Algorithm Theoretical Based Document (ATBD) - CESBIO

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SO-TN-ESL-SM-GS-0001 Issue 1.a Date: 31/08/2006 SMOS level 2 processor Soil moisture <strong>ATBD</strong> Reference values for space varying quantities entering the relevant forward models are obtained or computed over the DFFG relevant area for aggregated fractions FV which depend on the incidence angle. The idea is that with varying view angle the fractions might vary and, in some cases, with non negligible impact (even with the weighing) due to presence of water for instance. Fractions FV are very similar to FM or FM 0 fractions (from which the incidence angle dependency has been removed). However: • Concerning open water, distinct reference values are requested for sub-models MWS and MWP applied to distinct sub-fractions FWS and FWP of FWO; • For each DFFG node, the resampled "supplementary" covers override the complementary ones. However, the topographic supplementary covers, which were taken into account when computing the fractions FM 0 for driving the decision tree, are ignored when computing reference values. Atmospheric and sky parameters are computed a single time for the DGG node; Use is next made of WEF to compute average reference parameter values for each aggregated fraction FV. This summarized description concerns the most complicated occurrences, featuring several fractions, possible NPE occurrences, non uniform quantities. There are many practical cases where the full computation from DFFG can be avoided. Table 20 summarizes the categories of necessary reference values to be used in relevant fractions and forward models. Table 20: Categories of necessary reference values Cover Parameters Soil Open water Ice Forest Wetlands Barren Urban Frost Snow Atmosphere, sky Physical temperature MN MN MW MW MD MD MD MD MD* Dielectric constant MD MD MD MD MD* OS MW MW SM MN MN Others (dielectric layer) MN MN Roughness MN MN Vegetation layer MN MN MW MD MD MD MD MD MOUNTAIN ALL None * concerning snow, the table corresponds to present choices for wet and mixed snow ** concerning snow, this is a place holder waiting for a mixed snow model to become available. A number of particular cases will probably request clarifications. • When applying the MD model, a reference value for the dielectric constant (real and imaginary parts) is computed by weight averaging over every concerned aggregated fraction. Next, references values for A_card and U_card are computed using Eq 45. The reference value computed for U_card supersedes the default value. • Optical thickness retrieval: this parameter does not depend on polarisation. The initial (reference) value is obtained through summing the standing vegetation component and the average of the (polarisation dependent) litter component. 3.2.3.6 Decision tree stage 2 for retrieval conditions 3.2.3.6.1 Minimum, full and maximum retrievals When filtering out L1c pixels, we defined (section 3.2.2.1.5) an initial validation index MVAL0. Now it is possible to account for the overall fraction FRE selected for retrieval and to estimate a more realistic validation index MVAL: MVAL = MVAL0 * FRE In order to define retrieval conditions, one must compare MVAL to thresholds. Although 2 thresholds will be initially sufficient, it is wiser to define 3 of them: TH_MMIN1, TH_MMIN2 and TH_MMIN3. Then 4 cases may occur: MVAL < TH_MMIN1: L1c pixel is finally invalidated . 81
SO-TN-ESL-SM-GS-0001 Issue 1.a Date: 31/08/2006 SMOS level 2 processor Soil moisture <strong>ATBD</strong> TH_MMIN1 TH_34 10.0 10.0 10.0 [0 TH_23] nil nil nil nil nil nil nil nil nil [TH_23 TH_34] 0.1 0.5 0.5 0.1 0.5 0.5 0.1 0.5 0.5 > TH_34 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 [0 TH_23] nil nil nil nil nil nil nil nil nil [TH_23 TH_34] nil nil 1.0 nil nil 1.0 nil nil 1.0 > TH_34 nil nil nil nil nil nil nil nil nil [0 TH_23] nil nil nil nil nil nil nil nil nil [TH_23 TH_34] nil nil 2.0 nil nil 2.0 nil nil 2.0 > TH_34 nil nil nil nil nil nil nil nil nil [0 TH_23] nil nil nil nil nil nil nil nil nil [TH_23 TH_34] nil nil 0.1 nil nil 0.1 nil nil 0.1 > TH_34 nil nil 0.1 nil nil 0.1 nil nil 0.1 [0 TH_23] nil nil nil nil nil nil nil nil nil [TH_23 TH_34] nil nil 0.1 nil nil 0.1 nil nil 0.1 > TH_34 nil nil nil nil nil nil nil nil nil [0 TH_23] nil 0.1 0.1 nil 0.1 0.1 [TH_23 TH_34] nil nil nil nil nil nil > TH_34 nil nil nil nil nil nil The names of parameters considered for retrieval (given in col A) are self explanatory: T_PHYS stands for T gc (or water, surface, snow temperature), TAU for τ NAD . Retrieval choices are defined by the a priori standard deviations ASTD, which are shown in the table (col D to L). In many cases (ASTD = "nil"), the parameter is not retrieved.. Large values mean the parameter is practically left free. While the above table has been filled for illustration, numerical ASTD values are specified in UPF. The number of free parameters NP can be obtained from this data. 3.2.3.6.2 Condition on a priori optical thickness . 82
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