1. Page 1© Crown copyright 2006 Boundary layer mechanisms in extra-tropical cyclones Bob Beare

2. Page 2© Crown copyright 2006 Motivation  Boundary layer significantly reduces the depth of a cyclone (~50% reduction in growth rates, Valdes and Hoskins 1988).  Mechanism provides framework for understanding model changes. Drag  Ekman pumping typically used at Met Office.  Conceptual models.  Recent research (Stephen Belcher, NWP seminar July 2005) indicates that potential vorticity (PV) should be considered also. Highlights warm sector and warm conveyor belt.  Aim: compare Ekman and PV approaches.

3. Page 3© Crown copyright 2006 Boundary layer structure in extra-tropical cyclone L Potential temperature Height Warm sector (stable) Cold sector (unstable)

4. Page 4© Crown copyright 2006 Ekman pumping Boundary layer stress h u*2u*2 Geostrophic wind Coriolis Pressure gradient drag Momentum balance & continuity L = horizontal scale f 0 =Coriolis parameter Convergent wind L u * =friction velocity

5. Page 5© Crown copyright 2006 Ekman pumping  Convergence from boundary layer drag  ascent (Ekman pumping)  spin down by vortex squashing  Distribution of friction velocity  Ekman pumping  Weakness: momentum budget only, what about thermodynamics?

6. Page 6© Crown copyright 2006 Potential vorticity Potential temperature black, PV anomaly red, winds orange  Conservation when no heating or friction  Inversion (balance condition and boundary conditions) Potential temperature gradient Absolute vorticity Increasing

7. Page 7© Crown copyright 2006 Potential vorticity in boundary layer Heating from boundary layer:  Surface heat fluxes Boundary layer stress:  Ekman pumping contribution  Horizontal potential temperature gradients  All terms dependent on gradients in potential temperature. Weakness: what about regions with zero gradient of potential temperature (unstable layers)?

8. Page 8© Crown copyright 2006 Summary Ekman pumping Potential vorticity Friction velocity Horizontal scale Coriolis paramter Potential temperature gradient Absolute vorticity

9. Page 9© Crown copyright 2006 Model set up  Met Office Unified model  Idealised: dry, only boundary layer scheme and dynamics operating.  Why idealised? Isolate boundary layer-dynamics interaction, control over jets and sea surface temperature, framework for interpreting global model or NAE.  Met Office boundary layer scheme: separate treatment of stable and unstable boundary layers.  Limited area 18000 km x 9000 km, 45 km horizontal resolution, 10 minute time step.

10. Page 10© Crown copyright 2006 Basic state

11. Page 11© Crown copyright 2006 Triggering cyclogensis Near surface Potential temperature Upper level trough Tropopause PV

12. Page 12© Crown copyright 2006 Idealised UM cyclone lifecycle Cold conveyor belt jet (height 1 km) Warm seclusion Shading friction velocity > 0.5 m/s

13. Page 13© Crown copyright 2006 Ekman pumping vs potential vorticity Ekman pumping max 9 cm/s Boundary layer averaged PV in warm sector

14. Page 14© Crown copyright 2006 Distribution with stability Unstable (cold sector)Stable (warm sector) Peak friction velocity in neutral/ unstable boundary layer h = boundary layer depth L= Obukhov length PV confined to stable boundary layers

15. Page 15© Crown copyright 2006 Summary  Ekman pumping and boundary layer averaged PV in distinct locations: the cold sector/ seclusion region (unstable) and warm sector (stable) respectively.  Which sector of the cyclone contributes the most to the cyclone depth? Next switch off boundary layer mixing in each region.

16. Page 16© Crown copyright 2006 Switching off boundary layer mixing u * 2 averaged over area of cyclone

17. Page 17© Crown copyright 2006 PV inversion Horizontal section through domain PV anomaly

18. Page 18© Crown copyright 2006 Comparison with global model Idealised UM Global N216 Friction velocity >0.5 m/s shaded, solid 20 m theta, dashed divides between stable and unstable BLs Stable warm sector

19. Page 19© Crown copyright 2006 Sensitivity to “operational” boundary layer and dynamics changes PS 10 changes (stability dependence) Interpolation in Semi-lagrangian scheme Changes which affect the neutral boundary layer

20. Page 20© Crown copyright 2006 Conclusions  Unified model has been set up with an idealised jet, forming a realistic cyclone lifecycle.  Ekman pumping and potential vorticity in different locations at occlusion: Ekman in cold sector/occlusion and PV in warm sector.  Switching off mixing in unstable boundary layer (cold sector/ occlusion) has more impact than for stable boundary layer (warm sector).  Operational type boundary layer mixing changes produce ~+/-2hPa change over 72 hours.

21. Page 21© Crown copyright 2006 Future work  Invert warm sector PV to determine its impact on flow.  Harmonise PV and Ekman views via bottom boundary temperatures.  Revisit the role of the neutral boundary layer mixing on cyclones.  CASE studentship with Reading University: Ian Boutle.