Adding Mesoscale Heating in E3SMv1 Improves MJO and Precipitation

  • August 24, 2021
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  • Diagram of multiscale coherent atmospheric structure

    Figure 1. Diagram of multiscale coherent structure with a slantwise overturning layer including a trailing stratiform region, an overturning ascent, and a mesoscale downdraft (adapted from MLB17). The structure is propagating from left to right at speed “C” in a sheared wind environment depicted by “Uz”.

    Scientists added a mesoscale heating parameterization to E3SMv1 improving the MJO representation.


    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.
    Idealization of mesoscale heating (H) and cooling (C) regions of the prototype Multiscale Coherent Structure Parameterization

    Figure 2. (a) Idealization of mesoscale heating (H) and cooling (C) regions of the prototype Multiscale Coherent Structure Parameterization (MSCP) adopted for the prototype MCSP of Moncrieff et al. (2017) (MLB17). The heating/cooling dipole in (b) is consistent with the mesoscale ascent (thick red lines) and cool mesoscale descent (thick blue line) in (a). (Adapted from Figure 12 of MLB17)


    The Energy Exascale Earth System Model (E3SM) currently does not represent heat or momentum transport associated with mesoscale convective organization, so the MCSP introduces missing physical processes which can potentially reduce E3SM’s biases, in particular, over the Intertropical Convergence Zone (ITCZ), south Pacific convergence zone, and the maritime continent. Parameterization of mesoscale transport is needed for the next generations of E3SM. In a recently published paper (Chen, et al., 2021), E3SM scientists incorporated the heating component of the MCSP (Fig. 2) into E3SM version 1 (E3SMv1). The heating component of MCSP is represented by a second baroclinic normal mode with amplitude proportional to the vertically averaged convective heating provided by the convective parameterization.
    Precipitation Biases Improved with MCSP

    Figure 3. Precipitation biases (mm/day) simulated by E3SMv1 for the boreal (Northern Hemisphere) winter. Left panel shows the control model minus the Global Precipitation Climatology Project (GPCP) 2.3 observations. The right displays the Multiscale Coherent Structure Parameterization (MCSP) minus E3SMv1 using an MCSP with α = 0.5 and a zonal wind shear trigger of 3 m/s.


    As a result of the addition of mesoscale heating, the representation of the MJO in E3SMv1 has improved and the Kelvin wave spectra were enhanced. Additionally, tropical precipitation biases near the maritime continent and tropical west Pacific were reduced (Fig. 3). Since there are strong positive precipitation biases in these regions, which MCSP cannot fully correct, it is difficult to see the improvements MCSP makes by plotting biases against GPCP for both model configurations [i.e. E3SMv1 – GPCP and (E3SMv1 with MCSP) – GPCP]. Instead, Figure 3 shows E3SMv1 compared to observations [E3SMv1 – GPCP] and E3SMv1 with MCSP compared to E3SMv1 [MCSP – E3SMv1] to highlight the contribution MCSP makes in reducing biases in these areas.

    Since the MJO is a dominant mode of sub-seasonal variability in the tropics and it impacts weather and extreme weather in many parts of the world via teleconnections, it is important to model the MJO accurately. The improved representation of the MJO in E3SMv1 is likely to lead to a more realistic representation of MJO impacts.



    • U.S. Department of Energy support was provided by DOE’s Office of Biological and Environmental Research (BER), Earth System Model Development Program Area and the Energy Exascale Earth System Model (E3SM) project for its next generation of development (NGD) of atmospheric physics.


    • Chih-Chieh (Jack) Chen, National Center for Atmospheric Research (NCAR)
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