Regional Climate Downscaling

Research Programs

Climate & Air Quality

To meet it's growing need for regional climate projections to support impact assessments, EPA is developing climate downscaling capabilities using both dynamic downscaling and statistical downscaling techniques. EPA's Atmospheric Modeling and Analysis program is developing a methodology for using the Weather Research and Forecasting (WRF) model to downscale Global Climate Model (GCM) simulations provided by colleagues at NASA's Goddard Institute for Space Studies.

When using coarse-scale data (either from a reanalysis or a GCM) as lateral boundary conditions (LBCs) for a regional model without any further constraint, the interior meteorological fields simulated by the regional model can deviate significantly from those of the driving fields. Four-dimensional data assimilation (FDDA) techniques provide one way to constrain the RCM and keep it from diverging too far from the coarse-scale fields. If the regional model is constrained too strongly to the GCM fields, however, there is the possibility that the benefit of using the higher-resolution RCM will not be realized. What is needed is a delicate balance between the amount of constraint given to the RCM and the freedom of the RCM to simulate its own mesoscale features.

Analysis nudging and spectral nudging are two forms of interior nudging available within the WRF model. These methods have been applied in the literature (e.g., Miguez-Macho et al., 2004; Lo et al., 2008), but they have rarely been compared to each other for climate simulations. Our research will apply each nudging method to reanalysis- and GCM-driven WRF model simlations, with physics options chosen for air quality applications.

Seasonally-averaged (April-June) wind fields at 300hPa as simulated by (a) North American Regional Reanalysis; (b) WRF without nudging; (c) WRF with analysis nudging; and (d) WRF with spectral nudging. Analysis nudging improves WRF's ability to simulate the location and intensity of the jet stream.

Preliminary simulations indicate that nudging is likely needed for both reanalysis- (above) and GCM-driven (below) simulations to maintain large-scale consistency between the driving fields and those simulated within the WRF model.

Differences in July average 500-hPa geopotential height simulated by WRF and ModelE

Mean July 500-hPa geopotential height (m) for (a) GISS ModelE; (b) base WRF run, without any interior nudging; (c) WRF with analysis nudging; and (d) WRF with spectral nudging. Though both nudging techniques are only applied above the planetary boundary layer, both serve to keep the 500-hPa geopotential height simulated by WRF from diverging as far from ModelE.

Differences in July average 2-m temperature simulated by WRF and ModelE

Mean July 2-m temperature (K) for (a) GISS ModelE; (b) base WRF run, without any interior nudging; (c) WRF with analysis nudging; and (d) WRF with spectral nudging. Without nudging, average near-surface temperatures simulated by WRF for the Pacific Northwest are more than 6 K warmer than in the GCM.

Contacts: Jared Bowden, Tanya Otte, Chris Nolte.

Relevant Publications & Presentations:

J. H. Bowden, T. L. Otte, C. G. Nolte, J. A. Herwehe, J. E. Pleim, An assessment of nudging techniques for regional climate modeling: Reanalysis-Driven Simulations American Meteorological Society 22nd Conference on Climate Variability and Change, Atlanta, GA, January 21, 2010.

T. L. Otte, J. H. Bowden, C. G. Nolte, J. A. Herwehe, J. E. Pleim, G. Faluvegi, D. T. Shindell, A demonstration of the value of nudging in downscaled regional climate simulations from a global climate model American Meteorological Society 18th Conference on Applied Climatology, Atlanta, GA, January 19, 2010.

T. L. Otte, J. H. Bowden, J. A. Herwehe, C. G. Nolte, G. Faluvegi, Dynamical Downscaling of NASA/GISS ModelE Using WRF, 2009 CMAS Users' Conference, Chapel Hill, NC, October 20, 2009.

Lo, J.C., Z. Yang, and R.A. Pielke Sr. (2008), Assessment of three dynamical climate downscaling methods using the Weather Research and Forecasting (WRF) model, J. Geophys. Res., 113, D09112.

Miguez-Macho, G., G.L. Stenchikov, A. Robock (2004), Spectral nudging to eliminate the effects of domain position and geometry in regional climate model simlations, J. Geophys. Res. 109, D13104

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Regional Climate Downscaling

Research Programs

Climate & Air Quality

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