Since we will be using an aqua-planet version of the GCM to study changes in the behaviour of convection, we must satisfy ourselves that chnages to the basic state of the aqua-planet model are similar to those in the full GCM. Figure 9 summarizes some of the changes to the mean state of the aqua-planet model when going from 19 to 30 levels. Figure 9(a) shows the zonal mean convective precipitation distribution for both 19 and 30 level integrations. The heavy precipitation near the equator reduces by 2-3 mm per day, but the belt of heavy precipitation becomes slightly wider with the net effect being that the global mean convective precipitation actually increases slightly at L30. Changes in dynamic precipitation between L19 and L30 runs in both aqua-planet and full GCM are negligible in the tropics.
The zonal mean temperature changes in the aqua-planet model are shown in fig 9(b). There is a general cooling of the tropical troposphere at L30, which is largest at around 200 hPa. This is very similar to the pattern seen in the full GCM, bearing in mind the asymmetries in the full GCM associated with inter-hemispheric differences in continentality.
In section 3.2 it was shown that the tropical troposphere becomes drier at L30 in the full GCM. Figure 9(c) shows the change to the distribution of specific humidity in the aqua-planet GCM when going from L19 to L30. Comparison of this figure with fig. 6 shows that there are differences between changes to the water vapour distribution in the aqua-planet model and those in the full GCM. However, much of the difference can be explained by the asymmetry of the full GCM and that deep convection is occurring right around the equator in the aqua-planet integration but is suppressed in some locations on the equator in the full GCM. The main signal when going from L19 to L30 in both full GCM and aqua-planet is that the tropical troposphere becomes drier, particularly above 650 hPa.
In summary, the L30 aqua-planet run shows a slightly broader belt of tropical convection, with peak values of convective precipitation slightly lower than the L19 run. The tropical troposphere has lower temperature and specific humidity values in the L30 run, particularly above 600 hPa. These changes are generally in the same sense as the changes in the full GCM when increasing the vertical resolution.