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.