Diffuse attenuation coefficient (KD) for downwelling irradiance at 490-nm
preliminary results, 16 Jun 2009
The operational algorithm for deriving the diffuse attenuation coefficients of downwelling irradiance at 490-nm (KD; Mueller 2000) was updated using in situ data from NOMAD version 2. NOMAD is a publicly available, global bio-optical data set constructed from data archived in SeaBASS. The data span a wide range of water types from coastal and offshore regions and were collected by participants in the NASA Ocean Biology & Biogeochemistry Program and by voluntary and international collaborators. The algorithm form describes the polynomial best fit that relates the log-transformed geophysical variable to a log-transformed ratio of remote-sensing reflectances. The polynomial form replaces the power-law form employed by past versions of KD.
requirements
* polynomials bisect the full dynamic range of the development data set
* KD2S (SeaWiFS) = KD2M (MODIS) = KD2E (MERIS) = KD2O (OCTS)
algorithm
form:
* Rrs1 = 490-nm (SeaWiFS, MERIS, OCTS), 488-nm (MODIS), or 443-nm (CZCS)
* Rrs2 = 555-nm (SeaWiFS), 547-nm (MODIS), 560-nm (MERIS), 520-nm (CZCS), or 565-nm (OCTS)
* X = log10(Rrs1 / Rrs2)
* Kd_490 = 10^(a0 + a1*X + a2*X^2 + a3*X^3 + a4*X^4) + 0.0166
changes from operational:
* use of Rrs instead of nLw
* log-log polynomial expression instead of power law
v6 coefficients:
sensor | blue | green | a0 | a1 | a2 | a3 | a4 | |
---|---|---|---|---|---|---|---|---|
KD2S | SeaWiFS | 489 | 555 | -0.8515 | -1.8263 | 1.8714 | -2.4414 | -1.0690 |
KD2M | MODIS | 489 | 547 | -0.8813 | -2.0584 | 2.5878 | -3.4885 | -1.5061 |
KD2E | MERIS | 489 | 560 | -0.8641 | -1.6549 | 2.0112 | -2.5174 | -1.1035 |
KD2V | VIIRS | 486 | 550 | -0.8730 | -1.8912 | 1.8021 | -2.3865 | -1.0453 |
KD2O | OCTS | 489 | 565 | -0.8878 | -1.5135 | 2.1459 | -2.4943 | -1.1043 |
KD2C | CZCS | 443 | 520 | -1.1358 | -2.1146 | 1.6474 | -1.1428 | -0.6190 |
algorithm development
KD2S | KD2M | KD2E |
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KD2V | KD2O | KD2C |
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algorithm verification: internal consistency

algorithm verification: comparison with previous versions
compare with K490 from Werdell 2005 (operational) and Mueller 2000

algorithm verification: comparison with alternate models
compare with Kd_490 from Morel et al. 2007 (both Rrs- and Chl-driven) and Lee et al. 2005

algorithm verification: level-2 match-ups
using NOMAD-free subset of SeaWiFS match-ups

using all available SeaWiFS match-ups

NASA Ocean Biology Processing Group