Increasing potential for intense tropical and subtropical thunderstorms under global warming

Abstract

Intense thunderstorms produce rapid cloud updrafts and may be associated with a range of destructive weather events. An important ingredient in measures of the potential for intense thunderstorms is the convective available potential energy (CAPE). Climate models project increases in summertime mean CAPE in the tropics and subtropics in response to global warming, but the physical mechanisms responsible for such increases and the implications for future thunderstorm activity remain uncertain. Here, we show that high percentiles of the CAPE distribution (CAPE extremes) also increase robustly with warming across the tropics and subtropics in an ensemble of state-of-the-art climate models, implying strong increases in the frequency of occurrence of environments conducive to intense thunderstorms in future climate projections. The increase in CAPE extremes is consistent with a recently proposed theoretical model in which CAPE depends on the influence of convective entrainment on the tropospheric lapse rate, and we demonstrate the importance of this influence for simulated CAPE extremes using a climate model in which the convective entrainment rate is varied. We further show that the theoretical model is able to account for the climatological relationship between CAPE and a measure of lower-tropospheric humidity in simulations and in observations. Our results provide a physical basis on which to understand projected future increases in intense thunderstorm potential, and they suggest that an important mechanism that contributes to such increases may be present in Earth's atmosphere.

Keywords: CAPE; climate change; intense thunderstorms; severe weather; tropical atmosphere.

Fig. 1.

(A) Ensemble mean of CAPE⁹⁵ (colors) and 5 mm$\cdot {\text{d}}^{-1}$ contour of time mean and ensemble mean precipitation (gray) based on 12 CMIP5 models for the period 1981–2000. (B) Change in the ensemble mean CAPE⁹⁵ from the current climate (1981–2000) to the future climate (2081–2100) under the RCP8.5 scenario. Stipling in B indicates regions where 11 of the 12 models agree on the sign of the response.

Fig. 2.

(A) CAPE⁹⁵ (colors) and 5 mm$\cdot {\text{d}}^{-1}$ contour of time mean precipitation (gray) in simulations with modified entrainment for the control value of the entrainment parameter $\alpha =0.2$. (B) Difference in CAPE⁹⁵ for the case $\alpha =0.4$ compared with the case $\alpha =0.2$.

Fig. 3.

(A–D) The 95th percentile of CAPE in strongly precipitating regions (${\text{CAPE}}_p^{95}$; left axis) and (E–H) frequency (freq.) of daily precipitation exceeding 5 mm$\cdot {\text{d}}^{-1}$, as a function of the lower-tropospheric saturation (sat.) deficit $q_{\text{def}}$ for (A and E) modified entrainment simulations for different values of the entrainment parameter $\alpha$, (B and F) superparameterized GCM simulations of preindustrial climate (black) and quadrupled CO₂ concentration (red), (C and G) CMIP5 ensemble mean in the current climate (1981–2000; thick black; individual models shown in gray) and in the future climate under the RCP8.5 scenario (2081–2100; thick red), and (D and H) observations. Statistics are calculated using 100 bins, each containing roughly the same proportion of the available data. In D, lightning frequency in strongly precipitating regions is plotted as a function of lower-tropospheric saturation deficit (green, right axis), and lines are smoothed with a five-point Lowess filter, with symbols giving unsmoothed values.

Fig. 4.

CAPE as a function of lower-tropospheric saturation deficit $q_{\text{def}}$ calculated using the zero-buoyancy plume model. Calculations are performed with a surface relative humidity of 80% and surface temperatures of 300 K (solid) and 303 K (dashed) for values of the environmental relative humidity in the free troposphere varying between 20% and 100% and entrainment rates $ϵ$ as given in the legend. Arrows connect solutions with the same relative humidity for different surface temperatures.