Tropical cyclones are a prime example of high-impact weather and of how improved predictions can save lives and economic losses. Our main research interest is in improving our fundamental understanding of the dynamics and predictability of these hazardous storms. Of particular interest are interactions between the midlatitudes and tropical or tropical-like cyclones because these interactions appear to exhibit particularly low predictability.
‘Ventilation’ of tropical cyclones
‘Ventilation’ refers to the systematic intrusion of cooler and drier environmental air into the inner-core convection of tropical cyclones. Ventilation is deemed to be a leading-order thermodynamic mechanism to impact intensity evolution. A series of papers (Riemer et al., 2010, 2013; Riemer and Montgomery, 2011; Riemer 2016) has established a novel conceptual framework of how vertical shear of the environmental winds promote ventilation through a thermodynamic modification of the tropical cyclone’s inflow layer.
The left figure shows devastating Hurricane Katrina approaching New Orleans on 28 August 2005. The animation to the right shows a radar loop of Hurricane Dean affecting the Lesser Antilles in Aug 2007. It reveals a distinct asymmetry in reflectivities outside of the eyewall to the North-east of the center that develops in response to vertical shear. Such asymmetries have been a focus of this series of papers.
Ventilation is a truly Lagrangian process. We have thus developed a trajectory-based diagnostic to identify and quantify individual ventilation pathways (Riemer and Laliberté, 2015). Idealized experiments indicate that the propensity of individual pathways depends on the vertical profile of the environmental winds (Fu et al., 2019).
Predictability of 'hybrid' tropical cyclones
Cyclones of midlatitude origin sometimes undergo ‘tropical’ transition, by which they acquire characteristics of a tropical cyclone. This transition is sensitive to the interaction with upper-tropospheric trough features (PV streamers of cut-offs). In a case study of Hurricane Chris (2012) (Maier-Gerber et al., 2019) we demonstrate how the complex precursor dynamics of upper-tropospheric PV features can pose a predictability barrier to forecasting tropical transition. In the European region, tropical-like cyclones form in the Mediterranean Sea, referred to as Medicanes, and arguably since recently in the Bay of Biscay (Biscanes, Maier-Gerber et al., 2017). In a systematic study of all Medicanes from 2011-2017 and the Biscane (Di Muzio et al., 2019), predictability barriers are found in the ECMWF ensemble forecast system for all cases. A multi-scale analysis of these cases (Di Muzio et al., 2020) indicates that predictability that could be inherited from the larger-scale environment is masked by the strongly nonlinear interplay with processes that organize near-core convection, which ultimately pose a limit of predictability on Medicanes.
When tropical cyclones move towards the midlatitudes they undergo “extratropical transition” (ET), i.e. they gradually lose their tropical characteristics and may transform into an extratropical cyclone. The impact of ET on the midlatitude flow can be significant: ET modifies Rossby wave packets and thus ET’s impact disperses downstream along the midlatitude jet. Frequently, midlatitude predictability is severely compromised by ET.
In a series of papers (Riemer et al., 2008; Riemer and Jones, 2010, 2014; Riemer et al. 2014) we analyzed in detail the processes that govern the impact on the midlatitude jet, how this impact modifies downstream cyclogenesis, and importantly how the generic midlatitude wave pattern generates bifurcation points in the cyclones steering flow, which provides a fundamental explanation of the observed low intrinsic predictability. More recently, we explored the importance of ‘phase-locking’ between the transitioning cyclone and the midlatitude wave pattern and how phase-locking enhances the formation of downstream blocking (Riboldi et al. 2019).
The dynamics of the cyclone during transition is rather complex and there is large case-to-case variability. Different pathways exist by which a tropical cyclone may resist vertical shear early during ET and, although the environment becomes increasingly hostile for the tropical cyclone, increasing vertical shear tends to increase the vertical mass flux in the cyclone’s inner core (Davis et al., 2008). The associated upper-tropospheric divergent outflow then starts modifying the jet. Different pathways exist also how a thermodynamic interaction with the cooler and drier midlatitude air can be deferred, e.g., either by orographic blocking (Lentink et al., 2018) or by a fast relative translation speed (Euler et al., 2019).
Two recent reviews summarize the current state of the art concerning the transitioning cyclone (Evans et al., 2017) and its downstream impact (Keller et al., 2019)
- Davis, C. A., S. C. Jones, and M. Riemer, 2008: Hurricane Vortex Dynamics during Atlantic Extratropical Transition, J. Atmos. Sci., 65, 714-736
- Di Muzio, E., M. Riemer, A. H. M. Fink, and M. Maier-Gerber, 2019: Assessing the predictability of Medicanes in ECMWF ensemble forecasts using an object-based approach, Quart. J. Roy. Meteor. Soc., 145, 1202-1217
- Di Muzio, E., M. Riemer, A. H. M. Fink, and M. Maier-Gerber, 2020: The role of large-scale dynamics in the predictability of Medicanes in ECMWF ensemble forecasts, Quart. J. Roy. Meteor. Soc., to be submitted
- Euler, C., M. Riemer, T. Kremer, and E. Schömer, 2019: Lagrangian Description of Air Masses Associated with Latent Heat Release in Tropical Storm Karl (2016) during Extratropical Transition, Mon. Wea. Rev., 147, 2657-2676
- Evans, C.; K. Wood; S. D. Aberson; H. M. Archambault; S. M. Milrad; L. F. Bosart; K. L. Corbosiero; C. A. Davis; J. R. Dias Pinto; J. Doyle; C. Fogarty; T. J. Galarneau, Jr.; C. M. Grams; K. S. Griffin; J. Gyakum; R. E. Hart; N. Kitabatake; H. S. Lentink; R. McTaggart-Cowan; W. Perrie; J. F. D. Quinting; C. A. Reynolds; M. Riemer; E. Ritchie; Y. Sun; F. Zhang, 2017: The Extratropical Transition of Tropical Cyclones. Part I: Cyclone Evolution and Direct Impacts, Mon. Wea. Rev., 145, 4317-4344
- Fu, H., Y. Wang, M. Riemer, and Q. Li, 2019: Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change – Lower-Layer Shear versus Upper-Layer Shear, J. Geophys. Res. Atmos., 124, 6265-6282
- Keller, J. H., C. M. Grams, M. Riemer, H. M. Archambault, L. Bosart, J. Doyle, J. L. Evans, T. Galarneau, K. S. Griffin, P. A. Harr, N. Kitabatake, R. McTaggart-Cowan, F. Pantillon, J. F. D. Quinting, C. A. Reynolds, E. A. Ritchie, R. Torn, F. Zhang, 2019: The Extratropical Transition of Tropical Cyclones: Interaction with the midlatitude flow, downstream impacts and implications in predictability, Mon. Wea. Rev., 147, 1077-1106
- Kumpf, A., M. Rautenhaus, M. Riemer, R. Westermann, 2019: Visual Analysis of the Temporal Evolution of Ensemble Forecast Sensitivities, IEEE Transactions on Visualization and Computer Graphics, 25, 98-108
- Lentink, H. S., C. M. Grams, M. Riemer, and S. C. Jones, 2018: The effects of orography on the extratropical transition of tropical cyclones: a case study of Typhoon Sinlaku (2008), Mon. Wea. Rev., 146, 4231-4246
- Maier-Gerber, M., M. Riemer, A. H. Fink, P. Knippertz, E. Di Muzio, R. McTaggart-Cowan, 2019: „Tropical Transition of Hurricane Chris (2012) over the North Atlantic Ocean: A Multi-Scale Investigation of Predictability“, Mon. Wea. Rev., 147, 951-970
- Maier-Gerber, M., F. Pantillon, E. Di Muzio, M. Riemer, A. H. Fink, P. Knippertz, 2017: Birth of the Biscane, Weather, 72, 236-241
- Riboldi, J., C. M. Grams, M. Riemer, and H. M. Archambault, 2019: A phase-locking perspective on Rossby wave amplification and atmospheric blocking downstream of recurving western North Pacific tropical cyclones, Mon. Wea. Rev., 147, 567-589
- Riemer, M., 2016: Meso-beta-scale environment for the stationary band complex of vertically-sheared tropical cyclones, Quart. J. Roy. Meteor. Soc., 142, 2442-2451
- 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
- Riemer, M. and S. C. Jones, 2010: Downstream impact of tropical cyclones on a developing baroclinic wave in idealized scenarios of extratropical transition, Quart. J. Roy. Meteor. Soc., 136, 617–637
- Riemer, M. and S. C. Jones, 2014: Interaction of a tropical cyclone with a high-amplitude, midlatitude wave pattern: Waviness analysis, trough deformation and track bifurcation, Quart. J. Roy. Meteor. Soc., 140, 1362-1376
- Riemer, M., S. C. Jones, and C. A. Davis, 2008: The impact of extratropical transition on the downstream flow: an idealised modelling study with a straight jet, Quart. J. Roy. Meteor. Soc., 134, 69-91
- Riemer, M. and F. Laliberté, 2015: Secondary circulation of tropical cyclones in vertical wind shear: Lagrangian diagnostic and pathways of environmental interaction, J. Atmos. Sci., 72, 3517-3536
- Riemer, M. and M.T. Montgomery, 2011: Simple kinematic models for the environmental interaction of tropical cyclones in vertical wind shear, Atmos. Chem. Phys., 11, 9395-9414
- Riemer, M., M. T. Montgomery, and M. A. Nicholls, 2010: A new paradigm for intensity modification of tropical cyclones: Thermodynamic impact of vertical wind shear on the inflow layer, Atmos. Chem. Phys, 10, 3163-3188
- Riemer, M., M. T. Montgomery, and M. A. Nicholls, 2013: Further examination of the thermodynamic modification of the inflow layer of tropical cyclones by vertical wind shear, Atmos. Chem. Phys., 13, 327-346