Factors Controlling the Intensity of Idealized Hurricanes in Numerical Models

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Factors Controlling the Intensity of Idealized
Hurricanes in Numerical Models

• Richard Rotunno
• NCAR

Based on Bryan, G. H., and R. Rotunno, 2008
The influence of near-surface, high-entropy air
in hurricane eyes on maximum hurricane intensity.
The Journal of the Atmospheric Sciences, in
press. Bryan, G. H., and R. Rotunno, 2008 The
maximum intensity of tropical cyclones in
axisymmetric numerical model simulations.
Submitted to Monthly Weather Review. Rotunno,
R., Y. Chen, W. Wang, C. Davis, J. Dudhia and G.
J. Holland, 2008 Resolved turbulence in a
three-dimensional model of an idealized tropicaI
cyclone. Submitted to Geophysical Research
Letters.
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Tangential Velocity from an Axisymmetric
Numerical Model
Bryan and Rotunno (2008 MWR)
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Rotunno and Emanuel 1987 (RE87) / Axisymmetric
Numerical Model Vortex Amplifies under
Moist-Neutral Conditions Steady State Intensity
Agrees with Theory (E-PI)
Bryan and Rotunno (2008 MWR)
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RE87
Persing and Montgomery (2003, JAS) using RE87
Model Vmax Sensitive to Grid Resolution
Bryan and Rotunno (2008 MWR)
RE87 paper
RE87 code
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Numerical Model

• Axisymmetric version of Bryan and Fritsch (2002,
  MWR) cloud model
• Uses newly developed, more accurate numerical
  techniques
• (compared to the RE87 model)
• Governing equations conserve mass and energy for
  saturated flows
• Includes dissipative heating
• Following RE87, new simulations use SST
  26C, moist-neutral initial sounding, same
  grid-staggering, simple microphysics, sea-surface
  transfer, and domain
• . New simulations
  at high grid resolution
  
• 
  ( RE87 grid resolution
  )

(PM03 grid resolution
)
Bryan and Rotunno (2008 MWR)
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Numerical Models
3D
Axisymmetric
37km
Turbulent fluxes represented by mixing-length
theory
Turbulent fluxes computed, but high resolution
required
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Sensitivity to Turbulence Length Scales
Bryan and Rotunno (2008 MWR)
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Sensitivity of Wind Speed to Turbulence Mixing
Length lh
Bryan and Rotunno (2008 MWR)
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Evaluation of the High-Entropy Reservoir
shaded black dot location of vmax
black line trajectory
default configuration
no surface fluxes in eye
Persing and Montgomery (2003)
Bryan and Rotunno (2008 JAS)
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Structure of and Angular Momentum
Bryan and Rotunno (2008 MWR)
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Horizontal Diffusion Weakens Gradients of and

Bryan and Rotunno (2008 MWR)
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Vmax , E-PI vs
Bryan and Rotunno (2009 in prep)
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Components of E-PI

• 1. Moist slantwise neutrality
• 2. PBL Model
• 3. Gradient-wind and hydrostatic balance

Bryan and Rotunno (2009 in prep)
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Flow with small horizontal diffusion not in
gradient-wind balance
Bryan and Rotunno (2009 in prep)
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Rotating Flow Above a Stationary Disk
Bödewadt (1940) (Schlichting 1968 Boundary
Layer Theory) see also Rott and Lewellen 1966
Prog Aero Sci)
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What is ?

• There are no observations of radial turbulent
  fluxes in a hurricane

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Numerical Models
3D
Axisymmetric
37km
Turbulent fluxes represented by mixing-length
theory
Turbulent fluxes computed, but high resolution
required
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Weather Research and Forecast (WRF) Forecasts

• Radar Reflectivity at z3km
• WRF
• WRF
• ELDORA

Davis et al. (2008 MWR)
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Domain
6075km
Idealized TC f-plane zero env wind fixed SST
Nested Grids
1500km
1000km
111km
333km
37km
50 vertical levels Dz60m1km Ztop27km
Rotunno et al. (2008 GRL)
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Initial Condition (after RE87)
Axisymmetric initial vortex
Initial sounding WRF neutral
Rotunno et al. (2008 GRL)
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Instantaneous Maximum 10-m Wind Speed
Rotunno et al. (2008 GRL)
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10-m Wind Speed t9.75d
max61.5
max86.7
ykm
max121.7
max86.2
ykm
Rotunno et al. (2008 GRL)
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10-m Wind Speed
instantaneous
1-min average
max121.7
max78.8
Max85.5
Max82.3
Max83.7
37km
37 km
Rotunno et al. (2008 GRL)
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10-m Tangentially Averaged Wind Speed vs Grid
Interval
Rotunno et al. (2008 GRL)
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Eddy Kinetic Energy Spectra
Rotunno et al. (2008 GRL)
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Estimation of Eddy Viscosity for Axisymmetric
Models
LES
grid-scale
subgrid-scale
grid-scale
TC Vortex
azimuthal average
grid-scale
grid-scale waves turbulence
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Conclusions

• Treatment of turbulence in numerical models of
  hurricanes is as important for intensity
  prediction as other factors (e.g. sea-surface
  transfer, ocean-wave drag, cloud physics)
• Quantitative information needed on turbulent
  transfer in hurricanes
• Observations are expensive, LES so far
  inconclusive