1. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 1 / 55 Synoptic-scale forcing mechanisms – development of severe weather  Ervin Zsoter  ECMWF, Meteorological Operations Section  ervin.zsoter@ecmwf.int With contributions from: Peter Bechtold and Mark Rodwell

2. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 2 / 55 Forecasting time and space scale Time scale Space scale Tornadoes Supercell thunderstorm Mesoscale convective complex Tropical cyclones Storm track Monsoon Extra-tropical cyclones Climate change minhoursdaysweeksmonths years ~100 m ~ 10 km ~100 km ~1000 km Global Pacific SSTs

3. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 3 / 55 Different NWP parameters showing different scale Z500 PV 330K Precipitation

4. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 4 / 55 Annual-mean of ACC for Europe Z 500 22 Precip SYNOP

5. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 5 / 55 What sort of events are we interested in?  Heavy rainfalls – floods, land slides, etc.  Thunder strikes  Tornadoes  Devastating wind storms  Tropical cyclones related events (wind and rain again)  …

6. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 6 / 55 What do we need for severe weather development?  Unstable atmosphere  Enough moisture  Rising motion trigger in the atmosphere By saying in a simplified way:

7. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 7 / 55 JJA DJF mm day -1 “MONSOON” FROM ARABIC WORD “MAUSIM”: SEASONAL REVERSAL OF WINDS FARMERS MORE INTERESTED IN SEASONAL CYCLE OF RAINS BOTH ASPECTS ARE LINKED MONSOONS: ASIAN & AUSTRALIAN, NORTH & SOUTH AFRICAN, SOUTH AMERICAN & MEXICAN WINDS IMPORTANT FOR ARAB MERCHANT SAILORS Observed Precipitation, V 925 and Z 500 SUSTAIN HALF THE WORLD’S POPULATION Monsoon

8. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 8 / 55 Monsoons forecast: a problem on different scales  Short range: Single rain events within an active phase of the monsoon (influenced by Mesoscale convective systems, Easterly waves)  Medium-range/extended-range: Alternation of active and quiescent monsoon phases (influenced by MJO, Kelvin waves)  Interannual variability: Annual variation of precipitation intensity and position (influenced by Astronomical factors, SST distribution, surface conditions, EL NINO)

9. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 9 / 55 Global: Precipitation and Convection (1)  It’s raining again… 2000/2001 rainfall rate as simulated by IFS CY30R2 and compared to GPCP obs  About 3 mm/day is falling globally, but most i.e. 5-7 mm/day in the Tropics

10. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 10 / 55 Global: Convective cloud types (2) proxy distribution of deep and shallow convective clouds as obtained from IFS

11. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 11 / 55 How well we predict the tropical convection in the short range

12. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 12 / 55 How well we predict the tropical convection in the short range

13. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 13 / 55 Basic mechanisms for the development of eddies  WWave instabilities important for synoptic-scale meteorology are zonally asymmetric perturbations (eddies) to zonally symmetric flow field  BBaroclinic instability –BBaroclinic instability is a wave instability associated with vertical shear of the mean flow. –BBaroclinic instabilities grow by converting potential energy associated with the mean horizontal temperature gradient. –TThe temperature gradient must exist to provide thermal wind balance for vertical shear  BBarotropic instabilities –BBaroclinic instability is a wave instability associated with a horizontal shear in a jet like current –BBarotropic instabilities grow by extracting kinetic energy from the mean flow field

14. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 14 / 55  TThe conservative (Lagrangian way – following the particle in move) quantities are the best in monitoring, detecting structures, evolution of flow, etc. Some basics of dynamic meteorology – termodynamic flow tracers Compression / expansion  TThe temperature is not conservative, the Lagrangian variation is driven by two factors –EEffect of pressure change of the particle –EEffect of heat exchange with diabatic sources + Diabatic sources  BBy combining the temperature change with the 1 st term we get the potential temperature (Θ) –The pot. temperature is a tracer for the particle if the evolution is adiabatic –Important role - static stability (N - buoyancy frequency or Brunt-Vaïsala frequency), on a synoptic-scale it is always positive, vertical motions are forced –If an air particle is raised or lowered under the effect of the vertical motion associated with a convergence or divergence area, the static stability compels it to return towards its initial level Diabatic sources

15. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 15 / 55 Example for (equivalent) potential temperature field

16. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 16 / 55  DDivergent field Divergence (convergence) and vorticity - flow tracers of the “dynamics”  Vorticity due to wind shear  Vorticity due to curvature Parcel gain positive (cyclonic) vorticity  CConvergent field  TThe rotation is described by the vorticity (ξ) –TThe rotation is linked both to the motion of the Earth, and to the rotation component of the wind –VVorticity is the measure of spin around the vertical axis of an object

17. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 17 / 55 Example of relative vorticity field

18. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 18 / 55  SSimilarly to the idea of potential temperature the potential vorticity can be defined (PV) Flow tracers of the “dynamics” – Potential vorticity For hidrostatic atmosphere with potential temperature used as vertical coordinate  FFor typical midlatitude, synoptic flow PV has an order of 10 -6 Values less than ~ 1.5 PVU (or 2.0) are associated with the troposphere  IIt is conserved in frictionless and adiabatic motion and on constant Θ surface it is advected like a passive tracer  Its field shows more structure than the more traditional but equivalent approach of considering the geopotential height on constant pressure surface

19. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 19 / 55 Flow tracers of the “dynamics” – Potential vorticity  There is a strong transition between the high potential vorticity values (in the stratosphere) and the low values (in the troposphere)  It is particularly rapid when following the iso-ө  Zonal average of 10 cold seasons (1986-1995) From METEO France training material

20. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 20 / 55 How to diagnose synoptic-scale motions  Quasi-Geostrophic Assumption –The assumption of the balance, in the atmosphere, between the horizontal Coriolis force and the horizontal pressure force selectively used in the momentum and thermodynamic equations  Hydrostatic balance –No explicit vertical accelerations are allowed –Specifically, horizontal winds are replaced by their geostrophic values in the horizontal acceleration terms of the momentum equations, and horizontal advection in the thermodynamic equation is approximated by geostrophic advection. –This approximation is not accurate in situations in which the ageostrophic wind plays an important advective role, for example, around fronts and jets. –It helps us to understand how the mass and momentum fields interact on the synoptic scale to create vertical circulations which result in sensible weather.

21. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 21 / 55 Quasi-Geostrophic Height Tendency Equation A B C Term A: three-dimensional Laplacian of the height tendency Term B: advection of the absolute geostrophic vorticity by the geostrophic wind Term C: vertical variation of the geostrophic thickness advection P2P2  Derived from the quasi-geostrophic termodynamic equation and the quasi- geostrophic vorticity equation Which is the thickness of a layer defined by two pressure surfaces P1P1 ΔΦΔΦ

22. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 22 / 55 Quasi-Geostrophic Height Tendency Equation -12 -14 -16 -12 VgVg VgVg ΔηΔη -f 0 |V g | | Δη | cos 180 0 > 0 < 0, height falls -f 0 |V g | | Δη | cos 0 0 < 0 > 0, height rises  Interpretation of Term B advection of the Absolute Geostrophic Vorticity by the Geostrophic Wind ΔηΔη Φ-1 ΦΦ+1 Φ Φ-1

23. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 23 / 55 warm air advection cold air advection 500 mb 700 mb 850 mb thickness increases thickness decreases Cold air advection decreasing with height height falls at 700 mb Same result for warm air advection that increases with height 500 mb 700 mb 850 mb old position of 700 mb  Interpretation of Term C Vertical Variation of the Geostrophic Thickness Advection Quasi-Geostrophic Height Tendency Equation

24. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 24 / 55 Quasi-Geostrophic Diagnostic Omega Equation A B C Term A: three-dimensional Laplacian of omega Term B: vertical variation of the geostrophic advection of the absolute geostrophic vorticity Term C: Laplacian of the geostrophic advection of thickness

25. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 25 / 55 Quasi-Geostrophic Diagnostic Omega Equation 500 mb 300 mb 700 mb negative vorticity advection positive vorticity advection < 0 > 0 Therefore, ω < 0 (i.e., upward vertical motion) PVANVA  Interpretation of Term C Vertical Variation of the Geostrophic Advection of the Absolute Geostrophic Vorticity

26. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 26 / 55 Quasi-Geostrophic Diagnostic Omega Equation Since Φ increases as pressure decreases For cold air advection For warm air advection Since the three-dimensional Laplacian operator changes the sign of the function on which it operates, we can see that: Δ 2 (CAA) will be 0 (downward vertical motion) Δ 2 (WAA) will be > 0; therefore LHS of omega equation is >0 and ω will be < 0 (upward vertical motion)  Interpretation of Term C Vertical Variation of the Geostrophic Advection of the Absolute Geostrophic Vorticity

27. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 27 / 55 Quasi-Geostrophic Theory  As consequence of the Q-G vorticity equation The only way that the relative vorticity can change locally in a quasi-geostrophic atmosphere is through: geostrophic advection divergence/convergence (i.e., shrinking or stretching the column)  As a consequence of the Q-G height tendency equation The only way that the thickness can change locally in a quasi-geostrophic atmosphere is through: geostrophic advection adiabatic heating/cooling of the layer through vertical motion

28. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 28 / 55 Tropospheric systems trough ridge PVA NVA Region of upper-level divergence Region of upper-level convergence 500 hPa geop. height HL Surface

29. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 29 / 55 Jet streams warm Surface cold Forcing of ageostrophic circulations/convection in the right entrance and left exit side of upper-level Jet Thermally indirect circulation Thermally direct circulation

30. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 30 / 55 Ex. French Floods: 3 December 2003 (1) upper/lower-level 48h Forecast c 45N 10E10W0 20E 45N 10W10E0 b 250 hPa Wind, 330 K PV, 850 Thetae925 hPa Wind, 330 K PV, 850 Thetae Upper-level divergence and lower level convergence

31. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 31 / 55 45N 010E a 10W 20E French Floods: 3 December 2003 (2) Comparison 48h Forecast and Analysis 45N 10W10E0 b 925 hPa Wind, 330 K PV, 850 Thetae Analysis 48h Forecast

32. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 32 / 55 French Floods: 1/2 December 2003 (3) Precipitation verification for deterministic forecast old Thin numbers=Obs Thick numbers= max. Forecast values

33. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 33 / 55 French Floods: 1-4 December 2003 (4) The role of the EPS= provide probabilities. Here lagged EPS forecasts verifying at the same time : “the closer to the event the better”

34. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 34 / 55 South Africa “tomorrow ”

35. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 35 / 55 South Africa “tomorrow ”

36. Severe Weather Forecasting in Africa – Development of Severe Weather – Ervin Zsoter 36 / 55 Weather situation on Thursday in South Africa Rapid cyclognesis along the vorticity advection