Thick Snow Cover Research Articles

Impact of climate warming on the ground surface temperature in the sporadic permafrost zone of the Tatra Mountains, Poland and Slovakia

This paper examines the influence of climate change on the temperature of a debris slope surface in the zone of sporadic permafrost occurrence in the Tatra Mountains. This involved applications of linear regression equations which describe statistical dependence of the ground surface temperature (GST) upon the current air temperature in snow-free periods, or upon the previous air temperature and snow depth. These equations are validated using the daily air temperature and snow depth at a reference synoptic station and GST at 5 sites located in different topographic conditions. In order to estimate mean winter half-year, mean summer half-year and mean annual GST at these sites in hydrological years 1955–2010, archival meteorological data were used. The maximum errors in these estimations, determined for two reference years, were 0.5 °C, 0.2 °C and 0.1 °C respectively. The results show that the debris slopes thermal sensitivity to climate warming decreases with the increase of: a) shading, 2) altitude, and 3) depth of seasonal snow cover. In general, the changes of the air temperature influence the changes of mean annual ground surface temperature more in snow-free periods than in snow periods, whereas the degradation of orographically conditioned permafrost patches occurs faster in the places where snow cover is thin during the whole winter. The decrease of the depth of a thick snow cover may be accompanied by the decrease of its bottom temperature, despite the increase of the winter air temperature. We infer that in special orographic-nival conditions, the increase of mean annual air temperature may be accompanied by the decrease of mean annual ground surface temperature.

Prediction of temperature variation within a snowpack in open areas and under different canopy covers

AbstractSnow temperature is a major component of many physical processes in a snowpack. The temperature and the change in temperature across a layer have a dominant effect on physical properties of snow grains as well as its hardness, strength, and failure resistance. In this study, temperature and snow cover thickness were measured during the snow season of 2007–2008 in 11 elevation classes and in three different sampling locations, one in an open area and two under different forest canopy covers for each class along Kartalkaya road, Bolu. Each sampling site was visited 44 times to collect data including snow depth, snow surface temperature, ground temperature, and temperature within snowpack at 20‐cm intervals. Seven different models are developed to determine snowpack temperature variations under forest canopy covers and in an open area with different leaf area index values. All models were performed using a multilayer perceptron (MP) method for the Bolu–Kartalkaya area, Turkey. MP approach constitutes a standard form of neural network modeling and can modify two‐layer linear perceptron methods using three and more layers. The ability of MP is to handle complex nonlinear interactions, which ease the natural process of modeling. This method can overcome complex computations using neuron networks, and they can easily nonlinearly link input and output variables. The predictive errors are determined on the basis of mean absolute error and mean square error criteria. The Nash–Sutcliffe sufficiency score showing compliance between observed and predicted values is also calculated. According to the mean absolute error, the mean square error, and the Nash–Sutcliffe sufficiency score criteria, the predictive errors are within reasonable error intervals, justifying the use of the developed MP models for engineering applications. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of seasonal snow cover on suspended sediment runoff in a mountainous catchment

► Effects of snow cover on SS runoff at a temperate mountainous area were studied. ► Snow accumulation and ablation had a significant impact on SS runoff. ► SS runoff mechanism variation within the snow-melting period was revealed. ► Thick snow cover caused strong clockwise CQ hysteresis in the early melting period. ► Counterclockwise CQ hysteresis was observed only in the late snow-melting period. The effect of snow cover and snowmelt on suspended sediment (SS) runoff at a mountainous catchment in the temperate zone was investigated. The observation was carried out from March 1999 to May 2002 (3 years) at an experimental catchment with the area of 0.347 km 2 ranging from 280 m to 618 m above mean sea level with average stream gradient of 1/4. It was concluded that the snow accumulation and ablation process in the catchment had a significant impact on SS runoff characteristics especially during the snow-melting period. It was estimated that more than 60% of the annual SS load flowed out during the snow-melting period because of the enhanced water discharge. The daily average SS flux during the snow-melting period was calculated to be 2.54 g s −1 km −2 , corresponding to a 6–7 times greater SS flux than that during other periods. The C–Q relation analysis in each runoff event revealed a dynamic change in the SS runoff mechanism during the snow-melting period. Strong clockwise hysteresis in C–Q relation in the early stage of the snow-melting period may be explained by consumption of the bed load by the first flush followed by quick depletion of the material on the stream bed due to suppression of further SS supply by thick snow cover. Counterclockwise hysteresis in C–Q relation was observed only in the late stage of the snow-melting period. SS production at steep slopes beside the stream by avalanches and surface soil slides, induced by an abundance of meltwater, was considered to be one of the main causes of the counterclockwise loops. It would be hypothesized that exposure of other steep slopes or bluffs in the catchment apart from the stream and stream bank failure by repeating surges of the stream water stage could also be other possible SS sources causing counterclockwise hysteresis in the late snow-melting period.

Monitoring Winter Ice Conditions Using Thermal Imaging Cameras Equipped with Infrared Microbolometer Sensors

Snow cover duration and thickness affects the permafrost thermal state, the depth and timing of seasonal soil freeze/thaw/break-up, and melting of on land and sea ice. Monitoring the ice conditions in lakes and rivers during a winter season is critical for the safety of people living in those regions. Infrared cameras equipped with microbolometer sensors, placed near lakes and rivers during winter, captures and send those thermal images wirelessly to a server, where image processing and analysis algorithms measure the ice conditions in real-time. This study presents results from an ice classification system using captured ice images.

Comparative analysis of temperature courses in Antarctic lakes of different morphology: Study from James Ross Island, Antarctica

Monitoring of physical properties of terrestrial lakes belongs to one of key research activities performed by Czech scientists at James Ross Island. Throughout 2011, water temperature was measured and recorded by dataloggers in 1 h interval in two Antarctic lakes of different size and morphology. The first lake is a large shallow one located in a depression of sedimentary rock (Lachman Lake 1) at the altitude of 10m a.s.l. Lake Dulanek, the second one, is typical small-area water body located on stony glacier surface at the altitude of 220 m a.s.l. Dulanek Lake, thanks to higher altitude, remained frozen for longer period (274 d) of austral winter than Lachman Lake 1 (205 d). Presence of thick snow cover over Dulanek lake during winter period, however, caused higher values of winter minimal temperature (-17.5°C) than those recorded for Lachman lake 1 (several periods below -20.0°C). Mean annual temperature reached -4.6°C and -5.2°C for Lake Dulanek, and Lachman Lake 1, respectively. Shorter summer season accompanied by lower water temperature in Lake Dulanek however, did not bring unfovourable conditions for growth and reproduction of autotrophic organisms. Biomass of autotrophic organisms found in Lake Dulanek was quite high.

Soil frost control: agricultural adaptation to climate variability in a cold region of Japan

In the northernmost region of Japan (Hokkaido Island), earlier onsets of thick snowcover in recent years (post 1980) have reduced the penetration depth of soil frost, resulting in over-winter survival of unharvested small potato (Solanum tuberosum) tubers that emerge as weeds in the spring in rotation crop fields. To prevent the occurrence of potato weeds, a method was developed to manipulate soil-frost depths by artificially controlling snowcover thickness, guided by a simple numerical model that simulates soil freezing-thawing processes using daily mean air temperature and snowcover thinckness as input variables. The method involves removal of snowcover to expose the soil surface in the beginning of winter until the soil freezes to a sufficient depth. After that time, snow is deposited back or allowed to accumulate naturally to prevent further penetration of frost, which may cause undesirable delay in the seeding of spring crops. Field trials indicated that the model predicted frost depths within several centimeters of observed values, when measured temperature and snowcover thickness were used as model input. Based on the field and laboratory data, a soil temperature of −3°C is necessary for complete elimination of potato tubers. To achieve this temperature in potato-burial zones without causing excessive freezing, an optimal frost depth is 0.3 to 0.4 m. The method is being adopted by progressive potato producers in the region, who use tractor-mounted snow ploughs to manipulate snowcover over a large scale. This is an emerging new technology for agricultural adaptation to climate variability.

Does the removal of snowpack and the consequent changes in soil frost affect the physiology of Norway spruce needles?

Climate change may cause a decrease in snow cover in northern latitudes. This, on the other hand, may result in more severe soil frost even in areas where it is not common at present, and may lead to increased stress on the tree canopy. We studied the effects of snow removal and consequent changes in soil frost and water content on the physiology of Norway spruce ( Picea abies [L.] Karst.) needles and implications on root biomass. The study was conducted at a 47-year-old Norway spruce stand in eastern Finland during the two winters of 2005/06 and 2006/07. The treatments in three replicates were: (i) natural snow accumulation and melting (CTRL), (ii) artificial snow removal during the winter (OPEN), and (iii) the same as OPEN, but the ground was insulated in early spring to delay soil thawing (FROST). In spite of the deeper soil frost in the OPEN than in the CTRL treatment, soil warming in spring occurred at the same time, whereas soil warming in the FROST was delayed by 2 and 1.5 months in 2006 and 2007, respectively. The soil water content was affected by snow manipulations, being at a lower level in the OPEN and FROST than CTRL in spring and early summer. The physiological measurements of the needles (e.g. starch, carbon and nitrogen content and apoplastic electrical resistance) showed differences between soil frost treatments. The differences were mostly seen between the CTRL and FROST, but also in the case of the starch content in early spring 2007 between the CTRL and OPEN. The needle responses in the FROST were more evident after the colder winter of 2006. The physiological changes seemed to be related to the soil temperature and water content in the early growing season rather than to the wintertime soil temperature. No difference was found in the fine root (diameter < 2 mm) biomass between the treatments assessed in 2007. In the future, conditions similar to the OPEN treatment may be more common than at present in areas experiencing a thick snow cover. The present experiment took place over the course of two years. It is possible that whenever thin snow cover occurs yearly, the reduced starch content during the early spring may be reflected in the tree growth itself as a result of reduced energy reserves.

Onset and termination of the late-glacial climate reversal in the high-resolution diatom and sedimentary records from the annually laminated SG06 core from Lake Suigetsu, Japan

High-resolution diatom, sediment and pollen analyses of two sections from the annually laminated SG06 core from Lake Suigetsu were used to study the onset and termination of the late-glacial climate reversal in central Japan. Its broadly recognised counterpart is the Younger Dryas or Greenland Stadial-1 (ca. 12.85–11.65 cal. kyr BP based on the NGRIP ice core records). Our study suggests that accumulation of the analysed late-glacial sediments occurred in a deep and relatively cold water meso-eutrophic lake with a strong mixing regime and relatively high silica content. Combining these results together with available pollen-based environmental reconstructions we suggest that climate cooling, together with an intensified winter monsoon and thicker snow cover could influence changes in regional vegetation, sedimentation processes and trophic status of the lake during the transition from the last interstadial to stadial around Lake Suigetsu. A decrease in total pollen concentration and increase in Fagus pollen percentage indicate local vegetation stress/disturbances and suggest that cooling started at least 2–3 decades prior to the major shift in the inorganic sediment (accumulation of detrital layers) and in diatom assemblages (change from Aulacoseira ambigua to Aulacoseira subarctica dominance), which took about 10 years. The transition from the last stadial to the Holocene again shows that vegetation in the lake catchment area reacted first to the regional climate change, i.e. to the weakening of the winter monsoon and decrease in winter snow accumulation. The increase in the vegetation cover density and reduced volume of surface runoff associated with the decrease in melt water supply is likely responsible for the reduced soil erosion activity which caused the cessation in detrital layer accumulation and consequent decrease in the amount of nutrients brought to the lake and lowering of the water nutrient status. The latter process finally influenced changes in the diatom assemblages, including the return to dominance of A. subarctica ca. 30 years after the virtual disappearance of detrital layers from the sediment. Our results demonstrate the rapid response of the Lake Suigetsu system to the global cooling and subsequent warming, and allow clear definition of the onset and termination of the late-glacial climate reversal. Despite the fact that the lake system shows a more abrupt shift from the warm to cold (and cold to warm) environments than terrestrial records of vegetation demonstrate, we do not see any delayed response of local vegetation to the climate change. This last conclusion is of particular importance for application of the SG06 pollen record for quantitative climate reconstruction.

Interactions between snow, canopy, and vegetation in a boreal coniferous forest

Background: Snow is known to have a major impact on the distribution of plants in arctic and alpine ecosystems; however, its impact on understorey vegetation in boreal forests is little reported. Aims: To study the effects of trees on the distribution of snow and examine the small-scale spatial relation between snow distribution and ground vegetation. Methods: Detailed spatial variation in snow depth and summer precipitation, canopy dimensions and locations of individual trees, and ground vegetation cover were observed in a coniferous forest, and the spatial correlations between these variables were examined. Results: Spatial patterns of snow remained unchanged throughout the winter and across two different winters. Snow depth showed significant correlations with different tree influence indices calculated based on the distance to the trunk, height, diameter or canopy extent. Dwarf shrub cover correlated positively with snow, and moss cover correlated negatively with the tree influence indices. The highest covers of Vaccinium myrtillus, V. vitis-idaea and Hylocomium splendens were observed on patches with thick snow cover. Linnaea borealis, in contrast, was absent from these patches. Pleurozium schreberi and Dicranum polysetum were most abundant on patches with moderate snow. Conclusions: Trees do not only affect ground vegetation through competition, but also have indirect effects associated with uneven snow cover. Our results suggest that, like arctic and alpine species, boreal forest understorey species show differences in their snow affinity.

Influence of rain, air temperature, and snow cover on subsequent spring-snowmelt infiltration into thin frozen soil layer in northern Japan

► Temporal liquid precipitation in cold winter can increase ice in/on the ground. ► A lot of ice impeded snowmelt infiltration even though it is not thick (∼0.15 m). ► Thin snow cover is required to freeze large amount of water near the ground surface. In cold regions, thick frozen soil layers can significantly impede snowmelt infiltration and thus increase runoff of spring snowmelt water. This study aimed to clarify the relationship between soil freezing and spring-snowmelt infiltration in regions with mild climates and thin frozen soil layers. Spring-snowmelt infiltration into agricultural fields was quantified in Sapporo and Memuro in northern Japan. At each site, snow was removed in early winter to enhance soil freezing in one (treatment) plot and was undisturbed in another (control) plot. As compared to Memuro, the thicker snow cover and warmer winter air temperatures in Sapporo kept the soil unfrozen in the control plot, resulting in a substantial amount of snowmelt infiltration in that plot. However, the thick (>0.4 m) frozen layer impeded snowmelt infiltration in the Memuro treatment plot. The frost depth at the Sapporo treatment plot was almost the same as that at the Memuro control plot (∼0.15 m in the early snowmelt period). Although most of the snowmelt infiltrated into the ground in the Memuro control plot, infiltration was impeded in the Sapporo treatment plot by a thin ice-rich layer that was formed in the following processes: (1) The air temperature briefly rose above 0 °C and rain fell in January and February. (2) The air temperature then dropped to between −8 and −3 °C. (3) The snowmelt and rain water froze near the soil surface. A simple calculation revealed that the thin snow cover was required to freeze a large amount of water near the soil surface in a short period under the air temperature observed after the rain event at the Sapporo site.

Beringia: Impact on paleoclimates of northeast Asia and North Pacific during Last Pleistocene glaciation

The Beringian land bridge that emerged repeatedly during ice ages was of great importance in the energy and matter redistribution within the “continent – ocean” system and had a pronounced effect on air streams and surface marine currents in the North Pacific region. The paper considers an important and still unsettled paleogeographic problem of the Arctic Northeast of Asia: could Beringia have been covered with an ice sheet at the last glacial maximum? and what was the natural refrigerator impact on regional climate and paleo-oceanography of the Northern Pacific? Based on studies of diatoms and pollen spectra recovered from deposits dated to the last glacial time, Western Beringia was essentially ice-free at that time. This may be attributed to a deficiency of moisture supply from the Pacific due to changes in vectors and intensity of the North Pacific currents. The thick snow cover needed for glacier formation could not accumulate. Western Beringia was dominated by a unique landscape of cold and dry tundra-steppe, its topography being complicated with arcuate ridges – end moraines of alpine and cirque glaciers. The glaciation of the region developed according to the North Yakutia scenario.

A Race for Space? How Sphagnum fuscum stabilizes vegetation composition during long-term climate manipulations

Strong climate warming is predicted at higher latitudes this century, with potentially major consequences for productivity and carbon sequestration. Although northern peatlands contain one-third of the world's soil organic carbon, little is known about the long-term responses to experimental climate change of vascular plant communities in these Sphagnum-dominated ecosystems. We aimed to see how long-term experimental climate manipulations, relevant to different predicted future climate scenarios, affect total vascular plant abundance and species composition when the community is dominated by mosses. During 8 years, we investigated how the vascular plant community of a Sphagnum fuscum-dominated subarctic peat bog responded to six experimental climate regimes, including factorial combinations of summer as well as spring warming and a thicker snow cover. Vascular plant species composition in our peat bog was more stable than is typically observed in (sub)arctic experiments: neither changes in total vascular plant abundance, nor in individual species abundances, Shannon's diversity or evenness were found in response to the climate manipulations. For three key species (Empetrum hermaphroditum, Betula nana and S. fuscum) we also measured whether the treatments had a sustained effect on plant length growth responses and how these responses interacted. Contrasting with the stability at the community level, both key shrubs and the peatmoss showed sustained positive growth responses at the plant level to the climate treatments. However, a higher percentage of moss-encroached E. hermaphroditum shoots and a lack of change in B. nana net shrub height indicated encroachment by S. fuscum, resulting in long-term stability of the vascular community composition: in a warmer world, vascular species of subarctic peat bogs appear to just keep pace with growing Sphagnum in their race for space. Our findings contribute to general ecological theory by demonstrating that community resistance to environmental changes does not necessarily mean inertia in vegetation response.

Preparatory Tests with an Explorer Class Autonomous Underwater Vehicle for Missions under Sea Ice

The autonomous underwater vehicle (AUV) operations described here are preparatory missions to enable operation of an AUV under sea ice in polar regions. The proposed polar projects include operations under land fast ice in the Arctic through an ice moon pool and a planned project to assess sea ice mass balance and habitat assessment in the Southern Ocean in East Antarctica. This paper focuses on the preparatory missions, done in open water, and the planned Southern Ocean project. The plan is to use an autonomous underwater vehicle (AUV) for the under ice component of measurements. The ultimate goals are to quantify the size and shape of ridge keel structures and their contribution to the sea ice mass balance over a study region; to understand the processes that link sea ice with the distribution of ice algae and krill; to provide the necessary field measurements, over sufficiently large areas, for the calibration/validation of satellite and aircraft-based measurements of the sea ice and snow cover thickness; and to provide baseline measurements of sea ice thickness for future climate monitoring.

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

AbstractDespite 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 &amp; Sons, Ltd.

A comparison of annual and seasonal carbon dioxide effluxes between sub-Arctic Sweden and High-Arctic Svalbard

Recent climate change predictions suggest altered patterns of winter precipitation across the Arctic. It has been suggested that the presence, timing and quantity of snow all affect microbial activity, thus influencing CO2 production in soil. In this study annual and seasonal emissions of CO2 were estimated in High-Arctic Adventdalen, Svalbard, and sub-Arctic Latnjajaure, Sweden, using a new trace gas-based method to track real-time diffusion rates through the snow. Summer measurements from snow-free soils were made using a chamber-based method. Measurements were obtained from different snow regimes in order to evaluate the effect of snow depth on winter CO2 effluxes. Total annual emissions of CO2 from the sub-Arctic site (0.662–1.487 kg CO2 m–2 yr–1) were found to be more than double the emissions from the High-Arctic site (0.369–0.591 kg CO2 m–2 yr–1). There were no significant differences in winter effluxes between snow regimes or vegetation types, indicating that spatial variability in winter soil CO2 effluxes are not directly linked to snow cover thickness or soil temperatures. Total winter emissions (0.004– 0.248 kg CO2 m–2) were found to be in the lower range of those previously described in the literature. Winter emissions varied in their contribution to total annual production between 1 and 18%. Artificial snow drifts shortened the snow-free period by 2 weeks and decreased the annual CO2 emission by up to 20%. This study suggests that future shifts in vegetation zones may increase soil respiration from Arctic tundra regions.

Radar Volume Backscatter From Spatially Extended Geophysical Targets in a “Slice” Approach

This paper presents an assessment of the radar backscatter from a spatially extended geophysical target (SEGT) based on a semiempirical (SE) model. An SEGT is any geophysical object that is at least semitransparent to radar illumination (clouds, rain, snowfall in the atmosphere, thick snow cover of the ground). The existing SE model does not take into account the statistical properties of the SEGT's media. To improve the SE model, a so-called ldquoslicerdquo approach is applied. In this approach, the particles located close to the wavefront of the radar illumination are assumed to produce backscatter that is mainly coherent. This method allows the contribution of the microphysical parameters of the scattering media to the volume component of the radar cross section to be described more comprehensively than the SE model based on the incoherent approach. It is shown that the slice concept results in the original SE model in the particular case when the particle number fluctuation within the slices pertains to the Poisson law.

Two decades of responses (1986–2006) to climate by the Laurichard rock glacier, French Alps

AbstractThe Laurichard active rock glacier is the permafrost‐related landform with the longest record of monitoring in France, including an annual geodetic survey, repeated geoelectrical campaigns from 1979 onwards and continuous recording of ground temperature since 2003. These data were used to examine changes in creep rates and internal structure from 1986 to 2006. The control that climatic variables exert on rock glacier kinematics was investigated over three time scales. Between the 1980s and the early 2000s, the main observed changes were a general increase in surface velocity and a decrease in internal resistivity. At a multi‐year scale, the high correlation between surface movement and snow thickness in the preceding December appears to confirm the importance of snow cover conditions in early winter through their influence on the ground thermal regime. A comparison of surface velocities, regional climatic datasets and ground sub‐surface temperatures over six years suggests a strong relation between rock glacier deformation and ground temperature, as well as a role for liquid water due to melt of thick snow cover. Finally, unusual surface lowering that accompanied peak velocities in 2004 may be due to a general thaw of the top of the permafrost, probably caused both by two successive snowy winters and by high energy inputs during the warm summer of 2003. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Effect of air temperature on common bunt (Tilletia caries) infection in winter wheat

During the period 1993–2006 experiments designed to study the peculiarities of common bunt infection in winter wheat were carried out in a nursery with artificial inoculation at the Lithuanian Institute of Agriculture. The effects of air temperature from sowing to resumption of vegetation were determined using correlation analysis on the infection level of common bunt in winter wheat. A significant correlation between the mean nursery infection and daily mean temperature had a maximum value on the 36th day (r=0.60) at a 30–190-day interval after sowing. The minimum value was detected on the 150th day (r = -0.65) at a 150–190-day interval after sowing. The calculation of multi-intervals of mean temperatures allowed us to highlight the actual influence of temperature on the final level of common bunt infection. Cross-comparison of 6 contrasting years indicated that the impact of post-sowing temperatures was low. This was proved in 1994, 1998, and 2006 with similar temperatures but considerable differences in final infection levels – 46.9, 6.8, and 80.9%, respectively. Of the 6 test years only post-sowing temperatures in the autumn of the year 2003 were unfavourable for initial infection. The mean infection level was low when heavy frosts occurred together with a thin snow cover in 1995 (8.6%), 1998 (6.8%), and 2004 (8.9%). A high infection level occurred when heavy frosts coincided with a thicker snow cover in 1994 (46.9%), 1999 (22.9%), and 2006 (80.9%).

The environment and permafrost of the Mackenzie Delta area

AbstractThe Mackenzie Delta, prograding northwestwards into the Beaufort Sea, is North America's largest arctic delta. This Holocene feature is bounded by rolling uplands to the east and the Richardson Mountains to the west. Treeline traverses the region, separating the subarctic boreal forest in southern parts from low‐shrub tundra and sedge wetlands at the coast. The region is experiencing rapid climate change, and mean annual air temperature has increased by more than 2.5°C since 1970. The area was at the margin of the Wisconsinan ice sheet, so that in the uplands the mean annual ground temperature and glacial history control permafrost thickness, which varies from &gt;700 m to &lt;100 m. Ground temperatures in the delta are distinct from the uplands due to the thermal influence of numerous lakes and shifting channels. In the uplands, ground temperatures decrease northwards across treeline in association with a decrease in the thickness of snow cover. Ground temperatures have increased since 1970 in the uplands by approximately 1.5°C in association with rising annual mean air temperature. The increase has been less in the delta south of treeline due to the extensive thermal influence of water bodies on ground temperature. However, in the outer delta, the ground is currently more than 2.5°C warmer than in 1970. The impact of climate change on permafrost is also evident in the thickness of the active layer, which increased on average by 8 cm at 12 tundra sites on northern Richards Island from 1983–2008. Copyright © 2009 John Wiley &amp; Sons, Ltd. and Her Majesty the Queen in right of Canada.

Variability of selected extreme meteorological events in Poland

The principal aim of this paper is to analyze the trends of the multi-annual course of the selected characteristics of extreme precipitation, snow cover and atmospheric thunderstorms in the second half of the twentieth century in Poland. The results of these investigations show that in Poland it is only possible to determine a weak decreasing trend of extreme precipitation events in the S and especially in the SW part of the country. In northern Poland, opposite, although similarly weak, trends have also been observed. It is assumed that the most essential features of long-term changeability of extreme precipitation include a higher than average number of days with extremely high precipitation during the 1960s and 1970s, a distinctly lower frequency of such days during the 1950s, 1980s and in the first half of the1990s. In Poland it is possible to distinguish four broad homogenous areas in terms of the long-term changes in the occurrence of extreme precipitation events. There is considerable regional differentiation when it comes to the occurrence of thunderstorms in Poland, and their long-term changeability does not show any clear trends. Only three stations have determined a weak increase in the number of thunderstorms during the last 120 years. In some stations, an increase in the number of days with thunderstorms during the winter seasons was also observed. There were no significant trends in extreme snow cover in Poland. The periods that contained large and small areas of extreme snow cover thickness occurred alternately. Since the winter season 1987/88, the area of extremely thin snow cover has remained at a relatively high level.