COSMO General Meeting, WG3, Krakow, 2008 - 1 - Revised parameter settings in the COSMO-DE boundary...

COSMO General Meeting, WG3, Krakow, 2008 - 1 -

Revised parameter settings in the COSMO-DE

boundary layer (TKE) scheme

Axel SeifertDeutscher Wetterdienst, Germany

Deutscher WetterdienstGB Forschung und Entwicklung


Modifications of PBL scheme

Blackadars parameterization of the turbulent length scale:

ECMWF/IFS: lturlen = 375 m

COSMO model: lturlen = 500 m

New COSMO-DE: lturlen = 150 m

In the olde COSMO-DE setup

sub-grid cloudiness started early,

compared to the original Sommeria

and Deardorff (1977) scheme.

The revised settings use the linearization

as originally suggested by SD77


Fraction Skill ScoreA scale dependent precipitation skill score

for convective-scale NWP models

Δ = 1: 0 vs 1 → bad forecast on 1Δ-scaleΔ = 3: 4/9 vs 3/9 → OK forecast on 3Δ-scaleΔ = 5: 6/25 vs 6/25 → perfect forecast on 5Δ-scale

Roberts and Lean (2005)


Fraction Skill Score

Roberts and Lean (2005)

In the following we chose 21x21 grid points, i.e., 60x60 km2.


Precip and QPF-Scores 01. Juni 2008 06-18 UTC

Weak large-scale forcing by quasi-stationary occlusion zone


Precip and QPF-Scores 15. Mai 2008 06-18 UTC

Slowly moving warm front over Rheinland-Pfalz/Hessen/Sachsen



Occlusion zone in the Elbe region with several convergence lines



Weak slowly moving cold front (?)


Total scores from 1 May to 9 June 2008

• Significantly more strong (convective) precipitation events.• Slight improvement of ETS for thresholds larger than 5 mm / 12 h• Much better FSS especially for high thresholds

06-18 UTC


Total scores from 1 May to 9 June 2008

Including day and night in the statistics gives a less clear signal!

06-18 UTC und 18-06 UTC!


Hovmöller diagramms or precipiation rate1 May to 9 June 2008

• In the operational COSMO-DE almost no diurnal cycle, the afternoon maximum is very weak.

• Clear improvement in the model behavior using the modified PBL parameters. Reduced nocturnal drizzle, better afternoon maximum


Monthly precipitation accumulations form May 2008

• The total accumulated precipitation is robust. Both variants of COSMO-DE give a good monthly budget.


Conclusions

• Changing the PBL parameters (turlen, SGS cloud) has a strong effect on the initiation of convection and the diurnal cycle of precipiation during summer.

• For COSMO-DE this resolves the notorious problem of the underestimation of convective activity. Therefore the modification was made operational at DWD on 10 Sep 2008.

• The modifications have also a strong effect on the temperature profile and 2m-temperature. Both increase, making the profile more unstable. This might introduce a positive bias in the temperature forecasts.

• More research on improved PBL schemes is necessary (UTCS). The work presented here shows that the mixing length, and the moist turbulence assumption are important and need special attention.

Deutscher WetterdienstGB Forschung und Entwicklung


Case study 9 June 2007:

Prä-frontale Konvergenzline führt zu starker Konvektion:

Radar turlen=500m, clc=0.75 turlen=250m, clc=0.50

turlen=150m, clc=0.50 turlen=150m, clc=0.25 turlen=150m, SD77




Hovmöller diagrams of precipitation ratefrom 1 June to 21 June 2007


Hovmöller diagramms of precipitation frequency 1 May to 9 June 2008

• Confirms the findings seen in the precip rate diagramms.




Hovmöller diagrams of precipitation frequencyfrom 1 June to 21 June 2007


Standard SYNOP-Verifikation: 00 UTC Lauf (Achtung: Hier Experiment für Juni 2007)

2m-Temperatur und -Feuchte zeigen ein deutliche Änderung.


Akkumulierter Niederschlag und QPF-Scores 13. Mai 2008 06-18 UTC

COSMO General Meeting, WG3, Krakow, 2008 - 1 - Revised parameter settings in the COSMO-DE boundary...

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