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基于观测的华北地区大气冰核对对流云微物理过程和降水影响的模拟研究 *

Numerical simulation research on the effects of atmospheric ice nuclei based on observations on the convective cloud microphysical processes and precipitation in North China

  • 摘要: 大气冰核在对流云降水发生发展过程中起着至关重要的作用。本文基于华北地区大气冰核和气溶胶的协同观测构建的符合华北地区实际特征的冰核核化参数化公式改进了中尺度数值模式WRF中SBM分档方案的冰晶核化参数化描述,并利用改进的参数化方案和模式对华北地区典型的对流云降水过程进行模拟与分析。结果显示利用修改后的方案模拟的对流云中上升气流更强,更接近实况;大气冰核在上升气流的作用下传输至云中高层和云砧中,通过异质核化等过程增加了高空的冰晶,最终导致地面雨强和总降雨的增加。此外大气冰核的接触冻结、凝结/凝华冻结、浸润冻结三种异质成冰机制在不同的温度范围内发生,并在不同的高度占主导地位;当增加大气冰核浓度时,三种异质核化的冰核形成率有同步的增加,并且与云滴的同质冻结相比更具竞争力。高浓度的大气冰核会导致异质核化过程增大,冰晶和雪的质量浓度数浓度增大,异质核化消耗云水导致霰减少,雨水减少。

     

    Abstract: Atmospheric ice nuclei play a crucial role in the occurrence and development of convective cloud precipitation. This paper is based on the collaborative observation of ice nucleation and aerosols in North China, and constructs a parameterized formula for ice nucleation that conforms to the actual characteristics of North China. The parameterization description of ice nucleation in the SBM scheme of the WRF model is improved, and then are used to simulate typical convective cloud precipitation processes in North China. The results show that the simulated convective cloud with the modified scheme has stronger upward airflow and is closer to the actual situation; Under the action of updrafts, atmospheric ice nuclei are transported to the middle and upper layers of clouds and cloud anvils, increasing the number of ice crystals in the upper atmosphere through processes such as heterogeneous nucleation, ultimately leading to an increase in surface rainfall intensity and total rainfall. In addition, the three heterogeneous ice formation mechanisms of atmospheric ice nuclei, namely contact freezing, condensation/sublimation freezing, and immersion freezing, occur in different temperature ranges and dominate at different heights; When the concentration of atmospheric ice nuclei increases, the formation rates of the three heterogeneous nuclei increase synchronously and are more competitive compared to homogeneous freezing of cloud droplets. High concentrations of atmospheric ice nuclei lead to an increase in heterogeneous nucleation processes, an increase in the mass concentration of ice crystals and snow, and the consumption of cloud water by heterogeneous nucleation leads to a decrease in graupel and rainwater.

     

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