Downscaling Ability of the NCEP Regional Spectral Model v.97: The Big Brother Experiment

1. Large Limited Area High Resolution Model Small Limited-area High Resolution model Deborah Herceg* Adam Sobel*†‡ Liqiang Sun‡ Stephen Zebiak‡ *Department of Applied Physics and Applied Mathematics Columbia University New York, NY †Department of Earth and Environmental Sciences Columbia University New York, NY ‡International Research Institute For Climate Prediction Columbia University Palisades, NY dh2006@columbia.edu Tropical Domain Results BIG BROTHER LITTLE BROTHER Setup of the Big Brother Experiment: Motivation: The Big Brother Experiment methodology allows us to assess the errors in regional climate simulations which are solely due to the nesting strategy. This experiment was first performed by Denis et al. (2001), using a grid point model for a midlatitude winter simulation. Our study extends theirs by using both a tropical and a midlatitude domain case, and using a different (in this case, spectral) model. The Big Brother Experiment (BBE) • STEPS: • We first perform a large-domain high resolution regional model simulation, the “Big Brother”. • This reference simulation is then degraded by filtering out short-scales which would be unresolved in the GCM • This filtered reference is then used to drive the same regional climate model, integrated at the same resolution as the Big Brother, but over a smaller domain embedded in the large domain, called the Little Brother • We then compare the climate statistics of the Little Brother with those of the Big Brother on the Little Brother Domain • Since the same model is used for both the Big Brother and Little Brother experiments, the climate of the two should in principle be identical. Any difference in the solutions can only be attributed to the nesting strategy, and not to model error. Global Low-Resolution Data Set IC and LBC Conclusions: Large regional High Resolution Reference Simulation • In most fields, good agreement between Big and Little Brothers was obtained, for both small and large scales, after a brief initial spin-up period. • Precipitation was the exception. For our tropical domain experiment, the small-scale features of the precipitation field in the Big Brother were not reproduced by the Little Brother. The spatial and temporal variations of the large-scale component of the precipitation were well reproduced. • The reason for the poor reproduction of the small-scale component of the precipitation is that this component is inherently unpredictable, at least in this model, rather than because of any inherent flaw in the nesting strategy. This was shown by an ensemble of identically forced Little Brother simulations which were initialized at different times differing by 6-24 hours, in which the correlations of the small-scale precipitation between these different simulations was nearly as low as that between the Big and Little Brothers. Related to this, the agreement between Big and Little Brothers is fairly insensitive to the choice of the cutoff wave number used for the filtering. • Good reproduction of the small-scale precipitation was obtained for a wintertime midlatitude simulation. Thus, the relatively poor reproduction of this field for the tropical case appears to be due to specifically tropical (convective) precipitation processes. LEGEND: IC = Initial Condition LBC = Lateral Boundary Condition Filter Small Scales LBC IC Model Corresponding Data Set Regional Climate Validation Small regional High Resolution Simulation Downscaling Ability of the NCEP Regional Spectral Model v.97: The Big Brother Experiment Midlatitude Domain Results Our Experiment : Domains BIG BROTHER LITTLE BROTHER TROPICAL DOMAIN: MIDLATITUDE DOMAIN: • One month simulation completed for April, 1994 • Both domains centered at 37.5°W and 5°S • Horizontal resolution of 60 km • Big Brother domain at 181 x 182 grid points • Little Brother domain at 91 x 92 grid points • 18 vertical layers • One month simulation completed for February, 2001 • Both domains centered at 70°W and 47°N • Horizontal resolution of 50 km • Big Brother domain at 151 x 142 grid points • Little Brother domain at 81 x 77 grid points • 18 vertical layers LITTLE BROTHER BIG BROTHER LITTLE BROTHER BIG BROTHER Experimental Details: “Ensemble” Simulations Results • The Model: NCEP Regional Spectral Model, v. 97 – Juang et al., 1997. • ECHAM4.5 data, archived every 6 hours was used to drive the Big Brother simulation. • The Big Brother simulation data was archived every 6 hours for the nesting of the Little Brother simulation, as well. • All experiments were executed for a month. • All of the statistical computations were performed excluding the first day in order to reject any short time-scale spin-up phenomenon. A ten grid point lateral boundary zone was also excluded. • To mimic the resolution of the operational GCM (ECHAM at T-42 resolution), a cutoff wavelength of ~900 km was used in the filtering of the Big Brother output before using it to drive the Little Brother. • PRECIPITATION FIELD ENSEMBLES • The table on the left shows the “ensemble” results comparing the identically forced Little Brother simulations, but initialized at different times differing by 6-24 hours; the correlations of the small-scale precipitation between these different simulations was almost as low as that between the Big and Little Brother “ensemble” simulations (not shown here). Table Legend: • Total Field (TOTAL) is separated into a small-scale component (SW), and large-scale component (LW); defined as the wave numbers above and below the cutoff wave number. • Stationary Correlation (Stat. Corr.) = spatial correlation of time-mean fields. • Stationary Variance Ratio (Stat. Var. Ratio)=ratio of variances of time-mean fields. • Transient Correlation (Trans. Corr.), Transient Variance Ratio (Trans. Var. Ratio) = spatial correlation, variance ratio of transient fields (total-stationary). • Statistics provided for sea and land separately, as well as total. NOAA 29th Annual Climate Diagnostics and Prediction Workshop; October 18-22, 2004, Madison, Wisconsin