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Surface characteristics and mass balance of Icelandic ice caps

Surface characteristics and mass balance of Icelandic ice caps

Surface characteristics

The surface albedo or reflectance is a key parameter in the surface energy balance of glaciers and ice sheets because is a primary control on absorption of solar radiation. Remotely-sensed data are being used increasingly to monitor spatial and temporal changes in surface albedo over ice masses. One challenge is to ensure that different sensors, and the different methods used to correct such data for the influence of the atmosphere, produce consistent results.

Our research has taken a high-resolution (5 m) airborne multispectral dataset collected over Langjökull, Iceland, and compared it with near-contemporaneous Landsat Enhanced Thematic Mapper (ETM+) and NASA Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. All three datasets were corrected for atmospheric effects using two commonly-applied techniques, allowing assessment of the similarity of albedo values produced by different atmospheric correction schemes for the same instrument, and then also to compare results from different instruments. This research shows that the atmospheric correction scheme used can affect the calculated albedo values.

It also shows that the albedo values vary between instruments, that there are therefore biases depending on the instrument used, and that these biases vary between different surface types (snow, slush, clean ice, dirty ice). Understanding these effects, and developing improved methods to allow consistency between different satellite sensors and atmospheric correction schemes, will be very important for the ongoing monitoring and measurement of glacier albedo around the world.

Mass balance

In August 2013 we undertook a second airborne survey of Langjökull, Iceland's second largest icecap, six years after our last airborne survey in 2007. The instruments on board included a laser scanner (lidar). The lidar data have been used to generate a very high spatial resolution (2m) digital elevation model (DEM) of the ice cap surface. The 2013 DEM is compared with our previous 2007 DEM and with other lower spatial resolution DEMs for 2004 and 1997 derived from satellite and ground based measurements respectively.

Results suggest that the ~920km2 ice cap lost ~7.52km3 of ice (water equivalent) between 2007 and 2013, a rate of 1.25km3 per year. This compares with losses of 0.37 km3 per year between 2004 and 2007 and 1.64km3 per year from 1997 to 2004. Comparison of the mass balance derived by DEM comparison, with the surface mass balance calculated using a model, allows us to map the dynamic component of the mass balance. It distinguishes areas where the ice cap is losing mass primarily by changes in surface accumulation and runoff, from areas where it is losing mass by dynamic thinning.

Publications

Papers stemming from this project so far:

  • Pope, A., Willis, I.C., Pálsson, F., Arnold, N.S., Rees, W.G., Björnsson, H. and Grey, L., 2016a. Elevation change, mass balance, dynamics and surging of Langjökull, Iceland from 1997 to 2007. Journal of Glaciology, v. 62, p.497-511. doi:10.1017/jog.2016.55.
  • Pope, E.L., Willis, I.C., Pope, A., Miles, E.S., Arnold, N.S. and Rees, W.G., 2016b. Contrasting snow and ice albedos derived from MODIS, Landsat ETM+ and airborne data from Langjökull, Iceland. Remote Sensing of Environment, v. 175, p.183-195. doi:10.1016/j.rse.2015.12.051.
  • Pope, A., Willis, I.C., Rees, W.G., Arnold, N.S. and Pálsson, F., 2013. Combing airborne LiDAR and Landsat ETM+ data with photoclinometry to produce a digital elevation model for Langjökull, Iceland. International Journal of Remote Sensing, v. 34, p.1005-1025. doi:10.1080/01431161.2012.705446.

Figure

Ground surveying the surface of Langjökull, Iceland to complement the airborne lidar surveys being done simultaneously. Photo: Ian Willis.

Figure

Langjökull, Iceland showing a) Landsat ETM+ real colour image from 20 August 2000 (path 220, row 15); b) delineated drainage basins. From Pope et al, 2016b

Figure

Maps of derived albedo for part of Langjökull, Iceland showing the effects of instrument resolution on data variability: a) airborne thematic mapper (multispectral radiometer)(5m); b) Landsat satellite ETM+ (30m); MODIS satellite (500m). From Pope et al, 2016b.

Figure

Map of differences in albedo values between different datasets. a) airborne thematic mapper minus Landsat ETM+; b) airborne thematic mapper minus MODIS; c) Landsat ETM+ minus MODIS. The maps show that differences between the datasets are spatially correlated. There also appear to be four zones where the biases are distinctly different. At higher elevations corresponding to high albedo snowfacies, albedos from the high resolution instrument are less than those from the low resolution instrument. This is true also at mid elevations in the ablation area corresponding to moderately dirty ice facies. Between these two areas is the transition zone facies where the reverse is true; there, albedos from the high resolution instrument are greater than those from the low resolution instrument. Similarly, over very dirty ice facies at low elevations around the glacier margin high resolution instrument albedos are larger than those from the low resolution instrument. From Pope et al, 2016b.

Figure

Change in surface elevation of Langjökull: a) 2004-2007; b) 2007-2013. W.H. and E.H. identify the Western- and Eastern- Hagafellsjökull outlet glaciers, respectively.