Analysis of the Wind Field Evolution Associated with the Extratropical Transition of Bonnie (1998)

Abstract Extratropical transition brings about a number of environmentally induced structural changes within a transitioning tropical cyclone. Of particular interest among these changes is the acceleration of the wind field away from the cyclone’s center of circulation along with the outward movement of the radial wind maximum, together termed wind field expansion. Previous informal hypotheses aimed at understanding this evolution do not entirely capture the observed expansion, while a review of the literature shows no formal work done upon the topic beyond analyzing its occurrence. This study seeks to analyze the physical and dynamical mechanisms behind the wind field expansion using model simulations of a representative transition case, North Atlantic Tropical Cyclone Bonnie of 1998. The acceleration of the wind field along the outer periphery of the cyclone is found to be a function of the net import of absolute angular momentum within the cyclone’s environment along inflowing trajectories. This evolution is shown to be a natural outgrowth of the development of isentropic conveyor belts and asymmetries associated with extratropical cyclones. Asymmetries in the outer-core wind field manifest themselves via the tightening and development of height and temperature gradients within the cyclone’s environment. Outward movement of the radial wind maximum occurs coincident with integrated net cooling found inside the radius of maximum winds. Tests using a secondary circulation balance model show the radial wind maximum evolution to be similar yet opposite to the response noted for intensifying tropical cyclones with contracting eyewalls.

Download Full-text

Influence of Cloud–Radiative Forcing on Tropical Cyclone Structure

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-0265.1 ◽

2014 ◽

Vol 71 (5) ◽

pp. 1644-1662 ◽

Cited By ~ 43

Author(s):

Yizhe Peggy Bu ◽

Robert G. Fovell ◽

Kristen L. Corbosiero

Keyword(s):

Tropical Cyclone ◽

Wind Field ◽

Radiative Forcing ◽

Outer Core ◽

Shortwave Radiation ◽

Convective Heating ◽

Cloud Radiative Forcing ◽

Radiative Processes ◽

Tangential Wind ◽

Radial Outflow

Abstract The authors demonstrate how and why cloud–radiative forcing (CRF), the interaction of hydrometeors with longwave and shortwave radiation, can influence tropical cyclone structure through “semi idealized” integrations of the Hurricane Weather Research and Forecasting model (HWRF) and an axisymmetric cloud model. Averaged through a diurnal cycle, CRF consists of pronounced cooling along the anvil top and weak warming through the cloudy air, which locally reverses the large net cooling that occurs in the troposphere under clear-sky conditions. CRF itself depends on the microphysics parameterization and represents one of the major reasons why simulations can be sensitive to microphysical assumptions. By itself, CRF enhances convective activity in the tropical cyclone’s outer core, leading to a wider eye, a broader tangential wind field, and a stronger secondary circulation. This forcing also functions as a positive feedback, assisting in the development of a thicker and more radially extensive anvil than would otherwise have formed. These simulations clearly show that the weak (primarily longwave) warming within the cloud anvil is the major component of CRF, directly forcing stronger upper-tropospheric radial outflow as well as slow, yet sustained, ascent throughout the outer core. In particular, this ascent leads to enhanced convective heating, which in turn broadens the wind field, as demonstrated with dry simulations using realistic heat sources. As a consequence, improved tropical cyclone forecasting in operational models may depend on proper representation of cloud–radiative processes, as they can strongly modulate the size and strength of the outer wind field that can potentially influence cyclone track as well as the magnitude of the storm surge.

Download Full-text

Influence of Cloud Microphysics and Radiation on Tropical Cyclone Structure and Motion

Meteorological Monographs ◽

10.1175/amsmonographs-d-15-0006.1 ◽

2016 ◽

Vol 56 ◽

pp. 11.1-11.27 ◽

Cited By ~ 17

Author(s):

Robert G. Fovell ◽

Yizhe Peggy Bu ◽

Kristen L. Corbosiero ◽

Wen-wen Tung ◽

Yang Cao ◽

...

Keyword(s):

Tropical Cyclone ◽

Wind Field ◽

Radiative Forcing ◽

Inner Core ◽

Longwave Radiation ◽

Outer Core ◽

Cloud Microphysics ◽

Convective Activity ◽

Radiative Processes ◽

Structure And Motion

Abstract The authors survey a series of modeling studies that have examined the influences that cloud microphysical processes can have on tropical cyclone (TC) motion, the strength and breadth of the wind field, inner-core diabatic heating asymmetries, outer-core convective activity, and the characteristics of the TC anvil cloud. These characteristics are sensitive to the microphysical parameterization (MP) in large part owing to the cloud-radiative forcing (CRF), the interaction of hydrometeors with radiation. The most influential component of CRF is that due to absorption and emission of longwave radiation in the anvil, which via gentle lifting directly encourages the more extensive convective activity that then leads to a radial expansion of the TC wind field. On a curved Earth, the magnitude of the outer winds helps determine the speed and direction of TC motion via the beta drift. CRF also influences TC motion by determining how convective asymmetries develop in the TC inner core. Further improvements in TC forecasting may require improved understanding and representation of cloud-radiative processes in operational models, and more comprehensive comparisons with observations are clearly needed.

Download Full-text

Departures from Axisymmetric Balance Dynamics during Secondary Eyewall Formation

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0018.1 ◽

2014 ◽

Vol 71 (10) ◽

pp. 3723-3738 ◽

Cited By ~ 29

Author(s):

Sergio F. Abarca ◽

Michael T. Montgomery

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Three Dimensional ◽

Balance Model ◽

Wind Maximum ◽

Secondary Eyewall Formation ◽

Tangential Wind ◽

Eyewall Replacement Cycle ◽

Tangential Momentum ◽

Balance Dynamics

Abstract Departures from axisymmetric balance dynamics are quantified during a case of secondary eyewall formation. The case occurred in a three-dimensional mesoscale convection-permitting numerical simulation of a tropical cyclone, integrated from an initial weak mesoscale vortex in an idealized quiescent environment. The simulation exhibits a canonical eyewall replacement cycle. Departures from balance dynamics are quantified by comparing the azimuthally averaged secondary circulation and corresponding tangential wind tendencies of the mesoscale integration with those diagnosed as the axisymmetric balanced response of a vortex subject to diabatic and tangential momentum forcing. Balance dynamics is defined here, following the tropical cyclone literature, as those processes that maintain a vortex in axisymmetric thermal wind balance. The dynamical and thermodynamical fields needed to characterize the background vortex for the Sawyer–Eliassen inversion are obtained by azimuthally averaging the relevant quantities in the mesoscale integration and by computing their corresponding balanced fields. Substantial differences between azimuthal averages and their homologous balance-derived fields are found in the boundary layer. These differences illustrate the inappropriateness of the balance assumption in this region of the vortex (where the secondary eyewall tangential wind maximum emerges). Although the balance model does broadly capture the sense of the forced transverse (overturning) circulation, the balance model is shown to significantly underestimate the inflow in the boundary layer. This difference translates to unexpected qualitative differences in the tangential wind tendency. The main finding is that balance dynamics does not capture the tangential wind spinup during the simulated secondary eyewall formation event.

Download Full-text

A high-fidelity parametric model for tropical cyclone boundary layer wind field by considering effects of land cover and terrain

Atmospheric Research ◽

10.1016/j.atmosres.2021.105701 ◽

2021 ◽

pp. 105701

Author(s):

Jian Yang ◽

Yu Chen ◽

Yanan Tang ◽

Guirong Yan ◽

Zhongdong Duan

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Land Cover ◽

Wind Field ◽

Parametric Model ◽

High Fidelity ◽

Tropical Cyclone Boundary Layer

Download Full-text

Intensity and Structure Changes during Hurricane Eyewall Replacement Cycles

Monthly Weather Review ◽

10.1175/mwr-d-11-00034.1 ◽

2011 ◽

Vol 139 (12) ◽

pp. 3829-3847 ◽

Cited By ~ 95

Author(s):

Matthew Sitkowski ◽

James P. Kossin ◽

Christopher M. Rozoff

Keyword(s):

Wind Field ◽

Inner Core ◽

Core Structure ◽

Wind Maximum ◽

Temporal Sampling ◽

Structure Changes ◽

Level Data ◽

Flight Level ◽

The Times ◽

Eyewall Replacement Cycles

Abstract A flight-level aircraft dataset consisting of 79 Atlantic basin hurricanes from 1977 to 2007 was used to develop an unprecedented climatology of inner-core intensity and structure changes associated with eyewall replacement cycles (ERCs). During an ERC, the inner-core structure was found to undergo dramatic changes that result in an intensity oscillation and rapid broadening of the wind field. Concentrated temporal sampling by reconnaissance aircraft in 14 of the 79 hurricanes captured virtually the entire evolution of 24 ERC events. The analysis of this large dataset extends the phenomenological paradigm of ERCs described in previous observational case studies by identifying and exploring three distinct phases of ERCs: intensification, weakening, and reintensification. In general, hurricanes intensify, sometimes rapidly, when outer wind maxima are first encountered by aircraft. The mean locations of the inner and outer wind maximum at the start of an ERC are 35 and 106 km from storm center, respectively. The intensification rate of the inner wind maximum begins to slow and the storm ultimately weakens as the inner-core structure begins to organize into concentric rings. On average, the inner wind maximum weakens 10 m s−1 before the outer wind maximum surpasses the inner wind maximum as it continues to intensify. This reintensification can be quite dramatic and often brings the storm to its maximum lifetime intensity. The entire ERC lasts 36 h on average. Comparison of flight-level data and microwave imagery reveals that the first appearance of an outer wind maximum, often associated with a spiral rainband, typically precedes the weakening of the storm by roughly 9 h, but the weakening is already well under way by the time a secondary convective ring with a well-defined moat appears in microwave imagery. The data also show that winds beyond the outer wind maximum remain elevated even after the outer wind maximum contracts inward. Additionally, the contraction of the outer wind maximum usually ceases at a radius larger than the location of the inner wind maximum at the start of the ERC. The combination of a larger primary eyewall and expanded outer wind field increase the integrated kinetic energy by an average of 28% over the course of a complete ERC despite little change in the maximum intensity between the times of onset and completion of the event.

Download Full-text

Simulated Sensitivity of Tropical Cyclone Size and Structure to the Atmospheric Temperature Profile

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0186.1 ◽

2016 ◽

Vol 73 (11) ◽

pp. 4553-4571 ◽

Cited By ~ 8

Author(s):

Diana R. Stovern ◽

Elizabeth A. Ritchie

Keyword(s):

Tropical Cyclone ◽

Surface Energy ◽

Temperature Profile ◽

Convective Available Potential Energy ◽

Atmospheric Temperature ◽

Outer Core ◽

Specific Humidity ◽

Energy Fluxes ◽

Surface Energy Fluxes ◽

Size And Structure

Abstract This study uses the WRF ARW to investigate how different atmospheric temperature environments impact the size and structure development of a simulated tropical cyclone (TC). In each simulation, the entire vertical virtual temperature profile is either warmed or cooled in 1°C increments from an initial specified state while the initial relative humidity profile and sea surface temperature are held constant. This alters the initial amount of convective available potential energy (CAPE), specific humidity, and air–sea temperature difference such that, when the simulated atmosphere is cooled (warmed), the initial specific humidity and CAPE decrease (increase), but the surface energy fluxes from the ocean increase (decrease). It is found that the TCs that form in an initially cooler environment develop larger wind and precipitation fields with more active outer-core rainband formation. Consistent with previous studies, outer-core rainband formation is associated with high surface energy fluxes, which leads to increases in the outer-core wind field. A larger convective field develops despite initializing in a low CAPE environment, and the dynamics are linked to a wider field of surface radial inflow. As the TC matures and radial inflow expands, large imports of relative angular momentum in the boundary layer continue to drive expansion of the TC’s overall size.

Download Full-text

The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

Monthly Weather Review ◽

10.1175/mwr-d-21-0191.1 ◽

2021 ◽

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Wind Field ◽

Surface Wind ◽

Frictional Stress ◽

Homogeneous Surface ◽

Sudden Decrease ◽

The Right ◽

Tropical Cyclone Boundary Layer

Abstract The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Lastly, a brief analysis of in-situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in-situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally-homogeneous surface types - which have been well-documented by previous studies - are inappropriate for use near strong frictional heterogeneity.

Download Full-text

The Extratropical Transition of Tropical Cyclones. Part I: Cyclone Evolution and Direct Impacts

Monthly Weather Review ◽

10.1175/mwr-d-17-0027.1 ◽

2017 ◽

Vol 145 (11) ◽

pp. 4317-4344 ◽

Cited By ~ 50

Author(s):

Clark Evans ◽

Kimberly M. Wood ◽

Sim D. Aberson ◽

Heather M. Archambault ◽

Shawn M. Milrad ◽

...

Keyword(s):

Tropical Cyclone ◽

North Atlantic ◽

North Pacific ◽

Western North Pacific ◽

North Atlantic Ocean ◽

Wind Waves ◽

Structural Evolution ◽

Hurricane Sandy ◽

Extratropical Transition ◽

Hazard Communication

Extratropical transition (ET) is the process by which a tropical cyclone, upon encountering a baroclinic environment and reduced sea surface temperature at higher latitudes, transforms into an extratropical cyclone. This process is influenced by, and influences, phenomena from the tropics to the midlatitudes and from the meso- to the planetary scales to extents that vary between individual events. Motivated in part by recent high-impact and/or extensively observed events such as North Atlantic Hurricane Sandy in 2012 and western North Pacific Typhoon Sinlaku in 2008, this review details advances in understanding and predicting ET since the publication of an earlier review in 2003. Methods for diagnosing ET in reanalysis, observational, and model-forecast datasets are discussed. New climatologies for the eastern North Pacific and southwest Indian Oceans are presented alongside updates to western North Pacific and North Atlantic Ocean climatologies. Advances in understanding and, in some cases, modeling the direct impacts of ET-related wind, waves, and precipitation are noted. Improved understanding of structural evolution throughout the transformation stage of ET fostered in large part by novel aircraft observations collected in several recent ET events is highlighted. Predictive skill for operational and numerical model ET-related forecasts is discussed along with environmental factors influencing posttransition cyclone structure and evolution. Operational ET forecast and analysis practices and challenges are detailed. In particular, some challenges of effective hazard communication for the evolving threats posed by a tropical cyclone during and after transition are introduced. This review concludes with recommendations for future work to further improve understanding, forecasts, and hazard communication.

Download Full-text