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

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 ?

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

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 ?

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

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

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 ?

Distinctive Error Pattern (- at Day 3-4) PREDICAMENT II PV@320K on January 16, 2002 Analysis FC96 Forecast - Analysis Davies, 2006

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

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

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 ?)

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

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

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

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

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

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

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

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)

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 ?

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 !!!

A Specific Example: European Winter of 2005-06 PREDICAMENT III

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

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