Parameterization of turbulent fluxes with respect to cyclogenesis

1. Parameterization of turbulent fluxes with respect to cyclogenesis Andre Simon (Slovak Hydrometeorological Institute) A research study in framework of ALAdin Training NETwork (ALATNET) program EWGLAM, Lisbon

2. Motivation of the study • Forecasts of rapid cyclogenesis by ARPEGE/ALADIN model case studies: 20 December 1998 storm, « Christimas storms » of 1999 (December 26, 27) • Sensitivity to parameterization of turbulent fluxes of heat in deeply stable PBL layers (relationships to cyclogenesis generally less known as by unstable or neutral stratification)

3. Current turbulent diffusion in ARPEGE/ALADIN • Based on K-theory (Louis, 1979) • Limit for Ri number, so called « critical Richardson number Ricr », keeping necessary (but small) amount of turbulent fluxes in very stable layers • Decrease of the Ricr seems to support cyclogenesis(success of the 1999 Christimas storms forecasts), but with several drawbacks (erosion of inversion)

4. Links between turbulence and cyclogenesis • Study of the 20 December 1998 storm: • Influence of increased vertical transport of heat is not direct, with 48 hour advance to rapid deepening of the cyclone • Decrease of static stability in certain areas supports vertical motions in early stage of the cyclone, thus protecting it from entire destruction

5. Outputs of sensitivity tests and diagnostics for the 1998 storm • Scheme for examining the influence of turbulent fluxes on cyclogenesis using the adjoint of ALADIN model: with respect to input forecast (simple physics of Buizza ) Evolution of the storm:

6. Outputs of sensitivity tests and diagnostics for the 1998 storm Sensitivity gradients of forecast error Decrease of static stability in 0 – 24 h 84 h 66 h 24 h

7. Budget calculations / cross sections • What happens in areas of sensitivity: Increase of heat flux Increase of heat flux Decrease of static stability Support of vertical motions

8. Experiments with different parameterizations Stable setup of F(Ri) Increase of diffusion Verifying analysis Arpege/Climat physics (2_nd order closure scheme) Kessler’s precipitation scheme on A/C physics Influence of F(Ri)

9. Parameterization of turbulence - different approach • Current scheme – no reasonable compromise between forecasts of rapid cyclogenesis and situations with very stable PBL layers • More sophisticated parameterizations –not always better + complicated links to schemes of cloudiness and precipitation • New proposals – adaptation of turbulence for conditions withsymmetric instability(inspired by Bennets and Hoskins, 1979)

10. Use of modified Richardson number (Rip) • Modification of exchange coefficients • Computation of Rip : If dry/conditional symmetric instability appears (Rip<0) _ scheme is brought to neutrality

11. Results: dry symmetric instability Rip applied only if : Example: Second « Christimas storm » 27/12/1999 Rip applied Reference

12. Results: conditional symmetric instability • Example: 20 December 1998 storm • Storm is forecasted, its evolution is not realistic

13. Benefits and drawbacks • Application of modified Ri number supports rapid cyclogenesis and works against false mesoscale cyclogenesis • The scheme probably overestimates conditions for symmetric instability appearence, hence the global distribution of static stability decreases significantly • Assumption of well mixed PBL in areas with symmetric instability requires a correction

14. Conclusion and future tasks • Turbulence scheme dependent on the type of the flow can be a solution for problems with forecasting cyclogenesis and inversion • Correct setting of such parameterization would require observations of turbulent fluxes in difficult conditions ( in areas of deep cyclones over sea) • Future: Another approaches will be also verified – e.g. flow dependent mixing length profile