Radiation-Induced Condensational Growth and Cooling of Cloud-Sized Mist Droplets
Radiation-Induced Condensational Growth and Cooling of Cloud-Sized Mist Droplets
复制标题
辐射诱导云大小雾滴的凝结生长和冷却
DOI：
10.1175/jas-d-19-0288.1
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发表时间：
2020
影响因子：
3.1
通讯作者：
M. Rood
中科院分区：
文献类型：
--
作者：
M. Brewster;X. Li;K. K. Roman;Ezra Owen McNichols;M. Rood
一项实验室实验和理论模型研究对云状水雾的等压辐射冷却到一个遥远的散热器进行了研究，类似于在云的顶部可能发生的情况。对于初始温度为20°C的薄雾，在−20°C的辐射汇冷却下，平均（D43）雾滴直径在80 s内从5.5 μm增加到8.4 μm，雾温从20°C下降到3°C。模拟表明，传统的假设能够很好地预测测量到的温度下降，但不能预测液滴大小的变化，这表明体辐射冷却被很好地模拟了，但没有详细描述液滴大小相关的行为。在理论分析中，利用lewis数近单位得到了准稳态过饱和的解析表达式，该表达式与Davies在1985年的报告一致，但更简单，并且仅是液滴尺寸分布、表面张力和溶质参数的函数，而不是辐射传递的函数。相应的时间常数也得到了一个更简单的表达式，它仅是液滴直径分布的二元扩散系数和D31力矩的函数。时间常数的单位是毫秒，而不是秒。简单地修改准稳定过饱和（即，将液滴冷却效应均匀地应用于所有液滴尺寸）被证明不是在液滴生长方程中包含液滴特定辐射项的可接受替代。这些结果证实了云顶辐射冷却对液滴大小演变和温度变化有显著影响，并为进一步研究辐射模拟假设和参数化提供了数据和分析简化。
A laboratory-experimental and theoretical-modeling investigation was conducted of isobaric, radiative cooling of cloud-like water mists to a remote heat sink, similar to what can happen at the tops of clouds. For mist initially at 20°C cooled by a radiative sink at −20°C, the mean (D43) mist droplet diameter grew from 5.5 to 8.4 μm and the mist temperature decreased from 20° to 3°C in just 80 s. Modeling showed that conventional assumptions were able to predict the measured temperature decrease reasonably well but not droplet size changes, suggesting that bulk radiative cooling was being reasonably well modeled but not detailed, droplet-size-dependent behavior. In a theoretical analysis, Lewis-number near unity was exploited to obtain an analytic expression for quasi-steady supersaturation that agrees with what Davies reported in 1985 but is simpler and is a function of only droplet size distribution, surface tension, and solute parameters and not radiative transfer. A simpler expression for the corresponding time constant was also found that is a function of only the binary diffusion coefficient and D31 moment of the droplet diameter distribution. The time constant was found to be in milliseconds and not seconds. Simply modifying quasi-steady supersaturation (i.e., applying droplet cooling effects uniformly to all droplet sizes) was shown not to be an acceptable substitute for including droplet-specific radiation terms in the droplet growth equation. These results confirm that radiative cooling at cloud top can have a significant effect on droplet size evolution and temperature change and provide data and analytical simplifications for use in further needed investigations of radiation modeling assumptions and parameterizations.