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Model simulations with COSMO-SPECS: Impact of heterogeneous freezing modes and ice nucleating particle types on ice formation and precipitation in a deep convective cloud

机译:COSMO-SPECS的模型仿真:异构的影响 冻结模式和冰成核颗粒类型对冰的形成和 深对流云中的降水

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摘要

In deep convective clouds, heavy rain is often formed involving the ice phase as essential process. Simulations were performed using the 3D cloud resolving model COSMO-SPECS with detailed spectral microphysics including parametrizations of homogeneous and three heterogeneous freezing modes. The initial conditions were selected to result in a deep convective cloud reaching 14 km altitude with strong updrafts up to 40 m/s. In such altitudes with corresponding temperatures below −40 °C the major fraction of liquid drops freezes homogeneously. The goal of the present model simulations was to investigate how additional heterogeneous freezing will affect ice formation and precipitation although its contribution to total ice formation may be rather low. In such a situation small perturbations which do not show significant effects at first sight may trigger cloud microphysical responses. Effects of the following small perturbations were studied: (1) additional ice formation via immersion, contact, and deposition modes in comparison to sole homogeneous freezing, (2) contact and deposition freezing in comparison to immersion freezing, (3) small fractions of biological ice nucleating particles (INP) in comparison to higher fractions of mineral dust INP. The results indicate that the modification of precipitation proceeds via the formation of larger ice particles which may be supported by direct freezing of larger drops, the growth of pristine ice particles by riming, and by nucleation of larger drops by collisions with pristine ice particles. In comparison to the reference case homogeneous freezing such small perturbations may affect an enhancement of total precipitation but mostly the effects are limited to modifications of the temporal development of precipitation, i.e. a gradual increase already at early cloud stages instead a strong increase at later cloud stages. These effects are coupled with changes in the local distribution of precipitation, i.e. approximately 50 % more precipitation in the cloud center. The modifications depend on the active freezing modes, the fractions of active INP, and the composition of the internal mixtures in the drops.
机译:在深层对流云中,经常形成大雨,而冰期是必不可少的过程。使用具有详细光谱微观物理学的3D云解析模型COSMO-SPECS进行了模拟,包括均匀和三种非均匀冻结模式的参数化。选择初始条件是导致深对流云达到14 cloudkm的高度,并有高达40 m / s的强上升气流。在这样的高度下,相应的温度低于-40 C,大部分液滴均匀冻结。本模型仿真的目的是研究附加的非均质冻结将如何影响冰的形成和降水,尽管其对总冰形成的贡献可能很小。在这种情况下,乍看之下并未表现出明显影响的微小扰动可能会触发云的微物理响应。研究了以下小扰动的影响:(1)与单独的均质冷冻相比,通过浸没,接触和沉积方式形成了更多的冰,(2)与浸没冷冻相比,接触和沉积冷冻,(3)生物的小部分冰核颗粒(INP)与更高比例的矿物粉尘INP相比。结果表明,降水的改变是通过形成较大的冰粒而进行的,较大的冰粒可通过直接冻结较大的液滴,缘缘生长原始冰粒以及通过与原始冰粒碰撞而使较大的液滴成核来支持。与参考情况相比,均质冻结这种较小的扰动可能会影响总降水量的增加,但大多数影响仅限于降水时间变化的修正,即在云的早期已经逐渐增加,而在云的后期已经逐渐增加。这些影响与降水的局部分布变化有关,即云中心的降水增加了约50%。修改取决于活性冷冻模式,活性INP的比例以及液滴中内部混合物的组成。

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