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.

scholarly journals A Comparison of Inner and Outer Spiral Rainbands in a Numerically Simulated Tropical Cyclone

2012 ◽  
Vol 140 (9) ◽  
pp. 2782-2805 ◽  
Author(s):  
Qingqing Li ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Horizontal Wind ◽  
Cold Pool ◽  
Wind Maximum ◽  
Low Level ◽  
Convective Systems ◽  
Tangential Wind ◽  
Spiral Rainbands ◽  
Convective Cells

Abstract The simulated inner and outer spiral rainbands in a tropical cyclone are compared in this study. The inner rainbands are generally active immediately outside the eyewall in the rapid filamentation zone, while the outer rainbands are active in regions outside about 3 times the radius of maximum wind. The inner rainbands are characterized by the convectively coupled vortex Rossby waves. The movement of the outer rainbands follows the low-level vector winds associated with the azimuthally averaged low-level flow and the radially outward cross-band flow caused by the downdraft-induced cold pool in the boundary layer. Convective cells in outer rainbands are typical of convective systems and move cyclonically and radially outward (inward) at large (small) radii. Net upward vertical mass transports (VMTs) appear throughout the depth of the troposphere in the whole inner-rainband region, while net downward VMTs are found below 4-km height in the outer-rainband region. In the whole inner-rainband region, only a very shallow layer with net horizontal convergence appears below 2-km height, while a deep layer with net convergence is found below 7.5-km height with net divergence aloft in the outer-rainband region. The inner rainband shows two tangential wind maxima, respectively, located near the top of the inflow boundary layer and immediately below the upper-tropospheric outflow layer. A secondary horizontal wind maximum occurs at about 4-km height on the inner edge of the outer rainband. Distinct features of the upwind, middle, and downwind sectors of the outer rainband are also discussed.

Asymmetric Rainband Processes Leading to Secondary Eyewall Formation in a Model Simulation of Hurricane Matthew (2016)

2021 ◽  
Vol 78 (1) ◽  
pp. 29-49
Author(s):  
Chau-Lam Yu ◽  
Anthony C. Didlake ◽  
Fuqing Zhang ◽  
Robert G. Nystrom
Keyword(s):  
Wind Field ◽  
Model Simulation ◽  
Wind Maximum ◽  
Low Level ◽  
Secondary Eyewall Formation ◽  
Positive Acceleration ◽  
Radial Extent ◽  
Tangential Wind ◽  
Low Levels ◽  
Left Quadrant

AbstractThe dynamics of an asymmetric rainband complex leading into secondary eyewall formation (SEF) are examined in a simulation of Hurricane Matthew (2016), with particular focus on the tangential wind field evolution. Prior to SEF, the storm experiences an axisymmetric broadening of the tangential wind field as a stationary rainband complex in the downshear quadrants intensifies. The axisymmetric acceleration pattern that causes this broadening is an inward-descending structure of positive acceleration nearly 100 km wide in radial extent and maximizes in the low levels near 50 km radius. Vertical advection from convective updrafts in the downshear-right quadrant largely contributes to the low-level acceleration maximum, while the broader inward-descending pattern is due to horizontal advection within stratiform precipitation in the downshear-left quadrant. This broad slantwise pattern of positive acceleration is due to a mesoscale descending inflow (MDI) that is driven by midlevel cooling within the stratiform regions and draws absolute angular momentum inward. The MDI is further revealed by examining the irrotational component of the radial velocity, which shows the MDI extending downwind into the upshear-left quadrant. Here, the MDI connects with the boundary layer, where new convective updrafts are triggered along its inner edge; these new upshear-left updrafts are found to be important to the subsequent axisymmetrization of the low-level tangential wind maximum within the incipient secondary eyewall.

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 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.

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 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.

scholarly journals Role of Cumulus Congestus in Tropical Cyclone Formation in a High-Resolution Numerical Model Simulation

2014 ◽  
Vol 71 (5) ◽  
pp. 1681-1700 ◽  
Author(s):  
Zhuo Wang
Keyword(s):  
High Resolution ◽  
Tropical Cyclone ◽  
Deep Layer ◽  
Dominant Role ◽  
Rapid Development ◽  
Deep Convection ◽  
Cyclonic Circulation ◽  
Low Level ◽  
Cumulus Congestus

Abstract The role of cumulus congestus (shallow and congestus convection) in tropical cyclone (TC) formation is examined in a high-resolution simulation of Tropical Cyclone Fay (2008). It is found that cumulus congestus plays a dominant role in moistening the lower to middle troposphere and spinning up the near-surface circulation prior to genesis, while deep convection plays a key role in moistening the upper troposphere and intensifying the cyclonic circulation over a deep layer. The transition from the tropical wave stage to the TC stage is marked by a substantial increase in net condensation and potential vorticity generation by deep convection in the inner wave pouch region. This study suggests that TC formation can be regarded as a two-stage process. The first stage is a gradual process of moisture preconditioning and low-level spinup, in which cumulus congestus plays a dominant role. The second stage commences with the rapid development of deep convection in the inner pouch region after the air column is moistened sufficiently, whereupon the concentrated convective heating near the pouch center strengthens the transverse circulation and leads to the amplification of the cyclonic circulation over a deep layer. The rapid development of deep convection can be explained by the power-law increase of precipitation rate with column water vapor (CWV) above a critical value. The high CWV near the pouch center thus plays an important role in convective organization. It is also shown that cumulus congestus can effectively drive the low-level convergence and provides a direct and simple pathway for the development of the TC protovortex near the surface.

The Roles of an Expanding Wind Field and Inertial Stability in Tropical Cyclone Secondary Eyewall Formation

2012 ◽  
Vol 69 (9) ◽  
pp. 2621-2643 ◽  
Author(s):  
Christopher M. Rozoff ◽  
David S. Nolan ◽  
James P. Kossin ◽  
Fuqing Zhang ◽  
Juan Fang
Keyword(s):  
Energy Efficiency ◽  
Tropical Cyclone ◽  
Kinetic Energy ◽  
Wind Field ◽  
Temperature Structure ◽  
Latent Heating ◽  
Vortex Model ◽  
Secondary Eyewall Formation ◽  
Tangential Wind ◽  
Transverse Circulation

Abstract The Weather and Research and Forecasting Model (WRF) is used to simulate secondary eyewall formation (SEF) in a tropical cyclone (TC) on the β plane. The simulated SEF process is accompanied by an outward expansion of kinetic energy and the TC warm core. An absolute angular momentum budget demonstrates that this outward expansion is predominantly a symmetric response to the azimuthal-mean and wavenumber-1 components of the transverse circulation. As the kinetic energy expands outward, the kinetic energy efficiency in which latent heating can be retained as local kinetic energy increases near the developing outer eyewall. The kinetic energy efficiency associated with SEF is examined further using a symmetric linearized, nonhydrostatic vortex model that is configured as a balanced vortex model. Given the symmetric tangential wind and temperature structure from WRF, which is close to a state of thermal wind balance above the boundary layer, the idealized model provides the transverse circulation associated with the symmetric latent heating and friction prescribed from WRF. In a number of ways, this vortex response matches the azimuthal-mean secondary circulation in WRF. These calculations suggest that sustained azimuthal-mean latent heating outside of the primary eyewall will eventually lead to SEF. Sensitivity experiments with the balanced vortex model show that, for a fixed amount of heating, SEF is facilitated by a broadening TC wind field.

Thermodynamic Aspects of Tropical Cyclone Formation

2012 ◽  
Vol 69 (8) ◽  
pp. 2433-2451 ◽  
Author(s):  
Zhuo Wang
Keyword(s):  
Tropical Cyclone ◽  
Deep Convection ◽  
Potential Temperature ◽  
Air Entrainment ◽  
Spatial Average ◽  
Low Level ◽  
Equivalent Potential ◽  
Thermodynamic Conditions ◽  
Short Incubation Time ◽  
Convective Organization

Abstract The thermodynamic aspects of tropical cyclone (TC) formation near the center of the wave pouch, a region of approximately closed Lagrangian circulation within the wave critical layer, are examined through diagnoses of a high-resolution numerical simulation and dropsonde data from a recent field campaign. It is found that the meso-β area near the pouch center is characterized by high saturation fraction, small difference in equivalent potential temperature θe between the surface and the middle troposphere, and a short incubation time scale. Updrafts tend to be more vigorous in this region, presumably because of reduced dry air entrainment, while downdrafts are not suppressed. The thermodynamic conditions near the pouch center are thus critically important for TC formation. The balanced responses to convective and stratiform heating at the pregenesis stage are examined using the Sawyer–Eliassen equation. Deep convection is concentrated near the pouch center. The strong radial and vertical gradients of latent heat release effectively force the transverse circulation and spin up a surface protovortex near the pouch center. Stratiform heating induces modest midlevel inflow and very weak low-level outflow, which contributes to the midlevel spinup without substantially spinning down the low-level circulation. The analysis of dropsonde data shows that the midlevel θe increases significantly near the pouch center one to two days prior to genesis but changes little away from the pouch center. This may indicate convective organization and the impending TC genesis. It also suggests that the critical information of TC genesis near the pouch center may be masked out if a spatial average is taken over the pouch scale.

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