Representation of Sea Salt Aerosol in CAM coupled

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 AR 3, 2001]. Figure 1. Annual average source strength in kg km-2 hr-1 [IPCC AR 3, 2001]

Outline • CAM/CARMA Model Description • Production, Wind Particle swelling Dry deposition • Primary results • Problems

Model Description Namelist: carma_flag, carma_emission, carma_drydep, carma_vtran, … NCEP Weibull Wind CAM + Dry Deposition Production Wet Deposition Particle Swelling Community Aerosol and Radiation Model for Atmospheres + CARMA Optical Sedimentation Interface module Concentration Optical Depth 20 bins (0. 01 ~ 15 μm) Horizontal: 2 o x 2. 5 o Vertical: 28 layers Nucleation condensational growth/evaporation coagulation [Toon, 1988]

Sea Salt Production • Difference come within a factor of 2 for radius > 0. 5 μm • Significant submicron flux (Clarke 2006, Martensson 2003) • Gong’s source function applies to 0. 02 to 10 μm, doing well 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.

NCEP U, V Production Weibull Distribution CAM 10 meter wind Wind Field • 10 meter wind from ocean model. It is related to drag. Data Ocean Model Neutral Stability Drag Coefficient Friction Velocity • Production is sensitive to wind speed. Weibull wind distribution represents the sub-grid-scale characteristics. Figure 4. Sea salt concentration increases with the introduction of Weibull wind distribution.

Particle Swelling • Swelling affects the dry deposition and optical depth calculations. NCEP QFLX, T • Gerber’s scheme let particles swell too large at high relative humidity (RH). A constrain to the RH is needed. Particle Swelling CAM Relative humidity Sedimentation Dry Deposition dry 80% 98% Optical

Dry Deposition Scheme Sedimentation (Vg) CAM Land Model Aerodynamic resistance (ra) Friction velocity Dry Deposition (Vd) Seinfeld and Pandis scheme Figure 5. For large particles, Vd is equal to sedimentation, For small particles, Vd is dominated by mechanisms. CARMA

Global Distribution: Surface flux and mass concentration 4. e 6 2. 5 e 7 4. 3 e 7 5. 2 e 7 7. e 7 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.

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.

85% 15% 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. Canonical mass concentration [Lewis and Schwartz, 2004] Model mass concentration

Hoppel’s dry deposition Scheme ① Transport is upward ② Transport is downward Comparison of the deposition velocity by Hoppel et al. [2005] and Slinn [1980]

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 (32 m/s), providing information on the spume droplet production. • By adding Smith’s source function, we cover the missing mass of the spectrum.