Dynamical similarities and differences between cold fronts and density currents

1
Dynamical similarities and differences between
cold fronts and density currents

• Victoria Sinclair
• University of Helsinki

15.08.2011
Victoria.Sinclair_at_helsinki.fi

www.atm.helsinki.fi/vsi
nclai

2
Some cold fronts can be visually similar to
density currents
w and ?
Model simulation of a cold front (dx 2.5km)
Tower observations (Shapiro et al 1985)
3
Questions

• What is the force balance across a cold front?
• How does the force balance compare to theoretical
  predictions?
• Is the force balance across a cold front similar
  to that across a density current?
• Are cold fronts dynamically related to density
  currents?

4
3D Idealised experiments with WRF

• WRF-ARW v3.2, Non-hydrostatic
• YSU BL scheme over a sea surface
• No moisture
• Simulate a cold front and a density current

Potential temperature, wind
Potential temperature, surface pressure and
location of nested domains

• Drop a cold bubble and allow to spread
• dx 4km

• 3D baroclinic life cycle with nested domains
• dx 100km, 20km, 4km

5
Scale Analysis for fronts
l a ß ACC along PGF along COR along ACC across PGF across COR across
100km 1 0.1 1 1 1 0.01 1 1
Semi Geostrophic theory we can neglect the
acceleration only in the across front
direction (Hoskins and Bretherton, 1972)
6
dx 100km Force balance across cold front
Green Wind Vectors Blue
Coriolis Force Red Pressure gradient force
Black Resultant acceleration
7
dx 4km Force balance across cold front
Green Wind Vectors Blue
Coriolis Force Red Pressure gradient force
Black Resultant acceleration Purple BL force
8
Along front forces z100m
dx 100km dx 20km dx 4km
9
Across front forces z100m
dx 100km dx 20km dx 4km
10
Cold front vs. Density Current
Across front z 100m dx 4km
Blue Coriolis Force Red
Pressure gradient force Black Resultant
acceleration Purple BL force
11
Cold front vs. Density Current
Along front z 100m dx 4km
Blue Coriolis Force Red
Pressure gradient force Black Resultant
acceleration Purple BL force
12
Conclusions

• The force balance is resolution dependent
• For the cold front at dx 4km,
• PGF Coriolis force in the along front
  direction
• PGF gtgt Coriolis force in the across front
  direction
• In the across front direction, the cold front
  force balance approaches that of the density
  current as resolution increases.
• In the along front direction the cold front force
  balance differs to the density current force
  balance
• Across the cold front, the unbalance pressure
  gradient force is likely due to enhanced
  horizontal buoyancy gradients.

13
(No Transcript)
14
Cold fronts have a large variety of structures
Temperature
Temperature, pressure, rain rate
?T 10K ?t 5hrs
?T 2K ?t 30 mins
15
Density Current force balance
z 100m dx 4km
Blue Coriolis Force Red
Pressure gradient force Black Resultant
acceleration Purple BL force
16
Density Current force balance
z 1 km dx 4km
Blue Coriolis Force Red
Pressure gradient force Black Resultant
acceleration Purple BL force
17
Cold front force balance
z 100m dx 4km
Blue Coriolis Force Red
Pressure gradient force Black Resultant
acceleration Purple BL force
18
Cold front force balance
z 1 km dx 4km
Blue Coriolis Force Red
Pressure gradient force Black Resultant
acceleration Purple BL force
19
Scale Analysis - revisited
Use U, V and l from the WRF simulation
l a ß ACC along PGF along COR along ACC across PGF across COR across
20 km 7.5 0.5 1 1 0.13 1.8 3.3 1
Boundary layer processes are not included in Semi
Geostrophic theory
20
Why do the results differ to theory?

• Two assumptions

21
Scale Analysis - Hoskins and Bretherton (1972)
ACROSS FRONT
ALONG FRONT
Rossby number is assumed to be O(1) Along front
wind is assumed to be much greater than along
fronts wind
22
?x100km. No Boundary layer scheme
Green Wind Vectors Blue
Coriolis Force Red Pressure gradient force
Black Resultant acceleration
23
What do these terms really mean?
U
X
CF
Increases convergence Strengthens front
Decreases convergence Weakens front
24
What do these terms really mean?
V
X
CF
Decreases vorticity
Increases vorticity
25
Effect of resolution of frontal structure
Potential temperature and wind barbs at z100m
The cross front scale decreases with increasing
resolution and the wind shift becomes sharper
26
Effect of PBL on vertical structure of front
27
Density Current dx4km.t1.5 hours, z100m, YSU
BL scheme
Blue Coriolis Force Red Pressure gradient force
Black Resultant acceleration Purple BL force
Potential temperature and wind vectors