Linking Deep and Shallow Convective Mass Fluxes via an Assumed Entrainment Distribution in CAM5-CLUBB: Parameterization and Simulated Precipitation Variability

Abstract

We modify the Zhang-McFarlane (ZM) deep convection scheme in the Community Atmosphere Model version 5 to couple it with a unified parameterization for boundary-layer turbulence and shallow convection, that is, Cloud Layers Unified by Binormals (CLUBB). By assuming a lognormal distribution of entrainment rate across the entire moist convective regimes, we link mass fluxes between shallow and deep convection, which are partitioned by the entrainment rate of the shallowest deep convective plume. Hence, a new deep convective closure is established which is coupled to the sub-grid vertical motion variability in CLUBB. The convection feedback (or memory) effects are also considered to decrease the entrainment spectrum width and enhance the vertical velocity variability that further affect deep convection. Results show that the revised scheme improves the precipitation simulations in terms of the mean state and variability at various timescales, such as the alleviated double-intertropical convergence zone and more realistic simulations of the seasonal variation of monsoon precipitation over East Asia, Madden-Julian Oscillation, and precipitation diurnal phase propagations downstream of large terrains. The improvements are still seen in many aspects such as the mean-state precipitation when turning off the convection feedback impacts in the revised scheme, emphasizing the benefits of using the modified mass-flux closure. However, the convection feedbacks have considerable effects on the precipitation diurnal cycle simulations over regions with late-afternoon precipitation peaks. Overall, the revised scheme provides a unified treatment for sub-grid vertical motions across regimes of boundary-layer turbulence, shallow convection, and deep convection, leading to better-simulated precipitation at various timescales.