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The current and future treatment of systematic raingauge measuring errors

The current and future treatment of systematic raingauge measuring errors by the GPCC for operational global precipitation analysis Bruno Rudolf 1 , Tobias Fuchs 1 , Franz Rubel 2 1 Global Precipitation Climatology Centre, DWD, Offenbach, Germany

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The current and future treatment of systematic raingauge measuring errors

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  1. The current and future treatment of systematic raingauge measuring errors by the GPCC for operational global precipitation analysis Bruno Rudolf1, Tobias Fuchs1, Franz Rubel2 1 Global Precipitation Climatology Centre, DWD, Offenbach, Germany 2 Inst. for Med. Physics and Biostatistics, VUW Vienna, Austria (June 2002) Raingauge being protected from evaporation losses, designed by J. Leupold 1726

  2. Correction of raingauge-measured data due to systematic errors • Given conditions for the operational task of the GPCC: • Raingauge data from worldwide up to 50,000 stations are used. • Precipitation data have to be corrected for many different gauge types. • Instrument characteristics are unknown for many of the stations. • Auxiliary metorological parameters are not measured at most of • the raingauge stations. • An applicable correction method needs to be based on information • being actually available !

  3. Correction of raingauge-measured data due to systematic errors • The procedure still being used operationally at the GPCC: • The systematic error is quantified based on Legates‘ climatology • (corrected versus uncorrected long-term mean precipitation). • The station-related data used by GPCC are not being corrected. • The gridded precipitation data sets distributed by the GPCC also • are not yet corrected. • Mean monthly correction terms are supplied by the GPCC on the grid • complementary to the gridded precipitation data. • Users are recommended to apply the corrections terms on the • gridded product for bias reduction.

  4. The current procedure: GPCC delivers separately two gridded data sets: 1. The Monitoring Product: monthly precipitation in mm based on uncorrected data (Sample: January 2002) 2. Mean monthly percentual corrections: (Lcorr-Lmeas) / Lmeas in % with max-corr = 200% (Sample: mean January)

  5. Precipitation phase and windspeed are the most important parameters for correction of raingauge observations Correction ratio in % of the data measured by the Hellmann gauge (without windshield) as a function of wind speed for solid, liquid and mixed recipiation

  6. Probability of solid and liquid precipitation as function of air temperature and humidtity derived from SYNOP data (Fuchs, T., J. Rapp, F. Rubel and B. Rudolf, 2001) solid liquid

  7. Data processing scheme • for SYNOP reports: • Sorting data from the synoptic structure into time-series. • Quality-control of RRRTRcoded groups using information from wwW1W2 . • Decomposition of over-lapping RRRTRgroups. • Calculation of • daily precipitation totals from quality-checked and corrected RRRTRgroups.

  8. Correction using precipitation phase and wind speed from SYNOP reports: • Decoding of T, Td and v10m. • Separation of liquid, mixed and solid phase using wwW1W2 and T and Td • Correction using a wind speed reduced from v10m to instrument height. • Calculation of corrected daily precipitation totals.

  9. Percentual partitions of liquid and solid precipitation phases for Europe (31°-72°N, 11°W - 44°E) and period 1 Jan to 31 Aug 2001 based on GPCC's phase scheme. Mean daily and daily-based monthly precipitation corrections for Europe (31°-72°N, 11°W - 44°E) and period 1 Jan to 31 Dec 2001 and Legates' mean monthly climatologic corrections.

  10. Comparison of monthly percentual corrections in % of observed data derived from daily corrections for the years 1996 and 1997 and long-term mean monthly corrections after Legates (Ungersböck et al. 2001)

  11. Comparison of monthly percentual corrections in % of observed data derived from daily corrections for the years 1996 and 1997 and long-term mean monthly corrections after Legates (Ungersböck et al. 2001)

  12. Comparison of monthly percentual corrections in % of observed data derived from daily corrections for the years 1996 and 1997 and long-term mean monthly corrections after Legates (Ungersböck et al. 2001)

  13. Comparison of monthly percentual corrections in % of observed data derived from daily corrections for the years 1996 and 1997 and long-term mean monthly corrections after Legates (Ungersböck et al. 2001)

  14. Systematic gauge measuring error - Summary: • The new “on-event” correction method of GPCC is applicable because it is • only based on information beeing actually available. • The new method deliveres precipitation totals much closer to reality than the • use of climatic mean correction factors, because it is includes the observed • year-to-year variation of precipitation phases. • Still to do: • Development of an interpolation scheme for the new monthly correction • factors from synoptic stations to other stations. • Assessment of the accuracy of the new correction method by variation • of all input parameters used. • Completion of instrument parameter tables for all gauge types being used.

  15. Systematic gauge measuring error • GPCC‘s recommendations: • Precipitation data may be corrected best by the originator - • but the measured data should be delivered with the corrected data. • Corrections should be clearly indicated, the method and the parameters • should be documented; documentation should be supplied with the data. • Information about the instrument characteristics should be compiled by • the originator and be added in the meta data catalog for each station. • The precipitation measurement intercomparison program should continue • including all new instruments. Results should quickly be published.

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