1. Extreme cold events over Asia: role of stratospheric wave reflection By Debashis Nath & Chen WEN International Workshop on High Impact weather Research World Meteorological Organization, 20-23 January, 2015, Ningbo, China

2. Introduction January 2008 and 2013 experienced record breaking and persistent cold events, snowfalls and freezing rain over Eurasia and parts of China. 2008 extreme event caused extensive damage and general disruption over southern China. Most of the infrastructure damages were attributed to freezing rain, which led to a huge number of broken power transmission lines and chaotic traffic conditions for southern China. The disastrous heavy snow caused serious direct economic losses of 53.8 billion RMB (Zhou et al. 2009). 2013 cold event over Eurasia is triggered by sudden stratospheric warming event with persistent long duration. This favors the planetary waves to reflect back from the stratosphere and low level advection of cold air from the high latitudes. Data Used : NCEP & ECMWF daily fields.

3. Case 2013

4. 27 Dec.-31 Dec. 2012 (P1) (Prewarming) 26 Jan.-30 Jan. 2013 (P7) (SSW decay phase) 01 Jan.-05 Jan. 2013 (P2) 31 Jan.-04 Feb. 2013 (P8) 06 Jan.-10 Jan. 2013 (P3) (SSW developed phase) 05 Feb.-09 Feb. 2013 (P9) 11 Jan.-15 Jan. 2013 (P4) 10 Feb.-14 Feb. 2013 (P10) (Postwarming) 16 Jan.-20 Jan. 2013 (P5) 15 Feb.-19 Feb. 2013 (P11) 21 Jan.-25 Jan. 2013 (P6) 20 Feb.-24 Feb. 2013 (P12) Time evolution of the SSW 2013 event - ve index : Reflective state + ve index: Non Reflective state Perlwitz & Harnik, 2004

5. P1 P2 P3 P4 P5P6P7P8 P9P10P11P12 EP flux for total wavenumber i.e. the eddy flux (arrows) Zonal mean zonal wind (background contours) EP flux

6. Vertical component of Plumb flux Fz at 100 hpa (arrows) F horizontal at 100 hpa (contour) P1 P2 P3 P4 P6P5

7. Plumb Eddy flux, 60-80 N (P1-P6) Plumb wave activity Eddy flux (arrows) Eddy geopotential height field (contour)

8. Geopotential height field at 500 hpa (P1-P6) Eurasian trough due to reflection

9. Temperature anomaly at 850 hpa

10. Case 2008

11. Reflective Index Zonal mean, temporal mean of reflective Index may not be true always ??? Perlwitz & Harnik, 2004 - ve index : Reflective state + ve index: Non Reflective state

12. Eliassen Palm Flux, Zonal mean zonal wind Refractive Index - ve index : wave evanescent region (shaded) +ve index : wave propagation region (masked) Zonal mean zonal wind (westerly) Zonal mean zonal wind (easterly)

13. Plumb Vertical Wave activity flux Plumb Wave activity Flux Fx (zonal) Fy (meridional) Fz (vertical) Baffin island and the coast of Labrador Eurasia South Central Russia and China Upward Downward

14. Thermal advection, Temperature anomaly and Blocking pattern Source Sink Wave reflection Trigger Cold Advection, Cold weather over the tropics/subtropics (1) Horizontal component of Wave (2) Blocking Index Thermal Advection Fx-Fz (arrows), Fz (contour), WN1, 200 hpa, 30-70 N

15. Summary and Conclusions During winter 2008 and 2013 the stratospheric background state is reflective In 2008, planetary waves propagate up from Baffin Island and coast of Labrador and reflect back to Eurasia. In 2013 during SSW events planetary wave reflection occurs over Eurasia. This triggers low level advection and brings cold air from high latitude to Eurasian continent and parts of China. This reduces the temperature by 22K within 5 days and brought cold weather over troposphere.

16. Thank You