RF02How does it look in the real world?

Margreet van Zanten

Thanks to Bjorn Stevens and Gabor Vali

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

RF02 through the eyes of the radar

Mean thermodynamical state

• Well mixed profiles!

• q_l sub-adiabatic at CT -> drizzle

• Fuzzy cloud top height

RF02 vs RF01

• PBL moister and colder

• Deeper cloud• Moist layer

above PBL

Mean state hor. velocity

• Mean wind 7.3 m/s out of north- northwest

• Some shear at cloud top, comparable to RF01

RF02: intriguing case

Methodology

• Divide data into POC and the rest (nonPOC) based on brightness temperature difference

• Brightness temperature difference is indicator for occurrence of drizzle

Methodology

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

Manifestation of drizzle

• Cooler and moister in POC

• Upward motion in POC

Vertical velocity variance

• Well mixed, energetic PBL

• Similar to RF01

Vert. vel. var. POC vs nonPOC

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

Total water budget

• Drizzle flux important contribution to total water budget

• Net drying of PBL

Tot. water budget POC vs nonPOC

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

Heat budget

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

Heat budget POC vs nonPOC

• Net budget: warming or cooling of PBL?

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 ……

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!

Features to be reproduced by LES

• Well mixed PBL

• Energetic: high w’w’ values

• And when used to study POCs:

* mesoscale features

* longevity of drizzling cells