Ewan O’Connor, Robin Hogan, Anthony Illingworth

Drizzle comparisons

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

Liquid water contentMethod 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

LWC

Heig

ht

Cloud base(lidar)

Cloud top (radar)

Modellevels

Liquid water content– LWC not retrieved in rain

Drizzle below cloudDoppler radar and lidar - 4 observables (O’Connor et al. 2005)

• Radar/lidar ratio provides information on particle size

Drizzle below cloudDoppler 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.

Liquid water flux– LWF retrieved below cloud base and in the absence of rain

ECMWF model– Cloud similar to observations

Met Office mesoscale model– Cloud similar to observations

Met Office global model– Has rain at correct time!

Meteo France ARPEGE model– Thin layers - can be one grid box thick

Compare obs/models– Models have similar LWC / LWP– Models overestimate precipitation

Compare daily means– Models have similar LWC / LWP– Models overestimate precipitation

Monthly comparisons

– ECMWF model– Observations

Monthly comparisons

– Met Office mesoscale model– Observations

Monthly comparisons

– Met Office global model– Observations

Monthly comparisons

– Meteo France ARPEGE model– Observations

Drizzle drop size– Met Office model uses explicit size distributions– Treats all precipitation as rain

• Overestimates drop sizes • Underestimates evaporation

Disdrometer

Radar/lidar

Model

Met Office diagnostic precipitation

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

Met Office diagnostic precipitation

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

Met Office diagnostic precipitation

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

Met Office diagnostic precipitation

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

Met Office diagnostic precipitation

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

Conclusion– Models have reasonable LWC

• Produce far too much drizzle• Precipitation reaches surface• Underestimate evaporation

– Met Office model• Overestimate of drizzle drop size