1. Representation of low cloudiness/stratus in Aladin/AUT: Improvements and existing problems Areal extension of stratus layer The problem of horizontal diffusion Increasing the horizontal resolution Summer case 8.7.2004

2. Areal Extension Satellite VIS Image: 9.11.2003 12UTC

3. Operational Aladin-Austria model run: 9.11.2003 00UTC + 12 hours forecast

4. Satellite VIS Image: 10.11.2003 12UTC

5. Operational Aladin-Austria model run: 9.11.2003 00UTC + 36 hours forecast

6. Problem of Horizontal Diffusion Aladin-Austria: 9.11.2003 00UTC +08 hours with operational horizontal diffusion settings. The stratus layer hardly penetrates into the Danube valley. Aladin-Austria: 9.11.2003 00UTC +08 hours exp. run with horizontal diffusion of temperature set to 0. The stratus layer slightly propagates into the Danube valley.

7. Aladin-Austria: 9.11.2003 00UTC +32 hours with operational horizontal diffusion settings. The stratus layer hardly penetrates into the Danube valley. No signal of low clouds in the basin of Klagenfurt. Aladin-Austria: 9.11.2003 00UTC +32 hours exp. run with horizontal diffusion of temperature set to 0. Stratus forms in the Danube valley and in the basin of Klagenfurt.

8. Pseudo-Temp for Klagenfurt, 9.11.2003 00UTC + 32h, operational model. Surface inversion developed, 2m-temperature too cold (cloud free). Pseudo-Temp for Klagenfurt, 9.11.2003 00UTC + 32h, exp. run with HD of T =0. Elevated inversion with saturated layer below, 2m-temperature more realistic.

9. Increasing the horizontal resolution 9.11.2003 00UTC +08h: Exp. Model run with 2.3km horizontal resolution. Forecast of low clouds slightly improved in the Danube valley, but no signal in basin of Klagenfurt. 9.11.2003 00UTC +32h: Exp. Model run with 2.3km horizontal resolution. Forecast of low clouds significantly improved in the Danube valley, but again no signal in basin of Klagenfurt.

10. Summer Case: 8.7.2004 Satellite VIS Image: 8.7.2004 06 UTC. Low stratus developed during the night in the northern and eastern parts of Austria. ATP500 + MSLPRESSURE, Aladin-Austria operational model forecast. Run: 7.8.2004 00 UTC + 06 hours

11. Aladin-Austria operational model run: 8.7.2004 00UTC + 06 hours forecast. Low stratus layer covers northeastern parts of Austria, but not the Danube valley. Aladin-Austria exp. Run with HD of T = 0 8.7.2004 00UTC + 06 hours forecast. Low stratus layer covers northeastern parts of Austria and even extends through the Danube valley.

12. Additional experiments High horiz. Resolution (2.3km) with different vertical resolution (37 vs. 45 levels): no significant improvement Modification of the sub-inversion cloudiness scheme with flexible thickness of quasi-saturated layer: improvement of spatial extension (but danger of entering the greyzone of overestimation?)

13. Additional experiments Exp. with half/a tenth part of operational HD settings: improvements of low cloudiness forecast rather linear to decrease of HD of T. Exp. without SK-scheme but with HD=0 : vertical temperature profile only slightly improved, but existence of clouds necessary for the development of sharp and pronounced inversions (due to cloud top cooling and heating of sub-cloud layer).

14. Summary Spatial extension of stratus layer quite realistic, only few cases with overestimation Deficiencies in alpine valleys and basins: partly due to destruction of inversion by HD, partly due to wrong initial conditions. Simply increasing the horizontal resolution does not solve the problem in alpine valleys, need of other physical parameterizations.