Dynamical similarities and differences between cold fronts and density currents Victoria Sinclair University of Helsinki

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

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?

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

Scale Analysis for fronts lαβ ACC along PGF along COR along ACC across PGF across COR across 100km Semi – Geostrophic theory: we can neglect the acceleration only in the across front direction (Hoskins and Bretherton, 1972)

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

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

Along front forces z=100m dx =100km dx = 20km dx = 4km PGFCOR BL ACC

Across front forces z=100m dx =100km dx = 20km dx = 4km PGFCOR BL ACC

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

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

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.

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

Density Current force balance Blue: Coriolis Force Red: Pressure gradient force Black: Resultant acceleration Purple: BL force z = 100m dx = 4km

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

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

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

Scale Analysis - revisited lαβ ACC along PGF along COR along ACC across PGF across COR across 20 km Use U, V and l from the WRF simulation Boundary layer processes are not included in Semi Geostrophic theory

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

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

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

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

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

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

Effect of PBL on vertical structure of front YSU PBL dx = 4kmNo PBL dx = 4km Potential temperature (2K), system relative wind vectors. Ascent is shaded. Descent contoured. Scales differ by a factor of 10

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