Context
Clouds play an important role in the Earth’s radiative budget as they reflect incoming solar radiation from the sun and trap outgoing heat in the Earth’s atmosphere depending on the cloud height, composition and temperature. The magnitude of these two impacts are a function of cloud phase (ice crystals or liquid droplets, collectively called hydrometeors) and size of the hydrometeors. Between -38°C and 0 °C clouds can feature both ice crystals and super-cooled water droplets which undergo different physical processes changing their size, their phase and the radiative (reflective and absorptive) properties of the whole cloud. In this temperature regime, ice crystals can only form on Ice Nucleating Particles (INPs), which are tiny particles suspended in the atmosphere and act as centers for the ice crystals or cloud droplet to form upon. Unlike other aerosol particles (e.g. mineral dust, aged soot particles) particles of biological origin, e.g. fragments coming from pollen, bacteria or fungal spores can mediate ice crystal formation already at higher temperatures of -10 to -5 °C. These temperatures are often encountered in clouds over orographic terrain, where predictions of clouds and precipitation still challenge state-of-the-art weather models. This is also owed to the sparse knowledge of the distribution of biological INPs.
During a field measurement campaign called RACLETS (Role of Aerosol and Clouds Enhanced by Topography on Snow) in February – March 2019 in Davos, INP measurements were conducted to improve the understanding of INPs over orographic terrain.
