Measurements
The MODerate-resolution Imaging Spectroradiometer (MODIS) on the NASA EOS platforms, Terra and Aqua,
provide spectral radiances measurements in 32 solar and infrared bands since February 2000 and July 2002,
respectively. The common resolution of 1 km is used for the 5 bands (0.64, 3.8, 1.6 or 2.1, 10.8, and 12.0 μm)
employed in the Clouds and the Earth's Energy System (CERES) Edition 2 cloud detection and retrieval. Prior to processing,
the MODIS data sampled every other scan line and every 4th pixel, resulting in a nominal resolution of ≈2.8 km. Clouds are detected using a cascading threshold method applying sets of tests based on expected clear-sky radiances or brightness temperature differences and variability. Retrievals of cloud properties use subsets of the five channels and assume a plane-parallel single layer cloud completely covers each MODIS cloudy pixel. The sampled MODIS pixel results are averaged to obtain mean values for the clear portion of 1° x 1° grid and the clouds at up to two layers.
Local Observation Time and Length of Data Record
Terra, 10:30 AM and 10:30 PM ; 2001–2009 Aqua, 1:30 AM and 1:30 PM ; 2003–2008
Spatial Resolution
2 km (sampled), 4 km (sampled), 32-km clear sky radiance estimation
Cloud Detection
Thresholds based on standard deviation of expected clear radiances + expected clear radiance estimate at 0.64, 10.8 μm and for brightness temperature difference (BTD) between 3.8 and 10.8 μm during day; same during night and twilight (solar zenith angle = 82–89°) except no 0.64 μm data used and add BTD between 10.8 and 12 μm. Thin cirrus check used day and night as variable threshold compared to BTD(10.8–12). Multiple special checks are applied, especially over polar regions.
Retrieval Methodology
Daytime: 0.64, 3.8, and 10.8-μm radiances used simultaneously to estimate cloud phase, COD, CT, and CRE by matching to model calculations employing cloud reflectance LUTs and emittance parameterizations, and surface and clear-sky radiances. CEM computed from COD. CT → CZ using GEOS-4 temperature profile altered by simple 7.1 K/km lapse rate in the boundary layer; CZ → CP; COD + CRE → CWP. If no solution is found, a bispectral method is used to estimate the initial variables; other wise, the pixel is declared a no-retrieval pixel and no values are reported. Over snow and ice surfaces, either the 1.6-μm (Terra) or 2.1-μm (Aqua) reflectance replaces the 0.64-μm reflectance. Aqua snow-ice retrieval of COD underestimated due to code bug and insensitivity for ice clouds over snow at 2.1 μm. Nighttime: 10.8-μm radiance, BTD(3.8–10.8), and BTD(10.8–12) used simultaneously to estimate cloud phase, COD, CT, and CRE by matching to model calculations employing cloud emittance parameterizations. If COD > 8, default COD values of 8, 16, or 20 are assigned depending on CT and other factors. CEM computed from COD. CT → CZ using GEOS-4 temperature profile altered by simple 7.1 K/km lapse rate in the boundary layer; CZ → CP; COD + CRE → CWP.
Ancillary Input
References