Atmospheric stability sets maximum moist heat and convection in the midlatitudes

Abstract

Extreme near-surface moist heat and severe convective storms are among the leading causes of weather-related damages worldwide. Here, we show that episodes of extreme moist heat and severe convection frequently co-occur across midlatitude land regions, and develop a theoretical framework that links their maximum potential intensities to preexisting low-level energy inversions. By accounting for the stored-energy nature of midlatitude severe convection, where moist heat and atmospheric instability accumulate before convection initiates, our work advances the understanding of convective constraints on extreme heat events. The theory identifies low-level inversions as a critical factor shaping compound extreme heat and convective weather risks, and offers a pathway for improving the modeling and future projection of these events.

Figures (4)

• Figure 1: Extreme moist heat. ($A$): Annual maximum near-surface wet-bulb temperature ($WBT_s$). ($B$): Annual maximum near-surface moist static energy ($MSE_s$). ($C$): 500-hPa saturated moist static energy ($MSE_{500}^*$) associated with annual maximum $MSE_s$. ($D$): Joint histogram of annual maximum $MSE_s$ and the associated $MSE_{500}^*$, with a bin size of 1$\times$1 kJ kg$^{-1}$. Results are historical means based on ERA5 reanalysis data during 1980--2022 for land between 35$^{\circ}$N and 75$^{\circ}$N at elevations lower than 1000 m.
• Figure 2: Extreme convective instability. ($A$): Critical CAPE ($CAPE_c$), defined as CAPE at the time of annual maximum $MSE_s$. ($B$): Theoretical scaling for $CAPE_c$ following Eq.\ref{['scaling-cape']}. ($C$): Joint histogram of $CAPE_c$ and scaling $CAPE_c$, with a bin size of 250$\times$250 J kg$^{-1}$. Results are historical means based on ERA5 reanalysis data during 1980--2022 for land between 35$^{\circ}$N and 75$^{\circ}$N at elevations lower than 1000 m.
• Figure 3: Moist heat and convection buildup. ($A$--$B$): A case study associated with the annual maximum $MSE_s$ over central United States (41$^{\circ}$N, -96$^{\circ}$W) in 2004, temperature sounding (left) and MSE (right) profiles for the environment (solid lines) and the air parcel (dashed lines) adiabatically lifted from the near surface, at the time of ($A$) 5 days before the annual maximum $MSE_s$ and ($B$) the maximum $MSE_s$ during that year. Shaded areas indicate positive ($b>0$) and negative ($b<0$) buoyancy, and the dotted lines represent lifted condensation level ($LCL$). ($C$--$E$): Composite time series, centered at the time of annual maximum $MSE_s$, for ($C$) $MSE_s$, $MSE_{max}^{*}$, and $MSE_{500}^{*}$; ($D$) $CAPE$, scaling $CAPE$, and potential $CAPE$; ($E$) $CIN$ and the most negative buoyancy ($b_{min}$, scaled by 0.005). Shading denotes $\pm$ one standard error of the mean. The ($C$--$E$) composites are based on midlatitude continental cases where the annual maximum $MSE_s$ and $CAPE$ occur concurrently, with $CAPE_c>500$ J kg$^{-1}$, $CIN_c<5$ J kg$^{-1}$, and the maximum $CIN$ within 7 days before the peak heat of at least 50 J kg$^{-1}$ (detailed in $Materials$$and$$Methods$). Results are based on ERA5 reanalysis data. The ERA5 profiles for the case presented in ($A$--$B$) are validated against radiosonde observations form the National Weather Service in Omaha, NE, USA, as shown in Fig. S3.
• Figure 4: Maximum potential moist heat and convection. ($A$): Annual maximum potential $MSE_s$, defined as $MSE_{max}^{*}$ corresponding to the maximum $MSE_s$ following Eq.\ref{['eq1.2']}. ($B$): Annual maximum potential $CAPE_c$, defined by CAPE scaling based on $MSE_{max}^{*}$ and $MSE_{500}^{*}$ following Eq.\ref{['eq1.3']}. ($C$--$E$): Joint histograms of ($C$) annual maximum $MSE_s$ and the potential intensity with a bin size of 1.25$\times$1.25 kJ kg$^{-1}$, ($D$) annual maximum $WBT_s$ and the potential intensity with a bin size of 0.5$\times$0.5 K, and ($E$) $CAPE_c$ and the potential intensity with a bin size of 250$\times$250 J kg$^{-1}$. Results are historical means based on ERA5 reanalysis data during 1980--2022 for land between 35$^{\circ}$N and 75$^{\circ}$N at elevations lower than 1000 m.