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Drizzle comparisons

Evaluate operational forecast models' drizzle simulation accuracy through observed liquid water content, flux, and drop size comparisons. Explore radar/lidar data retrieval and effects on model performance.

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Drizzle comparisons

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  1. Drizzle comparisons Ewan O’Connor, Robin Hogan, Anthony Illingworth

  2. Overview • Do operational forecast models simulate drizzle correctly? • Models simulate drizzle through precipitation scheme • Treat coalescence drizzle as light rain • Observed liquid water content/flux • Cloud LWC retrieval • Drizzle liquid water flux retrieval • Compare observations and models • Daily basis • Monthly means • Drizzle drop size • Explicit in Met Office model • Effect of changing drizzle number concentration

  3. Liquid water content Model levels Method of Albrecht et al. (1990), Boers et al.(2000) • Use lidar/radar to determine cloud boundaries • Assume linear increase of LWC with height • Scale LWC profile to match LWP from radiometers Cloud top (radar) Height Cloud base (lidar) LWC

  4. Liquid water content • LWC not retrieved in rain

  5. Drizzle below cloud Doppler radar and lidar - 4 observables(O’Connor et al. 2005) • Radar/lidar ratio provides information on particle size

  6. Drizzle below cloud Doppler radar and lidar - 4 observables(O’Connor et al. 2005) • Radar/lidar ratio provides information on particle size Retrieve three components of drizzle DSD (N, D, μ). • Can then calculate LWC, LWF and vertical air velocity, w.

  7. Liquid water flux • LWF retrieved below cloud base and in the absence of rain

  8. ECMWF model • Cloud similar to observations

  9. Met Office mesoscale model • Cloud similar to observations

  10. Met Office global model • Has rain at correct time!

  11. Meteo France ARPEGE model • Thin layers - can be one grid box thick

  12. Compare obs/models • Models have similar LWC / LWP • Models overestimate precipitation

  13. Compare daily means • Models have similar LWC / LWP • Models overestimate precipitation

  14. Monthly comparisons • ECMWF model • Observations

  15. Monthly comparisons • Met Office mesoscale model • Observations

  16. Monthly comparisons • Met Office global model • Observations

  17. Monthly comparisons • Meteo France ARPEGE model • Observations

  18. Drizzle drop size • Met Office model uses explicit size distributions • Treats all precipitation as rain • Overestimates drop sizes • Underestimates evaporation Disdrometer Radar/lidar Model

  19. Met Office diagnostic precipitation • Met Office model uses explicit size • Treats all precipitation as rain

  20. Met Office diagnostic precipitation • Met Office model uses explicit size • Treats all precipitation as rain

  21. Met Office diagnostic precipitation • Met Office model uses explicit size • Treats all precipitation as rain

  22. Met Office diagnostic precipitation • Met Office model uses explicit size • Treats all precipitation as rain

  23. Met Office diagnostic precipitation • Met Office model uses explicit size • Treats all precipitation as rain

  24. Conclusion • Models have reasonable LWC • Produce far too much drizzle • Precipitation reaches surface • Underestimate evaporation • Met Office model • Overestimate of drizzle drop size

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