Adding Mesoscale Heating in E3SMv1 Improves MJO and Precipitation

The transport and mixing of heat and momentum throughout the atmosphere largely control the global circulation, and hence moisture and precipitation patterns. However, several transport processes occur on scales much smaller than a global circulation model (GCM) grid box, and therefore have to be parameterized. Improvements in the representation of subgrid heat and momentum transport can lead to significant model improvements in the representation of wind stresses, moisture and precipitation patterns, and organized modes of variability. In particular, convection is a large source of heat and momentum transport in the atmosphere, both on scales of individual convective plumes, as well as on scales of the order of 10–1,000 km (mesoscales).

The importance of convective organization on the global circulation has been recognized for more than three decades but parameterizations of the attendant processes are missing from GCMs. Contemporary convective parameterizations commonly use a convective plume model (or a spectrum of plumes). This is appropriate for unorganized convection. However, the assumption of a gap between the cumulus scale and the large-scale motion that underpins contemporary convective parameterizations fails to recognize mesoscale dynamics manifested in squall lines, mesoscale convective systems (MCS), mesoscale convective complexes (MCC), and multi-scale cloud systems associated with the Madden-Julian Oscillation (MJO). Over 50% of convective precipitation in the tropics is provided by MCS defined as heavily precipitating closely coupled cumulus ensembles embedded in the more moderately precipitating stratiform regions of these systems.

Moncrieff et al. (2017) (hereafter MLB17) recently implemented an approach in the Community Atmosphere Model (CAM) in the form of multiscale coherent structure parameterization (MCSP) where organized convection is treated as coherent structures in a turbulent environment. MCSP is approximated by a slantwise layer overturning dynamical model that exchanges tropospheric layers via convectively generated mesoscale circulations which are significantly controlled by environment vertical shear (Fig. 1). Because slantwise layer overturning is not represented by existing convective parameterizations in a GCM, it is appropriate to add the “missing” mesoscale tendencies to traditional convective parameterizations.  For the first time, differences between GCM simulations with and without MCSP directly measure the large-scale effects of convective organization.