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Structure and force balance of idealised cold fronts simulated with WRF. Victoria.Sinclair@helsinki.fi University of Helsinki, Division of Atmospheric Sciences University of Albany, Department of Atmospheric and Environmental Sciences. Victoria Sinclair 1 and Dan Keyser 2. Scientific Aims. - PowerPoint PPT Presentation
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www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Structure and force balance of idealised cold fronts simulated
with WRF
Victoria.Sinclair@helsinki.fi
1. University of Helsinki, Division of Atmospheric Sciences
2. University of Albany, Department of Atmospheric and Environmental Sciences
Victoria Sinclair1 and Dan Keyser2
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Scientific Aims
Understand how surface fluxes of heat and momentum and turbulent fluxes within the boundary layer modify the structure and motion of fronts
Identify to what extent the flow in frontal zones is unbalanced
Determine what controls the rate at which surface fronts collapse towards a discontinuity
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Scientific Aims
Understand how surface fluxes of heat and momentum and turbulent fluxes within the boundary layer modify the structure and motion of fronts
Identify to what extent the flow in frontal zones is unbalanced
Determine what controls the rate at which surface fronts collapse towards a discontinuity
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
WRF Experiments and Setup
Idealised version of WRF V3.2
Started from the em_b_wave test case Added YSU PBL scheme but no other physics
No moisture
Added two nested domains d01: 40 points x 80 points dx =100km
d02: 120 points x 100 points dx =20km
d03: 300 points x 160 points dx =4km
Increased the number of vertical levels to 62 Stretched grid, lots of levels in PBL
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Potential temperature and pressure at z=100m
Outermost domain
dx =100 km
Frames every 6 hours
East- West Periodic boundaries
Areas of high and low pressure develop
Cold front becomes evident
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Nested domains
Zoom in on the cold front
Only turn the nested domains after 4 days to save computer time
Future plans: increase size of nested domains and add a d04 domain
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Structure of the front in d03
Z = 100 m
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Vertical structure of the front
Potential temperature
System relative winds
Vertical motion
Frontal width ~ 20km
Temperature decrease ~ 6K
Strong forced ascent at leading edge
Gravity wave feature
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Identify unbalanced regions
I modified WRF to output all terms in the horizontal momentum equations
Compare the magnitude of each terms to determine the force balance across cold fronts
Unbalanced regions are defined to be where the Rossby number is much larger than 1
_
_
1
1
bl u
bl v
Du pfv F
Dt x
Dv pfu F
Dt y
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Force balance in d01 (dx=100km)
Green: Wind Vectors Blue: Coriolis ForceRed: Pressure gradient force Black: Resultant acceleration
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Force balance in d03 (dx=4km)
Green: Wind Vectors Blue: Coriolis ForceRed: Pressure gradient force Black: Resultant accelerationPurple: BL force
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Rossby Number = Acc / Cor
Narrow region of unbalanced flow co-located with the surface front and with the ascent plume above the surface front
Z=100m
Z=2.5 km
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Conclusions
Realistic frontal structure can be simulated with idealised WRF
The force balance changes with model resolution Low resolution: approximate geostrophic balance, no
unbalanced regions
High resolution: pressure gradient force in unbalanced
Rossby number > 1 in a narrow band along the surface front and above the surface front
www.helsinki.fi/yliopisto 24.10.11Department of Physics /Victoria Sinclairwww.atm.helsinki.fi/~vsinclai
Future plans and challenges with WRF
Increase size of nested domains and add a 4th domain.
Output the diffusion term
Attempt to simulate different cold fronts Different baroclinic life cycle e,g, LC1 / LC2 Deformation forced front 2-dimensional experiments – repeat previous experiments
e.g. Keyser & Anthes (1982)
Include moisture
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