ECMWF Training Course 2009 – NWP-PR

1. ECMWF Training Course 2009 – NWP-PR Atmospheric Variability: Extratropics Mark Rodwell 19 March 2009

2. Talk Outline • Free Barotropic Rossby Waves • Observations • Theory • The Rossby Wave Source • Theory • Explaining the extra-tropical response to the aerosol change • Diabatic Processes • Potential Vorticity • Explosive growth of cyclones • Causes of forecast “busts” • Precipitation • Deterministic verification • Combined prediction systems

3. Free Barotropic Rossby Waves

4. Group Velocity Phase Velocity Rossby waves. Upper tropospheric vΨ, vχ & RWS 10-10s-2 Contour 8ms-1 24 May 25 May 26 May 27 May 2008

5. z y x is the unit “vertical” vector and is the horizontal curl operator Curl of the 3D momentum equation in absolute frame of reference: Shallow atmosphere approximation & assuming non-divergent, horizontal, barotropic, frictionless flow: The Vorticity Equation Motivation (2D flow) :

6. Where Non-divergent barotropic vorticity equation Seeking wave-like solutions … We obtain the “dispersion relation” Where Rossby waves get advected downstream and propagate upstream The larger the spatial scale of the wave, the faster the upstream propagation For stationary waves Mid-latitude stationary zonal wavenumbers Free Barotropic Rossby Waves

7. Group Speed Phase Speed Rossby waves. Upper tropospheric vΨ, vχ & RWS 10-10s-2 Contour 8ms-1 24 May 25 May 26 May 27 May 10ms-1 agrees well with theory 2008

8. 10-11 s-2 Sig. 10% Non sig. Upstream Propagation Downstream Advection Stationary Rossby Waves: Vorticity advection Advection by Anomalous Rotational Wind Advection of Anomalous Vorticity 40-year mean response to change in aerosol climatology deduced using seasonal-mean data. Results are very similar when daily data are used. Anomalies integrated 100-300 hPa.

9. The Rossby Wave Source

10. Upper Troposphere Divergent Wind Anomaly New minus Old aerosol. Anomaly is integrated between 100 and 300 hPa

11. When divergent winds are not neglected in the vorticity equation Application to barotropic models: Sardeshmukh and Hoskins (1988) The “Rossby Wave Source” For use in complex GCMs, it is found here to be useful to vertically integrate this equation between 100 and 300 hPa

12. 10-11 s-2 Sig. 10% Wave Initiators Non sig. Upstream Propagation Downstream Advection JJA Balance in Vorticity Equation New-Old Rossby Wave Source The Rossby Wave Source is indeed seen as the tropically induced source of the extratropical stationary Rossby wave response Advection by Anomalous Rotational Wind Advection of Anomalous Vorticity 40-year mean response to change in aerosol climatology deduced using seasonal-mean data. Results are very similar when daily data are used. Anomalies integrated 100-300 hPa.

13. JJA New-Old RWS, vχ,Ψ and mean ζ 10-11 s-2 Rossby wave paths agree beautifully with those predicted by Hoskins and Ambrizzi (1995)

14. JJA Precipitation, v925 and Z500. New-Old mm day-1. 10% Sig.

15. Rossby wave path agrees with that shown by Hoskins and Ambrizzi (1993) Extratropical RWS anomaly coincides with precipitation changes. Is upper tropospheric divergent wind directly related to local physics? DJF New-Old RWS, vχ,Ψ and mean ζ 10-11 s-2

16. Diabatic Processes

17. ADIABATIC + DIABATIC ADIABATIC Account for adiabatic stretching by considering Potential Vorticity (P): “Stretching”, Tilting & Advection by Diabatic Processes (+ Friction) Adiabatic & Diabatic Contributions to RWS θ=340K 300 hPa PV ‘absorbs’ the adiabatic stretching θ=320K 600 30oN 60oN 90oN When averaging over a long period

18. -3 -2 -1 0 1 2 3 mm/day Total rainfall was double the climatological mean SYNOP Precipitation Anomaly Summer 2007 Based on 24hr accumulations 20070601-2007813 and our new global SYNOP climatology for the years 1979-2005

19. -3 -2 -1 0 1 2 3 mm/day DYNAMICAL WAVES? LOCAL PHYSICS? TROPICAL PHYSICS? Components of a Predictable Signal SCHEMATIC PHYSICS ALONG WAVE? Europe’s wet summer of 2007 could have been the unlucky mean of unpredictable variability. But if not, then this schematic shows some likely building blocks to predictability

20. Agreement with rainfall anomalies. Important for sustaining wave? Terms in PV equation @330K UNIT = 10-13Km2kg-1s-2 26 22 18 14 10 6 Clearer view of Rossby wave? 2 -2 -6 -10 -14 -18 -22 -26 Quadratic Diabatic (residual) Results are based on 0 and 12Z analyses. An over-bar indicates the 2001-2006 climatological mean and a prime indicates the instantaneous 2007 departure from the climatological mean. (June 1 to August 13)

21. TROPOPAUSE FOLDING ASSOCIATED WITH KURT AND ISENTROPIC DOWN-GLIDING(?) CYCLONE “KURT” LOW-LEVEL CYCLONE “LOTHAR” Analysis of winter storm “Lothar” 18Z, 25 DEC1999 PV=2 SURFACE V850 Wernli et al. (2002) Fig. 7a

22. CYCLONE “KURT” TROPOPAUSE FOLD NOW ALSO ASSOCIATED WITH LOTHAR LOW-LEVEL CYCLONE “LOTHAR” Analysis of winter storm “Lothar” 0Z, 26 DEC1999 PV=2 SURFACE V850 Wernli et al. (2002) Fig. 7b

23. CYCLONE “KURT” LOW-LEVEL CYCLONE “LOTHAR” UPPER AND LOWER PV ANOMALIES NEARLY JOIN INTENSE WINDS KILL 50 Analysis of winter storm “Lothar” 6Z, 26 DEC1999 PV=2 SURFACE V850 Wernli et al. (2002) Fig. 7c

24. Look at developments like this from PV perspective. • Classic example that led to a forecast “bust” over Europe a few days later • What is the diabatic forcing? • How well does the (first guess) forecast represent this forcing? • What are the implications of observation rejection? RWS, vχ and Meridional wind anomalies 30 May 5 June 6 June RWS shade interval 10-10 s-2. Meridional wind contour interval 8 ms-1. 100-300 hPa integrals

25. Precipitation

26. Deterministic Scores: Z500, θPV=2 & Precip

27. Extratropical Deterministic Precip Scores 24h Accumulated Precipitation Forecast Scored against SYNOP Observations 2008 1995 D+5 forecast in 2008 as good as D+1 forecast in 1995 Area = [SP--30oS & 30oN--NP]

28. ECMWF “Meteogram” of Precipitation Mumbai ENSEMBLE PREDICTION SYSTEM • Highly useful product but … • … “What should I believe?” • At D+2? • At D+5? ENSEMBLE CONTROL FORECAST HIGH RESOLUTION DETERMINISTIC FORECAST

29. Combined Prediction System – Concept Combining a 10-member ensemble of equally likely members (orange squares) with a single more accurate forecast (yellow rectangle)

30. Combined Prediction System - Theory B = Brier Skill Score averaged over all stations n = number of dates Mj = set of stations reporting on date j mj = number of stations in Mj pij = CPS probability vij = verification (0 or 1) bclim = bclim(location,month) (from climatology) K = number of forecast systems ( K≤3 here) wk is the weight applied to system k(independent of location) Find weights that maximize Brier Skill Score Apply in cross-validated mode (date for year y applied in year y+1)

31. Combined Prediction System - Weights 2001-2005

32. Combined Prediction System - Results 2001-2005

33. Combined “Meteogram” Mumbai

34. Summary • Free Barotropic Rossby Waves • Propagate upstream and get advected downstream • Larger waves can become stationary (e.g. Blocking) • The Rossby Wave Source • How the (tropical) divergent flow can influence the extratropics • Diabatic Processes • May be important for maintaining anomalous flow over a season • Clearly important for explosively growing cyclones • Poor representation (over North America) may lead to forecast “busts” • Precipitation • Deterministic scores show improving trends • Combined prediction systems can improve probabilistic forecasts