++ Vernagt Ferner *
Historical Mass Balances (1942-2011)
Figure 1. Vernagtferener on Sept 13, 2011 (composite photos by M. Kuhn, combined by M. Weber). Commission for Geodesy and Glaciology, Academy of Sciences, Munich, Germany
The historical Net, Ablation and Accumulation daily and annual balances of the Vernagtferner, Austria are determined for the 1942-2011 period with the PTAA model. Input to the model are daily precipitation and temperature observations collected at the Innsbruck weather station, located 100km NE of the glacier, together with the area-altitude distribution of the glacier. The mean annual (Net) balance for this 70-year period is -0.03 mwe, the Accumulation balance is +0.8 and the Ablation balance is -0.8 mwe. The glacier gained mass from 1952 to 1989, then thinned from 1990 to 2011. The annual balance in 2003 was -1.7 mwe, the most negative for the period of record (70 years), caused by a combination of one of the lowest Accumulation balances on record and much above normal temperatures during the ablation season. The average Equilibrium Line Altitude is 3040 meters.
Mass Balance Results
The PTAA Mass Balance Model is applied to the area-altitude distribution shown in Figure 2 to produce daily and annual balances, using as input the daily temperature and precipitation observations at Innsbruck, Austria, located approximately 100 km northeast of Vernagtferner. Available weather records for this station are for the 1942-2012 period.
The annual mass balance for Vernagtferner averaged for 70 years (1942-2011) is -0.03 mwe (Figure 3). oss (Figure 4). The average Accumulation Balance is 0.8 mwe and the Ablation Balance is -0.8 mwe (Figure 5).
Mass balance terminology used in this report deviates slightly from that proposed by the IACS Working Group of Mass Balance Terminology and Methods (Cogley et al, 2011). The main variables that determine mass balance, such as precipitation, temperature, lapse rates and snowfall, are directly dependent on elevation. Glacier balance models are constructed by applying relationships between these variables (for example precipitation and temperature) to calculate mass balance as a function of elevation. The IACS study suggests that elevation should be shown on the Y axis. In this report, balance and other variables are all shown to be dependent on elevation.
In addition, the terms Accumulation Balance and Ablation Balance are preferred over Winter Balance and Summer Balance, that are used in the IACS report. Snow accumulation continues at higher elevations for this and other Alpine glaciers throughout most of the year. For many Himalayan Range glaciers, snow accumulation is greatest during the Monsoon season, from June through September, and at lower elevations ablation often occurs during the winter months.
Figure 2. Area-altitude distribution of Vernagtferner. The total area of the glacier is 7.92 km2 and there are 19 altitude intervals spaced at 50 m, ranging from 2750 to 3650 meters in elevation. Latitude 46.866 N Longitude 10.833 W
Figure 3. Annual balances of the Vernagtferner, 1942-2011 period. The average annual balance is -0.03 mwe. The minimum annual balance for the period (-1.6 mwe) occurred in 2003. The extremely negative balance this year is due to unprecedented ablation produced by much above normal summer temperatures, and an unusually low winter snowfall.
Figure 4. Cumulative balance of Vernagtferner. The glacier gained mass from 1952-1989 and thinned 9 meters of ice from 1990 to 2011.
Figure 5. Accumulation and Ablation Balances. The average annual Accumulation balance for this period is +0.8 mwe and the average Ablation balance is -0.8 mwe. The minimum Ablation Balance for this period (-2.2 mwe) occurred in 2003, and is partly due to the below normal Accumulation balance of 0.55 mwe.
Balance versus Elevation
The net, accumulation and ablation balances as a function of elevation are shown in Figure 6, averaged for the 1942-2011 period, Average ablation at the terminus is 2 mwe and accumulation ranges from approximately 0.3 mwe at the terminus to a maximum of 1.0 mwe above 3600 meters. The ELA is 3040 meters.
Figure 6a. Net, Accumulation and Ablation balances as a function of elevation, averaged for the 1942-2011 period. Accumulation ranges from 0.5 mwe at the terminus to 1.0 mwe at 3600 meters. The ELA is 3040 meters compared to 3120 meters that is observed for the 1965-2011 period.
Figure 6b. Net, Accumulation and Ablation balances as a function of elevation, averaged for 2003. The ELA is 3570 meters.
The model is calibrated for Vernagtferner by calculating the daily balance using 15 coefficients that convert precipitation and temperature observations to snow accumulation and snow and ice ablation for each elevation interval. One iteration performs this operation for each day of the full period of record and for each elevation interval, from the terminus to the head of the glacier. Both the annual balance and calibration error are calculated for each iteration. In Figure 7a, the first 16 iterations use pre-set coefficients; the remaining are determined automatically by the Simplex optimizing procedure. In Figure 7b, the annual balance and calibration error is shown for each iteration.
Optimum coefficient values are found by minimizing the error that occurs when one balance variable is regressed against another in a linear, least squares fitting process. Nine different but not independent regressions are made for each day throughout the summer ablation season. From June 1 to September 30 approximately 120 regressions are made for each set of variables. Thus, over 1000 regressions are run for each iteration of the Simplex. The calibration error determined for each iteration is the average root-mean-square-error (in percent of the mean) for all regressions that are run. The calibration error is minimized by simultaneously and minutely adjusting the 15 coefficients for the next iteration. Minimizing the calibration error produces an internal consistency in the processes within the PTAA model that cause snow and ice ablation and snow accumulation.
Figure 7a. Calibration error versus iteration number of the optimizing Simplex. Annual balances 1-15 are calculated with preset coefficients. Balances from 16 to 300 are calculated with coefficients determined by the Simplex optimizing process. The final mean (1942-2011) balance after about 300 iterations equals -0.02 mwe.
Figure 7b. Mean annual balance versus calibration error. The mean annual balance equals -0.02 when the calibration error is a minimum at 52%. Each point represents one iteration. At the end of each iteration a retrospective is applied to seek ways to improve the process for the next iteration.
Real-time Glacier Balances
One goal of the PTAAGMB project is to continuously monitor all 200 glaciers in the set and display the current mass balance of each one, in real-time if up-to-date weather observations are available, or near real-time if weather observations are delayed. The daily balances for the 2003 balance year shown in Figure 7 demonstrate how real-time mass balances will be displayed in the future years
Figure 8. Daily Accumulation, Ablation and Net Balance for the 2003 balance year. On September 30, 3003 the Accumulation Balance = 0.5 mwe, Ablation Balance = -2.2 mwe, and Net Balance = -1.7 mwe. The low snowfall and high summer temperatures made this year's balance the most negative for the period of record.
Comparison of Manual With PTAA Balances
Annual balances have been measured for Vernagtferner using the glaciological method since 1965 (reference). Retrospective balances are calculated independently with the PTAA model for the 1942-2011 period. Comparison of annual balances for the 1965-2011 period produced by the two methods are shown in Figure 9a. It is emphasized that the PTAA balances are produced without beforehand knowledge of the manually measured balances. Calibration of the model is accomplished by minimizing the error of regressing balance variables that are calculated internally. The vaiance that is shown in Figure 9b is caused by balance errors in both methods.
Figure 9a Annual balances of the manually measured and the PTAA balances for the 1965-2011 period.
Figure 9b Manually measured annual balances versus PTAA balances. The R2 for a regression fit is 0.67 and the average PTAA balance tends to be 0.15 mwe lower than manually measured balances. Both methods contribute to the scatter shown for this regression.
Figure 9c Ablation balances of the manually measured and the PTAA for the 1965-2011 period.. Both methods contribute to the scatter shown for this regression.
Figure 9d Annual balances of the manually measured and the PTAA Accumulation balances for the 1965-2011 period. . Both methods contribute to the scatter shown for this regression.
The mass balance of Vernagtferner is generated daily and
annually using the daily
precipitation and temperature observations collected at the weather station in Innsbruck, Austria, and the area-altitude distribution of this glacier. The accuracy of the annual balances and the distribution of balance with altitude appears to be approximately equal to accuracy produced by manually measured balances using the glaciological method.
Beedle, Matthew ,www.GlacierChange.org
A new website that chronicles the rapid changes in the world's glaciers.
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