scholarly journals The Extratropical Transition of Tropical Cyclone Lili (1996) and Its Crucial Contribution to a Moderate Extratropical Development

2005 ◽  
Vol 133 (6) ◽  
pp. 1562-1573 ◽  
Author(s):  
Anna Agustí-Panareda ◽  
Suzanne L. Gray ◽  
George C. Craig ◽  
Chris Thorncroft
Keyword(s):  
Tropical Cyclone ◽  
Case Studies ◽  
North Atlantic ◽  
Initial Conditions ◽  
Life Cycles ◽  
Extratropical Transition ◽  
The North ◽  
Potential Vorticity Inversion ◽  
The Impact

Abstract The transition that a tropical cyclone experiences as it moves into the extratropical environment (known as extratropical transition) can result in the decay or intensification of a baroclinic cyclone. The extratropical transition (ET) of Tropical Cyclone Lili (1996) in the North Atlantic resulted in a moderate extratropical development of a baroclinic cyclone. The impact of Lili in the extratropical development that occurred during its ET is investigated. Numerical experiments are performed using potential vorticity inversion and the Met Office Unified Model to forecast the extratropical development with and without the tropical cyclone in the initial conditions. In contrast with other case studies in the literature, Lili is shown to play a crucial role during its ET in the development of a baroclinic cyclone that occurred in the same region. A hypothesis of the possible scenarios of ET is presented that links the case-to-case variability of ET case studies in the literature with a classification of the life cycles of baroclinic cyclones.

scholarly journals The Contribution of Ex–Tropical Cyclone Gert (1999) toward the Weakening of a Midlatitude Cyclogenesis Event

2008 ◽  
Vol 136 (6) ◽  
pp. 2091-2111 ◽  
Author(s):  
Anna Agustí-Panareda
Keyword(s):  
Tropical Cyclone ◽  
Vertical Velocity ◽  
Initial Conditions ◽  
Negative Impact ◽  
Positive Impact ◽  
Extratropical Cyclone ◽  
Synoptic Scale ◽  
Extratropical Transition ◽  
Peak Intensity ◽  
The Impact

Abstract Tropical Cyclone Gert (1999) experienced an extratropical transition while it merged with an extratropical cyclone upstream. The upstream extratropical cyclone had started to intensify before it merged with the transitioning tropical cyclone, and it continued intensifying afterward (12 hPa in 12 h, according to the Met Office analysis). The question addressed in this paper is the following: what was the impact of the transitioning tropical cyclone on this intensification of the upstream extratropical cyclone? Until now, in the literature, tropical cyclones that experience extratropical transition have been found to have either no impact or a positive impact on the development of extratropical cyclogenesis events. The positive impact involves either a triggering of the development of the extratropical cyclone or simply a contribution to its deepening. However, the case studied here proves to have a negative impact on the developing extratropical cyclone upstream by diminishing its intensification. Forecasts are performed with and without the tropical cyclone in the initial conditions. They show that when Gert is not present in the initial conditions, the peak intensity of the cyclone upstream occurs 9 h earlier and it is 10 hPa deeper than when Gert is present. Thus, Gert acts to weaken the development by contributing to the filling of the extratropical surface low upstream. Quasigeostropic (QG) diagnostics show that the negative impact on the extratropical development is linked to the fact that the transitioning tropical cyclone interacts with a warm front inducing a negative QG vertical velocity over the developing extratropical low upstream. This interpretation is consistent with other contrasting cases in which the transitioning tropical cyclone interacts with a cold front and induces a positive QG vertical velocity over the developing low upstream, thus enhancing its development. The results are also in agreement with idealized experiments in the literature that are aimed at studying the predictability of extratropical storms. These idealized experiments yielded similar results using synoptic-scale and mesoscale vortices as perturbations on warm and cold fronts.

scholarly journals A Case Study of Downstream Baroclinic Development over the North Pacific Ocean. Part I: Dynamical Impacts

2006 ◽  
Vol 134 (5) ◽  
pp. 1534-1548 ◽  
Author(s):  
Richard E. Danielson ◽  
John R. Gyakum ◽  
David N. Straub
Keyword(s):  
North Pacific ◽  
Initial Conditions ◽  
North Pacific Ocean ◽  
The North ◽  
Potential Vorticity Inversion ◽  
Surface Cyclone ◽  
Upper Level ◽  
The Impact ◽  
The North Pacific

Abstract The impact of eddy energy growth and radiation from a western North Pacific cyclone on the intensity of an eastern North Pacific cyclone a few days later is examined. Associated with the western cyclone is an upstream ridge and trough couplet, initially over Siberia on 8 March 1977. The amplitude of this couplet is perturbed in 5-day numerical simulations of the two marine cyclones. Balanced initial conditions are created by potential vorticity inversion. The magnitude of the upper-level couplet governs much of the subsequent growth of eddy energy in the western cyclone as well as the propagation of eddy energy between the two cyclones. This culminates in measurable changes in the maximum intensity of the eastern surface cyclone. The broader question of the sensitivity of this cyclone to upstream perturbations is also briefly addressed.

Observational Evidence for Oceanic Controls on Hurricane Intensity

2011 ◽  
Vol 24 (4) ◽  
pp. 1138-1153 ◽  
Author(s):  
Ian D. Lloyd ◽  
Gabriel A. Vecchi
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
North Atlantic ◽  
Translation Speed ◽  
Ocean Stratification ◽  
Cyclone Intensity ◽  
The North ◽  
Sst Cooling ◽  
The North Atlantic ◽  
The Impact

Abstract The influence of oceanic changes on tropical cyclone activity is investigated using observational estimates of sea surface temperature (SST), air–sea fluxes, and ocean subsurface thermal structure during the period 1998–2007. SST conditions are examined before, during, and after the passage of tropical cyclones, through Lagrangian composites along cyclone tracks across all ocean basins, with particular focus on the North Atlantic. The influence of translation speed is explored by separating tropical cyclones according to the translation speed divided by the Coriolis parameter. On average for tropical cyclones up to category 2, SST cooling becomes larger as cyclone intensity increases, peaking at 1.8 K in the North Atlantic. Beyond category 2 hurricanes, however, the cooling no longer follows an increasing monotonic relationship with intensity. In the North Atlantic, the cooling for stronger hurricanes decreases, while in other ocean basins the cyclone-induced cooling does not significantly differ from category 2 to category 5 tropical cyclones, with the exception of the South Pacific. Since the SST response is nonmonotonic, with stronger cyclones producing more cooling up to category 2, but producing less or approximately equal cooling for categories 3–5, the observations indicate that oceanic feedbacks can inhibit intensification of cyclones. This result implies that large-scale oceanic conditions are a control on tropical cyclone intensity, since they control oceanic sensitivity to atmospheric forcing. Ocean subsurface thermal data provide additional support for this dependence, showing weaker upper-ocean stratification for stronger tropical cyclones. Intensification is suppressed by strong ocean stratification since it favors large SST cooling, but the ability of tropical cyclones to intensify is less inhibited when stratification is weak and cyclone-induced SST cooling is small. Thus, after accounting for tropical cyclone translation speeds and latitudes, it is argued that reduced cooling under extreme tropical cyclones is the manifestation of the impact of oceanic conditions on the ability of tropical cyclones to intensify.

Seasonal Forecasting of Wind and Waves in the North Atlantic Using a Grand Multimodel Ensemble

2019 ◽  
Vol 34 (1) ◽  
pp. 31-59 ◽  
Author(s):  
Ray Bell ◽  
Ben Kirtman
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Wind Wave ◽  
Initial Conditions ◽  
Monte Carlo Model ◽  
Surface Wind ◽  
Multimodel Ensemble ◽  
The North ◽  
The North Atlantic ◽  
The Impact

Abstract This study assesses the skill of multimodel forecasts of 10-m wind speed, significant wave height, and mean wave period in the North Atlantic for the winter months. The 10-m winds from four North American multimodel ensemble models and three European Multimodel Seasonal-to-Interannual Prediction project (EUROSIP) models are used to force WAVEWATCH III experiments. Ten ensembles are used for each model. All three variables can be predicted using December initial conditions. The spatial maps of rank probability skill score are explained by the impact of the North Atlantic Oscillation (NAO) on the large-scale wind–wave relationship. Two winter case studies are investigated to understand the relationship between large-scale environmental conditions such as sea surface temperature, geopotential height at 500 hPa, and zonal wind at 200 hPa to the NAO and the wind–wave climate. The very strong negative NAO in 2008/09 was not well forecast by any of the ensembles while most models correctly predicted the sign of the event. This led to a poor forecast of the surface wind and waves. A Monte Carlo model combination analysis is applied to understand how many models are needed for a skillful multimodel forecast. While the grand multimodel ensemble provides robust skill, in some cases skill improves once some models are not included.

Decadal predictability in the North Atlantic as seen by EC-Earth3.

2021 ◽  
Author(s):  
Bo Christiansen ◽  
Shuting Yang ◽  
Dominic Matte
Keyword(s):  
North Atlantic ◽  
Lead Time ◽  
Initial Conditions ◽  
Statistical Significance ◽  
Lead Times ◽  
The North ◽  
Weak Evidence ◽  
The North Atlantic ◽  
The Impact ◽  
Decadal Predictability

<p>We study the decadal predictability in the North Atlantic region using  ensembles of historical and decadal prediction experiments with EC-Earth3  and other CMIP models. In particular, the focus is on the NAO and the sub-polar gyre region. In general the impact of initialization is weak  for lead-times larger than one to two years and we investigate different ways to isolate and estimate the statistical significance of this impact. For the sub-polar gyre region the prediction skill is found to be mainly due to an abrupt change in the late 90ies and models disagree on whether this skill is due to forcing or initial conditions. Also the predictability of the NAO is weak and varies with lead-time and length of the predicted period. We only see weak evidence of the 'signal-to-noise paradox'. The importance of the ensemble size is also studied.                                                              </p>

scholarly journals Upstream Cyclone Influence on the Predictability of Block Onsets over the Euro-Atlantic Region

2019 ◽  
Vol 147 (4) ◽  
pp. 1277-1296 ◽  
Author(s):  
J. W. Maddison ◽  
S. L. Gray ◽  
O. Martínez-Alvarado ◽  
K. D. Williams
Keyword(s):  
Case Studies ◽  
North Atlantic ◽  
Zonal Flow ◽  
Dynamical Theory ◽  
Atlantic Region ◽  
The North ◽  
Impact Experiment ◽  
Medium Range ◽  
Block Area ◽  
The Impact

AbstractAtmospheric blocking has been shown to be a phenomenon that models struggle to predict accurately, particularly the onset of a blocked state following a more zonal flow. This struggle is, in part, due to the lack of a complete dynamical theory for block onset and maintenance. Here, we evaluate the impact cyclone representation had on the forecast of block onset in two case studies from the North Atlantic Waveguide and Downstream Impact Experiment field campaign and the 20 most unpredictable block onsets over the Euro-Atlantic region in medium-range forecasts from the ECMWF. The 6-day forecast of block onset in the case studies is sensitive to changes in the forecast location and intensity of upstream cyclones (one cyclone for one case and two for the other case) in the days preceding the onset. Ensemble sensitivity analysis reveals that this is often the case in unpredictable block onset cases: a one standard deviation change in 1000-hPa geopotential height near an upstream cyclone, or 320-K potential vorticity near the tropopause, two or three days prior to block onset is associated with more than a 10% change in block area on the analyzed onset day in 17 of the 20 onset cases. These results imply that improvement in the forecasts of upstream cyclone location and intensity may help improve block onset forecasts.

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

2019 ◽  
Vol 32 (12) ◽  
pp. 3583-3597 ◽  
Author(s):  
Melanie Bieli ◽  
Suzana J. Camargo ◽  
Adam H. Sobel ◽  
Jenni L. Evans ◽  
Timothy Hall
Keyword(s):  
Phase Space ◽  
North Atlantic ◽  
North Pacific ◽  
Correlation Coefficients ◽  
Automated Identification ◽  
Extratropical Transition ◽  
The North ◽  
Objective Metrics ◽  
The North Atlantic

Abstract This study analyzes the differences between an objective, automated identification of tropical cyclones (TCs) that undergo extratropical transition (ET), and the designation of ET determined subjectively by human forecasters in best track data in all basins globally. The objective identification of ET is based on the cyclone phase space (CPS), calculated from the Japanese 55-yr Reanalysis (JRA-55) or the ECMWF interim reanalysis (ERA-Interim). The resulting classification into ET storms and non-ET storms underlies the global climatology of ET presented in Part I of this study. Here, the authors investigate how well the CPS classifications agree with those in the best track records calculated from JRA-55 or from ERA-Interim data. According to F1 scores and Matthews correlation coefficients (MCCs), the classification of ET storms in the CPS agrees best with the best track classification in the western North Pacific (MCC > 0.7) and the North Atlantic (MCC > 0.5). In other basins, the correlation between the CPS classification and the best track classification is only slightly higher than that of a random classification. The JRA-55 classification achieves higher performance scores than does the ERA-Interim classification, and the differences are statistically significant in all basins. The lower performance of ERA-Interim is mainly due to a higher false alarm rate, particularly in the eastern North Pacific. Overall, the results show that while the CPS-based classifications are good enough to be useful for many purposes, there is almost certainly room for improvement—in the representation of the storms in reanalyses, in our objective metrics of ET, and in our scientific understanding of the ET process.

Downstream Development Associated with the Extratropical Transition of Tropical Cyclones over the Western North Pacific

2009 ◽  
Vol 137 (4) ◽  
pp. 1295-1319 ◽  
Author(s):  
Patrick A. Harr ◽  
Jonathan M. Dea
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
High Amplitude ◽  
Extratropical Transition ◽  
The North ◽  
The Impact ◽  
The North Pacific ◽  
Downstream Development

Abstract The movement of a tropical cyclone into the midlatitudes involves interactions among many complex physical processes over a variety of space and time scales. Furthermore, the extratropical transition (ET) of a tropical cyclone may also result in a high-amplitude Rossby wave response that can extend to near-hemispheric scales. After an ET event occurs over the western portion of a Northern Hemisphere ocean basin, the high-amplitude downstream response often forces anomalous midlatitude circulations for periods of days to a week. These circulations may then be related to high-impact weather events far downstream of the forcing by the ET event. In this study, downstream development following ET events over the western North Pacific Ocean is examined. Local eddy kinetic energy analyses are conducted on four cases of North Pacific tropical cyclones of varying characteristics during ET into varying midlatitude flow characteristics during 15 July–30 September 2005. The goal is to examine the impact of each case on downstream development across the North Pacific during a period in which these events might increase the midlatitude cyclogenesis across the North Pacific during a season in which cyclogenesis is typically weak. Four typhoon (TY) cases from the summer of 2005 are chosen to represent the wide spectrum of variability in ET. This includes a case (TY Nabi 14W) that directly resulted in an intense midlatitude cyclone, a case in which a weak midlatitude cyclone resulted (TY Banyan 07W), a case in which the decaying tropical cyclone was absorbed into the midlatitude flow (TY Guchol 12W), and a case (TY Saola 17W) in which the tropical cyclone decayed under the influence of strong vertical wind shear. The variability in downstream response to each ET case is related to specific physical characteristics associated with the evolution of the ET process and the phasing between the poleward-moving tropical cyclone and the midlatitude circulation into which it is moving. A case of downstream development that occurred during September 2005 without an ET event is compared with the four ET cases.

scholarly journals The Impact of Anthropogenic Climate Change on North Atlantic Tropical Cyclone Tracks*

2013 ◽  
Vol 26 (12) ◽  
pp. 4088-4095 ◽  
Author(s):  
Angela J. Colbert ◽  
Brian J. Soden ◽  
Gabriel A. Vecchi ◽  
Ben P. Kirtman
Keyword(s):  
Tropical Cyclone ◽  
Greenhouse Gases ◽  
North Atlantic ◽  
Large Scale ◽  
Coupled Model ◽  
The North ◽  
Steering Flow ◽  
Southern Gulf Of Mexico ◽  
Projected Changes ◽  
The Impact

Abstract The authors examine the change in tropical cyclone (TC) tracks that results from projected changes in the large-scale steering flow and genesis location from increasing greenhouse gases. Tracks are first simulated using a Beta and Advection Model (BAM) and NCEP–NCAR reanalysis winds for all TCs that formed in the North Atlantic Ocean’s Main Development Region (MDR) for the period 1950–2010. Changes in genesis location and large-scale steering flow are then estimated from an ensemble mean of 17 models from phase 3 of the Coupled Model Intercomparison Project (CMIP3) for the A1b emissions scenario. The BAM simulations are then repeated with these changes to estimate how the TC tracks would respond to increased greenhouse gases. As the climate warms, the models project a weakening of the subtropical easterlies as well as an eastward shift in genesis location. This results in a statistically significant decrease in straight-moving (westward) storm tracks of ~5.5% and an increase in recurving (open ocean) tracks of ~5.5%. These track changes decrease TC counts over the southern Gulf of Mexico and Caribbean by 1–1.5 decade−1 and increase counts over the central Atlantic by 1–1.5 decade−1. Changes in the large-scale steering flow account for a vast majority of the projected changes in TC trajectories.

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