Revision History. Applicable Documents

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2 Revision History Version Date Revision History Remarks Update of the processing algorithm of CAI Level 3 NDVI, which yields the NDVI product Ver The major updates of this product from the previous version are as follows; i) The duration of CAI data to take into account for calculating NDVI is changed from 15 days to 30 days. ii) Mesh size of NDVI is changed from 1/240 degrees (15 seconds) to 1/30 degrees (2 minutes). iii) The value of NDVI is defined as 4 byte real type. In the previous version, it was given as DN (Digital Number), which is integer of actual NDVI value multiplied by Applicable Documents (1) Japan Aerospace Exploration Agency GOSAT Project Team, Algorithm Theoretical Basis Document for GOSAT TANSO (in Japanese and Not Published, Version R, SAT ) (2) Japan Aerospace Exploration Agency Earth Observation Research Center, GOSAT Level 1 Product Format Descriptions (MAS B and MAS C) (3) National Institute for Environmental Studies, NIES GOSAT Project Product Format Descriptions (Version 2.00, NIES-GOSAT-PO ) (4) GOSAT TANSO-CAI L1B Algorithm Theoretic Basis Document (Ver. 1.0 NIES-GOSAT-PO-010) (5) GOSAT TANSO-CAI L2 Cloud Flag Algorithm Theoretic Basis Document (Ver. 1.1 NIES-GOSAT-PO-016) i

3 Table of Contents 1. Introduction Stored Data Algorithm... 2 ii

4 1. Introduction This document is the Algorithm Theoretical Basis Document (ATBD) of TANSO-CAI Level 3 NDVI. The global earth is divided into 36 rectangles as the distribution unit of TANSO-CAI Level 3 NDVI, which is 30 degrees in latitude by 60 degrees in longitude. The mesh size in a rectangle is 1/30 degrees. Firstly, the minimum value of Band 2 (0.67μm) reflectance for each mesh is determined from CAI data of the past 30 days. Secondly, the reflectance of Band 3 (0.87μm) of each mesh is selected from the corresponding CAI L1B data. Then NDVI is calculated from the reflectances of Band 2 and 3. The atmospheric correction with the Rayleigh scattering is applied to these reflectance values. The algorithm described in this ATBD is referred as Ver.1.1, which corresponds to the explanation of the TANSO-CAI L3 NDVI products Ver Stored Data A product of TANSO-CAI Level 3 NDVI is formatted with HDF5. Its delivery unit is a rectangle with 30 degrees of latitude by 60 degrees of longitude. There exist 36 rectangles for the whole globe. The value of NDVI is stored as 4 bytes real value. The observation date, path number, Band 2 and Band 3 reflectances (for which the atmospheric correction is applied) of corresponding CAI L1B data and the clear sky confidence level of CAI L2 Cloud Flag are also stored as its auxiliary data. The detailed algorithm to evaluate the clear-sky confidence level is written in GOSAT TANSO-CAI L2 Cloud Flag Algorithm Theoretic Basis Document (Ver. 1.1 NIES-GOSAT-PO-016). (1) Rectangle Number A rectangle is defined as an area of 30 degrees of latitude by 60 degrees of longitude. The rectangle blocking starts from 25 degrees West in longitude with the interval of 60 degrees and starts from 90 degrees North in latitude with an interval of 30 degrees. Rectangle is numbered from 1 to 36 as is shown in Fig. 1. But the products of rectangle number 31 to 36 are omitted because there is no effective area due to Antarctic area. 1

5 Fig. 1 Rectangle division and its Number (2) Mesh size The product is image data in a rectangular area in Cylindrical Equidistant Projection with 1800 meshes for longitude by 900, for latitude. Therefore, the mesh resolution is 1/30 degrees (2 minutes) and the mesh origin in a rectangle is set at the north and west edge of the rectangle in southward and eastward directions. 3. Algorithm (1) NDVI value and the clear sky confidence level are calculated and stored for each mesh. In the previous version, NDVI was evaluated from a set of CAI data for a 5 satellite recurrence period (15 days), and there were some meshes with no effective NDVI values due to the cloud coverage during the whole period. To avoid such cloud-contaminated meshes with missing NDVI, a set of CAI data for 10 satellite recurrences (30 days) is used to get the cloud free data (i.e. clear sky data) for each mesh in the new version. NDVI calculated with the selected cloud free CAI data is stored in the product. The detailed procedure to determine the most possible cloud free data from a set of CAI L1B products for continuous 30 days is described as follows: i) The value of Land-Sea mask for the center of the gridded mesh is obtained. If its latitude is located between 60 deg. North and 60 deg. South, Shuttle Radar Topography Mission (SRTM) land/sea mask data is applied; otherwise Land-Sea Mask used in Global Land 1-KM AVHRR Project of U.S. Geological Survey (USGS) is applied. ii) If the center of gridded mesh is referred as Sea, the value of -0.3 is set as invalid for the 2

6 mesh. iii) The minimum reflectance of Band 2 for each mesh from one path data of CAI L1B is extracted as the preparation of step iv) in the below. Gridded mesh, in which the center pixel of Band 3 of CAI L1B is located, is identified for each pixel of CAI L1B. Since "Geometric information of the observation point" stored in a CAI L1B product is calculated for Band 3 as the reference, the center pixel of Band 3 is also applied for NDVI as the reference. The detail of calculating geometric information is written in GOSAT TANSO-CAI L1B Algorithm Theoretic Basis Document (Ver. 1.0 NIES-GOSAT-PO-010). There will be several pixels of CAI L1B in a gridded mesh as shown in Fig. 2. Then, the reflectance of Band 2 is calculated with the formula (1) for all the pixels in each mesh and their minimum is determined. And the reflectance of Band 3 is also calculated with the formula (1) for the minimum corresponding to the mesh selected from Band 2. Referring to the time and location of the minimum for each mesh, the zenith and azimuth angles of the sun, the zenith and azimuth angles to the satellite, the ozone concentration and the atmospheric pressure on the ground are applied for the atmospheric correction written in the next section. The ozone concentration is based on NASA s data, which is Ozone Monitoring Instrument (OMI) standard Level3 product (Ozone, 0.25 degree mesh, daily) and obtained on The atmospheric pressure is used based on Grid Point Value (GPV) (0.5 degree mesh, every 6 hours) by Japan Meteorological Agency. The ozone concentration and the atmospheric pressure of the mesh at the concerning observation time are given by their linear interpolations. NDVI gridded mesh with thick lines (1/30 degrees) Pixel of CAI L1B enclosed with thin lines (0.5km for Band 2 & 3) Fig. 2 Conceptual sketch of NDVI mesh and CAI L1B pixel Reflectance before the atmospheric correction is given as follows: 2 rad πr ref = irad cos( θ ), where (1) 3

7 R: Distance between earth and sun (Arbitrary Unit), θ: Solar zenith angle, ref: Reflectance before the atmospheric correction, rad: Brightness by TANSO-CAI sensor (W/m 2 /sr/μm), irad: Radiation of sun weighted by the response function of TANSO-CAI sensor system (radiation at 1 A.U., W/m 2 /μm). Note: irad is Radiation of the sun, which is calculated by referring to "Thuillier et al., The Solar Spectral Inradiance from 200 to 2400 nm as Measured by the Solspec Spectrometer from the Atlas and Eureca Missions, Solar Physics, 214 (1): 1-22, May 2003, weighted by the response function of TANSO-CAI sensor system for each band. The detailed values of the TANSO-CAI sensor system response function are available on the page of Technical Information of Sensor from GOSAT Data Archive Service (GDAS), which URL is as follows: iv) The minimum value of Band 2 reflectance for each mesh is searched from CAI data for the past 30 days as the clearest possible sky data. Corresponding value of Band 3 reflectance, zenith and azimuth angle of the sun, the zenith and azimuth angle to the satellite, the ozone concentration and the atmospheric pressure on the ground for each mesh are stored. If the suitable ozone concentration is not found, its value is set with the climatology data 300 D.U. v) If there is no valid data for a mesh as clear sky regarding to the 30 days CAI data, an invalid data -0.3 is stored for the mesh. (2) Atmospheric Correction The atmospheric correction considering the absorption by ozone and oxygen and the Rayleigh scattering is applied to the reflectance of Band 2 and 3 at each gridded mesh. The reflectances of Band 2 and Band 3 with atmospheric correction are described as ref2 and ref3, respectively. The atmospheric correction is applied when the sun s zenith angle is less than 70 degrees. Otherwise an invalid value is stored on the reflectance for the mesh. An invalid reflectance value is stored for the meshes in the southern area over 47 degrees south on the summer solstice or the northern area over 47 degrees north on the winter solstice. It is also stored for the meshes in the southern or northern area over 70 degrees on the Vernal Equinox day or the Autumn Equinox day, respectively. (3) Calculation of NDVI 4

8 a. NDVI Normalized Difference Vegetation Index (NDVI) is calculated for each mesh as follows: (2) where NDVI: Value of Normalized Difference Vegetation Index, ref2, ref3: Reflectances of Band 2 and 3 with atmospheric correction. b. Invalid value The value of NDVI is stored with an invalid value, which is -0.3, for the following cases: Calculated NDVI is less than -0.2 Calculated NDVI is greater than 1.0 If one of the following conditions is satisfied, NDVI, reflectances of Band 2 and 3 with atmospheric correction, and clear sky confidence level are considered as invalid and their values are stored with Reflectance of Band 2 or 3 is negative Reflectance of Band 2 or 3 is greater than 1.0 Sum of Reflectances of Band 2 and 3 is less than 0.1 Reflectance of Band 2 or 3 is stored with the invalid value ( -0.3 ) 5