Stephen R. Hudson

Spectral Bidirectional Reflectance of Antarctic Snow: Measurements and Parameterization

The bidirectional reflectance distribution function (BRDF) of snow was measured from a 32-m tower at Dome C, at latitude 75°S on the East Antarctic Plateau. These measurements were made at 96 solar zenith angles between 51° and 87° and cover wavelengths 350–2400 nm, with 3- to 30-nm resolution, over the full range of viewing geometry. The BRDF at 900 nm had previously been measured at the South Pole; the Dome C measurement at that wavelength is similar. At both locations the natural roughness of the snow surface causes the anisotropy of the BRDF to be less than that of flat snow. The inherent BRDF of the snow is nearly constant in the high-albedo part of the spectrum (350–900 nm), but the angular distribution of reflected radiance becomes more isotropic at the shorter wavelengths because of atmospheric Rayleigh scattering. Parameterizations were developed for the anisotropic reflectance factor using a small number of empirical orthogonal functions. Because the reflectance is more anisotropic at wavelengths at which ice is more absorptive, albedo rather than wavelength is used as a predictor in the near infrared. The parameterizations cover nearly all viewing angles and are applicable to the high parts of the Antarctic Plateau that have small surface roughness and, at viewing zenith angles less than 55°, elsewhere on the plateau, where larger surface roughness affects the BRDF at larger viewing angles. The root-mean-squared error of the parameterized reflectances is between 2% and 4% at wavelengths less than 1400 nm and between 5% and 8% at longer wavelengths.

A Look at the Surface-Based Temperature Inversion on the Antarctic Plateau

Data from radiosondes, towers, and a thermistor string are used to characterize the temperature inversion at two stations: the Amundsen-Scott Station at the South Pole, and the somewhat higher and colder Dome C Station at a lower latitude. Ten years of temperature data from a 22-m tower at the South Pole are analyzed. The data include 2- and 22-m temperatures for the entire period and 13-m temperatures for the last 2 yr. Statistics of the individual temperatures and the differences among the three levels are presented for summer (December and January) and winter (April–September). The relationships of temperature and inversion strength in the lowest 22 m with wind speed and downward longwave flux are examined for the winter months. Two preferred regimes, one warming and one cooling, are found in the temperature versus longwave flux data, but the physical causes of these regimes have not been determined. The minimum temperatures and the maximum inversions tend to occur not with calm winds, but with winds of 3–5 m s 1 , likely due to the inversion wind. This inversion wind also explains why the near-surface winds at South Pole blow almost exclusively from the northeast quadrant. Temperature data from the surface to 2 m above the surface from South Pole in the winter of 2001 are presented, showing that the steepest temperature gradient in the entire atmosphere is in the lowest 20 cm. The median difference between the temperatures at 2 m and the surface is over 1.0 K in winter; under clear skies this difference increases to about 1.3 K. Monthly mean temperature profiles of the lowest 30 km of the atmosphere over South Pole are presented, based on 10 yr of radiosonde data. These profiles show large variations in lower-stratospheric temperatures, and in the strength and depth of the surface-based inversion. The near-destruction of a strong inversion at South Pole durin g7ho n 8September 1992 is examined using a thermal-conductivity model of the snowpack, driven by the measured downward longwave flux. The downward flux increased when a cloud moved over the station, and it seems that this increase in radiation alone can explain the magnitude and timing of the warming near the surface. Temperature data from the 2003/04 and 2004/05 summers at Dome C Station are presented to show the effects of the diurnal cycle of solar elevation over the Antarctic Plateau. These data include surface temperature and 2- and 30-m air temperatures, as well as radiosonde air temperatures. They show that strong inversions, averaging 10 K between the surface and 30 m, develop quickly at night when the sun is low in the sky, but are destroyed during the middle of the day. The diurnal temperature range at the surface was 13 K, but only 3 K at 30 m.

Leads in Arctic pack ice enable early phytoplankton blooms below snow-covered sea ice

The Arctic icescape is rapidly transforming from a thicker multiyear ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production. One critical challenge is to understand how productivity will change within the next decades. Recent studies have reported extensive phytoplankton blooms beneath ponded sea ice during summer, indicating that satellite-based Arctic annual primary production estimates may be significantly underestimated. Here we present a unique time-series of a phytoplankton spring bloom observed beneath snow-covered Arctic pack ice. The bloom, dominated by the haptophyte algae Phaeocystis pouchetii, caused near depletion of the surface nitrate inventory and a decline in dissolved inorganic carbon by 16 ± 6 g C m−2. Ocean circulation characteristics in the area indicated that the bloom developed in situ despite the snow-covered sea ice. Leads in the dynamic ice cover provided added sunlight necessary to initiate and sustain the bloom. Phytoplankton blooms beneath snow-covered ice might become more common and widespread in the future Arctic Ocean with frequent lead formation due to thinner and more dynamic sea ice despite projected increases in high-Arctic snowfall. This could alter productivity, marine food webs and carbon sequestration in the Arctic Ocean.

Floating ice-algal aggregates below melting arctic sea ice.

During two consecutive cruises to the Eastern Central Arctic in late summer 2012, we observed floating algal aggregates in the melt-water layer below and between melting ice floes of first-year pack ice. The macroscopic (1-15 cm in diameter) aggregates had a mucous consistency and were dominated by typical ice-associated pennate diatoms embedded within the mucous matrix. Aggregates maintained buoyancy and accumulated just above a strong pycnocline that separated meltwater and seawater layers. We were able, for the first time, to obtain quantitative abundance and biomass estimates of these aggregates. Although their biomass and production on a square metre basis was small compared to ice-algal blooms, the floating ice-algal aggregates supported high levels of biological activity on the scale of the individual aggregate. In addition they constituted a food source for the ice-associated fauna as revealed by pigments indicative of zooplankton grazing, high abundance of naked ciliates, and ice amphipods associated with them. During the Arctic melt season, these floating aggregates likely play an important ecological role in an otherwise impoverished near-surface sea ice environment. Our findings provide important observations and measurements of a unique aggregate-based habitat during the 2012 record sea ice minimum year.

A comparison of the two Arctic atmospheric winter states observed during N‐ICE2015 and SHEBA

Winter time atmospheric observations from the 2015 Norwegian young sea-ICE campaign (N-ICE2015) are compared with data from the 1997-1998 Surface Heat Budget of the Arctic (SHEBA) campaign. Both datasets have a bimodal distribution of the net longwave radiative flux for January-February, with modal values of -40 W m-2 and 0 W m-2. These values correspond to the radiatively clear and opaquely cloudy states, respectively, and are likely to be representative of the wider Arctic. The new N-ICE2015 observations demonstrate that the two winter states operate in the Atlantic sector of the Arctic and regions of thin sea ice. We compare the N-ICE2015 and SHEBA data with ERA-Interim and output from the coupled Arctic regional climate model HIRHAM-NAOSIM. ERA-Interim simulates two Arctic winter states well and captures the timing of transitions from one state to the other, despite underestimating the cloud liquid water path. HIRHAM-NAOSIM has more cloud liquid water compared with ERA-Interim, but simulates the two states poorly. Our results demonstrate that models must simulate realistic synoptic forcing and temperature profiles to accurately capture the two Arctic winter states, and not only the presence of mixed-phase clouds. Using ERA-Interim, we find a positive trend in the number of opaquely cloudy days in the western Atlantic sector of the Arctic, and a strong correlation with the mean winter temperature over much of the Arctic Basin. Hence, the two Arctic winter states are important for understanding inter-annual variability in the Arctic. The N-ICE2015 dataset will help improve our understanding of these relationships.

Temperature, Humidity, and Pressure Response of Radiosondes at Low Temperatures

Abstract The response of radiosondes to an instantaneous change of environment was studied by taking the instruments from a warm building into the cold environment at South Pole Station. After being initialized inside, the radiosondes were carried outside and placed on the snow surface, where they were left until they reported stable values of temperature, pressure, and relative humidity. Three models of radiosondes were tested: Vaisala RS80, Atmospheric Instrumentation Research (AIR) 4A, and AIR 5A. The reported temperature equilibrated to the outside conditions within 30 s. However, it frequently took 30 min before the relative humidity outside was accurately reported. Additionally, the reported pressure rose by several hectopascals over a 5-min period when the sonde was taken outside. In the RS80s this bias was as large as 10 hPa, and disappeared in about 30 min. In the AIR sondes, the maximum pressure bias was never much over 2 hPa, but seemed not to diminish with time. The RS80s were also tested to s...

Autonomous observations of solar energy partitioning in first‐year sea ice in the Arctic Basin

A Spectral Radiation Buoy (SRB) was developed to autonomously measure the spectral incident, reflected, and transmitted spectral solar radiation (350-800 nm) above and below sea ice. The SRB was deployed on drifting first-year sea ice near the North Pole in mid-April 2012, together with velocity and ice mass balance buoys. The buoys drifted southward and reached Fram Strait after approximately 7 months, covering a complete melt season. At the SRB site, snowmelt started on 10 June, and had completely disappeared by 14 July. Surface albedo was above 0.85 until snowmelt onset and decreased rapidly with the progression of snowmelt. Albedo was lowest on 14 July, when the observed surface was likely a mixture of bare ice and melt pond(s). The transmitted irradiance measured under the ice was largest in July, with a monthly average of 20 W m(-2), compared to <0.3 W m(-2) premelt. Under-ice irradiance peaked on 19-20 July, with a daily average around 35 W m(-2). From mid-April to mid-September, the solar energy transmitted through the ice into the ocean contributed about two-thirds of the energy required for the observed bottom melt (0.49 m). The energy absorbed by the ice after snowmelt was enough to melt an additional 0.1 m of ice. Solar energy incident on open water and melt ponds provided significant additional heating, indicating solar heating could explain all of the observed bottom melt in this region in summer 2012.

Windows in Arctic sea ice: Light transmission and ice algae in a refrozen lead

The Arctic Ocean is rapidly changing from thicker multiyear to thinner first-year ice cover, with significant consequences for radiative transfer through the ice pack and light availability for algal growth. A thinner, more dynamic ice cover will possibly result in more frequent leads, covered by newly formed ice with little snow cover. We studied a refrozen lead (≤0.27 m ice) in drifting pack ice north of Svalbard (80.5–82.4 °N) in May-June 2015 during the Norwegian young sea ICE expedition (N-ICE2015). We measured downwelling incident and ice transmitted spectral irradiance, and colored dissolved organic matter (CDOM), particle absorption, ultraviolet (UV)-protecting mycosporine-like amino acids (MAAs) and chlorophyll a (Chl a) in melted sea ice samples. We found occasionally very high MAA concentrations (up to 39 mg m-3, mean 4.5 ± 7.8 mg m-3) and MAA to Chl a ratios (up to 6.3, mean 1.2 ± 1.3). Disagreement in modelled and observed transmittance in the UV range let us conclude that MAA signatures in CDOM absorption spectra may be artefacts due to osmotic shock during ice melting. Although observed PAR transmittance through the thin ice was 5–40 times that of the adjacent thicker ice with deep snow cover, ice algal standing stocks were low (≤2.31 mg Chl a m-2) and similar to the adjacent ice. Ice algal accumulation in the lead was possibly delayed by the low inoculum and the time needed for photoacclimation to the high-light environment. However, leads are important for phytoplankton growth by acting like windows into the water column.

In situ observations of black carbon in snow and the corresponding spectral surface albedo reduction

Black carbon (BC) particles emitted from incomplete combustion of fossil fuel and biomass and deposited on snow and ice darken the surface and reduce the surface albedo. Even small initial surface albedo reductions may have larger adjusted effects due to snow morphology changes and changes in the sublimation and snow melt rate. Most of the literature on the effect of BC on snow surface albedo is based on numerical models, and few in situ field measurements exist to confirm this reduction. Here we present an extensive set of concurrent in situ measurements of spectral surface albedo, BC concentrations in the upper 5 cm of the snowpack, snow physical parameters (grain size and depth), and incident solar flux characteristics from the Arctic. From this data set (with median BC concentrations ranging from 5 to 137 ng BC per gram of snow) we are able to separate the BC signature on the snow albedo from the natural snow variability. Our measurements show a significant correlation between BC in snow and spectral surface albedo. Based on these measurements, parameterizations are provided, relating the snow albedo, as a function of wavelength, to the equivalent BC content in the snowpack. The term equivalent BC used here is the elemental carbon concentration inferred from the thermo-optical method adjusted for the fraction of non-BC constituents absorbing sunlight in the snow. The first parameterization is a simple equation which efficiently describes the snow albedo reduction due to the equivalent BC without including details on the snow or BC microphysics. This can be used in models when a simplified description is needed. A second parameterization, including snow grain size information, shows enhanced correspondence with the measurements. The extracted parameterizations are valid for wavelength bands 400-900 nm, constrained for BC concentrations between 1 and 400 ng g(-1), and for an optically thick snowpack. The parameterizations are purely empirical, and particular focus was on the uncertainties associated with the measurements, and how these uncertainties propagate in the parameterizations. Integrated, the first parameterization (based only on the equivalent BC) gives a broadband (400-900 nm) snow albedo reduction of 0.004 due to 10 ng equivalent BC per gram of snow, while the effect is almost 5 times larger for BC concentrations 1 order of magnitude higher. The study shows that the reconstructed albedo from the second parameterization (including information on the snow grain size) corresponds better to the radiative transfer model Snow, Ice, and Aerosol Radiation albedo than the reconstructed albedo from the first parameterization (excluding grain size information).

Effects of an Arctic under‐ice bloom on solar radiant heating of the water column

The deposition of solar energy in the upper Arctic Ocean depends, among other things, on the composition of the water column. During the N-ICE2015 expedition, a drift in the Arctic pack ice north of Svalbard, an under-ice phytoplankton bloom was encountered in May 2015. This bloom led to significant changes in the inherent optical properties (IOPs) of the upper ocean. Mean values of total water absorption in the upper 20 m of the water column were up to 4 times higher during the bloom than prior to it. The total water attenuation coefficient increased by a factor of up to around 7. Radiative transfer modeling, with measured IOPs as input, has been performed with a coupled atmosphere-ice-ocean model. Simulations are used to investigate the change in depth dependent solar heating of the ocean after the onset of the bloom, for wavelengths in the region 350–700 nm. Effects of clouds, sea ice cover, solar zenith angle, as well as the average cosine for scattering of the ocean inclusions are evaluated. An increase in energy absorption in the upper 10 m of about 36% is found under 25 cm ice with 2 cm snow, for bloom conditions relative to pre-bloom conditions, which may have implications for ice melt and growth in spring. Thicker clouds and lower sun reduce the irradiance available, but lead to an increase in relative absorption. This article is protected by copyright. All rights reserved.