Representation of low Representation of low cloudiness/stratus in Aladin/AUT: cloudiness/stratus in Aladin/AUT:

Improvements and existing problemsImprovements and existing problems

• Areal extension of stratus layer

• The problem of horizontal diffusion

• Increasing the horizontal resolution

• Summer case 8.7.2004

Areal Extension

Satellite VIS Image: 9.11.2003 12UTC

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

Satellite VIS Image: 10.11.2003 12UTC

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

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.

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.

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.

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.

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

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.

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?)

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).

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.