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 ATBD R3 NO Try R2 R3 YES NO Try R2 R3 YES YES YES Filter, try again R3, build output (f) If YES, flag if NO, flag if NO, flag R3 YES YES NO Build output if NO, flag if NO, flag R2 NO Build output (g) R2 YES NO Build output (g) R2 YES YES YES Filter, try again R2, build output (f) If YES, flag if NO, flag if NO, flag R2 YES YES NO Build output if NO, flag if NO, flag Notes (a) Retrieval status: R2, 3 or 4 means the retrieval under analysis corresponds to the option 2, 3 or 4 in Table 21 (b) Rstd are acceptable if all smaller than thresholds TH_DQX_**.(TH_DQX_SM; same for A_card, T_PHYS, HR, TT_H, RTT, OMH, DIFF_OM) are supplied in UPF. Note that this test will be active only for values smaller than the a priori ASTD also supplied by UPF (c) Outliers are detected through comparing abs(TBM - TBF)/DTBs to TH_FIT. TH_FIT is a provided constant; suggested value is 4. If ratio > 1, delete data Note: outliers are removed altogether. After the 2 nd retrieval the outliers are also checked to eventually update flags and RFI counters, but no more retrieval is tried. (c') If wild data are still present, flag (d) Fit is good is CHI2 within given range. Chi2 formula and range (upper threshold TH_CVAL) to be provided (e) Acceptable ranges for parameters (SM_min, SM_max; same for A_card, T_PHYS, HR, TT_H, RTT, OMH, DIFF_OM) are supplied in UPF (f) It is assumed that removing outliers will not impact columns B & C (g) No product The following output data may in summary characterize the retrieval when successful: outside stipulated ranges, (FL_RANGE), outside acceptable retrieval Rstd (FL_DQX), poor fit quality (see below), and persisting outliers (FL_WILD). Concerning the systematic MD retrieval, when it is carried out in addition (MDa) to any other (MN or MW) retrieval, the following rules are proposed: • If the initial (MN or MW) retrieval fails, no MDa retrieval is attempted; • The additional MD retrieval MDa is only attempted once, i.e. for the highest level (4, 3, 2) for which the former retrieval (with MN or MW) was successful; • If then this additional retrieval fails, no further attempt with MDa for poorer retrievals is carried out. This seems to be the only case where recording a retrieval failure in the product cannot be avoided. References (initial) values for A_card and U_card are computed from the MN or MW retrieval results. 3.2.5.2 Updating current parameter maps Even though part of the ATBD, it must be clear that this section does not apply to the processor itself but needs to be documented. The RFI, Tau, HR and Flood maps will be updated off line after the data processing and in view of being used as inputs for the next processing round. In consequence, the processor outputs products. Then another processor read these products, checks quality and eventually updates the maps according to a procedure described below. The updated auxiliary data will be used in the next run of the processor. 3.2.5.2.1 Updating current RFI map . 87
SO-TN-ESL-SM-GS-0001 Issue 1.a Date: 31/08/2006 SMOS level 2 processor Soil moisture ATBD RFI map designates here a DGG map. No angular dependence is considered in the baseline. Provisions to take angular information into account might prove useful. Polarisation may be useful as RFI could only affect one polarisation. The table includes both the total number N SNAP of times the DGG node has been observed, and the total number N RFI-p of cases where an anomalously high TB has been unambiguously detected for polarisation p, as the results of any of 3 tests: TB range testing, L2 RFI test (see 3.2.2.1.4), post retrieval outliers. For each snapshot, the counter N SNAP is incremented; the N RFI-p counter is updated as well when RFI is detected. Over any point, if the DGG RFI_MAP indicates potential RFI while the tests are negative, the radiometric sensitivity is degraded by a factor depending upon the ration R RFI-p =N RFI-p / N SNAP using Eq 61 (see § 3.2.2.1.4). This algorithm is activated or deactivated by the global switch UPDATE_RFI defined in UPF. The use of RFI map to flag the input TB is activated or deactivated by the global switch USE_DEFLT_RFI defined in UPF. The map would thus contain for each node N SNAP , N RFIp for each polarisation requiring as SNAP will grow quickly 4 bytes. 3.2.5.2.2 Updating current optical thickness TAU_NADIR LV and FO map This external post-processing action still requires confirmation and detailed explanations. The basic concept is when the retrieval of optical thickness is possible and successful; this optical thickness updates the evolving map. The idea is that the new tau nadir will be more accurate/ up to date than the LAI derived one. It might also be affected by spurious events such as a heavy rainfall. The new TAU_NADIR is for a whole node but actually only for the class for which the retrieval process was carried out (e.g., low vegetation or forest).. It is also considered that for forests we will only update for fine tuning as the relevant variable is linked to the branch volume and thus should remain fairly constant. Any variation should be linked to changes in other variables (interception, litter water content…) So, when the retrieval of optical thickness is possible and successful, the TAU map class to be updated is the fraction for which TAU_nadir is retrieved, as provided by UPF (TGRD). The CURRENT_TAU_NADIR_LV is only updated following LV class retrievals: • Current TAU is updated with the retrieved TAU value, • Current DQX TAU is updated with a specific RSTD, obtained as indicated in 3.2.4.4.2 but with a matrix COV Z _CURRENT computed assuming that TAU is completely free (that is, practically, a very high ASTD). This quantity will be stored in the DAP • Current date is updated with swath acquisition date. • The CURRENT_TAU_NADIR_FO is only updated following FO class retrievals: • Current TAU is updated with the retrieved value, • Current DQX TAU is updated with the DQX of the retrieved value • Current date is updated with swath acquisition date. No update is allowed in the following cases: • Risk of interception • Presence of snow cover, or freezing conditions (vegetation is almost transparent and tau nadir is abnormally low) If no Tau_ nadir is retrieved, or update not considered as valid, the date of the last acquisition is checked. If the lag is larger than the allowed TH_LAG_TAU_LV, a default or missing value is put. This algorithm is activated or deactivated by the global switches UPDATE_TAU_NADIR_LV and UPDATE_TAU_NADIR_FO defined in UPF. The use of DGG_CURRENT TAU_NADIR_LV map and DGG_CURRENT TAU_NADIR_FO map in the retrieval is activated or deactivated by the global switches USE_DEFLT_TAU_NADIR_LV and USE_DEFLT_TAU_NADIR_FO defined in UPF. 3.2.5.2.3 Updating current H roughness HR map . 88
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