RF02

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RF02
How does it look in the real world?
Margreet van Zanten Thanks to Bjorn Stevens and
Gabor Vali
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DYCOMS-II

• July 2001, off the coast of CA
• Nine flights in sc topped PBLs
• Seven nighttime flights
• Basic flightpattern 30 min long circles at
  various heights

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RF02 through the eyes of the radar
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Mean thermodynamical state

• Well mixed profiles!
• q_l sub-adiabatic at CT -gt drizzle
• Fuzzy cloud top height

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RF02 vs RF01

• PBL moister and colder
• Deeper cloud
• Moist layer above PBL

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Mean state hor. velocity

• Mean wind 7.3 m/s out of north- northwest
• Some shear at cloud top, comparable to RF01

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RF02 intriguing case
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Methodology

• Divide data into POC and the rest (nonPOC) based
  on brightness temperature difference
• Brightness temperature difference is indicator
  for occurrence of drizzle

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Methodology

• Good correlation between indicator and drizzle
• Indicator useful, but not perfect

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Manifestation of drizzle

• Cooler and moister in POC
• Upward motion in POC

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Vertical velocity variance

• Well mixed, energetic PBL
• Similar to RF01

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Vert. vel. var. POC vs nonPOC

• In the POC, below cloud less variance due to
  reduced buoy-ancy production

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Total water budget

• Drizzle flux important contribution to total
  water budget
• Net drying of PBL

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Tot. water budget POC vs nonPOC

• Drizzle flux mainly in POC
• Net drying in POC, net moistening in nonPOC

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Heat budget

• Drizzle flux important contribution to heat
  budget
• Net warming of PBL

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Heat budget POC vs nonPOC

• Net budget warming or cooling of PBL?

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Summary

• RF02 is interesting for an intercomparison study
  because it is well mixed and energetic and
  drizzle fluxes are an important contribution to
  both the total water budget as the heat budget,
• but it is also physically very
• interesting

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A hypothesis reputed.

• Bjorn Stevens et. al. 1998
• persistent, well-mixed, shallow, radiatively
  driven stratocumulus-topped PBLs, in which
  drizzle is heavy and downdrafts are negatively
  buoyant through a deep layer, do not exist in
  nature
• RF02 shows they do!

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Features to be reproduced by LES

• Well mixed PBL
• Energetic high ww values
• And when used to study POCs
• mesoscale features
• longevity of drizzling cells