Carl S. Benson

Vapor transport, grain growth and depth-hoar development in the subarctic snow

Measurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow, the fundamental processes of dry-snow metamorphism. Due to extreme temperature gradients, the subarctic pack undergoes extensive depth-hoar metamorphism. By the end of the winter a five-layered structure with a pronounced weak layer near the base of the snow evolves. Grain-size increases by a factor of 2-3, while the number of grains per unit mass decreases by a factor of 10. Observed growth rates require significant net inter-particle vapor fluxes. Stable-isotope ratios show that there are also significant net layer-to-layer vapor fluxes. Soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow, while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack. End-of-winter density profiles for snow on the ground, compared with snow on tables, indicate the net layer-to-layer vapor flux averages 6 x 10 -7 kg m 2 s -1, , though detailed measurements show the net flux is episodic and varies with time and height in the pack, with peak net fluxes ten times higher than average. A model, driven by observed temperature profiles, reproduces the layer-to-layer flux pattern and predicts the observed weak layer at the base of the snow. Calculated layer-to-layer vapor fluxes are ten times higher than inter-particle fluxes, which implies that depth-hoar grain growth is limited by factors other than the vapor supply. This finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow, and it explains why grains can metamorphose into different forms so readily.

Assessment of Snow-Cover Mapping Accuracy in a Variety of Vegetation-Cover Densities in Central Alaska

Field and aircraft measurements were acquired in April 1995 in central Alaska to map snow cover with MODIS Airborne Simulator (MAS) data, acquired from high-altitude aircraft. The Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS) is a 36-channel system that will be launched on the EOS-AM-1 platform in 1999. A vegetation-density map derived from integrated reflectances (Ri), from MAS data, is compared with an independently-produced vegetation type and density map derived from Thematic Mapper (TM) and ancillary data. The maps agreed to within 13%, thus corroborating the effectiveness of using the reflectance technique for mapping vegetation density. Snow cover was mapped on a 13 April 1995 MAS image, using the original MODIS prototype algorithm and an enhanced MODIS prototype algorithm. Field measurements revealed that the area was completely snow covered. With the original algorithm, snow was mapped in 96% of the pixels having <50% vegetation-cover density according to the Ri map, while in the areas having vegetation-cover densities ⩾50%, snow was mapped in only 71% of the pixels. When the enhanced MODIS snow-mapping algorithm was employed, 99% of the pixels having <50% vegetation-cover density were mapped, and 98% of the pixels with ⩾50% vegetation-cover density were mapped as snow covered. These results demonstrate that the enhanced algorithm represents a significant improvement over the original MODIS prototype algorithm especially in the mapping of snow in dense vegetation. The enhanced algorithm will thus be adopted as the MODIS at-launch snow-cover algorithm. Using this simple method for estimating vegetation density from pixel reflectance, it will be possible to analyze the accuracy of the MODIS snow-cover algorithm in a range of vegetation-cover in places where information on vegetation-cover density is not available from ground measurements.

Analysis of glacier facies using satellite techniques

The different snow and ice types on a glacier may be subdivided according to the glacier-facies concept. The surticial expression of some facies may be detected at the end of the balance year by the use of visible and near-infrared image data from the Landsat multispectral scanner (MSS) and thematic mapper (TM) sensors. Ice and snow can be distinguished by reflectivity differences in individ ual or ratioed TM bands on Bruarj6kull, an outlet glacier on the northern margin of the Vatnajokull ice cap, Iceland. The Landsat scene shows the upper limit of wet snow on 24 August 1986. Landsat-derived reflectance is lowest for exposed ice and increases markedly at the transient snow line. Above the slush zone is a gradual increase in near-infrared reflectance as a result of decreasing grain-size of the snow, which characterizes drier snow. Landsat data are useful in measuring the areal extent of the ice facies, the slush zone within the wet-snow facies, the snow facies (combined wetsnow, percolation and dry-snow facies), and the respective positions of the transient snow line and the slush limit . In addition, fresh snowfall and/or airborne contaminants, such as soot and tephra, can limit the utility of Landsat data for delineation of the glacier facies in some cases. INTRODUCTION data enables some of these changes to be observed and measured. The ablation area and the accumulation area of a glacier are separated by the equilibrium line. This is the line across a valley or outlet glacier, or an irrregular line roughly parallel to the margin of an ice cap or ice sheet, where the net mass balance for the glacier equals zero. For many valley and outlet glaciers in temperate climates, the position of the snow line (tirn limit) at the end of an average mass-balance year represents the approximate location of the equilibrium line. For more polar glaciers, however, a superimposed ice zone exists between the snow line (firn line) and the equilibrium line. In this zone, ice has formed from water percolating through the wet snow and refrozen on the underlying glacier ice (Wakahama and others, 1976; Paterson, 1981). In these cases the equilibrium line is displaced down-glacier from the snow line (firn line). The surficial expression of a glacier changes throughout the year in response to changing meteorological conditions, especially the type and amount of precipitation and air temperature. The use of satellite 120 Although the annual mass balance of a glacier is normally determined from direct field or aerial measurements and observations, by photogrammetric, hydrological or reconnaissance methods (Paterson, 1981), the cost and effort of acquiring the data have meant that massbalance measurements are done on an annual basis for only a few glaciers. Meier (1984) was able to find only 25 glaciers worldwide that had mass-balance and volumechange data available for part or all of the past 50 years. The global coverage provided by satellites provides an opportunity for development of methods for measuring the mass balance of the Earths glaciers. After the first Landsat satellite was placed into orbit in July 1972, remotely sensed data have been assessed by various investigators for their application to the determination of glacier mass balance, beginning with Krimmel and Meier (1975) and 0strem (1975) . Although the multispectral scanner (MSS) and thematic mapper (TM) sensors are useful, research has shown that many factors https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0022143000042878 Downloaded from https://www.cambridge.org/core. IP address: 54.191.40.80, on 17 Sep 2017 at 01:03:27, subject to the Cambridge Core terms of use, available at Williams and others: Ana(ysis of glacier facies

Structure and wind transport of seasonal snow on the Arctic slope of Alaska

The invention provides a marine winch driven by a central operating spindle coupled at its lower end to drive reduction gearing in turn coupled to an outer casing of the winch to rotate said casing at one of two alternative output ratios selectable on rotating the operating spindle in opposite directions. The operating spindle has a separate upper portion movable vertically relative to a lower portion and in splined driving engagement therewith. When the upper part of the operating spindle is lowered it is coupled directly to drive the outer casing so as to provide a third, 1:1, output ratio. Unidirectional stepless clutch means are provided in the drive lines to enable the three output ratios to be acheived and also to enable the outer casing to over run.

Scales of spatial heterogeneity for perennial and seasonal snow layers

Abstract Local observations of snow layers are used as the basis for spatial extrapolation of snow properties and for establishing a time record of snow deposition, yet significant lateral variations in layer thickness, density and microstructure are well documented. Here we examine the nature of layer heterogeneity over distances of 10– 100 000 m using data from primarily flat locations in Alaska, Antarctica and Greenland. We find that at a scale of 10 m or less, perennial snow layers on glaciers and ice sheets are more uniform and laterally continuous than seasonal layers, which, in addition to heterogeneity introduced by wind and water percolation, are also affected by local topography and vegetation. At a scale of about 100 m, heterogeneity of seasonal and perennial snow layers converges and approaches a peak value. At larger scales (103–105m), local (order 100 m) forcing continues to produce most of the layer heterogeneity, with synoptic-scale variations adding small amounts. Cross-correlation at these larger scales is based on recognizing distinctive layer sequences or matching a few key layers of snow. Many layers cannot be correlated because they pinch out or change at scales (i.e. 100 m) smaller than the spacing between snow pits.

Comparison of in situ and Landsat derived reflectance of Alaskan glaciers

Abstract Calculation of snow and ice albedo is necessary for developing physically realistic simulations of the Earths climate using general circulation models, and useful for analysis of glacier mass balance change using remote sensing. Reflectances calculated from TM data and corrected for atmospheric effects correspond with in situ measured reflectances in the nadir-viewing mode, and are shown to be related to a glaciers mass balance if measured over a period of years. A reflectance of 0.895 for a test site in the Wrangell Mountains, Alaska, was calculated from TM Band 4 (0.76–0.90 μm) data and corrected for atmospheric effects. This value was comparable to the in situ reflectance of 0.90 measured in the same 0.76–0.90 μm wavelength region. For the same site, a reflectance value of 0.79 derived from integrating over most (0.40–3.0 μm) of the reflective portion of the electromagnetic spectrum was quite different from the integrated reflectance of 0.95 calculated for the spectral range 0.40–1.0 μm. This demonstrates the importance of using the full reflective energy spectrum for calculating the albedo of snow, and for obtaining a meaningful computation of a glaciers energy and mass balance change.

Characteristics and Growth of a Snowdrift in Arctic Alaska, U.S.A

In arctic Alaska, 15% of the total winter snowpack is contained in large drifts. Stratigraphic sections reveal that these can form during as few as five weather events during winter, while comparison of stratigraphy and weather records show that significant deposition (up to 43% of the total drift volume) can occur during a single event of short duration (<72 h). Based on three years of wind, snowfall, and snow transport records, a set of rules was developed for predicting when periods of drift growth would occur. The rules were: 10-m wind speed >5.3 m s−1 for at least 3 h, wind direction within 30° of the normal to drift trap axis, and recent snowfall available for transport. When used, these rules successfully identified all drift-growth events, plus a few “extra” events that did not contribute substantially to drift growth. The extra events were invariably periods when there was sufficient wind to move snow, but insufficient snow for transport. In arctic Alaska drift size currently appears to be limited by precipitation rather than wind, leading us to speculate that an increase in precipitation could increase drift size and intensify the ecological, hydrological, and climatic impact of drifts on this arctic system.

Observations of Pronounced Greenland Ice Sheet Firn Warming and Implications for Runoff Production

Field measurements of shallow borehole temperatures in firn across the northern Greenland ice sheet are collected during May 2013. Sites first measured in 1952–1955 are revisited, showing long-term trends in firn temperature. Results indicate a pattern of substantial firn warming (up to +5.7°C) at midlevel elevations (1400–2500 m) and little temperature change at high elevations (>2500 m). We find that latent heat transport into the firn due to meltwater percolation drives the observed warming. Modeling shows that heat is stored at depth for several years, and energy delivered from consecutive melt events accumulates in the firn. The observed warming is likely not yet in equilibrium with recent melt production rates but captures the progression of sites in the percolation facies toward net runoff production.

Seasonality of snow accumulation at Mount Wrangell, Alaska, USA

We recorded the burial times of temperature sensors mounted on a specially constructed tower to determine snow accumulation during individual storms in the summit caldera of Mount Wrangell, Alaska, USA, (628 N, 1448 W; 4100 m a.s.l.) during the accumulation year June 2005 to June 2006. The experiment showed most of the accumulation occurred in episodic large storms, and half of the total accumulation was delivered in late summer. The timing of individual events correlated well with storms recorded upwind, at Cordova, the closest Pacific coastal weather station (200 km south- southeast), although the magnitude of events showed only poor correlation. Hence, snow accumulation at Mount Wrangell appears to be a reflection of synoptic-scale regional weather systems. The accumu- lation at Mount Wrangells summit (>2.5 m w.e.) exceeded the precipitation at Cordova. Although the direct relationship between accumulation of individual storms at the summit of Mount Wrangell and precipitation events at Cordova may be unique in the region, it is useful for interpreting ice cores obtained on Mount Wrangell. This is especially the case here because the high rate of accumulation allows high time resolution within the core.

Glacier-volcano interactions in the North Crater of Mt Wrangell, Alaska

Abstract Glaciological and related observations from 1961 to 2005 at the summit of Mt Wrangell (62.00° N, 144.02°W; 4317m a.s.l.), a massive glacier-covered shield volcano in south-central Alaska, show marked changes that appear to have been initiated by the Great Alaska Earthquake (Mw = 9.2) of 27 March 1964. The 4×6 km diameter, ice-filled Summit Caldera with several post-caldera craters on its rim, comprises the summit region where annual snow accumulation is 1–2m of water equivalent and the mean annual temperature, measured 10 m below the snow surface, is –20°C. Precision surveying, aerial photogrammetry and measurements of temperature and snow accumulation were used to measure the loss of glacier ice equivalent to about 0.03 km3 of water from the North Crater in a decade. Glacier calorimetry was used to calculate the associated heat flux, which varied within the range 20–140Wm–2; total heat flow was in the range 20–100MW. Seismicity data from the crater’s rim show two distinct responses to large earthquakes at time scales from minutes to months. Chemistry of water and gas from fumaroles indicates a shallow magma heat source and seismicity data are consistent with this interpretation.