On the Increased Frequency of US Extreme Daily Precipitation Events (Invited Presentation)

Heavy daily precipitation events have become more frequent during the last century over most US regions (Hayhoe et al., 2018). Here we address two features of the trend pattern. One is the considerable regional contrast in trend magnitudes - large increases in the frequency of heavy precipitation events have occurred over the Northern US while little or no changes have occurred over the American Southwest. Given the comparatively uniform pattern of precipitable water vapor changes over the US related to the thermodynamic effects of global warming, this spatial heterogeneity suggests dynamical effects of atmospheric circulation change. Some evidence for dynamical controls on recent trend patterns will presented based on the findings in Hoerling et al. (2016).

A second feature concerns the magnitude of increasing trends in heavy daily precipitation over the Northern US since the beginning of the 20^thcentury - these are comparable to projected future increases by the end of the 21^stcentury under RCP8.5 emissions. Yet, global radiative forcing is currently estimated to be only about 2 W/m², roughly one-fourth the projections by 2100 under RCP8.5. The question thus arises whether the observed large increases in heavy daily precipitation events during the last century are indicative of an observed climate system that is much more sensitive to radiative forcing than indicated by climate models, implying a future risk of extreme precipitation events far beyond CMIP projections. Alternatively, the large magnitude of observed increases may have resulted from processes unrelated to climate change forcing. New results derived from large ensemble historical simulations will be shown to illustrate the signal-to-noise ratio of 1901-2018 trends in US extreme daily precipitation. These permit a physically-based reconciliation of the observed trends in precipitation extremes with projections of their anticipated future changes.