von Storch, Hans and Feser, Frauke

1. Downscaling Tropical Cyclones from global re-analysis – Statistics of multi-decadal variability of TC activity in E Asia, 1948-2007 von Storch, Hansand Feser, Frauke Institute for Coastal Research, GKSS Research Center, Germany clisap-Klimacampus, University of Hamburg, Germany

2. Background – wind stats needed: decades, homogeneous analyses • Any assessment of how weather patterns have changed in recent decades requires long and homogeneous time series. • Local time series representing wind speeds are usually not homogeneous, even for a few decades (sensitivity to instrumentation and surrounding). • Homogeneous description of variability of meso-scale storms in recent decades has also not been achieved.

3. Applications: past and future marine weather in N Europe Mixed dynamical/empirical. downscaling cascade for constructing variable regional and local marine weather statistics Simulation with barotropicmodel of North Sea Globale development(NCEP) Dynamical DownscalingREMO or CLM Pegel St. Pauli Empirical Downscaling Cooperation with a variety of governmental agencies and with a number of private companies

4. Climate = statistics of weather The genesis of climate Cs = f(Cl, Φs) with Cl = larger scale climate Cs = smaller scale climate Φs = physiographic detail at smaller scale  “downscaling” von Storch, H., 1999: The global and regional climate system. In: H. von Storch and G. Flöser: Anthropogenic Climate Change, Springer Verlag, ISBN 3-540-65033-4, 3-36

5. Concept of Dynamical Downscaling RCM 3-d vector of state Physiographic detail Known large scale state projection of full state on large-scale scale Large-scale (spectral) nudging

6. Background – wind stats needed: decades, homogeneous analyses Assessing changes of „typhoons“ and „polar lows“-statistics difficult because of insufficient homogeneity and length of data time series. • Polar low project presently done at GKSS. • Typhoon project – more to come.

7. Downscaling SE Asian marine weather • We have implemented the dynamical downscaling approach for SE Asian marine weather. • The key question is – will we master the description of typhoons? • Done: Case studies and seasons – formation of typhoons induced by large scale dynamics and NOT by initial values. • Presently under examination: Continuous 6-decade simulations constrained by NCEP global re-analyses. Feser, F.,and H. von Storch, 2008: Regional modelling of the western Pacific typhoon season 2004, Meteor. Z. 17 (3), 1-10. DOI: 10.1127/0941-2948/2008/0282 Feser, F., and H. von Storch, 2008: A dynamical downscaling case study for typhoons in SE Asia using a regional climate model. Mon. Wea. Rev. 136, 1806-1815

8. A case study: • Simulating tropical storm Winnie (August 1997) • with a regional a climate model • Sensitivity to spectral nudging • Sensitivity to horizontal resolution • with regional atmospheric model CLM; • initiated at or close to 1. August 1997(about 10 days prior to formation) Feser, F., and H. von Storch, 2008: A dynamical downscaling case study for typhoons in SE Asia using a regional climate model. Mon. Wea. Rev. 136, 1806-1815

10. Winnie on 16.8.1997 Conventional RCM-simulations with spectral nudging NCEP

11. Effect of spectral nudging and different initial states CLM run with 0.5º grid Dates refer to time of initialization of simulation

12. Core pressure development

13. Wind speed development

14. Tokage Megi Rainfall patterns in 0.5º grid resolution in double nested 0.5 º / 0.165 º grid resolution

15. 12 TCs in Season 2004 only 10 were found in CLM simulation Following Zhang et al., 2007. Meteor. Atmos. Phys.

18. Is wind = wind?

19. Comparison of core pressure and max wind along the track of 10 storms in 2004 – of original NCEP/NCAR re-analysis and CLM downscaling. Reference ist JMA best track. Brier skill score

20. Complete simulation of 1948-2007using CLM with 0.5º grid resolutionand NCEP/NCAR reanalysis Spectral nudging of scales larges than about 800 km. Results are new; thus may be subject to future revisions!

21. Automatic detection and tracking algorithm Based on digital spatial filtering (spatial scales smaller than 270 km removed) Criteria have subjectively been selected so that the mean number of TCs detected in the simulation is about equal to the mean number given in the best track data.

26. Conclusions • (Some) typhoons can be described by regional atmospheric models run in the climate mode. • Lateral boundary value control is insufficient to reconstruct number and track of typhoons • Higher resolution, obtained through double nesting, gives better results. • But, downscaled (simulated) TCs are too weak in terms of core pressure, maximum pressure fall and maximum sustained wind. • We need better data for validation.

27. Conclusions Preliminary results concerning change • Strong year-to-year variability • Little decadal variability • No overall trend in numbers • Slight upward trends in minimum pressure and max wind since 1990s. • Too early to make an association with global warming. • Not yet examined: regional trends

28. Next workshop: 11+12 March 2009, Taipei or Tainan, Taiwan