Rapid Filamentation Zone in a Numerically Simulated Tropical Cyclone*

2008 ◽  
Vol 65 (4) ◽  
pp. 1158-1181 ◽  
Author(s):  
Yuqing Wang
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Inner Core ◽  
Deep Convection ◽  
Quantitative Measure ◽  
Considerable Effect ◽  
Convective Clouds ◽  
Radar Images ◽  
Shearing Deformation ◽  
Spiral Rainbands

Abstract In a recent study, Rozoff et al. proposed a possible mechanism to explain the formation and maintenance of the weak-echo annulus (or moat) outside of the primary eyewall of a tropical cyclone observed in radar images. By this mechanism, the moat is determined to be a region of the strain-dominated flow outside of the radius of maximum wind in which essentially all fields are filamented and deep convection is hypothesized to be highly distorted and even suppressed. This strain-dominated region is defined as the rapid filamentation zone wherein the filamentation time is shorter than the overturning time of deep convection. An attempt has been made in this study to test the hypothesis in a full-physics tropical cyclone model under idealized conditions and to extend the concept to the study of the inner-core dynamics of tropical cyclones. The foci of this paper are the evolution of the rapid filamentation zone during the storm intensification, the potential roles of rapid filamentation in the organization of inner spiral rainbands, and the damping of high azimuthal wavenumber asymmetries in the tropical cyclone inner core. The presented results show that instead of suppressing deep convection, the strain flow in the rapid filamentation zone outside the elevated potential vorticity core provides a favorable environment for the organized inner spiral rainbands, which generally have time scales of several hours, much longer than the typical overturning time scale of individual convective clouds. Although the moat in the simulated tropical cyclone is located in the rapid filamentation zone, it is mainly controlled by the subsidence associated with the overturning flow from eyewall convection and downdrafts from the anvil stratiform precipitation outside of the eyewall. It is thus suggested that rapid filamentation is likely to play a secondary role in the formation of the moat in tropical cyclones. Although the deformation field is determined primarily by the structure of the tropical cyclone, it can have a considerable effect on the evolution of the storm. Because of strong straining deformation, asymmetries with azimuthal wavenumber >4 are found to be damped effectively in the rapid filamentation zone. The filamentation time thus provides a quantitative measure of the stabilization and axisymmetrization of high-wavenumber asymmetries in the inner core by shearing deformation and filamentation.

scholarly journals How Do Outer Spiral Rainbands Affect Tropical Cyclone Structure and Intensity?*

2009 ◽  
Vol 66 (5) ◽  
pp. 1250-1273 ◽  
Author(s):  
Yuqing Wang
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Pressure ◽  
Diabatic Heating ◽  
Inner Core ◽  
Phase Changes ◽  
Wave Radiation ◽  
Atmospheric Column ◽  
Tangential Wind ◽  
Spiral Rainbands

Abstract A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropical cyclones is studied through a series of numerical experiments using the cloud-resolving tropical cyclone model TCM4. Because diabatic heating due to phase changes is the main driving force of outer spiral rainbands, their effect on the tropical cyclone structure and intensity is evaluated by artificially modifying the heating and cooling rate due to cloud microphysical processes in the model. The view proposed here is that the effect of diabatic heating in outer spiral rainbands on the storm structure and intensity results mainly from hydrostatic adjustment; that is, heating (cooling) of an atmospheric column decreases (increases) the surface pressure underneath the column. The change in surface pressure due to heating in the outer spiral rainbands is significant on the inward side of the rainbands where the inertial stability is generally high. Outside the rainbands in the far field, where the inertial stability is low and internal atmospheric heating is mostly lost to gravity wave radiation and little is left to warm the atmospheric column and lower the local surface pressure, the change in surface pressure is relatively small. This strong radially dependent response reduces the horizontal pressure gradient across the radius of maximum wind and thus the storm intensity in terms of the maximum low-level tangential wind while increasing the inner-core size of the storm. The numerical results show that cooling in the outer spiral rainbands maintains both the intensity of a tropical cyclone and the compactness of its inner core, whereas heating in the outer spiral rainbands decreases the intensity but increases the size of a tropical cyclone. Overall, the presence of strong outer spiral rainbands limits the intensity of a tropical cyclone. Because heating or cooling in the outer spiral rainbands depends strongly on the relative humidity in the near-core environment, the results have implications for the formation of the annular hurricane structure, the development of concentric eyewalls, and the size change in tropical cyclones.

An Initialization Scheme for Tropical Cyclone Numerical Prediction by Enhancing Humidity in Deep-Convection Region

2013 ◽  
Vol 52 (10) ◽  
pp. 2260-2277 ◽  
Author(s):  
Jianyong Liu ◽  
Shunan Yang ◽  
Leiming Ma ◽  
Xuwei Bao ◽  
Dongliang Wang ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Deep Convection ◽  
Background Field ◽  
Weather Research And Forecasting ◽  
China Meteorological Administration ◽  
Brightness Temperatures ◽  
Simulated Distribution ◽  
Initialization Scheme ◽  
Spiral Rainbands

AbstractA nudging scheme for humidity fields is implemented in the Advanced Hurricane Weather Research and Forecasting model (WRF) for tropical cyclone (TC) initialization. The scheme improves TC simulation by enhancing the TC humidity profile in deep-convection regions, where it uses satellite Fengyun 2 cloud-top brightness temperatures as a judging criterion. The impacts of the nudging on predicting TC intensity and structure are evaluated through the simulation of TC Khanun (2005) during its movement toward landfall at the coast of Zhejiang Province, China. During the nudging, the humidity distributions at the TC's inner core and along its outer spiral rainbands, where deep convections occur, are both enhanced. As a result, the intensity of the vortex is enhanced, being more consistent to the best-track data from the China Meteorological Administration. Specifically, the nudging modifies the simulated distribution of humidity according to convective activities captured by the satellite and therefore adjusts the development of deep convection in the model, which then influences the intensity and size of TC vortex through diabatic heating. During WRF simulation, the TC vortex initialized from the humidity nudging is dynamically and thermodynamically balanced with the background field, favoring a steady development of the vortex's intensity and structure. Because of the better simulation of TC inner core and outer spiral rainbands, the WRF simulation skills of TC intensity and track are improved.

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

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.

scholarly journals A High-Resolution Simulation of Asymmetries in Severe Southern Hemisphere Tropical Cyclone Larry (2006)

2009 ◽  
Vol 137 (12) ◽  
pp. 4171-4187 ◽  
Author(s):  
Hamish A. Ramsay ◽  
Lance M. Leslie ◽  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Southern Hemisphere ◽  
Inner Core ◽  
Mesoscale Model ◽  
Deep Convection ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Low Level ◽  
Frictional Convergence

Abstract Advances in observations, theory, and modeling have revealed that inner-core asymmetries are a common feature of tropical cyclones (TCs). In this study, the inner-core asymmetries of a severe Southern Hemisphere tropical cyclone, TC Larry (2006), are investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the Kepert–Wang boundary layer model. The MM5-simulated TC exhibited significant asymmetries in the inner-core region, including rainfall distribution, surface convergence, and low-level vertical motion. The near-core environment was characterized by very low environmental vertical shear and consequently the TC vortex had almost no vertical tilt. It was found that, prior to landfall, the rainfall asymmetry was very pronounced with precipitation maxima consistently to the right of the westward direction of motion. Persistent maxima in low-level convergence and vertical motion formed ahead of the translating TC, resulting in deep convection and associated hydrometeor maxima at about 500 hPa. The asymmetry in frictional convergence was mainly due to the storm motion at the eyewall, but was dominated by the proximity to land at larger radii. The displacement of about 30°–120° of azimuth between the surface and midlevel hydrometeor maxima is explained by the rapid cyclonic advection of hydrometeors by the tangential winds in the TC core. These results for TC Larry support earlier studies that show that frictional convergence in the boundary layer can play a significant role in determining the asymmetrical structures, particularly when the environmental vertical shear is weak or absent.

scholarly journals Passive Microwave Quantification of Tropical Cyclone Inner-Core Cloud Populations Relative to Subsequent Intensity Change

2016 ◽  
Vol 144 (11) ◽  
pp. 4461-4482 ◽  
Author(s):  
Daniel S. Harnos ◽  
Stephen W. Nesbitt
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Statistical Significance ◽  
Deep Convection ◽  
Detection Algorithm ◽  
Passive Microwave ◽  
Intensity Change ◽  
Atmospheric Column ◽  
Full Intensity ◽  
Relative Prevalence

Abstract Characteristics of over 15 000 tropical cyclone (TC) inner cores are evaluated coincidentally using 37- and 85-GHz passive microwave data to quantify the relative prevalence of cold clouds (i.e., deep convection and stratiform clouds) versus predominantly warm clouds (i.e., shallow cumuli and cumulus congestus). Results indicate greater presence of combined liquid and frozen hydrometeors associated with cold clouds within the atmospheric column for TCs undergoing subsequent rapid intensification (RI) or intensification. RI episodes compared to the full intensity change distribution exhibit approximately an order of magnitude increase for inner-core cold cloud frequency relative to warm cloud presence. Incorporation of an objective ring detection algorithm shows the robust presence of rings associated with hydrometeors for 85-GHz polarization corrected temperatures () and 37-GHz vertically polarized brightness temperatures () for differentiating RI with significance levels ≥99.99%, while 37-GHz false color rings of a combined cyan and pink appearance surrounding a region that is not cyan or pink lack statistical significance for discriminating RI against lesser intensification. Rings of depressed and enhanced tied to RI suggest the combined presence of liquid and frozen hydrometeors within the atmospheric column, indicative of cold clouds. The rings also exhibit preferences for those with collocated more widespread ice scattering signatures to be more commonly associated with RI and general intensification.

scholarly journals The Role of Near-Core Convective and Stratiform Heating/Cooling in Tropical Cyclone Structure and Intensity

2018 ◽  
Vol 75 (1) ◽  
pp. 297-326 ◽  
Author(s):  
Guanghua Chen ◽  
Chun-Chieh Wu ◽  
Yi-Hsuan Huang
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Core Region ◽  
Intensity Change ◽  
Moderate Intensity ◽  
Convective Heating ◽  
Core Size ◽  
Heating And Cooling ◽  
Diabatic Processes ◽  
Spiral Rainbands

The effects of convective and stratiform diabatic processes in the near-core region on tropical cyclone (TC) structure and intensity change are examined by artificially modifying the convective and stratiform heating/cooling between 40- and 80-km radii. Sensitivity experiments show that the absence of convective heating in the annulus can weaken TC intensity and decrease the inner-core size. The increased convective heating generates a thick and polygonal eyewall, while the storm intensifies more gently than that in the control run. The removal of stratiform heating can slow down TC intensification with a moderate intensity, whereas the doubling of stratiform heating has little effect on the TC evolution compared to the control run. The halved stratiform cooling facilitates TC rapid intensification and a compact inner-core structure with the spiral rainbands largely suppressed. With the stratiform cooling doubled, the storm terminates intensification and eventually develops a double-eyewall-like structure accompanied by the significantly outward expansion of the inner-core size. The removal of both stratiform heating and cooling generates the strongest storm with the structure and intensity similar to those in the experiment with stratiform cooling halved. When both stratiform heating and cooling are doubled, the storm first decays rapidly, followed by the vertical connection of the updrafts at mid- to upper levels in the near-core region and at lower levels in the collapsed eyewall, which reinvigorates the eyewall convection but with a large outward slope.

scholarly journals Impact of Stratiform Rainband Heating on the Tropical Cyclone Wind Field in Idealized Simulations

2019 ◽  
Vol 76 (8) ◽  
pp. 2443-2462 ◽  
Author(s):  
Chau-Lam Yu ◽  
Anthony C. Didlake
Keyword(s):  
Tropical Cyclone ◽  
Wind Field ◽  
Deep Convection ◽  
Cold Pool ◽  
Low Level ◽  
Secondary Eyewall Formation ◽  
Heating And Cooling ◽  
Storm Evolution ◽  
Spiral Rainbands ◽  
Diabatic Forcing

Abstract Using idealized simulations, we examine the storm-scale wind field response of a dry, hurricane-like vortex to prescribed stratiform heating profiles that mimic tropical cyclone (TC) spiral rainbands. These profiles were stationary with respect to the storm center to represent the diabatic forcing imposed by a quasi-stationary rainband complex. The first profile was typical of stratiform precipitation with heating above and cooling below the melting level. The vortex response included a mesoscale descending inflow and a midlevel tangential jet, consistent with previous studies. An additional response was an inward-spiraling low-level updraft radially inside the rainband heating. The second profile was a modified stratiform heating structure derived from observations and consisted of a diagonal dipole of heating and cooling. The same features were found with stronger magnitudes and larger vertical extents. The dynamics and implications of the forced low-level updraft were examined. This updraft was driven by buoyancy advection because of the stratiform-induced low-level cold pool. The stationary nature of the rainband diabatic forcing played an important role in modulating the required temperature and pressure anomalies to sustain this updraft. Simulations with moisture and full microphysics confirmed that this low-level updraft response was robust and capable of triggering sustained deep convection that could further impact the storm evolution, including having a potential role in secondary eyewall formation.

Rapid Filamentation Zones in Intense Tropical Cyclones

2006 ◽  
Vol 63 (1) ◽  
pp. 325-340 ◽  
Author(s):  
Christopher M. Rozoff ◽  
Wayne H. Schubert ◽  
Brian D. McNoldy ◽  
James P. Kossin
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Coherent Structures ◽  
Deep Convection ◽  
Relative Vorticity ◽  
Moist Convection ◽  
Vortex Interaction ◽  
Tangential Wind ◽  
Stratiform Precipitation ◽  
Deep Moist Convection

Abstract Intense tropical cyclones often possess relatively little convection around their cores. In radar composites, this surrounding region is usually echo-free or contains light stratiform precipitation. While subsidence is typically quite pronounced in this region, it is not the only mechanism suppressing convection. Another possible mechanism leading to weak-echo moats is presented in this paper. The basic idea is that the strain-dominated flow surrounding an intense vortex core creates an unfavorable environment for sustained deep, moist convection. Strain-dominated regions of a tropical cyclone can be distinguished from rotation-dominated regions by the sign of S21 + S22 − ζ2, where S1 = ux − υy and S2 = υx + uy are the rates of strain and ζ = υx − uy is the relative vorticity. Within the radius of maximum tangential wind, the flow tends to be rotation-dominated (ζ2 > S21 + S22), so that coherent structures, such as mesovortices, can survive for long periods of time. Outside the radius of maximum tangential wind, the flow tends to be strain-dominated (S21 + S22 > ζ2), resulting in filaments of anomalous vorticity. In the regions of strain-dominated flow the filamentation time is defined as τfil = 2(S21 + S22 − ζ2)−1/2. In a tropical cyclone, an approximately 30-km-wide annular region can exist just outside the radius of maximum tangential wind, where τfil is less than 30 min and even as small as 5 min. This region is defined as the rapid filamentation zone. Since the time scale for deep moist convective overturning is approximately 30 min, deep convection can be significantly distorted and even suppressed in the rapid filamentation zone. A nondivergent barotropic model illustrates the effects of rapid filamentation zones in category 1–5 hurricanes and demonstrates the evolution of such zones during binary vortex interaction and mesovortex formation from a thin annular ring of enhanced vorticity.

Spiral Rainbands in a Numerical Simulation of Hurricane Bill (2009). Part II: Propagation of Inner Rainbands

2015 ◽  
Vol 72 (1) ◽  
pp. 191-215 ◽  
Author(s):  
Yumin Moon ◽  
David S. Nolan
Keyword(s):  
Numerical Simulation ◽  
Tropical Cyclone ◽  
Gravity Waves ◽  
Wind Field ◽  
Rossby Waves ◽  
Alternative Hypothesis ◽  
Convective Clouds ◽  
Squall Lines ◽  
Spiral Rainbands

Abstract This is the second part of a study that examines spiral rainbands in a numerical simulation of Hurricane Bill (2009). This paper evaluates whether the propagation of inner rainbands in the Hurricane Bill simulation is consistent with previously proposed hypotheses. Results indicate that the propagation of inner rainbands is not consistent with gravity waves, vortex Rossby waves, or squall lines. An alternative hypothesis is offered, arguing that inner rainbands are simply convective clouds that are advected by the rapidly rotating tropical cyclone wind field while being deformed into spiral shapes. A summary and a discussion of the results of both Parts I and II are provided.

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.

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