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-BrotherFiltered Big-BrotherLittle-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 SUMMERWINTER ANIMATE !

700-hPa wind magnitude (temporal deviation) Small scales Large scales SUMMERWINTER

Small-scale transient eddies of the 700-hPa wind magnitude BB LB1LB2 LB3LB4 (m/s) R* LB1: 79% LB2: 68% LB3: 65% LB4: 78% SUMMER BB LB1LB2 LB3LB4 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 LB1LB2 LB3LB4 BB LB1LB2 LB3LB4

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

Small-scale transient variance ratio: LB / BB (relative humidity) (%) SUMMERWINTER Did summer really heal our RCM ? 1- Flow characteristics (a high residency time) 2- Intense convective processes 3- 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, 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, 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, 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, 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 rvort SUMMERWINTER

Summer and winter flows over Québec SUMMERWINTER