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 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

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 Contribution to merging of satellite and gauge data (GPCP) Distribution of the gridded products (via internet) Assessment of uncertainty (or error) of the gridded products The GPCC has been established in 1988 at DWD on request of WMO as a contribution to the WCRP Global Precipitation Climatology Project. GPCC‘s origin and specific functions 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).

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 Global Precipitation Climatology Centre (GPCC)

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

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

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

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 Global Precipitation Climatology Centre (GPCC)

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

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 available by end of 2004 based on 6,000 stations) Global Precipitation Climatology Centre (GPCC) GPCC Product Overview

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

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

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

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

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

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

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

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

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

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

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) Global Precipitation Climatology Centre (GPCC)

global data of CRU global data of FAO national data rt data Composition of the 50 years climatology 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.

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

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

Calculation of gridded area mean precipitation Global Precipitation Climatology Centre (GPCC) 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 empirical scheme of Shepard, which has been well proved in various intercomparison studies for application on global or continental analysis of precipitation.

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

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

However, data gaps do have a large effect: interpolation with temporally 'switched off' stations

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

Rudolf et al., 1994/ stations 4 stations 8 stations 12 stations = spatial variability of precipitation Sampling error for area mean precipitation and network density 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) 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 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 :complented new complemented version :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. 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. Global Precipitation Climatology Centre (GPCC)