Abstract
Abstract. Precipitation over the mid-latitudes originates mostly from the ice phase within mixed-phase clouds, signifying the importance of initial ice crystal formation. Primary ice crystals are formed on ice-nucleating particles (INPs), which measurements suggest are sparsely populated in the troposphere. INPs are emitted by a large number of ground-based sources into the atmosphere, from where they can be lifted up to cloud heights. Therefore, it is vital to understand vertical INP transport mechanisms, which are particularly complex over orographic terrain. We investigate the vertical transport and mixing mechanisms of INPs over orographic terrain during cloudy conditions by simultaneous measurements of in situ INP concentration at a high valley and a mountaintop site in the Swiss Alps in late winter 2019. On the mountaintop, the INP concentrations were, on average, lower than in the high valley. However, a diurnal cycle in INP concentrations was observed at the mountaintop, which was absent in the high valley. The median mountaintop INP concentration equilibrated to the concentration found in the high valley towards the night. We found that, in nearly 70 % of the observed cases, INP-rich air masses were orographically lifted from low elevation upstream of the measurement site. In addition, we present evidence that, over the course of the day, air masses containing high INP concentrations were advected from the Swiss plateau towards the measurement sites, contributing to the diurnal cycle of INPs. Our results suggest a local INP concentration enhancement over the Alps during cloud events.
Highlights
The formation of precipitation is linked to the evolution of cloud microphysical properties, which is complex for mixed-phase clouds (MPCs) due to the co-occurrence of supercooled liquid droplets and ice crystals (e.g., Wegener, 1911; Pruppacher and Klett, 2010; Lohmann et al, 2016b)
Across the temperature spectrum, median ice-nucleating particles (INPs) concentrations at WOP were approximately 3 times higher than at WFJ
We investigated the spatiotemporal distribution of INPs over the Swiss Alps near Davos in February and March 2019
Summary
Precipitation serves as a major source of fresh water in the global hydrological cycle. In the absence of upslope winds, and given the weak stratification of the air masses leeward of the ridge, the air masses will be pushed further down the leeward valley This synoptic forcing can result in a similar effect to the diurnal warming of the slopes, yet it is a consequence of the flow being deflected by the mountain topography (Chow et al, 2013). Measurements were conducted over 8 weeks at a high sampling frequency during cloud events (for 20 min every 1.5 h) in late winter 2019 (February and March) This allowed us to draw conclusions about the vertical distribution and mixing based on two extensive datasets of INP concentrations at both sites. Based on the local topography, we describe how dynamic transport processes can enhance the INP concentration at the mountaintop site
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