1. www.helsinki.fi/yliopisto 24.10.11 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 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. www.helsinki.fi/yliopisto 24.10.11 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. www.helsinki.fi/yliopisto 24.10.11 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. www.helsinki.fi/yliopisto 24.10.11 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. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.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

6. www.helsinki.fi/yliopisto 24.10.11 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. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Structure of the front in d03 Z = 100 m

8. www.helsinki.fi/yliopisto 24.10.11 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. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.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

10. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Force balance in d01 (dx=100km) Green: Wind Vectors Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration

11. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Force balance in d03 (dx=4km) Green: Wind Vectors Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

12. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai Force balance split into along- front and cross-front components PGF ACC COR BL Cross -front Along -front

13. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.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

14. www.helsinki.fi/yliopisto 24.10.11 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

15. www.helsinki.fi/yliopisto 24.10.11 Department of Physics /Victoria Sinclair www.atm.helsinki.fi/~vsinclai 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