COSMO_2005 DWD 15 Sep 2005Page 1 (5)

COSMO General MeetingZürich, 20 - 23 September 2005

Erdmann HeiseGerman Weather Service

Report on Workpackage 3.5.1

Validation of boundary layer clouds

Test results with the minimum vertical diffusion coefficient set to zero

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Workpackage 3.5.1 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.

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Workpackage 3.5.1 Validation of boundary layer clouds

The case of 11 Novem-berg led to vehementcomplains of the fore-casters. Whereas Ger-many was mainlycovered 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 forecastwas improved consid-erably (right hand part).

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Workpackage 3.5.1 Validation of boundary layer clouds

The verification results are shownfor December 2004 in the oppositeTable as an example. There are largeimprovements in cloud coverprediction, negligible changes for2m temperature and dew point,but drastic degradations inprecipitation and gusts. The results for November look similar. In October the frequency of overcastsituations is overpredicted in theexperiment, but temperature anddew point predictions improve alittle and there is no drastic degra-dation for precipitation and gusts.

December 2004 Reference ExperimentFrequency bias (opt=1)total cloud cover (0-2/8) 1.64 1.12total cloud cover (7-8/8) 0.82 1.01low level cloud cover (0-2/8) 1.31 0.93low level cloud cover (7-8/8) 0.75 1.05Percent correct (opt=100)total cloud cover 63.0 71.0low level cloud cover 58.7 65.4mid level cloud cover 68.6 67.2high level cloud cover 77.7 77.02m temperature 63.9 64.22m dew point 76.4 78.0Root mean square error (K)2m temperature 2.67 2.702m dew point 2.89 2.85True skill statistics (opt=100)precip > 0.1 mm/6h 60.7 47.0precip > 2.0 mm/6h 63.9 63.9precip > 10.0 mm/6h 44.6 42.7Equitable threat score (opt=100)gusts > 12 m/s 34.2 35.5gusts > 15 m/s 29.8 29.8gusts > 20 m/s 20.2 16.3gusts > 25 m/s 8.7 4.2

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Workpackage 3.5.1 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 3.10.1 (cure overestimation of low precipi-tation in winter) and to WP 3.5.2 (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.