北京高速公路大气能见度演变特征及其物理分析

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, 对大气能见度演变特征以及相应的物理因子进行分析。结论如下: (1) 大气能见度具有明显的日变化和逐月变化特征。就日变化而言, 能见度以14时最大, 但最低值出现时段却不一定。对于季节变化, 全年以夏季大气能见度状况最好。 (2) 无论是季节变化还是日变化, 高速公路上的大气能见度与气象要素之间都是复杂的非线性关系, 而不是简单的线性关系。通过对21个月中能见度最低月资料的分析表明, 大气能见度与湿度间呈明显的乘幂分布关系, 其与气温呈U型相关。但在月平均能见度最大月, 能见度则与湿度呈指数关系。 (3) 从物理上来看, 大气能见度与湿度的关系, 主要是通过水汽分子的Rayleigh散射和雾的Mie散射两个方面来表现; 风速则是由于压力阻力卷起大气气溶胶来影响大气能见度的; 而0℃附近温度影响大气能见度则主要是通过Bergeron三相过程。 (4) 200 m以下的低能见度基本上是湿度在100%的情况下发生的, 即都是大雾天气影响的直接结果, 但对200 m以上的低能见度则不同, 200～1000 m其间有一半是雾, 而1～4 km的能见度中不到三分之一是雾, 主要是由灰霾、 沙尘暴等天气现象造成。

Evolution Characteristics of Atmospheric Visibility in the Beijing Expressway and the Corresponding Physical Analysis

Based on the fine-resolution temporal observation data provided by the auto monitoring stations for road weather information in Beijing airport expressway, the evolution characteristics of atmospheric visibility in the Beijing expressway and the corresponding physical factors are studied. The main conclusions are as follows: (1) It is demonstrated that the visibility in the Beijing expressway has distinct daily and monthly variation features, that is to say, the visibility at 1400 LST is the highest in a day, and the time with the lowest visibility is uncertain in any day. For the seasonal variation, the summer visibility is the highest in a year. (2) Of monthly and daily variation, the relationship between the visibility and the meteorological factors is the complex nonlinear correlation, rather than the simple linear correlation. Through analyzing the lowest monthly visibility with 21-month data, it is shown that the relationship between the visibility and the humidity is in the distribution of power, and is U type between the visibility and the temperature. However, in the highest monthly visibility, the relationship between the visibility and the humidity becomes exponential. (3) From the view of physics, the visibility reduction is affected by humidity through the water vapors Rayleigh scattering and the fog's Mie scattering, is affected by the aerosol through the pressure resistance from the wind, and is affected by the water phase change through the Bergeron three-phase processes when the temperature is around 0℃. (4) It is shown that the visibility below 200 m happens mostly in the conditions that relative humidity of atmosphere is close to 100% (fog weather phenomenon), while the visibility above the 200 m is different. It is found that about 50% of visibility between 200 m and 1000 m happens in the case of fog, and about 30% of 1-4 km visibility is caused by fog. Thus, 1-4 km visibility mainly results from haze, sand storm and other weather phenomena.