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

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

3. 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. 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. 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. 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. 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. 8 ROC and pseudo-ROC curves 2 definitions for the False Alarm Rate !

9. 9 Reliability diagram and Brier score decomposition BS=  (p i -o i ) 2 /N BS=REL-RES+UNC REL=  n k (p k -o k ) 2 /N RES=  n k (o k -o) 2 /N UNC=o(1-o)

10. 10 Evaluation tasks More efficient if performed centrally  Could be performed by a WG-i participant not involved in the intercomparison due time defined format  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. 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