Case #:PM_MCD43_21259_NRT  Opening date: 09/16/21  Last update: 09/17/21 |
Users of MODIS LANCE NRT BRDF/Albedo products (MCD43*) are requested to note that because of a significant change in the science algorithm of the C61 NRT BRDF/Albedo LANCE NRT products, they can observe significant differences/discontinuities between the C6 and C61 products (refer to the figures below). Although some of these differences in the LANCE NRT product can be attributed to the differences in the upstream C61 L1B, some are addressing problems with the LANCE NRT implemented in C6 (which is why production of C6 LANCE NRT was suspended). Overall, the changes implemented in C61 are aimed at making the C61 LANCE NRT BRDF/Albedo products more comparable to their equivalents, for the same dates of interest, as coming out from the MODAPS standard (STD) processing stream.
In summary, the C61 daily LANCE NRT algorithm now more appropriately emphasizes the date of interest. After the daily BRDF retrieval, the C61 NRT BRDF code has been modified to apply a further magnitude inversion, further emphasizing the last day of the retrieval period. This extra magnitude inversion will only use the set of 9 days that are overlapping between STD and NRT retrieval periods, with the highest weight being assigned to the last day for the NRT versions. This change only applies to the LANCE NRT C6.1 version (not the MCD43 STD C61 version).
As an example: the daily LANCE NRT algorithm for date of interest 2021016 uses input days 1-16, while the STD algorithm for date of interest 2021016 uses input days 8-23. Thus, for date of interest 2021016, only inputs from days 8-16 are common inputs for both the NRT and STD versions. Hence, with this revision, the daily LANCE NRT code will now utilize a first round of attempted BRDF full inversion, using days 1-16, and will follow up with a second application of a magnitude inversion using only the overlapping days 8-16. The NRT date of interest (day 16) will be assigned the highest weight of emphasis.
Through this extra magnitude inversion, the daily LANCE NRT BRDF will be forced to emphasize the input data days that are common with their STD equivalent and assign the highest weight to the last day of the retrieval period, thus reducing inconsistencies between the STD and NRT values for the same date of interest. However, even with this further emphasis, the STD and NRT should not be expected to show a perfect match, and furthermore, although the per-band BRDF retrieval quality flags in MCD43A2 will continue to the indicate whether the BRDF retrieval for the date of interest was a highest quality full inversion or a lower quality magnitude inversion, the mandatory QA flags provided in MCD43A1, A3, and A4 will now always indicate a lower quality magnitude inversion in the daily LANCE NRT version (reflecting the additional emphasis being applied).
For more discussion on the differences in approach between the NRT and STD BRDF process and differences in input data days and weighting, users are requested to refer to this document. The images below show the differences between the C6 and C61 NRT for day (2021256). The images below show the differences between the C6 and C61 NRT for day (2021256). The examples show the difference images for the MCD43A3 White Sky Albedo (WSA) for the shortwave broadband as well as the differences for MCD43A4 (NBAR reflectance) for bands 1 and 2.
Test = C61 NRT, Baseline = C6 NRT
Difference images = (Test - Baseline)/Baseline
Occurrence: C6,C61 NRT MCD43 products.