A new study by Polashenski and others find that “Neither dust nor black carbon causing apparent albedo decline in Greenland’s dry snow zone; implications for MODIS C5 surface reflectance“.
Just one point of their work I expand on here is the issue of declining MODIS Terra sensor sensitivity (Wang and others 2012; Lyapustin et al. (2014). First, Polashenski and others (2015) develop the MODIS albedo decline issue nicely. Now, rewinding back to early 2012, after becoming aware of Wang and others (2012) results, I evaluated whether the albedo decline was present in the completely independent Greenland Climate Network (GC-Net) data after Steffen and others (1996). In 2012, I wrote:
“Degrading MODIS instrument sensitivity identified by Wang et al. (2012) introduces the possibility that the declining albedo trends may be erroneous. To validate the MODIS albedo trends, coinciding observations from GC-Net AWS are examined. The ground truth data are situated across a range of elevations, spanning the ablation and accumulation areas. Analysis of the GC-Net data confirms declining albedo trends in the 2000–2010 period to be widespread in individual months from May–September. Trend statistics are computed where at least 7 yr of annual data are available from both GC-Net and MODIS Terra. Significance is designated here more strictly where the trend measured by the linear regression slope has a magnitude that exceeds 2 of the residuals from the regression. In 41 of 43 (95 %) of monthly cases May–September, the trend is found to be significant and decreasing (Table 1). In 10 of 14 (71 %) cases, for which both GC-Net and MOD10A1 trends are significant, the GC-Net declining trend is larger than the MOD10A1 trend. It therefore does not seem that MODIS sensor degradation is enhancing an existing trend.” – Box and others (2012)
Still, an update after 2010 is in order. Now, the evaluation through 2014 yields that there is still a real albedo decline for the southern part of the ice sheet, including places like Saddle or South Dome where surface melting is uncommon. Note how not only do both GC-Net and MODIS MOD10A1 show a decline, they share peaks and troughs. Given that the ground data having a footprint size of just a few square meters and the satellite data that have an effective footprint size of 5 x 5 km and that they pick up the same high and low years is impressive.
Wonk Alert: Still the bias is very likely a latitude-dependent. Notice how the GC-Net trend is even stronger than the MODIS trend in the far south (South Dome and Saddle). Consider that we have sunlight reflecting off of a highly reflective part (an ice sheet) of a sphereoid (the earth) that arcs more than 20 degrees north south. Snow and ice have reflectance depending strongly on viewing and illumination angles. So, the satellite data compensation for simple albedo are susceptible to amplification of small sensor degradation biases.
Let me add that the detected bias is smaller than the type of anomalies produced by for example the large July 2015 melting for NW Greenland. The red and yellow areas below are real local albedo anomalies due primarily to melting.
MODIS having a more negative trend than the ground data for the northern high and usually dry bright snow interior sites (See NASA-E, Tunu-N, Humboldt above) is also a real issue that Polashenski and others (2015) nicely report. While the bias we are looking forward to be compensated in the version 6 MODIS data (Lyapustin and othes 2014). As to the role of black carbon in Greenland’s albedo decline, I would say there is more to the story than what Polashenski and others (2015) report. Stay tuned.
- Box, J. E., X. Fettweis, J.C. Stroeve, M. Tedesco, D.K. Hall, and K. Steffen. 2012. Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839. doi:10.5194/tc-6-821-2012
- Lyapustin, A., Wang, Y., Xiong, X., Meister, G., Platnick, S., Levy, R., Franz, B., Korkin, S., Hilker, T., Tucker, J., Hall, F., Sellers, P., Wu, A., and Angal, A.: Scientific impact of MODIS C5 calibration degradation and C6+ improvements, Atmos. Meas. Tech., 7, 4353-4365, doi:10.5194/amt-7-4353-2014, 2014.
- Polashenski, C.M. J.E. Dibb, M.G. Flanner, J.Y. Chen, Z.R. Courville, A.M. Lai, J.J. Schauer, M.M. Shafer and M. Bergin, 2015, Neither dust nor black carbon causing apparent albedo decline in Greenland’s dry snow zone; implications for MODIS C5 surface reflectance, DOI: 10.1002/2015GL065912
- Steffen, K., Box, J. E., and AbdalatiW.: Greenland climate network: GCNet, US Army Cold Regions Reattach and Engineering (CRREL), CRREL Special Report, 98–103, 1996.
- Wang, D. D., Morton, D., Masek, J., Wu, A. 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, 5561, 2012.