Ground-Based Observations and Modeling of the Visibility and Radar Reflectivity in a Radiation Fog Layer
Ground-Based Observations and Modeling of the Visibility and Radar Reflectivity in a Radiation Fog Layer
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DOI：
10.1175/jtech-d-12-00081.1
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发表时间：
2013-02-01
影响因子：
2.2
通讯作者：
Moerman, M.
中科院分区：
文献类型：
--
作者：
Boers, R.;Baltink, H. Klein;Moerman, M.
2011 年 3 月 23 日，通过地面原位和遥感观测观测到卡包夫大气研究实验场（北纬 51.97 度，东经 4.93 度）辐射雾层的形成，以研究能见度与雷达反射率之间的关系。雾层厚度小于200 m。即使能见度小于 100 m，雷达反射率值也不超过 -25 dBZ。雾的出现和蒸发产生不同的雷达反射率-能见度关系。雾层的演变采用液滴激活模型进行建模，该模型使用在 60 米高度塔水平观察到的气溶胶尺寸分布作为输入。使用米氏散射理论根据模型液滴尺寸光谱计算雷达反射率和可见度。由于与充满气溶胶的空气的绝热升力导致的冷却速率相比，辐射冷却速率较小，因此模拟的过饱和度仍然较低，因此很少有气溶胶颗粒被激活为云滴。建模结果表明，不同的雷达反射率-能见度关系是雾形成和蒸发过程中水蒸气和云滴之间相互作用差异的结果。在水滴激活过程中，在与较小的激活水滴成功竞争水蒸气后，只剩下一些大的云滴。这些小液滴最终会再次蒸发（失活）。在雾溶解/蒸发阶段，仅需要蒸发这些大液滴。因此，为了将雷达反射率转换为交通安全产品的可见度，需要了解局部雾演变的状态。
The development of a radiation fog layer at the Cabauw Experimental Site for Atmospheric Research (51.97 degrees N, 4.93 degrees E) on 23 March 2011 was observed with ground-based in situ and remote sensing observations to investigate the relationship between visibility and radar reflectivity. The fog layer thickness was less than 200 m. Radar reflectivity values did not exceed -25 dBZ even with visibilities less than 100 m. The onset and evaporation of fog produce different radar reflectivity-visibility relationships. The evolution of the fog layer was modeled with a droplet activation model that used the aerosol size distribution observed at the 60-m altitude tower level as input. Radar reflectivity and visibility were calculated from model drop size spectra using Mie scattering theory. Since radiative cooling rates are small in comparison with cooling rates due to adiabatic lift of aerosol-laden air, the modeled supersaturation remains low so that few aerosol particles are activated to cloud droplets. The modeling results suggest that the different radar reflectivity-visibility relationships are the result of differences in the interplay between water vapor and cloud droplets during formation and evaporation of the fog. During droplet activation, only a few large cloud droplets remain after successfully competing for water vapor with the smaller activated droplets. These small droplets eventually evaporate (deactivate) again. In the fog dissolution/evaporation stage, only these large droplet need to be evaporated. Therefore, to convert radar reflectivity to visibility for traffic safety products, knowledge of the state of local fog evolution is necessary.