1 Radiative impact of mineral dust on surface energy balance and PAR, implication for land-vegetation- atmosphere interactions Xin Xi Advisor: Irina N. Sokolik School of Earth and Atmospheric Sciences College of Science Georgia Institute of Technology 6 th Graduate Student Symposium Nov.14, 2008

2 Motivation 1. Climatic link of vegetation: global carbon cycle (photosynthesis and respiration), global energy balance (surface reflection), hydrological cycle (evapotranspiration). 2. Aerosol affects vegetation growth through direct (light scattering and absorption) and indirect (cloud and precipitation) effects. (aerosol deposition also disturbs plant functioning) 3. Aerosol diffuse effect: Aerosol reduces total photosynthetically active radiation (PAR, 0.4 µm ~ 0.7 µm ), but increase the diffuse component, which uniformly distributes among the leaves, thus increasing the total photosynthetic rate. (Cohan etal 2002; Gu etal 2003; Yamasoe etal 2006) e.g. Mount Pinatubo eruption in 1991  increase of noontime photosynthesis of Harvard forest by 23% in 1992 a). Past studies didn’t consider the aerosol-induced change in both surface net radiation and PAR. b). No study in the dust aerosol. This study is a starting point to investigate the dust aerosol effect in both surface net radiation and PAR, and how this effect potentially relates to vegetation functioning.

3 Approach 1. Optical modeling Mie-theory: complex refractive indices of each species and particle size distribution (lognormal) dust composition: calcite, quartz and two clay-iron oxide aggregates (illite-geothite and illite-hematite) (Lafon, et al 2006, JGR) 2. Dust surface forcing 1-D radiative transfer model: SBDART (Ricchiazzi et al 1998, BAMS) Net radiative flux: Surface net radiation (0.2 µm ~100 µm ): Dust surface radiative forcing:

4 Size distributionDust loadingVertical profileSurface cover Reid et al 2008highmixeddryland Lafon et al 2006moderatemultilayerrangeland Clarke et al 2004lowliftedgrassland (km) MixedMultilayerLifted Fine modeCoarse mode Reid etal µm:: % 0.84µm:: % Lafon etal µm:: % 1.05µm:: % Clarke etal µm:: % 0.89µm:: % 4.3µm: % Factors to be considered: AOD 0.5µ m ReidLafonClarke High Moderate Low

5 LW (-76.74) PAR (486.06) diffuse PAR (38.1) SW (716.29) SW+LW (637.95) Dust surface forcing in SW+LW, SW, LW and PAR, and downward diffuse PAR: comparison of dust loading solar zenith: 20 degree surface: grassland dust size: Lafon etal 2006 vertical profile: mixed 1.Negative forcing in shortwave (SW) and positive forcing in longwave (LW). 2. Net PAR is reduced, but the diffuse component dramatically increases e.g., by 139 Wm -2 at low dust loading case (AOD 0.5µm =0.4).

6 SW+LW (637.95) SW (716.29) LW (-76.74) PAR (486.06) diffuse PAR (38.1) Dust surface forcing in SW+LW, SW, LW and PAR, and downward diffuse PAR: comparison of dust size distribution surface: grassland dust loading: moderate (AOD 0.5µm =1.34) vertical profile: mixed 1.“Reid” contains largest fraction of coarse particles, which are more efficient in absorption and extinction (SW and PAR) than fine particles. 2.Coarse particles also cause larger LW forcing than fine particles (e.g., “Reid” is about 1 Wm -2 larger than “Clarke”), due to stronger absorption and scattering. 1 Wm -2 difference

7 SW+LW (637.95) SW (716.29) LW (-76.74) PAR (486.06) diffuse PAR (38.1) Dust surface forcing in SW+LW, SW, LW and PAR, and downward diffuse PAR: comparison of dust vertical profile surface: grassland size: Lafon et al 2006 dust loading: high (AOD 0.5µm =2.0) 1.Compared with “mixed” case, “lifted” dust layer causes less LW forcing (by 6 Wm -2 ), and as a result, a larger forcing in SW+LW. - dust forcing varies during transport, not only due to composition change. 2. “lifted” case induces more diffuse PAR (by about 2 Wm -2 at high loading case).

8 SW+LW (637.95) SW (716.29) LW (-76.74) PAR (486.06) diffuse PAR (38.1) Dust surface forcing in SW+LW, SW, LW and PAR, and downward diffuse PAR: comparison of surface albedo 1. The spectral dependence of surface reflectance causes different forcing in, e.g., SW vs. PAR. 2. Surface structure (e.g., canopy shape) significantly alters the radiation direction field, which is not resolved in 1D model. size: Lafon et al 2006 dust loading: moderate vertical profile: mixed different surface emissivities SW+LW SWPARdiffuse PAR

9 Implication for land-vegetation-atmosphere interactions aerosol effects on vegetationvegetation feedback

10 Conclusion and discussion 1.Composition, size, vertical profile and surface properties all affect dust surface forcing, which need to be constrained by measurements in real case studies. 4.Need to consider particle shape for more realistic scattering phase function (e.g., T-matrix, DDA). 5.Need to consider surface 3D structure (Bi-directional reflectance distribution function or BRDF) in the radiative transfer scheme for plant canopies, and couple it to the ecological models. (Kobayashi & Iwabushi, 2008, Matsui etal, 2008) 2.Dust forcing differs in SW (-) from LW (+). This is important for estimating diurnal dust radiative forcing. 3.Even at low loading, dust substantially increases diffuse PAR. Coarse particles cause more scattering and diffuse light. This may significantly modify vegetation behaviors.