Role of dry‑air entrainment in intensifying extratropical moist heat extremes

Extratropical continental atmospheres in summertime are close to moist convective neutrality, under which near-surface moist static energy (MSE_2m) is tightly constrained by mid-tropospheric saturation moist static energy (MSE₅₀₀). During moist heat extremes, however, MSE_2m often exceeds this moist-convective limit (MSE₅₀₀), indicating a breakdown of the neutrality assumption. Recent work has attributed this breakdown to the presence of energy inversions in the lower free troposphere during extratropical moist heat extremes.  

Here, we advance this understanding by examining the role of dry-air entrainment in modulating moist heat extremes. Building on recent work focused on the tropics, we develop an entrainment-based theoretical framework to diagnose near-surface MSE during extreme moist heat events. We analyse maximum wet-bulb temperature days over land regions between 35°N and 65°N using ERA5 reanalysis data (1991-2020) and CESM2 simulations. Simulations are performed with prescribed sea-surface temperatures and three different entrainment rates: control (entrainment rate=−1 km⁻¹), no-entrainment (0 km⁻¹), and doubled-entrainment (−2 km⁻¹). Our results show that entrainment of dry air intensifies boundary-layer instability (MSE_2m – MSE₅₀₀) during extratropical moist heat extremes. The theoretical framework using entrainment rates diagnosed from the zero-buoyancy plume model successfully explains the breakdown of moist convective neutrality during moist heat extremes. We attribute changes in MSE_2m under warming to the combined effects of changes in free-tropospheric MSE, lower-tropospheric saturation deficit, and entrainment strength. This framework improves fundamental understanding of extratropical moist heat extremes and provides a physically-grounded basis for interpreting their future changes.