Subtropical Low Cloud Response to a Warmer Climate in a Superparameterized Climate Model. Part II: Column Modeling with a Cloud Resolving Model

A cloud resolving model (CRM) is used to investigate the low-cloud increase due to a 2 K SST increase in the SP-CAM superparameterized climate model. Of particular interest is the sensitivity of cloud changes to the CRM resolution (4 km horizontal, 30 vertical levels) in SP-CAM. The CRM is run in column-modeling framework using SP-CAM composite cloud regimes. The experiments are run to steady state using composite advective tendencies, winds, and sea-surface temperature from the control and +2 K climates of SP-CAM. A new weak temperature gradient algorithm based on an idealized form of gravity wave adjustment is used to adjust vertical motion in the column to keep the simulated virtual temperature profile consistent with the corresponding SP-CAM composite profile. Humidity is also slowly relaxed toward the SP-CAM composite above the boundary layer. With SP-CAM grid resolution, the CRM shows +2 K low cloud increases similar to SP-CAM. With fine grid resolution, the CRM-simulated low cloud fraction and its increase in a warmer climate are much smaller. Hence, the negative low cloud feedbacks in SP-CAM may be exaggerated by under-resolution of trade cumulus boundary layers.