1. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Dr. Lee Tryhorn NOAA Climate Literacy Workshop 12-14 April 2010 NOAA Northeast Regional Climate Center

2. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Contents Motivation Different types of downscaling Statistical Downscaling Dynamical Downscaling Examples of downscaling in northeast assessments Northeast Regional Climate Center

3. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Why do we need downscaling?

4. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Why do we need downscaling?

5. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Smoothed topography

6. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Regionally, resolution matters 200 x 300 km 75 km

7. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu There is a mismatch in scales between the information that global climate models can supply and what is needed for local decision making. 300km 50km 10km Point source

8. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Downscaling Resolves the mismatch in scale between the coarse spatial resolution of climate model output and the need for weather and climate information at a higher resolution Extreme rainfall estimates Impact Assessment Models

9. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Two main types... Statistical (Empirical) Uses a statistical model to link large-scale climate variables to local climate characteristics Dynamical Most common type uses a mesoscale, physically-based regional climate model (RCM) Both have their origins in operational weather prediction.

10. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Statistical Downscaling Relationships derived between large-scale atmospheric variables (predictors) and observed local weather variables (predictands). These relationships are then applied to equivalent variables in the GCM. Predictand = f(Predictor) Sub-grid variable = f(large-scale variables)

11. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Statistical Downscaling PROSCONS Computationally inexpensiveAssume that the derived relationships do not change as the climate is perturbed. Easily tailored for specific locations, scales, and problems Do not describe the physical processes that affect climate Ability to downscale ‘exotic’ predictands (e.g. tidal surges, air quality) Requires observational data at the required scale Can be used to create site specific information. Requires access to the necessary predictor variable data

12. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Predictands Typically temperature or precipitation Can be unusual variables such as cloud cover, snow-cover duration, sea-level anomalies. Will govern the choice of statistical method used.

13. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Predictors Predictors are variables of relevance to the local climate variable being derived (the predictand) Should be reproduced realistically by the GCM Should be able to account for the observed variation in the predictand No universally optimal set of predictors

14. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Commonly used SD techniques Spatial interpolation Change factors Weather classification Transfer functions (e.g. regression methods) Weather generators

15. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Spatial Interpolation NYCDEP Climate Change Task Force

16. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Change factors (Delta Method) ClimAID (NYS Climate Adaptation Assessment)

17. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu GCM grid downscaled to regional grid Start with three datasets: (1) an observationally based gridded dataset of 20 th century surface climate conditions, aggregated from a spatial resolution of 1/8° (~12 km) to the GCM grid; (2) a given GCM's simulation of 20 th century climate simulation; and (3) the GCM's 21 st century climate simulation. Bias-Corrected Spatial Disaggregation

18. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Bias-Corrected Spatial Disaggregation Bias-correction: model distribution is changed to match observed distribution Assumes that shifts in climate variables occur with different magnitudes at different points along the distribution

19. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu F A E D C B (C) Scale factor = (A) BC GCM/ (B) Obs mean (F) = (D) *(E) Scale factor is interpolated

20. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Bias-Corrected Spatial Disaggregation North-East Climate Impacts Assessment (NECIA)

21. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Calibrate model Set model structure Choose predictors (NCEP) Choose predictand (obs) Analyze results Generate downscaled predicand Apply model Extract GCM predictors The Statistical Downscaling Model (SDSM)

22. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu SDSM Output

23. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Dynamical Downscaling Typical grid ~20-50km Regional climate models work like numerical weather prediction (NWP) models, except that they run for a much longer time Other types include global models with variable resolution (e.g. ARPEGE) North American Regional Climate Change Assessment Program (NARCCAP)

24. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu

25. Nested RCMs need boundary conditions from GCMs or Reanalysis The World Ends Here

26. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu How does dynamical downscaling with nested RCMs work? Run the global model, storing output several times per day (e.g. every 6 hours) Interpolate global model results to regional grid Periodically update the regional model in a zone around its boundaries using results from the global model Results from global model

27. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Dynamical downscaling PROSCONS Can produce internally consistent climate behavior in response to the full range of climate forcings Computationally expensive Many variables availableAny GCMs biases will be inherited by the RCM Better representation of some weather extremes than GCMs Current parameterizations may not hold in future climates Can be used in areas with little observational data High frequency (e.g. 6-hourly) time dependent GCM fields are needed.

28. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Hayhoe et al. 2008

29. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Summary Statistical and dynamical downscaling each have relative strengths and weaknesses. No “best” approach to downscaling Hybrid techniques – the future?? Combining statistical and downscaling to take advantage of the strengths of each E.g. Statistical downscaled temp and precip are fed into a high resolution hydrological model to get snow/evaporation

30. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Questions to consider Carefully consider the objectives of the project Is high-resolution climate information really needed for the application? Is the effort involved in producing the high-resolution climate information appropriate in the context of all the uncertainties associated with the project?

31. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Where are downscaled data available? NARCCAP http://www.narccap.ucar.edu/ SDSM software http://www.sdsm.org.uk NRCC (a future source – beginning to work on it!)

32. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu NOAA Northeast Regional Climate Center (NRCC) Located in the Department of Earth and Atmospheric Sciences at Cornell University. Mission is to facilitate and enhance the collection, dissemination and use of climate data and information, as well as to monitor and assess climatic conditions and impacts in the Northeast

33. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Addressing present and future climate impacts

34. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu By linking research with models, and data

35. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Working with stakeholders to provide relevant tools

36. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Serving as an authoritative source of climate products and information

37. NOAA Northeast Regional Climate Center www.nrcc.cornell.edu Questions?