Increased global warming is likely to be accompanied by changes in the
global distribution of clouds, precipitation, and water vapor. If we
had a clearer understanding of structural changes in the atmospheric
hydrological cycle due to surface warming, we could obtain a
“fingerprint” for evidence of global warming. The Goddard Cumulus
Ensemble Model (GCEM) was instituted to develop this understanding,
hoping to obtain a quantitative description of the basic structure and
water budget of tropical cumulus convection. Results of this study are
discussed in “A Preliminary Study of the Tropical Water Cycle and its
Sensitivity to Surface Warming.
The GCEM study consisted of two experiments. The first experiment (the control case) was modeled using present day conditions. The second experiment (the warm case) was modeled with conditions identical to the first, but with sea surface temperature increased to 30oC. Comparing results from both GCEM experiments show the response of the tropical water cycle to surface warming.
Results from the GCEM experiments indicate substantial changes in the tropical water cycle in a climate with above normal sea surface temperature. Temperature rises throughout the troposphere, with the largest warming (4oC) taking place in the upper troposphere. Water vapor increases approximately 20% near the surface and decreases monotonically with height up to 10 km. These changes are directly influenced by the change in surface evaporative flux. While unchanged in the lower troposphere, relative humidity increased 5-10% in the region above the freezing level and below the tropopause. This corresponds with an increase of upper-level cloud water between 6 and 10 km due to more clouds reaching higher levels. A sharp reduction in cloudiness between 4 and 5 km is also evident, with an increase in cloud water at low levels.
Experiment results show the warm case resulting in surface evaporation increasing 13%. The local precipitation gain resulting from enhanced surface evaporation is large (22%), and mostly in the form of convective rain. This suggests that surface warming induces a large positive feedback between dynamic moisture convergence and precipitation. This is attributable primarily to the increased vertical advection of moisture, and occurs in the presence of fixed large-scale vertical motion. Results also show the freezing level is elevated from 0.5 to 1 km. This produces changes in the proportion and elevation of ice and water clouds, with cloud cover increasing above, and decreasing below the freezing level.
While providing considerable information about structural changes in the tropical atmospheric hydrological cycle due to surface warming, the absence of feedback to the large-scale circulation prevents these results from being applied directly to the global domain. It should also be noted that despite the detailed microphysics and cloud-scale dynamical processes included in the GCEM, results regarding tropospheric water vapor and temperature are consistent with those from general circulation models (GCM) that treat microphysical processes crudely. This is in agreement with previous findings that equilibrium water vapor distribution is a strong function of temperature, so that if large-scale temperature is well predicted from either model, the two approaches will yield similar results.
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