An Update on Modifications to the ERBE Monthly Means

In Spring 2007, we reprocessed the ERBE record from February 1985 to April 1989. This product serves as an update to the previous CAS modifed ERBE fields. In this revision, a better accounting is made for the discontinuity that occurred with the loss of NOAA 9 in January 1987 and in the implied transport of energy from land to ocean regions based on ERBS retrievals. [The transport of energy can be inferred approximately from the 12-month running mean of net radiative flux over land, adjusted by the implied atmospheric energy tendency as the net flux of energy into land areas is small relatively (<0.1 PW, Huang et al. 2006).]. As in the previous version, corrections are made for the global mean imbalance and missing data are infilled.

The missing data all occur for the Absorbed Solar Radiation (ASR) and thus net radiation near the delimiter of the incoming radiation where it is difficult to obtain an accurate albedo. To fill the missing data, we firstly took advantage of the fact that the areas missing varied from year to year and we formed a climatology of the available data on albedo. A least squares fitted first harmonic was then derived for each point, and the missing climatological mean values determined and these were used to fill in the missing albedo. The ASR and net radiation were then derived. The mean annual cycle probably involves more than a single harmonic, but use of the least squares fitting approach with more than one harmonic could occasionally result in albedos for missing points that exceeded unity. Therefore the approach used is conservative, but it produces quite reasonable numbers. Because the missing points are weighted by small incoming radiation, the impact on the ASR is not very great, but it is desirable to do this step rather than treat the data as missing in order to provide fields that are continous in space and time.

For the first two years of ERBE data, the global mean outgoing longwave radiation (OLR) mean was 233.9 W/m^2 but it jumped to 236.5 W/m^2 after the loss of NOAA 9. Presuming that the values with 3 satellites are superior, we have adjusted the OLR uniformly over the globe and over land (by an additional 2.95 W/m^2) from February 1987 on, justifying this as a bias most likely arising from the disproportionately large diurnal cycle that exists over land. The net imbalance in annual mean net radiation was initially 4.2 to 6.0 W/m^2, and after the first adjustment to OLR, this was corrected for by applying a decrease in the albedo uniformly such that the mean radiation budget balances (in order to be consistent with the ocean energy tendency over the ERBE period presented in Levitus et al. 2001). This method has also yielded robust agreement with the ERBS-satellite estimate of the inferred transport of energy from land to ocean for 60N-60S.

divider barReferences:

Fasullo, J. T., and K. E. Trenberth, 2008: The Annual Cycle of the Energy Budget: Global Means and Land-Ocean Exchanges, J. Clim., 21, 2314-2326 [pdf]

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

Huang, S., 2006: Land warming as part of Global Warming, EOS, 87, 31 Oct 2006, p44.

Wielicki, B. A., K. Priestley, P. Minnis, N. Loeb, D. Kratz, T. Charlock, D. Doelling, D. Young, 2006: CERES Radiation Budget Accuracy Overview, 12th Conf. Atmospheric Radiation, Madison , Wisconsin.

Levitus, S. and co-authors, 2001: Anthropogenic warming of the Earth's climate system, Science, Vol 292, 13 Apr 2001, p 267.