Ronald Kwok

Ronald Kwok
University of Washington Seattle | UW · Applied Physics Laboratory

Ph. D. (Duke University)

About

405
Publications
58,286
Reads
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23,973
Citations
Citations since 2016
66 Research Items
11893 Citations
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201620172018201920202021202205001,0001,5002,000
201620172018201920202021202205001,0001,5002,000
Additional affiliations
June 1985 - June 2020
California Institute of Technology
Position
  • Senior Researcher

Publications

Publications (405)
Article
We present our estimates of the thickness and volume of the Arctic Ocean ice cover from CryoSat-2 data acquired between October 2010 and May 2014. Average ice thickness and draft differences are within 0.16 m of measurements from other sources (moorings, submarine, electromagnetic sensors, IceBridge). The choice of parameters that affect the conver...
Article
Regional variability in monthly CryoSat-2 sea ice thickness is partitioned into contributions from dynamics and thermodynamics using ice deformation calculated from large-scale ice drift. For five winters (December to April, 2011-2015), over a region of persistent convergence north of the coasts of Greenland and the Canadian Arctic Archipelago, def...
Article
Full-text available
Large-scale changes in Arctic sea ice thickness, volume and multiyear sea ice (MYI) coverage with available measurements from submarine sonars, satellite altimeters (ICESat and CryoSat-2), and satellite scatterometers are summarized. The submarine record spans the period between 1958 and 2000, the satellite altimeter records between 2003 and 2018,...
Article
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We present a first examination of Arctic sea ice snow depth estimates from differencing satellite lidar (ICESat‐2) and radar (CryoSat‐2) freeboards. These estimates cover the period between 14 October 2018 and the end of April 2019. Snow depth is related to freeboard differences by the refractive index/bulk density of the snow layer—the only free p...
Article
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We present the first winter season of surface height and sea ice freeboards of the Arctic Ocean from the new Ice, Cloud, and Land Elevation Satellite (ICESat‐2; IS‐2) mission. The Advanced Topographic Laser Altimeter System onboard has six photon‐counting beams for surface profiling with a 10‐kHz pulse rate (interpulse distance ~0.7 m) and footprin...
Article
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Using ICESat‐2 and CryoSat‐2 freeboards, we examine the variability of monthly Arctic sea ice snow depth, thickness and volume between October 2018 and April 2021. For the 3 years, satellite‐derived estimates captured a decrease in mean April snow depth (∼2.50 cm) and ice thickness (∼0.28 m) equivalent to an ice volume loss of ∼12.5%. Results show...
Article
Both in situ and remote sensing observations of Arctic Ocean hydrography and circulation have improved dramatically in recent decades, and combining the two can yield the most complete picture of Arctic Ocean change. Recent results derived from classical hydrography and satellite ocean altimetry illustrate this synergy and also reveal a fundamental...
Article
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Understanding and predicting Arctic change and its impacts on global climate requires broad, sustained observations of the atmosphere-ice-ocean system, yet technological and logistical challenges severely restrict the temporal and spatial scope of observing efforts. Satellite remote sensing provides unprecedented, pan-Arctic measurements of the sur...
Article
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Plain Language Summary The polar oceans, with warming and dramatic declines in sea ice coverage, are experiencing some of the most rapid environmental changes on Earth. These changes have direct impacts on ocean circulation and freshwater distribution, with observable changes in sea surface height. Measuring and monitoring basin‐scale variability o...
Article
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The National Aeronautics and Space Administration (NASA)’s Operation IceBridge (OIB) was a 13‐year (2009–2021) airborne mission to survey land and sea ice across the Arctic, Antarctic, and Alaska. Here, we review OIB’s goals, instruments, campaigns, key scientific results, and implications for future investigations of the cryosphere. OIB’s primary...
Article
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Microwave radiometry has provided valuable spaceborne observations of Earth's geophysical properties for decades. The recent SMOS, Aquarius, and SMAP satellites have demonstrated the value of measurements at 1400 MHz for observing surface soil moisture, sea surface salinity, sea ice thickness, soil freeze/thaw state, and other geophysical variables...
Article
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Abstract NASA's Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) mission launched in September 2018 and is now providing high‐resolution surface elevation profiling across the entire globe, including the sea ice cover of the Arctic and Southern Oceans. For sea ice applications, successfully discriminating returns between sea ice and open water...
Article
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In Release 001 and 002 of the ICESat-2 sea ice products, candidate height segments used to estimate the reference sea surface height for freeboard calculations included two surface types: specular and smooth dark leads. We found that the uncorrected photon rates, used as proxies of surface reflectance, are attenuated due to clouds resulting in the...
Article
Arctic Ocean surface circulation change should not be viewed as the strength of the anticyclonic Beaufort Gyre. While the Beaufort Gyre is a dominant feature of average Arctic Ocean surface circulation, empirical orthogonal function analysis of dynamic height (1950-1989) and satellite altimetry-derived dynamic ocean topography (2004-2019) show the...
Article
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Hemisphere‐wide observations of melt ponds on sea ice are needed to understand their influence on the surface radiation budget of the Arctic Ocean and to extend the satellite sea ice thickness data record. Here we present a first assessment of NASA's Ice, Cloud, and land Elevation Satellite‐2 (ICESat‐2) over individual Arctic sea ice melt ponds wit...
Conference Paper
In September 2019, the ambitious multidisciplinary Arctic MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate) started. The research icebreaker Polarstern got anchored to an ice floe in the central Arctic for a complete seasonal cycle (with minimal interruption) drifting with the ice. The expedition provides a...
Article
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We offer a view of the Antarctic sea ice cover from lidar (ICESat-2) and radar (CryoSat-2) altimetry, with retrievals of freeboard, snow depth, and ice thickness that span an 8-month winter between 1 April and 16 November 2019. Snow depths are from freeboard differences. The multiyear ice observed in the West Weddell sector is the thickest, with a...
Article
The Arctic Ocean is overwhelmingly forced by its lateral boundaries, and interacts with, the global system. For the development of nested conceptual models of the Arctic Ocean ecosystem we here choose the full pan-Arctic as our focal scale. Understanding the pan-Arctic scale, however, requires that we look at the underlying scales of its major comp...
Article
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Continental slopes – steep regions between the shelf break and abyssal ocean – play key roles in the climatology and ecology of the Arctic Ocean. Here, through review and synthesis, we find that the narrow slope regions contribute to ecosystem functioning disproportionately to the size of the habitat area (∼6% of total Arctic Ocean area). Driven by...
Article
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CryoSat-2 (CS2) is the first mission equipped with a pulse-limited radar altimeter capable of operating in Synthetic Aperture Radar (SAR) Interferometric (SARIn) mode. Over ice sheets and ice caps, CS2 SARIn data have been used to retrieve surface elevations over an across-track ground “swath.” This work demonstrates that retracking multiple cohere...
Preprint
Full-text available
In Release 1 and 2 of the ICESat-2 sea ice products, candidate height segments used to estimate the reference sea surface height for freeboard calculations included two surface types: specular and smooth dark leads. We found that the uncorrected photon rates, used as proxies of surface reflectance, are attenuated due to clouds resulting in the pote...
Article
Full-text available
The changing Arctic freshwater content and distribution have a significant implications for ocean circulation, climate, and water and biogeochemical cycles. The paucity of in situ salinity measurements in the Arctic Ocean has limited our ability to study Arctic‐Ocean freshwater variability. Although satellite‐derived sea surface height (SSH) and oc...
Article
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Plain Language Summary As the air temperature warms with the onset of summer, the Arctic Ocean ice cover undergoes its annual transformation from a highly reflective snow‐covered surface in winter to a darker, more absorbent surface during summer composed of bare ice, melt ponds, and open water. Several weeks after the onset of melt, much of the sn...
Preprint
Full-text available
We offer a view of the Antarctic sea ice cover from lidar (ICESat-2) and radar (CryoSat-2) altimetry, with retrievals of freeboards, snow depth, and ice volume that span an 8-month winter between April 2019 and November 2019. Snow depths are from freeboard differences. The multiyear ice in the West Weddell sector stands out with a mean sector thick...
Article
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The Fram Strait sea ice volume export 1992–2014 is derived by combining sea ice thickness from upward looking sonars (ULS) with satellite observations of sea ice drift and area. Fram Strait is the main gate for sea ice export from the Arctic. The average yearly sea ice export is 2,400 ± 640 km³. The mean and modal ULS ice thickness in Fram Strait d...
Article
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National Aeronautics and Space Administration's (NASA's) Ice, Cloud, and land Elevation Satellite‐2 (ICESat‐2) mission was launched in September 2018 with the primary goal of monitoring our rapidly changing polar regions. The sole instrument onboard, the Advanced Topographic Laser Altimeter System, is now providing routine, very high‐resolution, su...
Article
Full-text available
Uncertainty in snow properties impacts the accuracy of Arctic sea ice thickness estimates from radar altimetry. On first-year sea ice (FYI), spatiotemporal variations in snow properties can cause the Ku-band main radar scattering horizon to appear above the snow/sea ice interface. This can increase the estimated sea ice freeboard by several centime...
Article
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The Beaufort Gyre freshwater content has increased since the 1990s, potentially stabilizing in recent years. The mechanisms proposed to explain the stabilization involve either mesoscale eddy activity that opposes Ekman pumping or the reduction of Ekman pumping due to reduced sea ice–ocean surface stress. However, the relative importance of these m...
Article
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The Arctic has warmed more than twice as fast as the global average since the late twentieth century, a phenomenon known as Arctic amplification (AA). Recently, there have been considerable advances in understanding the physical contributions to AA, and progress has been made in understanding the mechanisms that link it to midlatitude weather varia...
Article
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Identifying the mechanisms controlling the timing and magnitude of snow accumulation on sea ice is crucial for understanding snow’s net effect on the surface energy budget and sea-ice mass balance. Here, we analyze the role of cyclone activity on the seasonal buildup of snow on Arctic sea ice using model, satellite, and in situ data over 1979–2016....
Article
The Arctic has warmed more than twice as fast as the global average since the late 20th century, a phenomenon known as Arctic amplification (AA). Recently, there have been significant advances in understanding the physical contributions to AA and progress has been made in understanding the mechanisms linking AA to mid-latitude weather variability....
Article
Full-text available
Plain Language Summary NASA's Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) was launched in September of 2018. For sea ice, the topic of focus here, the lidar instrument onboard ICESat‐2 is tasked to measure surface height and freeboard—the vertical height of the floating snow and ice above the sea surface—which will be used to estimate the...
Article
A first look at data from NASA’s laser altimeter mission ICESat-2 reveals very high resolution 3-D profiles of ice on land and sea, forests, and shallow bodies of water.
Article
An ultrawideband radiometer was used to measure microwave brightness temperature spectra over Arctic sea ice in the Lincoln Sea near the north coast of Greenland. Spectra over the range of 0.5-2 GHz were compared to thermal infrared images collected during the airborne campaign and also compared to nearly concurrent Sentinel-1 C-band synthetic aper...
Article
The Surface Water Ocean Topography mission (SWOT), scheduled for launch in 2021, is the first space-borne radar interferometer capable of providing wide-swath height maps of water surfaces with centimetric precision. In addition to its primary objectives in oceanography and hydrography, the SWOT instrument offers opportunities for other application...
Article
The development of the technologies of remote sensing of the ocean was initiated in the 1970s, while the ideas of observing the ocean from space were conceived in the late 1960s. The first global view from space revealed the expanse and complexity of the state of the ocean that had perplexed and inspired oceanographers ever since. This paper presen...
Article
Snow is the most reflective, and also the most insulative, natural material on Earth. Consequently, it is an integral part of the sea-ice and climate systems. However, the spatial and temporal heterogeneities of snow pose challenges for observing, understanding and modelling those systems under anthropogenic warming. Here, we survey the snow–ice sy...
Article
The Amundsen Sea Low (ASL) is an important driver of Antarctic sea ice variations largely because of wind-driven sea ice and ocean transport anomalies. However, the nature of the relationship between the ASL and sea ice is complicated by large seasonality in the ice cover and the ASL location and depth. Here we explore these relationships as a func...
Article
The airborne Operation IceBridge and spaceborne CryoSat-2 missions observe polar sea ice at different spatial and temporal scales as well as with different sensor suites. Comparison of data products from IceBridge and CryoSat-2 is complicated by the fact that they use different geophysical corrections: reference ellipsoid, geoid model, tide model,...
Article
Full-text available
We examine the variability of sea ice freeboard, snow depth, and ice thickness in three years (2011, 2014, and 2016) of repeat surveys of an IceBridge (OIB) transect across the Weddell Sea. Averaged over this transect, ice thickness ranges from 2.40±1.07 (2011) to 2.60±1.15m (2014) and snow depth from 35.8±11.5 (2016) to 43.6±10.2cm (2014), suggest...
Article
Full-text available
Specular (mirror-like) reflections in radar altimeter returns are sensitive indicators of flat open water in leads and melt ponds within the Arctic sea ice cover. Here we find increased specular and near-specular returns in CryoSat-2 waveforms as the sea ice cover transitions from a high albedo snow-covered surface to a lower albedo surface dominat...
Article
The Arctic Oscillation (AO) is the leading mode of extratropical northern hemisphere atmospheric variability, affecting surface pressure, winds, temperature, and precipitation. Here we use an altimeter sea level record spanning 2003–2014, covering the ice-covered and ice-free ocean, to examine the influence of the AO on Arctic sea level and surface...
Article
Full-text available
We examine the variability of sea ice freeboard, snow depth, and ice thickness in three years (2011, 2014, and 2016) of repeat surveys of an IceBridge (OIB) transect across the Weddell Sea. Averaged over this transect, ice thickness ranges from 2.4 ± 1.07 (2011) to 2.60 ± 1.15 m (2014), and snow depth from 30.0 ± 8.51 (2016) to 43.6 ± 10.2 cm (2014...
Article
Model and observational evidence has shown ocean current speeds in the Beaufort Gyre have increased and recently stabilized. Because these currents rival ice drift speeds, we examine the potential for the Beaufort Gyre's shift from a system in which the wind drives the ice and the ice drives a passive ocean to one in which the ocean often, in the a...
Article
This study combines sea surface height (SSH) estimates of the ice-covered Southern Ocean with conventional open ocean SSH estimates from CryoSat-2 to produce monthly composites of dynamic ocean topography (DOT) and sea level anomaly (SLA) on a 50km grid spanning 2011-2016. This dataset reveals the full Southern Ocean SSH seasonal cycle for the firs...
Article
Full-text available
Since 2009, the ultra-wideband snow radar on Operation IceBridge (OIB; a NASA airborne mission to survey the polar ice covers) has acquired data in annual 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 retrieva...
Article
The potential of deriving snow depth estimates using differences in freeboard heights from CryoSat-2 (CS-2) and ICESat-2 (IS-2) is examined. In our analysis, we use lidar freeboard from the Airborne Topographic Mapper (ATM) on Operation IceBridge (OIB) as proxy of IS-2 total (snow + ice) freeboard. Snow depths are estimates from the OIB snow radar....
Article
Full-text available
A realistic representation of sea-ice deformation in models is important for accurate simulation of the sea-ice mass balance. Simulated sea-ice deformation from numerical simulations with 4.5, 9, and 18 km horizontal grid spacing and a viscous–plastic (VP) sea-ice rheology are compared with synthetic aperture radar (SAR) satellite observations (RGP...
Article
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Understanding long-term changes in large-scale sea ice drift in the Southern Ocean is of considerable interest given its contribution to ice extent, to ice production in open waters, with associated dense water formation and heat flux to the atmosphere, and thus to the climate system. In this paper, we examine the trends and variability of this ice...
Article
Full-text available
Since 2009, the ultra-wideband snow-radar on Operation IceBridge has acquired data in annual 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 retrieval of snow depth. Existing retrieval algorithms differ in the w...
Research
Full-text available
The 2015 Arctic Observing Open Science Meeting (AOOSM), held in Seattle, WA, 17–19 November, provided the research community with a forum to discuss the advances supported by sustained, broad, contemporaneous observations in the Arctic and to identify areas for integration into an Interagency Arctic Observing Network.
Article
Losing its character The eastern Eurasian Basin of the Arctic Ocean is on the far side of the North Pole from the Atlantic, but it is becoming more like its larger neighbor as the climate warms. Polyakov et al. show that this region is also evolving toward a state of weakened stratification with increased vertical mixing, release of oceanic heat, a...
Article
Full-text available
The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocea...
Article
In the above paper [1] , there is an error in Table I . The value “30” in the bottom row, fifth