2. State Key Laboratory of Frozen Soils Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China;
3. Gen'he Meteorological Administration, Gen'he, Inner Mongolia 022350, China
Seasonal snow cover has significant influences on the ground thermal regime in cold regions. The main influences of snow cover can be attributed to:(1)low thermal conductivity of snow as a porous medium for heat transfer, depending on snow density and microstructure(Zhang et al.,1996a; Fierz and Lehning, 2001);(2)high surface albedo, emissivity, and absorptivity(Zhang et al.,1996b); and (3)the energy sink provided by the latent heat of fusion during the snow-melting process(Mellor, 1977; Sturm et al.,1997; Zhang, 2005). Where there is significant snow cover in the winter, the mean annual ground surface temperature(MAGST)is higher than the mean annual air temperature(MAAT)owing to the insulating effect of the snow cover. Gold(1963)found that snow cover was the principal reason for the difference between average annual air and soil temperatures in cold regions. In the east-central part of the Mackenzie Delta, Northwest Territories, Canada, snow cover was the major factor controlling the permafrost temperature during the winter and thus the mean annual permafrost surface temperature(Smith, 1975). Moreover, seasonal snow cover can be responsible for the absence of permafrost in certain locations where the air temperature is close to 0 ℃(Zhang et al.,1985; Tong et al.,1986).
In China, the presence of snow cover may result in great differences in the distribution of the lower limit of permafrost in east-west trending mountains, such as the Qilian, Tianshan, and Altai mountains, and the enhanced differentiation in sunny and shadowy slopes(Qiu and Cheng, 1995). The overall effect of snow cover on the ground thermal regime depends on the timing, duration, accumulation, and melting processes of snow cover(Goodrich, 1982; Zhang et al.,2001; Sokratov and Barry, 2002; Ling and Zhang, 2003, 2004, 2006; Luetschg et al.,2008); the thickness of snow cover, which if less than 0.6 m lacks sufficient insulation to prevent low atmospheric temperatures from cooling the soil(Haeberli, 1973; Keller and Gubler, 1993; Hanson and Hoelzle, 2004); and interactions of snow cover with micrometeorological conditions, local micro reliefs, vegetation, and geographical locations(Zhang et al.,1996a; Zhang, 2005; Ling and Zhang, 2007).
Snow cover is the most variable l and surface condition in both time and space in the Northern Hemisphere(Gutzler and Rosen, 1992; Cohen, 1994; Cohen and Entekhabi, 2001). It is of great importance in studies of permafrost degradation, ground thermal regimes, water regimes, freezing/thawing processes, and climate change(Walsh et al.,1985; Yang and Woo, 1993; Kang et al.,2000; Lan et al.,2002; Qin et al.,2006; Jin et al.,2008; Luetschg et al.,2008; Li et al.,2010; Mutter et al.,2010). Most quantitative researches relevant to snow cover-permafrost have focused on alpine, arctic, and subarctic areas using numerical models(Goodrich, 1982; Zhang and Stamnes, 1998; Zhang et al.,2001; Ling and Zhang, 2003, 2007). However, there is a lack of knowledge about the influences of snow cover on the ground thermal regimes in the mid-latitudes and boreal forest regions. The goal of this paper is to estimate the quantitative effects of snow cover on ground thermal regimes, ground freeze-thaw processes, and its surface offset on the analysis of meteorological data in Gen'he on the western flank of the Da Xing'anling(Hinggan)Mountains, northeastern China.2 Site and data description 2.1 Site description
Gen'he City is located in the Gen'he River Valley on the west flank of the Da Xing'anling Mountains in the Inner Mongolia Autonomous Region in northeastern China(50°47′N, 121°30′E), with an elevation of approximately 720-840 m a.s.l.. The topography of Gen'he and environs is characterized by rugged terrain, well-developed water channel systems, and complicated frozen ground conditions(the upper part of Figure 1). It is under the control of a cold, temperate, continental monsoon climate with long, dry, cold winters and short, moist, hot summers. The decadal MAATs at Gen'he were −5.5, −5.0, −4.0, −3.3 and −3.0 °C, respectively, from 1961 to 2010, with an increasing rate of 0.5 ℃/10a. Gen'he City had a population of 65, 000 in 2005 and an urban areal extent of 10.2 km2, with an estimated thermal isl and of 2 °C during the last 50 years(Codification Committee of Gen'he City History, 2007). Due to climate warming and anthropic influences, the permafrost under Gen'he City has been thawed, as evidenced by our drilling and ground temperature measurements.
The Gen'he Weather Station is situated to the west of this city. The soils at the station predominately consist of medium and fine s and or s and y clay with angular gravels, generally 10-12 m in thickness, and with a layer of humic topsoil 0.1-0.2 m in thickness(the lower part of Figure 1). Permafrost in the Da Xing'anling Mountains, which are in the southern margin of the permafrost zone of the Eurasian continent, is greatly affected by local climate, geology, and geography(Guo et al.,1981; Northeast China Permafrost Research Taskforce, 1983). It is largely discontinuous in Gen'he, with a mean annual ground temperature(MAGT)around −1.3 ℃(Lu et al.,1993). The Gen'he Weather Station is located in a talik, and its observation field is covered by sparse meadow with periodically cropped plant heights less than 0.1 m.
The MAAT between 2001 and 2010 was −3.0 °C with a st and ard deviation of 0.6 ℃; the mean monthly air temperature varied from −27.7 °C(January), with a st and ard deviation of 2.2 ℃, to 18.3 ℃(July), with a st and ard deviation of 0.8 ℃. The extreme maximum air temperature(32 ℃) and minimum air temperature(−50 ℃)were recorded on June 27, 2010 and January 10, 2001, respectively. The annual precipitation ranged from 450 to 550 mm, of which 12% fell between October and April as snow. The duration of seasonal snow cover generally lasted five or six months of a year, and the maximum thickness of the snow cover ranged fr om 10 to 36 cm at the Gen'he Weather Station during 2001 through 2010.2.2 Data description
Data were provided by the Genhe Weather Station, including mean daily air temperature(MDAT), daily precipitation, daily snow depth, and mean daily ground temperatures at the depths of 0, 5, 10, 20, 40, 80, 160, and 320 cm from the first day of 2001 to the last day of 2010.
It must be mentioned that daily ground temperatures before December 31, 2004 were manually read and recorded by a glass thermometer, and after that were monitored using a platinum resistance thermometer. Glass thermometers are classified into two groups by shape, the angular and the straight. Before December 31, 2004, the ground temperatures at the depths of 5, 10, and 20 cm were measured by angular glass thermometer, and at 40, 80, 160, and 320 cm by straight glass thermometer. Angular glass thermometers are very easily snapped, so they must be installed after thawing and put away before freezing. Hence the ground temperatures at 5, 10, and 20 cm were missed from September to May in 2001, 2002, 2003, and 2004. Prior to 2005 the ground temperatures at 0 cm were read at the snow surface instead of the snow-ground interface in winter. All the other data on air temperatures, snow depth, and ground temperatures during January 1, 2005 through December 31, 2010 are available.
3 Results and analysis 3.1 Freezing index
In general, the freezing index(FI) and thawing index(TI)are respectively defined as the cumulative number of subzero and above-zero degree days for a given time period(Associate Committee on Geotechnical Research, 1988; Van Everdingen, 2005). The annual FI or TI is thus a measure of freezing or thawing in a year, i.e., a measure of cold and warm season duration and temperatures(Johnson and Hartman, 1971). It is of great importance for underst and ing and for refining predictions of the response of permafrost soils to climate change(Brown et al.,2000). Empirically, the FI is simply the absolute value of the summation of the daily subzero air temperatures over those days. In this paper, the FIs of air, snow surface, and (mineral soils)ground surface are respectively calculated to analyze the impact of snow cover on the ground thermal regime(Table 1).
|Type||Beginning date of freezing||Ending date of freezing||FI(℃·d)||FId(℃C·d)||R(%)||SD Max.(cm)|
|Snow surface||2001-10-23||2002-03-29||3, 241|
|Snow surface||2002-10-12||2003-03-29||3, 640|
|Snow surface||2003-10-10||2004-04-07||3, 626|
|Air||2005-11-01||2006-04-17||3, 272||1, 588||48.5||24|
|Ground surface||2005-10-23||2006-04-14||1, 684|
|Air||2006-10-08||2007-04-15||3, 125||1, 823||58.3||25|
|Ground Surface||2006-10-10||2007-04-04||1, 302|
|Ground Surface||2007-10-17||2008-03-25||2, 164|
|Air||2008-10-24||2009-04-02||3, 241||2, 313||71.4||36|
|Air||2009-10-17||2010-04-18||3, 643 td>||2, 162||59.3||23|
|Ground surface||2009-10-26||2010-04-08||1, 481|
|FId: Difference of the FIs between air and snow or ground surface; R: ratio of FId to air FI; SD Max.: Maximum snow depth in the year.|
The values of the air FI are about 9.4%-11.8% less than those of the snow surface FI, which is not at all related to snow depth. However, they are considerably higher than the ground surface FI due to the insulating effect of snow cover at different depths. When the maximum snow depth(SD Max.)was no more than 10 cm during the winter of 2007-2008 in Gen'he, the FI of the ground surface was slightly lower than the air FI. However, the FI difference between the air and ground surface generally increases with the increasing maximum snow depth as a result of the effective reduction of heat loss from the ground by a thicker snow cover. Based on the FI statistics, the SD Max. of 10, 23, 24, 25, and 36 cm reduced heat loss from the ground in winter by as much as 28.1%, 59.3%, 48.5%, 58.3%, and 71.4%, respectively.3.2 Seasonal snow and freezing process
Snow cover fluctuates seasonally, interannally, and decadally in Gen'he. Variations in the timing of the seasonal snow cover result in variations in the ground freezing process, and they have been recorded at Gen'he Weather Station from 2005 to 2010(Table 2). In Table 2, the first and last snowfalls were not incorporated in the duration of stable snow cover because they melted after a short residence on the ground in late autumn and early winter and spring. In this paper the duration of ground freezing refers to the time period when the mean daily ground surface temperature is below 0 ℃.
|SE date||SD date||DS(d)||SDi(cm)||Bf date||Bt date||DGF(d)||FD Max.(m)|
|SE date: date of stable snow cover establishment; SD date: date of stable snow cover disappearance; DS: duration of stable snow cover; SDi: initial snow depth; Bf date: beginning date of ground freezing; Bt date: beginning date of ground thawing; DGF: duration of ground freezing; FD Max.: maximum freezing depth.|
In most winters, stable snow cover is often established after ground surface freezing and disappears before ground surface thawing. Thus, the duration of stable snow cover(DS)is generally shorter than the duration of ground freezing(DGF). However, the exact opposite is true in the winter of 2008-2009(Table 2). In that winter, the initial snow depth(SDi)with the thickness of 13 cm effectively protected the ground against heat-loss and kept the ground surface temperature above 0 ℃ for a long time, resulting in a delay of at least 10 days in the ground freezing occurrence. Meanwhile, the maximum freezing depth(FD Max.)is the shallowest in that winter. The greatest FD Max. can be found in the winter of 2007-2008 with the SD Max. not more than 10 cm(Figure 2), and the FD Max. in the winter of 2005-2006 is the second greatest with the longest DGF. It is suggested that the FD Max. is closely related to the SD Max. and the DGF in winter, meaning the thinner snow cover and the longer DGF, the greater FD Max..3.3 Snow depth and ground thermal regime
On the basis of continuous data records from January 1, 2005 through December 31, 2010 at the Gen'he Weather Station, snow depth, air temperatures, and ground temperatures at various depths are shown in Figure 1. Strictly speaking, only the data spanning five complete winters were available for assessing the influence of snow depth on the ground thermal regime. Therefore, in order to obtain a better correspondence between stable snow cover in winter and the ground thermal regime, the hydrologic year counted from September 1 to August 31 in the next year has been introduced to calculate MASTs(mean annual soil temperatures) and MAATs(mean annual air temperatures)in Gen'he.
In any winter, the ground is warmer than the air due to the presence of snow cover. In general, soil temperature increases with depth, with a dampened amplitude and the phase lag. The MAAT at Gen'he in the hydrological year of 2005-2006 was about −2.5 ℃, with an annual geographical amplitude of about 59.3 °C, and snow covered the ground surface for six months(from the beginning of November to the end of April)with a maximum snow thickness of 24 cm. The MASTs from the ground surface to the depth of 3.2 m varied from 3.1 to 5.4 ℃(Table 3). In 2006-2007, due to thicker and longer duration of snow cover(Figure 2 and Table 2), the ground was generally warmer than in 2005-2006, although the air temperatures were similar in the two hydrologic years(Table 3). In these five hydrologic years, the MASTs were almost the lowest in 2007-2008 because of the thinnest snow cover(no more than 10 cm) and the shortest DS. In contrast, the MAGT in 2008-2009 was significantly higher than that in any other hydrologic year due to the thickest snow cover and the longest DS, and this probably had a certain influence on ground temperatures at the depths of 160 and 320 cm in the next hydrologic year. It was extremely cold in 2009-2010, with −3.8 ℃ for the MAAT, and the snow conditions were similar to those in the winter season of 2005-2006. However, the MASTs were significantly higher than in 2005-2006, especially at the depths of 160 and 320 cm.
|Year||Ta(℃)||Tg at various depth(℃)|
|0 cm||5 cm||10 cm||20 cm||40 cm||80 cm||160 cm||320 cm|
|Ta: air temperature; Tg: ground temperature.|
Frozen ground is usually associated with a cold climate. The climate-frozen ground relation has been successfully defined in an explicit and functional manner by Smith and Riseborough(2002), and can be represented schematically by the mean annual temperature regimes at three levels(Smith and Riseborough, 2002). Referring to that literature, the surface offset is equal to the difference between the mean annual ground surface temperature(MAGST) and the MAAT in a hydrologic year.
During the study period, the MAAT varied interannually in Gen'he, with a small fluctuation ranging from −3.8 to −2.0 ℃, but the MAGSTs were all positive and fluctuated mainly between 2.7 and 5.2 ℃ due to different snow depths(Table 4). According to the data, the surface thermal offset in the winter of 2007-2008 was only 4.7 ℃, the smallest due to the thinnest snow cover, but it gradually increased with increasing snow depth in Gen'he. When the maximum snow depth increased from 10 to 36 cm in different winters, the surface thermal offset increased as much as 3.5 ℃. However, snow depth is not the only determinant of surface thermal offset; the air temperature and duration of ground freezing in this case are also evidently important. In the winter of 2009-2010, although the DS, SDi, and maximum snow depth were similar to those in the winters of 2005-2006 and 2006-2007(Tables 1 and 2), the surface thermal offset was significantly greater than that of 2005-2006 and 2006-2007 due to its lower MAAT and shorter DGF, and the ground heating in the previous snow-free period(especially in the summer).
|Max. snow depth(cm)||24||25||10||36||23|
Snow cover has a significant influence on the ground thermal regime(Zhang, 2005). The snow surface has a high albedo that leads to a reduction in absorbed solar energy and a lowering snow surface temperature, and thus snow surface FI is significantly higher than air FI. Although snow depth contributes nothing to the snow surface FI, the heat accumulated in the ground in previous seasons and transferred to the atmosphere in winter seasons is regulated by the thickness of the snow cover. Therefore, the ground surface FI is considerably lower than the FIs of both air and the snow surface. The increase of snow accumulation results in a reduced correlation between winter air temperature and ground surface temperatures, and less heat loss from the ground. At the Gen'he Weather Station, if the ground was covered by snow 10 cm in thickness for more than 100 days, then about 28% of the heat accumulated in the ground could be retained instead of lost to the air. When the snow cover thickness ranged from 23-25 cm in a similar duration, more than 55% of the energy in the ground could be kept. And once snow depth increased to 36 cm, 71% of the energy in the ground could be effectively kept from loss.
Based on the results of past investigations in the Alps, it assumed that if the depth of snow cover exceeds 0.8 m and is maintained until late winter, the snow-ground interface will be insulated from short-term changes in the energy flux(Haeberli, 1973; Hoelzle et al.,2001). The basal temperatures of snow cover(BTSs)are controlled, therefore, only by the heat flux from the subsurface, and they reflect ground thermal conditions. Moreover, BTS measurements have been considered as a reliable method for mapping and predicting the distribution of permafrost(Lewkowicz and Ednie, 2004; Brenning et al.,2005) and are widely used in the Alps and in other areas of the world(Hoelzle, 1992; Hoelzle et al.,1993; Etzelmüller et al.,2006; Julián and Chueca, 2007; Lewkowicz and Bonnaventure, 2008).
Because of the important role of snow cover in determining the thermal state of the ground, it has resulted in higher ground temperatures compared to the air temperatures effect. Actually, the difference in temperature between the ground surface and the atmosphere(surface thermal offset)has always been connected to the accumulation of snow cover. In the winter season of 2008-2009 in Gen'he, a thick snow cover developed earlier and lasted longer than in the immediately preceding winter. Therefore, the average ground surface temperature values in 2008-2009 were significantly higher than in the previous year, and the surface thermal offset was also 3.5 ℃ higher. On the other h and , although the accumulations of snow cover were similar in the winter seasons of 2005-2006, 2006-2007, and 2009-2010, the ground at the Gen'he Weather Station in 2006-2007 was warmer than in 2005-2006, but colder than in 2009-2010. That may be attributed to the ground heating in the previous snow-free period(especially in the summer) and lower MAAT at the same time.
Temporal variation of stable snow cover has a significant impact on the ground freezing process in Gen'he, especially the beginning date for stable snow cover establishment(SE date) and the initial snow depth(SDi). Taking the case in 2008-2009 for example, snow cover began to accumulate before the ground surface temperature was below 0 ℃, with an SDi of 13 cm in the winter season of 2008-2009, resulting in about a 10-day delay in ground freezing due to the insulating effect of the snow cover(Figure 3). Also during 2008-2009, the course of ground freezing progressed slowly, and the FD Max. was only 1.58 m, with an SD Max. of as much as 36 cm. When snow accumulation or snow depth decreased, the ground freezing was accelerated, and the FD Max. deepened due to the weakened insulating effect of snow cover. For instance, when the SD Max. was reduced to 10 cm in the winter season of 2007-2008, the ground was frozen rapidly and the FD Max. increased to 2.50 m(Figure 3). At the same time, the duration of ground freezing was prolonged for 20 days or more.5 Conclusions
Seasonal snow cover is an important factor determining the ground thermal regime in the western flank of the Da Xing'anling Mountains, northeastern China. Since there is only sparse vegetative cover, less than 0.1 m in thickness, at the Gen'he Weather Station, interannual changes of ground temperatures may be related to the accumulation changes of snow cover and air temperature changes in both snow-covered and snow-free periods.
The presence of snow cover results in a reduced correlation between winter air temperature and ground surface temperatures and less heat loss from the ground, due to its insulating effect, which depends significantly on snow depth and its duration. Snow cover of 10-cm thickness that stays on the ground for five or six months in winter can prevent 28% of the heat accumulated in the ground from being lost to the atmosphere; if the snow depth increases to 36 cm, 71% of the ground heat can be retained. Moreover, the occurrence of snow cover resulted in MAGSTs 4.7-8.2 ℃ higher than MAATs at the Gen'he Weather Station, according to a study in the past five years.
The beginning date for stable snow cover establishment(SE date) and the initial snow depth(SDi)also have a great influence on the ground freezing process in Gen'he. Heavy snowfall before ground freeze-up can postpone and retard the freezing process. As a result, the duration of ground freezing can be shortened by at least 20 days and the maximum depth of frost penetration can be as much as 90 cm shallower.Acknowledgments:
This study was supported by the National Natural Science Foundation of China(Nos. 41201066, 41401028, and J0930003/J0109) and the Open Fund of the State Key Laboratory of Frozen Soils Engineering(No. SKLFSE-ZT-14). The authors are appreciative of the editors of this journal for their hard work and generous assistance in improving the quality of the paper.
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