Understanding Extratropical Transition and its Downstream Impact through Diagnostic Analysis and International Collaboration

When a tropical cyclone moves polewards and interacts with the midlatitude flow it itself undergoes significant structural changes as well as modifies the midlatitude flow. Through these changes, extratopical transition can play a role in the development of high impact weather events far downstream and introduce significant uncertainty into the forecast of such events. In order to mitigate the societal impact of extratropical transition it is important to better understand the processes involved and their impact on the predictability downstream. The diagnostic methods pioneered by Daniel Keyser (e.g., the generalization of Petterssen’s frontogenesis function (Keyser et al. 1988) and Q-Vector partitioning to isolate the contributions of different dynamical processes to the total vertical motion field (Keyser et al. 1992)), can be applied to output from models and observations to gain insight into the processes involved. In this presentation, we present studies from real cases and idealized modelling. We use diagnostic analysis to quantify the role of low-level frontogenesis for the structural changes during extratropical transition. Q-Vector diagnostics suggest that forced ascent during warm frontogenesis triggers the rapid development of deep convection which reinforces the cyclone’s circulation through the diabatic generation of cyclonic potential vorticity. The generalization of Petterssen’s frontogenesis function further indicates an amplification of the low-level thermal wave associated with the interaction of a tropical cyclone with a zonally orientated baroclinic zone (Quinting et al. 2014). Analysis using flow partitioning shows that the divergent outflow associated with latent heat release in the cyclone then makes an important contribution to upper-tropospheric frontogenesis and thus to the formation of a jet streak downstream (Riemer and Jones 2010). Q-Vector partitioning further reveals the prominent but highly variable role that the jet streak plays for cyclone development in the downstream region (Riemer et al. 2014).

Progress in our understanding of how weather systems function is gained not only from diagnostic analysis but also from the exchange of ideas between scientists. As part of this presentation we reflect on how scientific work can be enhanced through enabling early career scientists to visit other institutions and through regular meetings such as the Cyclone Workshop.

References:

Keyser, D., M. J. Reeder, and R. J. Reed, 1988: A Generalization of Petterssen's Frontogenesis Function and Its Relation to the Forcing of Vertical Motion. Mon. Wea. Rev., 116, 762–781, https://doi.org/10.1175/1520-0493(1988)116<0762:AGOPFF>2.0.CO;2.

Keyser, D., B. D. Schmidt, and D. G. Duffy, 1992: Quasigeostrophic Vertical Motions Diagnosed from Along- and Cross-isentrope Components of the Q Vector. Mon. Wea. Rev., 120, 731–741, https://doi.org/10.1175/1520-0493(1992)120<0731:QVMDFA>2.0.CO;2.

Quinting, J. F., M. M. Bell, P. A. Harr, and S. C. Jones, 2014: Structural Characteristics of T-PARC Typhoon Sinlaku during Its Extratropical Transition. Mon. Wea. Rev., 142, 1945–1961, https://doi.org/10.1175/MWR-D-13-00306.1.

Riemer, M., and S. C. Jones, 2010: The downstream impact of tropical cyclones on a developing baroclinic wave in idealized scenarios of extratropical transition. Quart. J. Roy. Meteor. Soc., 136, 617-637, https://doi.org/10.1002/qj.605.

Riemer, M., M. Baumgart, and S. Eiermann, 2014, Cyclogenesis downstream of extratropical transition analyzed by Q-vector partitioning based on flow geometry. J. Atmos. Sci, 71, 4204-4220. https://doi.org/10.1175/JAS-D-14-0023.1.