Study On Dust Emissions And Pattens Of Sand-Dust Weather

Sand-dust weather has important influence on biological growth, ocean thermohaline circulation, the environment, human health, agriculture, industry, transportation, fire and political and economic to some extent as well as climate, atmospheric dynamical field and thermal field, radiation, cloud microphysics and carbon circle. It relates to blown sand physics, meteorology, electricity, pathology, marine science, biology, agriculture and animal husbandry forestry, transportation, science and political economics and other disciplines. So the study on dust emissions and characteristics and patterns of sand-dust weather has important values.After a serial of study related to air temperature and sand-dust weather and dust PM10concentrations, relations of air temperature, atmospheric pressure and the concentrations in dust weathers, state equation of dust atmosphere, the sequence of entrainment of blown sand particles, the pattern of the upward motions of sand-dust particles, patterns of dune, dust emissions by saltation bombardment and force associated with air, sand-dust model’s development and its advancement, sand-dust forecast imposed by sand-dust concentration assimilation, the mechanism of sand-dust storm, the deduce of closed physical constraint equations of sand-dust atmosphere based on both interaction of wind-blown-sand two-phase flows and mass change, the theoretical analysis of the equations and the numerical model for the two-phase flow and its associated ideal experiment, the following results are obtained:The relationship between temperature and dust weather indicates that there are generally three types between them, namely, cold type （sand-dust weather occurring with cold air）, warm type （sand-dust weather occurring with warm air） and mixed type （with cold air and warm air, together）. Among them, the cold and warm types are more common, and the mixed rare.Some empirical relationships between temperature and dust aerosols are obtained by using automated observational records of dust PM10concentrations and temperatures, which are taken every five minutes at the Minqin observational station during the time period, January2004through June2006. Results of the study disclose that:（1） empirical relationships are obtained by finding statistical regressions of temperature and dust PM10concentrations, taken at either 5-minute intervals,24-hour intervals and daily intervals of daily-average temperatures and dust PM10concentrations;（2） higher temperatures and larger fluctuations of temperatures are likely to correspond to higher dust PM10concentrations, which agrees well with the knowledge that high temperatures are helpful to the appearance of dust events;（3）5-minute intervals of temperatures and dust PM10concentrations are also linearly and positively correlated as well as their24-hour intervals;（4） the characteristic24-hour intervals for floating dust, blowing dust and dust storm are-2.1～4.4℃,-2.4～4.6℃and-2.7～5.0℃, respectively;（5） the relationship between daily-average temperature and dust PM10concentrations is not significant for low dust PM10concentrations, but for high dust PM10concentrations corresponding to dust storms.Air temperature, air pressure and concentration in the start, the strongest and the end of sand-dust weather show a very significant correlation in their own, suggesting that there is a certain structure in the dust cloud or its evolution.Different statistical relationships are found in density, temperature and pressure for dust weather and nondust weather, which maybe advance the entrainment on the isothermal surface and isobaric surface, and make the dust cloud has different actions on different isothermal surfaces such as rising with contraction or sinking with expansion, divergence or convergence along vertical direction, and also has different actions on different isobaric surface such as vertical development of dust cloud, or sink, divergence or convergence in the middle of the cloud.The state equation of dust atmosphere is found in statistical view based on air temperature, pressure and dust PM10concentration.Initial entrainment of sand depends on its size, the slope of the sands plane, the magnitude and the direction of the drag force, loose degree between sands and rough degree of sands plane. Given a fixed drag force and a fixed particle size, the sequence of locations of most likely entrainment at the outset is:point of inflection on a sands ridge, bump on the slope of sands plane and peak on the sands plane, points on the slope, points on horizontal sands plane and trough points.Bring forward smoothening movement, phase-retreating movement and phase-holding movement of the sand dune.The favorable conditions for sand-dust particles’emissions and uplifted motions are:updraft of air, big wind, particles’swirling with same direction’s sign to horizontal wind, vortical field with direction’s sign opposite to the horizontal wind, atmosphere static instability and strong shear of horizontal wind.Bring forward a transition model to predict dust emission by saltation bombardment. The transition model of dust emission is an analogy of the electron transition, which results from a detailed analysis of the stress transferring process in sand bed. In this transition model, a particle in the bed will be released if its energy, transferred to it from the impacting particle, is bigger than the transition energy which is the least energy to make a particle have a "transition", and the remained energy of the particle with a transition is treated as its initial kinetic energy. Thus the expressions of the number of ejected particles, their velocity distributions and the shapes of bombardment crater in two-dimension and three-dimension situations are then easily obtained based on the transition model. And the dust emission by saltation bombardment then becomes a problem to research into the transition energy and the energy distribution around the point where collision happens.Proposed sand source in the presence of surface and atmospheric gas exchange is the internal mechanism of pneumatic sand.A dust assimilation system named GRAPES₃DVAR_DUST was developed through adding a new control variable （namely, dust concentration） in the three-dimensional variational data assimilation system of GRAPES model in order to improve its forecast accuracy.Through the use of the dust model, namely, GRAPES_SDM, a dust-storm weather happening in summer, Jun17,2005, was modeled in order to study the structure of dust storm and its associated mechanism. The results show that there is a coupled system similar to Benard cell existing in the dust storm and the system caused this dust storm. So a theory of votex cell is proposed to explain the formation of the dust storm, which is the dust storm is a result from the the system’s sweeping sand-dust source.A whole mechanism of dust emissions for dust storm is proposed after the above studies. The mechanism is:when the vortex cell passes by the sand-dust source, the sand-dust particles will be emitted through the gas exchange between the land and atmosphere on the area where the wind speed is bigger than the threshold wind speed; the sand-dust particles then are litf up by the corherent structure of turbulence such as gust and plump within atmospheric surface layer; once they step into Ekman boundary layer, because the air current turns to the the lower-pressuer center due to the turbulent friction and then the secondary circulation will be produced, the particles will still be lifted up through the lower-pressure system and its associated secondary circulation （both are coupled into a vortex-cell system）, and even be drived into the free atmosphere through Ekman pumping; at last the particles will move with the vortex-cell system and the sand-dust storm weather is formed.The closed physical constraint equations of sand-dust atmosphere were established based on both interaction of wind-blown-sand two-phase flows and mass change. Ideal analysis of the equations shows that:（1） the density of the sand-dust atmosphere is bigger than that of the pure atmosphere, which will reduce the velocities of air parcels of the former to some extent;（2） velocity difference between sand-dust particles and air can make the fine particles be located in high-speed region and the coarse particles be located in low-speed region;（3） temperature difference between sand-dust particles and air can make the particles tend to enhance convection through acting as heat source in updraft and cold source in downdraft;（4） the constant-volume mass-specific heat capacity of the sand-dust atmosphere can give rise to a generation of new temperature gradient on an isobaric surface and then enhance entrainment at the boundary of a sand-dust cloud;（5） the gas constant of the sand-dust atmosphere can lead to a generation of new pressure gradient on a isothermal surface and also enhance entrainment at the boundary of a sand-dust cloud;（6） mass change can affect largely density, velocity and temperature of the sand-dust atmosphere. In brief, compared with the sand-dust cloud given by equations based on passive scalar, the real sand-dust cloud is higher and greater. Besides this, its inner convection and its entrainment at the boundary are more active, and its horizontal motion is slower.Establish the numerical model for wind-sand two-phase flow based on the above physical constraint equations, and the ideal experiment is performed. The results of Ideal experiment show that compared with the results from passive-scalar model, the horizontal velocity of two-phase-flow model is bigger within the vortex center and smaller outer the center; the potential temperature and vertical velocity increase under the height2km above the surface, decrease within the heights2-6km above the surface and then increase again above6km.