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Bonnie Light

Principal Physicist

Affiliate Associate Professor, Atmospheric Sciences

Email

bonnie@apl.washington.edu

Phone

206-543-9824

Department Affiliation

Polar Science Center

Publications

2000-present and while at APL-UW

Soluble salts at the Phoenix Lander site, Mars: A reanalysis of the Wet Chemistry Laboratory data

Toner, J.D., D.C. Catling, and B. Light, "Soluble salts at the Phoenix Lander site, Mars: A reanalysis of the Wet Chemistry Laboratory data," Geochimica et Cosmochimica Acta, 136, 142-168, doi:10.1016/j.gca.2014.03.030, 2014.

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1 Jul 2014

The Wet Chemistry Laboratory (WCL) on the Phoenix Mars Scout Lander analyzed soils for soluble ions and found Ca2 , Mg2 , Na , K , Cl-, SO42-, and ClO4-. The salts that gave rise to these ions can be inferred using aqueous equilibrium models; however, model predictions are sensitive to the initial solution composition. This is problematic because the WCL data is noisy and many different ion compositions are possible within error bounds. To better characterize ion concentrations, we reanalyzed WCL data using improvements to original analyses, including Kalman optimal smoothing and ion-pair corrections. Our results for Rosy Red are generally consistent with previous analyses, except that Ca2 and Cl- concentrations are lower. In contrast, ion concentrations in Sorceress 1 and 2 are significantly different from previous analyses. Using the more robust Rosy Red WCL analysis, we applied equilibrium models to determine salt compositions within the error bounds of the reduced data.

Modeling with FREZCHEM predicts that WCL solutions evolve Ca-Mg-ClO4-rich compositions at low temperatures. These unusual compositions are likely influenced by limitations in the experimental data used to parameterize FREZCHEM. As an alternative method to evaluate salt assemblages, we employed a chemical divide model based on the eutectic temperatures of salts. Our chemical divide model predicts that the most probable salts in order of mass abundance are MgSO4 x 11H2O (meridianiite), MgCO3 x nH2O, Mg(ClO4)2 x 8H2O, NaClO4 x 2H2O, KClO4, NaCl x 2H2O (hydrohalite), and CaCO3 (calcite). If ClO3- is included in the chemical divide model, then NaClO3 precipitates instead of NaClO4 x 2H2O and Mg(ClO3)2 x 6H2O precipitates in addition to Mg(ClO4)2 x 8H2O. These salt assemblages imply that at least 1.3 wt.% H2O is bound in the soil, noting that we cannot account for water in hydrated insoluble salts or deliquescent brines. All WCL solutions within error bounds precipitate Mg(ClO4)2 x 8H2O and Mg(ClO3)2 x 6H2O salts. These salts have low eutectic temperatures and are highly hygroscopic, which suggests that brines will be stable in soils for much of the Martian summer.

The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars

Toner, J.D., D.C. Catling, and B. Light, "The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars," Icarus, 233, 36-47, doi:10.1016/j.icarus.2014.01.018, 2014.

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29 Jan 2014

Salt solutions on Mars can stabilize liquid water at low temperatures by lowering the freezing point of water. The maximum equilibrium freezing-point depression possible, known as the eutectic temperature, suggests a lower temperature limit for liquid water on Mars; however, salt solutions can supercool below their eutectic before crystallization occurs. To investigate the magnitude of supercooling and its variation with salt composition and concentration, we performed slow cooling and warming experiments on pure salt solutions and saturated soil-solutions of MgSO4, MgCl2, NaCl, NaClO4, Mg(ClO4)2, and Ca(ClO4)2.

By monitoring solution temperatures, we identified exothermic crystallization events and determined the composition of precipitated phases from the eutectic melting temperature. Our results indicate that supercooling is pervasive. In general, supercooling is greater in more concentrated solutions and with salts of Ca and Mg. Slowly cooled MgSO4, MgCl2, NaCl, and NaClO4 solutions investigated in this study typically supercool 5–15¼C below their eutectic temperature before crystallizing. The addition of soil to these salt solutions has a variable effect on supercooling. Relative to the pure salt solutions, supercooling decreases in MgSO4 soil-solutions, increases in MgCl2 soil-solutions, and is similar in NaCl and NaClO4 soil-solutions. Supercooling in MgSO4, MgCl2, NaCl, and NaClO4 solutions could marginally extend the duration of liquid water during relatively warm daytime temperatures in the Martian summer.

In contrast, we find that Mg(ClO4)2 and Ca(ClO4)2 solutions do not crystallize during slow cooling, but remain in a supercooled, liquid state until forming an amorphous glass near –120¼C. Even if soil is added to the solutions, a glass still forms during cooling. The large supercooling effect in Mg(ClO4)2 and Ca(ClO4)2 solutions has the potential to prevent water from freezing over diurnal and possibly annual cycles on Mars. Glasses are also potentially important for astrobiology because of their ability to preserve pristine cellular structures intact compared to solutions that crystallize.

Synthesis of primary production in the Arctic Ocean: I. Surface waters, 1954-2007

Matrai, P.A., E. Olson, S. Suttles, V. Hill, L.A. Codispoti, B. Light, and M. Steele, "Synthesis of primary production in the Arctic Ocean: I. Surface waters, 1954-2007," Prog. Oceanogr., 110, 93-106, doi:10.1016/j.pocean.2012.11.004, 2013.

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1 Mar 2013

The spatial and seasonal magnitude and variability of primary production in the Arctic Ocean (AO) is quantified with a pan-arctic approach. We synthesize estimates of primary production (PP), focusing on surface waters (0–5 m), using complementary methods that emphasize different spatial and temporal scales. These methods include (1) in situ observations of 14C uptake mostly and possibly some O2 production reported in units of carbon (in situ PP), (2) remotely sensed primary production (sat-PP), and (3) an empirical algorithm giving net PP as a function of in situ chlorophyll a (in situ Chl-PP). The work presented herein examines historical data for PP collected in surface waters only, as they form the majority of the values of a larger ensemble of PP data collected over >50 years (ARCSS-PP) by many national and international efforts. This extended set of surface and vertically-resolved data will provide pan-Arctic validation of remotely sensed chlorophyll a and PP, an extremely valuable tool in this environment which is so difficult to sample. To this day, PP data in the AO are scarce and have uneven temporal and spatial coverage which, when added to the AO's regional heterogeneity, its strong seasonal changes, and limited access, have made and continue to make obtaining a comprehensive picture of PP in the AO difficult.

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Synthesis of primary production in the Arctic Ocean: III. Nitrate and phosphate based estimates of net community production

Cadispoti, L.A., V. Kelly, A. Thessen, P. Matrai, S.Suttles, V. Hill, M. Steele, and B. Light, "Synthesis of primary production in the Arctic Ocean: III. Nitrate and phosphate based estimates of net community production," Prog. Oceanogr., 110, 126-150, doi:10.1016/j.pocean.2012.11.006, 2013.

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1 Mar 2013

Combining nitrate, nitrite and phosphate data from several sources with additional quality control produced a database that eliminates many questionable values. This database, in turn, facilitated estimation of net community production (NCP) in the Arctic Marine System (AMS). In some regions, the new database enabled quantitative calculation of NCP over the vegetative season from changes in nutrient concentrations. In others, useful inferences were possible based on nutrient concentration patterns. This analysis demonstrates that it is possible to estimate NCP from seasonal changes in nutrients in many parts of the Arctic, however, the data were so sparse that most of our estimates for 14 sub-regions of the AMS are attended by uncertainties >50%. Nevertheless, the wide regional variation of NCP within the AMS (~two orders of magnitude) may make the results useful.

Arctic climate response to forcing from light-absorbing particles in snow and sea ice in CESM

Goldenson, N., S.J. Doherty, C.M. Bitz, M.M. Holland, B. Light, and A.J. Conley, "Arctic climate response to forcing from light-absorbing particles in snow and sea ice in CESM," Atmos. Chem. Phys., 12, 7903-7920, doi:10.5194/acp-12-7903-2012, 2012.

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5 Sep 2012

The presence of light-absorbing aerosol particles deposited on arctic snow and sea ice influences the surface albedo, causing greater shortwave absorption, warming, and loss of snow and sea ice, lowering the albedo further. The Community Earth System Model version 1 (CESM1) now includes the radiative effects of light-absorbing particles in snow on land and sea ice and in sea ice itself. We investigate the model response to the deposition of black carbon and dust to both snow and sea ice. For these purposes we employ a slab ocean version of CESM1, using the Community Atmosphere Model version 4 (CAM4), run to equilibrium for year 2000 levels of CO2 and fixed aerosol deposition. We construct experiments with and without aerosol deposition, with dust or black carbon deposition alone, and with varying quantities of black carbon and dust to approximate year 1850 and 2000 deposition fluxes. The year 2000 deposition fluxes of both dust and black carbon cause 1–2°C of surface warming over large areas of the Arctic Ocean and sub-Arctic seas in autumn and winter and in patches of Northern land in every season. Atmospheric circulation changes are a key component of the surface-warming pattern. Arctic sea ice thins by on average about 30 cm. Simulations with year 1850 aerosol deposition are not substantially different from those with year 2000 deposition, given constant levels of CO2. The climatic impact of particulate impurities deposited over land exceeds that of particles deposited over sea ice. Even the surface warming over the sea ice and sea ice thinning depends more upon light-absorbing particles deposited over land. For CO2 doubled relative to year 2000 levels, the climate impact of particulate impurities in snow and sea ice is substantially lower than for the year 2000 equilibrium simulation.

Improved sea ice shortwave radiation physics in CCSM4: The impact of melt ponds and aerosols on Arctic sea ice

Holland, M.M., D.A. Bailey, B.P. Briegleb, B. Light, and E. Hunke, "Improved sea ice shortwave radiation physics in CCSM4: The impact of melt ponds and aerosols on Arctic sea ice," J. Clim., 25, 1413-1430, doi:10.1175/JCLI-D-11-00078.1, 2012.

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1 Mar 2012

The Community Climate System Model, version 4 has revisions across all components. For sea ice, the most notable improvements are the incorporation of a new shortwave radiative transfer scheme and the capabilities that this enables. This scheme uses inherent optical properties to define scattering and absorption characteristics of snow, ice, and included shortwave absorbers and explicitly allows for melt ponds and aerosols. The deposition and cycling of aerosols in sea ice is now included, and a new parameterization derives ponded water from the surface meltwater flux. Taken together, this provides a more sophisticated, accurate, and complete treatment of sea ice radiative transfer. In preindustrial CO2 simulations, the radiative impact of ponds and aerosols on Arctic sea ice is 1.1 W m-2 annually, with aerosols accounting for up to 8 W m-2 of enhanced June shortwave absorption in the Barents and Kara Seas and with ponds accounting for over 10 W m-2 in shelf regions in July. In double CO2 (2XCO2) simulations with the same aerosol deposition, ponds have a larger effect, whereas aerosol effects are reduced, thereby modifying the surface albedo feedback. Although the direct forcing is modest, because aerosols and ponds influence the albedo, the response is amplified. In simulations with no ponds or aerosols in sea ice, the Arctic ice is over 1 m thicker and retains more summer ice cover. Diagnosis of a twentieth-century simulation indicates an increased radiative forcing from aerosols and melt ponds, which could play a role in twentieth-century Arctic sea ice reductions. In contrast, ponds and aerosol deposition have little effect on Antarctic sea ice for all climates considered.

Arctic sea-ice melt in 2008 and the role of solar heating.

Perovich, D.K., J.A. Richter-Menge, K.F. Jones, B. Light, B.C. Elder, C. Polashenski, D. Laroche, T. Markus, and R. Lindsay, "Arctic sea-ice melt in 2008 and the role of solar heating." Ann. Glaciol., 52, 355-359, 2011.

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1 Jun 2011

There has been a marked decline in the summer extent of Arctic sea ice over the past few
decades. Data from autonomous ice mass-balance buoys can enhance our understanding of this decline. These buoys monitor changes in snow deposition and ablation, ice growth, and ice surface and bottom melt. Results from the summer of 2008 showed considerable large-scale spatial variability in the amount of surface and bottom melt. Small amounts of melting were observed north of Greenland, while melting in the southern Beaufort Sea was quite large. Comparison of net solar heat input to the ice and heat required for surface ablation showed only modest correlation. However, there was a strong correlation between solar heat input to the ocean and bottom melting. As the ice concentration in the Beaufort Sea region decreased, there was an increase in solar heat to the ocean and an increase
in bottom melting.

Solar partitioning in a changing Arctic sea-ice cover

Perovich, D.K., K.F. Jones, B. Light, H. Eicken, T. Markus, J. Stroeve, and R. Lindsay, "Solar partitioning in a changing Arctic sea-ice cover," Ann. Glaciol., 52, 192-196, 2011.

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1 Jan 2011

The summer extent of the Arctic sea-ice cover has decreased in recent decades and there have been alterations in the timing and duration of the summer melt season. These changes in ice conditions have affected the partitioning of solar radiation in the Arctic atmosphere-ice-ocean system. The impact of sea-ice changes on solar partitioning is examined on a pan-Arctic scale using a 25 km x 25 km Equal-Area Scalable Earth Grid for the years 1979-2007. Daily values of incident solar irradiance are obtained from NCEP reanalysis products adjusted by ERA-40, and ice concentrations are determined from passive microwave satellite data. The albedo of the ice is parameterized by a five-stage process that includes dry snow, melting snow, melt pond formation, melt pond evolution, and freeze-up. The timing of these stages is governed by the onset dates of summer melt and fall freeze-up, which are determined from satellite observations. Trends of solar heat input to the ice were mixed, with increases due to longer melt seasons and decreases due to reduced ice concentration. Results indicate a general trend of increasing solar heat input to the Arctic ice-ocean system due to declines in albedo induced by decreases in ice concentration and longer melt seasons. The evolution of sea-ice albedo, and hence the total solar heating of the ice-ocean system, is more sensitive to the date of melt onset than the date of fall freeze-up. The largest increases in total annual solar heat input from 1979 to 2007, averaging as much as 4%a-1, occurred in the Chukchi Sea region. The contribution of solar heat to the ocean is increasing faster than the contribution to the ice due to the loss of sea ice.

Migration of air bubbles in ice under a temperature gradient, with application to 'Snowball Earth'

Dadic, R., B. Light, and S.G. Warren, "Migration of air bubbles in ice under a temperature gradient, with application to 'Snowball Earth'," J. Geophys. Res., 115, doi:10.1029/2010JD014148, 2010.

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29 Sep 2010

To help characterize the albedo of "sea glaciers" on Snowball Earth, a study of the migration rates of air bubbles in freshwater ice under a temperature gradient was carried out in the laboratory. The migration rates of air bubbles in both natural glacier ice and laboratory-grown ice were measured for temperatures between -36 deg C and -4 deg C and for bubble diameters of 23–2000 micrometers. The glacier ice was sampled from a depth near close-off (74 m) in the JEMS2 ice core from Summit, Greenland. Migration rates were measured by positioning thick sections of ice on a temperature gradient stage mounted on a microscope inside a freezer laboratory.

The maximum and minimum migration rates were 5.45 micrometers h-1 (K cm-1)-1 at -4 deg C and 0.03 micrometers h-1 (K cm-1)-1 at -36 deg C. Besides a strong dependence on temperature, migration rates were found to be proportional to bubble size. We think that this is due to the internal air pressure within the bubbles, which may correlate with time since close-off and therefore with bubble size. Migration rates show no significant dependence on bubble shape. Estimates of migration rates computed as a function of bubble depth within sea glaciers indicate that the rates would be low relative to the predicted sublimation rates, such that the ice surface would not lose its air bubbles to net downward migration. It is therefore unlikely that air bubble migration could outrun the advancing sublimation front, transforming glacial ice to a nearly bubble-free ice type, analogous to low-albedo marine ice.

Theoretical and observational techniques for estimating light scattering in first-year Arctic sea ice

Light, B., "Theoretical and observational techniques for estimating light scattering in first-year Arctic sea ice," In Light Scattering Reviews, vol. 5, edited by A. Kokhanovsky. Springer-Praxis, Berlin, 331-392, 2010.

15 Jan 2010

Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature gradient, with application to 'Snowball Earth'

Light, B., R.E. Brandt, and S.G. Warren, "Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature gradient, with application to 'Snowball Earth'," J. Geophys. Res., 114, doi:10.1029/2008JC005211, 2009.

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17 Jul 2009

When NaCl precipitates out of a saturated solution, it forms anhydrous crystals of halite at temperatures above 0.11°C, but at temperatures below this threshold it instead precipitates as the dihydrate "hydrohalite," NaCl x 2H2O. When sea ice is cooled, hydrohalite begins to precipitate within brine inclusions at about –23°C.

In this work, hydrohalite crystals are examined in laboratory experiments: their formation, their shape, and their response to warming and desiccation. Sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite, which has the character of a fine powder. The precipitation of hydrohalite in brine inclusions raises the albedo of sea ice, and the subsequent formation of a surface accumulation further raises the albedo. Although these processes have limited climatic importance on the modern Earth, they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a Snowball Earth event. However, brine inclusions in sea ice migrate downward to warmer ice, so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration. The migration rates are measured in a laboratory experiment at temperatures from –2°C to –32°C; the migration appears to be too slow to prevent formation of a salt crust on Snowball Earth.

Transpolar observations of the morphological properties of Arctic sea ice

Perovich, D.K., T.C. Grenfell, B. Light, et al., "Transpolar observations of the morphological properties of Arctic sea ice," J. Geophys. Res., 114, 10.1029/2008JC004892, 2009.

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30 Jan 2009

During the 5 August to 30 September 2005 Healy Oden Trans-Arctic Expedition a trans-Arctic survey of the physical properties of the polar ice pack was conducted. The observational program consisted of four broad classes of snow and ice characterization activities: observations made while the ship was in transit, ice station measurements, helicopter survey flights, and the deployment of autonomous ice mass balance buoys. Ice conditions, including ice thicknesses, classes, and concentrations of primary, secondary, and tertiary categories were reported at 2-hour intervals.

Pond fractions were large early in the cruise at the southern edge of the ice pack, reaching peak values of 0.5 and averaging 0.25. Ice concentrations ranged from 0.8 to 1.0 north of 79°N, save for an area between 88°30'N and 89°30'N, where polynyas and thin ice were observed. Surveys of snow depth, ice thickness, and ice properties were conducted at ice stations. Thickness observations suggest a general latitudinal trend of increasing ice thickness moving northward, with considerable variability from floe to floe and within a single floe. Average floe thicknesses varied from 1.0 to >2.8 m, and the standard deviation of thickness on an individual floe was as large as 1 m. Ice crystallography showed a large amount of granular ice. The average optical-equivalent soot content was 4 ng C g-1 for new snow, 8 ng C g-1 for the surface granular layer of multiyear ice, and 18 ng C g-1 for the interior of multiyear ice, indicating a tendency of the particulates to concentrate at the surface with melting.

Sunlight, water, and ice: Extreme arctic sea ice melt during the summer of 2007

Perovich, D.K. J.A. Richter-Menge, K.F. Jones, and B. Light, "Sunlight, water, and ice: Extreme arctic sea ice melt during the summer of 2007," Geophys. Res. Lett., 35, doi:10.1029/2008GL034007, 2008.

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3 Jun 2008

The summer extent of the Arctic sea ice cover, widely recognized as an indicator of climate change, has been declining for the past few decades reaching a record minimum in September 2007. The causes of the dramatic loss have implications for the future trajectory of the Arctic sea ice cover. Ice mass balance observations demonstrate that there was an extraordinarily large amount of melting on the bottom of the ice in the Beaufort Sea in the summer of 2007. Calculations indicate that solar heating of the upper ocean was the primary source of heat for this observed enhanced Beaufort Sea bottom melting. An increase in the open water fraction resulted in a 500% positive anomaly in solar heat input to the upper ocean, triggering an ice–albedo feedback and contributing to the accelerating ice retreat.

Transmission and absorption of solar radiation by arctic sea ice during the melt season

Light, B. T.C. Grenfell, and D.K. Perovich, "Transmission and absorption of solar radiation by arctic sea ice during the melt season," J. Geophys. Res., 113, doi:10.1029/2006JC003977, 2008.

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21 Mar 2008

The partitioning of incident solar radiation between sea ice, ocean, and atmosphere strongly affects the Arctic energy balance during summer. In addition to spectral albedo of the ice surface, transmission of solar radiation through the ice is critical for assessing heat and mass balances of sea ice. Observations of spectral irradiance profiles within and transmittance through ice in the Beaufort Sea during the summer of 1998 during the Surface Heat Budget of the Arctic Ocean (SHEBA) are presented. Sites representative of melting multiyear and first-year ice, along with ponded ice were measured. Observed spectral irradiance extinction coefficients (Kλ) show broad minima near 500 nm and strong increases at near-infrared wavelengths. The median Kλ at 600 nm for the bare ice cases is close to 0.8 m-1 and about 0.6 m-1 for ponded ice. Values are considerably smaller than the previously accepted value of 1.5 m-1. Radiative transfer models were used to analyze the observations and obtain inherent optical properties of the ice. Derived scattering coefficients range from 500 m-1 to 1100 m-1 in the surface layer and 8 to 30 m-1 in the ice interior. While ponded ice is known to transmit a significant amount of shortwave radiation to the ocean, the irradiance transmitted through bare, melting ice is also shown to be significant. The findings of this study predict 3–10 times more solar radiation penetrating the ice cover than predicted by a current GCM (CCSM3) parameterization, depending on ice thickness, pond coverage, stage of the melt season, and specific vertical scattering coefficient profile.

Increasing solar heating of the Arctic Ocean and adjacent seas, 1979-2005: Attribution and role in the ice-albedo feedback

Perovich, D.K., B. Light, H. Eicken, K.F. Jones, K. Runciman, and S.V. Nghiem, "Increasing solar heating of the Arctic Ocean and adjacent seas, 1979-2005: Attribution and role in the ice-albedo feedback," Geophys. Res. Lett., 112, doi:10.1029/2007GL031480, 2007.

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11 Oct 2007

Over the past few decades the Arctic sea ice cover has decreased in areal extent. This has altered the solar radiation forcing on the Arctic atmosphere-ice-ocean system by decreasing the surface albedo and allowing more solar heating of the upper ocean. This study addresses how the amount of solar energy absorbed in areas of open water in the Arctic Basin has varied spatially and temporally over the past few decades. A synthetic approach was taken, combining satellite-derived ice concentrations, incident irradiances determined from reanalysis products, and field observations of ocean albedo over the Arctic Ocean and the adjacent seas. Results indicate an increase in the solar energy deposited in the upper ocean over the past few decades in 89% of the region studied. The largest increases in total yearly solar heat input, as much as 4% per year, occurred in the Chukchi Sea and adjacent areas.

Mapping sediment-laden sea ice in the Arctic using AVHRR remote-sensing data: Atmospheric correction and determination of reflectances as a function of ice type and sediment load

Huck, P., B. Light, H. Eicken, and M. Haller, "Mapping sediment-laden sea ice in the Arctic using AVHRR remote-sensing data: Atmospheric correction and determination of reflectances as a function of ice type and sediment load," Remote Sens. Environ., 107, 484-495, doi:10.1016/j.rse.2006.10.002, 2007.

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12 Apr 2007

Exploiting the fact that the spectral characteristics of light backscattered from sediment-laden ice differ substantially from those of clean ice and that sediment tends to accumulate at the ice surface during the first melt season, remote-sensing techniques provide a valuable tool for mapping the extent of particle-laden ice in the Arctic basin and assessing its particulate loading. This study considers two fundamental problems that still need to be addressed in order to make full use of satellite observations for this type of assessment: (i) the effects of the atmosphere on surface reflectances derived from radiances measured by the satellite sensor need to be quantified and ultimately corrected for, and (ii) the spectral reflectance of the ice surface as a function of particle loading and sub-pixel distribution needs to be determined in order to derive quantitative estimates from the at-sensor satellite signal. Here, spectral albedos have been computed for different ice surfaces of variable sediment load with a radiative transfer model for sea ice coupled with an optical model for particulates included in sea ice. In a second step, the role of the atmosphere in modulating the surface reflectance signal is assessed with the aid of an atmospheric radiative transfer model applied to a "standard" Arctic atmosphere and surface boundary conditions as prescribed by the sea ice radiative transfer model. A series of sensitivity studies helps assess differences between top-of-the-atmosphere and true surface reflectance and has been utilized to derive a look-up table for atmospheric correction of Advanced Very High Resolution Radiometer (AVHRR) data over sediment-laden sea ice surfaces. In particular, the effects of solar elevation, viewing geometry, and atmospheric properties are considered. The atmospheric corrections are necessary for certain geometries and surface types. Large discrepancies between raw and corrected data are particularly evident in the derived coverage of clean ice and ice with small sediment loading.

A Delta-Eddington Multiple Scattering Parameterization for Solar Radiation in the Sea Ice Component of the Community Climate System Model

Briegleb, B.P., and B. Light, "A Delta-Eddington Multiple Scattering Parameterization for Solar Radiation in the Sea Ice Component of the Community Climate System Model," Technical Note NCAR/TN-472-STR, National Center for Atmosphere Research, Boulder, CO, 2007.

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30 Jan 2007

Many climate model predictions of future climate change due to increasing greenhouse gases indicate polar warming two to three times the global mean. One important factor in this enhanced polar warming is thought to be the snow and sea ice albedo feedback. The essence of this feedback is the strong contrast in how open water and snow-covered or bare sea ice reflect, absorb, and transmit incoming solar radiation. Snow and sea ice have high albedo; open water has low albedo. The high albedo of snow and sea ice is caused by multiple scattering attributed to individual snow grains and inclusions of gas, brine and precipitated salt crystals embedded in sea ice. An accurate representation of solar radiation transfer in the snow/sea ice system requires a multiple scattering parameterization.

Interactions between snow and sea ice and solar radiation in the present version of the Community Climate System Model (Version 3) are not based on a multiple scattering calculation. Rather, these interactions are based on empirical parameterizations which depend solely on the depth of snow (if any) overlying sea ice, sea ice thickness and its surface temperature. Considerable arbitrariness and inconsistency are inherent in these parameterizations since it is possible to alter one part of this parameterization independent of other parts, which is often done when tuning sea ice albedo to achieve acceptable CCSM present-day simulations. Because of this arbitrariness and inconsistency, it is likely that the solar radiation parameterization for snow and sea ice in the present CCSM may not adequately represent the radiation physics necessary for an accurate estimate of the snow and sea ice albedo feedback.

A Delta-Eddington multiple scattering radiative transfer model is presented here as an alternative treatment for the interactions between solar radiation and snow and sea ice. Optical properties for snow and sea ice are prescribed based on physical measurements. These optical properties are then used in the radiative transfer model to compute the albedo, absorption within snow and sea ice and transmission to the underlying ocean. Snow and sea ice surface albedos and transmissions in this parameterization agree well with observations made during SHEBA. The effects of absorption due to impurities such as carbon soot can be included without loss of consistency. This parameterization also provides opportunities for further improvements in the CCSM treatment of snow and sea ice physics, such as snow aging, vertical gradients in snow pack properties, and the effects of surface melt ponds. Employing the Delta-Eddington solar radiation parameterization for sea ice in CCSM will afford more consistent tuning for present climate, more accurate simulation of control climate annual cycle and variability, and provide increased confidence in simulations of future climate change.

Spectral transmission and implications for the partitioning of shortwave radiation in arctic sea ice

Grenfell, T.C., B. Light, and D.K. Perovich, "Spectral transmission and implications for the partitioning of shortwave radiation in arctic sea ice," Ann. Glaciol., 44, 1-6, doi:10.3189/172756406781811763, 2006.

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1 Nov 2006

We present a new set of values for the spectral extinction coefficients for the interior of first-year (FY) and multi-year (MY) Arctic sea ice during the summer melt season measured during SHEBA (Surface Heat Budget of the Arctic Ocean program) and at Barrow, Alaska, USA. Results for FY ice are consistent with previously reported values, and differences can be understood in terms of variations in the concentration of biological and suspended particulate material. The values for the interior of MY ice are lower than previously reported for both bare and ponded ice. For bare MY ice the new spectral extinction coefficient values predict a substantial increase in the solar radiation transmitted through the ice into the upper mixed layer. Ponded MY ice is only slightly more transparent than previously reported, and FY ice values are generally consistent with previously reported values. Assuming an asymmetry parameter of 0.94, the extinction coefficients are consistent with a volume-scattering coefficient of 77 m-1 that is constant from 400 to at least 720 nm.

A temperature-dependent, structural-optical model of first-year sea ice

Light, B., G.A. Maykut, and T.C. Grenfell, "A temperature-dependent, structural-optical model of first-year sea ice," J. Geophys. Res., 109, 10.1029/2003JC002164, 2004.

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10 Jun 2004

A model has been developed that relates the structural properties of first-year sea ice to its inherent optical properties, quantities needed by detailed radiative transfer models. The structural-optical model makes it possible to calculate absorption coefficients, scattering coefficients, and phase functions for the ice from information about its physical properties. The model takes into account scattering by brine inclusions in the ice, gas bubbles in both brine and ice, and precipitated salt crystals. The model was developed using concurrent laboratory measurements of the microstructure and apparent optical properties of first-year, interior sea ice between temperatures of –33°C and –1°C. Results show that the structural-optical properties of sea ice can be divided into three distinct thermal regimes: cold (T < –23°C), moderate (–23°C < T < –8°C), and warm (T > –8°C). Relationships between structural and optical properties in each regime involve different sets of physical processes, of which most are strongly tied to freezing equilibrium of the brine and ice. Volume scattering in cold ice is dominated by the size and number distribution of precipitated hydrohalite crystals. Scattering at intermediate temperatures is controlled by changes in the distribution of brine inclusions, gas bubbles, and mirabilite crystals. Total volume scattering in this regime is approximately independent of temperature because of a balance between increasing and decreasing scattering related to the thermal evolution of these inclusions and scattering by drained inclusions. In warm ice, scattering is controlled principally by temperature-dependent changes in the real refractive index of brine and by the escape of gas bubbles from the ice. Model predictions indicate that scattering coefficients can exceed 3000 m-1 for cold ice, averaging ~450 m-1 for moderate and warm ice and reaching a minimum of ~340 m-1 at –8°C. Scattering in all three regimes is very strongly forward peaked, with values of the asymmetry parameter g generally falling between 0.975 (T = –8°C) and 0.995 (T = –33°C).

A two-dimensional Monte Carlo model of radiative transfer in sea ice

Light, B., G.A. Maykut, and T.C. Grenfell, "A two-dimensional Monte Carlo model of radiative transfer in sea ice," J. Geophys. Res., 108, 10.1029/2002JC001513, 2003.

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8 Jul 2003

A two-dimensional, Monte Carlo radiative transfer model was developed for the analysis of optical data from cylindrical samples of sea ice. The backward Monte Carlo method was used to solve the radiative transfer equation in a cylindrical, azimuthally symmetric domain. Horizontal layers between two depths and vertical shells between two radii can be used to simulate spatial gradients in scattering, absorption, and refractive index in the model. The top of the cylinder can be illuminated by either normally incident, collimated radiation or by diffuse radiation. Irradiance and radiance detectors can be located anywhere within or on the cylindrical domain. The model was tested by comparing predicted apparent optical properties with solutions from existing one-dimensional and two-dimensional radiative transfer models. Domains with the largest optical depths and smallest radii were found to be impacted most by the horizontally finite geometry. The model was used to interpret backscattered and transmitted spectral radiance data taken in the laboratory from cylindrical core samples of first-year sea ice at ~15°C. Use of a similarity parameter facilitated comparison between observations and model predictions by reducing the number of independent variables by one. Concurrent observations of ice microstructure indicated that light scattering due to inclusions of brine, gas, and precipitated salts should result in a scattering coefficient of ~4.6 cm-1 in our samples. Combining this value with the inferred similarity parameter yielded an asymmetry parameter of 0.98 for first-year sea ice at ~15°C. Agreement between observed and predicted spectral radiances demonstrates the viability of this model as a tool for analyzing the optical properties of samples with finite geometry.

Effects of temperature on the microstructure of first-year Arctic sea ice

Light, B., G.A. Maykut, and T.C. Grenfell, "Effects of temperature on the microstructure of first-year Arctic sea ice," J. Geophys. Res., 108, doi:10.1029/2001JC000887, 2003.

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27 Feb 2003

While the apparent optical properties of sea ice vary with ice type and temperature throughout the annual cycle, they depend more fundamentally on how inclusions of brine, gas, precipitated salts, and other impurities are distributed within the ice. Since little is known about these distributions or about how they evolve with temperature, experiments were designed to collect detailed information on the microstructure of Arctic sea ice over a wide range of temperatures. An imaging system, capable of resolving inclusion sizes of less than 0.01 mm in diameter, was used to examine the microstructure of first-year ice in a temperature-controlled laboratory. Experiments were initially carried out at -15°C to obtain size distributions for brine inclusions and gas bubbles in cold ice. Brine inclusion dimensions were found to range from less than 0.01 mm to nearly 10 mm, with number densities averaging about 24 pockets per mm3. This is an order of magnitude larger than number densities previously reported. Gas bubbles in the samples were generally smaller than 0.2 mm and had number densities of approximately 1 per mm3, also an order of magnitude larger than previously reported. Large changes in microstructure were observed as samples were cooled to -30°C and subsequently warmed to -2°C. Observational results document the thermal evolution of the ice, as well as interactions between brine inclusions, gas bubbles, and precipitated salts. The link between the structural and optical properties of sea ice is closely tied to the total cross-sectional area of the inclusions. We show that this quantity increases dramatically when the ice cools below -23°C or warms above -5°C, but because changes in brine inclusions offset changes in precipitated salts, it remains surprisingly constant between these temperatures.

Spatial distribution and radiative effects of soot in the snow and sea ice during the SHEBA experiment

Grenfell, T.C., B. Light, and M. Sturm, "Spatial distribution and radiative effects of soot in the snow and sea ice during the SHEBA experiment," J. Geophys. Res., 107, 10.1029/2000JC000414, 2002.

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30 Aug 2002

Soot observations around the periphery of the Arctic Ocean indicate snowpack concentrations ranging from about 1 to more than 200 ng carbon/g snow (ngC/g), with typical values being near 40–50 ngC/g. Values of this magnitude would significantly affect not only the albedo and transmissivity of the ice cover but also surface melt rates and internal heat storage in the ice. During the Surface Heat Budget of the Arctic Ocean (SHEBA) drift, there was concern that soot emitted from the ship could adversely impact the heat and mass balance measurements, producing results that would not be representative of the region as a whole. To investigate this possibility, a series of soot measurements was carried out starting in the spring of 1998 during the time of maximum snowpack thickness. On the upwind side of the ship, where the heat and mass balance program was carried out, soot concentrations averaged over the depth of the snowpack spanned a range from 1 to 7 ngC/g, with average values of 4–5 ngC/g. On the downwind side, concentrations increased to 35 ngC/g and above. Measurements made up to 16 km from the ship yielded average background soot levels of approximately 4.4 ngC/g, with a standard deviation of 2.9 ngC/g evenly distributed throughout the different snow layers. These concentrations were not statistically distinguishable from the values measured in the observing areas on the upwind side of the ship. This indicates that soot concentrations in the central Arctic Basin are substantially lower than those reported for the coastal regions and are not sufficient to produce a significant decrease in the albedo. Although measurements of sea ice samples gave similarly low values, parameter studies show that the snow soot levels could be significant if the summer melt caused all the soot to be concentrated at the ice surface.

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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