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 &amp 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:

sensorbluegreena0a1a2a3a4
KD2SSeaWiFS489555-0.8515-1.82631.8714-2.4414-1.0690
KD2MMODIS489547-0.8813-2.05842.5878-3.4885-1.5061
KD2EMERIS489560-0.8641-1.65492.0112-2.5174-1.1035
KD2VVIIRS486550-0.8730-1.89121.8021-2.3865-1.0453
KD2OOCTS489565-0.8878-1.51352.1459-2.4943-1.1043
KD2CCZCS443520-1.1358-2.11461.6474-1.1428-0.6190


algorithm development

KD2S KD2M KD2E

KD2V KD2O KD2C


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




Questions? Contact Jeremy Werdell
NASA Ocean Biology Processing Group