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

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. Plain Language Summary The physical consistency between shallow and deep convection is an important but largely misrepresented aspect in climate models. In the atmosphere, convective updraft plumes usually differ in their fractional entrainment rate, with weaker entrainment favoring a more vertical extension of the plume. Here, we modify the ZM convective parameterization in CAM5 to couple it with a unified parameterization for boundary-layer turbulence and shallow convection, that is, CLUBB. A continuous distribution is used to represent the spectrum of entrainment rates across the entire moist convective regimes and to link mass fluxes between shallow and deep convection. This couples deep convection to CLUBB and provides a unified treatment for sub-grid vertical motions across regimes of boundary-layer turbulence, shallow convection, and deep convection. In addition, the convection memory effects are included to affect deep convection via changing the entrainment spectrum width and vertical velocity variability. The revised scheme performs much better than the original one in the simulated precipitation variability at various timescales, such as the alleviated double-intertropical convergence zone and improved monsoon precipitation, Madden-Julian Oscillation, and continental precipitation diurnal variation. Both of the modifications related to the mass flux closure and convection feedback impacts are important for the improved model performance.