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Tropical Transition of Hurricane Chris (2012) Over the North Atlantic Ocean: a Multi-Scale Investigation of Predictability

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Tropical Transition of Hurricane Chris (2012) Over the North Atlantic Ocean: a Multi-Scale Investigation of Predictability Tropical transition of Hurricane Chris (2012) over the North Atlantic Ocean: A multi-scale investigation of predictability Michael Maier-Gerber*1, Michael Riemer2, Andreas H. Fink1, Peter Knippertz1, Enrico Di Muzio1,2, Ron McTaggart-Cowan3 1 Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany 2 Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany 3 Numerical Weather Prediction Research Section, Environment and Climate Change Canada, Dorval, Quebec, Canada *[email protected] This work has been submitted to Monthly Weather Review. Copyright in this work may be transferred without further notice. Abstract Tropical cyclones that evolve from a non-tropical origin may pose a special chal- lenge for predictions, as they often emerge at the end of a multi-scale cascade of at- mospheric processes. Climatological studies have shown that the ’tropical transition’ (TT) pathway plays a prominent role in cyclogenesis, in particular over the North At- lantic Ocean. Here we use operational European Centre for Medium-Range Weather Forecasts ensemble predictions to investigate the TT of North Atlantic Hurricane Chris (2012), whose formation was preceded by the merger of two potential vorticity (PV) maxima, eventually resulting in the storm-inducing PV streamer. The principal goal is to elucidate the dynamic and thermodynamic processes governing the predictability of cyclogenesis and subsequent TT. Dynamic time warping is applied to identify ensemble tracks that are similar to the analysis track. This technique permits small temporal and spatial shifts in the development. The formation of the pre-Chris cyclone is predicted by those members that also pre- dict the merging of the two PV maxima. The position of the storm relative to the PV streamer determines whether the pre-Chris cyclone follows the TT pathway. The transitioning storms are located inside a favorable region of high equivalent potential temperatures that result from a warm seclusion underneath the cyclonic roll-up of the PV streamer. A systematic investigation of consecutive ensemble forecasts indicates that forecast improvements are linked to specific events, such as the PV merging. The present case exemplifies how a novel combination of Eulerian and Lagrangian ensemble forecast analysis tool allows to infer physical causes of abrupt changes in predictability. 1 Introduction (Davis and Bosart, 2003, 2004). Dur- ing TT, the extratropical cyclone trans- arXiv:1811.12513v1 [physics.ao-ph] 29 Nov 2018 ”Tropical transition” (TT) describes the forms from a cold to a warm core sys- phenomenon when a tropical cyclone (TC) tem. A cascade of events commonly pre- emerges from an extratropical cyclone cedes the TT: anticyclonic wave break- 1 ing (e.g., Thorncroft et al., 1993; Postel pathway is exceptionally high in the North and Hitchman, 1999) causes an upper-level Atlantic basin (almost 40 %). Because of precursor potential vorticity (PV) trough the extratropical origin of the precursor PV to penetrate into the tropics (Galarneau troughs, North Atlantic TCs that emerge et al., 2015), which initially induces the from a TT generally tend to form at higher development of either an antecedent ex- latitudes (Bentley et al., 2016), and reach tratropical (Davis and Bosart, 2004) or weaker intensities on average compared to subtropical cyclone (Evans and Guishard, all TCs (McTaggart-Cowan et al., 2008). 2009; Gonz´alez-Alem´anet al., 2015; Bent- However, Davis and Bosart (2004) point ley et al., 2016, 2017). The interplay be- out the challenge of accurately forecasting tween the upper-tropospheric PV trough these events, because they primarily occur and a low-level baroclinic zone facilitates in proximity to the eastern seaboard of the organization of convection embedded North America (McTaggart-Cowan et al., (Davis and Bosart, 2004; Hulme and Mar- 2008). tin, 2009) and is characteristic of a TT event, distinguishing it from other baro- In a recent study, Wang et al. (2018) ex- clinically influenced pathways of TC gen- amine reforecasts in terms of their skill to esis. The convection associated with the predict tropical cyclogenesis in the North precursor cyclone eventually diminishes the Atlantic, using the pathway classification PV gradients above the storm center and of McTaggart-Cowan et al. (2013). The hence reduces vertical wind shear (Davis authors conclude that the two TT cate- and Bosart, 2003, 2004), providing a favor- gories are less predictable than the oth- able environment for the cyclone to acquire ers, a finding that they attribute to fore- tropical nature. cast errors of the deep-layer shear and the moisture in the mid-troposphere. They TC developments are traditionally clas- also speculate that the interactions of pre- sified into ’tropical only’ and ’baroclini- cursor features and processes required for cally influenced’ categories (e.g., Hess et al., TT further reduce the overall predictabil- 1995; Elsner et al., 1996). McTaggart- ity as those constitute additional proba- Cowan et al. (2008, 2013) further suggest bility factors that multiplicatively extend a more precise classification of TC devel- the joint probability for non-TT pathways. opment pathways based on an analysis of Despite this probabilistic way of viewing two metrics, which assess baroclinicity in TT predictability, it still remains unclear the lower and upper troposphere, respec- which factors of uncertainty cause prac- tively. The two most baroclinically in- tical predictability (hereafter predictabil- fluenced categories are identified to repre- ity always refers to practical predictabil- sent ”weak TTs” and ”strong TTs”, distin- ity and not to the intrinsic predictability guished by the strength of the baroclinicity of the atmosphere) problems at the dif- in the lower troposphere. ferent stages of the event. The aim of From a climatological perspective, this study is to fill this gap by identify- McTaggart-Cowan et al. (2013) reveal that ing (thermo-)dynamic causes for eminently merely 16 % of all global TCs between 1948 rapid changes in the predictability of a) the and 2010 resulted from TT, but that the occurrence of the pre-Chris cyclone, b) its relative importance of the TT development TT, and c) the structural evolution, in Eu- 2 ropean Centre for Medium-Range Weather of Chris, before the results in terms of pre- Forecasts (ECMWF) ensemble forecasts of dictability are presented in section 4. The Hurricane Chris (2012) throughout the pre- findings and conclusions from this study TT portion of the storm’s life cycle. are discussed in section 5. Hurricane Chris was chosen for this multi-scale predictability study because of 2 Data and methods the complex antecedent PV dynamics and the strong baroclinic environment in the 2.1 Data upper and lower levels that facilitated the The present case study is based on grid- development of the extratropical precursor ded, six-hourly operational analysis and en- cyclone. The storm can be unambiguously semble forecast data from the European classified as a ”strong TT” case with regard Centre for Medium-Range Weather Fore- to the climatology of McTaggart-Cowan casts (ECMWF). To assess the evolution et al. (2013), and may therefore serve as of predictability, consecutive ensemble fore- a suitable archetype for investigation of casts initialized at 0000 UTC between 10 TT predictability. At the medium fore- June 2012 and 19 June 2012 – equivalent cast range, the model struggles to develop to nine and a half (seven) days prior to the pre-Chris cyclone; however, a strong in- TC (the pre-Chris cyclone’s) formation – crease in the number of ensemble members are systematically investigated. The min- predicting cyclone formation two to three imum horizontal grid spacing available for days before its actual development in the the first 10 days of the ensemble forecasts analysis means that focus can be shifted to is 0.25 . To allow for comparison, analy- the predictability of TT itself from a struc- ◦ sis and ensemble forecasts are analyzed us- tural perspective. Cyclone-relative ensem- ing that resolution. PV fields, however, are ble statistics of consecutive forecasts ini- only examined on the synoptic scale, and tialized up to nine days before TT occurs thus considered with a coarser resolution in the analysis will finally allow to link em- of 0.5 . inently rapid changes in predictability of ◦ structural evolution to antecedent (thermo- )dynamic events. Though individual en- 2.2 Ensemble partitioning: Cy- semble forecasts have been examined to as- clone vs. no-cyclone sess predictability of storm-structural evo- In a first step, every forecast ensemble con- lution before (e.g., Gonz´alez-Alem´anet al., sidered in this study is split into ”cyclone” 2018), the present case study is the first and ”no-cyclone” groups to elucidate dy- to investigate changes in predictability of namic causes limiting predictability of the TT with lead time and thus contributes to pre-Chris cyclone’s formation. The group a deeper understanding of the associated memberships are determined based on sim- sources of uncertainty. ilarity between forecast tracks and the anal- Following this introduction, section 2 will ysis track using a dynamic time warping describe the data used and the methods ap- technique (see section 2.3 for details). A plied in this study. The synoptic overview forecast member thus belongs to the ”cy- in section 3 highlights the key atmospheric clone” (”no-cyclone”) group when it pre- features that were associated with the TT dicts (lacks) such a ’similar track’. Group 3 names are italicized hereafter to better dis- solely occur in space. However, it may be tinguish them from text. that a forecast predicts a track accurately in space but slightly shifted in time, for 2.3 Cyclone tracking and evalua- instance due to a lag in the precursor tion of forecast tracks PV dynamics. In the present study, we therefore want to be less restrictive and The simple cyclone tracking algorithm de- also allow for small temporal shifts in scribed by Hart (2003) is employed here the forecast tracks, using a dynamic time because its performance compared to man- warping technique.
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