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Daniel Watters, Alessandro Battaglia, and Richard P. Allan

Abstract

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.

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Daniel Watters, Alessandro Battaglia, Kamil Mroz, and Frédéric Tridon

Abstract

Instantaneous surface rain rate estimates from the Global Precipitation Measurement (GPM) mission’s Dual-Frequency Precipitation Radar (DPR) and combined DPR and multifrequency microwave imager (CMB) version-5 products are compared to those from the Met Office Radarnet 4 system’s Great Britain and Ireland (GBI) radar composite product. The spaceborne and ground-based rainfall products are collocated spatially and temporally and compared at 5- and 25-km resolutions over GBI during a 3-yr period (from May 2014 to April 2017). The comparison results are evaluated as a function of both the intensity and variability of precipitation within the DPR field of view and are stratified spatially and seasonally. CMB and DPR products underestimate rain rates with respect to the Radarnet product by 21% and 31%, respectively, when considering 25-km resolution data taken within 75 km of a ground-based radar. Large variability in the discrepancies between spaceborne and ground-based rain rate estimates is the result of limitations of both systems and random errors in the collocation of their measurements. The Radarnet retrieval is affected by issues with measuring the vertical extent of precipitation at far ranges, while the GPM system struggles in properly quantifying orographic precipitation. Part of the underestimation by the GPM products appears to be a consequence of an erroneous DPR clutter identification in the presence of low freezing levels. Both products are susceptible to seasonal variations in performance and decreases in precision with increased levels of heterogeneity within the instruments’ field of view.

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