Application of the ensemble technique to CRCM simulations Study of internal variability in regional climate models: Application of the ensemble technique to CRCM simulations Léo Separovic Director: René Laprise Co-director: Ramon de Elía

Context Objective Approach As a consequence of internal variability, RCM variables can be regarded as composed of: (I) Reproducible, externally forced part, associated with the lateral boundary and surface forcing; (II) Irreproducible part, affected by internal chaotic variations, that must be considered in stochastic form. Objective To examine partition of spatiotemporal variability downscaled by RCM simulations between these two components by means of geographical and length-scale distribution of reproducibility of time-dependent RCM variables and their seasonal averages. Approach Ensemble of 20 CRCM simulations with perturbed initial conditions is conducted for one summer season (J-J-A 1993) over a large mid-latitude domain with lateral boundary conditions derived from NCEP reanalyses (Alexandru et al., 2007). The reproducible part is assessed by the ensemble mean, the stochastic part is sampled by the ensemble deviations. Reproducibility of a simulated variable is evaluated utilizing the ratio of variances of these two components. Power spectra of simulated variables and their reproducible and irreproducible parts provide length-scale distribution of reproducibility. Geographical distribution of reproducibility is assessed by temporal variances of the two components.

Illustration of results Two snapshots of geopotential at 925 hPa “seen” by different simulations (left) and corresponding power spectra (right) - total power, reproducible power and irreproducible power. Also shown are power spectra computed for relative vorticity. GEOP925 1993.07.25.0000GMT GEOP925 1993.07.25.0000GMT RVRT925 1993.07.25.0000GMT total reproducible irreproducible regridded NCEP GEOP925 1993.06.20.0000GMT GEOP925 1993.06.20.0000GMT RVRT925 1993.06.20.0000GMT

Time average partition of spectral power between the two components for instantaneous precipitation: Reproducibility (reproducible/total SQRT power) of instantaneous precipitation in (%) as function of the integration time at various length scales: yellow 2250km, black 900km, pink 450km, blue 225km, red 112.5km: Power spectra computed for seasonal precipitation. The irreproducible part is not negligible for small-scales: total reproducible irreproducible total reproducible irreproducible Average reproducibility of small scales in instantaneous anomalies: ratio of the reproducible and total temporal variance in (%). Length scales larger than 500km are filtered from simulated fields prior to the computation of reproducibility. GEOP 925hPa vertical cross-section along the arrow (on the left) over continent over ocean