黄土粉尘搬运过程的动力学物理模型

A PHYSCIAL MODEL OF LOESS DUST TRANSPORT PROCESS

通过对黄土粉尘重力沉降过程的动力学分析,给出了黄土粉尘粒度分布的数学表达,讨论了粉尘沉积通量随搬运距离、粒径变化的物理过程,首次确定了估算粉尘搬运距离和风力强度的计算方法,为区分粉尘搬运距离和风力强度对粒度的影响及其它们在冰期间冰期中的差异提供了物理学的判别依据。分析结果表明:1)在重力沉降作用下,粉尘沉降通量随搬运距离的变化服从几何分布,具有沉降通量随搬运距离的增加迅速减小,越粗的粉尘颗粒其沉降通量初值越大,同时下降速度也越迅速的特点;2)如果用携粉尘气流的水平通量作代表风力强度,则粉尘搬运距离与粒度分布曲线上重力沉降部分最高点的粉尘沉降通量成反比,风力强度与该点对应粒径的平方和粉尘搬运距离成正比。因此,根据该点的粉尘沉降通量和对应粒径,可以估算粉尘的搬运距离和风力强度。根据上述理论对渭南阳郭中学S_0～L_1黄土-古土壤剖面进行了粉尘搬运距离和风力强度的估算:首先,从粒度分布中提取出3个对数正态分布的独立组份;然后利用粗粒组份的参数计算粉尘搬运距离和风力强度。分析结果表明粉尘搬运距离具有冰期近、间冰期远的特点,风力强度的变化则具有冰期弱、间冰期强的特点,LGM时段的风力强度比MIS 3阶段大,但小于全新世适宜期,而LGM时期粉尘搬运距离并未明显减小,因此,可能黄土粒度的变化并非反映了冬季风的变化,而是反映了夏季风的变化,夏季风是通过影响粉尘源区来影响粉尘粒度的变化。

In this paper, the dynamic mechanism of the dust gravity settling process was analyzed, the mathematic expression of loess grain size distribution was studied, and physical relationships among dust deposition flux, dust-transport distance,wind intensity and dust grain-size was analyzed. A physical model that can estimate the dust transport distance and the wind intensity was firstly developed to distinguish their impacts on the grain size of the transported dusts during the glacial-interglacial transition. The dynamic analyses of dust gravity settling came to two conclusions. One is that the dust deposition flux varies with the dust transport distance, that is, the dust deposition flux decreases rapidly as the dust transport distance increases and the coarser the dust particle is, the larger the initial value of dust deposition flux and dust settling velocity are. Another is that if the wind intensity is expressed by the horizontal flux of the dust-carried air current, the dust transport distance is inversely proportional to the dust deposition flux at the highest point of the gravitational settlement dominated grain-size distribution curve and the wind intensity is proportional to the dust transport distance and the square of the grain size at this highest point. Therefore,the dust transport distance and the wind intensity can be estimated according to such a highest point. The two conclusions were applied in analysis of a So - L~ loess-paleosol section accumulated in the last 60ka. The section is located in Yangge Town of Weinan County. Three independent modes with logarithmic normal distribution were first extracted from the grain size distribution of loess and paleosol samples. Then the dust transport distance and the wind intensity were estimated by using parameters of the coarse mode. The results show that the dust transport distance was shorter during the glacial period and longer during the Early Holocene. And the wind intensity was weaker during the glacial period and stronger in the Early Holocene. The wind intensity was larger in the LGM than in the MIS 3 stage but lower than during the Early Holocene. It is the dust source distance but not the wind intensity that may be the major factor to affect the loess grain size. This implies that the grain size of loess and paleosol may indicate the fluctuation of summer monsoon but not the fluctuation of winter monsoon. As the Eastern Asian summer monsoon concerns, its affecting loess grain size should be completed by impacting the vegetation of the dust source and in turn leading the dust source to advance and retreat.