New Saturation Adjustment for the COSMO model

Ulrich Blahak, DWD, COSMO General Meeting, Moskow, 5.9.2010

Ulrich Blahak, Axel Seifert

German Weather Service, Offenbach

New Saturation Adjustment for the

COSMO model


Motivation

Present saturation adjustment is formulated as an isobaric process and does not conserve mass.

During evaporation, mass is created and during condensation, mass is destroyed.

Because in total, there is more condensation than evaporation (due to precipitation), net sink of mass.

COSMO seems to be too dry compared to observations.


Saturation adjustmentx in

k g /k g

T_0, x_v0, x_c0

T_adj, x_vadj, x_cadj


Saturation adjustment

Solution to achieve mass conservation during the process:

Make it isochoric!

Implications: Change of thermodynamic formulation based on the first principle, adding pressure diagnosis after adjustment (→ let the model deal with the introduced pressure perturbation)


Isochoric saturation adjustment


Saturation adjustmentx in

k g /k g

T_0, x_v0, x_c0

T_adj, x_vadj, x_cadj

adiab. expansion happening in the next timestep


Saturation adjustment


Testing idealized cases






Verification – COSMO EU June 2010








Verification – COSMO DE Jul/Aug 2010


















Summary + Outlook

Isochoric saturation adjustment does conserve mass and seems to increase precipitation for both EU and DE.

Also influence on precipitation structure

Verification results so far indecisive: FBI improved, biases of T and p reduced, but other scores show no clear improvement.

Nevertheless, it will be more thoroughly tested in the framework of „65 levels for COSMO-DE“

Documents

New Saturation Adjustment for the COSMO model