Algorithm Theoretical Based Document (ATBD) - CESBIO

SO-TN-ESL-SM-GS-0001
Issue 1.a
Date: 31/08/2006
SMOS level 2 processor
Soil moisture ATBD
Total 208166 100.0
A rough-cut test indicates that the percentage of land pixels is 208166/ (1080*540) = fraction of continent = 35.69 %
instead of the commonly accepted 29%.
The 6% discrepancy comes from the fraction resolution at 40 km that has a tendency to thicken the coastline that
overlaps a little the ocean. Consequently the surface of continent appears overestimated.
The Urban threshold was never activated. The urban ECOCLIMAP class is slightly different of those of DT, so its
threshold needs to be adjusted.
First analysis
The fraction of nominal behaviour, i.e. SM + τ, is: c4ann + c4bnn that is ~ 66%. It seems not very much, but the main
limiting factor is permanent snow on poles (~ 30%) that is not really relevant SM studies. If we do not consider
permanent snow areas, we have 95.8 % of nominal pixels and this is a good result, even though optimistic this time as
we will have topography, non permanent snow, ice etc…
What’s next:
The above assessment is preliminary and needs further refinement and verifications:
1. the ECOCLIMAP rocks label is used for dectree barren. The true meaning of ECOCLIMAP rock label as well
as the meaning of barren must be verified.
2. urban class has no impact. For ECOCLIMAP urban is the fraction of man made elements within towns. So
even considering a big city filling a 40 km x 40 km square, this fraction will not be 100%, but depend on
gardens, trees, etc leading rather to 60-80 %.
3. wetland, there is a problem 0.0005% is too small (we can think that large north-eastern Europe or Australian
region are not capture). So the wetland label in ECOCLIMAP is to be controlled.
4. The seasonal characteristics are not accounted for (seasonal snow, ice, frozen surface). This will be done very
soon since access to ECMWF data is being to be solved.
5. c7ddd represent what has not been identified and labelled by the decision tree, that is 1.3 % which seems little
high and need to be explained.
6. Topography has still to be accounted for
7. Seasonal effect are also to be accounted for
8. this analysis is purely geometrical; The WEF has to be accounted for, for completeness
Conclusion:
Finally, excluding ocean surfaces, polar ice & snow surfaces that are out of concern for SM, we can remember that the
SM + τ model will be used at best in 94 % of the cases. Those 94 % will necessarily decrease because:
• of seasonal aspects, frozen, ice, snow
• of the use of WEFIn the next assessment we will try to tackle the first two aspects.
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