Relevance between the Changes in Moisture Transport and Cirrus

Centralized deep convection supplies more moisture to the tropopause under the higher SST condition, and high thin clouds, which had invisible optical thickness, become thicker and visible (Figure 20.1). Moisture transport increases in the tropics, despite the decreasing vertical velocity over almost all latitudes. The rise in moisture transport through deep convection increases the detainment of moisture at the top of the convection until anvils form. This contributes to the increase in the cirrus in the tropopause. Further detailed study is required before a full understanding of the mechanisms involved in the changes in moisture transport is possible.

Uncertainties remain because of our insufficient understanding of the mi-crophysics of clouds. Sensitivity studies of radiative convective experiments over the tropics (not shown here) reveal that the extent of high clouds and the thickening of thin clouds (magnitude of the increase in albedo effect) are sensitive to our model's parameters of microphysics and boundary layer. However, almost all experiments show an increase in the clouds' greenhouse effect.

From these results, the decrease in cirrus and smaller difference in cloud greenhouse effect among GCMs might be attributed to the different treatment of cirrus and cumulus convection, which are represented by large-scale condensation and cumulus parameterization, respectively. In the GCM, whose grid scale is on the order of 100 km, we cannot explicitly resolve the deep convection and detrained anvils associated with them. Our results indicate the necessity of quantifying the amount of detrained moisture from the convection in the subgrid-scale distribution of moisture in a GCM grid in a more consistent manner.

Climate studies using CRMs will become more widespread in the near future. The acquisition of new cloud data with higher dimension and accuracy has already begun (CloudSat, CALIPSO), and subsequent evaluation will be essential to reduce the uncertainty in climate sensitivity simulations.

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