Field Report Snow and Ice Processes AGF212

Transcription

1 Field Report 2013 Snow and Ice Processes AGF212 (picture) Names...

2 Contents 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers Introduction Instruments and method Specific Balance 2013 getting Net balance 2013 getting Total Mass balance Positive Degree-day model Results Tellbreen Blekumbreen Fangenbreen Ice ablation model performance Discussion Errors on Total mass balance Ice Ablation model discussion appendix

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4 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers by Lucas DAVAZE et Marion DONAT-MAGNIN Abstract Introduction This fieldwork permits to establish the mass balance from march 2012 to march 2013 about three glaciers : Tellbreen, Blekumbreen and Fangenbreen localized around 10 km, East of Longyearbyen. A Static Mass Balance sensitivity was effected. We indeed neglect change in the glacier size, shape, geometry and only measured the ice/snow inter-annual thickness variation. Some stakes were installed in Marthabreen to obtain mass balance over the following years. The past studies (AGF-212 previous report) show that these three glaciers lost mass, Tellbreen lost in average 1.30 m.w.eq.a-1 from march 2011 to march 2012, 2.55 m.eq.w.a-1 for Blekumbreen. The Mass Balance were done over the past 5 years (except 2010) for Tellbreen, 2 years for Blekumbreen. A positive degree-day approach was also effected for Tellbreen and Blekumbreen, including a basic hourly temperature-index and the energy balance term. 1.2 Instruments and method The aim is to see how Svalbard glaciers reacts over temporal scale. We calculed the actual specific balance, net balance and total net balance for the entire glacier from the stakes measurements. We didn t include basal melting and mass loss by calving process because the glaciers have any contact with the sea Specific Balance 2013 getting 34 stakes were installed between 2009 and 2013 over Tellbreen, Blekumbreen, Fangenbren and Marthabreen (Map GPS chapter). To obtain specific balance on each stakes for all glacier, data from 2012 have been used. s b = (b 2012 b 2013) α (1.1) 1

5 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers where b 2012 is distance from ice to the top of stake in 2012 and b 2013 in 2013 (that we measured this year), α = 0.9 is a coeficient to convert s b in m.ice.eq to m.w.eq, so all values concerning specific balance and net balance are in m.w.eq. If s b <0 we lost ice mass between march 2012 and march 2013, if s b >0 we gained ice. With this method we can have specific balance for each stake drilled before However, values for stakes 2, B1, B2, C2, 31, 32, 33, 34, 35 are missing due to impossiblity to find the stake or the cable, this is the case for C2 and Fangenbreen s stakes, or caused by a lack of data. For Tellbreen 10 values on 15 stakes are available, all values for Blekumbreen, 1 values on 5 stakes for Fangenbreen, that s why we drilled new stake (4). Few stakes was hard to find on Fangenbreen, the mainly cause was the stake was a cable stocked inside the ice due to strong melting and refreezing during winter time. Concerning Marthabreen, all the stake were drilled this year Net balance 2013 getting B = A 0 s b d A (1.2) To use this definition we need to divided the glacier in different subarea figure 1.1. One stake is at least represented by one subarea. To have more accuracy we took the same subarea than last year for Tellbreen and Blekumbreen, we just divided Fangenbreen in 5 areas. Some steep parts next to Telllbreen was not taken because they follow different regime. When we have several stakes on a same subarea we took the mean specific balance between those stakes to establish the net balance B. Afterward mapping software OziExplorer (Des Newman, Brisbane Australia) has been used to get values of each area. Figure 1.1: Map of Tellbreen, Blekumbreen and Fangenbreen glaciers, all divided in smaller subarea 2

6 1.2 Instruments and method Total Mass balance Regarding the total mass balance of each glacier from march 2012 to march 2013 we sum previous net balance from each area in order to obtain the net Balance for the entire glacier. B t o t = n s b n,i Ai (1.3) k= Positive Degree-day model An other part of the mass balance diagnostic is related to the Melt rate of the ice and snow, in order to predict the future ablation/accumulation of the glacier for a close future. The Positive Degree-day method was used for several reasons. Firstly, because it is easily computable in reason of the temperature record abundance. It s then possible to forecast the glacier Mass via the Degree-day factor. Therefore, for such a student study, almost any instrument are required. We can quickly hint the PDD principle : "Daily ablation at is proportional to the daily mean temperature as long as the temperature is at or above the melting point" (Braithwaite and Olesen, 1989) Temperature Index The theoretical formula for the temperature index method is : M = P D D D D F I c e /Snow (1.4) M is the thickness of Ice/snow melted, PDD the positive degree-day number from an hourly average. D D F I c e /Snow is the degree-day factor of the Ice or the Snow. An empirical value was chosen for the snow (D D F Snow = 7.5mm.d 1.K 1. We preferred to determine the PDH (Positive Degree-Hour) to be more accurate and take care of some possible positive temperature peak in a day. With the two weather stations (557m,667m) present on Tellbreen all over the year, we determined a particularly lapse rate of C m 1, very close to the values usually found for Svalbard (0.008 C over 6 glaciers, De Woul and Hock, 2005). The precipitation data (mm.w.e q ) were taken from the Svalbard airport. Each missing or unusable values were replaced by 0. The fresh snow density in order to determine the snow precipitation has been defined as 362 kg.m 3, mean density of the snow pack over Tellbreen/Blekumbreen (Snow Pits chapter). The melting/freezing temperature was define as T o = 0 C. The rain over the glacier was neglected to simplify the accumulation/ablation process. A single temperature has been affiliate at each stake for the year in order to determine the specific PDH and then, the melt rate of ice. The mean value of the melt rate is then an average of each stake. The ice melting season was defined at the moment were the snow depth is zero. We can then plot the melting season and the ice melting season. 3

7 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers Energy Balance The model is including the energy balance as the sum of the net solar radiation Q and the Longwave incoming radiation L (70% of the energy balance term) via the Stefan-Boltzmann law : L = εσ T 4 (1.5) with ε the Emissivity of the surface (assuming that E s now = E i c e = 0.98) and the Stefan-Boltzmann constant σ = J.s 1.m 2.K 4. The total energy balance is H = V A = m ρ = Q L ρl (1.6) with H the loosing thickness due to the radiation impact (m), V the volume melted (m 3 ), A the area of the entire glacier (m 2 ), m the ice mass melted (kg ), ρ the density of the snow pack taken from an average of the snow pit study or the ice (assuming to be 900 kg.m 3 for pure glacier ice), the net solar radiation Q (W.m 2) and the latent heat of the ice/snow fusion L = 334K J.kg 1. The albedo factor is including in the net solar radiation term, difference between the incoming and outgoing solar radiation. The global melting was considered by the Hock relation, 1999 : M = (C 1 + C 2 I )(T T 0 ) (1.7) We can then plot the total amount of PDD over the 2 glacier and also the amount of PDD really use to melt ice figure??, figure??. Figure 1.2: Global and Ice effective positive degree-day on Tellbreen Figure 1.3: Global and Ice effective positive degree-day on Blekumbreen 4

8 1.3 Results 1.3 Results Tellbreen Figure 1.4: Specific Balance versus Elevation Figure 1.5: Net Balance versus Elevation Both figures above represent values from 2012 to march 2013 for Tellbreen glacier. We put all the values in the Appendix with specific balance sb that we calculated with equation 1.1 net balance with equation 1.2 and total net balance equation 1.3, that correspond at the global gain or loss for the entire glacier. We plotted specific balance and net balance versus elevation. We can see on figure 1.4 and figure 1.5 two accumulation area, one on glacier front (358 m) with positive Specific balance around 2.25 m.w.eq and positive Net balance at m 3.w.eq. This accumulation area is followed by an ablation zone from 382 m to 581 m, meaning negative specific balance and net balance. The top of Tellbreen is dominated by accumulation. The values on stake 7 is not really accurate, the stake was tilted.,two differents values were observed at 600m, positve value for stake A1 and negative values for stake C1. This difference between those two stakes coming from the same elevation can be local factor such as avalanching. We can see on the map that Stake A1 is located close to a steep slope, proper conditions to accumulate material and obtain positive mass balance. On those plot two distinct ELA (Equilibrium Line Altitude) are observed where the Mass change is zerol. For Tellbreen, the first ELA is located between 382m and 385m and the second one between 581m and 583m. We didn t consider ELA at 600m due to local factor at stake A1. On figure 1.6 we ploted the Net balance for entire glacier (Tellbreen) versus year. This year we measured from march 2012 to march Compare to previous values from 2009 to 2012 we obtain something different. This year according to our values, Tellbreen gains mass over the entire area. It gains m 3.w.eq. Last year Tellbreen lost m 3.w.eq. 5

9 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers Figure 1.6: Total Mass Balance (Btot) versus years from 2009 to 2013 Figure 1.7: Specific Balance versus Elevation 6

10 1.3 Results Figure 1.8: Net Balance versus Elevation Figure 1.9: Total mass balance for Blekumbreen from march 2011 to march

11 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers Blekumbreen We ploted the same things than Tellbreen. We have only abalation on blekumbreen with maximum of ablation about m.w.eq on stake 215 figure 1.7. Equilibrium line doesn t exist on this case. We observe less ablation on stake 22 figure 1.8 due to the size of this area, area 22 is bigger than others Fangenbreen We obtain only one values due to lack of data from last year (Appendix) and cable problems. We get an accumulation on stake 36 about m.w.eq. The weak amount of values doesn t permit to obtain a model. It s hard to conclude on something concrete exept accumulation area on glacier top Ice ablation model performance The Degree-day factor needs to be used with precaution. The data used for the PDD of Tellbreen and Blekumbreen has been record only for 344 days while the PDD statement are usually established for at least 5 or 7 years (Braithwaite, 2005; De Woul and Hock 2005). The full model (energy balance with temperature index) is only computing over 263 days. Nevertheless, this period cover the summer, usually the only period of melting in the arctic region. The alblation seasons is clearly shorter and less intense on Tellbreen, figure 1.2 than for Blekumbreen figure 1.3. This can be explained by the wind pattern, mainly in the Blekumbreen axes (East-West), figure??, blowing out the snow and increasing the ice melting over the glacier, in contrast of Tellbreen, probably receiving snow from wind transport. 1.4 Discussion The obtention of two ELA for Tellbreen look quite different than previous year, in 2012, the previous team found ablation over the entire glacier so no ELA was defined. This year Tellbreen get accumulation so we can expect than this accumulation come from an increase amount of precipitation. We can test our hypothesis with data from Longyearbyen airport which daily measured the amount of precipitation. We get m from march 2011 to march 2012 and march 2012 to march 2013 figure 1.12, that can not explain why Tellbreen gains m 3.w.eq. Local variation of snow distribution can maybe explain this phenomenon. Accumulation comes from glacier front and top that can be explain by mechanical processes as topography, avalaching or blowing snow from winds. About topography there are no uphill after glacier front. So front accumulation can be not explain by that. Same probleme for avalanching, this part is surrounding by slope but not enough steep to accumulate 2.25 m.w.eq by avalanche process, but for the top of the glacier it s steep enough, that can explain accumulation on top. About wind pattern figure?? figure??, we obtain area dominated by easterly winds. Winds can carry snow, particules, so modifies locally the snow 8

12 1.4 Discussion distribution. This process can mainly explain why we have accumulation on Tellbreen front and top. Figure 1.10: Wind mean direction and speed at the Weather Station I, 557m Figure 1.11: Wind mean direction and speed at the Weather Station II, 660m Concerning Blekumbreen we obtain something quite different than Tellbreen for a glacier located not so far. Compare to the previous yearl Blekumbreen lost m 3.w.eq instead of m 3.w.eq over the entire glacier. Blekumbreen continues to loose mass, but this year it lost less than last year. The wind pattern dominated by easterly wind can explain why it s loosing more than Tellbreen, but can not explain why it lost less mass than Unfortunatey the amount of precipitation can not explain that either.we have less precipitation in We can expect that the difference results between Blekumbreen and Tellbreen comes from wind pattern, making some accumulation on Tellbreen which is the more exposed to the wind. It can be because of climate, sun exposition. We calculed the balance gradient for both glacier to find out the degree of continentality. We found a higer balance gradient for blekumbreen (coeficient about 8698 m 3.w.e q /m) than for Tellbreen (about 6631m 3.w.e q /m). Blekumbreen is considered with low degree of continentality compare to Tellbreen. It behave more like maritime glaciers with high abalation and high accumulation. Concerning accummulation on Tellbreen, we purpose another reason. Last year student found at stake 1 a layer of superimposed ice about 6.5 cm. This kind of ice come from melting event following by refreezing event which creates melt and refreeze water into snowpack. If the superimposed ice layer from last survived at last summer that become a part of the glacier accumulation (Hagen and Reeh,2003). This formation usually happens in Arctic Glacier and can be " dominant form of accumlation" (Koerner,1970). We tested our hypothesis with empirical relationship between air temperature (θ ) and maximum thickness of superimposedice S. S = 0.69(θ ) (1.8) We find a maximum thickness of superimposed ice about 4.78 cm with θ = 6.91 C 9

13 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers Figure 1.12: Average of Precipitation from 2005 to 2013 in Longyearbyen Airport 10

14 1.5 appendix. This is really less than our 2.25m.w.eq of accumulation. So that can not explain accumulation on glacier front. Another causes for that is maybe we had colder summer 2012 than summer Summer times are most important factors for glacier balance Errors on Total mass balance During the fieldwork we measured and obtained some data including some errors. We are here establishing errors measured for the total mass Balance on Tellbreen and Blekumbreen. Firstly, we determined the net balance error on each stake equation(1.5). To do that the absolut error on b (prob measurement from ice to the top of stake) is 0.05m, and the absolut error on Area calculated with Oziexplorer is 1000m 2. We used b measurement to obtain the absolut error on specific balance s b equation (1.7). And thus, to obtain the absolut error on total mass balance we used equation (1.7) whethe the sum of previous results. We have for Tellbreen a gain of ± m 3.w.eq and for Blekumbreen a loss of 2.114± m 3.w.eq. So we estimate more than 10% of error that can be quite a lot compare accuracy method like geodetic method where we an have 6% of error (Funk et al.,1997) but it still acceptable if we consider the lake of data for some stake, fieldwork condition, lake of accuracy about area division etc... δb = ( δs b S b + δa A ) B (1.9) δb 2012 δb 2013 δs b = ( + b 2012 b 2013 ) S b (1.10) δb t o t = δb (1.11) Ice Ablation model discussion In despite of the good results obtained by the Positive Degree-day method, some problems still persist. Few researches show that the Degree-day factor change for different seasons (Braithwaite and Olesen, 1993). Ablation seems to not depends only from the Positive-Degree day but also from the mean temperature of the season, even from month. Braithwaite (1995) experienced the ratio between the number of day with PDD and the PDD itself and the results clearly depends on the mean temperature of the day. Furthermore, the phase change Ice/Snow at 0 C is complex and melting or freezing can occur at ± 0 C. The wind can play a consequent impact, on the snow transport but even in the heat transport appendix 11

15 1 Mass Balance and Positive degree day approach on Spitzbergen Glaciers Figure 1.13: Appendix 12

16 1.5 appendix Table 1.1: Total mass balance Tellbreen Tellbreen total gain [m 3.w.e q ] h lost [m] 23466,8655 0,0243 Table 1.2: Total mass balance Blekumbreen Blekumbreen total lost [m 3.w.e q ] h lost [m] ,704-2,44 13