1. Aspects of Dynamical Processes and Predictability in Medium Range Forecasts of High Impact Weather
Edmund K.M. Chang
School of Marine and Atmospheric Sciences
Stony Brook University
Stony Brook, New York, USA
WWRP/THROPEX HIW Workshop
Karlsruhe, Germany
March 18, 2013

2. Alternative Title
Some thoughts on predictability and dynamical processes
Using Rossby Wave Trains (RWTs, or baroclinic wave packets) as an illustrative example

3. From THORPEX International Science Plan
(Shapiro and Thorpe, 2004)
Cyclogenesis near Japan
Flooding over Europe

4. Dundee Satellite Station: 1241 UTC 11 Aug. 2002
The Storm
France
Italy
Dundee Satellite Station: 1241 UTC 11 Aug. 2002
Courtesy of Mel Shapiro

5. Dresden Germany
Courtesy of Mel Shapiro

6. THORPEX Research Objectives:
Investigate the evolution of dynamical and physical processes and their influence on forecast skill
The skill of forecast systems in predicting Rossby wave properties (amplitudes, ray paths, group velocities …)
The initiation of Rossby wave trains by various processes (tropical convection, extratropical cyclones, large-scale topography …)
The initiation of tropical convection by Rossby wave-trains propagating into the tropics
From Shapiro and Thorpe (2004)

7. Examples of progress since 2004

8. New techniques developed to highlight RWTs

9. Zimin et al. (2006): Extracting envelopes of nonzonally propagating Rossby Wave Packets

10. Martius et al (2006): A refined Hovmoller diagram:
- Hovmoller diagram constructed following the 2 PVU PV contour instead of along a fixed latitude band

11. Linkage between RWTs and high impact weather events strengthened

12. Martius et al (2008)
- Wave packet signal precedes Alpine heavy precipitation events, especially in SON and DJF

13. Eichorn and Wirth (2013)
Chang (2005): RWT signal traced back to Asia at least 3 days prior to strong cyclone events over NW Pacific
Eichorn and Wirth (2013):
RWT signal traced back to Northeastern Pacific at least 6 days prior to strong cyclone events over central Europe
Presence of RWT over NE Pacific significantly enhance probability of strong cyclone over Europe 6 days later

14. Characteristics of RWTs examined

15. Glatt et al. (2011): Used Hovmoller diagrams to examine multiple processes associated with RWTs

16. ET vs. Winter Rossby Waves (Torn and Hakim 2013) - composites based on 112 ET and 281 winter cyclones
Amplitude

17. Linkage suggested between RWTs and growth of uncertainties in medium range EPS

18. Analysis of V300
Majumdar et al. (2010):
- “… distinctive targets could be traced upstream near Japan at lead time of 4-7 days. In these cases, the flow was predominantly zonal and a coherent Rossby wave packet was present over the northern Pacific Ocean.”
Zheng et al. (2013):
Used ensemble sensitivity analysis to study U.S. east coast snowstorm on December 2010
Sensitivity signal developed and propagated across the Pacific accompanying the development and propagation of a RWT
Sensitivity signal
From Zheng et al. (2013)

19. Many issues still remain:
Process Studies:
Initiation? Dissipation?
What physical processes control propagation, duration, coherence, amplitude, frequency?
How do atmospheric low frequency variability impact RWTs?
How do RWTs impact low frequency variability?
Forecast Science:
How well do our forecast models predict RWTs?
Do (How do) more accurate forecasts of RWTs transfer to better forecasts of high impact weather events?
Does the growth of a significant RWT imply increased or decreased predictability downstream?
Are RWTs initiated by ET less predictable?
SERA:
Can we quantify the socio-economical impacts of RWTs?

20. Challenges: It is difficult to define RWTs exactly (Glatt et al. 2011)
An objective climatology of RWTs is still lacking
Need objective identification and tracking

21. Glatt et al. (2011):
RWT object identification based on Hovmoller diagram

22. An objective tracking algorithm has been developed at SBU
Example of objective tracking results (Courtesy Matt Souders)
Jan 29 – Feb 12, 2009: Focus on packet number 107

23. Ongoing research at SBU:
Process studies:
Climatology and variability
Links to low frequency variability
Dynamics of growth and decay …
Forecast Science:
Verify and calibrate ensemble forecasts of RWTs
Links to forecast uncertainty and error growth

24. Challenges (continued)
Multiple scales in space and time

25. Time/Longitude: 250-mb Meridional Wind (m s-1); 55-40N.
Oct. 12
Oct. 18
Oct. 24
Nov. 3
Japan
Cal.
W. Africa
Courtesy
Mel Shapiro

26. Synoptic-scale phase velocity
Short-range
Synoptic-scale phase velocity
Wave-train group velocity
Medium-range
Time-mean planetary-waves
Sub-seasonal to
Seasonal
Courtesy
Mel Shapiro

27. Multiple scales (continued)
Rodwell et al (2013)
- ECMWF large error forecasts over Europe associated with errors in convective heating over central U.S. which acts to slow the progression of the RWT

28. Challenges (continued)
Forecast verification
Object (process) oriented forecast verification?
Cyclones (Froude et al. 2007, 2009, 2010)
RWTs (or any other object or process)
Need to identify model errors and biases in forecasting dynamical processes
HIW? How (and what) to verify?
Especially in the medium range
Verification of impact?

29. Opportunities (THORPEX and legacy)
International field campaigns (e.g. TNAWDEX)
Availability of high quality data for PDP research
State of the art reanalysis
TIGGE
Reforecast – multi-model multi-ensemble?
Develop effort parallel to CMIP?
Basically free and unrestricted availability of data for broad research community
Broad participation by international community (including data providers and data users)