Why Vertical Resolution May Matter More Than Horizontal for Midlatitude Circulation Biases: The Critical Role of Convective Upscaling

Accurately representing upscale interactions between convection and the large-scale flow remains a major challenge for global atmospheric models. While global kilometer-scale simulations that resolve deep convection can improve coupling with midlatitude circulation, the influence of vertical resolution remains far less explored. Here, we investigate how deep convection interacts with midlatitude circulation using a suite of Model for Prediction Across Scales (MPAS) experiments with systematically varied horizontal and vertical resolution.

We analyze four simulations initialized following the DYAMOND Phase 3 protocol and run for 40 days: (1) 3.75 km horizontal resolution, 127 vertical levels; (2) 15 km, 127 levels; (3) 15 km, 55 levels; and (4) 15 km, 55 levels with boundary-layer vertical refinement. The 127-level simulations have an average tropospheric grid-spacing of 190 m, compared to 310 m in the 55-level runs.

Simulations with 127 levels exhibit enhanced upscale kinetic energy transfer from deep convection, particularly in warm-conveyor-belt genesis regions about the tropopause. This drives quasi-stationary Rossby waves downstream, producing dry anomalies over Western Europe and Western North America. In contrast, 55-level simulations show weaker upscaling, favoring more zonal flow and wetter conditions. Horizontal refinement appears to only have a secondary effect. Preliminary diagnostics suggest that eddy viscosity treatment by the Planetary Boundary Layer scheme in MPAS is highly sensitive to vertical spacing, substantially influencing the mesoscale kinetic energy spectrum and, in turn, shaping the upscaling and midlatitude circulation response. Ongoing work looks to expand the ensemble of simulations. 

The results highlight the critical role of vertical resolution when configuring kilometer-scale global models. Understanding how vertical resolution interacts with model physics may also be key in reducing contemporary biases in midlatitude Rossby wave and blocking frequency.