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Convective Available Potential Energy: Mistreated by Models
Gettelman, A., Seidel, D.J., Wheeler, M.C. and Ross, R.J.  2002.  Multidecadal trends in tropical convective available potential energy.  Journal of Geophysical Research 107: 10.1029/2001JD001082.

The authors of this interesting observational-plus-modeling paper begin by stating that "convective available potential energy (CAPE), which can be calculated from radiosonde observations, is a measure of the conditional stability of the troposphere to a finite vertical displacement, as occurs during moist convection," noting that "long-term changes in CAPE might be associated with changes in convective activity and the atmospheric energy budget" and that "CAPE is thus a potential indicator of climate change."

What was done
The authors used radiosonde observations from fifteen stations in the tropics with "long stable records" to calculate CAPE over the period 1958-1997.  These records were then "compared to a calculation of CAPE from a climate model simulation forced by observed sea surface temperatures."  The model used for this purpose was the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3).

What was learned
The authors observed increases in CAPE at 12 of the 15 tropical radiosonde stations over the period of their study.  These increases, they say, appear to have been driven "by increases in near-surface temperature and/or humidity."  They also report that the overall increase in CAPE appears to have been caused largely "by a shift in the middle 1970s" that was "consistent with the time of an apparent shift of the background state of the climate."  The climate model, however, even though forced by observed sea surface temperatures, did not reproduce the overall increase in CAPE.  The authors then coupled the atmospheric model to an ocean model and ran a simulation "with changing greenhouse gases and aerosols [as observed] in the twentieth century."  Once again, however, they report that, "like the atmospheric GCM, the coupled model did not reproduce the observed trends in CAPE over the period examined."

What it means
The authors report that "observed CAPE has mostly statistically significant positive trends over the period of 1958-1997 in the tropics, yet a modern climate model is not able to reproduce these trends."  They thus state, in an implied challenge to climate modelers to correct this situation, that "ensuring future models can faithfully reproduce such trends is perhaps quite important for enhancing confidence in model predictions of future climate changes."

Reviewed 4 June 2003