Simple CCN budget in the MBL Model accounts for: Entrainment Surface production (sea-salt) Coalescence scavenging Dry deposition Model does not account.

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Presentation transcript:

Simple CCN budget in the MBL Model accounts for: Entrainment Surface production (sea-salt) Coalescence scavenging Dry deposition Model does not account for: New particle formation – significance still too uncertain to include Advection – more later  0

Production terms in CCN budget FT Aerosol concentration MBL depth Entrainment rate Wind speed at 10 m Sea-salt parameterization-dependent constant 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) 5,10,25,50,75,90,95 th %, triangle=mean cyan = 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) 5,10,25,50,75,90,95 th %, triangle=mean cyan = 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 composition RF03: , depending on composition RF14: in plume

Model and observed N d

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: cm -3, depend. on comp. RF03: cm -3, depend. on comp. RF14: in plume buffered by sea-salt buffered by precip

N d hovmoller MODIS and VOCALS aircraft obs (symbols) Strong coherent synoptic variability, especially west of 80 o W

N d histograms for remote region (78-86 o W, o S) Mean values: MODIS: 98 cm -3 C-130: 99 cm -3 C-130, q l >0.02 MODIS, f liquid >0.99 skewed tails

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

Reduction of N d from precipitation sink % Precipitation from midlatitude low clouds reduces N d by a factor of 5 In coastal subtropical Sc regions, precip sink is weak

Predicted and observed N d Monthly climatological means ( for MODIS, 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 N d [45 o S-45 o N, subsiding regions] Cloud droplet concentration [cm -3 ] dp/dN d MODIS MODEL tail driven by regions of very low precip

Histograms of predicted and observed mean N d [45 o S-45 o N, subsiding regions, log scale] MODIS MODEL 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 N FT 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 28 th POC case All VOCALS clouds