Evaluating the Effects of Deep Convection Parameter Perturbations on Cloud Radiative and Physical Properties in the Tropical Warm Pool

Abstract

The Pliocene Epoch was the last time in Earth's history that atmospheric CO2 levels exceeded the elevated levels seen today. With geography similar to the present day, the Pliocene serves as a potential analog for future warming. However, sea surface temperature (SST) reconstructions reveal less warming in the Tropical Warm Pool (TWP) than climate models predict for future warming. This discrepancy suggests that climate models could be missing a cloud radiative damping mechanism in the TWP. To explore this, we use the Single Column Atmosphere Model v6 to simulate the TWP and systematically perturb parameters in the Zhang-McFarlane deep convection scheme. We do so without temperature relaxation at control and +4 degrees C prescribed SSTs. We then test the significance (alpha = 0.05) of the ordinary least-squares regression coefficients with respect to cloud properties. We find that precipitation efficiency (c0_ocn) is not significant with respect to changes in most cloud variables. Increased reevaporation of precipitation (ke) correlates most strongly with greater specific humidity, and hence, greater net clearsky forcing. Net cloud forcing (CF) decreases as the entrainment parameter (dmpdz) becomes less negative due to substantially less negative shortwave CF. A longer deep convection timescale (tau) weakly correlates to a decrease in longwave CF and increased shortwave CF. These correlations and their physical explanations provide a foundation for research into a cloud radiative damping mechanism in climate models.