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Overview Motivation of the new reference atmosphere and description

A new reference atmosphere for the COSMO model Günther Zängl Deutscher Wetterdienst, Offenbach, Germany. Overview Motivation of the new reference atmosphere and description Tests considering its impact, including a comparison between Leapfrog and Runge-Kutta cores

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Overview Motivation of the new reference atmosphere and description

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  1. Günther Zängl, DWD A new reference atmosphere for the COSMO modelGünther ZänglDeutscher Wetterdienst, Offenbach, Germany

  2. Günther Zängl, DWD • Overview • Motivation of the new reference atmosphere and description • Tests considering its impact, including a comparison between Leapfrog and Runge-Kutta cores • Comparison with impact of unapproximated pressure tendency equation (Lucio Torrisi; see subsequent talk)

  3. Günther Zängl, DWD • New reference atmosphere - motivation • The existing reference atmopshere, which is based on the assumption , has the inconvenient property that dT0/dz gets increasingly negative in the stratosphere • This severely limits the allowable vertical extent of the model domain; for the default values used in COSMO, T0 reaches 0 K at a height of 28.9 km • Another physically questionable property is that the reference pressure is still nonzero ( 1.05 hPa) where T0 = 0 K

  4. Günther Zängl, DWD • New reference atmosphere - definition • The new reference atmosphere is based on • This expression also allows for an analytical integration of the hydrostatic equation, yielding • and • Present default values: T00 = 213.15 K, ΔT = 75 K, H = 10 km

  5. Günther Zängl, DWD • New reference atmosphere (cont’d) • As the new reference profile approaches an isothermal stratosphere, there is no longer a limit to the vertical extent of the model domain (and it is much closer to reality) • In the context of implementing the new reference atmosphere, an inconsistency in the calculation of the reference pressure at full levels was discovered: it is taken to be the arithmetic mean of the adjacent half levels, but this is also done for geopotential height in the RK core • Instead, we now use either the analytical formula or integrate the hydrostatic equation to get the full-level reference pressure

  6. Günther Zängl, DWD • Test program • Selected cases: 03/03/2008, 30/05/2008 (each +72h) • Leapfrog core (operational configuration) • Runge-Kutta core (standard implementation) • RK core using discretized hydrostatic equation to compute full-level reference pressure • RK core using analytical formula to compute full-level reference pressure • RK core with new reference atmosphere, combined with both consistent methods to compute reference pressure • Unapproximated pressure tendency equation (L. Torrisi) • Generally: Initialization from GME assimilation run (cold start), lateral boundary conditions from operational GME forecast

  7. Günther Zängl, DWD COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 12h Sea-level pressure Pressure difference forecast-analysis

  8. Günther Zängl, DWD COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 24h Sea-level pressure Pressure difference forecast-analysis

  9. Günther Zängl, DWD COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 36h Sea-level pressure Pressure difference forecast-analysis

  10. Günther Zängl, DWD COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 48h Sea-level pressure Pressure difference forecast-analysis

  11. Günther Zängl, DWD COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 60h Sea-level pressure Pressure difference forecast-analysis

  12. Günther Zängl, DWD COSMO-EU „cold start“ forecast initialized at 00 UTC 03/03/2008, operational setup, validation against GME assimilation run t + 72h Sea-level pressure Pressure difference forecast-analysis

  13. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 12h Runge-Kutta Leapfrog

  14. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 24h Runge-Kutta Leapfrog

  15. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 36h Runge-Kutta Leapfrog

  16. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 48h Runge-Kutta Leapfrog

  17. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 60h Runge-Kutta Leapfrog

  18. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with (unmodified) RK core t + 72h Runge-Kutta Leapfrog

  19. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 12h Runge-Kutta new Leapfrog

  20. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 24h Runge-Kutta new Leapfrog

  21. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 36h Runge-Kutta new Leapfrog

  22. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 48h Runge-Kutta new Leapfrog

  23. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 60h Runge-Kutta new Leapfrog

  24. Günther Zängl, DWD Comparison between reference run (operational setup; right) and experiment with RK core and new reference atmosphere (discrete hydrost. eqn.) t + 72h Runge-Kutta new Leapfrog

  25. Günther Zängl, DWD Temporal evolution of pressure bias, case 1 (3/3/08)

  26. Günther Zängl, DWD RMS error (after bias removal), case 1 (3/3/08)

  27. Günther Zängl, DWD Temporal evolution of pressure bias, case 2 (30/5/08)

  28. Günther Zängl, DWD RMS error (after bias removal), case 2 (30/5/08)

  29. Günther Zängl, DWD • Summary of test results • The new reference atmosphere tends to improve the pressure forecast; in both cases, the results are similar to those obtained with the old leapfrog core • The way of computing the reference pressure (analytical / numerical integration of the hydrostatic equation) has a marked systematic impact on the pressure bias • The unapproximated pressure tendency equation affects the temporal evolution of the pressure bias; however, more tests are needed for quality assessment • Tests (not shown here) with higher model top (28 instead of 22 km) do not indicate a systematic impact

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