The influence of local factors on snow cover and seasonal ground freezing in the Tien Shan

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1 Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN The influence of local factors on snow cover and seasonal ground freezing in the I.V. Severskiy Institute of geography, Almaty, Kazakhstan E.V. Severskiy Institute of permafrost, Yakutsk, Russia ABSTRACT: The distinctive feature of mountains the zoning of cryogenic phenomena according to altitude and exposition is a result of the cumulative effect from zonal, regional and local factors. It significantly depends upon the characteristics of snow-cover distribution. The latter is determined by the influence of latitude, orography, location of the region in the mountain system, and also by local factors. Due to the influence of local factors in mountains of Eurasia with low to moderate snowfall, horizontal contrasts of snowiness and seasonal freezing are significantly greater than vertical ones. 1 INTRODUCTION The physical principles of heat/energy exchange between the atmosphere, the snow cover and the underlying grounds are well investigated. The main ideas of Voeikov (1889) and Sumgin (1937) served as a basis for further research by Kudryavtsev (1967) and Feldman (1977) and others in this area. Although most of this work is related to areas of lowland permafrost, the described regularities of heat- and mass exchange in the system atmosphere snow cover underlying grounds are also valid for mountain topography. The main result is that the role of snow cover in freezing is of two kinds: in the cold period the snow cover warms the ground, and at the beginning of the warm season it produces a cooling effect by impeding heat penetration into the ground. Early formation of a continuous snow cover in autumn and large snow accumulation at the beginning of winter provides better thermal insulation (Kudryavtsev 1967). The regime and characteristics of ground freezing depend upon the depth of snow cover, the intensity of snow accumulation, and the duration of a continuous snow cover (Nekrasov 1981). Apparently, there are no reasons to question the validity of these conclusions with reference to mountain territories. However, in mountains these general regularities are considerably complicated by the influence of local factors. The most important factors are slope exposure (slope orientation and inclination), type of vegetation, structure and hydrophysical properties of the underlying ground (Severskiy & Severskiy 1990). The main reason is that the aforementioned local factors decisively affect the regime and distribution of snow cover in mountains (Severskiy et al. 2000). As a matter of fact, sharp contrasts in the duration and depth of seasonal freezing, which are typical for mountains, are the reflection of the main regularities of snow cover distribution in space and time. Stated somewhat differently, the distribution of seasonal ground freezing (the depth, intensity of freezing, content of ice) which depends upon the absolute altitude, exposition of slopes and type of vegetation is mainly determined by the influence of the above-mentioned factors on the snow cover regime and distribution. The results of the influence from local factors on snow-cover duration and distribution as well as on seasonal ground freezing are based on multi-year observations at specially created experimental stations. Such stations were located at different altitudes in the basins of the Turgen and the Bolshaya and Malaya Almatinka rivers on northern slope of the Zailiiskiy Alatau (Northern ). The structure of experimental monitoring systems, the content and methods of natural observations are described in the monographs (Severskiy & Severskiy 1990, Severskiy et al. 2000). Therefore, only two features must be pointed out here: 1. The experimental station for snow cover measurements consisted of 37 to 60 sites (the number varied from year to year) uniformly located at altitudes from 1600 to 3500, taking into account different exposures for different slopes, types of vegetation (meadow, deciduous forest, bushes, coniferous forest) and closeness of tree and bush crowns. More than 30 stations for geocryological observations were located taking into account differences in altitude, slope exposure, type of vegetation and ground composition. 2. The above stationary sites were also used for carrying out simultaneous hydrometeorological observations. Continuous year-around observations at the above monitoring stations have been carried out for more than 30 years. 1023

2 2 RESEARCH AND RESULTS Snow-cover characteristics (dates of the beginning and end of continuous snow cover, the depth and snowwater equivalent of snow cover (water equivalent of snow cover at the period of maximal snow accumulation, SWE, in mm)) are analyzed using data from different mountain areas of Eurasia (Severskiy et al. 2000). As a result of this research, a system of calculation methods was developed. It has made it possible to quantitatively estimate the above-mentioned characteristics of snow cover for any mountain region, including insufficiently studied ones, using the data from standard observations. The main results of these investigations were published earlier (Severskiy 1982, Severskiy & Severskiy 1990, Severskiy et al. 2000). Only the main features that are necessary to describe spatial and temporal characteristics of seasonal ground freezing in the mountains of the region will be highlighted here. An analysis of available information on snow cover in Eurasia from the Alps in the west to the Altai- Sayany Mountains in the east, and from the Khibins and Fennoscandia Mountains in the north to the Pamirs and Karakorum in the south made it possible to draw the conclusion that snow-cover distribution in the mountains of inner-continental regions is caused by the following general features (Severskiy et al. 2000): similar conditions of relief and location of an area relative to the mountain periphery call for specific types of distribution of snow parameters, which are closely related to the altitude and geographical coordinates of the site in question. The geographic distribution of snow-cover characteristics (dates of formation and disappearance of continuous snow cover, the depth and snow-water equivalent of snow cover) strongly depends on the latitude and altitude of the point of observation. Thus, if the influence of altitude is not taken into account (other things being equal), the average latitudinal gradient of the dates of stable snow cover disappearance, which are common for the mountains of Eurasia (within the territory of the former USSR), is equal to 6.3 days per one degree of latitude (Severskiy et al. 2000). On the basis of latitudinal snowiness zoning one can clearly see the effects of orographic barriers and massiveness of mountains. The first one causes significant differences in snow accumulation on windward slopes of peripheral ridges and their leeward slopes. The second one is revealed in sharp contrast between snow-cover distribution on peripheral areas of the mountains and in intermountain regions (Severskiy et al. 2000). On the periphery of the mountains, the value of snowiness mainly depends upon the orientation of river valleys with respect to the predominant direction of atmospheric moisture transfer during the cold period. The direction of moisture transport in the continental mountains is not constant. During cold periods in the Caucasus, the main axis of the moisture transport is directed from the south-west to the northeast at an angle of 60 to the meridian, while in the Pamir, Gissaro-Alai, and this angle is 40. Further to the north, the prevailing direction of moisture flow is closer to the west. In the Dzhungarsky Alatau, the maximum SWE corresponds to the azimuths of valleys with an azimuth of 250 and in the Altai with an azimuth of 270 (Severskiy 1982, Severskiy et al. 2000). Under conditions existing in Eurasia, the maximum snow cover on the periphery of mountains is observed for the river basins located on the macroslopes of western and southwest orientation. Macroslopes of northernoriented peripheral ranges have much lower snowiness, although snow accumulation there is greater than in the intra-mountain areas. The minimal snow cover can be found in the intra-mountain regions and in the river basins located on the macroslopes of peripheral ranges of eastern orientation (Severskiy et al. 2000). The typical feature of mountains, i.e. the altitudinal zoning of the total amount of precipitation and snow cover strongly depends on macro-orientation of ridge slopes and the location of the region with respect to the periphery of the mountain areas. It should be noted that the distribution of the snowcover duration is more stable in comparison to the distribution of maximum snow storage. The dates of formation of continuous snow cover in the mountains of Central Asia are divided into three groups (Table 1). The first type is characteristic for peripheral basins oriented unfavourably with regard to the prevailing directions of moisture-bearing air streams. The second type, with the greatest vertical gradients under the considered conditions, is typical of orographically open basins on the western periphery of mountain areas. The third type, with the smallest vertical gradients, is valid for orographically closed interior mountain regions. Table 1. The typical equations for calculation of average dates of stable snow cover formation/disappearance in the mountains. Type of distribution Equations used for calculations Altitude, m 1 T f H 0.035H * T d 0.11H 2 1.1H T f H 0.025H * T d 0.88H 0.086H T f H T d 0.039H H 4 T f and T d the difference between the terms of formation and disappearance stable snow cover at altitude 1000 m and calculated altitude H, [days]; H altitude of the given point, [hundred m]. *for the Dzhungarskiy Alatau. 1024

3 The distribution of the maximum snow-water equivalent (SWE) versus altitude varies considerably more: for the same territory, eight types of the distribution of SWE at the same altitude have been found, three of which are shown in Table 2. The vertical SWE gradient in the major snow basins decreases from west to east (Fig. 1). The largest values Table 2. Typical dependencies of the maximum water equivalent of the snow cover on the altitude. Standard error Limitation Type of Equations used by altitude distribution for calculations mm % band, m 1. Central W 5.2H Inner W 5.0H Alai W 13.0H Talas W 11.7H North W 13.6H Kuramino- W 25.0H Chatkal 7. Fergana W 40.83H Western W 94.5H Note: W the difference in the average of the maximum SWE on «zero» (600 m) and calculated altitudes, [mm]; H altitude (calculated level), [km]. Figure 1. Ratios between the typical dependencies of the long-term mean water equivalent of the snow cover and the altitude in different mountain regions of Eurasia. 1 8 correspond to the numbers of typical dependencies for the mountains in Central Asia and Kazakhstan (please refer to Table 2); 9 intermountain regions of the Northern Caucasus; 10 peripheral regions of the Northern Caucasus; 11 the Central Transcaucasus (the southern slope of the Big Caucasus mountain range); 12 the Western Transcaucasus; 13 inner regions of Southern Norway; 14 inner regions of Northern Norway; 15 western near-shore regions of Southern Norway. of the gradient dw/dh characterize western slopes of the Scandinavian mountains. These values are reduced by half in the West Caucasus and even more at the western periphery of the and Altai. Within separate regions, changes of SWE in the direction from west to east are not significant in comparison with the change induced by elevation and geographic latitude. In most mountain regions, substantial snow-cover redistribution by wind is observed only above the forest line; it is especially strong in the ridge zones of the glacial-nival belt. Relatively high values of SWE at the forest line are a result of snow-cover redistribution by wind. In many basins the second SWE maximum is observed in the lower third of slopes in closed river valleys and at distances of m below the ridges. Common tendencies in snow-cover dependence on its location in the mountains of the south of Eurasia are considerably complicated by the influence of the aforementioned local factors. According to the results of this study, significant exposure contrasts in snow-cover distribution are especially clearly expressed in regions with shallow or moderate snow cover and are typical of all mountain systems of Eurasia (Severskiy 1982, Severskiy et al. 2000). It is sufficient to say, that the coefficient of precipitation retention (the ratio between the snowwater equivalent of the snow cover and the sum of precipitation at a certain moment of time, in mm) in the mid-mountain belt of the Zailiiskiy Alatau range, Northern, is % on northern slopes and only 30 50% on southern ones (Sosedov 1967). As the Zailiiskiy Alatau (Northern ) is characterized by vertical gradients in the maximum SWE, the change in orientation from north to east (or west) is equivalent to a decrease in altitude by almost 1000 m (in mid-mountain belts). Therefore, the difference in maximum snow accumulation (SWE) between grass-covered surfaces and slopes covered with a deciduous forest is almost the same as between northern and eastern (or western) slopes. The snow-water equivalent in coniferous forest, other things being equal, is less than in deciduous forest by almost a factor of two. In other words, the decrease in SWE as related to the transition from meadow to coniferous forest (at a typical plant density of ), more than twice exceeds the change in SWE, which corresponds to the decrease in altitude from 2500 to 1500 m. Thus, the influence of slope orientation and the type of vegetation on the snow cover in regions with moderate snow cover is significantly greater than the influence of altitude. In regions with abundant snow accumulation, the influence of slope orientation on the snow cover distribution is less pronounced. In the most snowy regions in the western periphery of the mountains, the influence of slope orientation is very small and mainly determined by the exposure 1025

4 differences in snow losses through evaporation (Severskiy et al. 2000). It is necessary to note another important feature, namely the altitudinal contrasts in snow cover. The depth of snow cover increases with altitude up to the end of March only in the low belt of mountains and, reaches its maximum at moderate altitudes ( m) and then decreases again with the transition to highmountains. Due to such a mode of snow accumulation, the most favorable conditions for ground freezing are developed in the high-mountain zone where the depth of snow cover in winter is significantly less than at lower altitudes but the temperature is significantly lower. In this zone, snow cover is formed by compression due to its intensive wind redistribution, and it has considerably smaller thermal insulation properties than the snow cover in the mid-mountain zone (I. Severskiy & E. Severskiy 1990). These snow-cover features are reflected in spatial differentiation of the duration and depth of seasonal freezing and thawing of the ground. Soils in the river basin on the western periphery of the mountains are characterized by the smallest depth of freezing. The greatest depth of freezing is observed in the intramountain and orographically closed basins within the eastern periphery of the mountains. In the whole range of altitudes, the soils of southern slopes where the depth of discontinuous snow cover is much less than on the slopes of other orientation are subjected to the most heavy freezing. This general background of freezing conditions caused by snow-cover distribution is affected by the structure and hydrophysical properties of the ground as well as by the differences in the snow cover caused by the type of vegetation (Severskiy & Severskiy 1990, Severskiy et al. 2000). The above factors affect spatial and temporal characteristics of the cryogenesis and are clearly manifested in the specific structure of altitudinal geocryological belts in the mountains of Northern Asia and Kazakhstan (Severskiy & Severskiy 1990). In the coniferous forest where precipitation is retained by the spruce crowns and no wind-redistribution of snow takes place, usually no continuous snow cover forms at the level of the trunks. The depth of the snow cover sharply increases towards the periphery of the projection of spruce crowns and at m distance from the edges of the forest, it attains the depth of snow cover on open meadow slopes. Similar characteristics of snow-cover distribution are typical of the slopes covered with Juniper stands. As a result of long-term observations, it was found that the intensity of freezing in the wood under spruce crowns and Juniper is higher than on open meadow surfaces. Thus, on the average, for the multi-year period in the Zailiiskiy Alatau (Northern ), the intensity of freezing under spruce crowns was cm/day, under Juniper stands it was , and on the neighboring grass-covered sites cm/day. The sum of negative monthly ground temperatures at a depth of 20 cm as averaged for the specified period of observations was 14.5 C under Juniper stands, 16.7 C under coniferous forest against 9.9 C on a grass-covered slope. On average and for the period of observations from 1975 to 1988 on the northern slope of the Zailiiskiy Alatau in the altitude range from 1400 to 2700 m, the depth of freezing under spruce crown-cover was times more intense than on meadow surfaces at the same altitude and orientation of slopes. As a whole, coniferous forests and Juniper stands in the mountains of the region strengthen cooling effects. It is no coincidence that just under spruce forest and Juniper stands, growing on coarse deposits, permafrost parcels and areas of perennially frozen grounds occur at lowest altitudes. With other things being equal (similar slope exposure, altitude and composition of ground materials), the sites under crowns in the coniferous forest are subjected to the strongest freezing. Thus, the higher the plant density, the deeper is ground freezing. The longest period with frozen soil is a characteristic of a coniferous forest with a high density of planting. Similar conditions of freezing are developed on the slopes covered with Juniper. Due to the specific features of snow cover distribution, the soil under Juniper shrubs gets frozen almost as deep as under spruce crowns. The duration of the period with frozen soil under Juniper is much longer than that on nearby meadow surfaces. The conditions in the deciduous forest are far less favorable for ground freezing. The depth of ground freezing here is less than under the crowns in the coniferous forest by almost a factor of two. The least favorable conditions for freezing exist on grass-covered slopes with normal conditions of snow accumulation. In the moss-spruce forest, very special conditions of heat exchange are developed. Thermal insulation characteristics of a thick (up to 40 cm) moss cover are so intensive, that soils which are frozen during winter remain frozen during summer. Thus, permafrost parcels can be observed in the spruce forest at abnormally low altitudes down to 1800 m. The areas of permafrost parcels are clearly shown in the forest structure (bonitet, canopy closure, structure and composition of underwood). The perennial regime of frozen soils is displayed in the dynamics of radial wood increment: compared with normally developed species in the park forest, Schrenk s spruce, growing on perennially frozen grounds, is characterized by a high content of late wood in the structure of annual radial increment (up to 45%), a high coefficient of sensitivity (18 33%), 1026

5 and minimal radial increments (from 0.03 to 0.50 mm) (Severskiy & Severskiy 1990). It has been determined for plains that coarse-grained soils are subject to deeper and more intensive freezing (other things being equal), this feature is clearly pronounced in the conditions of mountain relief. According to the results of long-term studies in the mountains of Central Asia and Kazakhstan, this dependence is extremely pronounced for stony and especially rocky soils. (Gorbunov et al. 1999, Severskiy & Severskiy 1990). Long-term observations on grass-covered sites of stony-loam soils indicate that, sharp daily fluctuations in air temperature only penetrate to a depth of m, even with snow-free conditions. In nearby coarse gravelly talus, they penetrate to more than 2 m. Coarse deposits of crystalline rocks are subject to most intense and deep freezing. So, at an altitude of 2600 m in the basin of the Great Almatinka River which has positive mean annual air temperature, the corresponding temperature of deposits (averaged for the period from 1974 to 1994) was negative at a depth of 3 m varying from 1.2 C at 3.5 m to 2.7 C at 6 m. At the nearby grass-covered sites, mean annual temperatures of aleuritic soils remained positive during the specified period. As soil and ground texture in the mountains of Central Asia and Kazakhstan is getting considerably coarser with altitude, the intensity and depth of seasonal freezing are increasing in the same direction. Above the upper tree line, the belt of intermittent/patchy occurrence of perennially frozen ground transforms into areas of widespread permafrost distribution. The influence of zonal, regional and local factors on the snow-cover duration and distribution in the mountains is expressed in the altitudinal/exposition zoning of the depth and character of seasonal freezing which is well pronounced in the mountains of Central Asia and Kazakhstan. This zoning in the Zailiiskiy Alatau (Northern Tien Shan) is shown in Figure 2. On northern slopes and under conditions of the Northern in the mid-elevation mountain belt (up to altitudes of m) at a typical regime of snow accumulation, the snow cover can protect the ground from intensive cooling. Therefore, freezing is not deep there; it steadily increases from cm at the altitudes lower than 1000 m to cm at the altitude of 2000 m. With further rise in altitude, the intensity and depth of freezing within loose deposits increases sharply due to the above-mentioned reasons. As a result, at the upper tree-line (at m altitude) the depth of freezing on northern grasscovered slopes reaches m, and above about m, the seasonally frozen layer is in contact with the underlying permafrost. Figure 2. Dependencies of the seasonal freezing depth (Z) on the altitude (H), ground structure, slope orientation and vegetation on the northern slopes of the Zailiyskiy Alatau with the changes in the snow cover height (h, cm) and sums of the negative daily mean air temperatures ( t C). 1 meadow-steppe south slopes; 2 meadow north slopes; 3 meadow east and west slopes; 4 juniper shrubs; 5 spruce forests; 6 large-detrital deposits. Clearly pronounced altitudinal zoning of seasonal ground freezing is typical of the whole variety of natural complexes. The ratio between the vertical gradients of the examined characteristics (Fig. 2) reflects the changes in the cumulative influence of the abovementioned local conditions on the snow cover characteristics, and through them on the territorial and temporal changes in seasonal freezing. 3 CONCLUSION The main regularities of spatial and temporal dynamics of seasonal freezing are largely determined by the snow-cover distribution. In the mountains of Central Asia and Kazakhstan, the influence of local factors on the snow cover is so strong that especially in the regions with low or moderate snow it has large effects during most of the cold period. The local factors affect the processes of mass/energy exchange in the system atmosphere snow cover underlying ground and, finally, lead to the spatial differentiation of the intensity and depth of seasonal freezing, so typical for mountains. Due to the influence of local factors, the depth and type of freezing in the mountains sharply change over short distances, and the horizontal differences in the parameters of seasonal freezing often surpass the altitudinal differences. Snow cover is a factor that defines the depth and the character of seasonal freezing. As a matter of fact, the 1027

6 aforementioned local factors influence the processes of freezing and thawing through the characteristics of snow cover. The obtained results, in the authors opinion, reliably characterize the main regularities of the snow cover distribution and seasonal freezing in the mountains of the region. Most parts of the described experimental work was carried out in the Zailiiskiy Alatau. However, taking into account common spatial and temporal changes in climatic conditions and parameters of snow cover, it is possible to state with a high degree of certainty that the characteristics obtained in this research are valid for the mountains of Kazakhstan and Central Asia as a whole. REFERENCES Feldman, G.M The forecast of a temperature regime of grounds and development of cryogenic processes (in Russian). Novosibirsk: Nauka. Gorbunov, A.P., Severskiy, E.V. & Titkov, S.N Geocryological conditions of the and Pamirs. Yakutsk: Nauka. Kudryavtsev, V.A Influence of a snow cover on seasonal freezing-thawing and temperature regime of ground (in Russian). Permafrost study 7: Nekrasov, I.A A snow cover and deep freezing of lithosphere (in Russian). Thematic and regional researches of frozen ground of Northern Eurasia: Severskiy, I.V The problem of snow cover and avalanche hazard assessment in mountains. Abstract of the thesis for the doctorate in geography (in Russian). Alma-Ata: Nauka. Severskiy, I.V. & Severskiy, E.V Snow cover and seasonal freezing of ground in the Northern (in Russian). Yakutsk: Nauka. Severskiy, I.V., Blagoveshchenskiy, V.P., Severskiy, S.I., Pimankina, N.V., Xie Zichu, Zhang Zhizhong & Hu Ruji Snow Cover and Avalanches in Tien Shan Mountains. (in English). Almaty: VAC Publishing House. Sosedov, I.S Research of the balance of snow moisture on mountain slopes (in Russian). Alma-Ata: Nauka. Sumgin, M.I Permafrost in the limits of the USSR (in Russian). Moscow-Leningrad: USSR Ac.sci. publishing. Voeikov, A.I A snow cover and its influence on soil, climate and weather; ways of research (in Russian). Zapiski Geographical society in physical geography 18(2). 1028