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Intercomparison of Low Visibility Prediction Methods: COST-722

This proposal aims to assess and compare various methods for predicting horizontal surface visibility at airports, focusing on data collected over four winter seasons from October 1999 to March 2003. Participants will provide hourly visibility observations and forecasts while learning from the strengths of different forecasting approaches. The initiative emphasizes collaboration rather than competition, encouraging participants to present unique methods and forecast variations. Results will be verified through contingency tables, reliability diagrams, and various performance metrics to improve forecasting accuracy for critical visibility thresholds.

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Intercomparison of Low Visibility Prediction Methods: COST-722

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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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