Tropical intraseasonal variability in 22 IPCC, DEMETER and NCEP global models

We have evaluated the tropical intraseasonal variability in 22 global weather and climate forecasting models, including:

(1) 14 climate models participating in the IPCC Fourth Assessment Report (AR4) to be released in 2007; (2) Seven European NWP models participating in the DEMETER project; and (3) Four NCEP GFS and CFS models.

We found three encouraging results:

(1) Many of the models have signals of convectively coupled waves, with Kelvin and MRG-EIG waves especially prominent. (2) The eastward MJO precipitation variance in many models is significantly larger than its westward counterpart, and even approaches the observed value in two models. (3) The NWP models generally have better MJO signals than the climate models.

However, we also found two common biases:

(1) The MJO variances in many models do not come from a pronounced spectral peak, but from part of a too red spectrum (i.e., too red “background noise”), which in turn are associated with a too strong persistence of precipitation. (2) The equivalent depths for all equatorial waves are too large, which is indicative of a too strong “effective static stability” and thus too weak wave-heating feedback.

Possible ways to improve the simulated tropical intraseasonal variability are discussed, and some of them are demonstrated by GCM sensitivity experiments.