization/modelling of coastal currents.
The objective of this activity is to perform a consolidated and improved understanding and modelling of coastal processes and physics responsible for sea level changes on various temporal/spatial scales. The study runs from October 2015 to march 2017 and involves the following elements
Develop an approach to estimate a consistent DT at tide gauges, coastal areas, and open ocean; Validate the approach in well-surveyed areas where DT can be determined at tide gauges; Determine a consistent MDT using GOCE with consistent error covariance fields; improving altimetry (SAR) along the coast for MSS/MDT improvement and finally connecting the global set of tide gauges and investigate trends
Coastal Altimetry from CryoSat-2
1University of Bonn, Germany; 2HeSpace/EUMETSAT, Darmstadt, Germany; 3TU Darmstadt, Institute of Geodesy, Darmstadt, Germany; 4EUMETSAT, Darmstadt, Germany; 5ESA/ESRIN, Frascati, Italy
CryoSat-2 SAR altimetry provides in coastal zone improved measurements of height, wave height and wind speed compared to conventional altimetry. We investigate the North-Eastern Atlantic shelf from Lisbon to Bergen and river estuaries in the German Bight.
We consider SAR CryoSat-2 altimetry products from the GPOD and SCOOP projects. The reduced SAR altimetry (RDSAR) are in-house products. The conventional altimetry data are Jason-2 and Envisat data from the Climate Change Initiative CCI project and standard ESA products.
Improved processing include application of sub-waveform re-trackers for RDSAR and various approaches in SAR altimetry, as Hamming weighting window on the burst data prior to the azimuth FFT, zero-padding prior to the range FFT, doubling of the extension for the radar range swath.
We cross-validate the various products with different options and validate them with in-situ data to assess the improved accuracy and precision and their performance for long-term water level change studies.
Improved Oceanographic Measurements with CryoSat SAR Altimetry
1Satellite Oceanographic Consultants, United Kingdom; 2isardSAT, Spain; 3Noveltis, France; 4DTU Space, Denmark; 5National Oceanography Centre, UK; 6Starlab, UK; 7TU Munich, Germany; 8TU Delft, The Netherlands; 9SERCO/ESRIN, Italy; 10DEIMOS/ESRIN, Italy; 11ESA-ESRIN, Italy
The ESA CryoSat mission is the first space mission to carry a radar altimeter that can operate in Synthetic Aperture Radar “SAR” (or delay-Doppler) and interferometric SAR (SARin) modes. Studies have confirmed the improved ocean measuring capability offered by SAR mode altimetry, through increased resolution and precision in sea surface height and wave height measurements, and have also added significantly to our understanding of the issues around the processing and interpretation of SAR altimeter echoes.
The objective of the CryoSat Plus for Oceans (CP4O) project was to develop and evaluate new ocean products from CryoSat data and so maximize the scientific return of CryoSat over oceans. The main focus of CP4O has been on the additional measurement capabilities that are offered by the SAR mode of the SIRAL altimeter, with further work in developing improved geophysical corrections.
This paper first presents a short overview of the major results from CP4O, and outlines a proposed roadmap for the further development and exploitation of these results in operational and scientific applications.
We then present results from an extension to CP4O in four themes, each investigating different aspects of the opportunities offered by this new technology.
The first two studies address the coastal zone. Firstly, a thorough analysis was made of the performance of “SAR” altimeter data (delay-Doppler processed) in the coastal zone. This quantified the performance, confirming the significant improvement over “conventional” pulse-limited altimetry. In the second study a processing scheme was developed with CryoSat SARin mode data to enable the retrieval of valid oceanographic measurements in coastal areas with complex topography. Thanks to further development of the algorithms, a new approach was achieved that can also be applied to SAR and conventional altimetry data (e.g., Sentinel-3, Jason series, Envisat).
The third part of the project developed and evaluated improvements to the SAMOSA altimeter re-tracker as implemented in the Sentinel-3 processing chain. The modifications to the processing scheme should support improved performance in terms of accuracy and efficiency in retrieving oceanographic geophysical parameters from altimeter data.
Finally, we describe the development of a state of the art tidal atlas for the Arctic Ocean with CryoSat altimeter data. Through its high inclination orbit, the CryoSat mission provides the most complete altimeter data set ever used in this region, and so should enable the production of a highly accurate Arctic tidal model. This in turn will improve the quality of CryoSat Sea Surface Height measurements and all derived products (e.g. mean sea surface, mean dynamic topography).
Together these studies provide an important foundation for exploiting data from the Sentinel-3 and Sentinel-6/Jason-CS missions.
The work described in this presentation was supported by ESA through the STSE programme. We also acknowledge the support of CNES who provided the CNES-CPP CryoSat Products used in these studies. CNES-CPP products were developed by CNES and CLS in the frame of the “Sentinel-3 SRAL SAR mode performance assessment” study.
Quality Assessment of CryoSat-2 Ocean Altimetry
1Arospace/TU Delft, The Netherlands; 2RHEA/ESA, Italy; 3ESRIN/ESA, Italy
CryoSat-2 has been monitoring the Earth’s cryosphere and oceans with unprecedented accuracy and precision since its launch in 2010. In our paper we assess the quality of the CryoSat Ocean Processor (COP) LRM and pLRM products by cross-calibration with the LRM ocean altimeter data in the RADS database and by comparing the COP sea level data with a selected set of tide gauges. The goal is long-term monitoring; evaluating the stability of the measurement system and identifying biases & drifts. In summary we observe a very stable system which is on par with the Jason’s ocean reference missions. Also we have been generating since 2010 precise orbits for CryoSat-2 and have cross-validated them with the CNES orbit given in the COP product. The orbits compare very well and can be considered Jason-class with a difference standard deviation < 1.5cm.
Global Evaluation Of The New CryoSat Geophysical Ocean Products
1National Oceanography Centre; 2British Oceanographic Data Centre; 3ESA/ESRIN, RHEA-System; 4ESA/ESRIN
Marine products from CryoSat-2 generated by a dedicated processor (CryoSat Ocean Processor or COP) have been available since April 2014. Here we present the results of a verification and scientific validation of the Geophysical Ocean Products (GOP), which have consolidated orbits and are available 30 days after acquisition. This assessment, carried out within the ESA-funded CryOcean-QCV project, is performed for the sea surface height (SSH), the significant wave height (SWH), and the wind speed. The mean value of the 20 Hz SSH anomaly (SSHA) noise corresponding to a SWH of 2 m is 6.3 cm for LRM (Low Resolution Mode) data and 10.2 cm for pseudo-LRM data. The standard deviation of the crossovers is 5.4 cm. The SSH is validated at the coast against the sea level measured by a set of carefully selected and quality controlled tide gauges, and compared with Jason-2 observations. Correlations between satellite SSH and tide gauge records are statistically significant at nearly all stations, with a median value of 0.78 and 0.76 for CryoSat-2 and Jason-2, respectively. The Global Mean Sea Level (GMSL) curve from the GOP matches well the same curve from other altimetry missions, suggesting that CryoSat-2 is suitable for GMSL monitoring. In the open ocean the SSH is compared globally with the steric heights derived from temperature and salinity profiles as measured by Argo floats. The median correlation between SSH and steric heights is 0.68. The correlation, however, shows a strong latitudinal dependence, with higher values at low latitudes (median > 0.8 in the 10˚S–10˚N band). Regarding SWH and wind speed, they are both validated against buoy observations. The SWH shows an RMS of 15 cm whereas wind speed exhibits a bias of about 2 m/s relative to both Jason-2 and to buoy data. In addition, the SWH is also compared with the values provided by the Wavewatch III model. Differences between CryoSat-2 and the WW3 are less than 20% of the SWH over most areas of the ocean.
Tropical Cyclones Above Australia During Last 12 Years
1Space Research Institute, Moscow, Russian Federation; 2The “Seeingear”elibrary, Chain Lane, Battle, UK
A tropical cyclone (TC) is the global generic term for an intense circulating weather system over tropical seas and oceans. It is accompanied with very strong winds, heavy rains & large ocean waves.
It is well known that the tropical zone of the global ocean–atmosphere system plays a key role in the dynamics and evolution of synoptic and climatic meteorological processes on Earth. The ocean–atmosphere system of the Earth’s tropical zone has a unique
property. It can generate sufficiently organized and stable mesoscale vortex structures, TCs, from the atmospheric turbulent chaos in the global circulation system. For a long time the process of generation of these vortex systems in the Earth’s atmosphere was considered a purely meteorological phenomenon when it was investigated based on standard meteorological approaches(1).
Depending on the influence source position, the atmospheric layers can interact both from below and from above. Upward-propagating planetary, tidal, and gravity waves that penetrate through the systems of zonal winds and are reflected by them are considered the
physical basis of the influence from below. These waves are caused by intense dynamic processes in the troposphere, and the transfer characteristics are determined by the thermodynamic regime of the middle atmosphere.
Based on the results obtained in these investigations, it was concluded that the gravity waves (GW) are the main possible influence factors on the ionosphere by the active underlying cyclones. In turn, TCs are not only long-living area sources of disturbances, but also moving in a wide area. Thus, we will have to deal with addition of waves and anisotropic pattern of the ionospheric excitation. Simultaneously, it should be borne in mind that not all of the GWs from the TC will reach ionospheric heights because of filtering by strong tropospheric winds. The IGW occurrence mechanism is most likely based on the hypothesis of the growth and decay of convective towers with a spatial scale of about 100 km and time scales of a few hours.
In this presentation the author analyzes the dynamic ocean parameters and ionosphere parameters, received in the process of satellite remote sensing above TC (above Australia) in the last 12 years in the south-eastern area of the East hemisphere.
1. L. B. Vanina-Dart and E. A. Sharkov,Main Results of Recent Investigations into the Physical Mechanisms of the Interaction of Tropical Cyclones and the Ionosphere//ISSN 0001-4338, Izvestiya, Atmospheric and Oceanic Physics, 2016, Vol. 52, No. 9, pp. 1119–1126. © Pleiades Publishing, Ltd., 2016.