scholarly journals A direct aircraft observation of helical rolls in the tropical cyclone boundary layer

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Jie Tang ◽  
Jun A. Zhang ◽  
Pakwai Chan ◽  
Kaikwong Hon ◽  
Xiaotu Lei ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Numerical Models ◽  
Correlation Method ◽  
Outer Core ◽  
Eddy Correlation ◽  
Turbulence Structure ◽  
Planetary Boundary Layer Parameterization ◽  
Tropical Cyclone Boundary Layer

AbstractHelical rolls are known to play a significant role in modulating both the mean and turbulence structure of the atmospheric boundary layer in tropical cyclones. However, in-situ measurements of these rolls have been limited due to safety restrictions. This study presents analyses of data collected by an aircraft operated by the Hong Kong Observatory in Typhoon Kalmaegi (1415) and Typhoon Nida (1604). Examination of the flight-level data at ~ 600 m altitude confirmed the existence of sub-kilometer-scale rolls. These rolls were mostly observed in the outer-core region. Turbulent momentum fluxes were computed using the eddy correlation method. The averaged momentum flux of flight legs with rolls was found to be ~ 2.5 times that of legs without rolls at a similar wind speed range. This result suggests that rolls could significantly modulate turbulent transfer in the tropical cyclone boundary layer. This roll effect on turbulent fluxes should be considered in the planetary boundary layer parameterization schemes of numerical models simulating and forecasting tropical cyclones.

scholarly journals Multilevel Tower Observations of Vertical Eddy Diffusivity and Mixing Length in the Tropical Cyclone Boundary Layer during Landfalls

2018 ◽  
Vol 75 (9) ◽  
pp. 3159-3168 ◽  
Author(s):  
Jie Tang ◽  
Jun A. Zhang ◽  
Sim D. Aberson ◽  
Frank D. Marks ◽  
Xiaotu Lei
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Numerical Models ◽  
Theoretical Method ◽  
Eddy Diffusivity ◽  
Eddy Correlation ◽  
Mixing Length ◽  
Momentum Fluxes ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

Abstract This study analyzes the fast-response (20 Hz) wind data collected by a multilevel tower during the landfalls of Tropical Storm Lionrock (1006), Typhoon Fanapi (1011), and Typhoon Megi (1015) in 2010. Turbulent momentum fluxes are calculated using the standard eddy-correlation method. Vertical eddy diffusivity Km and mixing length are estimated using the directly measured momentum fluxes and mean-wind profiles. It is found that the momentum flux increases with wind speed at all four levels. The eddy diffusivity calculated using the direct-flux method is compared to that using a theoretical method in which the vertical eddy diffusivity is formulated as a linear function of the friction velocity and height. It is found that below ~60 m, Km can be approximately parameterized using this theoretical method, though this method overestimates Km for higher altitude, indicating that the surface-layer depth is close to 60 m in the tropical cyclones studied here. It is also found that Km at each level varies with wind direction during landfalls: Km estimated based on observations with landward fetch is significantly larger than that estimated using data with seaward fetch. This result suggests that different parameterizations of Km should be used in the boundary layer schemes of numerical models forecasting tropical cyclones over land versus over the ocean.

scholarly journals Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part I: Hurricane Georges

2006 ◽  
Vol 63 (9) ◽  
pp. 2169-2193 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Radial Profile ◽  
Lower Altitude ◽  
Near Surface ◽  
Wind Structure ◽  
The Right ◽  
Tropical Cyclone Boundary Layer ◽  
Upper Boundary

Abstract The GPS dropsonde allows observations at unprecedentedly high horizontal and vertical resolution, and of very high accuracy, within the tropical cyclone boundary layer. These data are used to document the boundary layer wind field of the core of Hurricane Georges (1998) when it was close to its maximum intensity. The spatial variability of the boundary layer wind structure is found to agree very well with the theoretical predictions in the works of Kepert and Wang. In particular, the ratio of the near-surface wind speed to that above the boundary layer is found to increase inward toward the radius of maximum winds and to be larger to the left of the track than to the right, while the low-level wind maximum is both more marked and at lower altitude on the left of the storm track than on the right. However, the expected supergradient flow in the upper boundary layer is not found, with the winds being diagnosed as close to gradient balance. The tropical cyclone boundary layer model of Kepert and Wang is used to simulate the boundary layer flow in Hurricane Georges. The simulated wind profiles are in good agreement with the observations, and the asymmetries are well captured. In addition, it is found that the modeled flow in the upper boundary layer at the eyewall is barely supergradient, in contrast to previously studied cases. It is argued that this lack of supergradient flow is a consequence of the particular radial structure in Georges, which had a comparatively slow decrease of wind speed with radius outside the eyewall. This radial profile leads to a relatively weak gradient of inertial stability near the eyewall and a strong gradient at larger radii, and hence the tropical cyclone boundary layer dynamics described by Kepert and Wang can produce only marginally supergradient flow near the radius of maximum winds. The lack of supergradient flow, diagnosed from the observational analysis, is thus attributed to the large-scale structure of this particular storm. A companion paper presents a similar analysis for Hurricane Mitch (1998), with contrasting results.

The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

2021 ◽  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Wind Field ◽  
Surface Wind ◽  
Frictional Stress ◽  
Homogeneous Surface ◽  
Sudden Decrease ◽  
The Right ◽  
Tropical Cyclone Boundary Layer

Abstract The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Lastly, a brief analysis of in-situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in-situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally-homogeneous surface types - which have been well-documented by previous studies - are inappropriate for use near strong frictional heterogeneity.

Contribution of vertical advection to supergradient wind in tropical cyclone boundary layer: A numerical study

2021 ◽  
Author(s):  
Rong Fei ◽  
Yuqing Wang ◽  
Yuanlong Li
Keyword(s):  
Boundary Layer ◽  
Steady State ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Vertical Structure ◽  
Numerical Study ◽  
Substantial Reduction ◽  
Vertical Advection ◽  
Physics Model ◽  
Tropical Cyclone Boundary Layer

AbstractThe existence of supergradient wind in the interior of the boundary layer is a distinct feature of a tropical cyclone (TC). Although the vertical advection is shown to enhance supergradient wind in TC boundary layer (TCBL), how and to what extent the strength and structure of supergradient wind are modulated by vertical advection are not well understood. In this study, both a TCBL model and an axisymmetric full-physics model are used to quantify the contribution of vertical advection process to the strength and vertical structure of supergradient wind in TCBL. Results from the TCBL model show that the removal of vertical advection of radial wind reduces both the strength and height of supergradient wind by slightly more than 50%. The removal of vertical advection of agradient wind reduces the height of the supergradient wind core by ~30% but increases the strength of supergradient wind by ~10%. Results from the full-physics model show that the removal of vertical advection of radial wind or agradient wind reduces both the strength and height of supergradient wind but the removal of that of radial wind produces a more substantial reduction (52%) than the removal of that of agradient wind (35%). However, both the intensification rate and final intensity of the simulated TCs in terms of maximum 10-m wind speed show little differences in experiments with and without the vertical advection of radial or agradient wind, suggesting that supergradient wind contributes little to either the intensification rate or the steady-state intensity of the simulated TC.

scholarly journals Shock-like structures in the tropical cyclone boundary layer

2013 ◽  
Vol 5 (2) ◽  
pp. 338-353 ◽  
Author(s):  
Gabriel J. Williams ◽  
Richard K. Taft ◽  
Brian D. McNoldy ◽  
Wayne H. Schubert
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Tropical Cyclone Boundary Layer

scholarly journals Time and Space Scales in the Tropical Cyclone Boundary Layer, and the Location of the Eyewall Updraft

2017 ◽  
Vol 74 (10) ◽  
pp. 3305-3323 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Nonlinear Models ◽  
Vertical Motion ◽  
Oscillation Period ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Radial Profiles ◽  
Sinusoidal Oscillations ◽  
Tropical Cyclone Boundary Layer

Abstract The transient response of the tropical cyclone boundary layer is studied using linearized and nonlinear models, with particular focus on the frictionally forced vertical motion. The impulsively started, linearized tropical cyclone boundary layer is shown to adjust to its equilibrium solution via a series of decaying oscillations with the inertial period . In the nonlinear case, the oscillation period is slightly lengthened by inward advection of the slower-evolving flow from larger radii, but the oscillations decay more quickly. In an idealized cyclone with small sinusoidal oscillations superimposed on the gradient wind, the equilibrium nonlinear boundary layer acts as a low-pass filter with pass length scaling as , where is the 10-m frictional inflow. This filter is absent from the linearized boundary layer. The eyewall frictional updraft is similarly displaced inward of the radius of maximum winds (RMW) by a distance that scales with , owing to nonlinear overshoot of the inflowing air as it crosses the relatively sharp increase in I near the eyewall. This displacement is smaller (other things being equal) when the RMW is small, and greater when it is large, including in secondary eyewalls. The dependence of this distance on may explain, at least partially, why observed RMW are seldom less than 20 km, why storms with relatively peaked radial profiles of wind speed can intensify more rapidly, and why some secondary eyewalls initially contract rapidly with little intensification, then contract more slowly while intensifying.

scholarly journals A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region. Part II: An Analytic Method for Determining the Time Variation of the Intensity of a Tropical Cyclone*

2010 ◽  
Vol 138 (1) ◽  
pp. 22-41 ◽  
Author(s):  
France Lajoie ◽  
Kevin Walsh
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Pressure ◽  
Time Variation ◽  
Surface Wind ◽  
Ground Truth Data ◽  
Maximum Wind ◽  
Australian Region ◽  
Central Surface

Abstract The observed features discussed in Part I of this paper, regarding the intensification and dissipation of Tropical Cyclone Kathy, have been integrated in a simple mathematical model that can produce a reliable 15–30-h forecast of (i) the central surface pressure of a tropical cyclone, (ii) the sustained maximum surface wind and gust around the cyclone, (iii) the radial distribution of the sustained mean surface wind along different directions, and (iv) the time variation of the three intensity parameters previously mentioned. For three tropical cyclones in the Australian region that have some reliable ground truth data, the computed central surface pressure, the predicted maximum mean surface wind, and maximum gust were, respectively, within ±3 hPa and ±2 m s−1 of the observations. Since the model is only based on the circulation in the boundary layer and on the variation of the cloud structure in and around the cyclone, its accuracy strongly suggests that (i) the maximum wind is partly dependent on the large-scale environmental circulation within the boundary layer and partly on the size of the radius of maximum wind and (ii) that all factors that contribute one way or another to the intensity of a tropical cyclone act together to control the size of the eye radius and the central surface pressure.

scholarly journals Why is the Tropical Cyclone Boundary Layer Not “Well Mixed”?

2016 ◽  
Vol 73 (3) ◽  
pp. 957-973 ◽  
Author(s):  
Jeffrey D. Kepert ◽  
Juliane Schwendike ◽  
Hamish Ramsay
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mixed Layer ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Static Stability ◽  
Layer Depth ◽  
Turbulent Dissipation ◽  
Unstable Layer ◽  
Tropical Cyclone Boundary Layer

Abstract Plausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed, but that the main cause of this unstable layer is heating from turbulent dissipation. The top of the well-mixed layer is thus distinct from the top of the boundary layer in tropical cyclones. The top of the inflow layer is a better proxy for the top of the boundary layer but is not without limitations. These results may have implications for boundary layer parameterizations that diagnose the boundary layer depth from thermodynamic, or partly thermodynamic, criteria.

scholarly journals A Highly Configurable Vortex Initialization Method for Tropical Cyclones

2013 ◽  
Vol 141 (10) ◽  
pp. 3556-3575 ◽  
Author(s):  
Eric D. Rappin ◽  
David S. Nolan ◽  
Sharanya J. Majumdar
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Steady State ◽  
Tropical Cyclones ◽  
Mesoscale Model ◽  
Layer Structure ◽  
Numerical Models ◽  
Secondary Circulation ◽  
Free Atmosphere ◽  
Vortex Initialization

Abstract A highly configurable vortex initialization methodology has been constructed in order to permit manipulation of the initial vortex structure in numerical models of tropical cyclones. By using distinct specifications of the flow in the boundary layer and free atmosphere, an array of parameters is available to modify the structure. A nonlinear similarity model that solves the steady-state, height-dependent equations for a neutrally stratified, axisymmetric vortex is solved for the boundary layer flow. Above the boundary layer, a steady-state, moist-neutral, hydrostatic and gradient wind balanced model is used to generate the angular momentum distribution in the free atmosphere. In addition, an unbalanced mass-conserving secondary circulation is generated through the assumption of conservation of mass and angular momentum above the boundary layer. Numerical simulations are conducted using a full-physics mesoscale model to explore the sensitivity of the vortex evolution to different prescriptions of the initial vortex. Dynamical adjustment is found to be dominant in the early evolution of the simulations, thereby masking any sensitivity to initial changes in the secondary circulation and boundary layer structure. The adjustment time can be significantly reduced by arbitrarily enhancing the moisture in the eyewall region.

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