COSMO_2005 DWD 15 Sep 2005Page 1 (5) COSMO General Meeting Zürich, September 2005 Erdmann Heise German Weather Service Report on Workpackage Validation of boundary layer clouds Test results with the minimum vertical diffusion coefficient set to zero

COSMO_2005 DWD 15 Sep 2005Page 2 (5) Workpackage Validation of boundary layer clouds There is a long lasting problem of LM to rapidly dissolve low level stratus or stratocumulus in late autumn and in winter. According to earlier experiments by Dmitrii Mironov the problem might be fixed by setting back the minimum value of the vertical diffusion coefficient to zero. This coefficient was introduced some years ago to avoid long lasting overcast situations over water areas. Following a reduction of evaporation over water in April 2004 this minimum vertical diffusion might turn up to be unnecessary. The effect of a zero minimum vertical diffusion was tested in a parallel experiment for the period 1 October 2004 to 31 December 2004.

COSMO_2005 DWD 15 Sep 2005Page 3 (5) Workpackage Validation of boundary layer clouds The case of 11 Novem- berg led to vehement complains of the fore- casters. Whereas Ger- many was mainly covered by low clouds, the operational LM- forecast showed a mainly cloudless sky (left hand part). With the minimum vertical diffusion coefficient set to zero the forecast was improved consid- erably (right hand part).

COSMO_2005 DWD 15 Sep 2005Page 4 (5) Workpackage Validation of boundary layer clouds The verification results are shown for December 2004 in the opposite Table as an example. There are large improvements in cloud cover prediction, negligible changes for 2m temperature and dew point, but drastic degradations in precipitation and gusts. The results for November look similar. In October the frequency of overcast situations is overpredicted in the experiment, but temperature and dew point predictions improve a little and there is no drastic degra- dation for precipitation and gusts.

COSMO_2005 DWD 15 Sep 2005Page 5 (5) Workpackage Validation of boundary layer clouds On average the results of the objective verification are disappointing, but they still wait for a thorough evaluation. The reason for the degradation of precipitation prediction and of the gust diagnosis is not yet understood. There are connections to WP (cure overestimation of low precipi- tation in winter) and to WP (testing sub-grid scale cloudiness approaches). Connections are also obvious to the two proposed projects ‘Tackle deficiencies in precipitation forecasts’ and ‘Generalised moist PBL scheme’. An Interim Report on the results is available.