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Seoul National Univ UAW2008 An assessment of uncertainties in the estimation of dust emission rate due to vegetation 2008. 7. 2 Eunjoo Jung & Soon-Chang Yoon School of Earth and Environmental Sciences Seoul National University
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Seoul National Univ UAW2008 Purpose 1. Purpose - Improvement of dust emission scheme by evaluating uncertainties in dust emission estimation due to vegetation Method 2. Method - Sensitivity tests of dust emission scheme - Numerical simulation of East Asian dust event in Mar. 2004 using regional-scale dust transport model Validation 3. Validation - PM10, LIDAR - WMO SYNOP observations Discussion and conclusion 4. Discussion and conclusion Outline
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Seoul National Univ UAW2008 Model dust fluxes during dust events in 2002 Seasonal vegetation cover in East Asia 15-25 March 2002 4-14 April 2002 (Uno et al., 200, JGR)
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Seoul National Univ UAW2008 Drag partition scheme n : number of roughness elements b : mean breadth of roughness elements h: mean height of roughness elements S: ground area : frontal area index h b where : the ratio of the drag coefficient for an roughness element, C D to that for the ground surface, C S having no roughness element where : total wind shear stress acting on ground area S R : a part of acting on the roughness elements averaged over S S : a part of acting on the bare surface averaged over S (Raupach et al.,1993)
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Seoul National Univ UAW2008 where - (=A b /A f ): the ratio of roughness-element basal area to frontal area -m: the parameter accounting for the spatial inhomogeneity of surface stress =202, =1.45, and m=0.16 m eff = f( ) [Okin, 2008, JGR] Measured wind shear stress ratio (SSR) vs. Frontal Area Index Raupach et al. (1993, JGR)
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Seoul National Univ UAW2008 Dependence of streamwise sand flux on SSR (Lancaster and Baas,1998: 0.38< u * <0.62m/s) Q: streamwise sand flux in [ML -1 T] u *t : threshold friction velocity for a smooth surface u *ts : threshold friction velocity for a rough surface with roughness elements
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Seoul National Univ UAW2008 The effect of vegetation on vertical dust flux Two field measurements 1. Gillette (1977): bare crusted soil in Texas 2. Nickling and Gillies (1993):various vegetation types in Mali, West Africa Dust emission scheme (Shao, 2004) F(d, D, u*)=c y f (d)[(1- )+ p ](1+ m )(g/u * 2 )Q(u *,D, )
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Seoul National Univ UAW2008 Model: regional-scale dust model (Shao et al., 2002) - Horizontal resolution: 50 km x 50 km - Vertical resolution: 25 levels Period - Asian dust event during 7-11 March, 2004 Data - Atmospheric data interpolated from CMA T213 analysis - GIS data with horizontal resolution of 5km x 5km Experimental Design Validation - PM10, LIDAR, WMO SYNOP observations Numerical simulation of a dust event in Mar. 2004 Experimentm=0.16m=m_eff Frontal Area Index data1EXP1AEXP1B Frontal Area Index data2EXP2AEXP2B
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Seoul National Univ UAW2008 T G BTU Hu Ho Spatial distribution of vegetation cover and frontal area indices in East Asia T: the Taklamakan desert G: the Gobi desert BTU: sandy deserts in N China Hu: Hunshandake desert Ho: Horqin desert
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Seoul National Univ UAW2008 NCEP MSLP reanalysis and dust weather codes during the Asian dust event L H L L L L
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Seoul National Univ UAW2008 Simulated vs. measured PM10
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Seoul National Univ UAW2008 Time-height cross section of model PM20 vs. LIDAR in Beijing
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Seoul National Univ UAW2008 Comparison of model dust fluxes Experiment Area-averaged (95-125E,38-48N) Emission rate ( g/m 2 /s) Area-averaged (95-125E,38-48N) Dry deposition rate ( g/m 2 /s) EXP1A 104.6 94.8 EXP1B 76.0 ( -27.4%) 68.8 (-27.4%) EXP2A122.2 (+16.8%)112.2 (+18.4%) EXP2B 97.3 ( +7.0%) 89.4 ( -5.6%)
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Seoul National Univ UAW2008 model dust emission rates vs. friction velocity
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Seoul National Univ UAW2008 Spatial distribution of threshold friction velocity
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Seoul National Univ UAW2008 Conclusion The dust transport model predicted well the dust event during 9-11 March 2004. Comparison of the four simulations shows significant differences in dust fluxes from 7% to 61% each other. It is essential to quantitatively estimate the non- uniformity of wind shear stress for different vegetation types and vegetation covers It is desirable to replace frontal area index with a more directly measurable quantity such as vegetation cover.
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Seoul National Univ UAW2008 Thank you!!
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Seoul National Univ UAW2008 Source of uncertainties due to vegetation in dust emission scheme 1.Estimation of vegetation cover, v c, from MODIS/Terra fPAR (fraction of Photosynthetically Active Radiation) based on Darke et al. (1997) and Wittich & Hansing (1995) 2. Estimation of frontal area index,, as a function of vegetation cover, v c, following (Shao, 2000) = - C ln(1-v c ) where C is dependent on vegetation type (desert, grass, shrubs, crops, trees) 3. Non-uniformity of wind shear stress, m SSR = [(1+m )(1-m )] -1/2
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