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

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Presentation on theme: "Www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Structure and force balance of idealised cold."— Presentation transcript:

1 Department of Physics /Victoria Sinclair Structure and force balance of idealised cold fronts simulated with WRF 1.University of Helsinki, Division of Atmospheric Sciences 2.University of Albany, Department of Atmospheric and Environmental Sciences Victoria Sinclair 1 and Dan Keyser 2

2 Department of Physics /Victoria Sinclair 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

3 Department of Physics /Victoria Sinclair 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

4 Department of Physics /Victoria Sinclair 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

5 Department of Physics /Victoria Sinclair 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

6 Department of Physics /Victoria Sinclair 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

7 Department of Physics /Victoria Sinclair Structure of the front in d03 Z = 100 m

8 Department of Physics /Victoria Sinclair 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

9 Department of Physics /Victoria Sinclair 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

10 Department of Physics /Victoria Sinclair Force balance in d01 (dx=100km) Green: Wind Vectors Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration

11 Department of Physics /Victoria Sinclair Force balance in d03 (dx=4km) Green: Wind Vectors Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

12 Department of Physics /Victoria Sinclair Force balance split into along- front and cross-front components PGF ACC COR BL Cross -front Along -front

13 Department of Physics /Victoria Sinclair 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

14 Department of Physics /Victoria Sinclair 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

15 Department of Physics /Victoria Sinclair Future plans and challenges with WRF  Increase size of nested domains and add a 4 th 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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