Global hydro-climatological indicators and changes in the global hydrological cycle and rainfall patterns

There are only a few climate indicators that describe the state of the global hydrological cycle. In this presentation, we argue that important climate indicators based on global daily precipitation are lacking and propose three new indicators: 1) the daily global precipitation amount, 2) the daily global surface area receiving precipitation, and 3) the global mean daily precipitation intensity. Historically, assessing these indicators is limited by the extent of global observational networks. However, recent advancements in satellite observations and reanalysis data, particularly the ERA5 reanalysis, have enabled better estimations.

We present an analysis of the proposed indicators using ERA5 data and other data sources. We also discuss limitations and biases of the data sources, e.g. ERA5's tendency to overestimate precipitation. Further, a wavelet analysis of spatial characteristics of 24-hour precipitation is conducted, offering insights into the spatial extent and intensity of precipitation systems and their variations over time. To address the question whether long-term changes reflect real changes in Earth's global hydrological cycle due to warming, or may be artefacts from changes in the assimilated (satellite) data in ERA5, we examine an ensemble of CMIP6 simulations under scenarios of increasing greenhouse gas concentration.

Our analysis reveals that ERA5 shows a decrease in the global area of daily precipitation from 43% to 41% between 1950 and 2020. At the same time, the total daily global precipitation amount increased from 1440 Gt to 1510 Gt. The wavelet analysis of ERA5 data indicates that individual precipitation systems have become smaller in spatial extent but more intense over this period, suggesting an accelerated global hydrological cycle with reduced global rainfall area. The CMIP6 simulations show a robust decrease in the precipitation area towards the end of the 21st century in agreement with ERA5. However, compared to the reanalysis the changes are smaller and less rapid.  Nevertheless, our results suggest that in a warming climate the daily precipitation area may shrink, contributing to an increase in the mean daily precipitation intensity.