Rain-on-snow Research Articles

Spatio‐temporal controls of snowmelt and runoff generation during rain‐on‐snow events in a mid‐latitude mountain catchment

AbstractA network of 30 standalone snow monitoring stations was used to investigate the snow cover distribution, snowmelt dynamics, and runoff generation during two rain‐on‐snow (ROS) events in a 40 km2 montane catchment in the Black Forest region of southwestern Germany. A multiple linear regression analysis using elevation, aspect, and land cover as predictors for the snow water equivalent (SWE) distribution within the catchment was applied on an hourly basis for two significant ROS flood events that occurred in December 2012. The available snowmelt water, liquid precipitation, as well as the total retention storage of the snow cover were considered in order to estimate the amount of water potentially available for the runoff generation. The study provides a spatially and temporally distributed picture of how the two observed ROS floods developed in the catchment. It became evident that the retention capacity of the snow cover is a crucial mechanism during ROS. It took several hours before water was released from the snowpack during the first ROS event, while retention storage was exceeded within 1 h from the start of the second event. Elevation was the most important terrain feature. South‐facing terrain contributed more water for runoff than north‐facing slopes, and only slightly more runoff was generated at open compared to forested areas. The results highlight the importance of snowmelt together with liquid precipitation for the generation of flood runoff during ROS and the large temporal and spatial variability of the relevant processes. Copyright © 2015 John Wiley & Sons, Ltd.

Model simulations of the modulating effect of the snow cover in a rain-on-snow event

Abstract. In October 2011, the Swiss Alps underwent a marked rain-on-snow (ROS) event when a large snowfall on 8 and 9 October was followed by intense rain on 10 October. This resulted in severe flooding in some parts of Switzerland. Model simulations were carried out for 14 meteorological stations in two affected regions of the Swiss Alps using the detailed physics-based snowpack model SNOWPACK. We also conducted an ensemble sensitivity study, in which repeated simulations for a specific station were done with meteorological forcing and rainfall from other stations. This allowed the quantification of the contribution of rainfall, snow melt and liquid water storage on generating snowpack runoff. In the simulations, the snowpack produced runoff about 4–6 h after rainfall started, and total snowpack runoff became higher than total rainfall after about 11–13 h. These values appeared to be strongly dependent on snow depth, rainfall and melt rates. Deeper snow covers had more storage potential and could absorb all rain and meltwater in the first hours, whereas the snowpack runoff from shallow snow covers reacts much more quickly. However, the simulated snowpack runoff rates exceeded the rainfall intensities in both snow depth classes. In addition to snow melt, the water released due to the reduction of liquid water storage contributed to excess snowpack runoff. This effect appears to be stronger for deeper snow covers and likely results from structural changes to the snowpack due to settling and wet snow metamorphism. These results are specifically valid for the point scale simulations performed in this study and for ROS events on relatively fresh snow.

Warmer and wetter winters: characteristics and implications of an extreme weather event in the High Arctic

One predicted consequence of global warming is an increased frequency of extreme weather events, such as heat waves, droughts, or heavy rainfalls. In parts of the Arctic, extreme warm spells and heavy rain-on-snow (ROS) events in winter are already more frequent. How these weather events impact snow-pack and permafrost characteristics is rarely documented empirically, and the implications for wildlife and society are hence far from understood. Here we characterize and document the effects of an extreme warm spell and ROS event that occurred in High Arctic Svalbard in January–February 2012, during the polar night. In this normally cold semi-desert environment, we recorded above-zero temperatures (up to 7 °C) across the entire archipelago and record-breaking precipitation, with up to 98 mm rainfall in one day (return period of >500 years prior to this event) and 272 mm over the two-week long warm spell. These precipitation amounts are equivalent to 25 and 70% respectively of the mean annual total precipitation. The extreme event caused significant increase in permafrost temperatures down to at least 5 m depth, induced slush avalanches with resultant damage to infrastructure, and left a significant ground-ice cover (∼5–20 cm thick basal ice). The ground-ice not only affected inhabitants by closing roads and airports as well as reducing mobility and thereby tourism income, but it also led to high starvation-induced mortality in all monitored populations of the wild reindeer by blocking access to the winter food source. Based on empirical-statistical downscaling of global climate models run under the moderate RCP4.5 emission scenario, we predict strong future warming with average mid-winter temperatures even approaching 0 °C, suggesting increased frequency of ROS. This will have far-reaching implications for Arctic ecosystems and societies through the changes in snow-pack and permafrost properties.

Large-scale analysis of changing frequencies of rain-on-snow events with flood-generation potential

Abstract. In January 2011 a rain-on-snow (RoS) event caused floods in the major river basins in central Europe, i.e. the Rhine, Danube, Weser, Elbe, Oder, and Ems. This event prompted the questions of how to define a RoS event and whether those events have become more frequent. Based on the flood of January 2011 and on other known events of the past, threshold values for potentially flood-generating RoS events were determined. Consequently events with rainfall of at least 3 mm on a snowpack of at least 10 mm snow water equivalent (SWE) and for which the sum of rainfall and snowmelt contains a minimum of 20% snowmelt were analysed. RoS events were estimated for the time period 1950–2011 and for the entire study area based on a temperature index snow model driven with a European-scale gridded data set of daily climate (E-OBS data). Frequencies and magnitudes of the modelled events differ depending on the elevation range. When distinguishing alpine, upland, and lowland basins, we found that upland basins are most influenced by RoS events. Overall, the frequency of rainfall increased during winter, while the frequency of snowfall decreased during spring. A decrease in the frequency of RoS events from April to May has been observed in all upland basins since 1990. In contrast, the results suggest an increasing trend in the magnitude and frequency of RoS days in January and February for most of the lowland and upland basins. These results suggest that the flood hazard from RoS events in the early winter season has increased in the medium-elevation mountain ranges of central Europe, especially in the Rhine, Weser, and Elbe river basins.

Snow distribution, melt and surface water inputs to the soil in the mountain rain–snow transition zone

Summary The timing, magnitude, and spatial distribution of snow cover and the resulting surface water inputs (SWI) are simulated at a small catchment located in the rain–snow transition zone of southwest Idaho, USA. A physically based snow model is run on this 1.5 ha study catchment, which has an elevation range of 1600–1645 masl. The catchment is divided into relatively steep (mean slope angle of 21 degrees) northeast and southwest facing hill slopes by an ephemeral stream that drains to the southeast. SWI are fundamental controls on soil moisture, streamflow generation, groundwater recharge, and nutrient cycling. Although the timing of melt events is similar across the basin, southwest facing slopes receive smaller magnitude and more frequent SWI from mid winter snow melt, while the northeast facing slope receives greater SWI during the spring. Three spatial patterns are observed in the modeled SWI time series: (1) equal between slopes, (2) majority of SWI on southwest facing slopes, and (3) majority of SWI on northeast facing slopes. Although any of these three spatial patterns can occur during the snow season, four emergent SWI patterns emerge through the melt season: (1) near uniform, (2) controlled by topographic differences in energy fluxes, (3) transitional, and (4) controlled by snow distribution. Rain on snow (ROS) events produce similar SWI between the northeast and southwest facing slopes, with the difference being attributed primarily to snow distribution. Turbulent fluxes dominate the snowpack energetics in four of the five rain on snow events, and advective fluxes from precipitation are greater than 17% during the 2 rain on snow events in December and January. Net radiation fluxes dominate spring melt events. Variations in the method used to distribute precipitation may result in large differences in total precipitation to the basin.

Controls on soil nitrification and stream nitrate export at two forested catchments

Dormant season inorganic nitrogen (N) leaching varies considerably among forested catchments with similar bedrock, forest cover and deposition history. Recent work has highlighted the importance of winter rain-on-snow (ROS) events as a source of winter nitrate (NO3-N) export, but differences among streams are likely due to differences in baseflow NO3-N concentrations, and thus soil N processes. The objective of this study was to investigate rates of N-mineralization and nitrification as well as their potential environmental controls throughout the year, but with particular focus on the winter season in south-central Ontario, Canada. Field incubations were utilized to assess differences in NO3-N and ammonium production over time and across topographic positions in two catchments with contrasting patterns of N export. Rates of nitrification were similar to rates of total mineralization, and nitrification rates were significantly higher during the summer and spring compared with the winter and fall; however, winter nitrification was substantial, and ranged from 19 to 36 % of annual rates. Seasonal differences in nitrification were largely driven by temperature, soil moisture and inorganic N concentration in soil. Rain and melting snow infiltrated the soil during ROS events, which were associated with increased NO3-N availability, particularly in well-drained soils, and ROS-induced increases in stream nitrate concentrations were largest at the catchment dominated by well-drained soil. Annual nitrification fluxes were almost two orders of magnitude greater than N deposition or NO3-N leaching fluxes at either catchment. Similar rates of NO3-N production within the two catchments suggest that consumption of NO3-N within wet soils is responsible for the 10-fold difference in NO3-N export between the two streams. Notably, these results suggest that consumption processes were important for reducing NO3-N export even during winter ROS events.

Variability of Observed Energy Fluxes during Rain-on-Snow and Clear Sky Snowmelt in a Midlatitude Mountain Environment

Abstract Hourly observations of 65 snow monitoring stations were used to investigate the spatiotemporal variability of the surface energy balance during snowmelt in the Black Forest region of southwestern Germany. The study focuses on two rain-on-snow (ROS) events in December 2012 and a clear sky period at the beginning of March 2013 using the same study locations. ROS and clear sky were chosen since they are completely different snowmelt conditions in terms of energy exchanges and dynamics. The results show that snowmelt was dominated by turbulent exchanges at the open field sites and by both turbulent exchanges and net longwave radiation in the forest during ROS. The energy available for snowmelt can be almost identical at open and forest locations during ROS, and a constant energy flux even during night was directed toward the snowpack. During the clear sky conditions, net shortwave radiation was the dominating term in the open, whereas net shortwave and net longwave radiation were most important in the forest. A diurnal signal with positive energy balance during daylight and negative energy balance in the night was observed, with considerably reduced energy available for snowmelt in the forest. Furthermore, the stratified sampling design revealed the strong influence of the canopy and the topography at the locations on the observed energy fluxes. Elevation, aspect, and leaf area index (LAI) were the most important predictor variables during ROS, whereas aspect and LAI were most influential during the clear sky period. The study highlights the distinct spatial variability of the individual energy balance terms over a relatively small area during the differing snowmelt conditions.

Soil, snow, weather, and sub-surface storage data from a mountain catchment in the rain–snow transition zone

Abstract. A comprehensive hydroclimatic data set is presented for the 2011 water year to improve understanding of hydrologic processes in the rain–snow transition zone. This type of data set is extremely rare in scientific literature because of the quality and quantity of soil depth, soil texture, soil moisture, and soil temperature data. Standard meteorological and snow cover data for the entire 2011 water year are included, which include several rain-on-snow (ROS) events. Surface soil textures and soil depths from 57 points are presented as well as soil texture profiles from 14 points. Meteorological data include continuous hourly shielded, unshielded, and wind-corrected precipitation, wind speed and direction, air temperature, relative humidity, dew point temperature, and incoming solar and thermal radiation data. This data is often viewed as "forcing data", and is gap filled and serially complete. Sub-surface data included are hourly soil moisture data from multiple depths from seven soil profiles within the catchment, and soil temperatures from multiple depths from two soil profiles. Hydrologic response data include hourly stream discharge from the catchment outlet weir, continuous snow depths from one location, intermittent snow depths from 5 locations, and snow depth and density data from ten weekly snow surveys. Snow and hydrologic response data are meant to provide data on the catchment hydrologic response to the weather data. This data is mostly presented "as measured" although snow depths from one sensor and streamflow at the catchment outlet have been gap filled and are serially complete. Though the weather, snow, and hydrologic response data only covers one water year, the presentation of the additional subsurface data (soil depth, texture, moisture, and temperature) makes it one of the most detailed and complete hydro-climatic data sets from the climatically sensitive rain–snow transition zone. The data presented are appropriate for a wide range of modeling (energy balance snow modeling, soil capacitance parametric modeling, etc.) and descriptive studies. Data is available at doi:10.1594/PANGAEA.819837.

Sources of nitrate export during rain-on-snow events at forested catchments

Rain-on-snow (ROS) events are major drivers of nitrate (NO3-N) export from seasonally snow-covered forested catchments and may cause episodic declines in stream pH. High intensity monitoring of throughfall, snow pack and stream water draining two proximal catchments (Harp 3A and Harp 6A) with very different NO3-N export revealed that a very small percentage of ROS-induced stream discharge originates from throughfall and melting snow (new water; average = 6.4 %). However, this new water has a very high concentration of NO3-N (throughfall/snowmelt average = 498 μg/L) compared with baseflow (average = 7.3 μg/L in Harp 6A; average = 41 μg/L in Harp 3A) and as a result, throughfall and snowmelt contribute the majority of NO3-N export (average = 62 %) during ROS events. In contrast, concentrations of sulphate, dissolved organic carbon and calcium in rain and snowpack are similar to baseflow and therefore ROS-induced declines in pH (often to below pH 6.0) are attributed entirely to increases in NO3-N concentration. Differences in absolute magnitude of ROS NO3-N export between catchments are explained through differences in baseflow NO3-N concentrations. The frequency and magnitude of ROS events in this region are affected by both NO3-N deposition and winter temperature, and thus the impact of these events in the future depends on changes in both atmospheric deposition and winter climate.

Early snowmelt events: detection, distribution, and significance in a major sub-arctic watershed

High latitude drainage basins are experiencing higher average temperatures, earlier snowmelt onset in spring, and an increase in rain on snow (ROS) events in winter, trends that climate models project into the future. Snowmelt-dominated basins are most sensitive to winter temperature increases that influence the frequency of ROS events and the timing and duration of snowmelt, resulting in changes to spring runoff. Of specific interest in this study are early melt events that occur in late winter preceding melt onset in the spring. The study focuses on satellite determination and characterization of these early melt events using the Yukon River Basin (Canada/USA) as a test domain. The timing of these events was estimated using data from passive (Advanced Microwave Scanning Radiometer—EOS (AMSR-E)) and active (SeaWinds on Quick Scatterometer (QuikSCAT)) microwave remote sensors, employing detection algorithms for brightness temperature (AMSR-E) and radar backscatter (QuikSCAT). The satellite detected events were validated with ground station meteorological and hydrological data, and the spatial and temporal variability of the events across the entire river basin was characterized. Possible causative factors for the detected events, including ROS, fog, and positive air temperatures, were determined by comparing the timing of the events to parameters from SnowModel and National Centers for Environmental Prediction North American Regional Reanalysis (NARR) outputs, and weather station data. All melt events coincided with above freezing temperatures, while a limited number corresponded to ROS (determined from SnowModel and ground data) and a majority to fog occurrence (determined from NARR). The results underscore the significant influence that warm air intrusions have on melt in some areas and demonstrate the large temporal and spatial variability over years and regions. The study provides a method for melt detection and a baseline from which to assess future change.

An evaluation of the hydrologic relevance of lateral flow in snow at hillslope and catchment scales

AbstractLateral downslope flow in snow during snowmelt and rain‐on‐snow (ROS) events is a well‐known phenomenon, yet its relevance to water redistribution at hillslope and catchment scales is not well understood. We used dye tracers, geophysical methods, and hydrometric measurements to describe the snow properties that promote lateral flow, assess the relative velocities of lateral flow in snow and soil, and estimate volumes of downslope flow. Results demonstrate that rain and melt water can travel tens of metres downslope along layers within the snowpack or at the snowpack base within tens of hours. Lateral flow within the snowpack becomes less likely as the snowpack becomes saturated and stratigraphic boundaries are destroyed. Flow along the base can be prevalent in all snowpack conditions. The net result of lateral flow in snow can be the deposition of water on the soil surface in advanced downslope positions relative to its point of origin, or direct discharge to a stream. Although both melt and ROS events can redistribute water to downslope positions, ROS events produced the most significant volumes of downslope flow. Direct stream contributions through the snowpack during one ROS event produced up to 12% of streamflow during the event. This can help explain rapid delivery of water to streams during ROS events, as well as anomalously high contributions of event water during snowmelt hydrographs. In catchments with a persistent snowpack, lateral redistribution of water within the snowpack should be considered a relevant moisture redistribution mechanism. Copyright © 2012 John Wiley & Sons, Ltd.

Variability in effect of climate change on rain-on-snow peak flow events in a temperate climate

► Historically extreme peak flows in a temperate climate are rain-on-snow events. ► Climate change is predicted to decrease the number of rain-on-snow events. ► The transient rain/snow zone is predicted to increase in elevation with climate change. ► Decreases in rain-on-snow events increased frequency of peak flows <5–10 year return. ► Decreases in rain-on-snow events decreased frequency of peak flows >10 year return. The frequency of rain-on-snow (ROS) hydrologic events, which produce high runoff volumes and lead to large-scale flooding and avalanching, are likely to change in the future as the types and timing of precipitation change. The relationship between ROS precipitation events and peak daily flow events ⩾1-year return were examined for historical and future runoff affected by climate change within the Santiam River Basin, Oregon. Historical streamflow records and modeled historical and future streamflow projections were analyzed for three sites across three elevation zones defined by the dominant precipitation types; rain, rain and snow transition, and snow. The results illustrate that, across elevation zones, historical peak daily flows ⩾1-year return have a high frequency (>60%) of association with ROS. The historical association between peak daily flows and ROS is highest within the transient rain and snow elevation band (350–1100 m), with 80% and 100% of ⩾1 and ⩾5-year return peak flows associated with ROS, respectively. In a future with increased air temperature due to climate change, our results indicate that a decrease in the frequency of high peak flow ROS events will occur in the low and middle elevation zones while the frequency of ROS associated peak flows will increase in high elevation areas. The transition of winter precipitation from snow to rain is predicted to increase peak daily flows <5–10-year return interval and decrease peak daily flows ⩾10-year return in the middle to high elevation zones.

Impact of winter warming on the timing of nutrient export from forested catchments

AbstractWinter climatic conditions can influence the timing and magnitude of water and nitrate (NO3–N) export from seasonally snow‐covered catchments. Specifically, mid‐winter rain‐on‐snow (ROS) events are a major source of NO3–N export to forested streams, but the impact of these events on other nutrients is not known. Climate projections for Ontario suggest that climate warming will be most pronounced during the winter months, which could result in more mid‐winter rain events and consequent changes in nutrient delivery to streams. The objective of this study was to examine the impact of winter climate variability on the timing of NO3–N export relative to water and other nutrients at six headwater catchments in south‐central Ontario that have long‐term water quality and hydrology records (1980–2002). The catchments represent a wide range of physiographic characteristics and stream chemistry, yet the timing of nitrate export from all catchments was coherent. In warmer winters with more ROS events, the bulk of NO3–N export relative to the export of water shifted earlier in the year from spring (i.e. the main period of snow melt) to winter. ROS events did not cause similar temporal shifts in the export of other nutrients, including dissolved organic carbon, total phosphorus and calcium. Instead, their export was synchronous with the bulk of water export. Future shifts to earlier export of NO3–N relative to water and other nutrients may impact aquatic productivity and cause more frequent episodic acidification of surface waters. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Kelp and seaweed feeding by High-Arctic wild reindeer under extreme winter conditions

One challenge in current Arctic ecological research is to understand and predict how wildlife may respond to increased frequencies of ‘‘extreme’’ weather events. Heavy rain-on-snow (ROS) is one such extreme phenomenon associated with winter warming that is not well studied but has potentially profound ecosystem effects through changes in snow-pack properties and ice formation. Here, we document how ice-locked pastures following substantial amounts of ROS forced coastal Svalbard reindeer ( Rangifer tarandus platyrhynchus ) to use marine habitat in late winter 2010. A thick coat of ground ice covered 98% of the lowland ranges, almost completely blocking access to terrestrial forage. Accordingly, a population census revealed that 13% of the total population ( n = 26 of 206 individuals) and 21% of one sub-population were feeding on washed-up kelp and seaweed on the sea-ice foot. Calves were overrepresented among the individuals that applied this foraging strategy, which probably represents a last attempt to avoid starvation under particularly severe foraging conditions. The study adds to the impression that extreme weather events such as heavy ROS and associated icing can trigger large changes in the realized foraging niche of Arctic herbivores. Keywords: Climate change; ground-ice; High Arctic; marine algae; Rangifer tarandus ; terrestrial herbivore. (Published: 9 March 2012) Citation: Polar Research 2012, 31 , 17258, DOI: 10.3402/polar.v31i0.17258

Climate, icing, and wild arctic reindeer: past relationships and future prospects

Across the Arctic, heavy rain-on-snow (ROS) is an "extreme" climatic event that is expected to become increasingly frequent with global warming. This has potentially large ecosystem implications through changes in snowpack properties and ground-icing, which can block the access to herbivores' winter food and thereby suppress their population growth rates. However, the supporting empirical evidence for this is still limited. We monitored late winter snowpack properties to examine the causes and consequences of ground-icing in a Svalbard reindeer (Rangifer tarandus platyrhynchus) metapopulation. In this high-arctic area, heavy ROS occurred annually, and ground-ice covered from 25% to 96% of low-altitude habitat in the sampling period (2000-2010). The extent of ground-icing increased with the annual number of days with heavy ROS (> or = 10 mm) and had a strong negative effect on reindeer population growth rates. Our results have important implications as a downscaled climate projection (2021-2050) suggests a substantial future increase in ROS and icing. The present study is the first to demonstrate empirically that warmer and wetter winter climate influences large herbivore population dynamics by generating ice-locked pastures. This may serve as an early warning of the importance of changes in winter climate and extreme weather events in arctic ecosystems.

Detection of snow surface thawing and refreezing in the Eurasian Arctic with QuikSCAT: implications for reindeer herding

Snow conditions play an important role for reindeer herding. In particular, the formation of ice crusts after rain-on-snow (ROS) events or general surface thawing with subsequent refreezing impedes foraging. Such events can be monitored using satellite data. A monitoring scheme has been developed for observation at the circumpolar scale based on data from the active microwave sensor SeaWinds on QuikSCAT (Ku-band), which is sensitive to changes on the snow surface. Ground observations on Yamal Peninsula were used for algorithm development. Snow refreezing patterns are presented for northern Eurasia above 60 degrees N from autumn 2001 to spring 2008. Western Siberia is more affected than Central and Eastern Siberia in accordance with climate data, and most events occur in November and April. Ice layers in late winter have an especially negative effect on reindeer as they are already weakened. Yamal Peninsula is located within a transition zone between high and low frequency of events. Refreezing was observed more than once a winter across the entire peninsula during recent years. The southern part experienced refreezing events on average four times each winter. Currently, herders can migrate laterally or north-south, depending on where and when a given event occurs. However, formation of ice crusts in the northern part of the peninsula may become as common as they are now in the southern part. Such a development would further constrain the possibility to migrate on the peninsula.

Modeling the Spatially Varying Water Balance Processes in a Semiarid Mountainous Watershed of Idaho1

Stratton, Benjamin T., Venakataramana Sridhar, Molly M. Gribb, James P. McNamara, and Balaji Narasimhan, 2009. Modeling the Spatially Varying Water Balance Processes in a Semiarid Mountainous Watershed of Idaho. Journal of the American Water Resources Association (JAWRA) 45(6):1390‐1408.Abstract: The distributed Soil Water Assessment Tool (SWAT) hydrologic model was applied to a research watershed, the Dry Creek Experimental Watershed, near Boise Idaho to investigate its water balance components both temporally and spatially. Calibrating and validating SWAT is necessary to enable our understanding of the water balance components in this semiarid watershed. Daily streamflow data from four streamflow gages were used for calibration and validation of the model. Monthly estimates of streamflow during the calibration phase by SWAT produced satisfactory results with a Nash Sutcliffe coefficient of model efficiency 0.79. Since it is a continuous simulation model, as opposed to an event‐based model, it demonstrated the limited ability in capturing both streamflow and soil moisture for selected rain‐on‐snow (ROS) events during the validation period between 2005 and 2007. Especially, soil moisture was generally underestimated compared with observations from two monitoring pits. However, our implementation of SWAT showed that seasonal and annual water balance partitioning of precipitation into evapotranspiration, streamflow, soil moisture, and drainage was not only possible but closely followed the trends of a typical semiarid watershed in the intermountain west. This study highlights the necessity for better techniques to precisely identify and drive the model with commonly observed climatic inversion‐related snowmelt or ROS weather events. Estimation of key parameters pertaining to soil (e.g., available water content and saturated hydraulic conductivity), snow (e.g., lapse rates, melting), and vegetation (e.g., leaf area index and maximum canopy index) using additional field observations in the watershed is critical for better prediction.

Rain on Snow: Little Understood Killer in the North

In October 2003, a severe rain‐on‐snow (ROS) event killed approximately 20,000 musk‐oxen (Figure 1) on Banks Island, which is the westernmost of the Canadian Arctic islands (approximately 380 kilometers by 290 kilometers in size). The event reduced the isolated herd by 25% and significantly affected the people dependent on the herd's well‐being. Because of the sparsity of weather stations in the Arctic and the lack of routinely deployed weather equipment that was capable of accurately sensing the ROS event, its detection largely was based on reports from hunters who were in the affected areas at the time.Such events can significantly alter a frozen ecosystem—with changes that often persist for the remainder of a winter—by creating ice layers at the surface of, within, or below the snowpack. The water and ice layers are known to facilitate the growth of toxic fungi, significantly warm the soil surface under thick snowpack, and deter large grazing mammals. Although ROS events of the magnitude that was experienced in Banks Island in 2003 likely have reverberations throughout the entire Arctic and subarctic ecosystem, little is presently known about them and their impacts. As understanding of ROS events expands, many ROS‐related aspects of the Arctic ecology and hydrology are likely to be discovered. They may include topics such as the fate of small mammals under the snowpack at the iced soil surface, the difficulty of ptarmigans to burrow into the iced snow, the limited infiltration of spring snowmelt into the iced over soil, and the changing drifting patterns of ice‐crusted snowpack.

A method for the detection of the severe rain‐on‐snow event on Banks Island, October 2003, using passive microwave remote sensing

Severe wintertime rain‐on‐snow (ROS) events create a strong ice layer (or layers) in the snow on arctic tundra that act as a barrier to ungulate grazing. They are linked with large‐scale ungulate (reindeer, caribou, elk, and musk‐ox) herd declines via starvation and reduced calf production rate when the animals are unable to penetrate the resulting subsnowpack ice layer. ROS events also produce considerable perturbation in the mean wintertime soil temperature under the snowpack. ROS is a sporadic but well‐known and significant phenomenon that is currently very poorly documented. Characterization of the distribution and occurrence of severe ROS events is based only on anecdotal evidence, indirect observations of carcasses found adjacent to iced snowpacks, and irregular detection by a sparse observational weather network. We have analyzed in detail a particular ROS event that took place on Banks Island in early October 2003 that resulted in the death of 20,000 musk oxen. We make use of multifrequency passive microwave imagery from the Special Sensor Microwave Imager satellite sensor suite in conjunction with a strong‐fluctuation‐theory (SFT) emissivity model. We show that a combination of time series analysis and cluster analysis based on microwave spectral gradients and polarization ratios provides a means to detect the stages of the ROS event resulting from the modification of the vertical structure of the snowpack, specifically wetting the snow, the accumulation of liquid water at the base of the snow during the rain event, and the subsequent modification of the snowpack after refreezing. SFT model analysis provides quantitative confirmation of our interpretation of the evolution of the microwave properties of the snowpack as a result of the ROS event. In addition to the grain coarsening owing to destructive metamorphism, we detect the presence of the internal water and ice layers, directly identifying the physical properties producing the hazardous conditions. This analysis offers the potential to characterize both the frequency and global distribution of ROS using multifrequency satellite passive microwave imagery.

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.