Relationships between snow distribution and climate in mountain areas

Transcription

1 Sea Level, Ice, and Climatic Change (Proceedings of the Canberra Symposium, December 1979). IAHS Publ. no, 131. INTRODUCTION Relationships between snow distribution and climate in mountain areas TOMOMI YAMADA, SHIGEO SUIZU, HIROSHI NISHIMURA & GOROW WAKAHAMA The Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan 060 ABSTRACT The distribution and the accumulation-ablation process of snow in mountain areas, under the influence of a winter monsoon in a middle latitude, have been investigated by measuring water equivalents of snow (Hw) along mountain slopes both in the forest zone below the timberline and in the alpine zone above it. In the forest zone, Hw is proportional to elevation (Z), and the slope of the regression lines increases with time, since Hw of every new snowfall is linearly distributed with Z, and is independent of the topography. The change of Hw at a given place shows no fluctuation and increases continuously with time. In the alpine zone, Hw is independent of Z. It varies with the locality, and widely fluctuates with time at a given place. Such a difference is explainable by the fact that the frequency of wind speed causing blowing snow (>7 m s -1 ) is practically zero below the timberline and exceeds 50% above it, so that deposited snow is not removed by the wind in the former, while it is removed by blowing snow in the latter. Hence, the timberline should be regarded as one of the important climatic boundaries in mountain areas. Snow constitutes a very important water resource in many countries such as the Soviet Union, the USA, Canada, European countries, Afganistan, Pakistan, Australia, Japan. A great amount of water is stored in mountain areas as solid-phase water with a useful potential energy. The mountain areas are regarded as good natural water reservoirs. For instance, in western and northern parts of Hokkaido, the northernmost island of Japan, nearly half of the annual precipitation occurs as solid precipitation, which accumulates as a snow cover because there is little snowmelt during winter months. Snow surveys have been conducted by previous investigators in connection with forecasting floods due to snowmelt as well as for seeking information on water resources. These surveys have attempted to estimate the total water equivalent of snow in a given mountain river basin at the end of the accumulation season (late March to early April), when the snow cover is at its maximum. However, much remains unknown in mountain areas about the accumulation-ablation process and the distribution of snow with time during the snow season. To clarify these, surveys were made of snow depth and the water 109

2 110 Tomomi Yamadaefa/. equivalent of snow at middle latitude mountain areas, under the influence of a winter monsoon. At Mt Asahidake and Mt Teine, in Hokkaido, measurements were made from the time that snow first accumulated at the foot of the mountains, until the time at which snow almost disappeared from the top of the mountains. OBSERVATION AREAS AND METHODS Snow surveys were carried out seven times during the snow season, from December 1977 to May 1978, along the west slope of Mt Asahidake (A in Fig. 1(a)), the highest mountain in Hokkaido (2290 m a.s.l., 43 40'N, 'E). Observations were made over an elevation range from 400 to 1800 m a.s.l., in the drainage basin of the River Chubetsu (Fig. 1(a)). In the alpine zone, above the timberline at about 1400 m a.s.l., the shrubbery zone gradually gives way to an alpine flora zone, which is entirely covered with snow in winter. The forest zone below the Fig. 1 (a) Map of the west slope of IVlt Asahidake. The crosses show the sites at which snow depth recorders were set up. Solid squares indicate the observation sites for snow depth and water equivalent of snow. A, B and C respectively show Mt Asahidake (2290 m a.s.l.), ropeway-station Sugatami (1595 m ) and Yukomanbetsu Spa (1070 m). (b) Map of Mt Teine. Solid and open squares show the observation sites along the ridge and valley, respectively.

3 Snow distribution climate relationships 111 timberline, is formed of coniferous trees of m in height, giving way to comparatively flat cultivated land, the Kamikawa Basin, below 400 m a.s.l. For measuring changes in snow depth as a function of time, snow depth recorders of the optical fibre type (Takahashi & Aburakawa, 1976) were set up at the four sites (440, 730, 1070 and 1620 m a.s.l.) shown by crosses in Fig. 1(a). The battery powered recorders can automatically record snow depths, to a resolution of 1 cm and cover a depth range of 0-2m, at hourly intervals for a period of 3600 h (about 5 months). Snow depths and water equivalents of snow were also measured at the sites indicated by solid squares in Fig. 1(a). In addition to the snow surveys, air temperatures and wind speeds were continuously measured at the Yukomanbetsu Spa 1070 m a.s.l. in the forest zone (C in Fig. 1(a)) and at the ropeway-station Sugatami 1595 m a.s.l. in the alpine zone (B in Fig. 1(a)), throughout the snow season. At Mt Teine (1024 m a.s.l., 43 05'N, 'E) on the western rim of Sapporo, the main observation sites were chosen on a wide gentle ridge on the east slope of the mountain at approximately loo m elevation intervals from 95 to 990 m a.s.l. (solid squares in Fig. 1(b)). Differences in water equivalent of snow between the ridge and the valley were obtained by selecting other observation sites, shown by open squares in the same figure, in the neighbouring valley. Having no timberline, Mt Teine is entirely covered with forests from the foot to the summit. Measurements were conducted every 20 days from January to May Most of the snowfall in the western part of Hokkaido derives from moisture evaporating from the Japan Sea, and transported by the cold northwesterly wind system of the winter monsoon. With the prevailing wind direction during the monsoon, the observed slopes are respectively to the windward of Mt Asahidake and to the leeward of Mt Teine. Both slopes are topographically simple, the elevation increasing continuously up to the summit. On both slopes, two or more measurements of the water equivalent of snow were made at the one time at each site using a cylindrical snow sampler with an inner area of 20 cm. Observation sites in the forest at Mt Asahidake and Mt Teine were chosen in open and comparatively flat spaces, free from the influence of snow deposition on trees, and can be regarded as representative of areas where the fallen snow continues to accumulate throughout winter without removal. In the alpine zone on Mt Asahidake, areas where snow piled up in drifts were avoided as observation sites. ACCUMULATION PROCESS OF SNOW Changes in snow depth as a function of time at the sites on Mt Asahidake are shown in Fig. 2(a). The observation site 1620 m a.s.l. and the nearest meteorological station at Higashikawa, 215 m a.s.l., are respectively located in the alpine zone and in the Kamikawa basin, while the other three sites are in the forest zone. Changes in the water equivalent of snow on Mt Teine are

4 112 Tomomi Yamada et al. Dec Jan Feb Mar Apr May I Fig, 2 (a) Change in snow depth as a function of time on Mt Asahidake as measured by snow depth recorders (except at the site 215 m a.s.l., the nearest meteorological station), (b) Change in the water equivalent of snow on Mt Teine. The site 15 m a.s.l. is the nearest meteorological station. shown in Fig. 2(b). The site 15 m a.s.l. is the Teine-Yamaguchi meteorological observation station, near the observed slope of Mt Teine. As shown in the figures, especially Fig. 2(a), the accumulation of snow follows the same pattern at all sites except the site in the alpine zone, and the amount of snow deposited on the upper part of the observed slope is greater than that on the lower part. This fact suggests that the solid precipitation at any time in winter takes place not on part but on the whole slope ; that is, snow is simultaneously deposited on all of the observed slope, except the alpine zone. In the early spring, from the middle of March into April, when snowmelt occurs on the lower part of the slope, accumulation still continues on the upper part, and this delay in snowmelt results in a lag in the time of maximum snow thickness on the upper slope. In the alpine zone, the pattern of variation in snow depth is much different from that in the part below the timberline ( r^1400 m a.s.l.). This is illustrated by measurements at the site 1620 m a.s.l., which is located on a wide and gentle ridge (Fig. 2(a)). Abrupt decreases in snow depth, for example, on 22 January and 1 March (indicated by arrows in the figure) were caused by erosion due to strong winds, as will be mentioned later.

5 DISTRIBUTION OF SNOW IN THE SNOW SEASON Snow distribution climate relationships 113 Period of solid precipitation The period from the start to the end of solid precipitation at the observation areas on Mt Asahidake and Mt Teine can be regarded as the accumulation season, and was estimated as the period between when the daily mean air temperature, T, fell below 0 C in late autumn until the_ time when T rose above 0 C in early spring. This period, when T remained below 0 C is shown in Fig. 3(a) by the lo-day running means of T values obtained from the meteorological stations mentioned earlier and from data from Sugatami (1595 m a.s.l.) and Yukomanbetsu (1070 m a.s.l.) in the Mt Asahidake area. The value of T at a given elevation on Mt Asahidake and Mt Teine was calculated assuming a lapse rate of air temperature of 0.55 C/100 m. T -summit of Mt Asahidake Mt Asahidake Mt Teine -timberline of Mt A summit of Mt Tetne -foot of Mt A. Oct Nov Dec Jan Feb Mar Apr May ,foot of Mt T. *L I L Number of day whent is below 0 C Fig. 3 Possible period for solid precipitation, (a) Period when the daily mean air temperature T is below 0 C. (b) Number of days when T is below 0 C. The times when T fell below 0 C and the times when it rose above 0 C along the slopes were fortuitously almost the same at both mountains. The 0 C isotherm descended the slopes at a linear rate of about 30 m of elevation per day in autumn and ascended at 60 m/day in spring. The possible period for solid precipitation during the winter on the summit of Mt Asahidake (2290 m a.s.l.) was longer by 94 days than the period of 127 days at the foot of the mountain. On Mt Teine (1023 m a.s.l.) the same period was 51 days longer than the 107-day period at the foot (Fig. 3 (b)). It has been shown by a previous investigation (Ishii, 1959) that snowmelt by solar radiation takes place on fine days when -3 < T < 0 C. The distribution of elevations at which T = -3 C along the slopes is also shown in Fig. 3(a). From this it can be seen that at a given elevation, snow starts melting about 1-2 weeks earlier than the time when T reaches 0 C. Distribution of snow in the accumulation season Vertical distributions of snow depth, H, and the water equivalent of snow, Hw, along the slopes in the accumulation season, are shown in Fig. 4(a) for Mt Asahidake and in Fig. 4(b) for Mt Teine.

6 114 Tomomi Yamada ef al >* 1» >~ Equ,t- i^- * < *-*-"*--*'". * V - 17 January February 25 February _ 14 March i i i Elevation (m) (b) Fig. 4 (a) Vertical distribution of snow depth on Mt Asahidake during the accumulation season, (b) Vertical distribution of the water equivalent of snow on Mt Teine during the accumulation season. O) 1 Equivalent 1 0 <? 10 March March 25 April 23 May?f^ X ^ / 1 X J' y» ' _ Elevation (m) '"(a) Fig. 5 (a) Vertical distribution of the water equivalent of snow, Hw, on Mt Asahidake during the ablation season, (b) Vertical distribution of Hw on Mt Teine during the ablation season. For the forest zone, on both the slopes observed, H and Hw increase linearly with increasing elevation, Z, at the beginning of the accumulation season and the linear relationships are maintained throughout the accumulation season, with the slope of the regression lines increasing with time. This indicates that the solid precipitation accumulates on the slopes with a linear relationship with Z during the several tens of days between one survey and the next. Meanwhile, for the alpine zone, H is independent of Z, fluctuating widely with time at a given observation site, and varying markedly with the locality. Here snow deposition is patchy and thought to be influenced by interaction between topographical features and blowing snow caused by strong wind, as mentioned later. The amount of deposited snow appears to decrease rather than to increase with Z as seen on the slope below the timberline. Distribution of snow in the ablation season Distributions of Hw with Z on Mt Asahidake and on Mt Teine are shown in Fig. 5(a) and (b) respectively, for the period from the end of the accumulation season to the end of the ablation season. As shown by the broken line (T = -3 C) in Fig. 3(a), snowmelt started in late March, beginning at the foot of the observed slope on both the mountains. The front of the snowmelt line gradually climbed up the slope, reaching the summit of Mt Teine (1023 m a.s.l.) about 9 April, the timberline of Mt Asahidake (1400 m a.s.l.) about 18 April and the summit of Mt Asahidake about 2 May. When T exceeded 0 C at a given elevation, any further solid precipitation was finished and only snowmelt took place. This occurred from 1 April at the foot of Mt Teine (~ sea level), from 8 April at the foot of Mt Asahidake (- 400 m a.s.l.),

7 Snow distribution climate relationships 115 from 18 April at the summit of Mt Teine, from 25 April at the timberline of Mt Asahidake and from 5 May at the summit of Mt Asahidake. During the period when -3 < T < 0 C, solid precipitation occasionally took place and then both melting and accumulation of snow occurred on the observed slopes. As shown in Fig. 5, the distribution of Hw with Z can be represented by linear regression lines throughout the ablation season for the region below the timberline, but again no linear distribution is seen for the alpine zone. In the early ablation season, snowmelt occurred at the foot of the mountains and at the same time snow accumulation continued in the upper regions. Therefore, the slope of the regression line increased rapidly until snow accumulation finished in higher regions, and attained a maximum when the 0 C isotherm reached the summit of Mt Teine (about 18 April) and the timberline of Mt Asahidake (about 25 April). In this early stage of the ablation season, the net mass balance for each period between surveys indicated a linear distribution with Z in the region below the timberline. However in the alpine zone, the net mass balance was independent of elevation and depended on the topography at the observation site. After solid precipitation ceased, (T > 0 0, it was noted that the rate of snowmelt became almost constant over the whole slope,- in both the forest zone and the alpine zone, in spite of air temperature decreasing with Z. The constant rate of snowmelt may be due to increasing wind speed with Z. DISCUSSION As mentioned previously, the alpine zone and the region below the timberline are fairly different from each other in both the accumulation process and the distribution of snow. These differences are explained by differences in wind action at the two locations. The critical wind speed causing blowing snow is observed to be about 7 m s"" 1 (Yamada, 1974), although it varies with the condition of the snow surface and air temperature. According to data obtained at Sugatami (1595 m a.s.l.) in the alpine zone, the frequency of wind speed greater than 7ms -1 exceeded 50% during the winter from December 1977 to March The mean wind speed in this period was about 7 m s~. No wind greater than 7 ms - in speed, however, was measured at Yukomanbetsu (1070 m a.s.l.) in the forest zone nor at the Higashikawa meteorological station (215 m a.s.l.) in the Kamikawa basin. This is also supported by the absence of surface features deriving from blowing snow at every snow survey along the slope up to the timberline. When snow is deposited under a strong wind or when deposited snow is removed by blowing snow, it is generally distributed nonuniformly under the influence of topographical features; snow is deposited mostly in the lee of rises and in depressions. In the alpine zone, blowing snow occurs frequently during winter months. Consequently, a patchy distribution of snow takes place as shown in Fig. 6, a cross section of a snow cover and the distribution of the snow depth along the measured line at a site 1620 m a.s.l.

8 116 Tomomi Yamada et al. in the alpine zone. The figure shows that snow depths at B and D, in valleys, are considerably greater than those at A and E on ridges or at the rise C. As the rate of mass transportation by blowing snow is proportional to the third power of wind speed (Kobayashi et al., 1969), the snow distribution in the alpine zone is completely changed by occasional winter snow storms with wind speeds greater than 30 m s~. For example, it was confirmed m Fig, 6 Cross section of a snow cover and the distribution of snow depth along a measured line at a site 1620 m a.s.l. in the alpine zone. 150r " Fig. 7 Vertical distribution of the water equivalent of snow measured on a ridge (solid circles) and in a valley (open circles) on Mt Teine. A 3 - > cr " UJ - a* a - S _ - ~ ::: S"B ~ 1 1 r. " I I I I ] :! i March in Mt Asahidake March i fin Mt Teine m K -16 March ' '"=- ï~~"~ 1 1 i i i i 1 i i i, i i "p Bevation(m) Fig. 8 Vertical distribution of new snow which fell in one series of events or over several days. - f I 1-!" 1 - /: / ~ - ~ -

9 Snow distribution climate relationships 117 that the abrupt decreases in snow depth at the site 1620 m a.s.l. on 22 January and 1 March (shown by arrows in Fig. 2(a)) were caused by erosion due to violent winds exceeding 30 m s -1 in speed with intense blowing snow. On the same days, the wind speed did not exceed 7 m s~ at Yukomanbetsu or at Higashikawa. In the region below the timberline, deposited snow is maintained during the winter at the place where it originally falls, and is not redistributed by the wind. Therefore, the original distribution of the total amount of solid precipitation falling along the slope is preserved during the period in which there is no melting. The absence of blowing snow results in the snow accumulation being independent of the topography as demonstrated in Fig. 7. No difference in Hw was found, for a given elevation, between sites on the ridge of Mt Teine and in the valley. These observations were made in the forest of Mt Teine at sites indicated in Fig. 1(b) by solid squares for the ridge (5 April) and by open squares for the valley (6 April). Though no wind data were obtained for Mt Teine, the frequency of blowing snow was estimated to be fairly low from consideration of wind data at the neighbouring meteorological station. The same fact is also revealed by snow surveys conducted along a measured line, 1.2 km long, set up along the contour line 1200 m a.s.l. in the forest zone of Mt Asahidake. This line crosses three ridges and two valleys. Almost the same value of Hw was obtained at all the observation sites, regardless of the topography. In conclusion, the distribution of snow below the timberline, where blowing snow is absent, does not depend on topography but only on elevation. A linear distribution of snow with elevation is another important characteristic of snow deposition in the region below the timberline. At the beginning of winter, when solid precipitation starts at the foot of the mountain, snow is distributed linearly with elevation as shown in Fig. 4(a) and (b). Each new snowfall shows a linear distribution of snow quantity with elevation, in Fig. 8, for data from Mt Asahidake (solid circles) and Mt Teine (open and solid squares). This results in the linear distribution being preserved throughout the winter, when the air temperature is below 0 C over the whole mountain area. Finally, the total amount of snow deposited in the forest on the mountain slope, throughout the winter, gives a linear distribution with elevation at the end of the accumulation season, as found by previous investigators (Sugaya, 1948; Higashi et al., 1956; Keeler & Weeks, 1968). From the viewpoint of the accumulation process and the distribution of snow, the timberline should be regarded as one of the important climatic boundaries in mountain areas. ACKNOWLEDGEMENTS The authors wish to express their appreciation to many colleagues in research groups of snow in mountain areas within The Institute of Low Temperature Science for their cooperation throughout the field study season. This work was done with support from a scientific research fund of the Ministry of Education in 1977 (Grant no ).

10 118 Tomomi Yamada et al. REFERENCES Higashi, A., Higuchi, K. & Itagaki, K. (1956) Shikaribetsu ryuiki no sekisetsu suirys chosa (Snow survey in the Lake Shikaribetsu basin). Hokudai Chikyubutsuri-gaku Hokoku (Geophys. Bull. Hokkaido Univ.) 5, Ishii, Y. (1959) Sekisetsu kiso chôsa, Yusetsu no kenkyu (A Study of Snow Melt). Published by The Hokkaido Electric Power Company Incorporated and Sapporo District Meteorological Observatory. Keeler, C. M. & Weeks, W. F. (1968) Investigations into the mechanical properties of alpine snow-packs. J. Glaciol. 1 (50), Kobayashi, D., Kobayashi, S. & Ishikawa, N. (1969) Mizo ni yoru jifubukiryo no sokutei (Measurements of snow drift using parallel trenches). Teion-kagaku (Low Temp. Sci.) A27, Sugaya, J. (1948) Sekisetsu suiryô no teiryô-teki chosa ni tsuite. Taisetsu-san sekisetsu suiryô" oyobi ryushutsu chosa (On the Calculation of the Total Water Equivalent of Snow in the Drainage Area of the River Chubetsu).- Published by Civil Engineering Section in Hokkaido Government. Takahashi, S. & Aburakawa, H. (1976) Kôgaku-sen'i o riyo shita sekisetsu shin kiroku-kei (A snow depth recorder using optical fibres). Teion-kagaku (Low Temp. Sci.) A34, Yamada, T. (1974) Syowa-kichi engan kara Mizuho Kansoku kyoten ni itaru chiiki no kishô jôtai ni tsuite (Surface meteorological condition in the region between Syowa station and Mizuho Camp, Mizuho Plateau, East Antarctica). Nankyoku-shiryo (The Antarctic Record) 50, 1-20.