2. Computation of Dust Content
Based on the atmospheric radiation transfer theory, in conjunction with NOAA/AVHRR remote sensing data, the reflectance of a dust storm over the inland arid region of the northwestern China is calculated and the optical depth of dust aerosol is derived, thus obtaining the dust content of the dust storm.

The reflectance of radiation received in the satellite instantaneous view field can be written as[1]:


Where R (m_s, m_v, f_v) is the reflectance at the satellite level; m_s, m_v express respectively solar and satellite zenith angle; f_v is satellite azimuth; (U₀₃) and (UH₂₀) are diffusion transmissivities of O₃ and Vapor respectively; R_a.m and R_a,a are atmospheric molecular and aerosol reflectance respectively; R_g and R_e are the effect of remote sensing environment on satellite view field and surface reflectance respectively; A_s is atmospheric spherical albedo; T(m_s) and T(m_v) are the total transmissivities of dust aerosol along the sun-surface-satellite paths respectively. The effects of O₃ and H₂O absorption in NOAA-AVHRR Ch1 (0.58-0.68 mm) as well as atmospheric molecular reflectance need to be determined.

The content of large particles (r>1mm, r is the radius of a dust particle) increases rapidly with the increasing intensity of fly dust and dust storm. The scattering and extinction characteristics of large particles, especially the particles with a radius of > 10 mm, play the major role. Given that dust aerosol layer is a plane parallel layer, the scattered light is natural unpolarized light and dust particles have the spherical shape, the phase function is expressed as: [2]


Where g₁ and g₂ are the asymmetry factors calculated from Mie scattering theory (r=10 mm) , indicate forward and backward scattering, b is weighted coefficient. Based on Mie scattering theory, the brightness temperatures of AVHRR Ch3 and Ch4 and the absorbing coefficient of particle when r=10 mm, the multiple scattering of incident photon in the dust layer is compared so as to calculate pixel by pixel the reflectance and absorptance of dust aerosol by using Monte-Carlo method. Use the corresponding surface image under clear sky during the time period of dust storm frequently occurring and carry out atmospheric correction and simplification of (1). From (1) and dust reflectance, as well as the approximate equations of atmospheric transmissivity and reflectance coefficients:[3]


Then t, the optical depth of a dust layer, can be obtained. In (3) and (4), R (t, m) is the boundary condition parameter are the first order scattering transmissivity and reflectance of spherical particles, xi is the parameter concerning the characteristics of particles. The resulting optical thickness of the dust storm in Kashi, Xinjiang Aut. Reg. In April 24, 1999 is about 2.0-5.0 (Fig 3).


Figure 3: The optical thickness over the southern Xinjiang Aut. Reg. in April 24. 1999

Figure 4: The dust mass loading over the southern Xinjiang Aut. Reg. in April 24, 1999
Using Junge's spectrum distribution model n (r)= Cr^(v+1), is the total mass of particles in the vertical column is [4].


According to (5) , the total mass of dust particles can be obtained pixel by pixel. The resulting dust content is 20-90 t/km2.

3. The Operational System of Disaster Assessment
The disaster assessment system is mainly based on a database containing such comprehensive factors as the on-the-spot investigation of farmland, grassland and forests, the distributions of population and urban areas, environmental factors, etc., in conjunction with certain common geographical information system (regionalization, rivers , railway, highway, etc.,). Using GIS software to process above mentioned data, the system can produce a detailed disaster situation assessment report according to the data on the extent of the areas suffered from dust storms provided by a monitoring system.

References

• Tanre D et. al. Atmospheric correction algorithm for NOAA-AVHRR products: theory and application IEEE Tran Geosci Remote Sensing, 1992, 30(2).
• Goody.R.M. et. al., Atmospheric Radiation Theoretical Basis. New York Oxford University Press, 1989.
• Sobolev V V et. al. A Treatise on Radiative transfer. D. VAN Nostrand Company, ING. 1963.
• Cachorro V E et. al. The correlation between particle mass loading and extinction application to desert dust aerosol content estimation. Remote Sensing environment, 1997, 60(2).

Fig. 5 The overall design for the meteorological satellite system of dust storms monitoring and the operational system of disaster assessment.

Fig.6 Short range duststorm forecasting system working flow chart and offering function.