Abstract/Summary

NASA Precipitation Measurement Mission observations are used to evaluate the diurnal cycle of precipitation from three CMIP6 models (NCAR-CESM2, CNRM-CM6-1, CNRM-ESM2-1) and the ERA5 reanalysis. NASA’s global-gridded IMERG product, which combines spaceborne microwave radiometer, infrared sensor and ground-based gauge measurements, provides high spatio-temporal resolution (0.1°, half-hourly) estimates that are suitable for evaluating the diurnal cycle in models, as determined against the CONUS ground-based radar network. IMERG estimates are coarsened to the spatial and hourly resolution of the state-of-the-art CMIP6 and ERA5 products, and their diurnal cycles are compared across multiple decades of June-July-August in the 60°N–S domain (IMERG and ERA5: 2000–2019; NCAR and CNRM: 1979–2008). Low precipitation regions (and weak amplitude regions when analyzing the diurnal phase) are excluded from analyses in order to assess only robust diurnal signals. Observations identify greater diurnal amplitudes over land (26–134% of the precipitation mean; 5th–95th percentile) than over ocean (14–66%). ERA5, NCAR and CNRM underestimate amplitudes over ocean, whilst ERA5 overestimates over land. IMERG observes a distinct diurnal cycle only in certain regions, with precipitation peaking broadly between 14–21 LST over land (21–6 LST over mountainous and varying-terrain regions) and 0–12 LST over ocean. The simulated diurnal cycle is unrealistically early compared with observations, particularly over land (NCAR-CESM2-AMIP: –1 hour; ERA5: –2 hours; CNRM-CM6-1-AMIP: –4 hours on average) with nocturnal maxima not well represented over mountainous regions. Furthermore, ERA5’s representation of the diurnal cycle is too simplified, with less interannual variability in the time of maximum compared to observations over many regions.