Upward transport and segregation of ice-nucleating particles in deep convective clouds

Abstract. Ice-nucleating particles (INPs) play a crucial role in Earth’s weather and climate by influencing cloud properties and precipitation. However, their abundance in the free troposphere, vertical distribution, and transport mechanisms are not well-characterized. This study presents immersion INP measurements from filter samples collected aboard the HALO research aircraft in the troposphere and the lower stratosphere (up to 14.5 km) over Europe during the CIRRUS-HL (Cirrus in High Latitudes) campaign in summer 2021. By sampling cloud particle residuals and aerosol particles in the inflow and outflow of deep convective clouds (DCCs) and performing offline INP analysis, we shed light on the vertical transport and segregation of INPs in DCCs.

While INP-temperature spectra of convective inflow included both INPs active at high (T > -15 °C) and low temperatures (T < -20 °C), the in-cloud and outflow spectra only featured INPs active at low temperatures. We explain the observed INP segregation in the updraft with precipitation scavenging of INPs active at high temperatures. In contrast, INPs active at lower temperatures (T < -20 °C) are efficiently transported upwards into the free troposphere with ambient temperatures below -40 °C, i.e., temperatures far below the temperatures at which these INPs initiate immersion freezing. In the DCC outflow, INP concentrations exceed the upper tropospheric background concentration by at least two orders of magnitude. These INPs are then available for ice formation in mid and upper tropospheric clouds.