1. Sensitivity Studies James Done NCAR Earth System Laboratory National Center for Atmospheric Research NCAR is Sponsored by NSF and this work is partially supported by the Willis Research Network and the Research Program to Secure Energy for America

2. Climate Change Scenario Emissions scenario uncertainty Climate change uncertainty

3. Choice of Base Period The 20-year period of 1975-1994 was chosen based on: - need to smooth out influences of El Niño - quality of data in early period is poor - exclude apparent climate shift in 1995 Caution: Multi-decadal variability, climate trends and shifts. - 1975-1994 had an average 8.9 TCs/yr - 1995-2005 had an average 14.3 TCs/yr Plan to test another base period.

4. Multi-Decadal Variability Global Model has a signal on multi-decadal timescales although the physical realism is in question. Can not expect global model variability to be in correct phase with real climate. NRCM time-slice approach shows 11-year windows into the full period. Complimentary downscaling approaches cover the full period.

5. Physics Parameterizations NRCM physics choices based on: Seasonal sensitivity tests. Model vs observed spatial distributions of tropical rainfall. A 10-year simulation of current climate driven by global reanalysis data showed reasonable global distributions of tropical cyclones. Observation 1995 - 2000 Regional Model 1995 - 2000

6. NRCM Resolution Simulations of the 2005 North Atlantic Hurricane Season 36 km based on available resources on NCAR’s bluefire and the success of previous simulations of tropical cyclones in current climate at 36 km Currently running at 12 km at Argonne National Lab 36 km 18 Storms 12 km 29 Storms 27 Storms Observed

7. Internal Variability (IV) Simulations are sensitive to small perturbations in the initial state due to non-linear dynamic and thermodynamic relations and multi- scale feedbacks that govern the climate system. Puts a predictability limit on details of weather to about 2 weeks. Open question: What is the impact of IV on tropical cyclones? - could be important for large regional domains. Aim: Distinguish the reproducible climate change signal associated with external forcing from the component due to IV.

8. Sensitivity to Initial Conditions 10 member ensemble Cold starts, 10 days apart, from 1 st Jan 2047 to 1 st Apr 2047 Total column water vapor for September 2047. (top) Cold start 1 st Jan 2045 (middle) Cold start 1 st Jan 2047 (bottom) Cold start 1 st Apr 2047

9. Sensitivity to Initial Conditions Tropical Cyclones in 2047: Average number15.3 Standard deviation 2.0 Range 13 - 20 variance/mean =0.26 Currently exploring methods to quantify IV and consequences for our climate variability and change results. Outliers suggest IV may be non-Gaussian.

10. Impact of Spectral Nudging 2005 Aug-Sept-Oct (mm dd) Observation 15 Storms

11. Impact of Spectral Nudging 2005 Aug-Sept-Oct (mm dd) Observation 15 Storms Spectral Nudging 9 Storms Control 12 Storms

12. Summary A thorough investigation into the sensitivities of our modeling system is essential for assessing confidence in our climate variability and change results. Source of Uncertainty Approach Climate change scenario3 emissions scenarios Base periodTests are planned Multi-decadal variability3 complimentary downscaling methods PhysicsPrevious experience Resolution2 resolutions Internal variabilityComprehensive assessment underway Spectral nudgingChoose not to apply