GRACE-FO and Future Satellite Gravity Missions Will Need to Account for Global Cloud Water Convergence

Abstract

Convective storms cause significant atmospheric and hydrological mass changes through the rapid accumulation of water vapor, cloud water, and heavy precipitation over hours or days. Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow-On (GRACE-FO) gravity field solutions enable holistic monitoring of terrestrial water storage changes, but only after mathematically removing modeled atmospheric mass variations from the observations. While this removal typically accounts for atmospheric water vapor, extreme convective events also lead to mass changes arising from liquid and frozen cloud water, which are currently neglected in gravity field processing. Using ERA5 cloud water data, we identified over 50,000 extreme events ($\ge$0.6 Gt) from 2002 to 2023, which we hypothesize to fall within the detection range of the GRACE-FO laser ranging interferometer. Our global catalog provides details on the biggest events and the evolution of different atmospheric and terrestrial water storages over the affected regions. We show that cloud water mass changes during these events can be comparable in magnitude to water vapor variations. We also observed a steady annual increase of about 52 events, from 1,796 in 2002 to 2,791 in 2023, alongside increasing intensity, which we attribute to the intensification of the water cycle driven by global warming. Our findings suggest that atmospheric cloud water, predominantly during large convective events in the tropics, map into GRACE-FO observations and that the integration of state-of-the-art cloud water simulations into the dealiasing products will improve the exploitation of GRACE-FO and Next Generation Gravity Mission data for ocean science and hydrological and climate research, particularly on submonthly timescales.