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Saturday, July 21, 2012

Jason Box: Latest Greenland Ice Sheet Reflectivity

Latest Greenland ice sheet reflectivity 

These albedo visualizations are discussed here and here.

[Readers, be sure to see the comments at the end.]

About the Data
Surface albedo retrievals from the NASA Terra platform MODIS sensor MOD10A1 product beginning 5 March 2000 are available from the National Snow and Ice Data Center (NSIDC) (Hall et al., 2011). The daily MOD10A1 product is chosen instead of the MODIS MOD43 or MCD43 8-day products to increase temporal resolution. Release version 005 data are compiled over Greenland spanning March 2000 to October 2011. Surface albedo is calculated using the first seven visible and near-infrared MODIS bands (Klein & Stroeve, 2002; Klein & Barnett, 2003). The MOD10A1 product contains snow extent, snow albedo, fractional snow cover, and quality assessment data at 500m resolution, gridded in a sinusoidal map projection. The data are interpolated to a 5 km Equal Area Scalable Earth (EASE) grid using the NSIDC regrid utility April and after September, there are few valid data, especially in Northern Greenland because of the extremely low solar incidence angles. The accuracy of retrieving albedo from satellite or ground-based instruments declines as the solar zenith angle (SZA) increases, especially beyond 75 degrees, resulting in many instances of albedo values that exceed the expected maximum clear sky snow albedo of 0.84 measured byKonzelmann and Ohmura (1995). Here, we limit problematic data by focusing on the June–August period when SZA is minimal.

Stroeve et al. (2006) concluded that the MOD10A1 data product captured the natural seasonal cycle in albedo, but exhibited significantly more temporal variability than recorded by ground observations. We now understand that a dominant component of this assessed error is the failure of the MODIS data product to completely remove cloud effects. Inspection of the raw MOD10A1 images reveal an abundance of residual cloud artifacts (shadows, contrails, thin clouds, cloud edges) in the albedo product, presumably because the similar spectral properties between snow and some clouds results in obvious cloud structures. Another problem consists of spuriously low values, for example below 0.4 in the accumulation area where albedo is not observed by pyranometers at the surface to drop below 0.7, seen as linear stripe artifacts in the imagery. Because both the cloud shadows and stripes introduce abrupt daily departures from the actual albedo time series, it is possible to reject them using a multi-day sample. Thus, on a pixel-by-pixel basis, 11-day running statistics are used to identify and reject values that exceed 2 standard deviations (2 sigma) from an 11-day average. To prevent rejecting potentially valid cases data within 0.04 of the median are not rejected. The 11-day median is taken to represent each pixel in the daily data and has a smoothing effect on the albedo time series. June–August (JJA or summer) seasonal averages are generated from monthly averages of the daily filtered and smoothed data. Redundant data from the Aqua satellite MODIS instrument are not used in this study for simplicity, to reduce computational burdens, and given an Aqua MODIS instrument near infrared (channel 6) failure (Hall et al., 2008) that reduces the cloud detection capability. ( The interpolation method employs a trend surface through the surrounding four 500-m grid cell values closest to the grid points. The resulting 5-km spatial resolution permits resolving the ablation area within the goals of this study. Major gaps in the time series occur July 29–August 18, 2000, and June 14–July 7, 2001. The frequency and quality of spaceborne albedo retrievals decreases in non-summer months as the amount of solar irradiance and solar incidence angles decrease. Also, in non-melting periods before

Works Cited
  • Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere Discuss., 6, 593-634, doi:10.5194/tcd-6-593-2012, 2012.
  • Hall, D. K., J. E. Box, K. Casey, S. J. Hook, C. A. Shuman, K. Steffen, Comparison of satellite-derived and in-situ observations of ice and snow surface temperatures over Greenland, Remote Sensing of Environment, 2008.
  • Hall, D. K., Riggs, G. A., and Salomonson, V. V.: MODIS/Terra Snow Cover Daily L3 Global 500m Grid V004, January to March 2003, Digital media, updated daily. National Snow and Ice Data Center, Boulder, CO, USA, 2011.
  • Klein, A. G. and Barnett, A. C.: Validation of daily MODIS snow cover maps of the Upper  Rio Grande River Basin for the 2000–2001 snow year, Remote Sens. Environ., 86(2), 162–176, 2003.
  • Klein, A. G. and Stroeve, J. C.: Development and validation of a snow albedo algorithm for the MODIS instrument, edited by: Winther, J. G. S. R., Ann. Glaciol., 34, 45–52, 2002.
  • Konzelmann, T. and Ohmura, A.: Radiative fluxes and their impact on the energy-balance of the Greenland ice-sheet, J. Glaciol., 41(139), 490–502, 1995.

3 Responses to “Latest Greenland ice sheet reflectivity”

  1. Greenland melt record likely Says:
    [...] the latest “noodle plot”2 (regularly updated here) for the ice sheet between elevations of 2,000 and 2,500 metres. 2012 (the black line) is well down [...]
  2. Rob Dekker Says:
    Dr. Box, thank you for showing the disturbing change in albedo at Greenland this year.
    Quick question with possibly significant consequences :
    We know that Greenland receives some 250–300 W/m^2 insolation ‘on the ice’ during June/July.
    With a change in albedo of 3 % to (as your recent numbers show) to 6 %, how much ice will melt over the entire ice sheet due to this change in albedo alone ?
    If we do the simple physics calculations of increased solar absorption of 3-6% albedo change, we get to a ice loss anomaly of some 7–14 cm or, over the entire 1.7 million km^2 ice sheet, an additional loss of 120–240 Gton per month (that the surface temps remain close to freezing on Greenland) due to this albedo anomaly alone.
    Please tell me that these calculations are not right, because if they are, we should be very concerned…
  3. Jason Box Says:
    Rob, I like this kind of calculation, do something similar in the attached, find the extra energy erodes 14 cm of the ‘cold content’ of the upper snow layers across the accumulation area… Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere Discuss., 6, 593-634, doi:10.5194/tcd-6-593-2012, 2012. DOWNLOAD LATEST, ACCEPTED VERSION


reisner said...


So under current conditions, we have a stabilizing feedback working in humanity's favor by slowing the amplifying albedo feedback and thus keeping the current rate of ice loss about linear (but accelerating).

In the near future from a policy perspective (10-20 years), additional warming will likely overide this dampening feedback loop and the rate of ice loss is likely to cross a tipping point and transition from linear to non-linear dynamics?

Tenney Naumer said...

Reisner, you should leave questions for Dr. Box on his blog - see link at bottom of post, please.