VIIRS True Color Atmospheric Correction Updates

Steve Miller14 June 2012

Overview• VIIRS natural color processing commenced shortly after receipt of first-

light data.• To expedite initial product suite, the atmospheric corrections necessary

for high-quality true color imagery were adopted from the legacy MODIS processing.

• The MODIS Blue/Green/Red bands (466, 554, 646 nm) are slightly different than VIIRS (488, 555, 672 nm).

• Associated rayleigh optical depths are different, leading to erroneous corrections when applying the MODIS coefficients and tables to VIIRS data.

• Correct handling of VIIRS requires a re-calculation of the necessary components, including: Fo, tau_ray, tau_ozone, tau_oxygen, tau_watvap.

• Codes adapted from SEADAS software (Eric Vermote and Norman Kuring).

• Monochromatic calculation used here (no band weighting), with values for various properties interpolated linearly from the original SeaWiFS band values. Sufficient for qualitative imagery product.

Modifications

Bands(MODIS): 3,4,1 (466nm, 554nm, 646nm)Bands(VIIRS): 3,4,5 (488nm, 555nm,672nm)

F_sun(MODIS): 202.3, 186.0, 160.2F_sun (VIIRS): 194.142, 185.930, 151.538

ray_taus(MODIS): 0.189789, 0.09490, 0.0492007ray_taus(VIIRS): 0.159419, 0.09420, 0.04532

New Rayleigh Look-Up Tables:

ftp://ftp.cira.colostate.edu/ftp/Miller/NRL/VIIRS_Rayleigh

raysa(MODIS): 0.142865, 0.0801539, 0.0492007raysa(VIIRS): 0.124679, 0.0797469, 0.0396954

aoz(MODIS): 0.015816, 0.09338, 0.056965aoz(VIIRS): 0.0244511, 0.09338, 0.04604

awv(MODIS): 0.0, 0.0008, 0.003539awv(VIIRS): 0.0, 0.0008, 0.00422421

ao2(MODIS): 0.0, 0.0, 0.0ao2(VIIRS): 0.0, 0.0, 0.0

Implementing the simple coefficient and table modifications listed above will result in dramatic improvements to VIIRS true color image quality. The following slides show before/after corrections applied (must run in slide-show mode to toggle imagery).

Coefficient Changes:

OLDNEW

California Coastal Blooms

OLDNEW

OLDNEW

Next Steps

• Evaluate the stability and performance at various sites.

• Apply these new corrections to the VIIRS dust enhancement prototype.

• Develop an I-band true color product based on re-bin of M-bands to 375 m resolution using similar techniques as MODIS 250 m.