Towards parameterising atmospheric concentrations of ice-nucleating particles active at moderate supercooling

Mignani, Claudia; Wieder, Jörg; Sprenger, Michael; Kanji, Zamin A.; Henneberger, Jan; Alewell, Christine; Conen, Franz

doi:10.5194/acp-21-657-2021

Cited by 13 publications

(28 citation statements)

References 31 publications

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“…10b). Depending on the rime mass fraction, riming can have a significant impact on the ice particle properties (Li et al, 2018;Moisseev et al, 2017).…”

Section: Discussionmentioning

confidence: 99%

“…1a). A vertically pointing Ka-band cloud radar Mira-36 (METEK GmbH, Germany;Melchionna et al, 2008;Görsdorf et al, 2015;Löhnert et al, 2015) provided vertical profiles of radar reflectivity factor, Doppler velocity, Doppler spectra, spectral width and linear depolarization ratio (LDR) with a vertical resolution of 31.17 m and a temporal resolution of 10 s. A PollyXT Raman and depolarization lidar (e.g., Engelmann et al, 2016) was deployed to study the aerosol and cloud properties. Moreover, a 14-channel microwave radiometer (HATPRO, Radiometer Physics GmbH, Germany; Rose et al, 2005) provided information about the vertical temperature and humidity profiles as well as the column-integrated water vapor content (IWV) and liquid water path (LWP).…”

Section: Instrument Setupmentioning

confidence: 99%

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Influence of low-level blocking and turbulence on the microphysics of a mixed-phase cloud in an inner-Alpine valley

et al. 2021

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Abstract. Previous studies that investigated orographic precipitation have primarily focused on isolated mountain barriers. Here we investigate the influence of low-level blocking and shear-induced turbulence on the cloud microphysics and precipitation formation in a complex inner-Alpine valley. The analysis focuses on a mid-level cloud in a post-frontal environment and a low-level feeder cloud induced by an in-valley circulation. Observations were obtained from an extensive set of instruments including ground-based remote sensing instrumentation, in situ instrumentation on a tethered-balloon system and ground-based precipitation measurements. During this event, the boundary layer was characterized by a blocked low-level flow and enhanced turbulence in the region of strong vertical wind shear at the boundary between the blocked layer in the valley and the stronger cross-barrier flow aloft. Cloud radar observations indicated changes in the microphysical cloud properties within the turbulent shear layer including enhanced linear depolarization ratio (i.e., change in particle shape or density) and increased radar reflectivity (i.e., enhanced ice growth). Based on the ice particle habits observed at the surface, we suggest that riming, aggregation and needle growth occurred within the turbulent layer. Collisions of fragile ice crystals (e.g., dendrites, needles) and the Hallett–Mossop process might have contributed to secondary ice production. Additionally, in situ instrumentation on the tethered-balloon system observed the presence of a low-level feeder cloud above a small-scale topographic feature, which dissipated when the low-level flow turned from a blocked to an unblocked state. Our observations indicate that the low-level blocking (due to the downstream mountain barrier) created an in-valley circulation, which led to the production of local updrafts and the formation of a low-level feeder cloud. Although the feeder cloud did not enhance precipitation in this particular case (since the majority of the precipitation sublimated when falling through a subsaturated layer above), we propose that local flow effects such as low-level blocking can induce the formation of feeder clouds in mountain valleys and on the leeward slope of foothills upstream of the main mountain barrier, where they can act to enhance orographic precipitation through the seeder–feeder mechanism.

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“…10b). Depending on the rime mass fraction, riming can have a significant impact on the ice particle properties (Li et al, 2018;Moisseev et al, 2017).…”

Section: Discussionmentioning

confidence: 99%

Section: Instrument Setupmentioning

confidence: 99%

Influence of low-level blocking and turbulence on the microphysics of a mixed-phase cloud in an inner-Alpine valley

et al. 2021

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“…Both drop-freezing instruments use a digital camera to detect freezing by a change in the light transmission through the aqueous solution. An intercomparison of an ambient aerosol sample between both instruments showed slightly higher INP concentrations for LINDA compared to DRINCZ for temperatures along the here-relevant freezing spectrum (i.e., a factor of 2 for −15 • C < T < −8 • C) (Miller et al, 2020), which can be likely attributed to instrumental differences.…”

Section: Inp Measurementsmentioning

confidence: 92%

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

et al. 2021

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Abstract. The seeder–feeder mechanism has been observed to enhance orographic precipitation in previous studies. However, the microphysical processes active in the seeder and feeder region are still being understood. In this paper, we investigate the seeder and feeder region of a mixed-phase cloud passing over the Swiss Alps, focusing on (1) fallstreaks of enhanced radar reflectivity originating from cloud top generating cells (seeder region) and (2) a persistent low-level feeder cloud produced by the boundary layer circulation (feeder region). Observations were obtained from a multi-dimensional set of instruments including ground-based remote sensing instrumentation (Ka-band polarimetric cloud radar, microwave radiometer, wind profiler), in situ instrumentation on a tethered balloon system, and ground-based aerosol and precipitation measurements. The cloud radar observations suggest that ice formation and growth were enhanced within cloud top generating cells, which is consistent with previous observational studies. However, uncertainties exist regarding the dominant ice formation mechanism within these cells. Here we propose different mechanisms that potentially enhance ice nucleation and growth in cloud top generating cells (convective overshooting, radiative cooling, droplet shattering) and attempt to estimate their potential contribution from an ice nucleating particle perspective. Once ice formation and growth within the seeder region exceeded a threshold value, the mixed-phase cloud became fully glaciated. Local flow effects on the lee side of the mountain barrier induced the formation of a persistent low-level feeder cloud over a small-scale topographic feature in the inner-Alpine valley. In situ measurements within the low-level feeder cloud observed the production of secondary ice particles likely due to the Hallett–Mossop process and ice particle fragmentation upon ice–ice collisions. Therefore, secondary ice production may have been partly responsible for the elevated ice crystal number concentrations that have been previously observed in feeder clouds at mountaintop observatories. Secondary ice production in feeder clouds can potentially enhance orographic precipitation.

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“…To calculate the growth time, we use the general equation of the ice particle growth by vapor diffusion given in Fukuta and Takahashi (1999). Using the mass-size relation of hexagonal plates from Mitchell et al (1990) and assuming water vapor saturation over liquid at a temperature of −2 • C, a splinter of 5 µm needs about 9 min to grow to a size of 93 µm and about 5 min to grow to a size of 60 µm. Thus, for any given time, a plate of 93 µm is 4 min older than a plate of 60 µm if the temperature is constant at −2 • C, and the water vapor is saturated over water.…”

Section: Estimation Of the Secondary-ice Crystal Number Concentrationmentioning

confidence: 99%

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

et al. 2021

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Abstract. An accurate prediction of the ice crystal number concentration in clouds is important to determine the radiation budget, the lifetime, and the precipitation formation of clouds. Secondary-ice production is thought to be responsible for the observed discrepancies between the ice crystal number concentration and the ice-nucleating particle concentration in clouds. The Hallett–Mossop process is active between −3 and −8 ∘C and has been implemented into several models, while all other secondary-ice processes are poorly constrained and lack a well-founded quantification. During 2 h of measurements taken on a mountain slope just above the melting layer at temperatures warmer than −3 ∘C, a continuously high concentration of small plates identified as secondary ice was observed. The presence of drizzle drops suggests droplet fragmentation upon freezing as the responsible secondary-ice mechanism. The constant supply of drizzle drops can be explained by a recirculation theory, suggesting that melted snowflakes, which sedimented through the melting layer, were reintroduced into the cloud as drizzle drops by orographically forced updrafts. Here we introduce a parametrization of droplet fragmentation at slightly sub-zero temperatures, where primary-ice nucleation is basically absent, and the first ice is initiated by the collision of drizzle drops with aged ice crystals sedimenting from higher altitudes. Based on previous measurements, we estimate that a droplet of 200 µm in diameter produces 18 secondary-ice crystals when it fragments upon freezing. The application of the parametrization to our measurements suggests that the actual number of splinters produced by a fragmenting droplet may be up to an order of magnitude higher.

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Towards parameterising atmospheric concentrations of ice-nucleating particles active at moderate supercooling

Cited by 13 publications

References 31 publications

Influence of low-level blocking and turbulence on the microphysics of a mixed-phase cloud in an inner-Alpine valley

Influence of low-level blocking and turbulence on the microphysics of a mixed-phase cloud in an inner-Alpine valley

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

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

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