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Structure and force balance of idealised cold fronts simulated with WRF

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.

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Structure and force balance of idealised cold fronts simulated with WRF

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  1. Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai 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 Sinclair1 and Dan Keyser2

  2. Department of Physics /Victoria Sinclair www.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

  3. Department of Physics /Victoria Sinclair www.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

  4. Department of Physics /Victoria Sinclair www.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

  5. Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Potential temperature and pressure at z=100m East- West Periodic boundaries Areas of high and low pressure develop Cold front becomes evident Outermost domain dx =100 km Frames every 6 hours

  6. Department of Physics /Victoria Sinclair www.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

  7. Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Structure of the front in d03 Z = 100 m

  8. Department of Physics /Victoria Sinclair www.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

  9. 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 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai

  10. Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Force balance in d01 (dx=100km) Green: Wind VectorsBlue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration

  11. Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Force balance in d03 (dx=4km) Green: Wind VectorsBlue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  12. Rossby Number = Acc / Cor Z=100m Z=2.5 km Narrow region of unbalanced flow co-located with the surface front and with the ascent plume above the surface front Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai

  13. Department of Physics /Victoria Sinclair www.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

  14. Department of Physics /Victoria Sinclair www.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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