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Diagnosing the Downstream Impact of Extratropical Transition Using Multimodel Operational Ensemble Prediction Systems

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Diagnosing the Downstream Impact of Extratropical Transition Using Multimodel Operational Ensemble Prediction Systems Diagnosing the Downstream Impact of Extratropical Transition Using Multimodel Operational Ensemble Prediction Systems Zur Erlangung des akademischen Grades eines DOKTORS DER NATURWISSENSCHAFTEN der Fakultät für Physik des Karlsruher Instituts für Technologie (KIT) Karlsruhe genehmigte DISSERTATION von Julia Henriette Keller aus Leonberg, Baden-Württemberg Tag der mündlichen Prüfung: 13. Januar 2012 Referent: Prof. Dr. Sarah Jones Korreferent: Prof. Dr. Christoph Kottmeier Abstract The extratropical transition of tropical cyclones (ET) often leads to a reduction in predictability for the synoptic development of the cyclones themselves, as well as for downstream regions. The improvement of numerical weather predictions for ET events requires a deeper understanding of the underlying phys- ical and dynamical processes during the interaction of the tropical cyclone with the mid-latitude flow. Furthermore, new alternative approaches are desired to better account for the inherent uncertainty during ET using already existing forecast tools. In this study the predictability during ET events is examined in a new multimodel ensemble data set. Furthermore, the interaction between the transitioning tropical cyclone and the mid-latitude flow is explored in an kinetic energy framework. Recent studies investigated the predictability during ET events based on the variability among mem- bers of single operational medium-range ensemble forecasts. The new THORPEX Interactive Grand Global Ensemble (TIGGE) combines several ensemble prediction systems (EPS) to a new multimodel ensemble. This new data set is used in this study to compare predictability during ET events in the different contributing EPS and to explore the potential benefits of the multimodel approach. TIGGE multimodel forecasts for five ET events are investigated using an empirical orthogonal function (EOF) analysis followed by a subsequent fuzzy clustering, enabling the extraction of the main possible devel- opment scenarios. In the present study, the characteristics of the TIGGE data set during the application of the described analysis technique are explored. It is examined, whether the TIGGE data set shows more variability and thus more possible development scenarios during ET events than the stand-alone ensemble forecast from the European Centre for Medium-Range Weather Forecast (ECMWF). Another important aspect of the study is to investigate, whether the individual EPS contribute differently to the development scenarios. It is shown that the individual EPS differ in their spread and their contributions to the distinct development scenarios. While some of the individual EPS are generally confined to only a few scenarios, other EPS contribute regularly to almost all scenarios. In principle, TIGGE contains more development scenarios than the ECMWF EPS. However, the full range of development scenarios is only found with the ECMWF included in the multimodel EPS. Ensemble forecasts provide possible development scenarios, which may differ strongly during episodes with decreased predictability, like ET events. These forecast scenarios are used in this study to further investigate the underlying physical and dynamical processes causing the distinct developments. There- fore, an analysis of the eddy kinetic energy budget is applied to forecast scenarios, which are extracted from ECMWF ensemble forecasts for the ET of two tropical cyclones. A comparison of the eddy ki- netic energy budget for these scenarios reveals the contribution of the transitioning tropical cyclone to the distinct developments and indicates the relevant processes during ET. It is shown that the duration of baroclinic conversion of eddy kinetic energy and the phasing between the transitioning tropical cyclone strongly influence the impact of the cyclone on the mid-latitude flow. Furthermore, the mid-latitude flow is sensitive to the interaction with a tropical cyclone only during a short time period, thereafter the impact of the albeit strong cyclone on the mid-latitude wave pattern decreases. This study reveals new aspects of predictability and the physical processes during ET and points out future directions for further studies in this field. Kurzfassung Die außertropische Umwandlung von tropischen Wirbelstürmen (extratropical transition - ET) ist oftmals von einer deutlichen Verschlechterung der Vorhersagbarkeit für die synoptische Entwicklung der Wirbel- stürme selbst, sowie für weiter stromabwärts liegende Regionen begleitet. Eine Weiterentwicklung der numerischen Wettervorhersage während ET erfordert ein verbessertes Verständnis der physikalischen und dynamischen Prozesse, die die Wechselwirkung des sich umwandelnden Sturms mit der Strömung der mittleren Breiten bestimmen. Weiterhin besteht Bedarf an der Entwicklung neuer Wege, die es er- möglichen, die Vorhersageunsicherheit während ET mit bereits existierenden Vorhersageprodukten noch besser zu erfassen. In der vorliegenden Arbeit wird zunächst die Vorhersagbarkeit während ET in einem neuen Multimodell-Ensemblevorhersagesystem untersucht. Im zweiten Teil der Arbeit wird die Wech- selwirkung zwischen einem sich umwandelnden tropischen Wirbelsturm und der Strömung der mittleren Breiten anhand eines kinetischen Energie-Budgets näher analysiert. Bisherige Studien konzentrierten sich auf die Untersuchung des Einflusses von ET auf die Vorher- sagbarkeit für stromabwärts gelegene Regionen anhand der Vorhersageunsicherheit in einzelnen oper- ationellen mittelfristigen Ensemblevorhersagen. Das neue “THORPEX Interactive Grand Global En- semble (TIGGE)” vereint mehrere dieser einzelnen Ensemblevorhersagesysteme zu einem neuen Multi- modell-Ensemble. Dieser neue Datensatz ermöglicht einen Vergleich der Vorhersagbarkeit während ET in den einzelnen zu TIGGE beitragenden Ensemblevorhersagesystemen. Zudem können die möglichen Vorteile einer Multimodell-Ensemblevorhersage für ET erörtert werden. In der vorliegenden Arbeit wer- den zehn TIGGE Vorhersagen für fünf ET-Fälle mittels einer Bestimmung der Empirischen Orthogo- nalfunktionen und einem anschliessenden Fuzzy-Clustering der Hauptkomponenten ausgewertet. Diese Analysemethode ermöglicht es, die dominierenden Vorhersageszenarien aus der Ensemblevorhersage zu extrahieren. Ziel der Arbeit ist es, das Verhalten des neuen Datensatzes während der Anwendung der beschriebenen Analysemethode zu untersuchen und die Vorhersagevariabilität in TIGGE während ET zu charakterisieren. Es wird untersucht, ob TIGGE mehr Vorhersagevariabilität und daher mehr mögliche Vorhersageszenarien als das einzelne Ensemblevorhersagesystem des Europäischen Zentrums für mittel- fristige Wettervorhersage (EZMW) zeigt und damit die tatsächliche Entwicklung öfter einschliesst. Ein weiterer wichtiger Aspekt dieser Studie ist es herauszufinden, wie die einzelnen Ensemblevorhersagesys- teme, aus denen sich TIGGE zusammensetzt, zur Vorhersagevariabilität in TIGGE beitragen. Es wird gezeigt, dass die einzelnen Vorhersagesysteme einen unterschiedlichen Beitrag zur Vorhersagevariabilität und den daraus resultierenden Entwicklungsszenarien liefern. Während manche der Vorhersagesysteme nur ein oder zwei mögliche Vorhersageszenarien erfassen und einen geringen Beitrag zur Vorhersagevari- abilität leisten, zeigen andere Systeme beinahe alle Vorhersageszenarien, die in TIGGE zu finden sind. Insgesamt zeigt eine TIGGE Multimodell-Ensemblevorhersage mehr mögliche Entwicklungsszenarien als das EZMW- Ensemble. Diese volle Bandbreite an Entwicklungsszenarien in TIGGE wird allerdings nur erreicht, wenn das EZMW-Ensemble als Mitglied in TIGGE mit berücksichtigt wird. Ensemblevorhersagen zeigen mehrere mögliche Vorhersageszenarien für die zukünftige Entwicklung einer bestimmten synoptischen Situation. In Episoden mit verstärkter Vorhersageunsicherheit, wie z.B. während ET, können diese Entwicklungsszenarien stark variieren. Verschiedene solcher Vorhersageszen- arien für ET-Fälle ermöglichen es, die physikalischen und dynamischen Prozesse während der außertropi- schen Umwandlung näher zu untersuchen. In dieser Arbeit wird ein Budget für die Eddy-Kinetische Energie in verschiedenen Vorhersageszenarien berechnet, die aus Vorhersagen des EZMW-Ensembles für zwei ET-Fälle extrahiert wurden. In einem Vergleich des Energie-Budget für die verschiedenen Szenarien zeigt sich der Beitrag des sich umwandelnden tropischen Wirbelsturms und der relevanten physikalischen Prozesse. Wichtige Ergebnisse sind, dass die Dauer der baroklinen Umwandlung der Eddy-Kinetischen Energie und die relative Phasenlage zwischen dem Sturm und der wellengeprägten Strömung in den mittleren Breiten die Auswirkung des Sturms auf die stromabwärts gelegene Strömung stark beeinflussen. Desweiteren scheint die Strömung der mittleren Breiten während eines kurzen Zeitin- tervalls besonders empfindlich auf die Wechselwirkung mit dem tropischen Wirbelsturm zu sein. Danach nimmt der Einfluss des oftmals weiterhin starken Sturms auf die Wellenstruktur in den mittleren Breiten ab. In der vorliegenden Arbeit werden neue Aspekte der Vorhersagbarkeit und die physikalischen Prozesse während ET untersucht und mögliche Ansätze für weiterführende Studien aufgezeigt. Contents 1 Introduction 1 2 Theoretical Background 5 2.1 Tropical Cyclones and their Extratropical Transition .................. 5 2.1.1 Aspects of Extratropical Transition ....................... 5 2.2 Numerical Weather Prediction and Ensemble Forecasts ............... 12 2.2.1 Basics of Numerical Weather Prediction .................... 13 2.2.2 Ensemble Forecasts ..............................
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