Volume 11, Number 14: 2 April 2008
How well do state-of-the-art climate models reproduce the workings of real-world energy and water cycles? This question is addressed in the study of L'Ecuyer and Stephens (2007), who state that "our ability to model the climate system and its response to natural and anthropogenic forcings requires [our italics] a faithful representation of the complex interactions that exist between radiation, clouds, and precipitation and their influence on the large-scale energy balance and heat transport in the atmosphere," and who further state that "it is also critical to assess [model] response to shorter-term natural variability in environmental forcings using observations."
In the spirit of this logical philosophy, the two researchers decided to use multi-sensor observations of visible, infrared and microwave radiance obtained from the Tropical Rainfall Measuring Mission satellite for the period running from January 1998 through December 1999, in order to evaluate the sensitivity of atmospheric heating -- and the factors that modify it -- to changes in east-west sea surface temperature gradients associated with the strong 1998 El Niņo event in the tropical Pacific, as expressed by the simulations of nine general circulation models of the atmosphere that were utilized in the Intergovernmental Panel on Climate Change's most recent Fourth Assessment Report. This protocol, in their words, "provides a natural example of a short-term climate change scenario in which clouds, precipitation, and regional energy budgets in the east and west Pacific are observed to respond to the eastward migration of warm sea surface temperatures," which is somewhat akin to the natural experiment approach of Idso (1998).
So what did they learn from this exercise?
L'Ecuyer and Stephens report that "a majority of the models examined do not reproduce the apparent westward transport of energy in the equatorial Pacific during the 1998 El Niņo event." They also state that "the intermodel variability in the responses of precipitation, total heating, and vertical motion is often larger than the intrinsic ENSO signal itself, implying an inherent lack of predictive capability in the ensemble with regard to the response of the mean zonal atmospheric circulation in the tropical Pacific to ENSO." In addition, they say that "many models also misrepresent the radiative impacts of clouds in both regions [the east and west Pacific], implying errors in total cloudiness, cloud thickness, and the relative frequency of occurrence of high and low clouds."
In light of these much-less-than-adequate findings, the two researchers from Colorado State University's Department of Atmospheric Science conclude that "deficiencies remain in the representation of relationships between radiation, clouds, and precipitation in current climate models," and they say that these deficiencies "cannot be ignored when interpreting their predictions of future climate."
We couldn't agree more.
Sherwood, Keith and Craig Idso
References
Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.
L'Ecuyer, T.S. and Stephens, G.L. 2007. The tropical atmospheric energy budget from the TRMM perspective. Part II: Evaluating GCM representations of the sensitivity of regional energy and water cycles to the 1998-99 ENSO cycle. Journal of Climate 20: 4548-4571.