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Initial Evaluation of NBAR NDVI GIP

Initial Evaluation of NBAR NDVI GIP. b y N. Shabanov NOAA-STAR Jan 03, 2013. Data: VIIRS GIP NBAR NDVI (17-day, Monthly and Rolling) for Oct 02-18, 2012 Master Land Index (MLI) for Sep 01, 2012 MOD13A2 TOC NDVI for Sept 29- Oct 14, 2012 MCD12Q1 8-biomes map for 2009.

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Initial Evaluation of NBAR NDVI GIP

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  1. Initial Evaluation of NBAR NDVI GIP by N. Shabanov NOAA-STAR Jan 03, 2013

  2. Data:VIIRS GIP NBAR NDVI (17-day, Monthly and Rolling) for Oct 02-18, 2012Master Land Index (MLI) for Sep 01, 2012MOD13A2 TOC NDVI for Sept 29- Oct 14, 2012MCD12Q1 8-biomes map for 2009 Objective:Evaluate test version of GIP NBAR NDVI using MODIS TOC NDVI 16-day compositing product

  3. Rolling GIP vs. MOD13A2 Rolling GIP MOD13A2 Rolling GIP – MOD13A2 Spatial patterns (this slide) and histograms (next slide) of Rolling NBAR NDVI GIP and MODIS TOC NDVI match at global scale. However regional discrepancies (anomalies) can be found at Western coast of Africa, Northern high latitudes and China.

  4. Histograms of differences (Rolling NBAR GIP-MOD13A2)

  5. Monthly GIP vs. MOD13A2 Monthly GIP MOD13A2 Monthly GIP – MOD13A2 Monthly product also matches MODIS TOC NDVI. However, regional anomalies are spatially located in the same areas as in case of rolling product.

  6. Histograms of differences (Monthly GIP-MOD13A2)

  7. Comparison o 17-day, monthly and Rolling versions of NBAR NDVI GIP 17-day NBAR Monthly NBAR Rolling NBAR As expected Monthly and Rolling versions don’t have large scale gaps as in case of 17-day version. While in general spatial patterens of Rolling and Monthly versions are similar, there exist difference especially at Northern high latitudes.

  8. Monthly GIP vs. MOD13A2 17-day GIP – MOD13A2 Monthly GIP – MOD13A2 Rolling GIP - MOD13A2 Discrepancies between Monthly and Rolling GIPs over Northern high latitudesare further highlighted with anomalies maps: in case of Monthly product, anomalies are positive, while in case of Rolling, they are mostly negative.

  9. Rolling GIP over gaps in 17-day GIP Total Rolling GIP Do anomalies in the Rolling GIP occur due to lack of NBAR retrievals? Rolling GIP was split into tw components: (1) over areas corresponding to fill values in 17-day GIP, and (2) remaining areas over land with valid retrievals in the 17-day product. Clearly, larges anomalies occur over gaps in the 17-day product. (1) Over gaps (2) Over retrieved areas

  10. Statistics on Fill Values Fill values are observed in Monthly and Rolling GIPs, this is against SOCBS specifications

  11. Examples of Fill values in Rolling GIP south to Hudson Bay Fill values color-coding: Black = Fill “SOB”, White = Fill “NA” Fill values “NA” typically occur near water bodies, and may be caused by inaccuracies/uncertainties of LW mask. However, Fill values “SOB” occur over valid land pixels.

  12. Results: • On global scale, spatial patterns and histograms of NBARD NDVI GIP (17-day, Monthly and Rolling) match that of MODIS TOC NDVI. However, two major problems were found: • (1) Products (Rolling and Monthly) exhibits large regional anomalies (discrepancies with MODIS) over Western coast of Africa, Northern High latitudes and China. Those anomalies are mostly found over gaps in 17-day product, which may be filled with (potentially inaccurate) climatology. • (2) There exist significatdiffrence between Monthly and Rolling GIP at Northen high laitudes. • (3) Fill values (“NA” and “SOUB” categories) exist over valid land pixels. “NA” fill values are typically found next to water bodies, and may be related to inaccuracies of LW mask. However, “SOUB” are found deep inland. DR concerning SOUB found in Granulated GIP was generated (Dr. William Thomas) and regrinding code bug fix was proposed (Dr. Alain Sei). However, this code fix does not fixes Monthly or Rolling NBAR NDVI GIP itself, it only affects Granulated GIP. • Recommendation of this study: • Current implementation of GIP NDVI is not ready to go operational • Need to trace the source of regional anomalies and fix them. To help in evaluating accuracy of GIP, extra layer- GIP Quality Control could be added (indicate accuracy of NBAR retrievals, if climatology was used, etc.) • Fill values in the Monthly and Rolling NBAR NDVI GIP should be fixed, as according to SOCB, those products are free from fill values.

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