Study of internal variability in regional climate models:Study of internal variability in regional climate models:

Application of the ensemble technique Application of the ensemble technique to CRCM simulationsto CRCM simulations

Léo SeparovicLéo Separovic

Director: René LapriseDirector: René LapriseCo-director: Ramon Co-director: Ramon de Elía de Elía

ContextContext• As a consequence of internal variability, RCM variables can be regarded as composed of: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; (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. (II) Irreproducible part, affected by internal chaotic variations, that must be considered in stochastic form.

ObjectiveObjective• To examine partition of spatiotemporal variability downscaled by RCM simulations between these two 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 components by means of geographical and length-scale distribution of reproducibility of time-dependent RCM variables and their seasonal averages.RCM variables and their seasonal averages.

ApproachApproach• Ensemble of 20 CRCM simulations with perturbed initial conditions is conducted for one summer season 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 (J-J-A 1993) over a large mid-latitude domain with lateral boundary conditions derived from NCEP reanalyses (Alexandru reanalyses (Alexandru et alet al., 2007). ., 2007).

• The reproducible part is assessed by the ensemble mean, the stochastic part is sampled by the 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 ensemble deviations. Reproducibility of a simulated variable is evaluated utilizing the ratio of variances of these two components. these two components.

• Power spectra of simulated variables and their reproducible and irreproducible parts provide length-scale Power spectra of simulated variables and their reproducible and irreproducible parts provide length-scale distribution of reproducibility.distribution of reproducibility.

• Geographical distribution of reproducibility is assessed by temporal variances of the two components.Geographical distribution of reproducibility is assessed by temporal variances of the two components.

Illustration of resultsIllustration of results

• Two snapshots of geopotential at 925 hPa “seen” by different simulations (left) and corresponding power spectra 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 (right) - total power, reproducible power and irreproducible power. Also shown are power spectra computed for relative vorticity.vorticity.

totaltotalreproduciblereproducibleirreproducibleirreproducibleregridded NCEPregridded NCEP

GEOP925 1993.07.25.0000GMTGEOP925 1993.07.25.0000GMTGEOP925 1993.07.25.0000GMTGEOP925 1993.07.25.0000GMT RVRT925 1993.07.25.0000GMTRVRT925 1993.07.25.0000GMT

GEOP925 1993.06.20.0000GMTGEOP925 1993.06.20.0000GMT

GEOP925 1993.06.20.0000GMTGEOP925 1993.06.20.0000GMT RVRT925 1993.06.20.0000GMTRVRT925 1993.06.20.0000GMT

• Average reproducibility of small scales in instantaneous anomalies: ratio of the reproducible and total temporal 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 variance in (%). Length scales larger than 500km are filtered from simulated fields prior to the computation of reproducibility. reproducibility.

GEOP 925hPaGEOP 925hPavertical cross-section along the arrow (on the left) vertical cross-section along the arrow (on the left)

• Power spectra computed for Power spectra computed for seasonal seasonal precipitationprecipitation. The irreproducible part is not . The irreproducible part is not negligible for small-scales:negligible for small-scales:

totaltotalreproduciblereproducibleirreproducibleirreproducible

• Time average partition of spectral Time average partition of spectral power between the two components power between the two components for for instantaneous precipitationinstantaneous precipitation: :

totaltotalreproduciblereproducibleirreproducibleirreproducible

• Reproducibility (reproducible/total SQRT Reproducibility (reproducible/total SQRT power) of instantaneous precipitation in (%) as power) of instantaneous precipitation in (%) as function of the integration time at various length function of the integration time at various length scales: yellow 2250km, black 900km, pink scales: yellow 2250km, black 900km, pink 450km, blue 225km, red 112.5km:450km, blue 225km, red 112.5km:

over over continentcontinent

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