Energy fluxes and melt rate of a seasonal snow cover in the Moroccan High Atlas

ABSTRACT

In this study, we used an energy balance model and two simple methods based on readily available data to identify the processes driving the point-scale energy and mass balance of the snowpack. Data were provided from an experimental site located at 3200 m. All models were evaluated by comparing observed and modelled snow water equivalents. Performances are variable from one season to the next and the energy balance model gives better results (mean of root mean square error, RMSE = 25 mm and r² = 0.90) than the two simplified approaches (mean of RMSE = 54 mm and r² = 0.70). There are significant amounts of snow sublimation but they are highly variable from season to season, depending on wind conditions (between 7 and 20% of the total). While the main source of energy for melting is net radiation, the amount of heat brought by sensible heat flux is significant for two of the most windy snow seasons.

Editor Z.W. Kundzewicz Associate editor not assigned     

Wake effects on turbulent transport in the convective boundary layer

The effect of the downstream propagation of a wake on the transport of momentum, energy and scalars (such as humidity) in the convective boundary layer (CBL) is studied using a direct numerical simulation. The incompressible Navier–Stokes and energy equations are integrated under neutral and unstable thermal stratification conditions in a rotating coordinate frame with the Ekman layer approximation. Wake effects are introduced by modifying the mean velocity field as an initial condition on a converged turbulent Ekman layer flow. With this initial velocity distribution, the governing equations are integrated in time to determine how turbulent transport in the CBL is affected by the wake. Through the use of Taylor’s hypothesis, temporal evolution of the flow field in a doubly periodic computational domain is transformed into a spatial evolution. The results clearly indicate an increase in the scalar flux at the surface for the neutrally stratified case. An increase in wall scalar and heat flux is also noted for the CBL under unstable stratification, though the effects are diminished given the enhanced buoyant mixing associated with the hot wall.     

Variable and conflicting shear stress estimates inside a boundary layer with sediment transport

This paper presents a comparison between two methods for estimating shear stress in an atmospheric internal boundary layer over a beach surface under optimum conditions, using wind velocities measured synchronously at 13 heights over a 1.7 m vertical array using ultrasonic anemometry. The Reynolds decomposition technique determines at‐a‐point shear stresses at each measurement height, while the Law‐of‐the‐Wall yields a single boundary layer estimate based on fitting a logarithmic velocity profile through the array data. Analysis reveals significant inconsistencies between estimates derived from the two methods, on both a whole‐event basis and as time‐series. Despite a near‐perfect fit of the Law‐of‐the‐Wall, the point estimates of Reynolds shear stress vary greatly between heights, calling into question the assumed presence of a constant stress layer. A comparison with simultaneously measured sediment transport finds no relationship between transport activity and the discrepancies in shear stress estimates. Results do show, however, that Reynolds shear stress measured nearer the bed exhibits slightly better correlation with sand transport rate. The findings serve as a major cautionary message to the interpretation and application of single‐height measurements of Reynolds shear stress and their equivalence to Law‐of‐the‐Wall derived estimates, and these concerns apply widely to boundary layer flows in general. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Controls of alluvial cover formation,morphology and bedload transport in a sinuous channel with a non-alluvial boundary

The alluvial cover in channels with non-alluvial beds is a major morphologic feature in these rivers and has important geomorphic and ecologic functions. Although controls on the extent of the alluvial cover have been previously researched, little is known about the role of channel meanders in shaping the three-dimensional morphology and bedload transport rates in these rivers. Flume experiments were conducted in a fixed-bed sinuous channel scaled from an engineered urban river. A fully graded sediment supply mixture was fed into the bare channel at rates ranging between 0.3 and 1.2 times the estimated channel capacity under constant discharge. The three-dimensional morphology and surface texture of the alluvial cover were captured using photogrammetry, and the sediment output was periodically measured and sieved. A stable alluvial cover was achieved under all sediment supply conditions that coincided with a sediment transport equilibrium. The sediment supply rate controlled the final areal extent, mass and volume of the alluvial cover, while cover developed as a periodic series of stable bars ‘fixed’ by the channel planform. The alluvial cover development followed consistent trajectories relative to angular position around bends but developed to a greater degree and higher elevation with increasing sediment supply. The stable cover extent had a logarithmic relationship with the relative sediment supply, while the final mass, volume and bar height had linear relationships. The final channel morphology was characterized by fine-textured point bars with flat tops and steep margins connected by coarse riffle features. The outside of banks between bend apexes remained bare, even at sediment supply conditions exceeding the channel capacity. The length of the exposed outer banks followed predictable linear relationships with the total cover extent. Insights from this study can provide guidance for the management of channels with non-alluvial boundaries and provide validation for models of sinuous bedrock channel abrasion. © 2020 John Wiley & Sons, Ltd.     

An analysis of snow cover patterns in a small alpine catchment

Snow cover patterns in a 9.4 km² basin in the Austrian Alps are examined during spring and summer 1989. Digital mono-plotting from oblique aerophotographs is used for mapping. on the basis of a square grid with 25 m spacing, snow cover as mapped during nine surveys is analysed as a function of elevation and slope. During winter conditions the snow cover is found to be much better related to these terrain features than during the late ablation period.     

Some effects of intra-red radiation on the dynamics of the atmospheric boundary layer

An approximate infra-red radiation scheme, employing essentially the cooling to space approximation, is included in a one-dimensional model of the atmospheric boundary layer. The approximate scheme is found not to produce significant errors in the behaviour of the dynamical model when integrated over a few hours. Radiative cooling is shown to be important in the development of a clear well-mixed layer which is capped by an essentially dry region; in particular, convetive instability is induced and this enhances the rate of entrainment of dry air. The development of fog is found to require sound models of both radiative transfer and turbulent diffusion.     

Trace element and major ion concentrations and dynamics in glacier snow and melt: Eliot Glacier,Oregon Cascades

We present concentrations of environmentally available (unfiltered acidified 2% v/v HNO₃) As, Cu, Cd, Pb, V, Sr, and major ions including Ca²⁺, Cl^?, and SO₄^2? in a July 2005 and a March 2006 shallow snow profile from the lower Eliot Glacier, Mount Hood, Oregon, and its proglacial stream, Eliot Creek. Low enrichment factors (EF) with respect to crustal averages suggests that in fresh March 2006 snow environmentally available elements are derived primarily from lithogenic sources. Soluble salts occurred in lower and less variable concentrations in July 2005 snow than March 2006. Conversely, environmentally available trace elements occurred in greater and more variable concentrations in July 2005 than March 2006 snow. Unlike major solutes, particulate‐associated trace elements are not readily eluted during the melt season. Additionally, elevated surface concentrations suggest that they are likely added throughout the year via dry deposition. In a 1‐h stream sampling, ratios of dissolved (<0·45 µm) V:Cl^?, Sr:Cl^?, and Cu:Cl^? are enriched in the Eliot Stream with respect to their environmentally available trace element to Cl^? ratios in Eliot Glacier snow, suggesting chemical weathering additions in the stream waters. Dissolved Pb:Cl^? is depleted in the Eliot Stream with respect to the ratio of environmentally available Pb to Cl^? in snow, corresponding to greater adsorption onto particles at greater pH values. Copyright © 2009 John Wiley & Sons, Ltd.     

The implications of permafrost thaw and land cover change on snow water equivalent accumulation,melt and runoff in discontinuous permafrost peatlands

In the discontinuous permafrost zone of the Northwest Territories (NWT), Canada, snow covers the ground surface for half the year. Snowmelt constitutes a primary source of moisture supply for the short growing season and strongly influences stream hydrographs. Permafrost thaw has changed the landscape by increasing the proportional coverage of permafrost-free wetlands at the expense of permafrost-cored peat plateau forests. The biophysical characteristics of each feature affect snow water equivalent (SWE) accumulation and melt rates. In headwater streams in the southern Dehcho region of the NWT, snowmelt runoff has significantly increased over the past 50 years, despite no significant change in annual SWE. At the Fort Simpson A climate station, we found that SWE measurements made by Environment and Climate Change Canada using a Nipher precipitation gauge were more accurate than the Adjusted and Homogenized Canadian Climate Dataset which was derived from snow depth measurements. Here, we: (a) provide 13 years of snow survey data to demonstrate differences in end-of-season SWE between wetlands and plateau forests; (b) provide ablation stake and radiation measurements to document differences in snow melt patterns among wetlands, plateau forests, and upland forests; and (c) evaluate the potential impact of permafrost-thaw induced wetland expansion on SWE accumulation, melt, and runoff. We found that plateaus retain significantly (p < 0.01) more SWE than wetlands. However, the differences are too small (123 mm and 111 mm, respectively) to cause any substantial change in basin SWE. During the snowmelt period in 2015, wetlands were the first feature to become snow-free in mid-April, followed by plateau forests (7 days after wetlands) and upland forests (18 days after wetlands). A transition to a higher percentage cover of wetlands may lead to more rapid snowmelt and provide a more hydrologically-connected landscape, a plausible mechanism driving the observed increase in spring freshet runoff.     

Representation of canopy snow interception,unloading and melt in a parsimonious snowmelt model

Recent improvements in the Utah Energy Balance (UEB) snowmelt model are focused on snow–vegetation–atmosphere interactions to understand how different types of vegetation affect snow processes in the mountains of Western USA. This work presents field work carried out in the Rocky Mountains of Northern Utah to evaluate new UEB model algorithms that represent the processes of canopy snow interception, sublimation, mass unloading and melt. Four years' continuous field observations showed generally smaller accumulations of snow beneath the forest canopies in comparison with open (sage and grass) areas, a difference that is attributed to interception and subsequent sublimation and redistribution of intercepted snow by wind, much of it into surrounding open areas. Accumulations beneath the denser forest (conifer) canopies were found to be less than the accumulation beneath the less dense forest (deciduous) canopies. The model was able to represent the accumulation of snow water equivalent in the open and beneath the deciduous forest quite well but without accounting for redistribution tended to overestimate the snow water equivalent beneath the conifer forest. Evidence of redistribution of the intercepted snow from the dense forest (i.e. conifer forest) to the adjacent area was inferred from observations. Including a simple representation of redistribution in the model gave satisfactory prediction of snow water equivalent beneath the coniferous forest. The simulated values of interception, sublimation and unloading were also compared with previous studies and found in agreement. Copyright © 2013 John Wiley & Sons, Ltd.     

Relative importance of snow accumulation and monsoon rainfall data for estimating annual runoff,Jhelum basin,Pakistan

Abstract

The relative importance of data on winter snow accumulation and summer (monsoon) rainfall for estimating annual runoff in the Jhelum River basin, Punjab Himalaya, Pakistan, has been investigated. Strong correlations were found between point measurements of the annual maximum of snowpack water equivalent and of total winter precipitation in the Kunhar sub-basin, and total annual discharge. In addition, total winter snowfall showed a generally significant correlation with annual discharge. Elevation did not appear to play a strong role in determining the usefulness of these measurements, whereas location within the basin relative to large scale precipitation patterns did, in some cases. Monsoon rainfall appeared to be a very poor indicator of annual discharge. The results also suggest that the operation of a continental scale negative feedback mechanism between Eurasian snow cover and the Indian monsoon might be felt in this region of the Himalaya.     

Effects of frozen soil and snow cover on cold‐season soil water dynamics in Tokachi,Japan

Despite the potential impact of winter soil water movements in cold regions, relatively few field studies have investigated cold‐season hydrological processes that occur before spring‐onset of snowmelt infiltration. The contribution of soil water fluxes in winter to the annual water balance was evaluated over 5 years of field observations at an agricultural field in Tokachi, Hokkaido, Japan. In two of the winters, soil frost reached a maximum depth of 0·2 m (‘frozen’ winters), whereas soil frost was mostly absent during the remaining three winters (‘unfrozen’ winters). Significant infiltration of winter snowmelt water, to a depth exceeding 1·0 m, occurred during both frozen and unfrozen winters. Such infiltration ranged between 126 and 255 mm, representing 28–51% of total annual soil water fluxes. During frozen winters, a substantial quantity of water (ca 40 mm) was drawn from deeper layers into the 0–0·2 m topsoil layer when this froze. Under such conditions, the progression and regression of the freezing front, regulated by the thickness of snow cover, controlled the quantity of soil water flux below the frozen layer. During unfrozen winters, 13–62 mm of water infiltrated to a depth of 0·2 m, before the spring snowmelt. These results indicate the importance of correctly evaluating winter soil water movement in cold regions. Copyright © 2010 John Wiley & Sons, Ltd.     

The effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer

The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively). © 1998 John Wiley & Sons, Ltd.     

边界层参数化方案对边界层热力和动力结构特征影响的比较

本文利用高分辨率中尺度WRF模式,通过改变边界层参数化方案进行多组试验,评估该模式对美国北部森林地区边界层结构的模拟能力,同时比较了五种不同边界层参数化方案模拟得出的边界层热力和动力结构.结果表明：除个别方案外,配合不同边界层方案的WRF模式都能成功模拟出白天对流边界层强湍流混合特征和夜间稳定边界层内强逆温、逆湿和低空急流等热力和动力结构.非局地YSU、ACM2方案在白天表现出强的湍流混合和卷夹,相比于局地MYJ、UW方案,模拟的对流边界层温度更高、湿度更低、混合层高度更高、感热通量更大,更接近实际观测,这表明在不稳定层结下考虑非局地大涡输送更为合理,但局地方案在风速和风向的预报上存在一定优势.TEMF方案得到的白天局地湍流混合强度为所有方案中最弱,混合层难以发展,无法体现对流边界层内气象要素垂直分布均匀的特点.对于夜间稳定边界层的模拟,不同参数化方案之间的差异较小,但是YSU方案在一定程度上高估了机械湍流,导致局地湍流混合偏强,从而影响了其对稳定边界层的模拟能力.     

Organic matter in snow cover in the influence zone of emissions from a pulp-and-paper mill

Qualitative and quantitative analysis of organic components of snow cover is carried out in the influence zone of emissions from a pulp-and-paper mill. The contribution of organic compounds to the pool of emissions of chemical pollutants during the period of snow cover formation is assessed. Criteria for establishing the pollution of natural complexes by organic substances in the influence zone of emissions from a pulp-and-paper mill are suggested.     

人为热源对城市边界层结构影响的数值模拟研究

用南京大学多尺度模式系统在不同区域进行了多种人为热源引入方案的研究,结果表明：考虑时空变化的人为热源按比例分别引入到地表能量平衡方程和大气热量守恒方程是将人为热源引入模式的最优方案.人为热通量密度变化的敏感性试验结果发现：人为热源的存在对城市热岛的生成有重要作用.计算结果表明,南京现有的人为热源排放量对该地区的城市热岛贡献率约为29.6％,若人为热通量密度在现有量值的基础上增大1倍,则其热岛贡献率可达42.9％;此外,人为热的排放对清晨城市边界层逆温结构有一定程度的破坏作用,能明显升高夜间近地层气温达0.5～1.0℃,并能使白天湍流活动的影响范围增大,混合层高度抬高,使夜间城市热岛环流的影响范围扩大.     

Assessment of temporal dynamics of snow cover and its validation with hydro-meteorological data in parts of Chenab Basin,western Himalayas

The present investigation was conducted to analyze the temporal patterns of snow cover area%(SCA%), air temperature, snowfall and river discharge in parts of Chenab basin, western Himalayas. The relationship of mean SCA% with mean air temperature and river discharge was also tested using Pearson's product-moment correlation at 95% confidence limit and further sensitivity analysis of river discharge to SCA and SCA to air temperature was performed. Moderate Resolution Imaging Spectroradiometer(MODIS) 8-day surface reflectance product MOD09A1 was used to delineate SCA during the period 2000–2013. Moreover, variation in the lowest elevation from where snow cover area starts(LESCA) was also analyzed and its relationship with mean air temperature was also studied. Non-parametric method, Mann-Kendall test was employed to determine the trend in the SCA%, air temperature, snowfall and river discharge. The investigation carried out for three meteorological stations i.e. Batote, Reasi and Tandi revealed significant findings. At Batote and Reasi, statistically significant decreasing trends were observed over the period 2000 to 2012, for maximum, minimum and mean air temperature. Mean minimum SCA% exhibited a significant upward trend during 2000–2013 which is corroborated by the significantly increasing trend of mean annual snowfall(Tandi station) from 2000 to 2010. Further, significant decreasing trend of river discharge for the winter season at Batote station from 2000 to 2011 and decreasing trends in the maximum, minimum and mean air temperature at Batote and Reasi stations are also consistent with the increasing trend of SCA%. At both Batote and Reasi stations, mean SCA% exhibited significant negative relationship with the mean air temperature. On the other hand, LESCA exhibited positive correlation with the mean air temperature except in a few months, where negative relationship was seen. Sensitivity analysis of river discharge to SCA exhibited very low values of sensitivity coefficient in most of the months, indicating less sensitivity of river discharge to SCA. On the other hand, sensitivity coefficient of SCA to air temperature exhibited comparatively higher values which indicate SCA is more sensitive to air temperature.     

Chemical composition of snow cover in the taiga zone of the Komi Republic

Quantitative chemical analysis of snow cover were carried out in background territories of the southern, middle, and northern taiga of Komi Republic. The snow cover in the taiga zone of the northeastern European Russia (Komi Republic) shows low mineralization and acid pH. The acidity of melt water is caused by the predominance of strong mineral acids and deficiency of neutralizing compounds. A statistically reliable latitudinal differentiation was revealed in the distribution of macro- and microelements in the snow from the south to the north.     

Winter methane dynamics beneath ice and in snow in a temperate poor fen

The influence of winter on methane (CH₄) stored in pore water and emitted through snow was investigated in a temperate poor fen in New Hampshire over two winters. Methane accumulated beneath ice layers (1 cm) deposited by freezing rain, resulting in snow-pore air mixing ratios as high as 140 ppmv during the first winter and 600 ppmv during the second. An early winter snow crust of 300 kg m^?3 caused no discontinuity in a linear mixing ratio profile and therefore was not observed to retard snowpack emissions. Methane concentration-depth profiles in pore water steepened and concentrations increased by as much as 400 μM at the 10 and 20 cm depths as the ice cover formed. This suggests that the peat-ice cover plays an important part in CH₄ build-up in pore water by limiting the transport of gases between the peat and the atmosphere. Pore water concentrations gradually declined through late winter. The seasonality of dissolved CH₄ in pore water over two winters and one summer showed an average annual amplitude of 1.3 gCH₄m^?2 (25–75cm depth range), with a winter maximum of 4.7gCH₄m^?2. Emissions during the winter with average snowfall accounted for a larger percentage (9.2% in 1993–1994) of total annual emission than the winter with below-average snowfall and warmer air temperature (2% in 1994–1995). Emissions averaged 56 and 26mg m^?2 day^?1 during the first and second winter (December, January and February), respectively.