Climate Data Records from Merged CERES and Geostationary Imager Data

Stable climate data records from satellite-based instruments are required for the detection of subtle climate trends. Changes in availability of data, orbital characteristics, sampling patterns, and calibration can all cause spurious trends in climate records derived from space-based instruments. However, greater stability in climate data records can be achieved by incorporating the varying strengths of multiple satellite-based instruments in different orbits. The operational products from the Clouds and the Earth’s Radiant Energy System (CERES) experiment are assimilating data from up to 8 separate spacecraft in order to produce the most accurate radiation budget data set to date. CERES has measured shortwave (SW: 0.3 – 5.0 µm) and longwave (LW: 5.0 – 200. µm) broadband radiance from five instruments on three spacecraft. The four instruments aboard the Terra and Aqua spacecraft are currently operating nominally with calibration accuracy of better than 1% in the SW and 0.5% in the LW with calibration stability of 0.1%/year. CERES has also greatly increased the accuracy of the estimates of instantaneous flux from the radiance measurements via the production of improved angular distribution models (ADM).

One area where the CERES climate record can be further improved through the use of additional data from other satellites is in the removal of temporal sampling errors. Low Earth orbit (LEO) satellites such as Terra and Aqua cannot adequately define the diurnal cycle of radiation. Estimates of monthly mean flux based on a single sun-synchronous satellite can be biased by more than 20 W/m2 in climatic regions with pronounced diurnal cycles. Instruments onboard geostationary (GEO) satellites have the capability of viewing regions during the complete diurnal cycle. Unfortunately, except for the recently launched Geostationary Earth Radiation Budget Instrument (GERB), there are no well-calibrated broadband measurements available from GEO. However, narrowband GEO data can be merged with CERES data to remove temporal sampling errors as long as careful consideration is given to calibration, spectral differences, and proper use of broadband ADM with narrowband data. Particular emphasis must be given to using the well-calibrated CERES observations as the absolute calibration source, and using the GEO data to provide information concerning diurnal variability. The GEO data will provide the diurnal variability but the variability is normalized to the CERES observations in order to maintain the CERES absolute calibration.

CERES goals include finer time interval averaging steps than monthly means. Daily means and synoptic fields of global fluxes are also planned. This further drives the need for unbiased estimates of broadband flux at times not observed by CERES. The successful incorporation of GEO data with CERES will also provide a means of maintaining a stable climate record that is not affected by changes in the temporal sampling from CERES.

This paper will present recently released CERES monthly mean and shorter time scale products. A detailed description of the methods used to incorporate GEO data into the CERES time record will be provided. The effectiveness of using GEO to eliminate temporal sampling errors from CERES will be shown using comparisons with surface broadband data, intercomparisons between multiple CERES instruments, and possibly preliminary comparisons with high temporal resolution broadband data from GERB.