The simulation of atmospheric temperature and water vapor is analyzed for the ten year GFDL AMIP integration. The most significant problem for water vapor and precipitation is a cold bias of about 2-3°K in the troposphere which results in roughly a 20% reduction of total precipitable water (TPW). This cold bias may also be responsible for the excessive precipitation rate which occurs in the areas of convection in the tropics due to the reduction of the moisture holding capacity of the middle and upper troposphere.
With regard to relative humidity, the middle tropospheric relative humidity
(MTH) tends to be too moist in the regions of subsidence in the subtropics,
but otherwise appears to resolve regional and seasonal variations quite
well. On the other hand, upper tropospheric relative humidity (UTH) is
systematically overestimated at all latitudes. This feature tends to be
especially pronounced over the convectively active regions of the tropics
which may be related to the cold bias in the model atmosphere. Although
precipitation in the tropics is overestimated, as noted above, geographic
patterns of precipitation appear to be in reasonable agreement with observation.
Interestingly enough, the overall radiative balance at the top of the model
atmosphere is quite good which suggests that the reduction of outgoing
longwave radiation due to the cold bias of the upper model atmosphere approximately
balances the increase of outgoing longwave radiation due to the reduction
of TPW.