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Intercomparison of low visibility prediction methods

Intercomparison of low visibility prediction methods. COST-722 (WG-i) Frédéric Atger & Thierry Bergot (Météo-France). Proposal. Horizontal surface (2m) visibility Main goal : learn about the value of the different existing methods Not a competition. Observations.

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Intercomparison of low visibility prediction methods

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  1. Intercomparison of low visibility prediction methods COST-722 (WG-i) Frédéric Atger & Thierry Bergot (Météo-France)

  2. Proposal • Horizontal surface (2m) visibility • Main goal : learn about the value of the different existing methods • Not a competition

  3. Observations • Participants propose airports where hourly visibility observations are available • WG-i select one airport (validated by MC) • Selected participant provides observations for 4 « winter » seasons (October to March) from Oct. 1999 to March 2003 • 2 seasons for adjusting statistical methods and models characteristics • 2 seasons for evaluating existing methods

  4. Observed parameters • Minimum required : hourly visibility (reference for verification) • Any potentially useful parameters (according to local observation capabilities) • for initializing models • as predictors for statistical methods

  5. Forecasts • Forecast basis : 00 UTC and 12 UTC • Lead times : +3h to +24h by 3h step • It is not a competition : • Participants indicate the main characteristics of the forecasting method (e.g. « 1D model coupled to ECMWF » or « MOS based on Aladin ») • Participants are encouraged to provide alternative sets of forecasts obtained by modifying these characteristics

  6. Visibility thresholds • Depend on WG-ii conclusions (requirements from the forecasters and from the customers) • Proposal : • 200 m (roads) • 600 m (airports) • 1000 m (fog) • 5000 m (mist) • Participants provide probabilistic or deterministic forecasts for as many thresholds as possible

  7. Verification • Comparison for a given validity (e.g. 06 UTC) and a given lead time (e.g. +6h) • 2 verification aspects : • Contingency tables  hit rate and false alarm rate  ROC or « pseudo-ROC » diagram • Reliability diagram  Brier Score + reliability and resolution components • Deterministic forecasts are considered as a special case of probabilistic forecasts

  8. ROC and pseudo-ROC curves 2 definitions for the False Alarm Rate !

  9. Reliability diagram and Brier score decomposition • BS=(pi-oi)2/N • BS=REL-RES+UNC • REL=nk(pk-ok)2/N • RES=nk(ok-o)2/N • UNC=o(1-o)

  10. Evaluation tasks • More efficient if performed centrally • Could be performed by a WG-i participant not involved in the intercomparison • Data (observations and forecasts) should be provided in due time and in a defined format by the participants • Alternatively, each participant conducts the evaluation of its own forecasts (following common verification rules) • All data and results should circulate among the participants

  11. Appendix : list of additional observed parameters • 2m temperature • 2m humidity • 10m wind • 1h/3h rainfall • Total cloud cover • Soil temperature • Surface pressure • Net short wave radiation near the ground • Soil type (soil + vegetation) • Radio-sounding observations

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