COSMO General Meeting 2008, Krakow - 1 -

Modifications to the COSMO-Model Cumulus

Parameterisation Scheme (Tiedtke 1989):

Implementation and Testing

Dimitrii Mironov and Axel SeifertDeutscher Wetterdienst, Offenbach am Main, Germany

Deutscher WetterdienstGB Forschung und Entwicklung

COSMO General Meeting 2008, Krakow - 2 -

Revision of the T89 mass-flux scheme

Motivation:• Once in a while, COSMO-EU simulates very high precipitation

amounts that are unrealistic, e.g. 200-400 mm in 24 h, making the COSMO-EU forecasts less useful for a number of applications, e.g. for flood prediction

• Orographic precipitation is primarily on the windward side of mountains, very little in the lee

• In general, a better coupling of convection, grid-scale microphysics and radiation is desirable

COSMO General Meeting 2008, Krakow - 3 -

Revision of the T89 mass-flux scheme (cont’d)

Modifications of the Tiedtke (1989) scheme:• Co-existence of cloud water and cloud ice (water-ice mixed phase) over a certain

temperature range (see next slide) is allowed during the saturation adjustment calculations within the T89 scheme (current version of COSMO-T89: cloud condensate is either water or ice, depending on the temperature being above/below the freezing point); this change slows down the T89 scheme

• Detrainment of convective cloud condensate (most importantly, cloud ice) which is passed to the other COSMO-model physics and dynamics in the form of tendencies of qc and qi (current version of COSMO-T89: detrained convective cloud condensate instantaneously evaporates); this change improves coupling of the T89 scheme with the other COSMO-model schemes

• 80% environment air relative humidity threshold for evaporation of convective rain (current version of COSMO-T89: 95% threshold); this change partially counteracts the above mixed-phase change by increasing the surface precipitation amount

All these modifications have long since been operational in the ECMWF IFS

COSMO General Meeting 2008, Krakow - 4 -

Mixed phase clouds in the Tiedtke scheme

The modified T89 scheme computes the saturation water vapour pressure over the (convective) cloud water-cloud ice mixed phase by interpolating between the saturation pressures over liquid water and over ice, using temperature as an interpolation variable.

Ratio of ice water content (IWC) and total water content (TWC) for convective updrafts in COSMO-DE (w > 10 m/s, TWC > 1 g/m3) taken from a single forecast of a squall line case (20.07.2007 00UTC run).

Lines show possible transitions from pure liquid to pure ice phase • The linear function used in the first experiments resulted in too weak convection • The quadratic function taken from the ECMWF IFS is now used in the COSMO model

COSMO General Meeting 2008, Krakow - 5 -

Total Precipitation 12 May 2008

COSMO-EU Parallel Routine:

mean=1.36, max=112.58

COSMO-EU Routine:

mean=1.49, max=243.91

COSMO General Meeting 2008, Krakow - 6 -

Total Precipitation May 2008

Boundary Effects

COSMO-EU Parallel Routine:

mean=1.46, max=193.49

COSMO-EU Routine:

mean=1.56, max=147.87

COSMO General Meeting 2008, Krakow - 7 -

Example 11 June 2007

avg 2.8 mm avg 3.9 mm avg 1.9 mmmax 44 mm max 274 mm max 27 mmavg 3.1 mmmax 60 mm

observation control new scheme

New scheme much better, but does not work that good all the time.

accumulated precipitation in mm

COSMO General Meeting 2008, Krakow - 8 -

Example 27 June 2007

avg 2.0 mm avg 2.4 mm avg 2.3 mmmax 33 mm max 29 mm max 38 mm

An old problem solved? Only for weak orographic precip.

observation control new scheme

accumulated precipitation in mm

COSMO General Meeting 2008, Krakow - 9 -

Monthly accumulation June 2007

• Obvious problems in control. Local overprediction, too strong orographic enhancement and bias of 21%.

• New convection scheme clearly improves the model, but does not solve all problems.

105 mm 128 mm (+21%) 116 mm (+11 %)

observation control new scheme

COSMO General Meeting 2008, Krakow - 10 -

Montly accumulation July 2007

• Confirms the good results for June 2007.

120 mm 131 mm (+9%) 120 mm (+0 %)

observation control new scheme

COSMO General Meeting 2008, Krakow - 11 -

Some QPF scores: Juni/Juli 2007

• Verification against 24h-REGNIE precipitation estimate at each model grid point.

• Bootstrap hypotheses test that confirms that the model change is statistically significant, i.e. both models version are indeed different.

• FBI clearly improved, TSS und ETS somewhat worse for low threshold (both scores are not independent from FBI). Slight improvement for high thresholds.

COSMO General Meeting 2008, Krakow - 12 -

BIAS,...: 00 UTC (‘LMQ-Domain’)Exp 6467(equivalent to COSMO-EU Routine) vs.Exp 6468 (modified T89 scheme)

High level cloudsslightly deteriorated

Low level clouds

slightly improved

COSMO General Meeting 2008, Krakow - 13 -

Conclusions

• The modified Tiedtke (1989) scheme helps improve the precipitation forecasts with the COSMO model

• The unrealistic high precipitation amounts are reduced

• The orographic precipitation structures are improved

• The FBI (frequency bias) is reduced and so are the monthly accumulations

• QPF scores, such as TSS and ETS, can be slightly worse due to the reduced FBI

• Prediction of low level clouds is slightly improved

• Prediction of high level clouds is slightly deteriorated

Deutscher WetterdienstGB Forschung und Entwicklung