1. Extra-tropical Flow Dynamics & THORPEX: Some Core Aspects
Huw C. Davies
Institute for Atmospheric and Climate Science, ETH Zurich
Switzerland
Opening Session, TTISS, Monterey. September, 2009

2. BACKGROUND : A Notable Success Story BUT
PREDICAMENTS
Evolution of ECMWF forecast skill for the extra-tropical Northern Hemisphere
Unsustainable rate of progress ?
20**
Strategy void ?
Relevance to the “beyond” time-frame ?

3. Recent Trends PREDICAMENT Update of Same Metric
- displayed on a log-scale

4. Can there be a Strategy for Day 4-10 NWP ?
Extra-tropical Flow Dynamics & THORPEX: Some Core Aspects
OUTLINE
PREDICAMENT I:
” End is nigh” for progress in Day 1-3 NWP ?
PREDICAMENT II:
Can there be a Strategy for Day 4-10 NWP ?
PREDICAMENT III:
Beyond the Medium-range - THORPEX Territory ?

5. Metrics & Potential Vorticity
PREDICAMENT I
Instantaneous PV
distribution on 320K
Normalized
PV gradient on 320K
PV distribution
on the 850 hPa surface

6. From a PV Perspective : Scope for progress !!! PREDICAMENT I
RMS ‘PV error’ for the
Atlantic-European Sector
Conventional time-trace for the RMS error
Upper
troposphere
Mid & lower
troposphere
Dirren et al., GRL, 2003

7. Can there be a Strategy for Day 4-10 NWP ?
Extra-tropical Flow Dynamics & THORPEX: Some Core Aspects
OUTLINE
PREDICAMENT I:
” End is nigh” for progress in Day 1-3 NWP ?
PREDICAMENT II:
Can there be a Strategy for Day 4-10 NWP ?
PREDICAMENT III:
Beyond the Medium-range - THORPEX Territory ?

8. Distinctive Error Pattern (- at Day 3-4)
PREDICAMENT II
on January 16, 2002
Analysis
FC96
Forecast - Analysis
Davies, 2006

9. TYPICAL Is the instantaneous pattern ..... & ...... ?
PREDICAMENT II
TYPICAL
DIFFERENCE or ERROR ?
PV distribution
on 310 K surface
Flight path
Davies, 2006

10. Cause(s) & Cure ? PREDICAMENT II POSSIBLE CAUSE :
Perturbation of the wave-guide by a juxtaposed PV anomaly ? PV
anomaly
Wave-
guide
Schwierz et al. J. Atmos. Sci., 2004

11. Why focus on precursors ?
DYNAMICS
&
WAY AHEAD
Why focus on precursors ?
A : Influence akin to “Forecast-Analysis” Difference fields
B : Plausible, Credible & Significant
- large-amplitude & proximity to wave guide betokens a major forcing,
- sub-synoptic scale & rich physics of precursors poses
observational/analysis/assimilation challenges, and
- downstream influence implies possible “dynamical memory”.
C : Anomalies have an extended temporal pre-existence
(exploitable ?)

12. High Impact Event A Phenomenological Perspective for the WAY AHEAD
PREDICAMENT II
TIME
High Impact Event
Synoptic/sub-synoptic evolution
leading to the event
Downstream propagation
on wave-guide
Short
Range
Precursor’s perturbation of wave-guide
Evolution & Origin
of Precursor
Medium
Range
Pacific/N. America? North Atlantic? Western Europe Alps
LONGITUDE

13. Character of Precursors
DYNAMICS
&
WAY AHEAD
Character of Precursors
TYPE I:
PV anomalies in the
Lowermost Stratosphere
localized high PV,
ubiquitous,
large amplitude,
life-time of a few days
significant frequency.
Kew et al., 2009

14. Take-home assertion : need to capture origin and evolution
DYNAMICS
&
WAY AHEAD
Role of high PV precursors
PV anomaly
Take-home assertion :
For Day 4-10 forecasts
need to capture origin and evolution
of positive PV anomalies !
14 Oct, 00 UTC
15 Oct, 12 UTC
16 Oct, 18 UTC

15. Credible & Significant Precursors ?
DYNAMICS
&
WAY AHEAD
Credible & Significant Precursors ?
TYPE II:
PV anomalies in the
Upper Troposphere
- band of “low” PV
numerous
large relative amp.
- aligned ahead of troughs,
- diabatic / tropical origin
Kew et al., 2009

16. Origin of “low-PV“ precursor bands
DYNAMICS
&
WAY AHEAD
Origin of “low-PV“ precursor bands
Lagrangian backward trajectories from a “low-PV” band

17. Evolution of “low-PV“ precursor bands
DYNAMICS
&
WAY AHEAD
Evolution of “low-PV“ precursor bands

18. Take-home assertions:
DYNAMICS
&
WAY AHEAD
Cause(s) of Error Pattern ?
Diagnosis of PV Error:
Lagrangian history (96 hour) from positive & negative “PV error” regions
Take-home assertions:
(i) Cloud physics / dynamics needs to capture
origin & evolution negative PV anomalies.
(ii) THE ultimate “dynamics test” of CLOUD PHYSICS
for Day 4-10 forecasts:
“Does the correct PV distribution develop aloft” !!
hPa AN FC
pvu
Didone & Davies, 2006

19. Rudiments of a Strategy ?
Desiderata for Day 4-9 HIE Forecasts
PREDICAMENT II
Rudiments of a Strategy ?
Need to
- capture origin, structure & dynamics of specific identifiable “event” precursor,
(improve mesoscale short-range skills)
- represent the subsequent far-downstream propagation of their effect,
(accurate representation of jet structure & evolution)
- exploit ensembles to help evaluate likelihood of an event’s occurrence
(flow dependent structure functions & tailored ensembles)

20. Can there be a Strategy for Day 4-10 NWP ?
Extra-tropical Flow Dynamics & THORPEX: Some Core Aspects
OUTLINE
OUTLINE
PREDICAMENT I:
” End is nigh” for progress in Day 1-3 NWP ?
PREDICAMENT II:
Can there be a Strategy for Day 4-10 NWP ?
PREDICAMENT III:
Beyond the Medium-range - THORPEX Territory ?

21. Beyond the Medium-range = THORPEX ?
PREDICAMENT
III
Beyond the Medium-range = THORPEX ?
STRATEGY & RATIONALE
seek to predict the prevailing patterns of climate variability (PCV),
CONVENTIONAL CAUSAL CHAIN
S-Forcing PCV Character of Weather Systems
AN ALTERNATIVE CAUSAL CHAIN
S-Forcing Weather Systems
PCV INFERENCE !!!

22. A Specific Example:
European Winter of
PREDICAMENT
III

23. Accurate representation/prediction of individual weather systems
Exploratory NWP Model Study
Weather Systems
& PCV
HYPOTHESIS
Positive upstream anomalies in SST and land surface temperature
impact positively upon Block formation
INFERENCE
Accurate representation/prediction
of individual weather systems
is important
to capture the PCV !!!
Verification
ECMWF Analysis
Control
Simulation
Croci-Maspoli & Davies, Mon. Wea. Rev., 2009

24. SUMMARY Predicament I : - scope for progress
Predicament II : A Way Ahead ?
- focus on precursors (and wave-guide)
Predicament III:
Beyond = THORPEX TERRITORY !!
- significant influence of weather systems