Abstract
Understanding the evolution of the ice phase within mixed-phase clouds (MPCs) is necessary to reduce uncertainties related to the cloud radiative feedback in climate projections and precipitation initiation. Both primary ice formation via ice nucleating particles (INPs) and secondary ice production (SIP) within MPCs are unconstrained, not least because of the lack of atmospheric observations. In the past decades, advanced remote sensing methods have emerged which provide high resolution data of aerosol and cloud properties and could be key in understanding microphysical processes on a global scale. In this study, we retrieved INP concentrations, and ice multiplication factors (IMFs) in wintertime orographic clouds using active remote sensing and in situ observations obtained during the RACLETS campaign in the Swiss Alps. INP concentrations in air masses dominated by Saharan dust and continental aerosol were retrieved from a polarization Raman lidar and validated with aerosol and INP in situ observations on a mountaintop. A calibration factor of 0.0204 for the global INP parameterization by DeMott et al. (2010) is derived by comparing in situ aerosol and INP measurements improving the INP concentration retrieval for continental aerosols. Based on combined lidar and radar measurements, the ice crystal number concentration and ice water content were retrieved and validated with balloon-borne in situ observations, which agreed with the balloon-borne in situ observations within an order of magnitude. For seven cloud cases the ice multiplication factors (IMFs), defined as the quotient of the ice crystal number concentration to the INP concentration, were calculated. The median IMF was around 80 and SIP was active (defined as IMFs > 1) nearly 85 % of the time. SIP was found to be active at all observed temperatures (−30 °C to −5 °C) with highest IMFs between −20 °C and −5 °C. The introduced methodology could be extended to larger datasets to better understand the impact of SIP not only over the Alps but also at other locations and for other cloud types.
Highlights
Introduction and backgroundThe increase of the Earth’s global mean temperature in recent years is unequivocal, yet the extent of a cloud cooling effect 20 remains most uncertain (IPCC, 2021)
Three main uncertainties are involved in the estimation: (i) uncertainties linked to the measurement of the aerosol extinction coefficient and its conversion to number or surface area concentration, (ii) uncertainties linked to the ice nucleating particles (INPs) parameterization itself, and (iii) using an param210 eterization not suitable for the dominant aerosol constituent
In this study we retrieved atmospheric INP concentrations in dust-dominated and continental air masses, and ice multiplication factors (IMFs) in winter530 time orographic clouds using active remote sensing and in situ observations obtained during the RACLETS field campaign in the Swiss Alps in February and March 2019
Summary
Introduction and backgroundThe increase of the Earth’s global mean temperature in recent years is unequivocal, yet the extent of a cloud cooling effect 20 remains most uncertain (IPCC, 2021). They are thermodynamically unstable (see e.g., Korolev et al, 2017) because of the lower vapor pressure with respect to ice than with respect to liquid water. This causes the ice crystals to grow at the expense of the evaporation of cloud droplets which is referred to as the Wegener-Bergeron-Findeisen process (Wegener, 1911; Bergeron, 25 1935; Findeisen, 1938). The Hallett-Mossop process can play an important role especially over orographic terrain, given that the orographic forcing induced updrafts could sustain the availability of liquid water droplets (Lohmann et al, 2016). The prevalence of SIP in the atmosphere remains uncertain as well as the environmental conditions for SIP to be active
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