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 An Observational Study of Vertical Eddy Diffusivity in the Hurricane Boundary Layer

2012 ◽  
Vol 69 (11) ◽  
pp. 3223-3236 ◽  
Author(s):  
Jun A. Zhang ◽  
William M. Drennan
Keyword(s):  
Boundary Layer ◽  
Latent Heat ◽  
Numerical Models ◽  
Eddy Diffusivity ◽  
Heat Fluxes ◽  
Sensible Heat ◽  
Eddy Diffusivities ◽  
Vertical Distributions ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

Abstract Although vertical eddy diffusivity or viscosity has been extensively used in theoretical and numerical models simulating tropical cyclones, little observational study has documented the magnitude of the eddy diffusivity in high-wind conditions (>20 m s−1) until now. Through analyzing in situ aircraft data that were collected in the atmospheric boundary layer of four intense hurricanes, this study provides the first estimates of vertical distributions of the vertical eddy diffusivities for momentum, sensible heat, and latent heat fluxes in the surface wind speed range between 18 and 30 m s−1. In this work, eddy diffusivity is determined from directly measured turbulent fluxes and vertical gradients of the mean variable, such as wind speed, temperature, and humidity. The analyses show that the magnitudes of vertical eddy diffusivities for momentum and latent heat fluxes are comparable to each other, but the eddy diffusivity for sensible heat flux is much smaller than that for the latent heat flux. The vertical distributions of the eddy diffusivities are generally alike, increasing from the surface to a maximum value within the thermodynamic mixed layer and then deceasing with height. The results indicate also that momentum and latent heat are mainly transferred downgradient of the mean flow and that countergradient transport of the sensible heat may exist. The observational estimates are compared with the eddy diffusivities derived from different methods as used in planetary boundary layer (PBL) parameterization schemes in numerical models as well as ones used in previous observational studies.

scholarly journals Effects of Vertical Eddy Diffusivity Parameterization on the Evolution of Landfalling Hurricanes

2017 ◽  
Vol 74 (6) ◽  
pp. 1879-1905 ◽  
Author(s):  
Feimin Zhang ◽  
Zhaoxia Pu
Keyword(s):  
Boundary Layer ◽  
Vertical Mixing ◽  
Eddy Diffusivity ◽  
Essential Components ◽  
Moisture Fluxes ◽  
Layer Velocity ◽  
Hurricane Boundary Layer ◽  
Heat And Moisture ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

Abstract As a result of rapid changes in surface conditions when a landfalling hurricane moves from ocean to land, interactions between the hurricane and surface heat and moisture fluxes become essential components of its evolution and dissipation. With a research version of the Hurricane Weather Research and Forecasting Model (HWRF), this study examines the effects of the vertical eddy diffusivity in the boundary layer on the evolution of three landfalling hurricanes (Dennis, Katrina, and Rita in 2005). Specifically, the parameterization scheme of eddy diffusivity for momentum Km is adjusted with the modification of the mixed-layer velocity scale in HWRF for both stable and unstable conditions. Results show that the change in the Km parameter leads to improved simulations of hurricane track, intensity, and quantitative precipitation against observations during and after landfall, compared to the simulations with the original Km. Further diagnosis shows that, compared to original Km, the modified Km produces stronger vertical mixing in the hurricane boundary layer over land, which tends to stabilize the hurricane boundary layer. Consequently, the simulated landfalling hurricanes attenuate effectively with the modified Km, while they mostly inherit their characteristics over the ocean and decay inefficiently with the original Km.

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 Seasonal variation of vertical eddy diffusivity in the troposphere, lower stratosphere and mesosphere over a tropical station

2001 ◽  
Vol 19 (8) ◽  
pp. 975-984 ◽  
Author(s):  
D. Narayana Rao ◽  
M. V. Ratnam ◽  
T. N. Rao ◽  
S. V. B. Rao
Keyword(s):  
Lower Stratosphere ◽  
Middle Atmosphere ◽  
Beam Width ◽  
Eddy Diffusivity ◽  
Lower Atmosphere ◽  
Spectral Parameters ◽  
Vhf Radar ◽  
June July ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

Abstract. Long-term VHF radar (53 MHz with 3° beam-width) observations at Gadanki (13.5° N, 79.2° E), India, during the period from September 1995 to August 1999 are used to study monthly, seasonal and annual medians of vertical eddy diffusivity, K in the troposphere, lower stratosphere and mesosphere. First, the spectral width contribution due to non-turbulent effects has been removed for further analysis and the monthly, seasonal medians of K are calculated. The monthly median of K in the troposphere shows maximum and minimum in June-July and November-December, respectively. In general, large values of K are seen up to 10 km and then decrease with height. Larger values of K are observed during monsoon and post-monsoon than in winter and summer. In general, the maximum and minimum values of the annual median of K (in logarithmic values) in the troposphere are found to be 0.25 and - 1.3 m2 s-1 respectively. In the mesosphere, the monthly median of K shows maximum and minimum during June-July and November-December, respectively, similar to the lower atmosphere. The value of K in the mesosphere becomes larger and it increases with height up to 75 km and again decreases above that height. The maximum values are seen during the summer, followed by equinoxes and a minimum during the winter. In general, the maximum and minimum values of K (in logarithmic values) are found to be 0.7 and 0.3 m2 s-1, respectively, in the mesosphere. A comparison of Doppler spectral parameters in different beam directions shows anisotropy in both signal-to- noise ratio (SNR) and spectral widths in the mesosphere, whereas it shows isotropy in SNR and anisotropy in the spectral widths in troposphere and lower stratosphere.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)

Seasonal and interannual variabilities of vertical eddy diffusivity observed by the MU radar

1994 ◽  
Vol 14 (9) ◽  
pp. 277-280 ◽  
Author(s):  
M.D. Yamanaka ◽  
S. Kurosaki ◽  
S. Fukao ◽  
H. Hashiguchi ◽  
T. Tsuda ◽  
...  
Keyword(s):  
Eddy Diffusivity ◽  
Mu Radar ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

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.

scholarly journals Mean Velocity Decomposition and Vertical Eddy Diffusivity of the Pacific Ocean from Surface GDP Drifters and 1000-m Argo Floats

2016 ◽  
Vol 46 (6) ◽  
pp. 1751-1768 ◽  
Author(s):  
Stephen M. Chiswell
Keyword(s):  
Eddy Diffusivity ◽  
Surface Velocity ◽  
Mean Velocity ◽  
Argo Data ◽  
Near Surface ◽  
Argo Floats ◽  
Velocity Decomposition ◽  
The Mean ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

AbstractWith the relatively recent development of Global Drifter Program (GDP) drifters that measure the near-surface ocean velocity and Argo floats that can be used to derive both the intermediate-ocean (1000 m) velocity and the mean dynamic height of the surface relative to 1000 dbar, there now exists the opportunity to directly observe the mean velocity decomposition of the ocean. This study computes the mean Ekman velocity by subtracting the mean referenced velocity derived from Argo data from the mean surface velocity derived from GDP data. This Ekman velocity is slightly stronger than previous observations and shows a spatial structure consistent with a vertical eddy diffusivity that is linearly dependent on wind stress. To do this analysis, the author has to deal with the fact that GDP drifters often lose their drogues, and a product of this research is validation of the wind-slip correction applied to GDP drifters that have lost their drogues.

Prediction of salinity intrusion in Danshuei estuarine system

2008 ◽  
Vol 39 (5-6) ◽  
pp. 497-505 ◽  
Author(s):  
A. Etemad-Shahidi ◽  
A. Dorostkar ◽  
Wen-Cheng Liu
Keyword(s):  
Fresh Water ◽  
Water Discharge ◽  
Eddy Diffusivity ◽  
Turbulence Closure ◽  
Estuarine System ◽  
Salinity Intrusion ◽  
Closure Scheme ◽  
Fresh Water Discharge ◽  
Vertical Eddy Diffusivity ◽  
Vertical Eddy

The main parameters that affect the flow conditions and intrusion of salt water in an estuary system are tides and the seasonal variation of water discharge. A laterally averaged two-dimensional numerical model called MIKE 11 XZ is used to simulate the hydrodynamics and salinity intrusion of Danshuei River estuarine system. This model can simulate hydrodynamics and water quality in estuaries, reservoirs and lakes. MIKE 11 XZ solves the Reynolds-averaged Navier–Stokes equations by using Abbott–Ionescu finite difference scheme in a non-dimensional vertical σ-coordinate. Vertical eddy diffusivity in the model can be determined by a constant value, a mixing length theory and a k or k−ɛ turbulence closure scheme with Richardson number correction. A series of comprehensive field data obtained from Danshuei estuarine system is used for evaluation, calibration and verification of the model. The friction coefficient was calibrated and verified using water surface elevation and velocity measurements, respectively. Then the vertical eddy diffusivity was calibrated and verified through comparison of salinity measurements in different layers of several stations. Reasonable agreement was obtained between the model results and the observed data using k−ɛ turbulence closure scheme. The model application was investigated with different discharges and the effect of discharge variation on salinity intrusion was determined. The results showed that the fresh water discharge is the main parameter that affects the salinity intrusion in this system. Finally, simple power equations are suggested to predict the salinity intrusion due to the fresh water discharge in different tributaries of the system.

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