Global mean frequency increases of daily and sub-daily heavy precipitation in ERA5

Changes in heavy precipitation frequency can be viewed as a change in the event return period, a common metric of risk. Compared with intensity, frequency changes are less well-studied and past work has largely been constrained to analysis of well-instrumented regions. We exploit the latest ERA5 reanalysis and its global hourly accumulations at 1/4 degrees spatial resolution and apply a metric that captures frequency changes across both wet and dry regions. According to ERA5 and in a global average sense, during 1989-2018, hourly events that occurred once per year in 1979-1988 increased in frequency by 71 (53-93, 95% range) %, while the one day per year heavy event frequency increased by 44 (37-54) %. Thus, hourly events that occurred once per year in the baseline decade are on track to double in frequency by 2021-2030, and the daily events by 2047-2056. Furthermore, our results replicate prior findings that relative frequency increases are larger for increasingly rare events, and for the first time we quantify that mean frequency increases have been greater over ocean than land. Ocean increases are larger by factors of 3.0 and 2.1 for the hourly and daily events that occurred once per year in 1979-1988, respectively.

Automated summary

The study found that changes in heavy precipitation frequency have been rapidly increasing globally in recent years, especially for hourly and once-per-day heavy events. Relative frequency increases are mainly present in rare events, and the frequency increases are more pronounced in oceanic regions.