Quantitatively evaluating the Community Earth System Model version 2 regarding the physical processes leading to heat extremes

Do heat extremes in climate model simulations form for the right physical reasons? Addressing this question is essential to further our confidence in heat extreme projections, to pinpoint potential model biases and thus for contributing to model improvements. Here, we perform a detailed process-based evaluation of the Community Earth System Model version 2 (CESM2) regarding heat extremes and employ a Lagrangian approach to quantify advective, adiabatic and diabatic contributions to near-surface temperature anomalies (T’) during heat extremes in CMIP6-type CESM2 simulations. Comparison of these results with results of an analogous analysis in ERA5 reveals that, qualitatively and at very large scales, heat extremes in CESM2 form in a physically similar way as in ERA5: Advective contributions dominate in storm track regions, diabatic contributions dominates over tropical and subtropical land regions, and adiabatic warming contributes significantly to heat extremes over subtropical oceans as well as extratropical land regions. However, quantitatively and at regional scales, there are considerable differences: For instance, in Europe, the diabatic contribution to heat extremes in CESM2 roughly doubles that in ERA5, while the adiabatic contribution to CESM2 heat extremes is only about half of that in ERA5. Thus, in Europe the dominant physical processes that form near-surface T’ during heat extremes differ between CESM2 and ERA5. Preliminary analyses of the underlying causes suggest that differences in climatological temperature gradients (i.e., a mean-state differences) as well as systematic differences in the trajectories of air parcels involved in heat extremes in Europe (i.e., a difference in the circulation leading up to heat extremes) both contribute to these discrepancies. Differences in the contributions to heat extremes are not uniform across space, e.g., in Australia, heat extremes in CESM2 feature much larger advective contributions than in ERA5, while the diabatic contribution is comparable for heat extremes in CESM2 and ERA5 there. We argue that such a detailed understanding of the physical function of simulated and observed heat extremes, and in particular their differences, is highly relevant to assessing the robustness of heat extreme projections. Our results thus call for analogous investigations also with other state of the art models.