Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers
Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers
复制标题
DOI：
10.5194/acp-13-9801-2013
复制
发表时间：
2012-10
影响因子：
6.3
通讯作者：
P. Spichtinger;M. Krämer
中科院分区：
文献类型：
--
作者：
P. Spichtinger;M. Krämer
在热带对流层顶层（TTL）的冷卷云内部和外部出现高的、持续的冰过饱和度仍然是一个谜，被激烈地称为“冰过饱和度之谜”。然而，最近证实，观测到的过饱和度与非常低的冰晶浓度相一致，这与均匀冻结是TTL中冰形成的主要方法的想法不相容。这样，热带对流层顶的“冰过饱和之谜”就变成了“冰成核之谜”。为了解释低冰晶浓度，人们提出了许多主要的非均相冷冻方法。在这里，我们通过广泛的模式模拟再现了冰晶浓度发生频率的原位测量，由TTL中的特殊动态条件驱动，即缓慢的大尺度上升气流与高频短波的叠加。从模拟中可以得出，当模式结果分别由连续的非均匀和均匀冰形成情景和纯均匀冰形成情景组成时，可以解释观测到的全部冰晶浓度。这些情景分别发生在非常慢（−1）和更快（bbb1cm s−1）的大尺度上升气流中。这一统计分析表明，大约80%的TTL卷云可以用“经典”均匀冰成核来解释，而剩下的20%则源于在相同环境中发生的非均匀和均匀冻结。在充满重力波的环境中，限制均匀冻结产生冰晶的机制是重力波的短暂性，这在形成更高冰晶浓度之前阻碍了冻结事件的发生。
The occurrence of high, persistent ice supersaturation inside and outside cold cirrus in the tropical tropopause layer (TTL) remains an enigma that is intensely debated as the "ice supersaturation puzzle". However, it was recently confirmed that observed supersaturations are consistent with very low ice crystal concentrations, which is incompatible with the idea that homogeneous freezing is the major method of ice formation in the TTL. Thus, the tropical tropopause "ice supersaturation puzzle" has become an "ice nucleation puzzle". To explain the low ice crystal concentrations, a number of mainly heterogeneous freezing methods have been proposed. Here, we reproduce in situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamic conditions in the TTL, namely the superposition of slow large-scale updraughts with high-frequency short waves. From the simulations, it follows that the full range of observed ice crystal concentrations can be explained when the model results are composed from scenarios with consecutive heterogeneous and homogeneous ice formation and scenarios with pure homogeneous ice formation occurring in very slow ( −1 ) and faster (> 1 cm s −1 ) large-scale updraughts, respectively. This statistical analysis shows that about 80% of TTL cirrus can be explained by "classical" homogeneous ice nucleation, while the remaining 20% stem from heterogeneous and homogeneous freezing occurring within the same environment. The mechanism limiting ice crystal production via homogeneous freezing in an environment full of gravity waves is the shortness of the gravity waves, which stalls freezing events before a higher ice crystal concentration can be formed.