Convective Momentum Transport and Its Impact on the Madden‐Julian Oscillation in E3SM‐MMF.

Convective momentum transport (CMT) is the process of vertical redistribution of horizontal momentum by small‐scale turbulent flows from moist convection. Traditional general circulation models (GCMs) and their multiscale modeling framework (MMF) versions poorly represent CMT due to insufficient information of subgrid‐scale flows at each GCM grid. Here the explicit scalar momentum transport (ESMT) scheme for representing CMT is implemented in the Energy Exascale Earth System Model‐Multiscale Modeling Framework (E3SM‐MMF) with embedded 2‐D cloud‐resolving models (CRMs), and verified against E3SM‐MMF simulations with 3‐D CRMs and observations. The results show that representing CMT by ESMT helps reduce climatological mean precipitation model bias over the western Pacific and the ITCZ regions, which is attributed to the weakened mean easterlies over the Pacific. Also, CMT from simulations with 2‐D and 3‐D CRMs impose a similar impact on Kelvin waves by reducing their variability and slowing down their phase speed, but opposite impacts on the Madden‐Julian Oscillation (MJO) variability. The ESMT scheme readily captures the climatological mean spatial patterns of the zonal and meridional components of CMT and their variability across multiple time scales, but shows some differences in estimating its magnitude. CMT mainly affects the MJO by decelerating its winds in the free troposphere, but accelerates its near‐surface winds. This study serves as a prototype for implementing CMT scheme in the MMF simulations, highlighting its crucial role in reducing model bias in mean state and spatiotemporal variability. Plain Language Summary: Small‐scale turbulent flows from moist convection typically lead to the vertical redistribution of large‐scale winds (referred to as convective momentum transport [CMT]). Due to the coarse grids that are too large to resolve small‐scale flows, traditional earth system models poorly represent the CMT, and thus rely on parameterizations that empirically describe the magnitude and vertical profiles of the CMT. In contrast, the default Energy Exascale Earth System Model‐Multiscale Modeling Framework (E3SM‐MMF) is an earth system model with a 2‐D (one horizontal dimension and one vertical dimension) cloud‐resolving model embedded within each coarse grid so as to better resolve small‐scale flows, although it still lacks the necessary information to fully calculate CMT due to the lack of the third dimension. Here we implemented the explicit scalar momentum transport (ESMT) scheme to represent CMT in the E3SM‐MMF associated with 2‐D small‐scale flows at each coarse grid. The results show that in general CMT helps reduce model biases in predicting time‐mean precipitation and winds as well as the spatiotemporal variability of tropical convection. The ESMT scheme reproduces the spatial patterns of CMT as simulated by the E3SM‐MMF model with 3‐D small‐scale flows. Lastly, we focused on the Madden‐Julian Oscillation, the dominant intraseasonal variability in the tropics, as an example to investigate the impact of CMT. Key Points: Convective momentum transport (CMT) affects large‐scale circulation and convective organization, and representing CMT reduces model biases in Energy Exascale Earth System Model‐Multiscale Modeling Framework simulationsExplicit scalar momentum transport scheme captures the spatial pattern of CMT comparable to 3‐D cloud‐resolving models that explicitly model CMTCMT damps the free tropospheric circulation associated with the Madden‐Julian oscillation, but accelerates its near‐surface winds [ABSTRACT FROM AUTHOR]