Robust increases in severe thunderstorm environments in response to greenhouse forcing

Abstract

Although severe thunderstorms are one of the primary causes of catastrophic loss in the United States, their response to elevated greenhouse forcing has remained a prominent source of uncertainty for climate change impacts assessment. We find that the Coupled Model Intercomparison Project, Phase 5, global climate model ensemble indicates robust increases in the occurrence of severe thunderstorm environments over the eastern United States in response to further global warming. For spring and autumn, these robust increases emerge before mean global warming of 2 °C above the preindustrial baseline. We also find that days with high convective available potential energy (CAPE) and strong low-level wind shear increase in occurrence, suggesting an increasing likelihood of atmospheric conditions that contribute to the most severe events, including tornadoes. In contrast, whereas expected decreases in mean wind shear have been used to argue for a negative influence of global warming on severe thunderstorms, we find that decreases in shear are in fact concentrated in days with low CAPE and therefore do not decrease the total occurrence of severe environments. Further, we find that the shift toward high CAPE is most concentrated in days with low convective inhibition, increasing the occurrence of high-CAPE/low-convective inhibition days. The fact that the projected increases in severe environments are robust across a suite of climate models, emerge in response to relatively moderate global warming, and result from robust physical changes suggests that continued increases in greenhouse forcing are likely to increase severe thunderstorm occurrence, thereby increasing the risk of thunderstorm-related damage.

Keywords: CMIP5; GCM; hazards; severe weather.

Fig. 1.

Response of severe thunderstorm environments in the late 21st century period of RCP8.5 during the winter (DJF), spring (MAM), summer (JJA), and autumn (SON) seasons. (A–D) Color contours show the difference in the number of days on which severe thunderstorm environments occur (NDSEV) between the 2070–2099 period of RCP8.5 and the 1970–1999 baseline, calculated as 2070–2099 minus 1970–1999. Black (gray) dots identify areas where the ensemble signal exceeds one (two) SD(s) of the ensemble noise, which we refer to as robust (highly robust). (E–H) Each gray line shows an individual model realization. For each realization, the anomaly in the regional average NDSEV value over the eastern United States (105–67.5°W, 25–50°N; land points only) is calculated for each year in the 21st century, with the anomaly expressed as a percentage of the 1970–1999 baseline mean value. A 31-y running mean then is applied to each time series of percentage anomalies. The black line shows the mean of the individual realizations.

Fig. 2.

Response of CAPE and wind shear in the late 21st century period of RCP8.5 during the winter (DJF), spring (MAM), summer (JJA), and autumn (SON) seasons. Differences are calculated as in Fig. 1, Left. (A–D) CAPE. (E–H) The magnitude of the vector difference of the horizontal wind at 6 km and the lowest model level (S06).

Fig. 3.

Zonal mean vertical profiles in the CMIP5 ensemble for the spring (MAM) and summer (JJA) seasons. The absolute difference in ensemble-mean magnitude between the 2070–2099 and 1970–1999 periods of the CMIP5 ensemble over the eastern United States (105–67.5°W, 25–50°N) is shown. (A and D) Atmospheric temperature. (B and E) Atmospheric specific humidity. (C and F) Eastward wind. Black and gray dots identify ensemble robustness as in Fig. 1.

Fig. 4.

Change in the frequency of occurrence of daily CAPE and shear in the winter (DJF; A), spring (MAM; B), summer (JJA; C), and autumn (SON; D) seasons in the late 21st century period of RCP8.5. Occurrences are counted for land grid points in the eastern United States (105–67.5°W, 25–50°N; land points only). The black curve shows the SEV threshold in the CAPE–S06 space. (A–D, Upper) The absolute difference in the ensemble-mean number of occurrences between the 1970–1999 and 2070–2099 periods, calculated as 2070–2099 minus 1970–1999, is shown for each season. (A–D, Lower) The absolute difference in the ensemble-mean PDF of occurrence between the 1970–1999 and 2070–2099 periods for days in which the SEV threshold is met, is shown for each season.

Fig. 5.

Change in the frequency of occurrence of daily CAPE and CIN in the winter (DJF; A), spring (MAM; B), summer (JJA; C), and autumn (SON; D) seasons in the late 21st century period of RCP8.5. Occurrences are counted for land grid points in the eastern United States (105–67.5°W, 25–50°N; land points only) for days in which the SEV threshold is met. The absolute difference in the ensemble-mean number of occurrences between the 1970–1999 and 2070–2099 periods, calculated as 2070–2099 minus 1970–1999, is shown for each season. CIN is expressed as the absolute magnitude of the most negative accumulated buoyant energy below the LFC, yielding positive values in units of joules per kilogram. The gray lines denote the levels at which CIN equals 200 J/kg and 100 J/kg, respectively.