The role of moisture-convection feedbacks in simulating the intraseasonal oscillation
Hannah, Walter Michael
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The sensitivity of the intraseasonal oscillation (ISO) in the National Center for Atmospheric Research’s (NCAR) Community Atmosphere Model (CAM) version 3.1 with Relaxed Arakawa-Schubert (RAS) convection modified with the moisture trigger of Tokioka et al. (1988) is analyzed with respect to changes to the specified minimum entrainment rate. Implementation of the Tokioka moisture trigger results in a drier and cooler troposphere due to the suppression of deep convection. A higher minimum entrainment threshold leads to more suppressed deep convection and improves the sensitivity of convection to free tropospheric humidity. This is accompanied by enhanced intraseasonal variability in the tropics. The simulated ISO which results from a non-zero minimum entrainment rate resembles a moisture mode. Variance of the column integrated saturation fraction is increased when minimum entrainment rate is increased, and precipitation becomes an increasingly non-linear function of saturation fraction which indicates that moisture-convection feedbacks are enhanced in the model. A reduction in the mean column moist static energy export by divergent motions indicates that the simulations with non-zero minimum entrainment thresholds are able to achieve negative gross moist stability which has been suggested as a necessary condition to be able to produce a moisture mode. This decrease in gross moist stability with increased minimum entrainment rate is accompanied by a lowering of the mean diabatic heating profile maximum. Additional simulations are analyzed to investigate the impact of a rain re-evaporation fraction parameter on the simulated ISO. A higher rain re-evaporation fraction leads a stronger ISO signal in the model. However, In contrast to the effect of increased minimum entrainment rate, increased rain re-evaporation fraction yields a mean state which is warmer and moister. This discrepancy in mean state humidity change indicates that intraseasonal variability has no unique dependence on basic state humidity, in contradiction to that suggested in previous studies.