representation of sea salt aerosol in cam coupled with a sectional aerosol microphysical model carma...
TRANSCRIPT
Representation of Sea Salt Aerosol in CAM coupled with a Sectional Aerosol
Microphysical Model CARMA
Tianyi Fan, Owen Brian Toon
LASP/ATOC, University of Colorado, Boulder
http://www.tulpule.com/contents/pix/cruises/ccl-ecstasy-dec-01/index.html
Introduction
• Sea salt aerosols (SSA) scatter solar radiation, modify the properties of clouds, transfer heat and moisture between ocean and atmosphere, and participate in heterogeneous chemistry.
• SSA dominates the particulate mass over the remote ocean, with a
global emission of 30~3,000 Tg/year [Lewis and Schwartz, 2004].
• The top-of-atmosphere, global annual radiative forcing due to sea salt is estimated between
-1.51 and -5.03 Wm-2 for high and low emission values [IPCC AR3, 2001].
Figure 1. Annual average source strength in kg km-2 hr-1
[IPCC AR3, 2001]
Outline
• CAM/CARMA Model Description
• Production, Wind
Particle swelling
Dry deposition
• Primary results
• Problems
Model Description
CAMProductionProduction
Particle SwellingParticle Swelling
Dry DepositionDry Deposition
CARMA
Optical Optical
Weibull WindWeibull Wind
ConcentrationConcentration
Optical DepthOptical Depth
Interface module
Community Aerosol and Radiation Model for Atmospheres
Nucleationcondensational growth/evaporationcoagulation [Toon, 1988]
+ +Wet DepositionWet Deposition
NCEP
20 bins (0.01 ~ 15 μm)Horizontal: 2o x 2.5o
Vertical: 28 layers
Namelist: carma_flag, carma_emission, carma_drydep, carma_vtran, …
SedimentationSedimentation
Sea Salt Production• Difference come within a factor of 2 for radius > 0.5 μm• Significant submicron flux (Clarke2006, Martensson2003)• Gong’s source function applies to 0.02 to 10 μm, doing w
ell for >1 μm.
Figure 2. A summary of recent Sea salt source functions [O’Dowd and de Leeuw, 2007]
Gong’s Source Function
• Number peaks at submicron particles. • Surface area and Mass peaks at > 1 μm.
Figure 3. Gong’s source function for number, surface area and mass concentration.
Data Ocean Model
ProductionProduction
Weibull DistributionWeibull DistributionCAM
10 meter wind10 meter wind
Wind FieldWind Field
Neutral StabilityDrag CoefficientFriction Velocity
• 10 meter wind from ocean model. It is related to drag.
• Production is sensitive to wind speed.
Weibull wind distribution represents the sub-grid-scale characteristics.
NCEPU, V
Figure 4. Sea salt concentration increases with the introduction of Weibull wind distribution.
)( 41.310UFndrdN
CAMParticle SwellingParticle Swelling
NCEPQFLX, T
Relative humidity
Particle SwellingParticle Swelling
Dry DepositionDry Deposition
SedimentationSedimentation
OpticalOptical
dry 80% 98%
•Swelling affects the dry deposition and optical depth calculations.
•Gerber’s scheme let particles swell too large at high relative humidity (RH). A constrain to the RH is needed.
Land Model
Dry Deposition Scheme
CAM
CARMA
Dry Deposition (Vd)Dry Deposition (Vd)
Sedimentation (Vg)Sedimentation (Vg)
Figure 5. For large particles, Vd is equal to sedimentation, For small particles, Vd is dominated by mechanisms.
Aerodynamic resistance (ra)Friction velocity
Seinfeld and Pandis scheme
ggbaba
d vvrrrr
v
1
Figure 6. Global distribution of surface flux in February and July. Northern hemisphere surface flux is enhanced in February and Southern hemisphere is enhanced in July.
Figure 7. Global distribution of concentration at the model bottom level in February and July. Trend is different from surface flux, indicating the effect of sinks.
Global Distribution: Surface flux and mass concentration
7.e75.2e74.3e72.5e74.e6 7.e75.2e74.3e72.5e74.e6
Model Result – Seasonal variation of mass concentration
Figure 8. Comparison between the model results and Prospero and Savoie’s observations at locations Cape Point, Mace Head, Bermuda, and Iceland. Model results underestimated the sea salt mass.
Canonical mass concentration [Lewis and Schwartz, 2004]
Model mass concentration
①Canonical size distribution: 15% of the mass is outside the range of Gong’s source function
② Model mass concentration vs. Canonical concentration:
Loss of mass due to overestimated dry deposition of large particles.
85% 15%
1 refgd fvv
gv
refref zf
Comparison of the deposition velocity by Hoppel et al. [2005] and Slinn and Slinn [1980]
Hoppel’s dry deposition Scheme
)1( refgref
gd fVvf
Vvv
① Transport is upward
② Transport is downward
zNvzNutN g *
Summary
• Comparisons with the observations show that sea salt concentrations are underestimated in the model over the global ocean except for the Antarctic.
• According to the canonical size distribution, the model results is missing large particles. It is possible due to the size range not described in the source function.
• Overestimated dry deposition for large particles may be another reason for low concentrations. An alternative scheme by Hoppel et al. is worth trying to give a lower deposition velocity for large particles.
To include Smith’s source function
• Smith’s source function measures under very high wind speed (32m/s), providing information on the spume droplet production.
• By adding Smith’s source function, we cover the missing mass of the spectrum.