RCM sensitivity to domain size in summer and winter

With the collaboration of:

Jean-Philippe Morin (simulations)and Mathieu Moretti (diagnostics)

By Martin Leduc and René Laprise

The Big-Brother experiment

« Perfect-model » approach:

driving and nested model are the same.

differences between LB and BB fields are directly attributable to the nesting procedure.

Big-Brother Filtered Big-Brother Little-Brother

Measuring the RCM sensitivity to various parameters:

• Spatial resolution and the update frequency of the LBC: Denis et al. (2003), Antic et al. (2004) and Dimitrijevic and Laprise (2005)

• Errors in the nesting data: Diaconescu et al. (2007)

• Size of the domain: Leduc and Laprise (2008)

The domain-size issue (1)- LARGE domain -

Model is free to develop its own large-scale dynamics, differing from the nesting data.

Does spectral nudging is appropriate to solve this ?

"If you don't believe in the value of global climate models then there's no point in downscaling them” -Filippo Giorgi

Maybe not !

May be good: Small-scale physical processes can affect the large-scale dynamics.

constrain the RCM to follow the “good” large scales

let the RCM free to correct the “wrong” large scales

In other words:

• LBC exert a strong control on the solution (as for SN).

• Small-scale features have not enough time/space to develop.

The domain-size issue (2)- SMALL domain -

Leduc and Laprise (2008) :Effects related to the domain size have been studied for a winter case where the atmospheric flow is :

strong and westerly

Need to repeat the experiment with a summer flow.

Experimental framework

Simulations:

Big-Brother BB: 196x196

Little-Brothers LB1: 144x144 LB2: 120x120 LB3: 96x96 LB4: 72x72

Validation area: QC: 38x38

2 Periods: July 90 to 93 February 90 to 93

Sponge zone: 10 grid points

Summer and winter flows over Québec

SUMMER WINTER

ANIMATE !

700-hPa wind magnitude(temporal deviation)

Small scales

Large scales

SUMMER WINTER

Small-scale transient eddies of the 700-hPa wind magnitude

BB

LB1 LB2

LB3 LB4

(m/s)

R*LB1: 79%LB2: 68%LB3: 65%LB4: 78%

SUMMER

BB

LB1 LB2

LB3 LB4

R*LB1: 56%LB2: 50%LB3: 71%LB4: 44%

WINTER

(m/s)

Small-scale transient eddies of the 700-hPa relative humidity

(%)

R*LB1: 61%LB2: 49%LB3: 49%LB4: 50%

WINTERSUMMER

R*LB1: 90%LB2: 87%LB3: 89%LB4: 90% (%)

BB

LB1 LB2

LB3 LB4

BB

LB1 LB2

LB3 LB4

Small-scale transient variance ratio: LB / BB (wind magnitude)

WINTERSUMMER

(%) (%)

(%)

Small-scale transient variance ratio: LB / BB (relative humidity)

(%)

SUMMER WINTER

Did summer really heal our RCM ?

1- Flow characteristics (a high residency time)2- Intense convective processes3- Vertical turbulent fluxes

General conclusions

- Smaller domain can be used in summer -(for comparable skills)

Lateral boundary conditions control :

increases when the domain size is reduced, and is stronger in winter applied on large scales, it affects similarly the small ones

Magnitude of the small-scale features:

spin-up area in winter homogeneous spin-up in summer

Antic, S., R. Laprise, B. Denis and R. de Elía, 2004: Testing the downscaling ability of a one-way nested regional climate model in regions of complex topography. Climate Dynamics, 23, 473-493.

Denis, B., R. Laprise and D. Caya, 2003: Sensitivity of a regional climate model to the resolution of the lateral boundary conditions. Climate Dynamics, 20, 107-126.

Diaconescu, E. P., R. Laprise and L. Sushama, 2007: The impact of lateral boundary data errors on the simulated climate of a nested regional climate model. Climate Dynamics, 28, 333-350.

Dimitrijevic, M. and R. Laprise, 2005: Validation of the nesting technique in a RCM and sensitivity tests to the resolution of the lateral boundary conditions during summer. Climate Dynamics, 25, 555-580.

Leduc, M. and R. Laprise, 2008: Regional Climate Model sensitivity to domain size. Accepted in Climate Dynamics.

References

Taylor diagrams (ls)

Taylor diagrams (ss)

Spectral filters“T30” : 2160 to 1080 km

LPF : 2160 to 540 km

Small-scale transient variance ratio: LB / BB (relative vorticity)

700-hPa rvortSUMMER WINTER

Summer and winter flows over Québec

SUMMER WINTER