沙尘暴垂直输运的两相流理论Ⅰ:气块模式

The two-phase flow theory about dust storm's vertical transport,partⅠ

沙尘暴是风蚀荒漠化中的一种天气现象,其形成受到两方面因素的共同影响:(1)自然因素,包括大风、降水减少及沙源;(2)人类活动因素,人类活动因素是指人类在发展经济过程中对植被的破坏以后,导致沙尘暴爆发频率数增加。沙尘暴已经成为中国西北、华北地区严重的气候灾害,而对沙尘的研究主要集中在起沙问题与沙尘沉降的问题,在理论上对前者研究较多,而对后者的研究则比较缺乏。假设空气块中带有沙尘,沙尘颗粒很小,随空气一起运动,可不计沙尘Stockes末速度,这样由空气和沙尘组成两相流。在两相流情况下,分析了均匀环境和非均匀环境条件下沙尘沉降的条件和速率。通过分析得到在均匀背景场下有利于沙尘沉降的条件为大的初始沙尘浓度扰动与小的初始温度扰动。进一步考虑更为真实的环境场,模式给出3种特征沉降区域:(1)速降区,沉降速率较快;(2)缓降区,沙尘先上升后下降,沉降时间较长;(3)波动区,沙尘在空气中不断地上下波动,此时很难判断沙尘何时沉降,取决于地面状况。同时也分析了沙尘不断上扬的背景条件。

Dust storm is a weather phenomenon over the desertification area. Nature factors tie together with human activities that have been put forward to account for the formation of dust storm. Nature factors include high wind, decreasing precipitation and enough sand sources. Human activities result in increasing of dust storm by decreasing vegetation covers in semiarid and arid regions during the economical development. Dust storm has been a serious climate disaster for northwest and north China, and the studies about dust storm focus on the blown sand-dust and sinking of sand-dust. Many studies prefer the former in theory, so the work on the sinking is necessary. Tiny dust particles whose Stockes terminal velocity can be overlooked will move with the air, and the air and the dust form a two-phase flow. We discussed the dust＇s sedimentation condition and velocity under mean and more real environment by two-phase flow. Under the mean environment the advantageous initial conditions for sinking are a strong disturbance of initial dust concentration and a weak disturbance of initial air parcel＇s temperature, further work is on the more real environment, and the model gives three characteristic sinking areas： （1） fast sinking area, where the sand-dust sinks fast; （2） slowly sinking area, where the sand-dust will rise first, then it sinks to the ground and the time for the total sinking is long; （3） wave sinking area, where the sanddust will wave in the environment and it is hard to determine when sand-dust will sink to the ground, so the time for sinking is determined by the condition of the ground. The conditions for the sand-dust＇s continuous rise are also discussed finally.