Structure and force balance of idealised cold fronts

Structure and force balance of idealised cold fronts simulated with WRF Victoria Sinclair 1 and Dan Keyser 2 Victoria. Sinclair@helsinki. fi 1. 2. University of Helsinki, Division of Atmospheric Sciences University of Albany, Department of Atmospheric and Environmental Sciences Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

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 Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

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 Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

WRF Experiments and Setup § Idealised version of WRF V 3. 2 § Started from the em_b_wave test case § Added YSU PBL scheme but no other physics § No moisture § Added two nested domains § d 01: 40 points x 80 points dx =100 km § d 02: 120 points x 100 points dx =20 km § d 03: 300 points x 160 points dx =4 km § Increased the number of vertical levels to 62 § Stretched grid, lots of levels in PBL Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

Potential temperature and pressure at z=100 m East- West Periodic boundaries Outermost domain Areas of high and low pressure develop dx =100 km Frames every 6 hours Cold front becomes evident Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

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 d 04 domain Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

Structure of the front in d 03 Z = 100 m Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

Vertical structure of the front § Potential temperature § System relative winds § Vertical motion § Frontal width ~ 20 km § Temperature decrease ~ 6 K § Strong forced ascent at leading edge § Gravity wave feature Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

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 www. helsinki. fi/yliopisto 24. 10. 11

Force balance in d 01 (dx=100 km) Green: Wind Vectors Red: Pressure gradient force Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai Blue: Coriolis Force Black: Resultant acceleration www. helsinki. fi/yliopisto 24. 10. 11

Force balance in d 03 (dx=4 km) Green: Wind Vectors Red: Pressure gradient force Purple: BL force Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai Blue: Coriolis Force Black: Resultant acceleration www. helsinki. fi/yliopisto 24. 10. 11

Rossby Number = Acc / Cor Z=100 m 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 www. helsinki. fi/yliopisto 24. 10. 11

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 Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11

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, LC 1 / LC 2 § Deformation forced front § 2 -dimensional experiments – repeat previous experiments e. g. Keyser & Anthes (1982) § Include moisture Department of Physics /Victoria Sinclair www. atm. helsinki. fi/~vsinclai www. helsinki. fi/yliopisto 24. 10. 11