The impact of tropical cyclones on extratropical predictability

Principal Investigator

Prof Sarah Jones
Institut für Meteorologie und Klimaforschung
Universität Karlsruhe
Kaiserstr. 12
76128 Karlsruhe
GERMANY

sarah.jones@imk.uka.de

Other researchers: Doris Anwender, Christian Grams, Simon Lang, Julia Keller

Project description

The aim of this project is to quantify the impact of tropical cyclones undergoing extratropical transition (ET) on the predictability of the midlatitude flow. A particular emphasis is placed on the predictability of developments downstream of an ET event (Harr et al. 2008; Anwender et al 2008). Thus we focus on the role of extratropical transition events in exciting upper-level midlatitude Rossby wave trains and investigate the representation of such events in operational forecasting systems. In our choice of case studies during the next phase of this project we will give priority to cases from the THORPEX Pacific Asian Regional Campaign taking place in August and September 2008.

Our specific aims in the next phase of the project are as follows.

• We will conduct new case studies in which the principal component / cluster analysis (Harr et al. 2008) is applied to new cases using the TIGGE dataset and the results compared to a structure based clustering (Arnott et al. 2004; Veren et al. 2008).
• We will continue experiments with the ECMWF ensemble prediction system (EPS) to investigate the relative roles of initial perturbations, stochastic physics, and resolution in the representation of the downstream impact of ET. The initial study of Typhoon Tokage (2004) is being prepared for publication (Anwender et al. 2008). In this phase we will investigate further case studies.
• We will extend our investigation of the structure of moist singular vectors (SVs) targeted on tropical cyclones. In the recent Masters thesis of Simon Lang a version of cycle 32r3 was used that had been modified by Martin Leutbecher so that the structure of the trajectory and the SVs could be outputted during the optimisation period. The evolution of the SVs could be related to the phase in the lifecycle of the tropical cyclone as well as to dynamical mechanisms. However, the structure of the tropical cyclone is not well represented in the T42 trajectory. Thus we will study the changes in the SV structure with increasing horizontal resolution and consider the sensitivity to the choice of optimisation region. For selected cases new EPS forecasts will be carried out.
• We will conduct data denial and data impact experiments to investigate the impact of observations during an ET event on the forecast for the downstream midlatitude flow.
• We will initiate a study to investigate whether nesting the COSMO model in selected EPS members can give a better representation of the structural changes of a tropical cyclone undergoing ET. We will follow the strategy of COSMO-LEPS but use the methodology described in Harr et al. (2008) to select the EPS members.

The computing resources requested are necessary to calculate the singular vectors, conduct new EPS forecasts and conduct data denial / data impact experiments using the ECMWF Integrated Forecast System. All of these require substantial computer time and can only be carried out at ECMWF. The COSMO experiments will be performed using local computer resources at the University of Karlsruhe.

The ensemble work will be carried out in collaboration with Martin Leutbecher and the data impact / denial studies with Carla Cardinali. The Karlsruhe scientists working on this project are members of the new German THORPEX research group PANDOWAE (Predictability and Dynamics of Weather Systems in the Atlantic – European Sector). In particular, 3 doctoral students will work on this project, so that the support over the three year period is very important for the success of their projects.

References

Anwender, D., P. A. Harr and S. C. Jones, 2008: Predictability associated with the downstream impacts of the extratropical transition of tropical cyclones:case studies. Mon. Wea. Rev., in press

Anwender, D., S. C. Jones, M. Leutbecher, and P. A. Harr, 2008: Sensitivity experiments for ensemble forecasts of the extratropical transition of Typhoon Tokage (2004). In preparation for submission to Quart. J. Roy. Meteor. Soc.

Arnott, J. M., J. L. Evans, F. Chiaromonte, 2004: Characterization of extratropical transition using cluster analysis. Mon. Wea. Rev. 132, 2916-2937.

Harr, P. A., D. Anwender and S. C. Jones, 2008: Predictability associated with the downstream impacts of the extratropical transition of tropical cyclones: Methodology and a case study. Mon. Wea. Rev., in press

Veren, D., J. L. Evans, S. C. Jones and F. Chiaromonte, 2008: Evaluation of Tropical Cyclone Ensemble Forecasts. Submitted to Mon. Wea. Rev.

For more details, please refer to the latest progress report.

Additional information

Project started in 2004.

Would not accept support for 1 year only.