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Dynamical Similarities Between Cold Fronts and Density Currents

Explore the forces and balance across cold fronts and density currents using model simulations and tower observations. Investigate their relationships and conduct idealized experiments to understand their dynamics. Analyze force balance, wind vectors, Coriolis force, pressure gradient force, and resultant acceleration. Draw conclusions on the resolution dependence of force balance and structural differences.

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Dynamical Similarities Between Cold Fronts and Density Currents

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  1. Victoria Sinclair University of Helsinki Dynamical similarities and differences between cold fronts and density currents 15.08.2011 Victoria.Sinclair@helsinki.fi www.atm.helsinki.fi/~vsinclai

  2. Some cold fronts can be visually similar to density currents w and θ Model simulation of a cold front (dx = 2.5km) Tower observations (Shapiro et al 1985)

  3. Questions • What is the force balance across a cold front? • How does the force balance compare to theoretical predictions? • Is the force balance across a cold front similar to that across a density current? • Are cold fronts dynamically related to density currents?

  4. 3D Idealised experiments with WRF • WRF-ARW v3.2, Non-hydrostatic • YSU BL scheme over a sea surface • No moisture • Simulate a cold front and a density current Potential temperature, wind Potential temperature, surface pressure and location of nested domains • 3D baroclinic life cycle with nested domains • dx = 100km, 20km, 4km Drop a cold bubble and allow to spread dx = 4km

  5. Scale Analysis for fronts Semi – Geostrophic theory: we can neglect the acceleration only in the across front direction (Hoskins and Bretherton, 1972)

  6. dx = 100km Force balance across cold front Green: Wind VectorsBlue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration

  7. dx = 4km Force balance across cold front Green: Wind VectorsBlue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  8. PGF COR ACC BL Along front forces z=100m dx =100kmdx = 20kmdx = 4km

  9. PGF COR ACC BL Across front forces z=100m dx =100kmdx = 20kmdx = 4km

  10. Density current Cold front Cold front vs. Density Current Across front z = 100m dx = 4km Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  11. Density current Cold front Cold front vs. Density Current Along front z = 100m dx = 4km Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  12. Conclusions • The force balance is resolution dependent • For the cold front at dx = 4km, • PGF ≈ Coriolis force in the along front direction • PGF >> Coriolis force in the across front direction • In the across front direction, the cold front force balance approaches that of the density current as resolution increases. • In the along front direction the cold front force balance differs to the density current force balance • Across the cold front, the unbalance pressure gradient force is likely due to enhanced horizontal buoyancy gradients.

  13. Cold fronts have a large variety of structures Temperature Temperature, pressure, rain rate ΔT= 10K Δt =5hrs ΔT= 2K Δt =30 mins

  14. Density Current force balance z = 1 km dx = 4km Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  15. Cold front force balance z = 100m dx = 4km Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  16. Cold front force balance z = 1 km dx = 4km Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force

  17. Scale Analysis - revisited Use U, V and l from the WRF simulation Boundary layer processes are not included in Semi Geostrophic theory

  18. U/V V Why do the results differ to theory? • Two assumptions

  19. Scale Analysis - Hoskins and Bretherton (1972) ACROSS FRONT ALONG FRONT Rossby number is assumed to be O(1) Along front wind is assumed to be much greater than along fronts wind

  20. ∆x=100km. No Boundary layer scheme Green: Wind VectorsBlue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration

  21. What do these terms really mean? U X CF Increases convergence Strengthens front Decreases convergence Weakens front

  22. What do these terms really mean? V X CF Decreases vorticity Increases vorticity

  23. Effect of resolution of frontal structure Potential temperature and wind barbs at z=100m The cross front scale decreases with increasing resolution and the wind shift becomes sharper

  24. No PBL dx = 4km YSU PBL dx = 4km Potential temperature (2K), system relative wind vectors. Ascent is shaded. Descent contoured. Scales differ by a factor of 10 Effect of PBL on vertical structure of front

  25. Density Current dx=4km.t=1.5 hours, z=100m, YSU BL scheme Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force Potential temperature and wind vectors

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