A Global Climatology of Extratropical Transition. Part II: Statistical Performance of the Cyclone Phase Space

Bieli, Melanie; Camargo, Suzana J.; Sobel, Adam H.; Evans, Jenni L.; Hall, Timothy M.

doi:10.1175/jcli-d-18-0052.1

Cited by 19 publications

(16 citation statements)

References 23 publications

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“…Following Bieli et al (, ), ET is defined as follows: ET onset is the first time a TC is either asymmetric ( B > 11) or has a cold core (

- V_{T}^{L} < 0

and

- V_{T}^{U} < 0

), and ET completion is when the second criterion is met. Bieli et al (, ) also require a minimum wind speed of 34 kt (≈17.5 m/s; tropical storm intensity) for ET onset in order to prevent weak, thermally asymmetric tropical depression‐like systems or monsoonal troughs from being incorrectly diagnosed as beginning ET events. Based on the objectively determined relationship between tropical storm intensity and model resolution given in Walsh et al (), the minimum wind speed for the onset of ET in FLOR‐FA is set to 16.5 m/s, as this is approximately the equivalent intensity of a tropical storm in a model with 50‐km horizontal resolution.…”

Section: Methodsmentioning

confidence: 99%

“…TCs undergoing ET are identified by means of their trajectories through the cyclone phase space (CPS Hart, ), calculated from global climate model output. The CPS has become widely used and has been applied to diagnose ET in operational analysis and reanalysis data (e.g., Bieli et al, , ; Hart, ; Kitabatake, ; Wood & Ritchie, ) as well as climate model output (Liu et al, , ; Zarzycki et al, ). We use the Forecast‐oriented Low Ocean Resolution version of CM2.5 with Flux Adjustment (FLOR‐FA), which was developed at the Geophysical Fluid Dynamics Laboratory.…”

Section: Introductionmentioning

confidence: 99%

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Application of the Cyclone Phase Space to Extratropical Transition in a Global Climate Model

Bieli

Sobel

Camargo

et al. 2020

J Adv Model Earth Syst

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The authors analyze the global statistics of tropical cyclones undergoing extratropical transition (ET) in the Forecast-oriented Low Ocean Resolution version of CM2.5 with Flux Adjustment (FLOR-FA). The cyclone phase space (CPS) is used to diagnose ET. A simulation of the recent historical climate is analyzed and compared with data from the Japanese 55-year Reanalysis (JRA-55), and then a simulation of late 21st century climate under Representative Concentration Pathway 4.5 is compared to the historical simulation. When CPS is applied to the FLOR-FA output in the historical simulation, the results diverge from those obtained from JRA-55 by having an unrealistic number of ET cases at low latitudes, due to the presence of strong local maxima in the upper-level geopotential. These features mislead CPS into detecting a cold core where one is not present. The misdiagnosis is largely corrected by replacing the maxima required by CPS with the 95th percentile values, smoothing the CPS trajectories in time, or both. Other climate models may contain grid-scale structures akin to those in FLOR-FA and, when used for CPS analysis, require solutions such as those discussed here. Comparisons of ET in the projected future climate with the historical climate show a number of changes that are robust to the details of the ET diagnosis, though few are statistically significant according to standard tests. Among them are an increase in the ET fraction and a reduction in the mean latitude at which ET occurs in the western North Pacific. Plain Language SummaryWhen tropical cyclones move into the midlatitudes, they change their physical structure and sometimes transform into extratropical cyclones. This process is called extratropical transition. One diagnostic tool for defining extratropical transition is the cyclone phase space. In this study, the authors apply the cyclone phase space to the tropical cyclones simulated by a global climate model. The output of this climate model has some features that misguide the cyclone phase space into diagnosing a cyclone as extratropical when in fact it is tropical. The authors examine various adjustments of the cyclone phase space that largely correct this misdiagnosis. They then study the changes in the global occurrence of extratropical transition between two 30-year time periods simulated by the climate model: a historical period from 1976 to 2005 and a future period from 2071 to 2100, whose simulation is based on a scenario of moderate increases in greenhouse gas emissions. In the western North Pacific, tropical cyclones are found to become more likely to undergo extratropical transition in the future climate. In general, though, the changes in global statistics of extratropical transition (e.g., where and how often it occurs) between the simulated future and historical climate are small.

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“…Following Bieli et al (, ), ET is defined as follows: ET onset is the first time a TC is either asymmetric ( B > 11) or has a cold core (

- V_{T}^{L} < 0

and

- V_{T}^{U} < 0

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Application of the Cyclone Phase Space to Extratropical Transition in a Global Climate Model

Bieli

Sobel

Camargo

et al. 2020

J Adv Model Earth Syst

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“…Notwithstanding these limitations, recent studies have assessed the changes of ET in a warmer climate using the cyclone phase space of Hart [119] as the classification standard of ET [120]. For warmer climate scenarios, it is expected that the number of TCs undergoing ET will increase over the North Atlantic [121][122][123] partly due to spatial shifts in TC genesis locations [123].…”

Section: Extratropical Transition Of Tropical Cyclonesmentioning

confidence: 99%

The Future of Midlatitude Cyclones

Catto

Ackerley

Booth

et al. 2019

Curr Clim Change Rep

112

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Purpose of Review This review brings together recent research on the structure, characteristics, dynamics, and impacts of extratropical cyclones in the future. It draws on research using idealized models and complex climate simulations, to evaluate what is known and unknown about these future changes. Recent Findings There are interacting processes that contribute to the uncertainties in future extratropical cyclone changes, e.g., changes in the horizontal and vertical structure of the atmosphere and increasing moisture content due to rising temperatures. Summary While precipitation intensity will most likely increase, along with associated increased latent heating, it is unclear to what extent and for which particular climate conditions this will feedback to increase the intensity of the cyclones. Future research could focus on bridging the gap between idealized models and complex climate models, as well as better understanding of the regional impacts of future changes in extratropical cyclones. Keywords Extratropical cyclones. Climate change. Windstorms. Idealized model. CMIP models This article is part of the Topical Collection on Mid-latitude Processes and Climate Change

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“…7. ET timing errors of the operational model (at lead time 0 h), the hazard model, and the CPS analysis in Bieli et al (2019b), which was performed using JRA-55 as well as ERA-Interim data. Timing errors (shown on the x axis) are defined as the differences between the predicted ET times and the true (i.e., best track) ET times.…”

Section: Discussionmentioning

confidence: 99%

“…Performance scores for the classifications into ET storms and non-ET storms obtained from the operational model (at lead time 0 h), the hazard model, and from the CPS analysis inBieli et al (2019b), which was performed using JRA-55 as well as ERA-Interim data. The table shows the F1 score (F1) and the Matthews correlation coefficient (MCC) for the North Atlantic (NAT) and western North Pacific (WNP) basins.…”

mentioning

confidence: 99%

A Statistical Model to Predict the Extratropical Transition of Tropical Cyclones

Bieli

Sobel

Camargo

et al. 2020

Weather and Forecasting

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This paper introduces a logistic regression model for the extratropical transition (ET) of tropical cyclones in the North Atlantic and the western North Pacific, using elastic net regularization to select predictors and estimate coefficients. Predictors are chosen from the 1979–2017 best track and reanalysis datasets, and verification is done against the tropical/extratropical labels in the best track data. In an independent test set, the model skillfully predicts ET at lead times up to 2 days, with latitude and sea surface temperature as its most important predictors. At a lead time of 24 h, it predicts ET with a Matthews correlation coefficient of 0.4 in the North Atlantic, and 0.6 in the western North Pacific. It identifies 80% of storms undergoing ET in the North Atlantic and 92% of those in the western North Pacific. In total, 90% of transition time errors are less than 24 h. Select examples of the model’s performance on individual storms illustrate its strengths and weaknesses. Two versions of the model are presented: an “operational model” that may provide baseline guidance for operational forecasts and a “hazard model” that can be integrated into statistical TC risk models. As instantaneous diagnostics for tropical/extratropical status, both models’ zero lead time predictions perform about as well as the widely used cyclone phase space (CPS) in the western North Pacific and better than the CPS in the North Atlantic, and predict the timings of the transitions better than CPS in both basins.

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A Global Climatology of Extratropical Transition. Part II: Statistical Performance of the Cyclone Phase Space

Cited by 19 publications

References 23 publications

Application of the Cyclone Phase Space to Extratropical Transition in a Global Climate Model

Application of the Cyclone Phase Space to Extratropical Transition in a Global Climate Model

The Future of Midlatitude Cyclones

A Statistical Model to Predict the Extratropical Transition of Tropical Cyclones

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