1. Terradellas, E; Bergot, T A comparison between two single-column models designed for short-term fog and low clouds forecasting Terradellas, E. and T. Bergot 6th. Annual Meeting of the E.M.S., Ljubljana 4-8 Sep. 2006

2. Introduction Once analysed the results of the previously presented intercomparison, the Spanish INM and Météo-France agreed to undertake a deeper comparison between their respective SCM (H1D and COBEL-ISBA) in the field of fog and low- clouds forecast. The model outputs would be compared during a full winter season for three sites with different climate and geographic conditions: Paris-CdG, Warsaw- Okecie and Casablanca-Nouasseur airports.

3. The sites

4. Paris-Charles de Gaulle airport Paris-Charles de Gaulle airport is located over relative flat terrain.

5. Test period 16 JAN. – 14 FEB. 2005 H1D runs: 0000, 0600, 1200, 1800 Runs start 3 h 30 m after nominal runtime. 24-hour forecast COBEL-ISBA runs: 0000, 0300, 0600, 0900, 1200, 1500, 1800, 2100. 12-hour forecast

6. Full season comparison 1 OCT. 2005 – 28 FEB. 2006 H1D runs: 0000, 0600, 1200, 1800 Runs start 3 h 30 m after nominal runtime. 24-hour forecast COBEL-ISBA runs: 0000, 0300, 0600, 0900, 1200, 1500, 1800, 2100. 6/8-hour forecast

7. Initialisation Dedicated obs. system:  30-m tower: T, RH  Soil T and humidity  SW and LW radiation

8. Initialisation

9. The problem of fog forecasting

10. H1D/COBEL. Night BL temperature Systematic difference in the cooling rate: nocturnal cooling is greater in H1D. Soil? Clouds? Lower part of the column is more stable in HIRLAM

11. H1D/COBEL. Daytime temperature Daytime behaviour is “normal”:  Bias is small and stable with time  Rmse increases with time

12. H1D/COBEL. Night BL sp. humidity “Normal” behaviour for the specific humidity H1D is slightly moister than COBEL

13. H1D/COBEL. Night BL IPW H1D is slightly moister than COBEL. The difference comes from the initialisation.

14. H1D/COBEL. Night BL liquid water Above 200 m, H1D presents less liquid water than COBEL  higher cooling rate  more liquid water at low levels (fog)

15. H1D/COBEL. Night BL wind Wind: normal behaviour. H1D is less windy. Possible contribution to a more stable low-level column.

16. H1D/COBEL. Night longwave radiation (downward positive) H1D: less liquid water  more loss of longwave radiation

17. Intramodel diagnostics Figures from the test period H1D is more “consistent”. This fact has positive and negative implications... Tq LW ws

18. Net radiation at ground Figure on the right is from the test period H1D overestimates both, the downward SW radiation and the upward LW radiation, probably because it underestimates cloudiness

19. Low C&V conditions Low C&V conditions for LFPG:  Visibility < 600 m or  Ceiling < 200 ft 7.2% of observations during the analysed period (0.4% only low visibility reported, 2.9% only low clouds reported and 3.9% both) That is 240 hours

20. Model skill LOW C&V 3-4 h FCST CIH1D POD5673 FAR3857 Similar performance for 3-4 h forecast. H1D: higher POD and FAR. Because its higher cooling rate? Before 3 h: COBEL performs better. Because its better initialisation? After 4 h: H1D performs better. Because its better treatment of horizontal unhomogeneity? H1D: HH+06 / HH+07, that is, 3-4 h after ending the assimilation cycle.

21. Conclusions  SCM, in particular COBEL-ISBA and H1D, are useful tools for short-term C&V forecast.  The initialisation is very important. Future development of H1D should, probably, focus on it.  COBEL development should, probably, focus on its treatment of horizontal unhomogeneity.  Model intercomparison experiments are excellent tools to identify the weakest part of a model, to find out which aspect is worth to work on.