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Bonnie Light

Principal Physicist

Affiliate Associate Professor, Atmospheric Sciences





Department Affiliation

Polar Science Center


Extreme Summer Melt: Assessing the Habitability and Physical Structure of Rotting First-year Arctic Sea Ice

Sea ice cover in the Arctic during summer is shrinking and thinning. The melt season is lengthening and the prevalence of "rotten" sea ice is increasing.

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30 Jul 2015

A multidisciplinary team of researchers is making a series of three monthly (May, June, and July) expeditions to Barrow, AK. They are measuring the summertime melt processes that transform the physical properties of sea ice, which in turn transform the biological and chemical properties of the ice habitat.

Investigating Arctic Ice Melt

"Investigating Arctic Ice Melt" is an interactive exhibit at the Pacific Science Center in Seattle, WA. Bonnie Light leads a tour through some of the installations and explains a few of the many pieces to the puzzle: What is causing the decreasing ice up north?

19 Mar 2014

Focus on Arctic Sea Ice: Current and Future States of a Diminished Sea Ice Cover

APL-UW polar scientists are featured in the March edition of the UW TV news magazine UW|360, where they discuss their research on the current and future states of a diminished sea ice cover in the Arctic.

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7 Mar 2012

The dramatic melting of Arctic sea ice over the past several summers has generated great interest and concern in the scientific community and among the public. Here, APL-UW polar scientists present their research on the current state of Arctic sea ice. A long-term, downward trend in sea ice volume is clear.

They also describe how the many observations they gather are used to improve computer simulations of global climate that, in turn, help us to asses the impacts of a future state of diminished sea ice cover in the Arctic.

This movie presentation was first seen on the March 2012 edition of UW|360, the monthly University of Washington Television news magazine.


2000-present and while at APL-UW

The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: 1. Laboratory measurements

Light, B., R. Carns, and S.G. Warren, "The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: 1. Laboratory measurements," J. Geophys. Res., 121, 4966-4979, doi:10.1002/2016JC011803, 2016.

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16 Jun 2016

The ice-albedo feedback mechanism likely contributed to global glaciation during the Snowball Earth events of the Neoproterozoic era (1 Ga to 544 Ma). This feedback results from the albedo contrast between sea ice and open ocean. Little is known about the optical properties of some of the possible surface types that may have been present, including sea ice that is both snow-free and cold enough for salts to precipitate within brine inclusions. A proxy surface for such ice was grown in a freezer laboratory using the single salt NaCl and kept below the eutectic temperature (–21.2°C) of the NaCl – H2O binary system. The resulting ice cover was composed of ice and precipitated hydrohalite crystals (NaCl ⋅ 2H2O). As the cold ice sublimated, a thin lag-deposit of salt formed on the surface. To hasten its growth in the laboratory, the deposit was augmented by addition of a salt-enriched surface crust. Measurements of the spectral albedo of this surface were carried out over 90 days as the hydrohalite crust thickened due to sublimation of ice, and subsequently over several hours as the crust warmed and dissolved, finally resulting in a surface with puddled liquid brine. The all-wave solar albedo of the subeutectic crust is 0.93 (in contrast to 0.83 for fresh snow and 0.67 for melting bare sea ice). Incorporation of these processes into a climate model of Snowball Earth will result in a positive salt-albedo feedback operating between –21°C and –36°C.

The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: 2. Optical modeling

Carns, R.C., B. Light, and S.G. Warren, "The spectral albedo of sea ice and salt crusts on the tropical ocean of Snowball Earth: 2. Optical modeling," J. Geophys. Res., 121, 5217-5230, doi:10.1002/2016JC011804, 2016.

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16 Jun 2016

During the Snowball Earth events of the Neoproterozoic, tropical regions of the ocean could have developed a precipitated salt lag deposit left behind by sublimating sea ice. The major salt would have been hydrohalite, NaCl⋅2H2O. The crystals in such a deposit can be small and highly scattering, resulting in an allwave albedo similar to that of snow. The snow-free sea ice from which such a crust could develop has a lower albedo, around 0.5, so the development of a crust would substantially increase the albedo of tropical regions on Snowball Earth. Hydrohalite crystals are much less absorptive than ice in the near-infrared part of the solar spectrum, so their presence at the surface would increase the overall albedo as well as altering its spectral distribution.

In this paper, we use laboratory measurements of the spectral albedo of a hydrohalite lag deposit, in combination with a radiative transfer model, to infer the inherent optical properties of hydrohalite as functions of wavelength. Using this result, we model mixtures of hydrohalite and ice representing both artificially created surfaces in the laboratory and surfaces relevant to Snowball Earth. The model is tested against sequences of laboratory measurements taken during the formation and the dissolution of a lag deposit of hydrohalite. We present a parameterization for the broadband albedo of cold, sublimating sea ice as it forms and evolves a hydrohalite crust, for use in climate models of Snowball Earth.

The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin, Utah, USA

Zatko, M., and 14 others, including B. Light, "The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin, Utah, USA," Atmos. Chem. Phys., 16, 13837-13851, doi:10.5194/acp-2016-320, 2016.

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17 May 2016

Reactive nitrogen (Nr = NO, NO2, HONO) and volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140 ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic flux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and mid-latitude snow, but snow-sourced reactive nitrogen fluxes in mid-latitude regions have not yet been quantified in the field. Here we present vertical profiles of snow nitrate concentration and nitrogen isotopes (δ15N) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic flux in snow and the production of Nr from the photolysis of snow nitrate. The observed δ15N(NO3−) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of snow-sourced Nr to the overlying boundary layer. Snow-surface δ15N(NO3−) measurements range from −5 ‰ to 10 ‰ and suggest that the local nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NOx sources from beyond the basin are dominant. Modeled daily-averaged snow-sourced Nr fluxes range from 5.6−71 × 107 molec cm−2 s−1 over the course of the field campaign, with a maximum noon-time value of 3.1 × 109 molec cm−2 s−1. The top-down emission estimate of primary, anthropogenic NOx in the Uintah and Duchesne counties is at least 300 times higher than the estimated snow NOx emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen fluxes are minor contributors to the Nr boundary layer budget in the highly-polluted Uintah Basin boundary layer during winter 2014.

More Publications

Optical properties of melting first-year Arctic sea ice

Light, B., D.K. Perovich, M.A. Webster, C. Polashenski, and R. Dadic, "Optical properties of melting first-year Arctic sea ice," J. Geophys. Res., 120, 7657-7675, doi:10.1002/2015JC011163, 2015.

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1 Nov 2015

The albedo and transmittance of melting, first-year Arctic sea ice were measured during two cruises of the Impacts of Climate on the Eco-Systems and Chemistry of the Arctic Pacific Environment (ICESCAPE) project during the summers of 2010 and 2011. Spectral measurements were made for both bare and ponded ice types at a total of 19 ice stations in the Chukchi and Beaufort Seas. These data, along with irradiance profiles taken within boreholes, laboratory measurements of the optical properties of core samples, ice physical property observations, and radiative transfer model simulations are employed to describe representative optical properties for melting first-year Arctic sea ice. Ponded ice was found to transmit roughly 4.4 times more total energy into the ocean, relative to nearby bare ice. The ubiquitous surface-scattering layer and drained layer present on bare, melting sea ice are responsible for its relatively high albedo and relatively low transmittance. Light transmittance through ponded ice depends on the physical thickness of the ice and the magnitude of the scattering coefficient in the ice interior. Bare ice reflects nearly three-quarters of the incident sunlight, enhancing its resiliency to absorption by solar insolation. In contrast, ponded ice absorbs or transmits to the ocean more than three-quarters of the incident sunlight. Characterization of the heat balance of a summertime ice cover is largely dictated by its pond coverage, and light transmittance through ponded ice shows strong contrast between first-year and multiyear Arctic ice covers.

A revised Pitzer model for low-temperature soluble salt assemblages at the Phoenix site, Mars

Toner, J.D., D.C. Catling, and B. Light, "A revised Pitzer model for low-temperature soluble salt assemblages at the Phoenix site, Mars," Geochim. Cosmochim. Act, 166, 327-343, doi:10.1016/j.gca.2015.06.011, 2015.

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1 Oct 2015

The Wet Chemistry Laboratory (WCL) on the Mars Phoenix Lander measured ions in a soil–water extraction and found Na , K , H (pH), Ca2 , Mg2 , SO42-,ClO4-, and Cl-. Equilibrium models offer insights into salt phases that were originally present in the Phoenix soil, which dissolved to form the measured WCL solution; however, there are few experimental datasets for single cation perchlorates (ClO4-), and none for mixed perchlorates, at low temperatures, which are needed to build models. In this study, we measure ice and salt solubilities in binary and ternary solutions in the Na-Ca-Mg-ClO4 system, and then use this data, along with existing data, to construct a low-temperature Pitzer model for perchlorate brines. We then apply our model to a nominal WCL solution. Previous studies have modeled either freezing of a WCL solution or evaporation at a single temperature. For the first time, we model evaporation at subzero temperatures, which is relevant for dehydration conditions that might occur at the Phoenix site.

Seasonal evolution of melt ponds on Arctic sea ice

Webster, M.A., I.G. Rigor, D.K. Perovich, J.A. Richter-Menge, C.M. Polashenski, and B. Light, "Seasonal evolution of melt ponds on Arctic sea ice," J. Geophys. Res., 120, 5968-5982, doi:10.1002/2015JC011030, 2015.

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4 Sep 2015

The seasonal evolution of melt ponds has been well documented on multiyear and landfast first-year sea ice, but is critically lacking on drifting, first-year sea ice, which is becoming increasingly prevalent in the Arctic. Using 1 m resolution panchromatic satellite imagery paired with airborne and in situ data, we evaluated melt pond evolution for an entire melt season on drifting first-year and multiyear sea ice near the 2011 Applied Physics Laboratory Ice Station (APLIS) site in the Beaufort and Chukchi seas. A new algorithm was developed to classify the imagery into sea ice, thin ice, melt pond, and open water classes on two contrasting ice types: first-year and multiyear sea ice. Surprisingly, melt ponds formed ~3 weeks earlier on multiyear ice. Both ice types had comparable mean snow depths, but multiyear ice had 0–5 cm deep snow covering ~37% of its surveyed area, which may have facilitated earlier melt due to its low surface albedo compared to thicker snow. Maximum pond fractions were 53 ± 3% and 38 ± 3% on first-year and multiyear ice, respectively. APLIS pond fractions were compared with those from the Surface Heat Budget of the Arctic Ocean (SHEBA) field campaign. APLIS exhibited earlier melt and double the maximum pond fraction, which was in part due to the greater presence of thin snow and first-year ice at APLIS. These results reveal considerable differences in pond formation between ice types, and underscore the importance of snow depth distributions in the timing and progression of melt pond formation.

'Albedo dome': A method for measuring spectral flux-reflectance in a laboratory for media with long optical paths

Light, B., R.C. Carns, and S.G. Warren, "'Albedo dome': A method for measuring spectral flux-reflectance in a laboratory for media with long optical paths," Appl. Opt., 54, 5260-5269, doi:10.1364/AO.54.005260, 2015.

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10 Jun 2015

A method is presented for accurate measurement of spectral flux-reflectance (albedo) in a laboratory, for media with long optical path lengths, such as snow and ice. The approach uses an acrylic hemispheric dome, which, when placed over the surface being studied, serves two functions: (i) it creates an overcast "sky" to illuminate the target surface from all directions within a hemisphere, and (ii) serves as a platform for measuring incident and backscattered spectral radiances, which can be integrated to obtain fluxes. The fluxes are relative measurements and because their ratio is used to determine flux-reflectance, no absolute radiometric calibrations are required. The dome and surface must meet minimum size requirements based on the scattering properties of the surface. This technique is suited for media with long photon path lengths since the backscattered illumination is collected over a large enough area to include photons that reemerge from the domain far from their point of entry because of multiple scattering and small absorption. Comparison between field and laboratory albedo of a portable test surface demonstrates the viability of this method.

Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts

Toner, J.D., D.C. Catling, and B. Light, "Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts," Icarus, 250, 451-461, doi:10.1016/j.icarus.2014.12.013, 2015.

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1 Apr 2015

Perchlorates, in mixture with sulfates, chlorides, and carbonates, have been found in relatively high concentrations in Martian soils. To determine probable soil salt assemblages from aqueous chemical data, equilibrium models have been developed to predict salt precipitation sequences during either freezing or evaporation of brines. However, these models have not been validated for multicomponent systems and some model predictions are clearly in error. In this study, we built a Pitzer model in the Na-K-Ca-Mg-Cl-SO4-ClO4-H2O system at 298.15 K using compilations of solubility data in ternary and quaternary perchlorate systems. The model is a significant improvement over FREZCHEM, particularly for Na-Mg-Cl-ClO4, Ca-Cl-ClO4, and Na-SO4-ClO4 mixtures. We applied our model to the evaporation of a nominal Phoenix Lander Wet Chemistry Laboratory (WCL) solution at 298.15 K and compare our results to FREZCHEM. Both models predict the early precipitation of KClO4, hydromagnesite (3MgCO3 ∙Mg(OH)2∙3H2O), gypsum (CaSO4∙2H2O), and epsomite (MgSO4∙7H2O), followed by dehydration of epsomite and gypsum to kieserite (MgSO4∙H2O) and anhydrite (CaSO4), respectively. At low residual water contents, our model predicts the precipitation of halite (NaCl), NaClO4∙H2O, and Mg(ClO4)2∙6H2O,whereas halite and NaClO4∙H2O never precipitate in FREZCHEM. Our model predicts that calcite does not precipitate from evaporating WCL solutions at 298.15 K, which conflicts with other evidence for calcite in Phoenix soils. Previous studies that modeled freezing of WCL solutions found that calcite does form. Furthermore, our model predicts that ~0.3 wt.% H2O is held in hydrated salts after the WCL solution has completely evaporated at 298.15 K, whereas previous studies have found that ~1.3 wt.% H2O is held in hydrated salts if WCL solutions freeze. Given minimum water contents in Mars soils of 1.5–2 wt.% H2O measured from orbital spectra and in situ measurements, our modeling results suggest that salts at the Phoenix site were not formed during evaporation near 298.15 K, whereas formation during freezing remains possible.

Evolution of summer Arctic sea ice albedo in CCSM4 simulations: Episodic summer snowfall and frozen summers

Light, B., S. Dickinson, D.K. Perovich, and M.M. Holland, "Evolution of summer Arctic sea ice albedo in CCSM4 simulations: Episodic summer snowfall and frozen summers," J. Geophys. Res., 120, 284-303, doi:10.1002/2014JC010149, 2015.

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1 Jan 2015

The albedo of Arctic sea ice is calculated from summertime output of twentieth century Community Climate System Model v.4 (CCSM4) simulations. This is compared with an empirical record based on the generalized observations of the summer albedo progression along with melt onset dates determined from remote sensing. Only the contributions to albedo from ice, snow, and ponds are analyzed; fractional ice area is not considered in this assessment. Key factors dictating summer albedo evolution are the timing and extent of ponding and accumulation of snow. The CCSM4 summer sea ice albedo decline was found, on average, to be less pronounced than either the empirical record or the CLARA-SAL satellite record. The modeled ice albedo does not go as low as the empirical record, nor does the low summer albedo last as long. In the model, certain summers were found to retain snow on sea ice, thus inhibiting ice surface melt and the formation or retention of melt ponds. These "frozen" summers were generally not the summers with the largest spring snow accumulation, but were instead summers that received at least trace snowfall in June or July. When these frozen summers are omitted from the comparison, the model and empirical records are in much better agreement. This suggests that the representation of summer Arctic snowfall events and/or their influence on the sea ice conditions are not well represented in CCSM4 integrations, providing a target for future model development work.

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.

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.

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