Joshua King

Evaluation of Operation IceBridge quick‐look snow depth estimates on sea ice

We evaluate Operation IceBridge (OIB) ‘quick-look’ (QL) snow depth on sea ice retrievals using in situ measurements taken over immobile first-year ice (FYI) and multi-year ice (MYI) during March of 2014. Good agreement was found over undeformed FYI (-4.5 cm mean bias) with reduced agreement over deformed FYI (-6.6 cm mean bias). Over MYI, the mean bias was -5.7 cm but 54% of retrievals were discarded by the OIB retrieval process as compared to only 10% over FYI. Footprint scale analysis revealed a root mean square error (RMSE) of 6.2 cm over undeformed FYI with RMSE of 10.5 cm and 17.5 cm in the more complex deformed FYI and MYI environments. Correlation analysis was used to demonstrate contrasting retrieval uncertainty associated with spatial aggregation and ice surface roughness.

UW-Scat: A Ground-Based Dual-Frequency Scatterometer for Observation of Snow Properties

The University of Waterloo scatterometer, which is a system developed for observation of snow and ice properties, is described. The system is composed of two frequency-modulated continuous-wave radars operating at center frequencies of 17.2 and 9.6 GHz. A field-deployable platform allows a rapid setup and observation at remote sites under harsh environmental conditions. A two-axis positioning system moves the radar beam across a user-programmable range of azimuth (±180°) and elevation angles (15°-105°). Typical azimuth scans of 60° angular width generate between 21 and 586 independent samples, depending on the wavelength and the elevation angle. The backscatter response of terrestrial snow in the Canadian Subarctic is demonstrated with two experiments conducted in Churchill, MB, Canada, between 2009 and 2011.

Effect of Snow Salinity on CryoSat‐2 Arctic First‐Year Sea Ice Freeboard Measurements

The European Space Agencys CryoSat-2 satellite mission provides radar altimeter data that are used to derive estimates of sea ice thickness and volume. These data are crucial to understanding recent variability and changes in Arctic sea ice. Sea ice thickness retrievals at the CryoSat-2 frequency require accurate measurements of sea ice freeboard, assumed to be attainable when the main radar scattering horizon is at the snow/sea ice interface. Using an extensive snow thermophysical property dataset from late winter conditions in the Canadian Arctic, we examine the role of saline snow on first-year sea ice (FYI), with respect to its effect on the location of the main radar scattering horizon, its ability to decrease radar penetration depth, and its impact on FYI thickness estimates. Based on the dielectric properties of saline snow commonly found on FYI, we quantify the vertical shift in the main scattering horizon. This is found to be approximately 0.07 m. We propose a thickness-dependent snow salinity correction factor for FYI freeboard estimates. This significantly reduces CryoSat-2 FYI retrieval error. Relative error reductions of ~ 11% are found for an an ice thickness of 0.95 m and ~ 25% for 0.7 m. Our method also helps to close the uncertainty gap between SMOS and CryoSat-2 thin ice thickness retrievals. Our results indicate that snow salinity should be considered for FYI freeboard estimates.

Observation and Modeling of X- and Ku-Band Backscatter of Snow-Covered Freshwater Lake Ice

This study is the first assessment of winter season backscatter (σ°) evolution for snow-covered lake ice observed by X(9.6 Gnz) and Ku-band (17.2 Gnz) ground-based scatterometers (UW-SCAT), collected during the Canadian Snow and Ice Experiment in 2010-2011. The σ° evolution of three ice cover scenarios is observed and simulated using a bubbled ice σ° model. The range resolution of UW-SCAT provides separation of interaction at the snow-ice interface (P1), and within the ice volume and ice-water interface (P2). Ice cores extracted at the end of the observation period indicate a P2 σ° increase of approximately 10-12 decibels (dB) (nn & VV) at Xand Ku-band coincident to tubular bubble development. Similarly, complexity of the ice surface (gray ice) results in increased P1 σ° (~7dB). P1 observations indicate that Ku-band is sensitive to snowpack overlying lake ice, with σ° exhibiting a 5 (6) dB drop for VV (nn) when 0.235 m snow is removed. X-band is insensitive to changes in overlying snowpack. A bubbled ice σ° model is presented using dense medium-radiative transfer theory under the quasicrystalline approximation (DMRT-QCA), which treats bubbles as spherical inclusions within an ice volume. Model runs demonstrate the capability of DMRT to produce observed σ° magnitude using snow and ice observations as input. This study improves the understanding of microwave interaction with bubbled ice near the surface or within the volume. Results from this study indicate that further model development involves bubble shape modification within the ice from spherical to tubular.

Observing Scattering Mechanisms of Bubbled Freshwater Lake Ice Using Polarimetric RADARSAT-2 (C-Band) and UW-Scat (X- and Ku-Bands)

A winter time series of ground-based (X- and Ku-bands) scatterometer and spaceborne synthetic aperture radar (SAR) (C-band) fully polarimetric observations coincident with in situ snow and ice measurements are used to identify the dominant scattering mechanism in bubbled freshwater lake ice in the Hudson Bay Lowlands near Churchill, Manitoba. Scatterometer observations identify two physical sources of backscatter from the ice cover: the snow–ice and ice–water interfaces. Backscatter time series at all frequencies show increases from the ice–water interface prior to the inclusion of tubular bubbles in the ice column based on in situ observations, indicating scattering mechanisms independent of double-bounce scatter. The co-polarized phase difference of interactions at the ice–water interface from both scatterometer and SAR observations is centered at 0° during the time series, also indicating a scattering regime other than double bounce. A Yamaguchi three-component decomposition of the RADARSAT-2 C-band time series is presented, which suggests the dominant scattering mechanism to be single-bounce off the ice–water interface with appreciable surface roughness or preferentially oriented facets, regardless of the presence, absence, or density of tubular bubble inclusions. This paper builds on newly established evidence of single-bounce scattering mechanism for freshwater lake ice and is the first to present a winter time series of ground-based and spaceborne fully polarimetric active microwave observations with polarimetric decompositions for bubbled freshwater lake ice.

Validation of physical model and radar retrieval algorithm of snow water equivalent using SnowSAR data

We validate an absorption based radar retrieval algorithm of snow water equivalent (SWE) using X- and Ku-band backscatter with airborne SAR data. The bicontinuous dense media radiative transfer (Bic-DMRT) model is first applied to generate a look-up table of snow properties against backscattering at X- and Ku-bands. In the retrieval algorithm, the background scattering is subtracted from the total scattering giving the volume scattering of snow. With the look-up table, we generate regression equations between multiple and single scattering and correlations between the scattering albedo and optical thickness at the two bands. With these relationships and the volume scattering of the snowpack, the best solution for the radar observation is found using a priori constrained least-squares cost function. Next, the absorption loss of the snowpack is derived from the solution, which is directly proportional to the SWE. We have applied the algorithm to airborne SAR observations from Finland and Canada. The retrieval algorithm is shown to be effective, achieving root mean square error (RMSE) of ∼19 mm for both SnowSAR data, which is smaller than the 20mm RMSE requirement of SCLP.

Exploring the influence of snow microstructure on dual-frequency radar measurements

Recent advancements to the understanding of snow-microwave interaction have helped to identify the considerable potential for radar-based retrieval of terrestrial snow mass. If applied to space-borne platforms, such retrievals could provide much needed improvements to the spatial and temporal availability of snow mass estimates. To further understanding of these interactions in tundra environments, this study evaluates an extensive set of coincident in situ snow measurements and airborne dual-frequency (17.2 and 9.6 GHz) radar observations near Inuvik, Northwest Territories, Canada. Given known uncertainties related to the role of microstructure in radar-based retrievals, an enhanced snow pit protocol was introduced to objectively characterize specific surface area (SSA) with a shortwave infrared integrating sphere (IRIS) system. Snow pit and bulk snow measurements including SSA are used to parameterize the Microwave Emission Model of Layered Snowpacks Adapted to Include Backscattering (MEMLS3&a) and evaluate observed spatial diversity in the airborne radar signal.

Spatially distributed dual frequency (17.2 and 9.2 GHZ) scatterometer observations of shallow tundra snow

Retrieval of snow volume properties at high spatial resolutions has become a priority in hydrological and climatological research. Recent studies have identified the 8 to 18 GHz range as particularly sensitive to snow water equivalent. In this study we present snow target observations using a dual frequency (17.2 GHz and 9.6 GHz) scatterometer system. The observations were collected in a unique tundra environment providing a novel dataset for comparison with in situ snow survey data. Moreover, the scatterometer was sled mounted allowing observations to be collected over a substantial spatial domain.