Boundary layer controls on extratropical cyclone development

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1 Boundary layer controls on extratropical cyclone development R. S. Plant (With thanks to: I. A. Boutle and S. E. Belcher) 28th May 2010 University of East Anglia

2 Outline Introduction and background Baroclinic wave simulations More than baroclinic instability The role of friction The role of boundary-layer moisture transport Conclusions Boundary layer controls on extratropical cyclone development p.1/37

3 Introduction and background Boundary layer controls on extratropical cyclone development p.2/37

4 Motivation Friction reduces growth rates of baroclinic waves by up to 50% (Valdes and Hoskins 1998) Improved surface winds and minimum pressure (Doyle 1995) from increased drag due to coupling with surface waves Precipitation and long-range moisture transport require evaporated moisture to be ventilated into free troposphere The boundary layer must mediate interactions with surface But what are the interaction mechanisms? Boundary layer controls on extratropical cyclone development p.3/37

5 Role of friction: Ekman pumping tropopause ξ 2 free troposphere ξ 1 ξ1 > ξ 2 boundary layer top boundary layer Boundary layer convergence leads to ascent leads to spin-down of a barotropic vortex Barotropic vorticity equation, Dζ w =ζ, Dt z ζ = f +ξ surface Boundary layer controls on extratropical cyclone development p.4/37

6 Potential vorticity 1 PV = ζ θ ρ Natural analogue of vorticity for a baroclinic system Conserved for adiabtic, frictionless flow Given a balance condition, PV can be inverted to deduce the full dynamics Conversely, PV generation reveals action of friction or diabatic processes High PV must be associated with high stability (stratosphere) or high vorticity (usual in troposphere) Boundary layer controls on extratropical cyclone development p.5/37

7 Baroclinic instability +ve anomaly of PV (eg descent of tropopause) implies a cyclonic circulation Warm temperature anomaly at surface equivalent to +ve PV anomaly (via the boundary condition for the inversion) Co-operative interaction of surface and tropospheric anomalies amplifies a baroclinic wave Strength of interaction governed by static stability Boundary layer controls on extratropical cyclone development p.6/37

8 Main airflows Main flows in a system-relative frame: Boundary layer controls on extratropical cyclone development p.7/37

9 Baroclinic wave simulations Boundary layer controls on extratropical cyclone development p.8/37

10 Simulation set-up Investigations based on simulations of real cases with Met Office Unified Model And also idealized simulations of baroclinic waves (most of the results today from these) These simulations are over ocean only Use a baroclinically-unstable initial state, which is zonally symmetric and based on the mean atmosphere in northern-hemisphere winter Start with small perturbation with wavenumber six Can be run with a dry atmosphere, and with or without a boundary-layer mixing scheme Boundary layer controls on extratropical cyclone development p.9/37

11 Initial conditions (a) 0 (b) Pressure (hpa) Pressure (hpa) N 25N 45N N 85N N N 45N 65N 85N Fixed SST equal to lowest-level atmospheric temperature in initial conditions Boundary layer controls on extratropical cyclone development p.10/37

12 More than baroclinic instability Boundary layer controls on extratropical cyclone development p.11/37

13 Effects of the boundary layer Control simulation, T+60 Simulation with no boundary layer turbulence, T mb 950mb Simulations with (left) and without (right) boundary layer, of storm of 30/10/00 Boundary layer controls on extratropical cyclone development p.12/37

14 Latent heat release Mid-level latent heat release produces +ve PV anomaly Associated cyclonic circulation enhances system development Boundary layer controls on extratropical cyclone development p.13/37

15 Latent heating can dominate By inversion, can measure contributions to the circulation A diabatic PV anomaly drives the intensification Interacts constructively with tropopause feature System does not develop without latent heating Boundary layer controls on extratropical cyclone development p.14/37

16 Diabatic and frictional effects 6 (b) EKE (105Jm-2) Dry BL 2 Dry NoBL Moist BL Moist NoBL Time (d) Does Ekman pumping explain the frictional effect? 2. How reliant is the latent-heating effect (and precipitation) on a boundary-layer moisture source? Boundary layer controls on extratropical cyclone development p.15/37

17 The role of friction Boundary layer controls on extratropical cyclone development p.16/37

18 Baroclinic effects Ekman pumping is barotropic mechanism, but cyclones are baroclinic! Evolution of PV due to friction, DP 1 = F θ Dt ρ Consider evolution of depth-integrated PV, [P] over boundary layer, 1 D[P] = (term wekman ) wh Ph (term vsurf vt ) h Dt Boundary layer controls on extratropical cyclone development p.17/37

19 Baroclinic PV generation? NS temperature gradient in basic state, implies westerly thermal wind Cyclonic circulation implies frictional PV generation to the north of a cyclone (dark shading) and PV destruction to the south (light shading) Boundary layer controls on extratropical cyclone development p.18/37

20 Baroclinic PV generation? Should also account for EW temperature gradients induced by the wave And frictional turning of the wind within the boundary layer Expect PV generation to the north and east of a cyclone Boundary layer controls on extratropical cyclone development p.19/37

21 Baroclinic PV generation Rate of PV generation at day 4 of a simulated dry baroclinic wave Boundary layer controls on extratropical cyclone development p.20/37

22 Transport of generated PV PV at σ = 0.98 (left), σ = (centre) and σ = 0.92 (right) after 6 days of a simulated dry baroclinic wave Boundary layer controls on extratropical cyclone development p.21/37

23 Transport of generated PV Negative low-level PV in vicinity of low generated by Ekman mechanism remains localised Positive PV North and East of low: generated by Baroclinic mechanism advected out of boundary layer on warm conveyor belt Boundary layer controls on extratropical cyclone development p.22/37

24 Effect on cyclone development Cross-section through low centre Thin PV anomaly, associated with enhanced static stability Boundary layer controls on extratropical cyclone development p.23/37

25 Zonal-mean stability Mid-level feature associated with dry intrusion Baroclinic frictional effects increase low-level stability over the low centre Reduces the strength of coupling between tropopause-level PV feature and surface temperature wave Boundary layer controls on extratropical cyclone development p.24/37

26 Comparison with Ekman effect 2 Frictionless Ekman Pumping 2 Increased N Both changes -1 Growth Rate (day ) 1.5 Increased N 2 shows a similar level of reduction Modified Eady model with Ekman-pumping included shows a reduction in growth rate 5e-07 1e e-06 2e e-06 3e-06-1 Zonal Wavenumber (m ) Both effects together can reduce growth rate by 50% Boundary layer controls on extratropical cyclone development p.25/37

27 Effects of boundary-layer friction PV attributed to frictional generation at T+24 in FASTEX IOP15 Barotropic term on 900mb. Baroclinic term on 850mb. Boundary layer controls on extratropical cyclone development p.26/37

28 The role of boundary-layer moisture transport Boundary layer controls on extratropical cyclone development p.27/37

29 Moisture source for a cyclone Precipitation occurs mainly in the ascending air on the warm conveyor belt In the WCB footprint area, boundary layer is stable (warm air moving over relatively cool surface) and evaporation is weak So where does the moisture come from? Boundary layer controls on extratropical cyclone development p.28/37

30 Moisture transport Schematic based on boundary-layer moisture budget analysis: Divergence from high and convergence towards low within the boundary layer necessary to supply WCB with moisture Boundary layer controls on extratropical cyclone development p.29/37

31 Ventilation mechanisms Warm-conveyor belt and shallow convection behind cold front ventilate similar amounts of moisture from the boundary layer Rainfall rate closely matches warmconveyor belt ventilation: moisture is precipitated out quickly and efficiently Boundary layer controls on extratropical cyclone development p.30/37

32 Convectively-ventilated moisture (b) Consider two tracers emitted at surface 20 ms-1 70N 60N One of them is not passed through convection scheme L 50N 40N H 30N 20N -15E 0 15E 30E 45N Tracer Concentration (All param-no convection) at 3.3 km Tracer difference at 3km Difference reveals that convectively-ventilated air is advected polewards and towards the cold front Boundary layer controls on extratropical cyclone development p.31/37

33 Initial moisture content Standard run has surface RH at 45N of 80% Total Moisture Transported (kg) WCB Advection Shallow Convection WCB Precipitation Rescale moisture content of atmosphere in initial conditions Little impact on total ventilation RH0(%) Less rain for low initial RH as some ventilated moisture retained in troposphere Boundary layer controls on extratropical cyclone development p.32/37

34 Final moisture content Compare final states with no initial moisture (left) and standard initialization (right) (b) (a) N N N Pressure (hpa) Pressure (hpa) N N N 75 35N 80 45N 80 55N 65N Climatological-mean mid-latitude moisture distribution can be regenerated in one wave lifecyle (14 days) Boundary layer controls on extratropical cyclone development p.33/37

35 Scalings for moisture transport Scalings with changes in absolute temperature Total Moisture Transported (kg) WCB Advection Shallow Convection WCB Precipitation Clausius-Clapeyron Like Clausius-Clapeyron based on temperature in the south, where main evaporation occurs T0 (K) 290 But steeper because latent-heat release feeds back on system strength Boundary layer controls on extratropical cyclone development p.34/37

36 Conclusions I Boundary layer friction dampens extratropical cyclones by two mechanisms: 1. Ekman spindown: a barotropic mechanism the directly reduces cyclonic circulations 2. Baroclinic PV generation and ventilation: an indirect mechanism that reduces the coupling between upper and lower levels Both mechanisms are robust Boundary layer controls on extratropical cyclone development p.35/37

37 Conclusions II Moisture source for WCB precipitaion may be well away from cyclone Shallow convection is also an important means of moisture ventilation from the mid-latitude boundary layer Scalings can be developed for these ventilation processes Is it possible to develop a bottom-up analysis of changes to the water cycle in a changing climate? Boundary layer controls on extratropical cyclone development p.36/37

38 Conclusions III Mid-latitude cyclogenesis is far from a dead, textbook subject Many important aspects of cyclones can only be understood by unravelling the interactions between large-scale, boundary-layer and moist dynamics Boundary layer controls on extratropical cyclone development p.37/37

Frictional Damping of Baroclinic Waves

Frictional Damping of Baroclinic Waves HHH, 26th May 2006 Bob Plant With thanks to Stephen Belcher, Brian Hoskins, Dan Adamson Motivation Control simulation, T+60 Simulation with no boundary layer turbulence,