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降水现象对大气消光系数和能见度的影响
Influences of Precipitation on Atmospheric Extinction Coefficient and Visibility
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摘要: 大气中各种粒子对大气消光系数和能见度有不同程度影响，除气溶胶粒子外，降水粒子对能见度影响也不可忽视。为了解降水粒子对能见度的影响，确定能见度变化与降水现象之间的关系，该文在分析降水粒子的大小、速度、形状、谱分布、光学特性等特征的基础上，忽略气溶胶粒子的影响，建立基于实测谱分布的降水与能见度的理论模型，讨论不同类型降雨、降雪对大气消光系数和能见度的影响。同时选取Parsivel降水粒子谱仪在南京地区的降雨和降雪观测记录，利用实测数据来对比验证本文所建立的降水能见度理论模型。结果表明：能见度随着降水强度的增大呈指数降低；受降水粒子特性和天气条件等多种因素影响，能见度与降水强度之间的关系并不是唯一对应的；降雨和降雪对能见度的影响各不相同，相比而言，降雨对能见度的影响比较容易确定，而降雪对能见度的影响比较复杂，主要因为雪花或冰晶的类型复杂多变，对大气消光系数有不同程度的影响。结合理论分析和实测数据对比验证，降水现象对能见度的影响得到了证实。Abstract: The variation of visibility is mainly influenced by the scattering and absorption of atmospheric particles at visible wavelengths. The principle of visibility sensors are based on the relationship between visibility and atmospheric extinction coefficient, which is integrated by scattering coefficient of atmospheric particles within a volume. The influence of aerosol, dust particles and smoke are usually taken into account in the traditional visibility analysis. However, as one important factor affecting visibility, precipitation particles are usually neglected, causing obvious inaccuracy when precipitation occurs. Physical characteristics (size, velocity, density, shape, size distribution) of precipitation particles are analyzed and then their influences on atmospheric extinction coefficient and visibility are discussed based on scattering characteristic of raindrops and snowflakes. The relationships of rainfall and snowfall with visibility are obtained by numerical simulation based on fitting size distribution parameters, which are validated by observation data of precipitation and visibility obtained by Parsivel precipitation particle spectrometer in Nanjing. The comparisons show that the numerical simulations are in good agreement with the observation data. The conclusions are as follows: The visibility decreases exponentially as the rainfall/snowfall intensity increases; however, the influence of rainfall and snowfall on visibility is different, because the atmospheric extinction is mainly affected by the types, density, velocity, sizes and distribution of the precipitation particles; with the same precipitation intensity, smaller particles leads to lower visibility; the theoretical relationship of precipitation intensity to visibility takes on a good accordance with the observation data while considering the size distribution and ice crystal types; and the influences of snowfall on visibility are more complex because the atmospheric extinction snowfall are related to the snow/crystal types. Finally the influences of precipitation on visibility are confirmed by the combination of observation data and theoretical analysis. The influences of precipitation particles on atmospheric extinction coefficient and visibility are discussed on the assumption that the aerosols are totally cleared by rain or snow in this model, which results in the overestimation of visibility. To obtain more accurate relationship of visibility and precipitation, the influence of aerosols and its variation with time should be considered, which deserves further investigation.
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Key words:
- rainfall;
- snowfall;
- extinction coefficient;
- visibility
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图 1 3次降雨实际谱分布和Gamma拟合谱分布的对比情况
Fig. 1 Comparisons of actual raindrop size distribution and fitting size distribution
图 2 降雨-能见度理论关系与实测数据对比
Fig. 2 Comparisons of theory relationships and rain observations by Parsivel
图 3 降雨样本a降雨强度和能见度随时间的变化情况
Fig. 3 Variation of rainrate and visibility during rainfall-a
图 4 3次降雪实际谱分布和Gamma拟合谱分布的对比情况
Fig. 4 Comparisons of actual drop size distribution and fitting size distribution of snowflake
图 5 降雪-能见度理论关系与实测数据对比
Fig. 5 Comparisons of theory relationships and snow observations by Parsive
图 6 降雪样本c中降雪强度和能见度随时间的变化情况
Fig. 6 Variation of snowfall rate and visibility during snowfall-c
表 1 气压为1000hPa时, 用于计算冰晶和雪花各种参数的系数
Table 1 Coefficients of ice crystal and snowflaKe parameters at 1000 hPa
表 2 Parsivel雨滴谱仪测量得到的降雨Gamma拟合谱参数
Table 2 Parameters of Gamma fitting spectrum based on raindrop size obtained by Parsivel
表 3 Parsivel雨滴谱仪测量的降雪Gamma拟合谱参数
Table 3 Parameters of Gamma fitting spectrum based on snowflaKe size by Parsivel
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