A CERES-Consistent Cloud Property Climate Data Record From AVHRR Data
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Wednesday, 7 January 2015
As part of the NOAA NCDC Climate Data Record (CDR) program, scientists at NASA LaRC are currently developing CDRs of cloud and clear-sky radiation properties and shortwave channel reflectance extending from 1978 to the present time using data from the Advanced Very High Resolution Radiometer (AVHRR) instrument. The CDR will be consistent with shortwave channel observations and cloud properties derived from MODIS for the Clouds and Earth's Radiant Energy System (CERES) program, though some modifications to these algorithms will be required to operate on the 5-channel and 4-km spatial resolution AVHRR Global Area Coverage (GAC) data. Accurate and stable shortwave channel calibration is essential for producing a robust, long-term analysis of global cloud amount and cloud properties. AVHRR does not have on-board calibration of the visible and near-IR channels so relative calibration techniques must be used. Reliable calibrations are ensured through matching AVHRR with Aqua MODIS using simultaneous nadir overpasses (SNOs) in addition to calibration trends derived from observations of deep convective clouds, deserts, polar ice caps which can also be analyzed prior to the MODIS era. Differences between MODIS and AVHRR spectral response functions are accounted for using SCIAMACHY spectral band adjustment factors (SBAF). SBAFs for the AVHRR IR channels have also been generated via Aqua MODIS SNOs. AVHRR navigation errors, pixel noise, and bad scan lines have been addressed to ensure that the highest quality input data is used for the CDR.
NASA LaRC cloud and clear-sky radiation property output consists of two categories of fields, 1) fields of sufficient maturity and quality to be used for long-term climate analysis, and 2) new fields that are expected to become “CDR-quality” in future versions. Current CDR-quality datasets consist of 0.63, 0.86, and 1.61 μm channel reflectance, cloud amount, phase, optical depth, and effective particle size, height, temperature, and pressure. Additional fields include land and water surface skin temperature, broadband shortwave albedo and longwave flux, cloud top and base temperature/height/pressure, overshooting cloud top pixel detection, and dataset quality control information.
The CDR generation process has recently begun at NASA LaRC, initially focusing on the NOAA-18 and NOAA-9 AVHRR time series. This presentation will describe the methodology used for the CDR, product validation using NASA A-Train observations, comparisons with other available cloud property climatologies (CERES MODIS, GSFC MODIS Science Team, EUMETSAT CMSAF, and ISCCP), and trends in cloud properties derived from NOAA-9 and -18 processing.