Benoit Laurent, Béatrice Marticorena, Gilles Bergametti

1. Frequencies and intensities of mineral dust emissions from Chinese and Mongolian deserts: A modeling approach  Benoit Laurent, Béatrice Marticorena, Gilles Bergametti blaurent@lisa.univ-paris12.fr Laboratoire Inter-universitaire des Systèmes Atmosphériques International Symposium on Sand and Dust Storm, Beijing, 12-14 Sept. 2004

2. Studied area: the main deserts of eastern Asia Studied area (35.5°N-47°N; 73°E-125°E)

3. Surface wind Vertical flux F Saltation Sandblasting Horizontal flux G Dust emission processes Dust emissions are sporadic and spatially heterogeneous soil particle movement: wind friction velocity (U*) > threshold friction velocity (U*t) Main processes of dust production

4. Dust emission processes Emission processes Model outputs Erosion threshold Emission frequencies (location and periods) Saltation Emission flux intensities (quantities) Sand-blasting

5. 1- Dust emission frequencies • - Erosion threshold • Aerodynamic roughness length (Z0) • Particle diameter (Dp) • Soil moisture • Snow cover • Surface wind velocity

6. Erosion threshold parameterization U*t = f (Dp;Z0) [Marticorena et al., J.G.R., 1997] For 50 µm < Dp <200 µm (generally always present in arid soils),Z0 is the key parameter to compute dust emission frequencies → Determination of Z0

7. What are the required input data ? To compute the erosion threshold: - aerodynamic roughness length (Z0) remote sensing 10 m erosion threshold wind velocity ● Dp = 210 µm σ = 1.8 - size-distribution - soil texture: f (depth) FAO ● Soil moisture - precipit., T°, albedo, geopot. ECMWF To compute the emission frequencies we also need: - snow depth ● Snow cover ECMWF ● - surface wind velocity ECMWF 10 m wind velocity

8. 12 2 13 11 10 5 8 3 7 1 4 6 9 Z0retrieved from theProtrusion Coefficient (PC) Remote sensing: POLDER-1 (POLarization and Directionality of the Earth’s Reflectance) Protrusion Coefficient (PC) derived from POLDER-1 measurements of bidirectional reflectance Empirical relation with a = 4.859.10-3 cm, and b = 0.052 is dimensionless Z0= a.exp (PC / b) [Marticorena et al., I.J.R.S.,2004] log10(Z0)

9. Z0 and 10 m erosion threshold wind velocities Z0 map (¼° × ¼°) U*t = f (Dp;Z0) and the neutral vertical wind velocity profile 10 m erosion threshold wind velocity map (¼° × ¼°) m.s-1 Ut(10m) [Laurent et al., J.G.R.,submitted]

10. Z0 and 10 m erosion threshold wind velocities In the Gobi: ● Our results: median ~15 m.s-1 ● Wind velocities associated with dust storms: 11-20 m.s-1[Natsagdorj et al., Atmos. Env.,2003] ●Wind tunnel and field studies: 10-12 m.s-1[Murayama, Met. Satell. Cent. Tech. Note,1988; Hu and Qu, Chin. Meteo. Press,1997] In the Taklimakan: ● Our results: median ~7 m.s-1 ●Wind velocities associated with dust storms: 6-8 m.s-1[Wang et al.,Water, Air, and Soil Poll.,2003] m.s-1 Ut(10m) [Laurent et al., J.G.R.,submitted]

11. % Latitude Longitude Simulation of dust emission frequencies (1997-1999) Frequent dust emission areas Dust storm events during 1960-1999 Dust storm occurrences during 1961-2000 [Sun J. et al., J.G.R., 2001] [Sun L. et al., Water, Air, and Soil Poll.,2003]

12. Simulation of dust emission frequencies (1997-1999) Seasonal cycle • Frequencies computed with soil moisture and snow cover [Laurent et al., J.G.R.,submitted]

13. Simulation of dust emission frequencies (1997-1999) Seasonal cycle • Frequencies computed with soil moisture and snow cover • Frequencies computed with soil moisture and without snow cover • Frequencies computed without soil moisture and snow cover [Laurent et al., J.G.R.,submitted]

14. In the Taklimakan: r ~ 0.95 slope ~ 0.44 Monthly average dust event frequency Monthly average frequency of TOMS AAI > 0.7 [Laurent et al., J.G.R.,submitted] Simulation of dust emission frequencies (1997-1999) Comparison with TOMS Absorbing Aerosol Index frequencies Location of the most frequent areas Latitude Longitude Frequencies of significant simulated dust emissions (flux > 10-10 g.cm-2.s-1) Latitude Longitude Frequencies of AAI TOMS > 0.7

15. 2- Dust emission fluxes - Flux parameterization Soil “dry” granulometry % erodible surface

16. Flux parameterization Parameterization of the saltation flux F= a.G = a.S Srel(Dp).C.U*2(1+U*/U*t (Dp,Z0))(1-U*²/U*t²(Dp,Z0)) [Marticorena and Bergametti, J.G.R.,1995] Parameterization of the sandblasting efficiency a = f(% clay) [Marticorena and Bergametti, J.G.R.,1995] → Determination of the soil “dry” granulometry

17. What are the required input data ? - aerodynamic roughness length (Z0) remote sensing 10 m erosion threshold wind velocity ● in-situ measurements - size-distribution - soil texture: f (depth) FAO ● Soil moisture - precipit., T°, albedo, geopot. ECMWF - snow depth ECMWF ● Snow cover ● - surface wind velocity ECMWF 10 m wind velocity To compute the emission fluxes we also need: in-situ measurements Soil “dry” granulometry - size-distribution ● % erodible surface - % no cover surface f(Z0) ●

18. Soil “dry” granulometry derived from in-situ measurements Derived from measurements of Gengsheng et al. [Global Alarm: Dust and Sandstorms from the World’s Drylands, report of United Nations, 2001]

19. 106 T Latitude Longitude Simulation of dust emission fluxes (1997-1999) Mean annual quantity

20. Simulation of dust emission fluxes (1997-1999) Seasonal cycle In frequency: end of spring In intensity: beginning of spring in 1998

21. Conclusion Simulations of dust emissions (1997-1999) ● Two main dust emission areas (both in frequency and intensity): the Taklimakan desert in the north western China and the Badain Jaran desert in the northern China ● A pronounced seasonal cycle of dust emissions with a maximum (both in frequency and intensity) in spring ● A weak influence of the soil moisture and the snow cover on simulations of dust emissions ● ~ 400 MT/year dust emitted from eastern Asian deserts

22. Frequencies and intensities of mineral dust emissions from Chinese and Mongolian deserts: A modeling approach  Benoit Laurent, Béatrice Marticorena, Gilles Bergametti blaurent@lisa.univ-paris12.fr Laboratoire Inter-universitaire des Systèmes Atmosphériques International Symposium on Sand and Dust Storm, Beijing, 12-14 Sept. 2004