Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics
Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics
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地形卷云中的初级冰核：微观物理的数值模拟
DOI：
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发表时间：
1999
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通讯作者：
C. Saunders
中科院分区：
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作者：
A. Spice;D. Johnson;P. Brown;A. Darlison;C. Saunders
采用简单的绝热上升-空气-包裹模式研究了地形卷云中均匀冻结和非均匀成核过程中冰粒子的产生。模型中的均匀冻结速率基于Jeffery和Austin的公式，该公式与观测到的速率相匹配，并根据溶液效应进行了修正。应用了两种备选冰核（IN）激活谱，一种依赖于温度，另一种依赖于冰过饱和度。对初始露点温度Td在- 20至- 45℃范围内，恒定垂直速度w为0.1至5.0 m s - 1的航空包裹进行了模拟。在Td大于或等于30°C的模拟云中，发现由In发起的非均质成核主导了冰的形成，因为均质冻结率如此之低。在Td≤−35°C的模拟云中，纯液态水的均匀冻结速率很大，但包含足够数量的In可以完全抑制水溶液液滴的生长、稀释和均匀冻结，即使在极低的温度下，非均质成核仍然占主导地位。对于固定的云凝结核（CCN）激活谱，均匀冻结抑制所需的IN数浓度随着低温下w值的增加而增加，因此与地形卷云相比，非地形卷云在低w值下更可行。通常的高空IN亏缺概念受到了挑战，而且，由于卷云微物理和辐射特性对这些气溶胶粒子的明显敏感性，除了CCN测量外，卷云水平的IN测量也被认为是至关重要的。该模型的结果表明，有可能根据主要的冰核模式来解释地形卷云中峰值液态水含量或最大液滴半径的测量结果。
Ice particle production by the processes of homogeneous freezing and heterogeneous nucleation in orographic cirrus clouds is studied using a simple adiabatic ascending‐air‐parcel model. Homogeneous freezing rates in the model are based on a formulation by Jeffery and Austin, which matches with observed rates, modified according to the solution effect. Two alternative ice nuclei (IN) activation spectra are applied, one dependent upon temperature and the other upon ice supersaturation. Simulations are performed for air parcels with initial dew‐point temperatures, Td, in the range from −20 to −45°C and constant vertical velocities, w, of 0.1 to 5.0 m s‐1. In modelled clouds with Td ⩾ 30°C, heterogeneous nucleation initiated by IN is found to dominate ice formation, since homogeneous freezing rates are so low. In modelled clouds with Td ⩽ −35°C, pure liquidwater homogeneous freezing rates are large, but inclusion of sufficient quantities of IN may completely suppress aqueous solution droplet growth, dilution and homogeneous freezing, with heterogeneous nucleation then remaining dominant even at very low temperatures. the IN number concentrations required for homogeneous freezing suppression, for a fixed cloud condensation nucleus (CCN) activation spectrum, are found to increase with increasing w‐heterogeneous nucleation at low temperatures is therefore much more viable at the low w values found in non‐orographic compared with orographic cirrus clouds. the common conception of a deficit of IN aloft is challenged, and, because of the apparent sensitivity of cirrus cloud microphysical and radiative properties to these aerosol particles, IN measurements at cirrus levels, in addition to CCN measurements, are suggested as vital. the model results suggest that it may be possible to interpret measurements of peak liquid‐water content, or possibly maximum droplet radius, in orographic cirrus clouds in terms of the dominant ice nucleation mode.