北京机场高速公路能见度与大气动力和热力因子的诊断及物理分析

The Diagnose and Physical Analyses about the Relationship between Atmospheric Visibility and the Corresponding Dynamical and Thermodynamical Factors in the Beijing Airport Expressway

根据首都国际机场高速公路专业气象自动监测站网所提供的高时间分辨率资料，结合大尺度背景场对大气能见度演变的动力热力条件和各级能见度条件下对应的主要气象要素特征进行分析。结论如下：1）在低频时间尺度，大气能见度的低频变化有如下的规律：850hPa垂直速度与能见度呈现同位相关系，并通过垂直速度影响水汽变化，进而通过水汽的Rayleigh散射与水粒子的Mie散射来影响大气能见度的变化，反映出大尺度动力因子与水汽共同作用影响了大气能见度变化。500-850hPa之间的位温差与能见度的反位相关系，反映出大气稳定度控制了大气边界层的分子扩散和湍流扩散，即通过热力因子影响大气能见度。但要注意，能见度的变化并不是由某一个单因子所决定的，必须综合考虑动力、热力和水汽等多个因子的相互制约。2）造成2005年1月和2006年1月能见度差异的主要原因是垂直速度和水汽的差异，即垂直下沉运动时，水汽易于燥，比湿较小，能见度较高，故2005年1月能见度比2006年1月高。3）能见度的等级分布与大气湿度、温度以及风速有重要的物理联系，尤其是温度在0℃以下，冰面饱和水汽压低于水面饱和水汽压时，有利于雾的形成，从而使得能见度较低。4）200-1000m的低能见度出现频数的日变化呈现以午时为低中心的U型分布，并且1000m以下的低能见度一定对应了低温、高湿和微风，但这只是必要条件，并不是其发生的充分条件，尤其对于200m以下的浓雾。而对大于4000m的较高能见度，其出现频数的日变化呈以午时为高中心的拱形分布。

Based on the fine resolution temporal observation data, which was observed by the auto monitoring sta tions for road weather information in Beijing airport expressway, combined with the background field in the large scale, the dynamical and thermodynamical conditions of atmospheric visibility＇s evolution and the characteristics of main meteorological factors under different classification of visibility in the Beijing expressway are studied. The main results are as follows： 1） In the low-frequency temporal scale, the law of the atmospheric visibility＇s low frequency oscillation is that the vertical velocity at 850 hPa and the corresponding visibility is positive phase. From the physical view of point, the mechanism is that the vertical velocity influences the vapor changing and the visibility is reduced by the effects of the water vapor＇s Rayleigh scattering and the water particle＇s Mie scattering. This reflects the effects of the large-scale dynamical factor and the vapor factor on the visibility. The potential temperature difference from 500 hPa to 850 hPa and the viability is negative phase. It is suggested that the atmospheric stability controls the molecular diffusion and turbulent diffusion of the boundary layer. Because of this reason, the corresponding visi bility is affected by the thermodynamical factor, but one thing should be noticed is that elements, which result in the variation of visibility, are extensive, the interactional physics process, including dynamical or thermodynamical and vapor must be taken into account, and single-factor analysis is not available. 2） The reason that results in difference between the visibility in January 2005 and that in January 2006 is the difference of the vertical velocity and the vapor between two months, that is to say, when there is sinking vertical motion, the vapor tends to get dryer and the humidity becomes less, hence the visibility turns better. So the visibility in January 2006 is finer than that in January 2005. 3） The classification of the visibility has important physical relationship with the humidity, temperature and the wind speed, especially when the temperature is below than 0℃, the saturation ice surface water pressure is less than the corresponding water, so that the cooling fog is easily formed, and the corresponding visibility becomes bad. 4） The daily frequency variation of visibility between 200 m and 1 000 m presents U type distribution, and the visibility below 1 000 m appears on the conditions that temperature is very low, humidity is very high and the wind velocity is very small, but it is only the essential condition not the full condition, especially for the heavy fog which visibility is below 200 m. And for the better visibility which is higher than 4 000 m, its daily variation of frequency presents arch form distribution.