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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