smc-meeting, bologna, 05/06. feb. 2014

M. Baldauf (DWD) 1

SMC-meeting, Bologna, 05/06. Feb. 2014with verification extensions for SMC-teleconf., 16. April 2014

Michael Baldauf (FE13)

Proposed changes for COSMO 5.1 from WG2

•Remove ‘hacks’ in the tracer module•Adaptation of the RK dynamical core for stochastic physics•‘Targeted diffusion’ to avoid cold pools in narrow valleys•Reformulated divergence damping coeff. in new fast waves solver

M. Baldauf (DWD) 2

Remove ‘hacks’ in the tracer module

A. Roches, O. Fuhrer (MeteoCH)

When implementing the new Tracer Module (Roches, Fuhrer (2013) COSMO Tech. Rep.) several decisions had to be made concerning specialized treatment of tracer species. In the first implementation of the Tracer Module, those had been treated with the Metadata functionality of the Tracer Module. However the question arises, if some (or the most) of these special treatments are still necessary.In particular, the following special treatments (‘hacks’) should be removed( from Roches, Fuhrer (2013) COSMO Tech. Rep.):

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Fixes concerning the Leapfrog dynamical core: •CLP_10E-12: clip qi to zero, if qi < 10-12

Fixes concerning the Runge-Kutta dynamical core:

•ADD_CLP_ADV: add clipping for sedimenting moisture species (qr, qs, qg) at the end of the advection routine.

•BD_0GRAD_FORCED: for the species qi, qr, qs, qg either boundary values are read from a file or the boundary condition (BC) grad=0 is used. In the original code version one of these is done in any case despite the fact, that one can prescribe also other boundary conditions.

•DAMP_FORCED: For precipitating species, Rayleigh damping is done in any case, even when a grad=0 BC is prescribed.

Remove ‘hacks’ inthe tracer module

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It is quite difficult to decide, if these measurements are still necessary (they have often been implemented during the development phase and sometimes accidentally remained in the code, even when they are not longer necessary). This is in particular the case if the original developer of the code is not longer available. Therefore often the only one needs real case test runs to decide, if anything strange happens or if the forecast quality suffers.

Those extensive tests have been performed by MeteoCH report by A. Roches and O. Fuhrer. Considering the above mentioned measurements together with these test results, it is recommended to follow their suggestions and to simplify the code by removing these ‘hacks’.

documentation: is available (Roches, Fuhrer (2013) COSMO Tech. Rep.,Roches, Fuhrer, verification report from 25. Nov. 2013)

recommendation for COSMO 5.1

Remove ‘hacks’ inthe tracer module

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Adaptation of the RK dynamical core for stochastic physics

Goal: separate physical and dynamical tendencies

First step: shift call of Coriolis force

Has no influence to stability, only small changes in results

recommended for COSMO 5.1

L. Torrisi (CNMCA), M. Baldauf (DWD)

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… occured in a few COSMO-DE-runs around 06 Dec. 2013 in a narrow Alpine valley and even led to a model crash in two 2 EPS-members (parallel routine).

Similar ‚cold pools‘ have been reported by MeteoCH.

Orography

M. Baldauf, DWD

‘Targeted diffusion’ to avoid cold pools in narrow valleys

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Problem: unrealistic strong cooling in an Alpine valley


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Problem: unrealistic strong cooling in an Alpine valley


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Main contributor to the cooling is the horizontal advection:

x


5th order advection intensifies the ‚ramp‘

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Linear advection equation (1-dim.)

Spatial discretisations of the advection operator (order 1 ... 6) (u>0 assumed)

upwind 1st order

centered diff. 2nd order

Hundsdorfer et al. (1995) JCPWicker, Skamarock (2002) MWR

discretized:

iadv_order=3

iadv_order=4

iadv_order=5 !

iadv_order=6

iadv_order=2

iadv_order=1


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Possible solutions (1):

•upwind 3rd order: this does not deepen the ‚ramp structure‘ in T;indeed it reduces the cold pool relatively fast,is used operationally in COSMO-EU,diadvantage: reduced accuracy in a convection-permitting setup

is possibly detrimental (?)

•dynamical bottom pressure boundary condition (A. Gassmann, 2004)ldyn_bbc=.TRUE. (works in both fast waves solvers)disadvantage: does not match with the Mahrer-Discretisation (introd. planned),

mechanism is not clear; does not prevent every ‚cold pool‘

•advection limitation (G. Zängl) disadvantage: does not prevent from every ‚cold pool‘, not efficient enough in the current form


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• ‚targeted diffusion‘ (analogous to G. Zängl in ICON)

criteria: diffusion in a (near bottom) grid point, if T‘ < <T‘environment> – 10K

is applied only in these grid points;reduces a cold pool very quickly and is not active later on

computation time consumption ~ 0.05% this is a relatively harmless action; recommendation for COSMO 5.1

Possible solutions (2):

documentation: a short section will be included in the COSMO Sci. Doc. Part I


Randbehandlung von p'/ (bzw. von p'/z) am Unterrand:

1.) einseitige finite Differenz (G. Zängl):

p'/ = 0 p'(ke) + 1 p'(ke-1) + 2 p'(ke-2)

2.) ‚dynamische untere Druckrandbedingung‘ (A. Gassmann, 2004, COSMO-Newsl.)Aus

und

folgt eine Bedingung für p'/ .


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Problem: model crash in a 7 km setup in the area ‚Oman‘, nearby Iranian mountains (‚Mekran‘)

Solution:corrected version of the slope dependent reduction of the divergence damping coefficient (Baldauf, 2013) in a staggered grid.

the divergence damping coefficient is critical for model stability Extensive testing in two longer running experiments at DWD:COSMO-DE: running for 2 months COSMO-EU: running for 1 ½ months without problems.Only small changes in the results

documentation: not necessary, current Tech. Report No. 21 is sufficient

recommendation for COSMO 5.1

M. Baldauf, DWD

Reformulated divergence damping coeff. in new fast waves solver

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Divergence damping coefficient in COSMO-DE along the Alps (current version)

23 km

~ 4 km

0 km

~ 2 km

~ 10 km


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23 km

~ 4 km

0 km

~ 2 km

~ 10 km

Divergence damping coefficient in COSMO-DE along the Alps (new version)


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Example for the influence of ‚targeted diffusion‘ and ‚reform. div. damping coeff‘06. Dec. 2013, 21 h forecast, T2m

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Example for the influence of ‚targeted diffusion‘ and ‚reform. div. damping coeff‘06. Dec. 2013, 21 h forecast, v10m

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NUMEX-runs for a longer period

COSMO-EU (7 km): Exp. 9537

Simulation period: ‚15.10.-15.12.2013‘

Verification results for period ‚01-30 Nov. 2013‘

upper air verification

(U. Pflüger)

SYNOP-Verif.(U. Damrath)Routine Exp.

COSMO-DE, Exp. 9536

simulation period: ‚15.10.-31.12.2013‘

upper air verification‚Nov. 2013‘

(U. Pflüger)

upper air verification‚Nov 2013‘

(U. Pflüger)

upper air verification‚Dec. 2013‘

(U. Pflüger)

Summary

Three proposed actions

•targeted diffusion on T‘:this solves cold pools in steep mountainous regionsnegligible increase in computational time

•bug-fixed slope-dependent divergence dampingsolves stability problems in strongly irregular terrain with steep slopes.no change in time consumption

•small change in calling order of Coriolis terms All actions does not significantly change the verification scores.

Probably they will come into the next version COSMO 5.0.2

smc-meeting, bologna, 05/06. feb. 2014

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