CERES Terra Edition1A Energy Balanced and Filled (EBAF) Data Quality Summary

Investigation: CERES

Data Product: CERES EBAF

Data Set: Terra (Instruments: CERES-FM1 or CERES-FM2)

Data Set Version: (Terra) Edition1A

Introduction

This document provides a high-level quality assessment of the CERES Energy Balanced and Filled (EBAF) data product. As such, it represents the minimum information needed by scientists for appropriate and successful use of the data product. For a more thorough description of the methodology used to produce EBAF, please see Loeb et al. (2008).

Description

Despite recent improvements in satellite instrument calibration and the algorithms used to determine reflected solar (SW) and emitted thermal (LW) top-of-atmosphere (TOA) radiative fluxes, a sizeable imbalance persists in the average global net radiation at the TOA from satellite observations. The 5-year global mean CERES net flux from the standard CERES SRBAVG-GEO Edition2D_rev1 product is 6.5 Wm^-2, much larger than our best estimate of 0.85 Wm^-2 based on observed ocean heat content data and model simulations. This imbalance is problematic in applications that use Earth Radiation Budget (ERB) data for climate model evaluation, estimate the Earth's annual global mean energy budget, and in studies that infer meridional heat transports.

A second problem users of SRBAVG data have noted is the occurrence of gaps in monthly mean clear-sky TOA flux maps due to the absence in some regions of cloud-free areas occurring at the CERES footprint scale (~20-km at nadir).

To address these problems, we have created a modified version of SRBAVG-GEO Edition2D_rev1, called the CERES Energy Balanced and Filled (EBAF) dataset, that uses an objective constrainment algorithm to adjust SW and LW TOA fluxes within their range of uncertainty to remove the inconsistency between average global net TOA flux and heat storage in the Earth-atmosphere system. The problem of gaps in clear-sky TOA flux maps is addressed by inferring clear-sky fluxes from both CERES and Moderate Resolution Imaging Spectrometer (MODIS) measurements to produce a new clear-sky TOA flux climatology that provides TOA fluxes in each region every month.

TOA Flux Adjustments

Table 1 provides an error budget for SW, LW and net TOA flux in the CERES SRBAVG-GEO product. It includes biases of known sign such as the use of TOA solar irradiance 1365 Wm^-2 to compute net TOA flux instead of 1361 Wm^-2 as has recently been reported by SORCE (Kopp et al., 2003). The largest uncertainties are associated with instrument calibration, which account for 4.2 Wm^-2 (2σ). When all uncertainties are tallied, the expected range in net TOA flux is -2.1 Wm^-2 to 6.7 Wm^-2.

To remove the inconsistency between average global net TOA flux and heat storage in the Earth-atmosphere system, an objective constrainment algorithm is used to derive optimal adjustments to the SRBAVG-GEO incoming solar, SW, and LW TOA fluxes. After removing the constant flux bias errors in Table 1, the constrainment algorithm assigns errors to each error source, accounting for the assessed range of uncertainty in each term and the overall difference between the average global net and the assumed heat storage in the Earth-atmosphere system. We assume the "true" global net flux imbalance is +0.85 Wm^-2, based on Hansen et al. (2005). The optimal adjustments are applied to SRBAVG-GEO to produce all-sky CERES-EBAF TOA fluxes.

CERES SRBAVG clear-sky monthly mean TOA fluxes are provided for 1°x1° latitude-longitude regions derived from CERES footprints that are completely cloud-free according to 1-km resolution MODIS data. Because of the coarse spatial resolution of CERES (20 km at nadir), this approach only considers flux contributions from cloud-free regions occurring over relatively large spatial scales and meteorological conditions and geographical regions where clouds occur less frequently. As a result, clear-sky maps from CERES SRBAVG contain many missing regions. We introduce an alternative approach that attempts to recover clear-sky flux contributions at smaller spatial scales. This approach is an extension to that used by Loeb and Manalo-Smith (2005) to estimate the SW TOA direct radiative effects of aerosols over ocean. We determine gridbox mean clear-sky fluxes using an area-weighted average of: (i) CERES broadband fluxes from completely cloud-free footprints, and (ii) MODIS-derived "broadband" clear-sky fluxes estimated from the cloud-free portions of partly and mostly cloudy CERES footprints. In both cases, clear regions are identified using the CERES cloud algorithm applied to MODIS pixel data (Minnis et al., 2003). Clear-sky fluxes in partly and mostly cloudy CERES footprints are derived using MODIS-CERES narrow-to- broadband regressions to convert MODIS narrowband radiances averaged over the clear portions of a footprint to broadband SW radiances. The "broadband" MODIS radiances are then converted to TOA radiative fluxes using CERES clear-sky ADMs (Loeb et al. 2005).

Table 2 provides 5-year mean TOA fluxes from CERES EBAF (last column) and SRBAVG-GEO (3^rd column) for March 2000 through February 2005. SW TOA flux increases by 1.8 Wm^-2 while LW increased by 2.5 Wm^-2. These changes together with a 1.3 Wm^-2 decrease in TOA solar irradiance leads to a net TOA flux imbalance of 0.9 Wm^-2, consistent with our best estimate for ocean heat storage. The EBAF TOA fluxes differ substantially from adjusted ERBE values based on Trenberth (1997). SW TOA fluxes in EBAF are 7 Wm^-2 lower and LW TOA fluxes are 5.2 Wm^-2 higher than the Trenberth et al. (1997) fluxes.

Contents of the Data Product

The CERES EBAF data product provides a 5-years of monthly mean top-of-atmosphere (TOA) radiative fluxes for March 2000 through October 2005. The fluxes are derived from the CERES Terra SRBAVG-GEO Edition2D_rev1 and CERES Terra SSF Edition2B_rev1 data products. The CERES EBAF netCDF file contains 68 monthly means (Mar00-Oct05) and 12 monthly 5-year means (average of all Januaries, Februaries, etc.) (Mar00-Feb05) as indicated in Table 3. CERES EBAF uses only CERES Terra SRBAVG-GEO data in crosstrack mode. To see what CERES instrument is in crosstrack mode for any given month, please refer to the CERES Instrument Scan Modes web page.

Cautions and Helpful Hints

Several cautions and helpful hints common to both CERES EBAF and CERES SRBAVG-GEO are provided in the CERES SRBAVG Data Quality Summary and are not repeated here. Rather, the following list addresses differences between CERES EBAF and SRBAVG that users of CERES EBAF should be aware of.

• CERES EBAF uses a solar irradiance at TOA based on SORCE (Kopp et al., 2003) which is 1361 Wm^-2. SRBAVG-GEO assumes a value of 1365 Wm^-2.

• CERES SRBAVG assumes a spherical Earth when averaging TOA insolation over the Earth's surface. This gives the well-known S_o/4 expression for mean solar irradiance, where S_o is the instantaneous solar irradiance at the TOA. When a more careful calculation is made by assuming the Earth is an oblate spheroid instead of a sphere, and the annual cycle in the Earth's declination angle and the Earth-sun distance are taken into account, the division factor becomes 4.0034 instead of 4. Consequently, the mean soar irradiance for CERES EBAF is 1361/4.0034 = 340.0 Wm^-2, compared to 1365/4 = 341.3 Wm^-2 for CERES SRBAVG.

• After the release of SRBAVG-GEO Edition2D, an error was discovered in the computation of the declination angle and earth-sun distance factor. The angle and factor were computed at 00:00 GMT instead of 12:00 GMT, which is appropriate for computing the solar incoming in local time. This has no effect on the annual mean insolation but significantly affects the monthly zonal solar incoming fluxes near the poles. This error is corrected in the final adjusted TOA fluxes and will also be rectified in the next SRBAVG version (Edition3).

• Adjustments to total solar irradiance associated with the spherical Earth assumption are applied zonally to improve the accuracy of incoming solar radiation at each latitude. While these adjustments are applied at the zonal level, the globally averaged correction is the same as in Table 1. Similarly, adjustments in SW TOA fluxes due to near-terminator flux biases are also applied zonally without modifying the global mean. Separate adjustments are made for clear and all-sky TOA fluxes.

References

Hansen, J., and co-authors, 2005: Earth's energy imbalance: Confirmation and implications. Science, 308, 1431-1435.

Kopp, G., G. Lawrence, and G. Rottman, 2005: The Total Irradiance Monitor (TIM): Science Results, Solar Physics, 230, 129-140.

Loeb, N.G., B.A. Wielicki, D.R. Doelling, G.L. Smith, D.F. Keyes, S. Kato, N. Manalo-Smith, and T. Wong, 2008: Towards optimal closure of the Earth's top-of atmosphere radiation budget, J. Climate (accepted).

Loeb, N.G., and N. Manalo-Smith, 2005: Top-of-atmosphere direct radiative effect of aerosols over global oceans from merged CERES and MODIS observations, J. Climate, 18, 3506-3526.

Loeb, N.G., S. Kato, K. Loukachine, and N.M. Smith, 2005: Angular distribution models for top-of-atmosphere radiative flux estimation from the Clouds and the Earth's Radiant Energy System instrument on the Terra Satellite. Part I: Methodology, J. Atmos. Ocean. Tech., 22, 338-351.

Minnis, P., D. F. Young, S. Sun-Mack, P. W. Heck, D. R. Doelling, and Q. Trepte, 2003: "CERES Cloud Property Retrievals from Imagers on TRMM, Terra, and Aqua" Proc. SPIE 10th International Symposium on Remote Sensing: Conference on Remote Sensing of Clouds and the Atmosphere VII, Barcelona, Spain, September 8-12, 37-48.

Trenberth, K. E., 1997: Using atmospheric budgets as a constraint on surface fluxes. J. Clim., 10, 2796-2809.

Browse Product Webpage

Maps of all parameters in the CERES EBAF dataset are available on the CERES Browse Products Webpage.

Expected Reprocessing

Reprocessing of CERES EBAF is anticipated well after release of CERES Edition3 SRBAVG-GEO in 2010.

Attribution

The CERES Team has gone to considerable trouble to remove major errors and to verify the quality and accuracy of this data. Please provide a reference to the following paper when you publish scientific results with the CERES EBAF data:

Loeb, N.G., B.A. Wielicki, D.R. Doelling, G.L. Smith, D.F. Keyes, S. Kato, N. Manalo-Smith, and T. Wong, 2008: Towards optimal closure of the Earth's top-of atmosphere radiation budget, J. Climate (accepted).

When Langley ASDC data are used in a publication, we request the following acknowledgment be included: "These data were obtained from the NASA Langley Research Center EOSDIS Distributed Active Archive Center."

The Langley ASDC requests two reprints of any published papers or reports which cite the use of data that we have distributed. This will help us determine the use of data that we distribute, which is helpful in optimizing product development. It also helps us to keep our product related references current.

Feedback and Questions

For questions or comments on the CERES Quality Summary, contact the User and Data Services staff at the Atmospheric Science Data Center.