An Ensemble Approach to Investigate Tropical Cyclone Intensification in Sheared Environments. Part II: Ophelia (2011)

Abstract The mechanisms leading to tropical cyclone (TC) intensification amid moderate vertical wind shear can vary from case to case, depending on the vortex structure and the large-scale conditions. To search for similarities between cases, this second part investigates the rapid intensification of Hurricane Ophelia (2011) in an environment characterized by 200–850-hPa westerly shear exceeding 8 m s−1. Similar to Part I, a 96-member ensemble was employed to compare a subset of members that predicted Ophelia would intensify with another subset that predicted Ophelia would weaken. This comparison revealed that the intensification of Ophelia was aided by enhanced convection and midtropospheric moisture in the downshear and left-of-shear quadrants. Enhanced left-of-shear convection was key to the establishment of an anticyclonic divergent outflow that forced a nearby upper-tropospheric trough to wrap around Ophelia. A vorticity budget showed that deep convection also contributed to the enhancement of vorticity within the inner core of Ophelia via vortex stretching and tilting of horizontal vorticity enhanced by the upper-tropospheric trough. These results suggest that TC intensity changes in sheared environments and in the presence of upper-tropospheric troughs highly depend on the interaction between convective-scale processes and the large-scale flow. Given the similarities between Part I and this part, the results suggest that observations from the three-dimensional moisture and wind fields could improve both forecasting and understanding of TC intensification in moderately sheared environments.

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On the Rapid Weakening of Very Intense Tropical Cyclone Hellen (2014)

Monthly Weather Review ◽

10.1175/mwr-d-18-0309.1 ◽

2019 ◽

Vol 147 (8) ◽

pp. 2717-2737 ◽

Cited By ~ 6

Author(s):

Adrien Colomb ◽

Tarik Kriat ◽

Marie-Dominique Leroux

Keyword(s):

Tropical Cyclone ◽

Wind Shear ◽

Large Scale ◽

Inner Core ◽

Potential Temperature ◽

Vertical Wind Shear ◽

Warm Core ◽

Operational Forecasting ◽

Intensity Changes ◽

The Impact

Abstract In late March 2014, very intense Tropical Cyclone Hellen threatened the Comoros Archipelago and the Madagascan northwest coastline as it became one of the strongest tropical cyclones (TCs) ever observed over the Mozambique Channel. Its steep intensity changes were not well anticipated by operational forecasting models or by La Reunion regional specialized meteorological center forecasters. In particular, the record-setting rapid weakening over the open ocean was not supported by usual large-scale predictors. AROME, a new nonhydrostatic finescale model, is able to closely reproduce these wide intensity changes. When benchmarked against available observations, the model is also consistent in terms of inner-core structure, environmental features, track, and intensity. In the simulation, a northwesterly 400-hPa environmental wind is associated with unsaturated air, while the classic 200–850-hPa wind shear remains weak, and does not suggest a specifically unfavorable environment. The 400-hPa constraint affects the simulated storm through two pathways. Air with low equivalent potential temperature (θe) is flushed downward into the inflow layer in the upshear semicircle, triggering the decay of the storm. Then, direct erosion of the upper half of the warm core efficiently increases the surface pressure and also plays an instrumental role in the rapid weakening. When the storm gets closer to the Madagascan coastline, low-θe air can be directly advected within the inflow layer. Results illustrate on a real TC case the recently proposed paradigm for TC intensity modification under vertical wind shear and highlight the need for innovative tools to assess the impact of wind shear at all vertical levels.

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Studying dynamic and thermodynamic influences on hurricane activity with a tropical cyclone emulator

10.5194/egusphere-egu21-8399 ◽

2021 ◽

Author(s):

Peter Pfleiderer ◽

Shruti Nath ◽

Abigal Jaye ◽

Carl-Friedrich Schleussner

Keyword(s):

Tropical Cyclone ◽

Large Scale ◽

Vertical Wind Shear ◽

Self Organizing Map ◽

Weather Patterns ◽

Wind Speeds ◽

Available Energy ◽

Global Circulation Models ◽

Open Questions ◽

Intensity Changes

Global warming influences tropical cyclones (TC) and their impacts in different ways. Warmer sea surface temperatures (SST) are expected to lead to stronger intensification, the increased water holding capacity of warmer air increases the precipitation brought by TCs. These are thermodynamic changes that are rather well understood.When it comes to the influence of circulation changes on tropical cyclone activity open questions remain: Will there be more or less TCs in a warmer world? And what would be the physical mechanism for a change in TC frequencies?TC formation and intensification not only depends on the available energy but also on the large-scale atmospheric circulation. For instance, TC development is strongly hampered when the vertical wind shear (difference between upper and lower level wind speeds) is high.Here we present a tropical cyclone season emulator for the Atlantic basin that produces TCs based on SSTs averaged over the Atlantic main development region and daily time series of weather patterns obtained from a self-organizing map clustering. The emulator is based on probabilities for storm genesis, storm length and intensity changes that were empirically assessed using the ERA5 reanalysis and IBTrACS TC observations.We see different applications for this emulator: 1) While most global circulation models (GCM) fail to adequately simulate TCs, their projections for SSTs and large-scale weather patterns contains valuable information. Using our emulator, we could indirectly analyse TC activity projections for all available GCMs. 2) In the emulator thermodynamic (SSTs) and dynamic influences (weather patterns) are distinct inputs. This allows us to construct different counterfactuals to attribute changes in TC activity to thermodynamic or dynamic changes. For example, the emulator could be used to simulate TC seasons with large scale circulation as observed in 2017 but with preindustrial SSTs, so as to analyse the extent to which warming of the ocean surface had contributed to the extreme hurricane season of 2017.

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Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part III: Diagnosis of Developing and Nondeveloping Storms

Journal of the Atmospheric Sciences ◽

10.1175/jas4023.1 ◽

2007 ◽

Vol 64 (9) ◽

pp. 3195-3213 ◽

Cited By ~ 25

Author(s):

K. J. Tory ◽

N. E. Davidson ◽

M. T. Montgomery

Keyword(s):

Tropical Cyclone ◽

Large Scale ◽

Deep Convection ◽

Weather Prediction ◽

Vertical Wind Shear ◽

Forecast Model ◽

Vertical Shear ◽

Limited Area ◽

Vortex Interactions ◽

Secondary Mechanism

Abstract This is the third of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. In Parts I and II, a primary and two secondary vortex enhancement mechanisms were illustrated, and shown to be responsible for TC genesis in a simulation of TC Chris. In this paper, five more TC-LAPS simulations are investigated: three developing and two nondeveloping. In each developing simulation the pathway to genesis was essentially the same as that reported in Part II. Potential vorticity (PV) cores developed through low- to middle-tropospheric vortex enhancement in model-resolved updraft cores (primary mechanism) and interacted to form larger cores through diabatic upscale vortex cascade (secondary mechanism). On the system scale, vortex intensification resulted from the large-scale mass redistribution forced by the upward mass flux, driven by diabatic heating, in the updraft cores (secondary mechanism). The nondeveloping cases illustrated that genesis can be hampered by (i) vertical wind shear, which may tilt and tear apart the PV cores as they develop, and (ii) an insufficient large-scale cyclonic environment, which may fail to sufficiently confine the warming and enhanced cyclonic winds, associated with the atmospheric adjustment to the convective updrafts. The exact detail of the vortex interactions was found to be unimportant for qualitative genesis forecast success. Instead the critical ingredients were found to be sufficient net deep convection in a sufficiently cyclonic environment in which vertical shear was less than some destructive limit. The often-observed TC genesis pattern of convection convergence, where the active convective regions converge into a 100-km-diameter center, prior to an intense convective burst and development to tropical storm intensity is evident in the developing TC-LAPS simulations. The simulations presented in this study and numerous other simulations not yet reported on have shown good qualitative forecast success. Assuming such success continues in a more rigorous study (currently under way) it could be argued that TC genesis is largely predictable provided the large-scale environment (vorticity, vertical shear, and convective forcing) is sufficiently resolved and initialized.

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Large-Eddy Simulation of Extreme Updrafts in the Tropical Cyclone Inner Core

10.5194/egusphere-egu2020-9581 ◽

2020 ◽

Author(s):

Liguang Wu

Keyword(s):

Tropical Cyclone ◽

Large Eddy Simulation ◽

Large Scale ◽

Inner Core ◽

Vertical Wind Shear ◽

Core Region ◽

Vertical Wind ◽

Eddy Simulation ◽

Large Eddy ◽

Wind Shears

Extreme updrafts (stronger than 10 m s-1) have been observed in the tropical cyclone core region, which have profound implications to tropical cyclone intensification and structure change. Since extreme updrafts in the tropical cyclone are difficult to observe, their features and the associated mechanisms for formation and influences on tropical cyclones remain poorly understood. This study presents an analysis of extreme updrafts in a strong tropical cyclone that was simulated with the large-eddy simulation technique and the finest grid spacing of 37 meters. The simulated tropical cyclone experiences the vertical wind shear of about 5 m s-1 in a typical large-scale evironment in the western North Pacific. The simulated extreme updrafts in the inner core region exhibit the high frequency at the altitudes of ~ 750 m, 6.5 km and 13 km. The extreme updrafts in the inflow and outflow layers are closely associated with the Richardson Number of less than 0.25, indicating their relationship with severe turbulence caused by strong vertical wind shears. The extreme updrafts in the middle layer are associated with the strong convective activity. The details of the structures of the extreme updrafts are discussed.

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Interactions between Simulated Tropical Cyclones and an Environment with a Variable Coriolis Parameter

Monthly Weather Review ◽

10.1175/mwr3359.1 ◽

2007 ◽

Vol 135 (5) ◽

pp. 1889-1905 ◽

Cited By ~ 37

Author(s):

Elizabeth A. Ritchie ◽

William M. Frank

Keyword(s):

Tropical Cyclone ◽

Tropical Cyclones ◽

Large Scale ◽

Inner Core ◽

Mesoscale Model ◽

Vertical Wind Shear ◽

State University ◽

Coriolis Parameter ◽

Fine Mesh ◽

Fifth Generation

Abstract Numerical simulations of tropical cyclones are performed to examine the effects of a variable Coriolis parameter on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When a variable Conolis parameter ( f ) environment is applied to a mature tropical cyclone, a persistent north-northwesterly shear develops over the storm center as a result of an interaction between the primary circulation of the storm and the gradient in absolute vorticity. As a result, the variable-f storm quickly develops a persistent wavenumber-1 asymmetry in its inner-core structure with upward motion and rainfall concentrated on the left side of the shear looking downshear, in agreement with earlier studies. In comparison, the constant-f storm develops weak transient asymmetries in structure that are only partially related to a weak vertical wind shear. As a result, it is found that the tropical cyclone with variable f intensifies slightly more slowly than that with constant f, and reaches a final intensity that is about 5 mb weaker. It is argued that this “beta shear” is not adequately represented in large-scale analyses and so does not figure into calculations of environmental shear. Although the effect of the beta shear on the tropical cyclone intensity seems small by itself, when combined with the environmental shear it can produce a large net shear or it can reduce an environmental shear below the apparent threshold to impact storm intensity. If this result proves to be generally true, then the presence of an additional overlooked beta shear may well explain differences in the response of tropical cyclone intensification to westerly versus easterly shear regimes.

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Mesoscale Features Associated with Tropical Cyclone Formations in the Western North Pacific

Monthly Weather Review ◽

10.1175/2007mwr2267.1 ◽

2008 ◽

Vol 136 (6) ◽

pp. 2006-2022 ◽

Cited By ~ 32

Author(s):

Cheng-Shang Lee ◽

Kevin K. W. Cheung ◽

Jenny S. N. Hui ◽

Russell L. Elsberry

Keyword(s):

Tropical Cyclone ◽

North Pacific ◽

Western North Pacific ◽

Large Scale ◽

North Pacific Ocean ◽

Deep Convection ◽

Mesoscale Convective System ◽

Convective System ◽

Synoptic Patterns ◽

The Mean

Abstract The mesoscale features of 124 tropical cyclone formations in the western North Pacific Ocean during 1999–2004 are investigated through large-scale analyses, satellite infrared brightness temperature (TB), and Quick Scatterometer (QuikSCAT) oceanic wind data. Based on low-level wind flow and surge direction, the formation cases are classified into six synoptic patterns: easterly wave (EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow (NE–SW), southwesterly flow (SW), monsoon confluence (MC), and monsoon shear (MS). Then the general convection characteristics and mesoscale convective system (MCS) activities associated with these formation cases are studied under this classification scheme. Convection processes in the EW cases are distinguished from the monsoon-related formations in that the convection is less deep and closer to the formation center. Five characteristic temporal evolutions of the deep convection are identified: (i) single convection event, (ii) two convection events, (iii) three convection events, (iv) gradual decrease in TB, and (v) fluctuating TB, or a slight increase in TB before formation. Although no dominant temporal evolution differentiates cases in the six synoptic patterns, evolutions ii and iii seem to be the common routes taken by the monsoon-related formations. The overall percentage of cases with MCS activity at multiple times is 63%, and in 35% of cases more than one MCS coexisted. Most of the MC and MS cases develop multiple MCSs that lead to several episodes of deep convection. These two patterns have the highest percentage of coexisting MCSs such that potential interaction between these systems may play a role in the formation process. The MCSs in the monsoon-related formations are distributed around the center, except in the NE–SW cases in which clustering of MCSs is found about 100–200 km east of the center during the 12 h before formation. On average only one MCS occurs during an EW formation, whereas the mean value is around two for the other monsoon-related patterns. Both the mean lifetime and time of first appearance of MCS in EW are much shorter than those developed in other synoptic patterns, which indicates that the overall formation evolution in the EW case is faster. Moreover, this MCS is most likely to be found within 100 km east of the center 12 h before formation. The implications of these results to internal mechanisms of tropical cyclone formation are discussed in light of other recent mesoscale studies.

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Dynamical and Physical Processes Leading to Tropical Cyclone Intensification under Upper-Level Trough Forcing

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0293.1 ◽

2013 ◽

Vol 70 (8) ◽

pp. 2547-2565 ◽

Cited By ~ 22

Author(s):

Marie-Dominique Leroux ◽

Matthieu Plu ◽

David Barbary ◽

Frank Roux ◽

Philippe Arbogast

Keyword(s):

Angular Momentum ◽

Tropical Cyclone ◽

Potential Vorticity ◽

Weather Prediction ◽

Three Dimensional ◽

Vertical Wind Shear ◽

Thick Layer ◽

Limited Area ◽

Southwest Indian Ocean ◽

Upper Level

Abstract The rapid intensification of Tropical Cyclone (TC) Dora (2007, southwest Indian Ocean) under upper-level trough forcing is investigated. TC–trough interaction is simulated using a limited-area operational numerical weather prediction model. The interaction between the storm and the trough involves a coupled evolution of vertical wind shear and binary vortex interaction in the horizontal and vertical dimensions. The three-dimensional potential vorticity structure associated with the trough undergoes strong deformation as it approaches the storm. Potential vorticity (PV) is advected toward the tropical cyclone core over a thick layer from 200 to 500 hPa while the TC upper-level flow turns cyclonic from the continuous import of angular momentum. It is found that vortex intensification first occurs inside the eyewall and results from PV superposition in the thick aforementioned layer. The main pathway to further storm intensification is associated with secondary eyewall formation triggered by external forcing. Eddy angular momentum convergence and eddy PV fluxes are responsible for spinning up an outer eyewall over the entire troposphere, while spindown is observed within the primary eyewall. The 8-km-resolution model is able to reproduce the main features of the eyewall replacement cycle observed for TC Dora. The outer eyewall intensifies further through mean vertical advection under dynamically forced upward motion. The processes are illustrated and quantified using various diagnostics.

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Intensification of Tilted Tropical Cyclones Over Relatively Cool and Warm Oceans in Idealized Numerical Simulations

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-21-0051.1 ◽

2021 ◽

Author(s):

David A. Schecter

Keyword(s):

Relative Humidity ◽

Tropical Cyclones ◽

Inner Core ◽

Structural Parameters ◽

Deep Convection ◽

Surface Wind ◽

Vertical Wind Shear ◽

Surface Wind Speed ◽

Cloud Resolving Model ◽

Complete Alignment

Abstract A cloud resolving model is used to examine the intensification of tilted tropical cyclones from depression to hurricane strength over relatively cool and warm oceans under idealized conditions where environmental vertical wind shear has become minimal. Variation of the SST does not substantially change the time-averaged relationship between tilt and the radial length scale of the inner core, or between tilt and the azimuthal distribution of precipitation during the hurricane formation period (HFP). By contrast, for systems having similar structural parameters, the HFP lengthens superlinearly in association with a decline of the precipitation rate as the SST decreases from 30 to 26 °C. In many simulations, hurricane formation progresses from a phase of slow or neutral intensification to fast spinup. The transition to fast spinup occurs after the magnitudes of tilt and convective asymmetry drop below certain SST-dependent levels following an alignment process explained in an earlier paper. For reasons examined herein, the alignment coincides with enhancements of lower–middle tropospheric relative humidity and lower tropospheric CAPE inward of the radius of maximum surface wind speed rm. Such moist-thermodynamic modifications appear to facilitate initiation of the faster mode of intensification, which involves contraction of rm and the characteristic radius of deep convection. The mean transitional values of the tilt magnitude and lower–middle tropospheric relative humidity for SSTs of 28-30 °C are respectively higher and lower than their counterparts at 26 °C. Greater magnitudes of the surface enthalpy flux and core deep-layer CAPE found at the higher SSTs plausibly compensate for less complete alignment and core humidification at the transition time.

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Observational Analysis of Tropical Cyclone Formation Associated with Monsoon Gyres

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0117.1 ◽

2013 ◽

Vol 70 (4) ◽

pp. 1023-1034 ◽

Cited By ~ 38

Author(s):

Liguang Wu ◽

Huijun Zong ◽

Jia Liang

Keyword(s):

Tropical Cyclone ◽

Tropical Cyclones ◽

Western North Pacific ◽

Large Scale ◽

Vertical Wind Shear ◽

Relative Vorticity ◽

Monsoon Trough ◽

Cyclonic Circulation ◽

Tropical Cyclone Formation ◽

Monsoon Gyre

Abstract Large-scale monsoon gyres and the involved tropical cyclone formation over the western North Pacific have been documented in previous studies. The aim of this study is to understand how monsoon gyres affect tropical cyclone formation. An observational study is conducted on monsoon gyres during the period 2000–10, with a focus on their structures and the associated tropical cyclone formation. A total of 37 monsoon gyres are identified in May–October during 2000–10, among which 31 monsoon gyres are accompanied with the formation of 42 tropical cyclones, accounting for 19.8% of the total tropical cyclone formation. Monsoon gyres are generally located on the poleward side of the composited monsoon trough with a peak occurrence in August–October. Extending about 1000 km outward from the center at lower levels, the cyclonic circulation of the composited monsoon gyre shrinks with height and is replaced with negative relative vorticity above 200 hPa. The maximum winds of the composited monsoon gyre appear 500–800 km away from the gyre center with a magnitude of 6–10 m s−1 at 850 hPa. In agreement with previous studies, the composited monsoon gyre shows enhanced southwesterly flow and convection on the south-southeastern side. Most of the tropical cyclones associated with monsoon gyres are found to form near the centers of monsoon gyres and the northeastern end of the enhanced southwesterly flows, accompanying relatively weak vertical wind shear.

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Evaluation of a Wind-Wave System for Ensemble Tropical Cyclone Wave Forecasting. Part I: Winds

Weather and Forecasting ◽

10.1175/waf-d-12-00054.1 ◽

2013 ◽

Vol 28 (2) ◽

pp. 297-315 ◽

Cited By ~ 4

Author(s):

Steven M. Lazarus ◽

Samuel T. Wilson ◽

Michael E. Splitt ◽

Gary A. Zarillo

Keyword(s):

Tropical Cyclone ◽

Inner Core ◽

Synthetic Data ◽

Radial Distance ◽

Wave System ◽

Computationally Efficient ◽

Wind Fields ◽

Translation Speed ◽

Maximum Wind ◽

Regional Reanalysis

Abstract A computationally efficient method of producing tropical cyclone (TC) wind analyses is developed and tested, using a hindcast methodology, for 12 Gulf of Mexico storms. The analyses are created by blending synthetic data, generated from a simple parametric model constructed using extended best-track data and climatology, with a first-guess field obtained from the NCEP–NCAR North American Regional Reanalysis (NARR). Tests are performed whereby parameters in the wind analysis and vortex model are varied in an attempt to best represent the TC wind fields. A comparison between nonlinear and climatological estimates of the TC size parameter indicates that the former yields a much improved correlation with the best-track radius of maximum wind rm. The analysis, augmented by a pseudoerror term that controls the degree of blending between the NARR and parametric winds, is tuned using buoy observations to calculate wind speed root-mean-square deviation (RMSD), scatter index (SI), and bias. The bias is minimized when the parametric winds are confined to the inner-core region. Analysis wind statistics are stratified within a storm-relative reference frame and by radial distance from storm center, storm intensity, radius of maximum wind, and storm translation speed. The analysis decreases the bias and RMSD in all quadrants for both moderate and strong storms and is most improved for storms with an rm of less than 20 n mi. The largest SI reductions occur for strong storms and storms with an rm of less than 20 n mi. The NARR impacts the analysis bias: when the bias in the former is relatively large, it remains so in the latter.

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