Influence of large-scale nudging on RCM’s internal variability UQÀM Université du Québec à Montréal Influence of large-scale nudging on RCM’s internal variability Adelina ALEXANDRU Chercheurs coordonnateurs: René LAPRISE Ramon de ELIA

Despite the fact that RCMs are constrained by lateral boundary conditions (LBC), recent studies have shown that RCMs also exhibit internal variability. In other words, the lateral boundaries do not determine a unique solution inside of the domain; several different states could be consistent in the interior for the same set of lateral boundary conditions. In fact, any source of noise, be it in initial conditions, boundary conditions or parameterizations, may cause deviations between RCM’s runs. The present study is focused on the Canadian RCM’s internal variability and its consequences on seasonal statistics, giving special emphasis to the impact of large-scale nudging on internal variability. The work is based on a series of experiments performed on several different domains over North America. Each experiment consists in an ensemble of 15 three-month simulations differing only in initial conditions (IC) performed under a given large-scale nudging configuration. The influence of internal variability at the seasonal scale is appreciated by the variation of the seasonal-mean field. The spread between seasonal averages of the ensemble members is estimated as the square root of the variance between individual member seasonal averages. Changes in large-scale nudging configuration mostly affect the intensity with which the CRCM simulation is forced to follow the driving fields. Preliminary results show that large-scale nudging diminishes in general the internal variability, although this does not occur in all cases. The general consequences and overall effect of the use large-scale nudging will also be analyzed.

NCEP Reanalyses without spectral nudging with spectral nudging L 159 Exemple of 5 similar 850-hPa Geopotential height runs with a delay of 24 hours in the initial conditions (valid at 0000 UTC 25 July 1993)

the ensemble (precipitation field) Without spectral nudging Domain size:140140 120120 100100 8080 mm/d Domain size:140140 With spectral nudging 120120 100100 8080 Square root of the variance between 15 individual member seasonal averages of the ensemble (precipitation field)