220.127.116.11.6.4 MERIS Ocean Colour Processing (step 2.9)
This chapter describes the processing to be applied to surface reflectances produced by the atmospheric corrections above water (see section 18.104.22.168.6.3 above) in order to derive ocean bio-optical parameters.
Different algorithms are used as shown in flow chart 8.5.2-1.
I. A band-ratio algorithm optimised for open ocean clear waters (so-called "Case 1") yields a geophysical quantity :
· Algal Pigment Index 1
II. Robust band-ratio algorithms valid for all water types, including yellow substance dominated (so-called Case 2 (y)) and waters with excessive back-scattering. These algorithms yield the following Product Confidence Data :
· flag for anomalous scattering waters
· flag for yellow substance-dominated waters
III. A robust algorithm based on the Inverse Modelling Technique (IMT), that can be applied to all water types, yields the following geophysical quantities :
· Algal Pigment Index 2 (mg.m-3)
· Yellow substance absorption (m-1)
· Sediment load (g.m-3)
IV. An algorithm to estimate the instantaneous value of the Photosynthetically Available Radiation (PAR)
Three different flags indicating the type of water sensed are used as they have an influence on processing quality:
· turbid waters (described in Section 7 of this document)
· yellow substance dominated waters
· anomalous scattering
Furthermore, range checks on input and output parameters are applied for quality control.
22.214.171.124.6.4.1 Case 2 (Yellow substance dominated) flag (step 2.9.4)
The presence of Case 2 water is flagged by Yellow substance (CASE2Y_F) flag (step 2.9.4). Input data are normalised water-leaving reflectance r'w(b, j, f). A LUT technique is applied for the retrieval. The procedure is described in RD8 (2.8).
126.96.36.199.6.4.2 Case 1 waters processing - Algal pigment index 1 (Chl1) retrieval (step 2.9.7)
Case 1 waters processing is based on a band ratio algorithm. Inputs are normalised water-leaving reflectance r'w(b, j, f) and ancillary data. The theory of data processing is described in RD8 (2.9). Processing is performed in two steps:
1. a band ratio estimate of Chl1 is selected among up to three possible ones, according to ratio value
2. an iterative procedure eliminates the influence of bi-directionality (parameter f_over_q1) on Chl1 estimate.
188.8.131.52.6.4.3 Case 2 anomalous scattering water flags (step 2.9.6)
The presence of Case 2 water is flagged by the Anomalous scattering (CASE2ANOM_F) flag (step 2.9.6). Input data are normalised water-leaving reflectance r'w(b, j, f) and algal pigment index 1. A LUT technique is applied for the retrieval. The procedure is described in RD8 (2.8).
184.108.40.206.6.4.4 Case 2 waters processing - Inverse modelling technique (IMT) (step 2.9.11)
IMT (RD 8, 2.12) uses an Inverse Radiative Transfer Model-Neural Network (IRTM-NN) to estimate the concentration of algal pigment index 2 (Chl2), yellow substance absorption (ODOC), and suspended particulate matter (SPM) concentration for Case 1 and Case 2 waters from normalised water-leaving reflectance at MERIS bands b442 to b665, b705, qs, qv and Dj. In this approach, Case 1 waters are treated as a special case of the Case 2 water algorithm. The IMT considers the complex nature of the water leaving reflectance and its parameterisation avoids any iterative procedures. The multiple non-linear regression method in this approach leads to high reduction in computing time and is therefore fast enough for operational mass production of Level 2 products, but it requires a careful and elaborate determination of the multiple coefficients (training phase). The IRTM-NN procedure is already programmed and requires only data input. The data output from this algorithm are Chl2 concentration, ODOC absorption, SPM concentration and a confidence flag.
The Neural Network applies generic Neural Network functions, as specified in AD 6, to specific auxiliary parameters and inputs, to obtain the required outputs. All specific aspects of the application are specified in section 220.127.116.11 below.
18.104.22.168.6.4.5 Photosynthetically Available Radiation (step 2.9.8)
Instantaneous Photosynthetically Available Radiation (PAR) is derived from the irradiance above each water pixels, under a tabulated relationship. This step is applied to all pixels where ACFAIL_F(j, f) is FALSE.