-
每种沙尘天气同时都含有多种尺度的粒子,沙尘暴对电磁辐射的作用影响与沙粒尺度分布密切相关。浮尘和扬沙天气的沙尘粒子尺寸较小,沙尘暴的沙粒尺寸较大,其粒径一般在0.01~300 μm范围内,研究表明:可采用对数正态分布函数来描述沙尘粒子的尺度分布,其表达式为[12]:
式中:N0为粒子数密度;
$p(r) = N(r)/{N_0}$ 为粒子尺度分布密度函数;$r$ 为沙粒半径。式中:m0和σ分别为ln(2r)的均值和标准偏差。沙源地不同,测量设备规格不同,测得沙尘暴粒子的尺寸分布也不同,文中选取董庆生和牛生杰分别在塔克拉玛干沙漠、贺兰山沙区等地测量的七种沙粒尺寸分布,其相应参数的统计结果如表1所示[13-14]。
图3是根据公式(5)和表1参数绘出的七种沙尘粒子的尺寸分布函数,图中横坐标D为沙粒直径。可以看出,1~6模态的沙粒尺寸比7模态的沙粒尺寸大许多,其中1黄土沙、2毛乌素两种模态的沙粒尺寸主要分布在5~150 μm范围之间,3腾格里、4塔克拉玛干和5甘肃三种模态的沙粒尺寸分布比较相近,粒径主要分布在30~210 μm范围之间,6海岸模态沙粒尺寸相对最大,粒径主要分布在80~250 μm范围之间;7贺兰山区模态沙粒尺寸较小,粒径主要分布在0.01~6 μm范围之间.
Number Sand source m0 σ 1 Loess –3.08 0.491 2 Maowusu –2.96 0.380 3 Tengger –2.31 0.296 4 Taklimakan –2.26 0.276 5 Gansu –2.19 0.279 6 Coast –1.83 0.166 7 Helan mountain –7.48 0.995 Table 1. Sand size distribution parameters
沙尘暴是具有多种尺寸分布的多相粒子群,总的消光是所有尺寸粒子消光共同贡献的结果,忽略多次散射对消光的影响,总的消光系数为:
式中:粒子数密度N0是一个很难测量的物理量,国内外学者通常借助于光学能见度Vb来表示,二者的关系为[15]:
除消光系数外,单次散射反照率ω和不对成因子g也是描述粒子散射和消光特性的重要物理量,对于多相粒子群,ω和g的计算式分别为:
根据以上各式,在能见度Vb=1 km时,计算得7种模态沙尘暴在0.86~20 μm红外波段的3个消光参量随波长的变化情况如图4和图5所示。
根据图4所示的6种大颗粒沙尘暴的消光特性,可以把6种模态的沙尘暴分成3组,第一组1和2模态由于粒径分布接近,3个消光参量随红外波长的变化基本一致,第二组是3、4和5模态,第三组是6模态。图4(a)表明第一组的两种沙尘暴其反照率在0.86~5 μm波段大于另两组的值,在长波红外波段,反照率又小于另两组的值,但在整个0.86~20 μm波段,反照率ω的值都在0.5~0.6之间,说明散射略强于吸收。图4(b)表明第一组的不对称因子小于另两组,但在整个0.86~20 μm波段内,反照率g的值都大于0.9,表明沙尘暴的前向散射较强。图4(c)表明第一组的消光系数明显大于另外两组的值,第三组沙尘暴的颗粒尺寸相对最大,消光系数相对最小,主要因为沙粒尺寸与该红外波段的波长差异最大的缘故。总之,在0.86~20 μm红外波段内,可见度相同时,这6种大颗粒模态沙尘暴的消光系数除在个别波长有小的震荡外,大体是随红外波长的增大而增大。
图5是小颗粒模态7沙尘暴消光参量随红外波长的变化情况。可以看出:在红外波段波长小于5 μm时,其反照率和不对称因子的值都比较大,反映沙尘暴在近、中波红外前向散射比较强;随红外波长增大,反照率震荡减小,不对称因子递减,说明该沙尘暴散射随波长增大而逐渐减弱且趋于各向同性;在红外波长小于2 μm时,消光系数明显大于其他波段的值,即该模态沙尘暴在近红外的消光能力较强,在中、长红外波段的消光能力较弱,且在13~20 μm波段范围内,消光对波长不敏感。
Analysis of extinction characteristics of sandstorm to infrared radiation(Invited)
doi: 10.3788/IRLA20201022
- Received Date: 2020-03-05
- Rev Recd Date: 2020-04-10
- Available Online: 2020-07-23
- Publish Date: 2020-07-23
-
Key words:
- infrared radiation /
- sandstorm /
- extinction coefficient /
- Monte Carlo /
- attenuation rate
Abstract: The extinction and attenuation characteristics of seven modes of sandstorm in China for 0.86 -20 μm band infrared radiation were analyzed based on Mie scattering theory and Monte Carlo method. The results show that for the small size dust particles, the extinction effect is mainly the result of scattering in the near and middle infrared, and for the large size dust particles, the infrared extinction is the result of absorption and scattering. The attenuation rate difference of single scattering and multiple scattering of sandstorm are compared. The attenuation rate of multiple scattering is less than that of single scattering under the same condition, and the difference decreases with the increase of visibility. The multiple scattering attenuation rate calculated based on Monte Carlo method is more comprehensive than the extinction coefficient to reflect the influence of sandstorm on infrared radiation intensity attenuation. The infrared attenuation intensity of six kinds of large particle mode sandstorm is greater than that of small particle mode sandstorm, and the attenuation rate increases with the increase of wavelength. The attenuation rate of the small particle mode duststorm fluctuates obviously with the change of wavelength, with a peak value in the range of 7.9–12.5 μm, and is not sensitive to the wavelength in the range of 13–20 μm.