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 An Idealized Numerical Study of Shear-Relative Low-Level Mean Flow on Tropical Cyclone Intensity and Size

2019 ◽  
Vol 76 (8) ◽  
pp. 2309-2334 ◽  
Author(s):  
Buo-Fu Chen ◽  
Christopher A. Davis ◽  
Ying-Hwa Kuo
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Numerical Study ◽  
Vertical Wind Shear ◽  
Development Stage ◽  
Surface Fluxes ◽  
Mean Flow ◽  
Low Level ◽  
Core Convection

Abstract Given comparable background vertical wind shear (VWS) magnitudes, the initially imposed shear-relative low-level mean flow (LMF) is hypothesized to modify the structure and convective features of a tropical cyclone (TC). This study uses idealized Weather Research and Forecasting Model simulations to examine TC structure and convection affected by various LMFs directed toward eight shear-relative orientations. The simulated TC affected by an initially imposed LMF directed toward downshear left yields an anomalously high intensification rate, while an upshear-right LMF yields a relatively high expansion rate. These two shear-relative LMF orientations affect the asymmetry of both surface fluxes and frictional inflow in the boundary layer and thus modify the TC convection. During the early development stage, the initially imposed downshear-left LMF promotes inner-core convection because of high boundary layer moisture fluxes into the inner core and is thus favorable for TC intensification because of large radial fluxes of azimuthal mean vorticity near the radius of maximum wind in the boundary layer. However, TCs affected by various LMFs may modify the near-TC VWS differently, making the intensity evolution afterward more complicated. The TC with a fast-established eyewall in response to the downshear-left LMF further reduces the near-TC VWS, maintaining a relatively high intensification rate. For the upshear-right LMF that leads to active and sustained rainbands in the downshear quadrants, TC size expansion is promoted by a positive radial flux of eddy vorticity near the radius of 34-kt wind (1 kt ≈ 0.51 m s−1) because the vorticity associated with the rainbands is in phase with the storm-motion-relative inflow.

A Numerical Study of the Impact of Vertical Shear on the Distribution of Rainfall in Hurricane Bonnie (1998)

2003 ◽  
Vol 131 (8) ◽  
pp. 1577-1599 ◽  
Author(s):  
Robert Rogers ◽  
Shuyi Chen ◽  
Joseph Tenerelli ◽  
Hugh Willoughby
Keyword(s):  
Mesoscale Model ◽  
Numerical Study ◽  
Vertical Wind Shear ◽  
Vertical Motion ◽  
Environmental Flow ◽  
Vertical Shear ◽  
Low Level ◽  
Hurricane Bonnie ◽  
Along Track ◽  
The Impact

Abstract Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem. A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm. These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow. The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm. This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie (1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall. The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to simulate the storm between 0000 UTC 22 August and 0000 UTC 27 August 1998. During this period significant changes in the vertical shear occurred in the simulation. It changed from strong west-southwesterly, and across track, to much weaker south-southwesterly, and along track. Nearly concurrently, the azimuthal distribution of convection changed from a distinct wavenumber-1 pattern to almost azimuthally symmetric by the end of the time period. The strongest convection in the core was generally located on the downshear left side of the shear vector when the shear was strong. The azimuthal distributions and magnitudes of low-level radial inflow, reflectivity, boundary layer divergence, and low-level vertical motion all varied consistently with the evolution of the vertical shear. Additionally, the vortex showed a generally downshear tilt from the vertical. The magnitude of the tilt correlated well with changes in magnitude of the environmental shear. The accumulated rainfall was distributed symmetrically across the track of the storm when the shear was strong and across track, and it was distributed asymmetrically across the track of the storm when the shear was weak and along track.

scholarly journals Sensitivity of Fine-Scale Structure in Tropical Cyclone Boundary Layer to Model Horizontal Resolution at Sub-Kilometer Grid Spacing

2021 ◽  
Vol 9 ◽  
Author(s):  
Hongxiong Xu ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Mesoscale Model ◽  
Horizontal Resolution ◽  
Scale Structure ◽  
Fine Scale ◽  
Grid Spacing ◽  
Large Eddies ◽  
Horizontal Grid

In view of the increasing interest in the explicit simulation of fine-scale features in the tropical cyclone (TC) boundary layer (TCBL), the effects of horizontal grid spacing on a 7–10 h simulation of an idealized TC are examined using the Weather Research and Forecast (ARW-WRF) mesoscale model with one-way moving nests and the nonlinear backscatter with anisotropy (NBA) sub-grid-scale (SGS) scheme. In general, reducing the horizontal grid spacing from 2 km to 500 m tends to produce a stronger TC with lower minimum sea level pressure (MSLP), stronger surface winds, and smaller TC inner core size. However, large eddies cannot be resolved at these grid spacings. In contrast, reducing the horizontal grid spacing from 500 to 166 m and further to 55 m leads to a decrease in TC intensity and an increase in the inner-core TC size. Moreover, although the 166-m grid spacing starts to resolve large eddies in terms of TCBL horizontal rolls and tornado-scale vortex, the use of the finest grid spacing of 55 m tends to produce shorter wavelengths in the turbulent motion and stronger multi-scale turbulence interaction. It is concluded that a grid spacing of sub-100-meters is desirable to produce more detailed and fine-scale structure of TCBL horizontal rolls and tornado-scale vortices, while the relatively coarse sub-kilometer grid spacing (e.g., 500 m) is more cost-effective and feasible for research that is not interested in the turbulence processes and for real-time operational TC forecasting in the near future.

scholarly journals The Structure, Evolution, and Dynamics of a Nocturnal Convective System Simulated Using the WRF-ARW Model

2017 ◽  
Vol 145 (8) ◽  
pp. 3179-3201 ◽  
Author(s):  
Benjamin T. Blake ◽  
David B. Parsons ◽  
Kevin R. Haghi ◽  
Stephen G. Castleberry
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Vertical Shear ◽  
Structure Evolution ◽  
Cold Pool ◽  
Density Current ◽  
Convective System ◽  
Low Level ◽  
Arw Model ◽  
Low Level Jet

Previous studies have documented a nocturnal maximum in thunderstorm frequency during the summer across the central United States. Forecast skill for these systems remains relatively low and the explanation for this nocturnal maximum is still an area of active debate. This study utilized the WRF-ARW Model to simulate a nocturnal mesoscale convective system that occurred over the southern Great Plains on 3–4 June 2013. A low-level jet transported a narrow corridor of air above the nocturnal boundary layer with convective instability that exceeded what was observed in the daytime boundary layer. The storm was elevated and associated with bores that assisted in the maintenance of the system. Three-dimensional variations in the system’s structure were found along the cold pool, which were examined using convective system dynamics and wave theory. Shallow lifting occurred on the southern flank of the storm. Conversely, the southeastern flank had deep lifting, with favorable integrated vertical shear over the layer of maximum CAPE. The bore assisted in transporting high-CAPE air toward its LFC, and the additional lifting by the density current allowed for deep convection to occur. The bore was not coupled to the convective system and it slowly pulled away, while the convection remained in phase with the density current. These results provide a possible explanation for how convection is maintained at night in the presence of a low-level jet and a stable boundary layer, and emphasize the importance of the three-dimensionality of these systems.

scholarly journals The Roles of Asymmetric Inflow Forcing Induced by Outer Rainbands in Tropical Cyclone Secondary Eyewall Formation

2013 ◽  
Vol 70 (3) ◽  
pp. 953-974 ◽  
Author(s):  
Xin Qiu ◽  
Zhe-Min Tan
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Early Phase ◽  
Inner Core ◽  
Deep Convection ◽  
Leading Edge ◽  
Three Dimensions ◽  
Secondary Eyewall Formation ◽  
Layer Flow ◽  
The Impact

Abstract This study analyzes the secondary eyewall formation (SEF) process in an idealized cloud-resolving simulation of a tropical cyclone. In particular, the unbalanced boundary layer response to asymmetric inflow forcing induced by outer rainbands (ORBs) is examined in order to understand the mechanisms driving the sustained convection outside the primary eyewall during the early phase of SEF. The enhancement of convection in the SEF region follows the formation and inward contraction of an ORB. The azimuthal distribution of the enhanced convection is highly asymmetric but regular, generally along a half circle starting from the downwind portion of the ORB. It turns out that the descending radial inflow in the middle and downwind portions of the ORB initiates/maintains a strong inflow in the boundary layer. The latter is able to penetrate into the inner-core region, sharpens the gradient of radial velocity, and reinforces convergence. Consequently, warm and moist air is continuously lifted up at the leading edge of the strong inflow to support deep convection. Moreover, the inflow from the ORB creates strong supergradient winds that are ejected outward downwind, thereby enhancing convergence and convection on the other side of the storm. The results provide new insight into the key processes responsible for convection enhancement during the early phase of SEF in three dimensions and suggest the limitations of axisymmetric studies. There are also implications regarding the impact of the asymmetric boundary layer flow under a translating storm on SEF.

Respective and combined impacts of regional SST anomalies on tropical cyclogenesis in different sectors of the western North Pacific

2021 ◽  
Author(s):  
Renguang Wu
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Environmental Variables ◽  
Western North Pacific ◽  
Environmental Changes ◽  
North Atlantic Ocean ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Low Level ◽  
Sst Anomalies

<p>Tropical cyclone activity over the western North Pacific (WNP) is subjected to impacts of sea surface temperature (SST) anomalies in the three tropical oceans. In this talk, the interannual variations in the tropical cyclone (TC) over the WNP and the influences of regional SST anomalies are documented by separating the WNP into four quadrants considering SST-induced non-uniform environmental changes. It will be shown that the TC variations in the northwest and southeast quadrants are related to both equatorial central-eastern Pacific (EPO) and tropical Indian Ocean (TIO) SST anomalies. The TC variation in the northeast quadrant is mainly related to tropical North Atlantic Ocean (TNA) SST anomalies. The main environmental variables differ for the TC variations in the four quadrants. Low-level (850-hPa) vorticity is important for the TC variations in the northwest, southwest and southeast quadrants. Mid-level (700-hPa) humidity contributes to the TC variations in the northwest, northeast and southeast quadrants. The vertical shear has a supplementary contribution to the TC variation in the southeast quadrant. The potential intensity negatively affects the TC variations in the southwest and southeast quadrants. The remote SST anomalies modulate different environmental variables over the WNP. The TIO SST influence is manifested in the low-level vorticity and vertical motion. The TNA SST impact occurs through the low-level vorticity change. The EPO SST effect occurs via changing the low-level vorticity and vertical motion as well as the mid-level moisture and vertical shear. The environmental variables experience more prominent changes when SST anomalies coexist in two remote regions. Numerical experiments confirm the EPO and TIO SST anomaly impacts on the environmental conditions affecting the WNP TC variations.</p>

scholarly journals Frictionally Induced Feedback in a Reduced Dynamical Model of Tropical Cyclone Intensification

2020 ◽  
Vol 77 (11) ◽  
pp. 3821-3831
Author(s):  
Chanh Kieu ◽  
Richard Rotunno ◽  
Quan Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Direct Consequence ◽  
Feedback Mechanism ◽  
Surface Fluxes ◽  
Convective Heating ◽  
Heating Source ◽  
Specific Feedback ◽  
Frictional Convergence

AbstractThis study examines the role of frictional feedback in the atmospheric boundary layer during tropical cyclone (TC) development. Using a reduced model of TC dynamics, it is shown that a feedback between frictional convergence and convective heating in the absence of slantwise moist neutrality is capable of producing a stable maximum-intensity limit, even without surface fluxes. However, the efficiency of this frictional-convergence feedback depends crucially on how effectively boundary layer moisture convergence is converted into convective heating, which decreases rapidly as the TC inner core approaches a state of moist neutrality. This decreasing efficiency during TC intensification explains why the effect of the frictional-convergence feedback is generally small compared to that of the wind-induced surface heat exchange (WISHE) feedback under the strict conditions of slantwise moist neutrality. Examination of the reduced TC model with a constant-heating source reveals that TC intensification is not peculiar to any specific feedback mechanism but, rather, is a direct consequence of the inward advection of absolute angular momentum, regardless of feedback mechanism.

scholarly journals Recent Advances in Our Understanding of Tropical Cyclone Intensity Change Processes from Airborne Observations

Atmosphere ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 650
Author(s):  
Robert F. Rogers
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Vertical Shear ◽  
Intensity Change ◽  
Change Processes ◽  
Cyclone Intensity ◽  
Secondary Eyewall Formation ◽  
Warm Core ◽  
Major Hurricane ◽  
Upper Level

Recent (past ~15 years) advances in our understanding of tropical cyclone (TC) intensity change processes using aircraft data are summarized here. The focus covers a variety of spatiotemporal scales, regions of the TC inner core, and stages of the TC lifecycle, from preformation to major hurricane status. Topics covered include (1) characterizing TC structure and its relationship to intensity change; (2) TC intensification in vertical shear; (3) planetary boundary layer (PBL) processes and air–sea interaction; (4) upper-level warm core structure and evolution; (5) genesis and development of weak TCs; and (6) secondary eyewall formation/eyewall replacement cycles (SEF/ERC). Gaps in our airborne observational capabilities are discussed, as are new observing technologies to address these gaps and future directions for airborne TC intensity change research.

Tracking a Long-Lasting Outer Tropical Cyclone Rainband: Origin and Convective Transformation

2019 ◽  
Vol 76 (10) ◽  
pp. 3267-3283 ◽  
Author(s):  
Cheng-Ku Yu ◽  
Che-Yu Lin ◽  
Jhang-Shuo Luo
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Radial Distance ◽  
Vertical Shear ◽  
Convective Precipitation ◽  
Squall Line ◽  
Mature Stage ◽  
Clear Trend ◽  
Dynamical Interaction ◽  
Cold Pools

Abstract This study used radar and surface observations to track a long-lasting outer tropical cyclone rainband (TCR) of Typhoon Jangmi (2008) over a considerable period of time (~10 h) from its formative to mature stage. Detailed analyses of these unique observations indicate that the TCR was initiated on the eastern side of the typhoon at a radial distance of ~190 km as it detached from the upwind segment of a stratiform rainband located close to the inner-core boundary. The outer rainband, as it propagated cyclonically outward, underwent a prominent convective transformation from generally stratiform precipitation during the earlier period to highly organized, convective precipitation during its mature stage. The transformation was accompanied by a clear trend of surface kinematics and thermodynamics toward squall-line-like features. The observed intensification of the rainband was not simply related to the spatial variation of the ambient CAPE or potential instability; instead, the dynamical interaction between the prerainband vertical shear and cold pools, with progression toward increasingly optimal conditions over time, provides a reasonable explanation for the temporal alternation of the precipitation intensity. The increasing intensity of cold pools was suggested to play an essential role in the convective transformation for the rainband. The propagation characteristics of the studied TCR were distinctly different from those of wave disturbances frequently documented within the cores of tropical cyclones; however, they were consistent with the theoretically predicted propagation of convectively generated cold pools. The convective transformation, as documented in the present case, is anticipated to be one of the fundamental processes determining the evolving and structural nature of outer TCRs.

scholarly journals Revisiting the Balanced and Unbalanced Aspects of Tropical Cyclone Intensification

2017 ◽  
Vol 74 (8) ◽  
pp. 2575-2591 ◽  
Author(s):  
Junyao Heng ◽  
Yuqing Wang ◽  
Weican Zhou
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Model Simulation ◽  
Surface Friction ◽  
Core Region ◽  
Physics Model ◽  
The One ◽  
Balanced Dynamics ◽  
Balanced Solution

Abstract The balanced and unbalanced aspects of tropical cyclone (TC) intensification are revisited with the balanced contribution diagnosed with the outputs from a full-physics model simulation of a TC using the Sawyer–Eliassen (SE) equation. The results show that the balanced dynamics can well capture the secondary circulation in the full-physics model simulation even in the inner-core region in the boundary layer. The balanced dynamics can largely explain the intensification of the simulated TC. The unbalanced dynamics mainly acts to prevent the boundary layer agradient flow in the inner-core region from further intensification. Although surface friction can enhance the boundary layer inflow and make the inflow penetrate more inward into the eye region, contributing to the eyewall contraction, the net dynamical effect of surface friction on TC intensification is negative. The sensitivity of the balanced solution to the procedure used to ensure the ellipticity condition for the SE equation is also examined. The results show that the boundary layer inflow in the balanced response is very sensitive to the adjustment to inertial stability in the upper troposphere and the calculation of radial wind at the surface with relatively coarse vertical resolution in the balanced solution. Both the use of the so-called global regularization and the one-sided finite-differencing scheme used to calculate the surface radial wind in the balanced solution as utilized in some previous studies can significantly underestimate the boundary layer inflow. This explains why the boundary layer inflow in the balanced response is too weak in some previous studies.

Sign in / Sign up

Export Citation Format

Share Document