Melt Energy Research Articles

Characteristics of the binary faceted eutectic: benzoic acid – salicylic acid system

AbstractThe consistent symmetry relations computed from the heterogeneous nucleation data of the non‐ideal benzoic acid – salicylic acid eutectic system verifies the validity of nucleation theory. The kinetics of crystal growth from the molten state of the system follows the dislocation mechanism. Anomalous behaviour of both viscosity and activation energy for the eutectic melt confirms the essence of specific interactions animating molecular clusters rich in predominating eutectic phase. Micromorphology of the system obeys the Hunt‐Jackson model. The plot between a mechanical property and variable anisotropic growth velocity for the eutectic composite evidentially complies with the Weibull probability distribution curve. The curve is perused with two cut‐off points corresponding to a lower strength limit in the slow and fast growth regions and an upper strength limit in the moderate growth region. The latter aspect reveals the theoretical strength of the eutectic crystallites. The strength‐growth relationship explicated thereby is linear, optimum and linear respectively in the slow, moderate and fast growth regions of solidification. The moderate anisotropic growth (∼2.96 X 10‐7m3s‐1) is of greater interest by virtue of its attribute to unifying and organizing the crystallites parallel to each other in the growth direction. The eutectic composite material obtained by this process attains remarkable superiority in the mechanical properties over its isotropic growth in an ice bath (∼273 K), and its constituent phases. The co‐relation between excess thermodynamic functions indeed predicting the liquidus character and the microstructural parameters inevitably structuring the morphology is presented. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Calculation of sensible-heat flux over a melting ice surface using simple climate data and daily measurements of ablation

AbstractSensible-heat flux is obviously important for glacier ablation but is difficult to measure routinely. Sensible-heat flux can be estimated from wind-speed and temperature data using a dimensionless heat-transfer coefficient. Values of the heat-transfer coefficient are evaluated for six sites by correlating measured melt energy with a wind–temperature variable (product of daily mean wind speed, temperature and mean atmospheric pressure for the altitude in question). Data are available for short periods from two sites in Arctic Canada and two sites in North Greenland, and for hundreds of days of record at Nordbogletscher and Qamanârssûp sermia in South and West Greenland, respectively. Average transfer coefficients for four out of the six sites are close to 0.003, which is in reasonable agreement with values reported elsewhere, while larger values of 0.0047 and 0.0057 are found at the other two sites. Heat-transfer coefficients are also estimated on a monthly basis for the two long records, and substantial variations are found, suggesting that the method should not be used for <20–30 days of data. The present study is based on manually observed ablation and climate data, but the approach could be updated to use data from automatic recording stations using modern sensors.

Glacier melt, air temperature, and energy balance in different climates: The Bolivian Tropics, the French Alps, and northern Sweden

This study investigates the physical basis of temperature‐index models for three glaciers in contrasting climates: Zongo (16°S, 5050 m, Bolivian Tropics), St Sorlin (45°N, 2760 m, French Alps), and Storglaciären (67°N, 1370 m, northern Sweden). The daily energy fluxes were computed during melt seasons and correlated with each other and with air temperature on and outside the glacier. The relative contribution of each flux to the correlations between temperature and melt energy was assessed. At Zongo, net short‐wave radiation controls the variability of the energy balance and is poorly correlated to temperature. On tropical glaciers, temperature remains low and varies little, melt energy is poorly correlated to temperature, and degree‐day models are not appropriate to simulate daily melting. At the yearly scale, the temperature is better correlated to the mass balance because it integrates the ablation and the accumulation processes over a long time period. At Sorlin, the turbulent sensible heat flux is greater because of higher temperatures, but melt variability is still controlled by short‐wave radiation. Temperature correlates well with melt energy mainly through short‐wave radiation, probably because of diurnal advection of warm air from the valley. At Storglaciären, high correlations between temperature and melt energy result from substantial variability of the sensible and latent heat fluxes (which both supply energy to the glacier), and their good correlations with temperature. In the three climates, long‐wave irradiance is the main source of energy, but its variability is small and poorly correlated to the temperature mainly because cloud emissions dominate its variability.

Partitioning of melt energy and meltwater fluxes in the ablation zone of the west Greenland ice sheet

Abstract. We present four years (August 2003–August 2007) of surface mass balance data from the ablation zone of the west Greenland ice sheet along the 67° N latitude circle. Sonic height rangers and automatic weather stations continuously measured accumulation/ablation and near-surface climate at distances of 6, 38 and 88 km from the ice sheet margin at elevations of 490, 1020 and 1520 m a.s.l. Using a melt model and reasonable assumptions about snow density and percolation characteristics, these data are used to quantify the partitioning of energy and mass fluxes during melt episodes. The lowest site receives very little winter accumulation, and ice melting is nearly continuous in June, July and August. Due to the lack of snow accumulation, little refreezing occurs and virtually all melt energy is invested in runoff. Higher up the ice sheet, the ice sheet surface freezes up during the night, making summer melting intermittent. At the intermediate site, refreezing in snow consumes about 10% of the melt energy, increasing to 40% at the highest site. The sum of these effects is that total melt and runoff increase exponentially towards the ice sheet margin, each time doubling between the stations. At the two lower sites, we estimate that radiation penetration causes 20–30% of the ice melt to occur below the surface.

Spatial Variability of Shortwave Irradiance for Snowmelt in Forests

Abstract The spatial variation of melt energy can influence snow cover depletion rates and in turn be influenced by the spatial variability of shortwave irradiance to snow. The spatial variability of shortwave irradiance during melt under uniform and discontinuous evergreen canopies at a U.S. Rocky Mountains site was measured, analyzed, and then compared to observations from mountain and boreal forests in Canada. All observations used arrays of pyranometers randomly spaced under evergreen canopies of varying structure and latitude. The spatial variability of irradiance for both overcast and clear conditions declined dramatically, as the sample averaging interval increased from minutes to 1 day. At daily averaging intervals, there was little influence of cloudiness on the variability of subcanopy irradiance; instead, it was dominated by stand structure. The spatial variability of irradiance on daily intervals was higher for the discontinuous canopies, but it did not scale reliably with canopy sky view. The spatial variation in irradiance resulted in a coefficient of variation of melt energy of 0.23 for the set of U.S. and Canadian stands. This variability in melt energy smoothed the snow-covered area depletion curve in a distributed melt simulation, thereby lengthening the duration of melt by 20%. This is consistent with observed natural snow cover depletion curves and shows that variations in melt energy and snow accumulation can influence snow-covered area depletion under forest canopies.

Surface energy balance in the ablation zone of Midtdalsbreen, a glacier in southern Norway: Interannual variability and the effect of clouds

We present a record of almost six years of data (2000–2006) from an automatic weather station (AWS) in the ablation zone of Midtdalsbreen, a glacier in southern Norway. Measured incoming longwave radiation is used to estimate cloudiness, revealing that high cloud fractions occur almost 50% of the time in all seasons. Measured wind speeds and humidity are higher for cloudy conditions, especially in winter. Net solar radiation dominates the surface energy balance in summer, contributing on average 75% of the melt energy. The turbulent fluxes supply 35% of the melt energy while net longwave radiation and the subsurface heat flux are energy sinks of 8% and 2%, respectively. Although the melt rate is generally larger under clear skies, almost 60% of the melt occurs under cloudy skies, a consequence of the prevailing cloudy conditions. Interannual variability in the total melt is found to be equally determined by variations in the date of ice reappearance and differences in the meteorological conditions during melt. Comparing the results for Midtdalsbreen with measurements from an AWS on Morteratschgletscher, Switzerland reveals that the larger ice ablation on Morteratschgletscher primarily results from an earlier start of the melt season and larger net solar radiation. The energy balance model used in this study is found to be more sensitive to changes in the stability correction than to an order‐of‐magnitude change in the roughness length for momentum.

Assessing the controls of the snow energy balance and water available for runoff in a rain-on-snow environment

Summary Rain-on-snow (ROS) melt production and its contribution to water available for runoff is poorly understood. In the Pacific Northwest (PNW) of the USA, ROS drives many runoff events with turbulent energy exchanges dominating the snow energy balance (EB). While previous experimental work in the PNW (most notably the H.J. Andrews Experimental Forest (HJA)) has quantified these energy balance components for a handful of ROS events, little is known about the EB components of snowmelt at HJA on an annual basis and how the relative importance of each component changes with different time periods of analysis. Beyond the few measured events at HJA and elsewhere in the PNW, there is still a lack of understanding of the dominant components of the EB during high-frequency ROS events and how much annual snowmelt is produced during ROS events. A physically based snow energy balance model (SNOBAL) was applied to data from three climate stations in the HJA to address these questions. Measurements of all required forcing data except incoming longwave radiation were made at each site. We employ the largest ROS dataset ever amassed with SNOBAL to use the model as a learning tool to characterize the snowmelt regime in the HJA. The results show that radiation dominated the melt energy balance over the period 1996–2003 while net turbulent energy exchanges were much lower than expected. Annual variability in EB components reflected duration of snowpack (snow covered period) – where later season snowpack resulted in higher radiation as percentage of the total EB. Radiation was the largest contributor to melt during ROS. These results question the general perception of turbulent energy exchange dominance of ROS and seasonal melt in the PNW. Overall, melt from ROS events was a small percentage of annual melt – for the period 1996–2003 snow season, 80–90% of snowmelt comes from non-ROS days. These results prove the highly variable spatial and temporal controls on the snowmelt regime in the Pacific Northwest.

Hydroclimate controls over seasonal sediment yield in two adjacent High Arctic watersheds

AbstractInterannual variations in seasonal sediment transfer in two High Arctic non‐glacial watersheds were evaluated through three summers of field observations (2003–2005). Total seasonal discharge, controlled by initial watershed snow water equivalence (SWE) was the most important factor in total seasonal suspended sediment transfer. Secondary factors included melt energy, snow distribution and sediment supply. The largest pre‐melt SWE of the three years studied (2004) generated the largest seasonal runoff and disproportionately greater suspended sediment yield than the other years. In contrast, 2003 and 2005 had similar SWE and total runoff, but reduced runoff intensity resulted in lower suspended sediment concentrations and lower total suspended sediment yield in 2005. Lower air temperatures at the beginning of the snowmelt period in 2003 prolonged the melt period and increased meltwater storage within the snowpack. Subsequently, peak discharge and instantaneous suspended sediment concentrations were more intense than in the otherwise warmer 2005 season. The results for this study will aid in model development for sediment yield estimation from cold regions and will contribute to the interpretation of paleoenvironmental records obtained from sedimentary deposits in lakes. Copyright © 2007 John Wiley & Sons, Ltd.

Physico-chemical characteristics, nutritional quality and shelf-life of pearl millet based extrusion cooked supplementary foods

The process variables for extrusion cooking of pearl millet were standardized and some of the physicochemical characteristics of the millet extrudates and also the nutritional qualities of the millet and legume-based extruded supplementary foods were determined. The millet grits less than 355 µm in size, equilibrated to 18±1% moisture content, extruded at 150±5°C temperature and at 200±10 rpm of the barrel of a twin-screw extruder yielded the extrudates of 1.75±0.21 expansion ratio and 7.5±1.5 kg breaking strength. The cold and cooked paste viscosity, the melt energy and also the carbohydrate digestibility of the extrudates indicated that the products were pre-cooked and were of ready-to-eat nature. The millet was blended with grain legumes (30%) and also with defatted soy (15%) separately and extruded to prepare ready-to-eat nutritious foods suitable as food supplements to children and mothers. The foods based on the millet and legumes and also the millet and soy contained 14.7% and 16.0% protein with 2.0 and 2.1 protein efficiency ratio values, respectively. The shelf-life of the foods was about 6 months in different flexible pouches at ambient storage conditions. The study showed that applications of extrusion cooking technology to pearl millet have promise for preparation of diversified and value-added food products from the millet.

A Case Study of the Energy Budget of a Snowpack in the Arid, Subtropical Climate of the Southwestern United States

ABSTRACT An extensive micrometeorological and numerical modeling case study was conducted at Happy Jack, Arizona, during hydrologic year 2003. The goal of the study was to provide an initial assessment of the potential difference in the energy budget of a snow covered surface between an arid subtropical climate and more popularly studied regions. Modeling the snowpack evolution using the SNTHERM snowmelt model produced snow water equivalent values that were in good agreement with measured values for three different modeled ablation periods. The importance of the net radiation flux in ablation of the snow pack was decreased compared to that in more traditionally studied regions due to the earlier timing of ablation and subsequent lower solar altitude angles at the Arizona site. Consequently, the relative importance of the sensible heat flux increased and the relative importance of the latent heat flux decreased. The ground heat flux, ignored in most studies, accounted for up to 18% of the melt energy in th...

The synoptic climate controls on hydrology in the upper reaches of the Peace River Basin. Part II: Snow ablation

AbstractThe timing and rate of spring snowmelt in the upper reaches of the Peace River Basin control the development of large upstream spring flood waves and subsequent downstream ice‐jam flooding events. Given that snowmelt is controlled by atmospheric processes, this study identified the mid‐tropospheric circulation patterns that control the timing and rate of the snowmelt season on a critical tributary of the Peace River Basin.Results showed that the date of spring snowmelt initiation in the upper Peace has become significantly earlier over the study period (1963–1996). This was explained by a significant increasing (decreasing) trend in the occurrence of mid‐tropospheric ridging (troughing and zonal flow) patterns, which are associated with above (below) average mean daily temperatures and available melt energy at Grande Prairie. Further analysis revealed that variances in the timing and rate of melt at Grande Prairie could be explained by variances in the local synoptic regime.Examination of the broad‐scale atmospheric controls identified that the variances in the occurrence of the synoptic patterns could be explained by variances in the Pacific/North American teleconnection pattern. Although El Niño events were associated with average synoptic and snow ablation conditions, La Niña events were found to significantly impact the March synoptic regime over western Canada, and subsequently, the melt timing at Grande Prairie. Copyright © 2006 John Wiley & Sons, Ltd.

Spatial-Temporal Variability in Turbulent Fluxes during Spring Snowmelt

Turbulent sensible and latent heat exchanges play an important role in melting snow covers, contributing 30–40% of overall melt energy with daily values reaching over 50% on warm, cloudy days (Morris, 1989). The spatial variability of these turbulent fluxes across a basin and the relative importance of the differences is not well known. This paper specifically addresses small-scale variabilities in sensible and latent energy fluxes related to topographically induced wind speed variations. A simple wind model was used to simulate topographic effects on the surface wind field. Hourly wind observations were areally distributed by the model and used to calculate spatially variable sensible and latent turbulent heat fluxes for a small (63 km2) research catchment dominated by open tundra vegetation. Simulations showed that, even though the study area is characterized by relatively low relief (average slope 3°), the small-scale sensible and latent heat fluxes varied considerably throughout the basin. The resulting variations in snowmelt rates play an important role in the development of a patchy snow cover. Overall, turbulent fluxes within the research area varied by as much as 20% from the mean, leading to differences in potential snowmelt of up to 70 mm snow water equivalent over the entire melt period.

Modelling the spatial–temporal variability of spring snowmelt in an arctic catchment

Arctic spring landscapes are usually characterized by a mosaic of coexisting snow-covered and bare ground patches. This phenomenon has major implications for hydrological processes, including meltwater production and runoff. Furthermore, as indicated by aircraft observations, it affects land-surface–atmosphere exchanges, leading to a high degree of variability in surface energy terms during melt. The heterogeneity and related differences when certain parts of the landscape become snow free also affects the length of the growing season and the carbon cycle. Small-scale variability in arctic snowmelt is addressed here by combining a spatially distributed end-of-winter snow cover with simulations of variable snowmelt energy balance factors for the small arctic catchment of Trail Valley Creek (63 km2). Throughout the winter, snow in arctic tundra basins is redistributed by frequent blowing snow events. Areas of above- or below-average end-of-winter snow water equivalents were determined from land-cover classifications, topography, land-cover-based snow surveys, and distributed surface wind-field simulations. Topographic influences on major snowmelt energy balance factors (solar radiation and turbulent fluxes of sensible and latent heat) were modelled on a small-scale (40 m) basis. A spatially variable complete snowmelt energy balance was subsequently computed and applied to the distributed snow cover, allowing the simulation of the progress of melt throughout the basin. The emerging patterns compared very well visually to snow cover observations from satellite images and aerial photographs. Results show the relative importance of variable end-of-winter snow cover, spatially distributed melt energy fluxes, and local advection processes for the development of a patchy snow cover. This illustrates that the consideration of these processes is crucial for an accurate determination of snow-covered areas, as well as the location, timing, and amount of meltwater release from arctic catchments, and should, therefore, be included in hydrological models. Furthermore, the study shows the need for a subgrid parameterization of these factors in the land surface schemes of larger scale climate models. Copyright © 2005 John Wiley & Sons, Ltd.

Climatic Controls on Streamflow and Suspended Sediment Transport in Three Large Middle Arctic Catchments, Boothia Peninsula, Nunavut, Canada

Streamflow, suspended sediment transport, and meteorological variables were measured during two field seasons in order to determine the climatic controls on daily discharge and suspended sediment load for three large middle arctic catchments. Substantive suspended sediment transfer only occurred during the short-lived nival peak, and the duration of the peak appears to be broadly scaled with interannual catchment snow water equivalence (SWE). Thermal energy was critical in generating streamflow and suspended sediment transfer, but only until watershed snowpack had been exhausted. Thus, total annual suspended sediment load in this environment is ultimately a function of total discharge through SWE rather than melt energy. Specific sediment yields were some of the lowest recorded in the arctic, ranging between 0.2 and 1.9 t km−2·a−1. This study demonstrates the sensitivity middle arctic watersheds to both winter snowpack and spring thermal conditions and provides a basis for understanding the hydrological impact of future climate changes.

Controlled line shaped electron beam for silicon zone melting recrystallization on float glass substrates

A line shaped electron beam system for the zone melting recrystallization (ZMR) of thick silicon films on float glass substrates is described. The low thermal stability of the glass requires a rapid regulated zone heating. The electron beam system allows the ZMR in a continuous scan without pulsing. The setup consists of the tungsten wire with a Pierce reflector as line cathode and the moving substrate as anode. A negative voltage applied on the reflectors limits the electron emission and provides the necessary control. The melt energy for optimized processing decreases with an increase of the velocity. The beam profiles and penetration depth of the electrons is discussed. Homogenous polycrystalline silicon areas of several 10cm2 are achieved on a tungsten coated borosilicate float glass for the intended use as solar cell absorber.

Modeling of laser deposition and repair process

Laser aided deposition is a material additive based manufacturing process via metallurgically bonding the deposited material to the substrate. Due to its capability to bond various materials together, it becomes an attractive technology for part repair in small scale. However, the details of the process remain an active area for research because of the complicated interactions involved. In this study, a mathematical model was established to investigate thermal and mass transportation phenomena, which include substrate melting and solidification as well as the powder heating process. Affected by the powder injection, the melt pool surface keeps fluctuating during the process and dynamic evolution of the melt pool shape is tracked by using the level set method. Melt pool shape and energy balance at liquid–vapor and solid–liquid interfaces are taken into account in the model. A typical hole repair process based on material deposition technology is presented. The detailed process together with the temperature distribution were simulated. The simulation results of the melt pool length and peak temperature are validated for the laser deposition process through the measurements on the melt pool geometry and surface peak temperature by the complementary metal–oxide–semiconductor camera and dual-wavelength temperature sensor.

Analysis of meteorological data and the surface energy balance of McCall Glacier, Alaska, USA

AbstractWe report on analysis of meteorological data for the period 27 May–20 August 2004, from two automatic weather stations on McCall Glacier, Alaska, USA, aimed at studying the relationship between climate and ablation. One station is located on a mountain ridge and the other in the ablation area where we also analyzed the energy balance. The weather station on the glacier measured an average temperature of 5.3°C (at 2 m height above surface) and wind speed of 3.1 m s−1 (at 3 m height). A sonic height ranger and ablation stakes indicate a specific mass balance of –1.94 ± 0.09 m w.e between 15 June and 20 August. The specific mass balance calculated from the surface energy balance, –2.06 ± 0.18 m w.e., is in close correspondence to this. The latter is the sum of 0.12 m w.e. of snowfall, 0.003 m w.e. of deposition and –2.18 m w.e. of melt. Net radiation contributes 74% of the melt energy. Compared to ablation measurements in the early 1970s, summer ablation was large. This increase is explained by a combination of a relatively higher net radiation, a lower albedo and larger turbulent heat fluxes that led to more energy being available for melting. No single meteorological variable can be isolated as being the principal reason for the high ablation, however. The lower ice albedo (0.19) is possibly due to ash deposits from forest fires.

Greenland ice sheet surface mass balance 1991–2000: Application of Polar MM5 mesoscale model and in situ data

The Polar Pennsylvania State University–National Center for Atmospheric Research Fifth‐Generation Mesoscale Model (Polar MM5) regional climate model was run over the North Atlantic region for 1991–2000. We analyze 24‐km output over the Greenland ice sheet to evaluate spatial and temporal variability of the surface mass balance and its subcomponents. The model output is compared with 3 years of automatic weather station (AWS) data from 17 sites to identify biases. Using the in situ data, we derive simple corrections for biases in melt energy and in water vapor fluxes from the surface and from blowing snow. The simulated accumulation rate is in agreement with AWS and snow pit observations. Estimates for runoff and the surface mass balance distribution over the ice sheet are produced using modeled melt volume and a meltwater retention scheme. From the decade investigated, the magnitude of interannual variability in surface mass balance components is tentatively established. The largest variability is concentrated along the ice sheet margin, where both accumulation and ablation rates are largest. The simulated interannual fluctuations suggest a large absolute variability, ±187 km3 yr−1 for total ice sheet surface mass balance. Variability in simulated equilibrium line altitude is suggestive of a dominance of thermal variability in the south with increasing importance of accumulation variability with increasing latitude. Empirical functions for the sensitivity of surface mass balance to temperature and precipitation anomalies are presented. The precise locations and regions of maximum and minimum surface energy and mass fluxes are suggested. Using an estimate for iceberg discharge and bottom melting, the total ice sheet mass balance is estimated be −78 km3 yr−1, producing 2.2 mm of eustatic sea level rise over the 1991–2000 decade and contributing 15% to the observed (1.5 mm yr−1) global sea level rise. The more negative mass balance is attributed to including blowing‐snow sublimation loss and to regional warming in the 1990s.

Snowmelt induced stemflow in northern hardwood forests: a theoretical explanation on the causation of a neglected hydrological process

Snowmelt induced stemflow is an under-studied and poorly understood hydrological process. The aim of the present manuscript is to set forth several testable hypotheses and conceptual models concerning the production of snowmelt induced stemflow. The first testable hypothesis is that tree species with lower canopy albedos of snow-covered and snow-free surfaces (portions of the canopy are usually exposed and snow-free while the remainder is covered) will generate larger snowmelt induced stemflow volumes than trees with higher canopy albedos of snow-covered and snow-free surfaces. It is also hypothesized that snow-to-rain events produce larger volumes of snowmelt induced stemflow than snow events of the same magnitude because of differences in precipitation type (snow-to-rain vs. snow) and the additional melt energy from the latent heat of fusion associated with rain infiltrating the intercepted snow mass. For deciduous tree species with similar branching geometries, it is hypothesized that larger longwave radiative flux will generate larger snowmelt induced stemflow volumes. The conceptual models describe the components and the flux directions necessary to generate snowmelt induced stemflow. Through a test of the hypotheses and conceptual models in subsequent work, it is hoped that quantitative models are developed that will be able to estimate snowmelt induced stemflow volumes and yield a new understanding of winter water flux in forested ecosystems.

Implications of spatial distributions of snow mass and melt rate for snow-cover depletion: theoretical considerations

AbstractSnow becomes patchy while melting, giving a heterogeneous surface with large contrasts in characteristics such as albedo. It is therefore necessary for surface schemes used in atmospheric and hydrological models to represent the influence of heterogeneities in snow cover on scales smaller than the model grid; this is typically done by introducing a snow-cover depletion (SCD) curve relating fractional snow cover to average depth. Here, ad hoc functional forms which have been used for this relationship are first compared with forms obtained for an assumed initial snow distribution undergoing homogeneous melt. The energy available for melt may be heterogeneous, however, and can be influenced by factors that also influence snow distribution, such as slope, aspect and vegetation cover. The modification of SCD curves by spatial variations in melt energy is investigated, and resulting curves are found to be similar to those which would be given by homogeneous melting of a snow cover with greater initial variance. This influence is increased by any negative correlation between snow depth and melt rate.