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Benjamin Smith

Senior Principal Physicist

Affiliate Associate Professor, Earth and Space Sciences

Email

bsmith@apl.washington.edu

Phone

206-616-9176

Department Affiliation

Polar Science Center

Education

B.S. Physics, University of Chicago, 1997

M.S. Geology & Geophysics, University of Wisconsin - Madison, 1999

Ph.D. Earth & Space Sciences/Geophysics, University of Washington - Seattle, 2005

Publications

2000-present and while at APL-UW

Quantifying volumetric scattering bias in ICESat-2 and Operation IceBridge altimetry over Greenland firn and aged snow

Fair, Z., M. Flanner, T. Neumann, C. Vuyovich, B. Smith, and A. Schneider, "Quantifying volumetric scattering bias in ICESat-2 and Operation IceBridge altimetry over Greenland firn and aged snow," Earth Space Sci., 11, doi:10.1029/2022EA002479, 2024.

More Info

19 Jun 2024

The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission has collected surface elevation measurements for over 5 years. ICESat-2 carries an instrument that emits laser light at 532 nm, and ice and snow absorb weakly at this wavelength. Previous modeling studies found that melting snow could induce significant bias to altimetry signals, but there is no formal assessment on ICESat-2 acquisitions during the melting season. We performed two case studies over the Greenland Ice Sheet to quantify bias in ICESat-2 signals over snow: one to validate Airborne Topographic Mapper (ATM) data against Next Generation Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-NG) grain sizes, and a second to estimate ICESat-2 bias relative to ATM. We used snow optical grain sizes derived from ATM and AVIRIS-NG to attribute altimetry bias to snowpack properties. For the first case study, the mean and standard deviation of optical grain sizes were 340 ± 65 μm (AVIRIS-NG) and 670 ± 420 μm (ATM). A mean altimetry bias of 4.81 ± 1.76 cm was found for ATM, with larger biases linked to increases in grain size. In the second case study, we found a mean grain size of 910 ± 381 μm and biases of 6.42 ± 1.77 cm (ICESat-2) and 9.82 ± 0.97 cm (ATM). The grain sizes and densities needed to recreate biases with a model are uncommon in nature, so we propose that additional surface attributes must be considered to characterize ICESat-2 bias over snow. The altimetry biases are within the accuracy requirements of the ICESat-2 mission, but we cannot rule out more significant errors over coarse-grained snow.

Estimating differential penetration of green (532 nm) laser light over sea ice with NASA's Airborne Topographic Mapper: observations and models

Studinger, M., B.E. Smith, N. Kurtz, A. Petty, T. Sutterly, and R. Tilling, "Estimating differential penetration of green (532 nm) laser light over sea ice with NASA's Airborne Topographic Mapper: observations and models," Cryophere, 18, 2625-2652, doi:10.5194/tc-18-2625-2024, 2024.

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31 May 2024

Differential penetration of green laser light into snow and ice has long been considered a possible cause of range and thus elevation bias in laser altimeters. Over snow, ice, and water, green photons can penetrate the surface and experience multiple scattering events in the subsurface volume before being scattered back to the surface and subsequently the instrument's detector, therefore biasing the range of the measurement. Newly formed sea ice adjacent to open-water leads provides an opportunity to identify differential penetration without the need for an absolute reference surface or dual-color lidar data. We use co-located, coincident high-resolution natural-color imagery and airborne lidar data to identify surface and ice types and evaluate elevation differences between those surfaces. The lidar data reveals that apparent elevations of thin ice and finger-rafted thin ice can be several tens of centimeters below the water surface of surrounding leads, but not over dry snow. These lower elevations coincide with broadening of the laser pulse, suggesting that subsurface volume scattering is causing the pulse broadening and elevation shift. To complement our analysis of pulse shapes and help interpret the physical mechanism behind the observed elevation biases, we match the waveform shapes with a model of scattering of light in snow and ice that predicts the shape of lidar waveforms reflecting from snow and ice surfaces based on the shape of the transmitted pulse, the surface roughness, and the optical scattering properties of the medium. We parameterize the scattering in our model based on the scattering length Lscat, the mean distance a photon travels between isotropic scattering events. The largest scattering lengths are found for thin ice that exhibits the largest negative elevation biases, where scattering lengths of several centimeters allow photons to build up considerable range biases over multiple scattering events, indicating that biased elevations exist in lower-level Airborne Topographic Mapper (ATM) data products. Preliminary analysis of ICESat-2 ATL10 data shows that a similar relationship between subsurface elevations (restored negative freeboard) and "pulse width" is present in ICESat-2 data over sea ice, suggesting that biased elevations caused by differential penetration likely also exist in lower-level ICESat-2 data products. The spatial correlation of observed differential penetration in ATM data with surface and ice type suggests that elevation biases could also have a seasonal component, increasing the challenge of applying a simple bias correction.

Simulating the processes controlling ice-shelf rift paths using damage mechanics

Huth, A., R. Dudu, B. Smith, and O. Sergienko, "Simulating the processes controlling ice-shelf rift paths using damage mechanics," J. Glaciol., EOR, doi:10.1017/jog.2023.71, 2023.

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21 Sep 2023

Rifts are full-thickness fractures that propagate laterally across an ice shelf. They cause ice-shelf weakening and calving of tabular icebergs, and control the initial size of calved icebergs. Here, we present a joint inverse and forward computational modeling framework to capture rifting by combining the vertically integrated momentum balance and anisotropic continuum damage mechanics formulations. We incorporate rift–flank boundary processes to investigate how the rift path is influenced by the pressure on rift–flank walls from seawater, contact between flanks, and ice mélange that may also transmit stress between flanks. To illustrate the viability of the framework, we simulate the final 2 years of rift propagation associated with the calving of tabular iceberg A68 in 2017. We find that the rift path can change with varying ice mélange conditions and the extent of contact between rift flanks. Combinations of parameters associated with slower rift widening rates yield simulated rift paths that best match observations. Our modeling framework lays the foundation for robust simulation of rifting and tabular calving processes, which can enable future studies on ice-sheet–climate interactions, and the effects of ice-shelf buttressing on land ice flow.

More Publications

In The News

UW-led project to study ozone, atmospheric layers a finalist for next-generation NASA satellite

UW News, Hannah Hickey

A project led by the University of Washington to better understand our atmosphere's complexity is a finalist for NASA's next generation of Earth-observing satellites. The four teams that reached the proof-of-concept stage will spend the next year refining their proposals. NASA will then review the concept study reports and select two for implementation.

14 May 2024

How ants inspired a new way to measure snow with space lasers

Wired, Matt Simon

Glaciologist Ben Smith comments on a clever new technique to measure fluffy snow on the Earth's surface with the orbiting ICESat-2 lidar instrument.

31 May 2022

Edge of Pine Island Glacier’s ice shelf is ripping apart, causing key Antarctic glacier to gain speed

UW News, Hannah Hickey

For decades, the ice shelf helping to hold back one of the fastest-moving glaciers in Antarctica has gradually thinned. Analysis of satellite images reveals a more dramatic process in recent years: From 2017 to 2020, large icebergs at the ice shelf’s edge broke off, and the glacier sped up.

11 Jun 2021

More News Items

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