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Dynamical similarities and differences between cold fronts and density currents

Victoria Sinclair University of Helsinki. Dynamical similarities and differences between cold fronts and density currents. 15.08.2011 Victoria.Sinclair@helsinki.fi

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Dynamical similarities and differences 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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