scholarly journals Characteristics of the Concentric Eyewall Structure of Super Typhoon Muifa during Its Formation and Replacement Processes

2019 ◽  
Author(s):  
Tonggui Bo ◽  
Yudi Liu ◽  
Dawei Li ◽  
Lang Huang ◽  
Yi Yu
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Lower Troposphere ◽  
Super Typhoon ◽  
Early Signal ◽  
Tangential Wind ◽  
Radar Echo ◽  
Vertical Wind Speed ◽  
Potential Vortex ◽  
Physical Quantities

To explore the characteristics of the concentric eyewall of a typhoon during its formation and replacement processes, with Super Typhoon Muifa in 2011 as the example case, the Weather Research and Forecast (WRF) mode was used to carry out a numerical simulation to reproduce the entire formation and replacement processes of the concentric eyewall. The physical quantities such as the tangential wind speed, radar echo, radial wind speed, vertical wind speed, and potential vortex were diagnosed and analyzed. The results of the analysis show that the outward expansion of the isovelocity in the lower troposphere was the early signal of the formation of the outer eyewall. After the outer eyewall formed, there was a center of second-highest tangential wind speed in the corresponding area. The second-highest wind speed increased as the strength of the outer eyewall increased, and the position of the second-highest wind speed center was retracted with the retraction of the outer eyewall. The tangential wind speed of the moat area was smaller than that corresponding to the concentric eyewall and this feature gradually disappeared with the increase of the height. The echo in the moat area was weak, and this characteristic was particularly evident when the moat area was relatively wide and the outer eyewall was relatively strong. With the formation and development of the outer eyewall, the intensity of the inflow in the boundary layer corresponding to the inner eyewall was reduced, the intensity of the outflow in the upper layers declined, and the intensities of the inflow and outflow corresponding to the outer eyewall were enhanced. After the second outer eyewall matured, there was a significant inflow in the upper layer of the moat area. Once the outer eyewall formed, a large amount of hydrometeors appeared in the corresponding area, and there was a strong ascending motion inside that area. The strength of the ascending motion and the content of hydrometeors increased as the outer eyewall increased. When the moat area was relatively wide, the divergent airflow generated by the developed outer eyewall in the upper layer would produce a significant descending motion in the moat area.

Characteristics of the Concentric Eyewall Structure of Super Typhoon Muifa during Its Formation and Replacement Processes

2020 ◽  
Author(s):  
Yudi Liu ◽  
Dawei Li ◽  
Lang Huang
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Lower Troposphere ◽  
Super Typhoon ◽  
Early Signal ◽  
Tangential Wind ◽  
Radar Echo ◽  
Vertical Wind Speed ◽  
Potential Vortex ◽  
Physical Quantities

<p>To explore the characteristics of the concentric eyewall of a typhoon during its formation and replacement processes, with Super Typhoon Muifa in 2011 as the example case, the Weather Research and Forecast (WRF) mode was used to carry out a numerical simulation to reproduce the entire formation and replacement processes of the concentric eyewall. The physical quantities such as the tangential wind speed, radar echo, radial wind speed, vertical wind speed, and potential vortex were diagnosed and analyzed. The results of the analysis show that the outward expansion of the isovelocity in the lower troposphere was the early signal of the formation of the outer eyewall. After the outer eyewall formed, there was a center of second-highest tangential wind speed in the corresponding area. The second-highest wind speed increased as the strength of the outer eyewall increased, and the position of the second-highest wind speed center was retracted with the retraction of the outer eyewall. The tangential wind speed of the moat area was smaller than that corresponding to the concentric eyewall and this feature gradually disappeared with the increase of the height. The echo in the moat area was weak, and this characteristic was particularly evident when the moat area was relatively wide and the outer eyewall was relatively strong. With the formation and development of the outer eyewall, the intensity of the inflow in the boundary layer corresponding to the inner eyewall was reduced, the intensity of the outflow in the upper layers declined, and the intensities of the inflow and outflow corresponding to the outer eyewall were enhanced. After the second outer eyewall matured, there was a significant inflow in the upper layer of the moat area. Once the outer eyewall formed, a large amount of hydrometeors appeared in the corresponding area, and there was a strong ascending motion inside that area. The strength of the ascending motion and the content of hydrometeors increased as the outer eyewall increased. When the moat area was relatively wide, the divergent airflow generated by the developed outer eyewall in the upper layer would produce a significant descending motion in the moat area.</p>

scholarly journals An Observational Study of the Symmetric Boundary Layer Structure and Tropical Cyclone Intensity

Atmosphere ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 158 ◽  
Author(s):  
Yifang Ren ◽  
Jun A. Zhang ◽  
Jonathan L. Vigh ◽  
Ping Zhu ◽  
Hailong Liu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Layer Structure ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Storm Intensity ◽  
Boundary Layer Structure ◽  
Warm Core ◽  
Tangential Wind

This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity.

scholarly journals Hurricane Boundary Layer Height Relative to Storm Motion from GPS Dropsonde Composites

Atmosphere ◽  
2019 ◽  
Vol 10 (6) ◽  
pp. 339 ◽  
Author(s):  
Yifang Ren ◽  
Jun A. Zhang ◽  
Stephen R. Guimond ◽  
Xiang Wang
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Layer Height ◽  
Boundary Layer Height ◽  
Tangential Wind ◽  
Hurricane Boundary Layer ◽  
The Right

This study investigates the asymmetric distribution of hurricane boundary layer height scales in a storm-motion-relative framework using global positioning system (GPS) dropsonde observations. Data from a total of 1916 dropsondes collected within four times the radius of maximum wind speed of 37 named hurricanes over the Atlantic basin from 1998 to 2015 are analyzed in the composite framework. Motion-relative quadrant mean composite analyses show that both the kinematic and thermodynamic boundary layer height scales tend to increase with increasing radius in all four motion-relative quadrants. It is also found that the thermodynamic mixed layer depth and height of maximum tangential wind speed are within the inflow layer in all motion-relative quadrants. The inflow layer depth and height of the maximum tangential wind are both found to be deeper in the two front quadrants, and they are largest in the right-front quadrant. The difference in the thermodynamic mixed layer depth between the front and back quadrants is smaller than that in the kinematic boundary layer height. The thermodynamic mixed layer is shallowest in the right-rear quadrant, which may be due to the cold wake phenomena. The boundary layer height derived using the critical Richardson number ( R i c ) method shows a similar front-back asymmetry as the kinematic boundary layer height.

Does the Rotating Convection Paradigm Describe Secondary Eyewall Formation in Idealized Three-dimensional Simulations?

2021 ◽  
Keyword(s):  
Boundary Layer ◽  
Wind Field ◽  
Three Dimensional ◽  
Lower Troposphere ◽  
Rotating Convection ◽  
Secondary Eyewall Formation ◽  
A Value ◽  
Tangential Wind ◽  
Rain Fall ◽  
Three Dimensional Simulations

Abstract The formation of a plausible secondary eyewall is examined with two principal simulation experiments that differ only in the fixed value of rain fall speed, one with a value of 70 m s−1 (approaching the pseudo-adiabatic limit) that simulates a secondary eyewall, and one with a value of 7 m s−1 that does not simulate a secondary eyewall. Key differences are sought between these idealized three-dimensional simulations. A notable expansion of the lower-tropospheric tangential wind field to approximately 400 km radius is found associated with the precursor period of the secondary eyewall. The wind field expansion is traced to an enhanced vertical mass flux across the 5.25-km height level, which leads, in turn, to enhanced radial inflow in the lower troposphere and above the boundary layer. The inflow spins up the tangential wind outside the primary eyewall via the conventional spin-up mechanism. This amplified tangential wind field is linked to a broad region of outwardly-directed agradient force in the upper boundary layer. Whereas scattered convection is found outside the primary eyewall in both simulations, the agradient force is shown to promote a ring-like organization of this convection when boundary layer convergence occurs in a persistent, localized region of super-gradient winds. The results support prior work highlighting a new model of secondary eyewall formation emphasizing a boundary layer control pathway for initiating the outer eyewall as part of the rotating convection paradigm of tropical cyclone evolution.

Revisiting the Dynamics of Eyewall Contraction of Tropical Cyclones

2019 ◽  
Vol 76 (10) ◽  
pp. 3229-3245 ◽  
Author(s):  
Yuanlong Li ◽  
Yuqing Wang ◽  
Yanluan Lin
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Directional Derivative ◽  
Three Dimensional ◽  
Lower Boundary ◽  
Lower Troposphere ◽  
Surface Friction ◽  
Three Dimensions ◽  
Tangential Wind ◽  
Lower Boundary Layer

Abstract The dynamics of eyewall contraction of tropical cyclones (TCs) has been revisited in this study based on both three-dimensional and axisymmetric simulations and dynamical diagnostics. Because eyewall contraction is closely related to the contraction of the radius of maximum wind (RMW), its dynamics is thus often studied by examining the RMW tendency in previous studies. Recently, Kieu and Stern et al. proposed two different frameworks to diagnose the RMW tendency but had different conclusions. In this study, the two frameworks are evaluated first based on theoretical analysis and idealized numerical simulations. It is shown that the framework of Kieu is a special case of the earlier framework of Willoughby et al. if the directional derivative is applied. An extension of Stern et al.’s approach not only can reproduce but also can predict the RMW tendency. A budget of the azimuthal-mean tangential wind tendency indicates that the contributions by radial and vertical advections to the RMW tendency vary with height. Namely, radial advection dominates the RMW contraction in the lower boundary layer, and vertical advection favors the RMW contraction in the upper boundary layer and lower troposphere. In addition to the curvature, the increase of the radial gradient of horizontal mixing (including the resolved eddy mixing in three dimensions) near the eyewall prohibits eyewall contraction in the lower boundary layer. Besides, the vertical mixing including surface friction also plays an important role in the cessation of eyewall contraction in the lower boundary layer.

scholarly journals The Axisymmetric and Asymmetric Aspects of the Secondary Eyewall Formation in a Numerically Simulated Tropical Cyclone under Idealized Conditions on an f Plane

2019 ◽  
Vol 76 (1) ◽  
pp. 357-378 ◽  
Author(s):  
Hui Wang ◽  
Yuqing Wang ◽  
Jing Xu ◽  
Yihong Duan
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Tropical Cyclone ◽  
Diabatic Heating ◽  
Lower Troposphere ◽  
Outer Edge ◽  
Top Down ◽  
Secondary Eyewall Formation ◽  
Tangential Wind ◽  
Surface Enthalpy

Abstract The axisymmetric and asymmetric aspects of the secondary eyewall formation (SEF) in a numerically simulated tropical cyclone (TC) under idealized conditions were analyzed. Consistent with previous findings, prior to the SEF, the tangential wind of the TC experienced an outward expansion both above and within the boundary layer near and outside the region of the SEF later. This outward expansion was found to be closely related to the top-down development and inward propagation of a strong outer rainband, which was characterized by deeper and more intense convection upwind and shallower and weaker convection downwind. In response to diabatic heating in the outer rainband was inflow in the mid- to lower troposphere, which brought the absolute angular momentum inward and spun up tangential wind in the inflow region and also in the convective region because of vertical advection. As a result, as the outer rainband intensified and spiraled cyclonically inward, perturbation tangential and radial winds also spiraled cyclonically inward and downward along the rainband. As it approached the outer edge of the rapid filamentation zone outside the primary eyewall, the downwind sector of the rainband in the boundary layer was rapidly axisymmetrized. Continuous inward propagation and axisymmetrization and secondarily the merging with inner rainbands led to the spinup of tangential wind in the boundary layer, enhancing surface enthalpy flux and convection and eventually leading to the simulated SEF. Our results demonstrate that the simulated SEF was a top-down process and was mainly triggered by asymmetric dynamics.

scholarly journals On Boundary Layer Separation in the Lee of Mesoscale Topography

2007 ◽  
Vol 64 (2) ◽  
pp. 401-420 ◽  
Author(s):  
Qingfang Jiang ◽  
James D. Doyle ◽  
Shouping Wang ◽  
Ronald B. Smith
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Wind Speed ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Lower Troposphere ◽  
Boundary Layer Separation ◽  
Surface Wind Speed ◽  
Trapped Waves ◽  
Layer Separation

Abstract The onset of boundary layer separation (BLS) forced by gravity waves in the lee of mesoscale topography is investigated based on a series of numerical simulations and analytical formulations. It is demonstrated that BLS forced by trapped waves is governed by a normalized ratio of the vertical velocity maximum to the surface wind speed; other factors such as the mountain height, mountain slope, or the leeside speedup factor are less relevant. The onset of BLS is sensitive to the surface sensible heat flux—a positive heat flux tends to increase the surface wind speed through enhancing the vertical momentum mixing and accordingly inhibits the occurrence of BLS, and a negative heat flux does the opposite. The wave forcing required to cause BLS decreases with an increase of the aerodynamical roughness zo; a larger zo generates larger surface stress and weaker surface winds and therefore promotes BLS. In addition, BLS shows some sensitivity to the terrain geometry, which modulates the wave characteristics. For a wider ridge, a higher mountain is required to generate trapped waves with a wave amplitude comparable to that generated by a lower but narrower ridge. The stronger hydrostatic waves associated with the wider and higher ridge play only a minor role in the onset of BLS. It has been demonstrated that although hydrostatic waves generally do not directly induce BLS, undular bores may form associated with wave breaking in the lower troposphere, which in turn induce BLS. In addition, BLS could occur underneath undular jump heads or associate with trapped waves downstream of a jump head in the presence of a low-level inversion.

Understanding Atypical Midlevel Wind Speed Maxima in Hurricane Eyewalls

2020 ◽  
Vol 77 (5) ◽  
pp. 1531-1557 ◽  
Author(s):  
Daniel P. Stern ◽  
Jeffrey D. Kepert ◽  
George H. Bryan ◽  
James D. Doyle
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Surface Wind ◽  
Local Maximum ◽  
Surface Friction ◽  
Surface Wind Speed ◽  
Small Radius ◽  
Inertial Oscillation ◽  
Tangential Wind ◽  
Wind Speed Maximum

Abstract In tropical cyclones (TCs), the peak wind speed is typically found near the top of the boundary layer (approximately 0.5–1 km). Recently, it was shown that in a few observed TCs, the wind speed within the eyewall can increase with height within the midtroposphere, resulting in a secondary local maximum at 4–5 km. This study presents additional evidence of such an atypical structure, using dropsonde and Doppler radar observations from Hurricane Patricia (2015). Near peak intensity, Patricia exhibited an absolute wind speed maximum at 5–6-km height, along with a weaker boundary layer maximum. Idealized simulations and a diagnostic boundary layer model are used to investigate the dynamics that result in these atypical wind profiles, which only occur in TCs that are very intense (surface wind speed > 50 m s−1) and/or very small (radius of maximum winds < 20 km). The existence of multiple maxima in wind speed is a consequence of an inertial oscillation that is driven ultimately by surface friction. The vertical oscillation in the radial velocity results in a series of unbalanced tangential wind jets, whose magnitude and structure can manifest as a midlevel wind speed maximum. The wavelength of the inertial oscillation increases with vertical mixing length l∞ in a turbulence parameterization, and no midlevel wind speed maximum occurs when l∞ is large. Consistent with theory, the wavelength in the simulations scales with (2K/I)1/2, where K is the (vertical) turbulent diffusivity, and I2 is the inertial stability. This scaling is used to explain why only small and/or strong TCs exhibit midlevel wind speed maxima.

scholarly journals Asymmetric Hurricane Boundary Layer Structure from Dropsonde Composites in Relation to the Environmental Vertical Wind Shear

2013 ◽  
Vol 141 (11) ◽  
pp. 3968-3984 ◽  
Author(s):  
Jun A. Zhang ◽  
Robert F. Rogers ◽  
Paul D. Reasor ◽  
Eric W. Uhlhorn ◽  
Frank D. Marks
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Mixed Layer ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Asymmetric Structure ◽  
Layer Depth ◽  
Tangential Wind ◽  
Hurricane Boundary Layer

Abstract This study investigates the asymmetric structure of the hurricane boundary layer in relation to the environmental vertical wind shear in the inner core region. Data from 1878 GPS dropsondes deployed by research aircraft in 19 hurricanes are analyzed in a composite framework. Kinematic structure analyses based on Doppler radar data from 75 flights are compared with the dropsonde composites. Shear-relative quadrant-mean composite analyses show that both the kinematic and thermodynamic boundary layer height scales tend to decrease with decreasing radius, consistent with previous axisymmetric analyses. There is still a clear separation between the kinematic and thermodynamic boundary layer heights. Both the thermodynamic mixed layer and height of maximum tangential wind speed are within the inflow layer. The inflow layer depth is found to be deeper in quadrants downshear, with the downshear right (DR) quadrant being the deepest. The mixed layer depth and height of maximum tangential wind speed are alike at the eyewall, but are deeper outside in quadrants left of the shear. The results also suggest that air parcels acquire equivalent potential temperature θe from surface fluxes as they rotate through the upshear right (UR) quadrant from the upshear left (UL) quadrant. Convection is triggered in the DR quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in θe. Energy is then released by latent heating in the downshear left (DL) quadrant. Convective downdrafts bring down cool and dry air to the surface and lower θe again in the DL and UL quadrants. This cycling process may be directly tied to shear-induced asymmetry of convection in hurricanes.

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