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Improved Sea Ice Shortwave Radiation Physics in CCSM4: The Impact of Melt Ponds and Aerosols on Arctic Sea Ice*

AbstractThe 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) ...

Solar partitioning in a changing Arctic sea-ice cover

Abstract 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 × 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.

The effect of included participates on the spectral albedo of sea ice

Sediments and other participates are often entrained into sea ice formed over shallow shelves in the Arctic, causing significant changes in the albedo of the ice and in the amount of shortwave radiation absorbed and transmitted by the ice. A structural-optical model was used in conjunction with a four-stream radiative transfer model to examine the effects of such particulates on the optical properties of sea ice. Albedo data from well-characterized ice with moderate particulate loading were combined with model calculations to infer a spectral absorption coefficient and effective size for the particulates. Results indicate that sediment particles contained in the ice have an effective radius (R) of ∼9 μm, assuming absorption coefficients similar to those of Saharan dust. With these values, model predictions are in close agreement with spectral albedo observations over a broad range of particulate loading. For particle size distributions commonly observed in sea ice, the calculations indicate that particles with R>30 μm have little effect on the bulk optical properties of the ice. The albedo data also suggest that even apparently “clean” ice contains trace amounts (5–10 g m−3) of particulates that reduce albedos by as much as 5–10% in the visible part of the spectrum. The calculations show that particulates in sea ice primarily affect radiative transfer at visible wavelengths, whereas apparent optical properties in the near-infrared tend to be governed by ice structure rather than by the presence of particulates. Particle-bearing layers occurring below ∼20–30 cm are found to have little effect on albedo, although they can still have a substantial effect on transmission. Estimates of total particle loading cannot be obtained from reflectance data without some additional information on particle size, vertical distribution, and ice structure.

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

Abstract 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.

Refractive-index measurements in freezing sea-ice and sodium chloride brines

Sea ice contains numerous pockets of brine and precipitated salts whose size and number distributions change dramatically with temperature. Theoretical treatment of scattering produced by these inclusions requires information on refractive-index differences among the brine, salts, and surrounding ice. Lacking specific data on refractive-index variations in the brine, we carried out laboratory measurements in freezing-equilibrium solutions between -2 and -32 °C. Index values at 589 nm increased from 1.341 to 1.397 over this temperature range, corresponding to salinities of 35 and 240 parts per thousand (ppt). Spectral data were also taken at 50-nm intervals between 400 and 700 nm in nonequilibrium solutions with salinities ranging up to 300 ppt. Spectral gradients increased slightly with salinity but showed no measurable dependence on temperature between +12 and -16 °C. The Lorentz-Lorenz equation, combined with data on density, molar refractivities, and brine composition, yielded temperature-dependent index predictions in excellent agreement with the experimental data. Similar index and density measurements in freezing sodium chloride brines yielded values nearly identical to those in the sea-ice brines. The absence of mirabilite crystals in sodium chloride ice, however, will cause it to have higher transmissivity and lower reflectivity than sea ice above -22 °C.

Seasonal evolution of melt ponds on Arctic sea ice

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.

Optical properties of melting first‐year Arctic sea ice

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.

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

Abstract We present a new set of values for the spectral extinction coefficients, Kλ, 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 Kλ 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.

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

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.

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

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 midlatitude 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 ( 15N) 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 15N(NO 3 ) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of Nr to the overlying boundary layer. Snow-surface 15N(NO 3 ) 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 NO x sources from beyond the basin are dominant. Modeled daily averaged snow-sourced Published by Copernicus Publications on behalf of the European Geosciences Union. 13838 M. Zatko et al.: Snow-sourced reactive nitrogen flux in the Uintah Basin Nr fluxes range from 5.6 to 71⇥ 107 molec cm 2 s 1 over the course of the field campaign, with a maximum noontime value of 3.1⇥ 109 molec cm 2 s 1. The top-down emission estimate of primary, anthropogenic NO x in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NO x 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.