Ewan OConnor, Robin Hogan, Anthony Illingworth

1
Drizzle comparisons

• Ewan OConnor, 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

• 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
  (OConnor et al. 2005)
• Radar/lidar ratio provides information on
  particle size

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Drizzle below cloud

• Doppler radar and lidar - 4 observables
  (OConnor 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.

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Liquid water flux

• LWF retrieved below cloud base and in the absence
  of rain

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ECMWF model

• Cloud similar to observations

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Met Office mesoscale model

• Cloud similar to observations

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Met Office global model

• Has rain at correct time!

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Meteo France ARPEGE model

• Thin layers - can be one grid box thick

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Compare obs/models

• Models have similar LWC / LWP
• Models overestimate precipitation

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Compare daily means

• Models have similar LWC / LWP
• Models overestimate precipitation

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Monthly comparisons

• ECMWF model
• Observations

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Monthly comparisons

• Met Office mesoscale model
• Observations

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Monthly comparisons

• Met Office global model
• Observations

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Monthly comparisons

• Meteo France ARPEGE model
• Observations

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Drizzle drop size

• Met Office model uses explicit size distributions
• Treats all precipitation as rain
• Overestimates drop sizes
• Underestimates evaporation

Disdrometer Radar/lidar Model
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Met Office diagnostic precipitation

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

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Met Office diagnostic precipitation

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

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Met Office diagnostic precipitation

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

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Met Office diagnostic precipitation

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

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Met Office diagnostic precipitation

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

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Conclusion

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