Simple CCN budget in the MBL

Model accounts for:• Entrainment• Surface production (sea-salt)• Coalescence scavenging• Dry depositionModel does not account for:• New particle formation – significance still too uncertain to

include• Advection – more later

0

Production terms in CCN budgetFT Aerosol concentration

MBL depth

Entrainment rate

Wind speed at 10 mSea-salt

parameterization-dependentconstant

We use Clarke et al. (J. Geophys. Res., 2007) at 0.4% supersaturation to represent an upper limit

Loss terms in CCN budget: (1) Coalescence scavenging

Precip. rate at cloud base

MBL depth

Constant

cloud thickness

Wood, J. Geophys. Res., 2006

Comparison against results from stochastic collection equation (SCE) applied to observed size distribution

Steady state (equilibrium) CCN concentration

Free-tropospheric CCN spectra

(West of 75W)

RF03 (high CO west of 80W)

RF05 (low CO west of 84W)

Box-whisker: CCN obs (Snider)5,10,25,50,75,90,95th %, triangle=meancyan = individual spectra

Black curve: Weber and McMurry (1996, Mauna Loa, subsiding conditions)

Same data, Log scale

Free-tropospheric CCN spectra

(West of 75W)

RF03 (high CO west of 80W)

RF05 (low CO west of 84W)

Box-whisker: CCN obs (Snider)5,10,25,50,75,90,95th %, triangle=meancyan = individual spectra

Black curve: Weber and McMurry (1996, Mauna Loa, subsiding conditions)

Number activating (S=0.3%)

Snider (mean/median): 140/120 cm-3

Weber/McMurry: 125 cm-3

RF05: 50-75, depending on compositionRF03: 350-420, depending on compositionRF14: 2000-3000 in plume

Model and observed Nd

With Tony’s size distributions

RF03

RF05

Number activating (S=0.3%)Snider (mean/median): 140/120 cm-3

Weber/McMurry: 125 cm-3

RF05: 50-75 cm-3, depend. on comp.RF03: 350-420 cm-3, depend. on comp.RF14: 2000-3000 in plume

buffered by sea-salt

buffered by precip

Nd hovmoller

• MODIS and VOCALS aircraft obs (symbols)

• Strong coherent synoptic variability, especially west of 80oW

Nd histograms for remote region (78-86oW, 18-22oS)

• Mean values:MODIS: 98 cm-3

C-130: 99 cm-3

C-130, ql>0.02MODIS, fliquid>0.99

skewed tails

Mean precipitation rate (CloudSat, 2C-PRECIP-COLUMN, Stratocumulus regions)

Reduction of Nd from precipitation sink

0 10 20 50 100 150 200 300 500 1000 2000 %

• Precipitation from midlatitude low clouds reduces Nd by a factor of 5• In coastal subtropical Sc regions, precip sink is weak

Predicted and observed Nd

• Monthly climatological means (2000-2009 for MODIS, 2006-2009 for CloudSat)

• Derive mean for locations where there are >3 months for which there is:

(1) positive large scale div.(2) mean cloud top height

<4 km (3) MODIS liquid cloud fraction > 0.4

• Use 2C-PRECIP-COLUMN and Z-R where 2C-PRECIP-COLUMN missing

Histograms of predicted and observed mean Nd [45oS-45oN, subsiding regions]

Cloud droplet concentration [cm-3]

dp/d

Nd

MODISMODEL

tail driven by regions of very low precip

Histograms of predicted and observed mean Nd [45oS-45oN, subsiding regions, log scale]

MODISMODEL

model cannot explain highest concentrations

(near coasts)

Sea-salt source strength compared with entrainment from FT

POCs and aerosols

Steady-state cloud condensation nucleus (CCN) model

Assumptions:

1. Wind-driven sea-salt production and entrainment of NFT CCN from free-troposphere are only aerosol sources

2. Coalescence scavenging is the only loss term

3. Examine steady state solution for CCN concentration

VOCALS, Oct 28th POC case

All VOCALS clouds