Www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair vsinclai Structure and force balance of idealised cold

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

[email protected]

1. University of Helsinki, Division of Atmospheric Sciences

2. University of Albany, Department of Atmospheric and Environmental Sciences

Victoria Sinclair1 and Dan Keyser2


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


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


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


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


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


Structure of the front in d03

Z = 100 m


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


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


Force balance in d01 (dx=100km)

Green: Wind Vectors Blue: Coriolis ForceRed: Pressure gradient force Black: Resultant acceleration


Force balance in d03 (dx=4km)

Green: Wind Vectors Blue: Coriolis ForceRed: Pressure gradient force Black: Resultant accelerationPurple: BL force


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


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


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

Documents

Www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair vsinclai Structure and force balance of idealised cold