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

2021 ◽  
Vol 21 (9) ◽  
pp. 6681-6706
Author(s):  
Fabiola Ramelli ◽  
Jan Henneberger ◽  
Robert O. David ◽  
Johannes Bühl ◽  
Martin Radenz ◽  
...  
Keyword(s):  
Mixed Phase ◽  
Swiss Alps ◽  
Ice Formation ◽  
Small Scale ◽  
Orographic Precipitation ◽  
Local Flow ◽  
Low Level ◽  
Cloud Radar ◽  
Ice Production

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.

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

2020 ◽  
Author(s):  
Fabiola Ramelli ◽  
Jan Henneberger ◽  
Robert O. David ◽  
Johannes Bühl ◽  
Martin Radenz ◽  
...  
Keyword(s):  
Mixed Phase ◽  
Swiss Alps ◽  
Ice Formation ◽  
Small Scale ◽  
Orographic Precipitation ◽  
Local Flow ◽  
Low Level ◽  
Cloud Radar ◽  
Ice Production

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 was 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 mountain-top observatories. Secondary ice production in feeder clouds can potentially enhance orographic precipitation.

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

2021 ◽  
Vol 21 (6) ◽  
pp. 5151-5172
Author(s):  
Fabiola Ramelli ◽  
Jan Henneberger ◽  
Robert O. David ◽  
Annika Lauber ◽  
Julie T. Pasquier ◽  
...  
Keyword(s):  
Cloud Microphysics ◽  
Small Scale ◽  
Orographic Precipitation ◽  
Tethered Balloon ◽  
Local Flow ◽  
Turbulent Layer ◽  
Alpine Valley ◽  
Low Level ◽  
Balloon System

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.

scholarly journals Sensitivity of precipitation formation to secondary ice production in winter orographic mixed-phase clouds

2021 ◽  
Vol 21 (19) ◽  
pp. 15115-15134
Author(s):  
Zane Dedekind ◽  
Annika Lauber ◽  
Sylvaine Ferrachat ◽  
Ulrike Lohmann
Keyword(s):  
Precipitation Amount ◽  
Model Performance ◽  
Swiss Alps ◽  
Ice Formation ◽  
Small Scale ◽  
Ice Crystal ◽  
Surface Precipitation ◽  
Ice Production ◽  
Mixed Phase Clouds ◽  
Precipitation Formation

Abstract. The discrepancy between the observed concentration of ice nucleating particles (INPs) and the ice crystal number concentration (ICNC) remains unresolved and limits our understanding of ice formation and, hence, precipitation amount, location and intensity. Enhanced ice formation through secondary ice production (SIP) could account for this discrepancy. Here, in a region over the eastern Swiss Alps, we perform sensitivity studies of additional simulated SIP processes on precipitation formation and surface precipitation intensity. The SIP processes considered include rime splintering, droplet shattering during freezing and breakup through ice–graupel collisions. We simulated the passage of a cold front at Gotschnagrat, a peak at 2281 m a.s.l. (above sea level), on 7 March 2019 with the Consortium for Small-scale Modeling (COSMO), at a 1 km horizontal grid spacing, as part of the RACLETS (Role of Aerosols and CLouds Enhanced by Topography and Snow) field campaign in the Davos region in Switzerland. The largest simulated difference in the ICNC at the surface originated from the breakup simulations. Indeed, breakup caused a 1 to 3 orders of magnitude increase in the ICNC compared to SIP from rime splintering or without SIP processes in the control simulation. The ICNCs from the collisional breakup simulations at Gotschnagrat were in best agreement with the ICNCs measured on a gondola near the surface. However, these simulations were not able to reproduce the ice crystal habits near the surface. Enhanced ICNCs from collisional breakup reduced localized regions of higher precipitation and, thereby, improved the model performance in terms of surface precipitation over the domain.

scholarly journals A comparison between polynya area and associated ice production with mooring-based measurements of temperature, salinity and currents in the southwestern Ross Sea, Antarctica

2011 ◽  
Vol 52 (57) ◽  
pp. 291-300 ◽  
Author(s):  
Stefan Kern ◽  
Stefano Aliani
Keyword(s):  
Ross Sea ◽  
Time Lag ◽  
Ice Formation ◽  
Terra Nova Bay ◽  
In Situ Observations ◽  
Satellite Microwave ◽  
Terra Nova ◽  
Ice Production ◽  
The Impact

AbstractWintertime (April–September) area estimates of the Terra Nova Bay polynya (TNBP), Antarctica, based on satellite microwave radiometry are compared with in situ observations of water salinity, temperature and currents at a mooring in Terra Nova Bay in 1996 and 1997. In 1996, polynya area anomalies and associated anomalies in polynya ice production are significantly correlated with salinity anomalies at the mooring. Salinity anomalies lag area and/or ice production anomalies by about 3 days. Up to 50% of the variability in the salinity at the mooring position can be explained by area and/or ice production anomalies in the TNBP for April–September 1996. This value increases to about 70% when considering shorter periods like April–June or May–July, but reduces to 30% later, for example July–September, together with a slight increase in time lag. In 1997, correlations are smaller, less significant and occur at a different time lag. Analysis of ocean currents at the mooring suggests that in 1996 conditions were more favourable than in 1997 for observing the impact of descending plumes of salt-enriched water formed in the polynya during ice formation on the water masses at the mooring depth.

Importance of Secondary Ice Production to Ice Formation and Phase of High-Latitude Mixed-Phase Clouds during SOCRATES and MARCUS

2021 ◽  
Author(s):  
Xi Zhao ◽  
Xiaohong Liu ◽  
Vaughan Phillips ◽  
Sachin Patade ◽  
Minghui Diao ◽  
...  
Keyword(s):  
High Latitude ◽  
Mixed Phase ◽  
Ice Formation ◽  
Ice Production ◽  
Mixed Phase Clouds

The observed impact of aerosols on cloud droplet formation during the RACLETS campaign

2020 ◽  
Author(s):  
Paraskevi Georgakaki ◽  
Aikaterini Bougiatioti ◽  
Athanasios Nenes
Keyword(s):  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Research Problem ◽  
Swiss Alps ◽  
Size Distributions ◽  
Ongoing Research ◽  
Microphysical Properties ◽  
Ice Production

<p>The influence of aerosols serving as cloud condensation nuclei (CCN) on the production of droplets in mixed-phase cloud systems is an ongoing research problem that influences their optical and microphysical properties. During February and March 2019, the Role of Aerosols and CLouds Enhanced by Topography on Snow (RACLETS) field campaign collected unique and detailed airborne and ground-based in-situ measurements of cloud and aerosol properties over the Swiss Alps. This study presents analysis of the observed CCN activity of the aerosol, which combined with observed aerosol size distributions, can be introduced into a cloud droplet activation parameterization to investigate the drivers of droplet variability in these clouds. The implications for secondary ice production are then discussed.</p>

Retrieval and Evaluation of Ice Water Content from the Airborne Wyoming Cloud Radar in Orographic Wintertime Clouds during SNOWNIE

2021 ◽  
Keyword(s):  
Water Content ◽  
Sensitivity Study ◽  
Mixed Phase ◽  
Zero Bias ◽  
Cloud Radar ◽  
Ice Water Content ◽  
The University ◽  
Mixed Phase Clouds ◽  
Ice Water

Abstract The ice water content (IWC) in ice and mixed-phase clouds is retrieved from airborne Wyoming Cloud Radar (WCR) measurements aboard the University of Wyoming King Air (UWKA), which has a suite of integrated in situ IWC, optical array probes (OAP) and remote sensing measurements and provides a unique dataset for this algorithm development and evaluation. A sensitivity study with different idealized ice particle habits shows that the retrieved IWC with aggregate ice particle habit agrees the best with the in situ measurement, especially in ice or ice-dominated mixed-phase clouds with a correlation coefficient (rr) of 0.91 and close-to-zero bias. For mixed-phase clouds with ice fraction ratio less than 0.8, the variances of IWC estimates increase (rr = 0.76) and the retrieved mean IWC is larger than in situ IWC by a factor of 2. This is found to be related to the uncertainty of in situ measurements, the large cloud inhomogeneity, and the retrieval assumption uncertainty. The simulated reflectivity (Ze) and IWC relationships assuming three idealized ice particle habits and measured particle size distributions show that hexagonal columns with the same Ze have a lower IWC than aggregates, whose Ze-IWC relation is more consistent with the observed WCR Ze and in-situ IWC relation in those clouds. The 2DS images also indicate that ice particle habit transition occurs in orographic mixed-phase clouds, hence the retrieved IWC assuming modified Gamma PSD of aggregate particles tends to be biased larger in this kind of clouds.

scholarly journals Observed aerosol suppression of cloud ice in low-level Arctic mixed-phase clouds

2018 ◽  
Vol 18 (18) ◽  
pp. 13345-13361 ◽  
Author(s):  
Matthew S. Norgren ◽  
Gijs de Boer ◽  
Matthew D. Shupe
Keyword(s):  
Mixed Phase ◽  
Crystal Nucleation ◽  
Cloud Base ◽  
Low Level ◽  
Aerosol State ◽  
Earth’S Climate ◽  
Ice Production ◽  
Mixed Phase Clouds ◽  
Ice Water ◽  
The Impact

Abstract. The interactions that occur between aerosols and a mixed-phase cloud system, and the subsequent alteration of the microphysical state of such clouds, are a problem that has yet to be well constrained. Advancing our understanding of aerosol–ice processes is necessary to determine the impact of natural and anthropogenic emissions on Earth's climate and to improve our capability to predict future climate states. This paper deals specifically with how aerosols influence ice mass production in low-level Arctic mixed-phase clouds. In this study, a 9-year record of aerosol, cloud and atmospheric state properties is used to quantify aerosol influence on ice production in mixed-phase clouds. It is found that mixed-phase clouds present in a clean aerosol state have higher ice water content (IWC) by a factor of 1.22 to 1.63 at cloud base than do similar clouds in cases with higher aerosol loading. We additionally analyze radar-derived mean Doppler velocities to better understand the drivers behind this relationship, and we conclude that aerosol induced reduction of the ice crystal nucleation rate, together with decreased riming rates in polluted clouds, are likely influences on the observed reductions in IWC.

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