What are we learning from recent marine boundary layer cloud campaigns?

Robert WoodUniversity of Washington

Artist: Christine Hella-Thompson

Themes• Precipitation

– More of it than we previously thought– A principal organizing force– (Sometimes) strong two-way interaction with aerosol

• Entrainment– Not as much as we previously thought (in Sc.)– Difficult to measure– Current observations insufficient to identify appropriate

closure

• Models for observational campaigns– New sampling approaches– New capabilities; the rise of multisensor remote sensing

Recent MBL Cloud focused field programs

Key physical processes: Stratocumulus

Large scale subsidence

Shallow, well-mixed STBL

Deep, cumulus-coupled STBL

PrecipitationFirst quantitative measurements in Sc. clouds (Brost et al. 1982)

z/z i

Water flux [g m-3 m s-1]

Precipitation increases strongly with cloud top

height in warm precipitating trade cumulus (Byers and

Hall 1955)

Precipitation rate geographical variation

• Over subtropical marine Sc. regions, mean cloud base precipitation rates increase from negligible values close to the coast to 1 mm day-1 about 1000 km from the coast.

Precipitation temporal variation

• Cloud cover 60-85% all year, with low clouds dominant

• 50% of clouds precipitate (radar echoes > -15

dBZ)

Rémillard et al. (J. Climate, 2012)

Azores, CAP-MBL AMF Deployment [May 2009-Dec 2010]

Anatomy of a subtropical marine

stratocumulus region

Bretherton et al. (2010)Toniazzo et al (2011)

Precipitation drives cloud transitionsDYCOMS-2 Field CampaignStevens et al. (2005, BAMS)

Precipitation in trade Cu drives cold pools, organizes cloud fields, creates resilient structures

• Sensor synergy: Ground-based S-band radar; ship based mm radar and in-situ; satellite)

RICO Field Campaign; Zuidema et al. (2012, J. Atmos. Sci.)

Precipitation both macrophysically and microphysically controlled

Precip. susceptibility decreases with cloud thickness in marine Sc

Terai et al. [Atmos. Chem. Phys. 2012]Brenguier and Wood [FIAS, 2009]

What controls CCN and cloud microphysical variability in the marine boundary layer?

• Simple budget model for CCN/Nd in the MBL:

Wood et al. (J. Geophys. Res. 2012)

• Assume constant FT aerosol conc. • Model Nd gradients mostly driven by precipitation sinks

Arctic Sc are in many ways rather similar to subtropical marine Sc.

Shupe et al. [J. Atmos. Sci., 2008]

Vali et al. [J. Atmos. Sci., 1998]

CalifornianStratocumulus

Arctic stratocumulus

Cloud top entrainmentDMSOzoneTotal waterMass budget

Faloona et al. [J. Atmos. Sci., 2005]

• Has a profound impact on Sc thickness and decoupling albedo

• A problem: cloud top entrainment is very difficult to measure

• Example compares flux-jump method using different tracers

Entr

ainm

ent r

ate

[cm

s-1

] 1.0

0.8

0.6

0.4

0.2

0.0

Observational estimates of cloud top entrainment

• Flux-jump approach for a tracer S:

• Turbulent flux typically measured at 2 or 3 levels below cloud top and extrapolated

• New results from POST (right) suggest that fluxes determined by very high resolution LWC measurements are in some cases nonlinear with height Hermann Gerber

Strong shear near cloud top in TO3 case

Entrainment flux from LWC holes [g kg-1 m s-1]

Little evidence for evaporative

cooling in entrained parcels

Mixing fraction 0.0 0.1 0.2 0.3 0.4 0.5

1

0

-1

buoy

ancy

[cm

s-2

]

LWC

0.6

0.3

0.0

-0.3

-0.6

T [K

]

FAST:

Turbulence

Humidity

LWC

Temperature

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

Turbulent mixing at small-scales

• ACTOS, UFT

Siebert et al. (2006)

High resolution measurements of the EIL atop stratocumulus

Katzwinkel et al. (BLM, 2011)“nibbling rather than englulfment”

2

1

0

-1

Nor

mal

ized

heig

ht 40-80 m

Freetroposphere

Cloudlayer

EntrainmentInterfacialLayer (EIL)

Large eddy scale << EIL thickness

Entrainment in trade cumulus: new considerations

Mas

s flu

xM

ass

flux

Distance from cloud edge [m]

Heus et al. (2009)Jonker et al. (2008)

LES model

Obs (RICO)

• Upward mass transport in cumulus clouds primarily balanced by near-cloud downdrafts

• Implications for species longevity in the trade Cu MBL, and for lateral entrainment

New capabilities open new frontiers

• ARM Climate Research Facilities, especially new remote sensing and aerosol sampling suites, offer long-duration sampling of cloud physical and dynamical processes. New statistical approaches to cloud sampling. New permanent ARM site in the Azores.

• Need for more routine measurements over the remote ocean (e.g. NSF Ocean Observatories Initiative “super buoys”)

• Aircraft facilities now offer longer range (e.g. Global Hawk, HIAPER), unprecedented remote sensing

• Big capabilities need big thinking

Where new field observations can help• Factors controlling cloud top entrainment in

stratocumulus and cumulus. – Role of shear and evaporative cooling uncertain.– Need to exploit new measurements approaches, both in-situ

(very high resolution measurements) and remote sensing (new Doppler radar and lidar approaches)

• Microphysical impacts (both CCN and ice formation) on marine low clouds over the extratropical oceans– IN remain poorly understood and measured– Role of aerosol in influencing extratropical cloud regimes

(e.g. open/closed cells, collapsed boundary layers)– Aerosol transport mechanisms to the remote ocean– What controls cloud droplet concentration over the

extratropical oceans?

Dave Leon, Chris Bretherton

Outbound leg

Return leg

Stratocumulus cloud thickness

What remains to be done?• Understanding of entrainment and key processes

affecting it remains poor– New measurement techniques required– Large eddy models do not yet explicitly model entrainment

• Large scale model treatments of low clouds are improving but low cloud feedback not narrowing significantly. Why?

• Aerosol impacts on Sc and (perhaps as importantly) Sc impacts on aerosols are not well known

Canonical profiles

Albrecht et al. (J. Geophys. Res., 1995)

Entrainment interfacial layer