Abstract

Cloud radiative properties, cloud lifetime, and precipitation initiation are strongly influenced by the cloud phase. Between ~ 235 and 273 K, ice nucleating particles (INPs) are responsible for the initial phase transition from the liquid to the ice phase in cloud hydrometeors. This study analyzes immersion-mode INP concentrations measured at 243 K at the High Altitude Research Station Jungfraujoch (3580 m a.s.l.) between February 2020 and January 2021, thereby presenting the longest continuous, high-resolution (20 min) data set of online INP measurements to date. The high time resolution and continuity allow to study the seasonal and the diurnal variability of INPs. After exclusion of special events, like Saharan dust events (SDEs), we found a seasonal cycle of INPs, highest in April (median in spring 3.1 INP std L−1), followed by summer (median: 1.6 INP std L−1) and lowest in fall and winter (median: 0.5 INP std L−1 and 0.7 INP std L−1, respectively). Pollen or subpollen particles were deemed unlikely to be responsible for elevated INP concentrations in spring and summer, as periods with high pollen loads from nearby measurement stations do not coincide with the periods of high INP concentrations. Furthermore, for days when the site was purely in the free troposphere (FT), no diurnal cycle in INP concentrations was observed, while days with boundary layer intrusions (BLI) showed a diurnal cycle. The seasonal and diurnal variability of INPs during periods excluding SDEs is with a factor of 7 and 3.3, respectively, significantly lower than the overall variability observed in INP concentration including SDEs of more than three orders of magnitude, when peak values result from SDEs. The median INP concentration over the analyzed 12 months was 1.2 INP std L−1 for FT periods excluding SDEs, and 1.4 INP std L−1 for both FT and BLI, and incl. SDEs, reflecting that despite SDEs showing strong but comparatively brief INP signals, they have a minor impact on the observed annual median INP concentration.

Highlights

  • The ratio of ice crystals to liquid droplets in a cloud strongly determines its radiative properties, lifetime, and precipitation 20 initiation (e.g., Lau and Wu, 2003; Lohmann and Feichter, 2005; Hoose and Möhler, 2012; Murray et al, 2012; Mülmenstädt et al, 2015; Heymsfield et al, 2020)

  • After exclusion of special events, like Saharan dust events (SDEs), we found a seasonal cycle of ice nucleating particles (INPs), highest in April, followed by summer and lowest in fall and winter

  • The seasonal and diurnal variability of INPs during periods excluding SDEs is with a factor of 7 and 3.3, respectively, significantly lower than the overall variability observed in INP concentration including SDEs of more than three orders of magnitude, when peak values result from SDEs. 15 The median INP concentration over the analyzed 12 months was 1.2 INP std L−1 for free troposphere (FT) periods excluding SDEs, and 1.4 INP std L−1 for both FT and boundary layer intrusions (BLI), and incl

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Summary

Introduction

The ratio of ice crystals to liquid droplets in a cloud strongly determines its radiative properties, lifetime, and precipitation 20 initiation (e.g., Lau and Wu, 2003; Lohmann and Feichter, 2005; Hoose and Möhler, 2012; Murray et al, 2012; Mülmenstädt et al, 2015; Heymsfield et al, 2020). Tobo et al (2020) studied seasonal INP concentrations with 72-hour filter samples taken on a building 458 m above ground in Tokyo between August 2016 and July 2017, followed by testing the particle collected on the filters for their ice nucleation activity on a cold stage They found a weak seasonal cycle for INPs active below 253 K, with spikes during 40 Asian dust events. A seasonal dependence in INP concentrations was apparent in all these studies, where highest INP concentrations were observed in spring and summer All these studies were performed using offline 50 sampling techniques, where changes to the ice nucleation ability of particles cannot be excluded between sampling and the analysis. The long duration, together with a sampling interval of 20 minutes allowed us to study the seasonal, and the diurnal variability of INP concentrations at the sampling site

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