1 / 14

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.

ulfah
Télécharger la présentation

Structure and force balance of idealised cold fronts simulated with WRF

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related