Archive for June, 2012

Greenland ice sheet albedo continues dropping at highest elevations

Wednesday, June 27th, 2012

Greenland ice sheet albedo in the elevation range from 2000-2500 m. This is the accumulation area where some melting is observed. Much more melting occurs at lower elevations.

With the most recent update to our near-real time monitoring of ice sheet albedo, we observe that the ice sheet albedo continues dropping into unprecedented low values especially at the higher elevations where there is little melting.

I wonder:

  1. Are the widespread wildfires, for example in Siberia or in Colorado adding to the albedo reduction?
  2. Or given that “Since 2000, global coal consumption has grown faster than any other fuel. ” … “Around 6 Gt of hard coal were used worldwide last year and 1 billion tonnes of brown coal. ” source: http://www.worldcoal.org/coal/uses-of-coal/ Is coal combustion part of the problem?

Specialists in snow impurities and albedo are on this problem. But, what are their results?

Greenland ice sheet reflectivity at record low, particularly at high elevations

Monday, June 25th, 2012

NASA MODIS MOD10A1 data from 1-22 June 2012 versus the previous 12 June periods spanning 2000-2011.

An updated compilation of NASA MODIS observations of Greenland ice surface reflectivity through 22 June, 2012 indicates that now, well into into the 2012 melt season, the ice sheet remains in a darkened state (see Greenland Ice Sheet Getting Darker).

Ice sheet reflectivity this year has been the lowest since accurate records began in March, 2000.
In this condition, the ice sheet will continue to absorb more solar energy in a self-reinforcing feedback loop that amplifies the effect of warming. It’s not a runaway loop, just an amplifier. A record setting melt season is likely if this pattern keeps up this year.
Perhaps most remarkable about the 2012 pattern is how much darker the snow and ice is becoming, not only at the lowest elevations around the ice sheet periphery where melting is always most intese, but in the higher elevation net snow accumulation area. June monthly average reflectivity is below the 2000-2011 average across the southern-central area where surface elevations are above 2,000 m (6,561 feet). A purple area about 1/4 the distance north of the ice sheet southern tip at an elevation of 2,400 m (7,874 ft) has reflectivity  7% below the already declining 2000-2011 June (12 year) average.
Consistent with the low albedo anomaly at high elevations is the shift of the summer radiation balance from negative (cooling) to positive (heating) (Box et al. 2012). In the 12 years between 2000 and 2011 the high elevation ice sheet net radiation (sum of upward and downward solar and infrared radiation) approached positive values. What I expect we will see if these low albedo conditions persist is 100% surface melting over the ice sheet. This would be a first in observations. It may not happen this year, but the trajectory the ice sheet is on, along with amplified Arctic warming, will have the ice sheet responding by melting more and more.
Updated visualizations of the ‘noodle plot’ to the right are maintained here.

The cause of the low reflectivity involves a combination of multiple factors:

  1. Abnormally intense melt at low elevations erases bright white snow, exposing a darker impurity rich bare ice surface. When the melt back of winter snow happens earlier, the anomaly grows.
  2. in areas where snow remains, temperature-driven snow metamorphism reduces reflectivity by rounding the sharp ice crystal edges that scatter visible light (Wiscombe and Warren, 1980; Dozier et al., 1981; Warren, 1982). This NOAA climate watch article includes a very useful photo. Fresh snow reflects ~84% of solar energy (Konzelmann and Ohmura, 1995).
    . This fraction, called the albedo, decreases with increasing snow effective grain size;
  3. Increased snow liquid water content in areas of enhanced melting increases absorption of visible light; and
  4. potentially less summer snowfall as in year 2011. Summertime snow events take the edge off the amplifying feedback by brightening the surface. With climate warming, the ratio of snowfall to rainfall decreases. It actually does rain on the lower elevations of the ice sheet. I measured 5 cm rainfall in a single 24 h period in June 1998 at Swiss Camp located at 1,150 m elevation along the central western slope of the ice sheet.
  5. atmospheric circulation that colleague Dr. Xavier Fetteweis at University of Liège, Belgium has been examining for Greenland and plans to post an analysis here.
  6. The possibility of increased snow impurities like carbonaceous soot from wildfires or diesel exhaust can lower ice sheet reflectivity.

I don’t know the relative contribution of impurities versus the reflectivity reduction resulting from the first 3 melt factors. Yet, the pattern of concentrated low reflectivity around the ice sheet periphery  indicates the earlier loss of winter snow in the ablation area of the ice sheet where bare ice is exposed each year sometime during the melt season. That exposure is just happening earlier in the year. The pattern over the far northwestern ice sheet, over the Humboldt glacier is a strong suggestion of increased melting. Tedesco et al. (2011) reported about the increase in bare ice area and reduced snow accumulation in allowing albedo to increase melting.

Enhanced ice sheet melting is likely promoted by changes in the surrounding marine environment:

  1. Abnormally high sea surface temperatures (see the DMI’s nice web product and select anomaly from the drop down menu).
  2. At-record setting low Arctic sea ice area (see NDISC’s operational sea ice extent visualization products).

There is certainly more to the story, such as the role of atmospheric circulation in pumping warm air up from the south as in the case of the former record setting year 2011 low albedo anomaly. That circulation anomaly is described in a paper I’m completing the rebuttal for. This is after an intensive external and anonymous review process for the paper:

  • 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

I paste below the abstract of that study that contains yet more relevant information:

  • Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. Using satellite–derived surface albedo with calibrated regional climate modeled surface air temperature and surface downward solar irradiance, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback over twelve summer periods beginning in 2000. We find that while albedo feedback defined by the change in net solar shortwave flux and temperature over time is positive over 97% of the ice sheet, when defined using paired annual anomalies, a second order negative feedback is evident over 63% of the accumulation area. This negative feedback damps the accumulation area response to warming due to a positive correlation between snowfall and surface air temperature anomalies. Positive anomaly–gauged feedback concentrated in the ablation area accounts for more than half of the overall increase in melting when satellite derived melt duration is used to define the timing when net shortwave flux is sunk into melting. Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007 enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area approached positive values during this period. Thus, it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.

According to a cross validation with independent GC-Net AWS data, degrading MODIS instru- ment sensitivity identified by Wang et al. (2012) is not here detected in the MOD10A1 product.

Works Cited

  • 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.
  • Dozier, J., Schneider, S. R., and McGinnis, D. F.: Effect of grain-size and snowpack water equivalence on visible and near-infrared satellite-observations of snow, Water Resour. Res., 17(4), 1213–1221, http://dx.doi.org/10.1029/WR017i004p01213doi:10.1029/WR017i004p01213, 1981.
  • Tedesco, M., X. Fettweis, M.R. van den Broeke, R.S.W. van de Wal , C.J.P.P. Smeets, W.J. van de Berg, M.C. Serreze and, J. E. Box, The role of albedo and accumulation in the 2010 melting record in Greenland, 2011: Environ. Res. Lett. 6 014005, doi: 10.1088/1748-9326/6/1/014005.
  • Wang, D., Morton, D., Masek, J., Wu, A., Nagol, J., Xiong, X., Levy, R., Vermote, E., and Wolfe, R.,: Impact of sensor degradation on the MODIS NDVI time series, Remote Sens. Environ., 119, 55–61, http://dx.doi.org/10.1016/j.rse.2011.12.001doi:10.1016/j.rse.2011.12.001, 2011.
  • Warren, S. G.: Optical-properties of snow, Rev. Geophys., 20(1), 67–89, http://dx.doi.org/10.1029/RG020i001p00067doi:10.1029/RG020i001p00067, 1982.
  • Wiscombe, W. J. and Warren, S. G.: A Model for the spectral albedo of snow, 1. Pure snow, J. Atmos. Sci., 37(12), 2712–2733, http://dx.doi.org/10.1175/1520- 0469(1980)037¡2712:amftsa¿2.0.co;2doi:10.1175/1520-0469(1980)037<2712:amftsa>2.0.co;2, 1980.

For more information about Greenland ice and climate studies, check out Jason Box’s homepage.

 

Latest Greenland ice sheet reflectivity

Monday, June 25th, 2012

 




These albedo visualizations are discussed here and here.

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 and Stroeve, 2002; Klein and 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. (http://nsidc.org/data/modis/ms2gt/). The interpolation method employs a trend surface through the surrounding four 500m 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 29 Jul – 18 August 2000 and 14 June – 7 July 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, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012. open access
  • 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.

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