Vertical transport and segregation of ice nucleating particles in deep convective clouds

Ice nucleating particles (INP) play a crucial role in shaping Earth's weather and climate by influencing cloud properties and precipitation behavior. However, their abundance in the free troposphere and transport mechanisms remain poorly characterized. INP sources are generally at ground level and large-scale atmospheric vertical motion, which could lift INP from the ground into the free troposphere is often accompanied by cloud formation. The fate of INP in clouds, whether they are lifted up or washed out, is still mostly unclear.

During the HALO aircraft campaign CIRRUS-HL in June and July 2021, aerosol particles and cloud particle residuals were collected on filters using the airborne High-volume flow aERosol particle filter sAmpler (HERA). Offline laboratory analysis of these filters yield immersion mode INP concentrations. Here we present a case study with which we investigate the vertical transport and cloud processing of INP in deep convective clouds (DCC) by analyzing residuals in clouds and aerosol particles in cloud-free inflow and outflow of  DCC.

Our results show that INP active above -15°C are diminished in anvil cirrus cloud particle residuals, while INP active below -20°C are found in high concentrations in DCC outflow air. We propose that precipitation formation wash out INP active at high temperatures, while INP active at low temperatures are efficiently transport upwards into the upper troposphere with ambient temperatures below -40°C, i.e., far below the INP immersion freezing temperature. With that, DCC outflow INP concentration are at least two order of magnitude above typical upper tropospheric/lower stratospheric INP concentrations derived throughout the CIRRUS-HL campaign.

This study provides new insights into the vertical transport and cloud processing of INP in the free troposphere with implications for balance between heterogeneous and homogeneous freezing in cirrus clouds.