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Impacts of compound extreme weather events on ozone in the present and future

Abstract. The Weather Research and Forecasting model coupled with Chemistry
(WRF-Chem) was used to study the effect of extreme weather events on
ozone in the US for historical (2001–2010) and future (2046–2055) periods
under the RCP8.5 scenario. During extreme weather events, including heat
waves, atmospheric stagnation, and their compound events, ozone concentration
is much higher compared to the non-extreme events period. A striking enhancement
of effect during compound events is revealed when heat wave and stagnation
occur simultaneously as both high temperature and low wind speed promote the
production of high ozone concentrations. In regions with high emissions,
compound extreme events can shift the high-end tails of the probability
density functions (PDFs) of ozone to even higher values to generate extreme
ozone episodes. In regions with low emissions, extreme events can still
increase high-ozone frequency but the high-end tails of the PDFs are
constrained by the low emissions. Despite the large anthropogenic emission
reduction projected for the future, compound events increase ozone more than
the single events by 10 to 13 %, comparable to the present, and high-ozone episodes with a maximum daily 8 h average (MDA8) ozone concentration over
70 ppbv are not eliminated. Using the CMIP5 multi-model ensemble, the
frequency of compound events is found to increase more dominantly compared to
the increased frequency of single events in the future over the US, Europe,
and China. High-ozone episodes will likely continue in the future due to
increases in both frequency and intensity of extreme events, despite
reductions in anthropogenic emissions of its precursors. However, the latter
could reduce or eliminate extreme ozone episodes; thus improving projections of
compound events and their impacts on extreme ozone may better constrain
future projections of extreme ozone episodes that have detrimental effects on
human health.
