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GPCC Global Precipitation Climatology Centre

GPCC Global Precipitation Climatology Centre. Calculation Of Gridded Precipitation Data for Global Land-Surface Using In Situ Gauge Observations IPWG, Monterey, October 2004. Bruno Rudolf with input from C. Beck, P. Finger, J. Grieser, H. Maechel, U. Schneider

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GPCC Global Precipitation Climatology Centre

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  1. GPCC Global Precipitation Climatology Centre Calculation Of Gridded Precipitation Data for Global Land-Surface Using In Situ Gauge Observations IPWG, Monterey, October 2004 Bruno Rudolf with input from C. Beck, P. Finger, J. Grieser, H. Maechel, U. Schneider Deutscher Wetterdienst, Offenbach am Main gpcc@dwd.de http://gpcc.dwd.de

  2. GPCC Global Precipitation Climatology Centre • Outline • Origin and functions of the GPCC • Data sources • Selection data for analysis • Data base of the individual products • Gridding and error aspects • Conclusions and Outlook

  3. GPCC‘s origin and specific functions The GPCC has been established in 1988 at DWD on request of WMO as a contribution to the WCRP Global Precipitation Climatology Project. GPCC focuses on analysis of preciptiation gauge observations for the global land surface. The scientific/technical functions comprise: • Acquisition and archiving of precipitation data world-wide • Data quality-control, correction of data errors • Transfer of gauge data to area-means on 1° and 2.5° grid cells • Assessment of uncertainty (or error) of the gridded products • Contribution to merging of satellite and gauge data (GPCP) • Distribution of the gridded products (via internet) By the meantime, the GPCC also contributes to other international programmes an organisations by supplying special gauge analysis products (e.g. for GCOS, GTOS, CLIVAR, FAO, UNESCO).

  4. Global Precipitation Climatology Centre (GPCC) GPCC‘s “Monitoring Product“ (2 months after obs.) Monthly precipitation Data base: 7,000 stations Spatial resolution: 1.0° or 2.5° lat/long used by the GPCP for combination with satellite products available on web http://gpcc.dwd.de

  5. Global Precipitation Climatology Centre (GPCC) GPCC‘s data sources • Near realtime: • GTS SYNOP (DWD RH Offenbach) • GTS CLIMAT (DWD RH Offenbach) • GTS CLIMAT (DWD RH Tokyo) • SYNOP-based (NOAA CPC Washington) • Non realtime: • Additional data from (so far) 173 countries • International project data (GEWEX and other) • Historical data collections (CRU, FAO, GHCN)

  6. Global Precipitation Climatology Centre (GPCC) SYNOP data from GTS Total number of records stored: 18,995,954 national data from 173 countries global data of FAO global data of CRU Total content of GPCC‘s data base at October 2004: Sources: GTS, NMHSs, historical data from CRU and FAO (GHCN not yet processed)

  7. Global Precipitation Climatology Centre (GPCC) Selection of data for analysis • For the different products, data are selected • with respect of • timeliness of data availability • quality control results • completeness of the data time-series.

  8. Global Precipitation Climatology Centre (GPCC) GPCC‘s Data Processing System with 4 QC-levels: 1a: Pre-control of SYNOPs 1b: Pre-control of CLIMATs 1c: Individual format-check of the additional data sets 2: Visualization of the indi- vidual data sets, control for location and outliers 3: Control of the meta data during the loading process 4: Semi-automatic QC of all monthly precipitation data: statistical pre-control and visual check of all data flagged as questionable

  9. Global Precipitation Climatology Centre (GPCC) Example of erroneous station meta information Erroneous coordinates of some stations within a data set which was delivered to the GPCC. The errors have been discovered by comparison of the station catalogs of CRU and GPCC.

  10. Global Precipitation Climatology Centre (GPCC) GPCC Product Overview • With respect of different needs or priorities of application, the GPCC provides various gridded data sets: • First guess of monthly precipitation anomaly • 6,000 SYNOP stations, automatic QC, less than 5 days after obs. month • Monitoring of monthly precipitation for combination with satallite data • 7,000 SYNOP and CLIMAT stations, high level QC, 2 months after obs. month • Full data product, high resolution, for global hydrological studies • 40,000 stations, high level QC, about 5 years after obs. month • Long-term gridded time-series for studies on climate variability and trend • using selected stations for a mostly homogeneous data coverage over time • (1st version 1951-2000 available by end of 2004 based on 6,000 stations)

  11. Global Precipitation Climatology Centre (GPCC) Data taken from the Global Telecommunication System (GTS) SYNOP reports received at DWD and at CPC, and monthly CLIMAT reports

  12. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Interim: July 1987

  13. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Monitoring: July 2004

  14. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Interim: July 1987 Monitoring: July 2004 (in front)

  15. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Interim: July 1987 (in front) Monitoring: July 2004

  16. Global Precipitation Climatology Centre (GPCC) The data base of GPCC‘s Full Data Product (Versions 1, 2 and 3) Data sources: GTS, national data, global hist. data from CRU and FAO

  17. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Full Data: July 1987

  18. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Full Data: July 2000

  19. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Full Data: July 1987 Full Data: July 2000 (in front)

  20. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) Interim: July 1987 (in front) Full Data: July 1987

  21. Global Precipitation Climatology Centre (GPCC) Overall comparison of GPCC‘s Full Data Version 2 and Monitoring Product. Global land-surface mean precipitation without Antarctica from 2.5° gridded results (mm/month)

  22. Global Precipitation Climatology Centre (GPCC) national data rt data global data of FAO global data of CRU Composition of the 50 years climatology 1951-2000 version 1 (2004/11): Monthly precipitation data selected from national/regional data and FAO and CRU global historical data collections - complemented by CLIMAT and SYNOP.

  23. Global Precipitation Climatology Centre (GPCC) GPCC Data Base: Location of Stations with Data (status 2004/10) 50 year climatology Version 1

  24. Global Precipitation Climatology Centre (GPCC) Gridding and error aspects Calculation of area means for grid cells Homogeneity of gridded time-series Sampling error Systematic measuring error

  25. Global Precipitation Climatology Centre (GPCC) Calculation of gridded area mean precipitation The procedure operationally used at the GPCC has 3 steps: 1. The irregularly distributed station data are interpolated onto regular grid points of a 0.5° latitude-longitude mesh following Shepard‘s method (Willmott et al. 1985). 2. Area mean precipitation is calculated for grid cells of 0.5° lat/long as an average from the interpolated grid point results at cell corners. 3. Area mean precipitation is calculated for grid cells of 1.0° and 2.5° grid cells as an land area-weighted average from the 0.5° area-means. The interpolation is based on the empiricalscheme of Shepard, which has been well proved in various intercomparison studies for application on global or continental analysis of precipitation.

  26. Global Precipitation Climatology Centre (GPCC) An interpolation experiment using real data shows the importance of a good data coverage and completenessPositionof 5 stations andthe interpolation point control

  27. Global Precipitation Climatology Centre (GPCC) Different analysis procedures don't have much effect: interpolation of monthly sums, anomalies, standardized, ratios

  28. Global Precipitation Climatology Centre (GPCC) However, data gaps do have a large effect: interpolation with temporally 'switched off' stations

  29. Global Precipitation Climatology Centre (GPCC) Required gauge network density (gauges per grid cell) with a given error range for monthly precipitation: 10% error -> 8 to 16 gauges for daily precipitation: 20% error -> 20 to 40 gauges WMO-No 115, 1985

  30. Global Precipitation Climatology Centre (GPCC) Sampling error for area mean precipitation and network density 4 stations 8 stations 12 stations 20 stations = spatial variability of precipitation Rudolf et al., 1994/1995

  31. Global Precipitation Climatology Centre (GPCC) The systematic gauge measuring error: undercatch depending on precipitation phase and windspeed Reference precipitation in % of the amount measured by the unshielded Hellmann as function of windspeed separately for solid, liquid and mixed precipitation based on comparison with the international reference gauge (DFIR). (Figure: T. Günther in Goodison et al, 1998)

  32. Global Precipitation Climatology Centre (GPCC) • Conclusions • There is not a one unique gauge data set being suitable for all applications. • Different subsets of the total data base need to be selected with respect of the purpose of the application. • Most important aspects to be considered are • quality of the data and meta data • spatial resolution (network density, sampling error) • accuracy of observation (gauge bias correction) • completeness of time-series • time-series homogeneity

  33. Global Precipitation Climatology Centre (GPCC) Outlook (major activities scheduled for 2004/2005) Complementation of the database for time-series 1901-present (inclusion of GHCN and GDCD precipitation data). Production of a set of long-term gridded precipitation data sets 1951-2000: complented new complemented version 2 1901-2000: using selected data (homogeneous coverage) 1901-present: based on all data (Full Data) Statistical evaluation of the long time-series (monthly and daily) Redesign of GPCC’s software decoding of synoptic data on daily basis. The redesign is needed for operational gauge bias correction and for partitioning of solid and liquid precipitation . Documentation of the procedures and methods used at GPCC.

  34. Global Precipitation Climatology Centre (GPCC) GPCC’s website offers download of gridded precipitation data sets. The “Visualizer” displays digital maps on-the-fly. Various options can be selected by the user. http://gpcc.dwd.de/

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