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History of modifications

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Acronyms

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The product is generated through merging of microwave and infrared precipitation estimates and outgoing longwave radiation from satellites, combined with rain gauge analyses from the Global Precipitation Climatology Centre (GPCC) over land. The temporal and spatial availability of all input data sources are described including a description of the evolution over time of surface measurements used by GPCC and of the satellite-based precipitation estimates. It is important to note that GPCP itself does not distinguish between a temporally fixed TCDR and a continuously updated ICDR. Instead, GPCP updates its long-term daily and monthly products continuously. The TCDR/ICDR distinction is only introduced in the scope of the brokering of the GPCP product to C3S, in order to harmonize its structure with other data products. Thus, the temporal coverage for the TCDR of monthly data is 1979-2017 1979–2017 while the corresponding ICDR covers 2018 onwards.

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It is emphasized that the dedicated effort within this C3S project for developing an algorithm for precipitation retrieval will not be based on the GPCP product. Instead, it will be a microwave-based TCDR exploiting MW imagers and sounders available at all latitudes in the time period 2000-20172000–2017. The use of both imagers and sounders will ensure high spatial coverage and temporal sampling of the precipitation. The new product will also utilize data from satellite-borne precipitation radars and improved methods for data merging and calibration.

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The GPCP global precipitation products merge rainfall estimates from several microwave- and infrared satellite-borne sensors and the gridded rain gauge-based product by the Global Precipitation Climatology Centre (GPCC). It is created and maintained by the GPCP team at the University of Maryland (UMD), where the intellectual property rights remain with, and brokered to the Climate Data Store (CDS) by the Copernicus Climate Change Service (C3S). The GPCP TCDR v1.0 covers the period 01/1979-121979–12/2017 (monthly data, v2.3), or 10/1996-12/2017 (daily data, v1.3), respectively. The GPCP ICDR v1.x starts in 01/2018 and at the time of writing extends until 09/2020 for both monthly and daily data.

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GCOS formulates requirements for precipitation products only for monthly resolution (GCOS, 2016, table 23). The given accuracy target of 0.5 mm/h [sic] is less strict than the one applied here; the stability target is stricter than the one applied here (0.02 mm/decade [sic]).

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2.1.3 Data format and content issues

The data are provided in one NetCDF file per month (monthly product) or per day (daily product). The files are compliant with the Climate and Forecast (CF) 1.6 convention and the Attribute Convention for Dataset Discovery (ACDD) 1.3. The products are both on a global equidistant latitude/longitude grid (Level 3), with a 2.5° (monthly), or a 1.0° (daily) spacing. The C-ATBDs [D1, D2] list all input and ancillary data at the respective epochs when they were used. Errors are only propagated and provided for the monthly product.

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1. Data from the Special Sensor Microwave/Imager (SSM/I) and its successor Special Sensor Microwave Imager/Sounder (SSMIS) on board the Defense Meteorological Satellite Program (DMSP) satellites. The SSMI and SSMIS instruments provide the key input data that are used for low latitudes (40°N-40°S40°N–40°S) within GPCP over both ocean and land for the period 1987–present, with the beginning of the operational activities of SSMIS sensor in 2009 (Adler et al., 2018).
2. The precipitation products from the TIROS Operational Vertical Sounder (TOVS, temporal coverage 1987-20031987–2003) and Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounding Unit (AMSU) flying on NASA’s Aqua satellite (operational since 2003) are exploited for filling in SSM/I-SSMIS data voids at higher latitudes (polar and cold land regions) caused by the shortcomings in precipitation retrieval over frozen surfaces (Adler et al., 2003; 2018).
3. For the whole duration of the GPCP dataset the geosynchronous IR-based estimates are obtained through the Geostationary Operational Environmental Satellite (GOES) precipitation Index (GPI; Arkin and Meisner, 1987) technique applied to GOES, METEOSAT, and Geostationary Meteorological Satellite (GMS)-Multifunction Transport Satellite (MTSAT)-Himawary-8 geostationary satellite series data.
4. In case of unavailable geostationary IR data, the NOAA Advanced Very High Resolution Radiometer (AVHRR) observations are used.
5. The Outgoing Longwave Radiation (OLR) Precipitation Index (OPI) monthly precipitation estimates based on IR data from all NOAA-series satellites are employed prior the SSM/I era to extend GPCP data back to 1979 (Adler et al., 2003; Xie and Arkin, 1998).
6. GPCC data: a new set of rain gauge data was introduced in the GPCP monthly v2.3, i.e. the GPCC Full analysis v7 for the period 1979-2013 and the GPCC Monitoring analysis v5 for 2014 and beyond (Adler et al., 2018)

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DMSP satellites carrying SSM/I and SSMIS mostly have local overpassing times in the early morning. It is likely that the respective systematic sampling introduces a bias into the retrieved average daily (or even monthly) precipitation rates in areas where precipitation follows a diurnal pattern. The MW-product to be developed within C3S will circumvent these sampling preferences at least partially by including precipitation rates retrieved by MHS and AMSU-B. These are onboard NOAA and METOP satellites which, due to their orbit distribution, will introduce a more complete coverage of the day and thus improve the diurnal sampling. The originally envisaged temporal coverage for the newly developed product was 1999-20161999–2016. However, due to just one MW sounder platform available in 1999, with many observations compromised, this time span has been shifted to 2000-2017 2000–2017 for which the availability of high-quality MW sounder data is much better. This change has been communicated to and accepted by C3S.

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3.5 Scientific Research needs

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3.5.1 Precipitation GPCP TCDR v1.0 + ICDR v1.x

The processing algorithm and its further development lie in the hands of the GPCP authors. In the scope of brokering the dataset to the Climate Data Store, we accept the product in its current form. Research related to the improvement of the precipitation CDR will be carried out within the development of the MW-based precipitation TCDR within C3S. The quality of satellite daily and monthly MW-based precipitation is critically dependent on the number and on the quality of MW radiometers used to ensure maximum spatial and temporal coverage, see section 3.4.1. The MW-based precipitation TCDR developed within C3S, and complementing the GPCP product, will be based on the exploitation of Level 1 FCDRs available for SSM/I / SSMIS MW imagers and MHS/AMSU-B MW sounders available between 2000 and 2017. There are two areas of research that will be explored
during the development of MW-based TCDRs:

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