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Radiative impact of mineral dust on surface energy balance and PAR, implication for land-vegetation-atmosphere interactions

Xin XiAdvisor: Irina N. Sokolik

School of Earth and Atmospheric SciencesCollege of Science

Georgia Institute of Technology6th Graduate Student Symposium

Nov.14, 2008

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Motivation1. 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, 0.4µm ~ 0.7µm), 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.

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Approach

1. Optical modelingMie-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 forcing1-D radiative transfer model: SBDART (Ricchiazzi et al 1998, BAMS)

Net radiative flux:

Surface net radiation (0.2µm~100µm):

Dust surface radiative forcing: aerosol clear sky

net netF F

down upnetF F F

41n sfc sfcR S L T T

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Size distribution

Dust loading Vertical profile

Surface cover

Reid et al 2008 high mixed dryland

Lafon et al 2006 moderate multilayer rangeland

Clarke et al 2004

low lifted grassland

(km)

5

4

3

2

1

Mixed Multilayer Lifted

Fine mode Coarse mode

Reid etal 2008

- 0.45µm::1.93

90.9%

0.84µm::1.78

9.1%

Lafon etal 2006

- 0.4µm::2.0

91.1%

1.05µm::2.15

8.9%

Clarke etal 2004

0.35µm::1.46

55.6%

0.89µm::1.85

44%

4.3µm:1.5

0.4%

Factors to be considered:

AOD0.5µm Reid Lafon Clarke

High 2.06 2.0 1.92

Moderate 1.34 1.3 1.25

Low 0.41 0.4 0.280.5 1 1.5 2 2.5 3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

wavelength (um)

refle

ctan

ce

drylandrangelandgrassland

0.3 .5 1 0.026

0.028

0.03

0.032

wavelength (micron)

norm

aliz

ed e

xtin

ctio

n co

effic

ient

0.3 0.5 10.76

0.8

0.84

0.88

0.9

sing

le s

catt

erin

g al

bedo

.3 .5 1 0.8

0.81

0.82

0.83

0.84

asym

met

ry p

aram

eter

Reid

LafonClarke

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-300 -200 -100 0 100 200 300

1

2

3

4

5

high

moderate

low

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 degreesurface: grasslanddust size: Lafon etal 2006vertical 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 (AOD0.5µm=0.4).

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-150 -100 -50 0 50 100 150 200 250 300

1

2

3

4

5

Reid etal 2008

Lafon etal 2006

Clarke etal 2004SW+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: grasslanddust loading: moderate (AOD0.5µm=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

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-220 -120 -20 80 180 280

1

2

3

4

5

lifted

multilayer

mixedSW+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: grasslandsize: Lafon et al 2006dust loading: high (AOD0.5µm=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).

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-160 -110 -60 -10 40 90 140 190 240

1

2

3

4

5

grassland

rangeland

drylandSW+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 2006dust loading: moderatevertical profile: mixed

different surface emissivities

0

100

200

300

400

500

600

700

800

1 2 3 4

grassland

rangeland

dryland

SW+LW SW PAR diffuse PAR

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Implication for land-vegetation-atmosphere interactions

aerosol effects on vegetation vegetation feedback

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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.