On the drivers of droplet variability in Alpine mixed-phase clouds

Abstract

Abstract. Droplet formation provides a direct microphysical link between aerosols and clouds (liquid or mixed phase), and its adequate description poses a major challenge for any atmospheric model. Observations are critical for evaluating and constraining the process. Towards this, aerosol size distributions, cloud condensation nuclei, hygroscopicity and lidar-derived vertical velocities were observed in Alpine mixed-phase clouds during the Role of Aerosols and Clouds Enhanced by Topography on Snow (RACLETS) field campaign in the Davos, Switzerland region during February and March 2019. Data from the mountain-top site of Weissfluhjoch (WFJ) and the valley site of Davos Wolfgang are studied. These observations are coupled with a state-of-the art droplet activation parameterization to investigate the aerosol-cloud droplet link in mixed-phase clouds. The mean CCN-derived hygroscopicity parameter, κ, at WFJ ranges between 0.2–0.3, consistent with expectations for continental aerosol. κ tends to decrease with size, possibly from an enrichment in organic material associated with the vertical transport of fresh ultrafine particle emissions (likely from biomass burning) from the valley floor in Davos. The parameterization provides droplet number that agrees with observations to within ~25 %. We also find that the susceptibility of droplet formation to aerosol concentration and vertical velocity variations can be appropriately described as a function of the standard deviation of the distribution of updraft velocities, σw, as the droplet number never exceeds a characteristic limit, termed limiting droplet number, of ~150–550 cm−3, which depends solely on σw. We also show that high aerosol levels in the valley, most likely from anthropogenic activities, increase cloud droplet number, reduce cloud supersaturation (<0.1 %) and shift the clouds to a state that is less susceptible to aerosol and become very sensitive to vertical velocity variations. The transition from aerosol to velocity-limited regime depends on the ratio of cloud droplet number to the limiting droplet number, as droplet formation becomes velocity-limited when this ratio exceeds 0.5. Under such conditions, droplet size tends to be minimal, reducing the likelihood that large drops are present that promote glaciation through rime splintering and droplet shattering. Identifying regimes where droplet number variability is dominated by dynamical – rather than aerosol – changes is key for interpreting and constraining when and which types of aerosol effects on clouds are active.

Microphysical investigation of the seeder and feeder region of an Alpine mixed-phase cloud

“…The data presented in this paper were collected during the RACLETS campaign, which took place in the Swiss Alps in the region around Davos from 8 February 2019 to 28 March 2019. The campaign was designed to observe the pathways of orographic precipitation formation covering the entire aerosol-cloud-precipitation process chain (see also Ramelli et al, 2021;Lauber et al, 2021;Mignani et al, 2021;Georgakaki et al, 2020). In the following, we will describe the relevant instruments and methods which have been used for the analysis of the presented case study.…”

“…Ground-based remote sensing measurements were obtained with a vertically pointing Ka-band polarimetric cloud radar that operated at 35.5 GHz (Mira-36 METEK GmbH, Germany; Görsdorf et al, 2015). The radar was operated at a pulse-repetition frequency of 6000 Hz and a pulse length of 208 × 10 −9 s, resulting in a vertical resolution of 31.17 m and a maximum unambiguous velocity range of 25.6 m s −1 , which spans from −12.8 to 12.8 m s −1 .…”

“…The 10 s averages of Doppler spectra are the prerequisite for the subsequent data analysis. The moments of the Doppler spectrum provide information about mean volume radar reflectivity, Doppler velocity and Doppler spectral width, based on which the abundance and turbulent properties of clouds can be inferred (Görsdorf et al, 2015). From the ratio of the co-and cross-polarized signal components, the linear depolarization ratio (LDR) is obtained.…”

“…Aerosols and INP properties were measured at the valley station of Wolfgang (1630 m) and at the mountaintop station of Weissfluhjoch (2700 m) (Fig. 1; see also Mignani et al, 2021;Georgakaki et al, 2020). Aerosol instruments were connected to heated inlets for measurements of ambient air at each site.…”

Continuous secondary-ice production initiated by updrafts through the melting layer in mountainous regions

“…Temperature measurements were taken from the MeteoSwiss station in Klosters (Hervo, 2019) and the snow drift station installed at Gotschnagrat (Walter and Huwald, 2019;Walter et al, 2020). For the analysis of the whole clouds, a vertically pointing cloud radar (Model Mira-36, METEK GmbH, Germany;Görsdorf et al, 2015) was installed in Wolfgang. INPCs were measured in the valley in Wolfgang (1630 m) and on the mountain top of Weissfluhjoch (2670 m), where ambient aerosol was sampled through heated inlets using a high-flow-rate impinger (Bertin Technologies, Model Coriolis µ, France).…”