A distinct type of heavy rainfall with large raindrops over extratropical regions revealed by 10 years of GPM spaceborne radar measurements

Abstract

Large-drop heavy rainfall, defined as heavy rainfall (>10 mm h⁻¹) with relatively large raindrop diameters and low number concentrations, has been known to occur mainly in continental deep convection based on previous ground-based studies. With spaceborne radar, global analysis of raindrop size has become possible, providing a unique opportunity to revisit this understanding. Using 10 years (2014–2023) of GPM Dual-frequency Precipitation Radar (DPR) observations, large-drop heavy rainfall events are classified into two types with a Gaussian Mixture Model based on storm height. The two resulting types—high storm height (HSH) and low storm height (LSH)—exhibit similar DSD characteristics but distinct structural and environmental properties in observations and reanalysis data. The HSH type is associated with continental deep convection under warm conditions favorable for collision–coalescence processes. In contrast, the LSH type features shallower storm structures in cold environments, with large drops likely formed by melting snow. Seasonal analyses show that HSH events occur mainly over continental regions in summer, whereas LSH events are mostly observed over midlatitude oceans in winter. For diurnal variation, HSH events exhibit a daytime peak, while LSH events show no diurnal cycle. Notably, more than half of the LSH events at midlatitudes are linked to extratropical cyclones. These findings demonstrate that large-drop heavy rainfall occurs not only over continental regions in summer, but also over midlatitude oceans in winter. These findings provide useful information for improving satellite precipitation retrieval algorithms and microphysical parameterizations in weather prediction models, particularly for better performance over midlatitude oceans.