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

2016 ◽  
Vol 56 ◽  
pp. 11.1-11.27 ◽  
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.

scholarly journals Influence of Cloud–Radiative Forcing on Tropical Cyclone Structure

2014 ◽  
Vol 71 (5) ◽  
pp. 1644-1662 ◽  
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.

scholarly journals The Influences of Boundary Layer Mixing and Cloud-Radiative Forcing on Tropical Cyclone Size

2017 ◽  
Vol 74 (4) ◽  
pp. 1273-1292 ◽  
Author(s):  
Yizhe Peggy Bu ◽  
Robert G. Fovell ◽  
Kristen L. Corbosiero
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Radiative Forcing ◽  
Vertical Mixing ◽  
Outer Core ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Convective Activity ◽  
Cloud Radiative Forcing ◽  
Mixing Coefficient

Abstract Tropical cyclone (TC) size is an important factor directly and indirectly influencing track, intensity, and related hazards, such as storm surge. Using a semi-idealized version of the operational Hurricane Weather Research and Forecasting Model (HWRF), the authors show that both enabling cloud-radiative forcing (CRF) and enhancing planetary boundary layer (PBL) vertical mixing can encourage wider storms by enhancing TC outer-core convective activity. While CRF acts primarily above the PBL, eddy mixing moistens the boundary layer from below, both making peripheral convection more likely. Thus, these two processes can cooperate and compete, making their influences difficult to deconvolve and complicating the evaluation of model physics improvements, especially since the sensitivity to both decreases as the environment becomes less favorable. Further study shows not only the magnitude of the eddy mixing coefficient but also the shape of it can determine the TC size and structure.

scholarly journals Impact of environmental moisture on tropical cyclone intensification

2015 ◽  
Vol 15 (11) ◽  
pp. 16111-16139 ◽  
Author(s):  
L. Wu ◽  
H. Su ◽  
R. G. Fovell ◽  
T. J. Dunkerton ◽  
Z. Wang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Convective Activity ◽  
Dry Air ◽  
Moving Storm ◽  
Lagrangian Boundary ◽  
Moisture Increase

Abstract. The impacts of environmental moisture on the intensification of a tropical cyclone (TC) are investigated in the Weather Research and Forecasting (WRF) model, with a focus on the azimuthal asymmetry of the moisture impacts. A series of sensitivity experiments with varying moisture perturbations in the environment are conducted and the Marsupial Paradigm framework is employed to understand the different moisture impacts. We find that modification of environmental moisture has insignificant impacts on the storm in this case unless it leads to convective activity in the environment, which deforms the quasi-Lagrangian boundary of the storm. By facilitating convection and precipitation outside the storm, enhanced environmental moisture ahead of the northwestward-moving storm induces a dry air intrusion to the inner core and limits TC intensification. However, increased moisture in the rear quadrants favors intensification by providing more moisture to the inner core and promoting storm symmetry, with primary contributions coming from moisture increase in the boundary layer. The different impacts of environmental moisture on TC intensification are governed by the relative locations of moisture perturbations and their interactions with the storm Lagrangian structure.

scholarly journals The Role of Inner-Core Moisture in Tropical Cyclone Predictability and Practical Forecast Skill

2017 ◽  
Vol 74 (7) ◽  
pp. 2315-2324 ◽  
Author(s):  
Kerry Emanuel ◽  
Fuqing Zhang
Keyword(s):  
Tropical Cyclone ◽  
Wind Field ◽  
Inner Core ◽  
Forecast Skill ◽  
Initial Condition ◽  
Tropical Cyclone Intensity ◽  
Cyclone Intensity ◽  
Storm Intensity ◽  
Tropical Cyclone Intensity Forecasts

Abstract Errors in tropical cyclone intensity forecasts are dominated by initial-condition errors out to at least a few days. Initialization errors are usually thought of in terms of position and intensity, but here it is shown that growth of intensity error is at least as sensitive to the specification of inner-core moisture as to that of the wind field. Implications of this finding for tropical cyclone observational strategies and for overall predictability of storm intensity are discussed.

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

2008 ◽  
Vol 136 (6) ◽  
pp. 2047-2065 ◽  
Author(s):  
Clark Evans ◽  
Robert E. Hart
Keyword(s):  
Tropical Cyclone ◽  
Wind Field ◽  
Structural Changes ◽  
Outer Core ◽  
Balance Model ◽  
Extratropical Transition ◽  
Wind Maximum ◽  
Outer Periphery ◽  
Conveyor Belts ◽  
Work Done

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.

scholarly journals Impact of environmental moisture on tropical cyclone intensification

2015 ◽  
Vol 15 (24) ◽  
pp. 14041-14053 ◽  
Author(s):  
L. Wu ◽  
H. Su ◽  
R. G. Fovell ◽  
T. J. Dunkerton ◽  
Z. Wang ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Moisture Transport ◽  
Inner Core ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Convective Activity ◽  
Dry Air ◽  
Moving Storm ◽  
Lagrangian Boundary ◽  
Moisture Increase

Abstract. The impacts of environmental moisture on the intensification of a tropical cyclone (TC) are investigated in the Weather Research and Forecasting (WRF) model, with a focus on the azimuthal asymmetry of the moisture impacts relative to the storm path. A series of sensitivity experiments with varying moisture perturbations in the environment are conducted and the Marsupial Paradigm framework is employed to understand the different moisture impacts. We find that modification of environmental moisture has insignificant impacts on the storm in this case unless it leads to convective activity that deforms the quasi-Lagrangian boundary of the storm and changes the moisture transport into the storm. By facilitating convection and precipitation outside the storm, enhanced environmental moisture ahead of the northwestward-moving storm induces a dry air intrusion to the inner core and limits TC intensification. In contrast, increased moisture in the rear quadrants favors intensification by providing more moisture to the inner core and promoting storm symmetry, with primary contributions coming from moisture increase in the boundary layer. The different impacts of environmental moisture on TC intensification are governed by the relative locations of moisture perturbations and their interactions with the storm Lagrangian structure.

scholarly journals Balanced Contribution to the Intensification of a Tropical Cyclone Simulated in TCM4: Outer-Core Spinup Process*

2011 ◽  
Vol 68 (3) ◽  
pp. 430-449 ◽  
Author(s):  
Hironori Fudeyasu ◽  
Yuqing Wang
Keyword(s):  
Angular Momentum ◽  
Tropical Cyclone ◽  
Diabatic Heating ◽  
Inner Core ◽  
Three Dimensional ◽  
Outer Core ◽  
Middle Troposphere ◽  
Tangential Wind ◽  
Spiral Rainbands ◽  
Balance Dynamics

Abstract The balanced contribution to the intensification of a tropical cyclone simulated in the three-dimensional, nonhydrostatic, full-physics tropical cyclone model version 4 (TCM4), in particular the spinup of the outer-core circulation, is investigated by solving the Sawyer–Eliassen equation and by computing terms in the azimuthal-mean tangential wind tendency equation. Results demonstrate that the azimuthal-mean secondary circulation (radial and vertical circulation) and the spinup of the midtropospheric outer-core circulation in the simulated tropical cyclone are well captured by balance dynamics. The midtropospheric inflow develops in response to diabatic heating in mid–upper-tropospheric stratiform (anvil) clouds outside the eyewall in active spiral rainbands and transports absolute angular momentum inward to spin up the outer-core circulation. Although the azimuthal-mean diabatic heating rate in the eyewall is the largest, its contribution to radial winds and thus the spinup of outer-core circulation in the middle troposphere is rather weak. This is because the high inertial stability in the inner-core region resists the radial inflow in the middle troposphere, limiting the inward transport of absolute angular momentum. The result thus suggests that diabatic heating in spiral rainbands is the key to the continued growth of the storm-scale circulation.

scholarly journals Impacts of Evaporation of Rainwater on Tropical Cyclone Structure and Intensity—A Revisit

2015 ◽  
Vol 72 (4) ◽  
pp. 1323-1345 ◽  
Author(s):  
Qingqing Li ◽  
Yuqing Wang ◽  
Yihong Duan
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Potential Temperature ◽  
Outer Core ◽  
Cold Pool ◽  
Equivalent Potential Temperature ◽  
Core Size ◽  
Size Change ◽  
Equivalent Potential ◽  
The Impact

Abstract The impact of evaporation of rainwater on tropical cyclone (TC) intensity and structure is revisited in this study. Evaporative cooling can result in strong downdrafts and produce low–equivalent potential temperature air in the inflow boundary layer, particularly in the region outside the eyewall, significantly suppressing eyewall convection and reducing the final intensity of a TC. Different from earlier findings, results from this study show that outer rainbands still form but are short lived in the absence of evaporation. Evaporation of rainwater is shown to facilitate the formation of outer rainbands indirectly by reducing the cooling due to melting of ice particles outside the inner core, not by the cold-pool dynamics, as previously believed. Only exclusion of evaporation in the eyewall region or the rapid filamentation zone has a very weak effect on the inner-core size change of a TC, whereas how evaporation in the outer core affects the inner-core size depends on how active the inner rainbands are. More (less) active inner rainbands may lead to an increase (a decrease) in the inner-core size.

scholarly journals Sensitivity of Tropical Cyclone Rainbands to Ice-Phase Microphysics

2005 ◽  
Vol 133 (8) ◽  
pp. 2473-2493 ◽  
Author(s):  
Charmaine N. Franklin ◽  
Greg J. Holland ◽  
Peter T. May
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Cloud Microphysics ◽  
Stratiform Region ◽  
Convective Scale ◽  
Sensitivity Studies ◽  
Mean Size ◽  
Cloud Ice ◽  
Rain Rates ◽  
Ice Phase

Abstract A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. As a first step, the model rainbands have been qualitatively compared with observed rainband characteristics. The model-generated rainbands show many of the mesoscale and convective-scale features of observed tropical cyclone rainbands. Sensitivity studies of numerically simulated tropical cyclone convection to ice-phase microphysical parameters showed that the model was most sensitive to changes in the graupel fall speed parameters. Increasing the fall speeds saw graupel being confined to the convective regions and producing higher rain rates in the inner core of the storm. A greater region of stratiform precipitation was produced when the efficiency for the collection of snow and cloud ice by graupel was reduced and when the mean size of graupel was reduced. Both of these simulations resulted in a higher concentration of snow being transported into the stratiform region. Although the precipitation structure changed across the simulations, the surface rainfall rate and the fundamental dynamical variables showed little sensitivity to the parameter variations.

scholarly journals A Model for the Complete Radial Structure of the Tropical Cyclone Wind Field. Part II: Wind Field Variability

2016 ◽  
Vol 73 (8) ◽  
pp. 3093-3113 ◽  
Author(s):  
Daniel R. Chavas ◽  
Ning Lin
Keyword(s):  
Tropical Cyclone ◽  
Physical Model ◽  
Wind Field ◽  
Inner Core ◽  
Outer Radius ◽  
Maximum Wind ◽  
Modes Of Variability ◽  
Simple Physical Model ◽  
Radial Structure ◽  
Field Variability

Abstract Part I of this work developed a simple physical model for the complete radial structure of the low-level azimuthal wind field in a tropical cyclone that compared well with observations. However, wind field variability in the model is tied principally to its external parameters given by the maximum wind speed and the radius of maximum wind, the latter of which lacks a credible independent physical model for its variability. Here the authors explore the modes of variability that arise from the alternative specification of the model, which takes the outer radius in lieu of the radius of maximum wind. Nondimensionalization of the model reveals two theoretical modes of structural variability in absolute angular momentum that are shown to closely match observations. These two modes correspond to three modes of wind field variability associated with variations in intensity, outer storm size, and latitude. These wind field modes are demonstrated to mirror the dominant modes of variability found in nature, in particular the intrastorm variation of inner-core structure and the interstorm variation of overall storm size. In combination, the model offers a credible physical solution for the complete time-dependent tropical cyclone wind field in conjunction with the external specification of intensity, outer size, and latitude. More broadly, the model offers theoretical and conceptual insight into the nature of the tropical cyclone wind field, including the oft-conflated terms “size” and “structure” and their distinct variabilities.

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