Known limitations of MIROC3.2 and speculated causes
* Besides these points, we believe MIROC3.2 is just comparable with or
even better than the other state-of-the-art climate models!
- TOA radiation imbalance and deep ocean temperature drift Due to the
insufficient spinup, TOA radiation budget has a downward imbalance of ~0.5Wm-2
in high-res. and ~1. Wm-2 in med-res. The ocean temperature has a warming
drift accordingly. However, this drift is limited in deep ocean; upper ocean
and above surface climate do not show any serious drift.
- Cold bias around tropopause A serious cold bias up to 10K is seen
around the tropopause. This is found to be mainly due to insufficient
absorption of shortwave radiation by ozone. (This will be fixed in the next
version of the model with revised radiation code, unfortunately not in time
for the IPCC AR4)
- Too shallow subtropical marine boundary layer The subtropical marine
boundary layer is too shallow and a serious dry bias is seen above it
(900-700hPa). It indicates a need for improving the parameterization of cloud
topped boundary layer in this model.
- Too much high cloud cover The high clouds seem too much in extent and
optically too thick. The middle level clouds are then obscured by them and
seem too little when looked at from the space. This may be because cloud cover
diagnostics and/or cloud overlapping treatment are not very appropriate.
- Thin NH sea ice in high-res./Small SH sea ice extent in med-res. The
Northern hemisphere sea ice in high-res. version is not as thick as some
observational data. This seems to cause earlier decrease of sea ice extent in
the NH responding to the enhancement of GHGs. This is better in the med-res.
version, which has, instead, sea ice extent in the Southern hemisphere smaller
- Small amplitude of ENSO The amplitude of ENSO, in terms of the
variation of NINO3 SST, for example, is smaller than reality. It seems to be
at least partly attributable to a loose thermocline temperature gradient at
- Problem in volcanic aerosol distribution In the 20C3M runs, the
distribution of optical thickness of volcanic aerosol is prescribed for each
volcanic eruption event. It was intended that its vertical distribution has
maximum just above the tropopause (diagnosed from model's temperature lapse
rate) and decreases with height. However, due to an error in a model
parameter, it has actually a maximum uniformly at 50hPa regardless of the
temperature profile. We have confirmed that the effect of this error on
overall results, including surface temperature change, is small. However, if
you look at the trend of 100hPa temperature in 20C, for example, it should be
problematic due to this error.
- Sea level rise diagnosis (zosga) Land ice melt is not explicitly
added in the 'zosga' data but it is implicitly treated in the model. Since the
mass balance of land ice is not considered in the model, snow over ice
exceeding a certain critical amount is considerd to be glacier and flows into
the oceans, while melted water from ice does not flow into the oceans. This
treatment should be OK for equilibrium control but could be problematic for
transient runs. We may revise this data with some correction of this term
- 20C historical temperature in high-res. The 20C3M run with our
high-res. version does not really reproduce the observed warming trend in
early 20C and cooling trend in middle 20C. The reason for this is unclear. The
20C3M run with our med-res. version, which uses basically the same natural and
anthropogenic forcing as in the high-res. run, does reproduce these trends.
This might be due to natural inter-decadal variability, but we cannot confirm
this because we do not have enough computer resource to make ensemble runs
with the high-res. version.