Nonlinear baroclinic dynamics of surface cyclones crossing a zonal jet

Nonlinear baroclinic dynamics of surface cyclones crossing a zonal jet Jean-Baptiste GILET, Matthieu Plu and Gwendal Rivière CNRM/GAME (Météo-France, CNRS) 3rd THORPEX International Science Symposium Monterey, CA, USA, 14-18 September 29

Cyclogenesis in the midlatitude : often complex life cycles. Study of a specific phase in the life of some mid-latitude synoptic scale surface cyclones : the jet-crossing phase. Cross-jet motion from warm (anticyclonic) to cold (cyclonic) sides linked with : structural changes of the vortex, transient growth during and just after the jet crossing. low-frequency jet (above 3m/s, interval: 1m/s) 2/19/1997 UTC high-frequency 35hPa geopotential (negative values, interval:5 m²/s²) high-frequency 9hPa geopotential (negative values, interval:25 m²/s²) From Rivière and Joly (27). 2/9

Cyclogenesis in the midlatitude : often complex life cycles. Study of a specific phase in the life of some mid-latitude synoptic scale surface cyclones : the jet-crossing phase. Cross-jet motion from warm (anticyclonic) to cold (cyclonic) sides linked with : structural changes of the vortex, transient growth during and just after the jet crossing. low-frequency jet (above 3m/s, interval: 1m/s) 2/19/1997 6UTC high-frequency 35hPa geopotential (negative values, interval:5 m²/s²) high-frequency 9hPa geopotential (negative values, interval:25 m²/s²) From Rivière and Joly (27). 2/9

Cyclogenesis in the midlatitude : often complex life cycles. Study of a specific phase in the life of some mid-latitude synoptic scale surface cyclones : the jet-crossing phase. Cross-jet motion from warm (anticyclonic) to cold (cyclonic) sides linked with : structural changes of the vortex, transient growth during and just after the jet crossing. low-frequency jet (above 3m/s, interval: 1m/s) 2/19/1997 12UTC high-frequency 35hPa geopotential (negative values, interval:5 m²/s²) high-frequency 9hPa geopotential (negative values, interval:25 m²/s²) From Rivière and Joly (27). 2/9

Cyclogenesis in the midlatitude : often complex life cycles. Study of a specific phase in the life of some mid-latitude synoptic scale surface cyclones : the jet-crossing phase. Cross-jet motion from warm (anticyclonic) to cold (cyclonic) sides linked with : structural changes of the vortex, transient growth during and just after the jet crossing. low-frequency jet (above 3m/s, interval: 1m/s) 2/19/1997 18UTC high-frequency 35hPa geopotential (negative values, interval:5 m²/s²) high-frequency 9hPa geopotential (negative values, interval:25 m²/s²) From Rivière and Joly (27). 2/9

Cyclogenesis in the midlatitude : often complex life cycles. Study of a specific phase in the life of some mid-latitude synoptic scale surface cyclones : the jet-crossing phase. Cross-jet motion from warm (anticyclonic) to cold (cyclonic) sides linked with : structural changes of the vortex, transient growth during and just after the jet crossing. low-frequency jet (above 3m/s, interval: 1m/s) 2/2/1997 UTC high-frequency 35hPa geopotential (negative values, interval:5 m²/s²) high-frequency 9hPa geopotential (negative values, interval:25 m²/s²) From Rivière and Joly (27). 2/9

Use of a highly idealized model : Talk framework Description of two basic physical mechanisms separetely : impact of barotropic shear on baroclinic interaction, role of the environmental PV gradient on the trajectory of vortices, known as β-drift for tropical cyclones and oceanic vortices, Examinination of a more complete simulation combining the two effects. 3/9

2-layer model (Phillips, 1951) : advection of potential vorticity q in two tropospheric layers : where q u = ψ u + (f + βy) λ 2 (ψ u ψ l ), (1) q l = ψ l + (f + βy) + λ 2 (ψ u ψ l ), (2) q k t + u k. q k =, (3) k {u, l} denotes the upper or the lower layer, u k = (u k, v k ) is the geostrophic wind, ψ k is the stream function in the k layer, f = f + βy is the Coriolis parameter, λ the Rossby deformation radius. 4/9

Baroclinic interaction modulated by barotropic shears Purpose : examination of the interaction between a favourably vertically tilted vortex and an environmental wind field, with both baroclinic and barotropic shear. Basic stationary states : u u = (±αy + αλ)i, (4) u l = (±.5αy)i. (5) y j i x 5/9

Baroclinic interaction modulated by barotropic shears 2 1 t=h -1-2 -2-1 1 2 3 4 5 6 t=12h t=24h 2 1-1 -2 1 2-2 -1 3 4 5 t=36h 6 positive perturbation vorticity -5-1 (interval: 2.1 s ) upper-layer lower-layer > large difference in vertical configuration and horizontal shape. 5/9

Baroclinic interaction modulated by barotropic shears 2.e-5 1.5e-5 1.e-5 anticyclonic shear internal conv. dk /dt 2.e-5 1.5e-5 1.e-5 internal conv. dk /dt cyclonic shear 5.e-6 vertical geopotential fluxes 5.e-6 vertical geopotential fluxes.e+.e+ -5.e-6 barotropic conv. -5.e-6 barotropic conv. -1.e-5 1 2 3 4 5 6-1.e-5 1 2 3 4 5 6 5/9

Impact of β on mid-latitude synoptic cyclones trajectories β-effect : can be induced by Coriolis parameter meridional variation or environmental PV gradient. In a purely baroclinic environment : q u y = β + α λ 1 c z, q l y = β α λ 1 c z. Basic motion component of tropical cyclones and oceanic vortices : extension to mid-latitude cyclones? 6/9

Impact of β on mid-latitude synoptic cyclones trajectories Experiment in a purely baroclinic environment on a β plane. 1 β = t=h β = 3β -1-2 -1 1 2-2 -1 1 2 perturbation vorticity -5-1 (interval: 5 1 s ) 6/9

Impact of β on mid-latitude synoptic cyclones trajectories Experiment in a purely baroclinic environment on a β plane. 1 β = t=12h β = 3β -1-2 -1 1 2-2 -1 1 2 perturbation vorticity -5-1 (interval: 5 1 s ) 6/9

Impact of β on mid-latitude synoptic cyclones trajectories Experiment in a purely baroclinic environment on a β plane. 1 β = t=24h β = 3β -1-2 -1 1 2-2 -1 1 2 perturbation vorticity -5-1 (interval: 5 1 s ) 6/9

Impact of β on mid-latitude synoptic cyclones trajectories Experiment in a purely baroclinic environment on a β plane. 1 β = t=36h β = 3β -1-2 -1 1 2-2 -1 1 2 perturbation vorticity -5-1 (interval: 5 1 s ) 6/9

Impact of β on mid-latitude synoptic cyclones trajectories Experiment in a purely baroclinic environment on a β plane. 1 β = t=48h β = 3β -1-2 -1 1 2-2 -1 1 2 perturbation vorticity -5-1 (interval: 5 1 s ) 6/9

Impact of β on mid-latitude synoptic cyclones trajectories Experiment in a purely baroclinic environment on a β plane. 1 β = t=6h β = 3β -1-2 -1 1 2-2 -1 1 2 perturbation vorticity -5-1 (interval: 5 1 s ) > Meridional displacement in an environmental PV gradient. No shift without β. 6/9

Jet crossing in a meridionally confined zonal jet on an f-plane. y j i 4 1 2 3 4 5 (m/s) 4-1 1 2 x1-11 -1-1 m.s x 3 3 2 2 1 1-1 -1-2 -2-3 -3-4 -4 7/9

Jet crossing in a meridionally confined zonal jet on an f-plane. t=12h 1 2 3 4 5 (m/s) t=36h 8 6 t=6h 4 2 2 4 6 8 1 12 14 16 7/9

Jet crossing in a meridionally confined zonal jet on an f-plane. 3e-5 2e-5 internal conv. 1e-5 e+ dk /dt barotropic conv. -1e-5 vertical geopotential fluxes -2e-5 1 2 3 4 5 6 7 7/9

Conclusions Focus on the evolution of surface cyclones on the southern side of a meridionally confined zonal jet : Cross-jet motion due to northward potential vorticity gradient, Structural modifications : stretching and sustained vertical configuration before the jet crossing, isotropization and loss of vertical configuration afterwards, Energetic regeneration at the jet-crossing. More details in Gilet, J.-B., M. Plu, and G. Rivière, 29 : Nonlinear baroclinic dynamics of surface cyclones crossing a zonal jet. J. Atmos. Sci., xx, in press. An automated algorithm has been developed to detect such evolutions in the ERA-Interim reanalysis, in order to assess the statistical robustness of the mechanisms highlighted here. 8/9