scholarly journals Clouds at Arctic Atmospheric Observatories. Part II: Thermodynamic Phase Characteristics

2011 ◽  
Vol 50 (3) ◽  
pp. 645-661 ◽  
Author(s):  
Matthew D. Shupe
Keyword(s):  
Liquid Water ◽  
Microwave Radiometer ◽  
Winter Season ◽  
Lower Troposphere ◽  
Mixed Phase ◽  
Ice Clouds ◽  
Cloud Properties ◽  
Significant Difference ◽  
Cloud Ice ◽  
Mixed Phase Clouds

Abstract Cloud phase defines many cloud properties and determines the ways in which clouds interact with other aspects of the climate system. The occurrence fraction and characteristics of clouds distinguished by their phase are examined at three Arctic atmospheric observatories. Each observatory has the basic suite of instruments that are necessary to identify cloud phase, namely, cloud radar, depolarization lidar, microwave radiometer, and twice-daily radiosondes. At these observatories, ice clouds are more prevalent than mixed-phase clouds, which are more prevalent than liquid-only clouds. Cloud ice occurs 60%–70% of the time over a typical year, at heights up to 11 km. Liquid water occurs at temperatures above −40°C and is increasingly more likely as temperatures increase. Within the temperature range from −40° to −30°C, liquid water occurs in 3%–5% of the observed cloudiness. Liquid water is found higher in the atmosphere when accompanied by ice; there are few liquid-only clouds above 3 km, although liquid in mixed-phase clouds occurs at heights up to about 7–8 km. Regardless of temperature or height, liquid water occurs 56% of the time at Barrow, Alaska, and at a western Arctic Ocean site, but only 32% of the time at Eureka, Nunavut, Canada. This significant difference in liquid occurrence is due to a relatively dry lower troposphere during summer at Eureka in addition to warmer cloud temperatures with more persistent liquid water layers at the far western locations. The most persistent liquid clouds at these locations occur continuously for more than 70 h in the autumn and more than 30 h in the winter. Ice clouds persist for much longer than do liquid clouds at Eureka and occur more frequently in the winter season, leading to a total cloud occurrence annual cycle that is distinct from the other observatories.

scholarly journals Arctic Mixed-Phase Stratiform Cloud Properties from Multiple Years of Surface-Based Measurements at Two High-Latitude Locations

2009 ◽  
Vol 66 (9) ◽  
pp. 2874-2887 ◽  
Author(s):  
Gijs de Boer ◽  
Edwin W. Eloranta ◽  
Matthew D. Shupe
Keyword(s):  
Liquid Water ◽  
Microwave Radiometer ◽  
Single Layer ◽  
Mixed Phase ◽  
Cloud Base ◽  
Microphysical Properties ◽  
Cloud Properties ◽  
Arctic Change ◽  
Mixed Phase Clouds ◽  
Water Contents

Abstract Macro- and microphysical properties of single-layer stratiform mixed-phase clouds are derived from multiple years of lidar, radar, and radiosonde observations. Measurements were made as part of the Mixed-Phase Arctic Clouds Experiment (MPACE) and the Study of Environmental Arctic Change (SEARCH) in Barrow, Alaska, and Eureka, Nunavut, Canada, respectively. Single-layer mixed-phase clouds occurred between 4% and 26% of the total time observed, varying with season and location. They had mean cloud-base heights between ∼700 and 2100 m and thicknesses between ∼200 and 700 m. Seasonal mean cloud optical depths ranged from 2.2 up. The clouds existed at temperatures of ∼242–271 K and occurred under different wind conditions, depending on season. Utilizing retrievals from a combination of lidar, radar, and microwave radiometer, mean cloud microphysical properties were derived, with mean liquid effective diameters estimated from 16 to 49 μm, mean liquid number densities on the order of 104–105 L−1, and mean water contents estimated between 0.07 and 0.28 g m−3. Ice precipitation was shown to have mean ice effective diameters of 50–125 μm, mean ice number densities on the order of 10 L−1, and mean water contents estimated between 0.012 and 0.031 g m−3. Mean cloud liquid water paths ranged from 25 to 100 g m−2. All results are compared to previous studies, and potential retrieval errors are discussed. Additionally, seasonal variation in macro- and microphysical properties was highlighted. Finally, fraction of liquid water to ice mass was shown to decrease with decreasing temperature.

scholarly journals The potential influence of Asian and African mineral dust on ice, mixed-phase and liquid water clouds

2010 ◽  
Vol 10 (2) ◽  
pp. 4027-4077 ◽  
Author(s):  
A. Wiacek ◽  
T. Peter ◽  
U. Lohmann
Keyword(s):  
Liquid Water ◽  
Mineral Dust ◽  
Dust Emission ◽  
Ice Nucleation ◽  
Mixed Phase ◽  
Specific Humidity ◽  
Dust Particles ◽  
Ice Clouds ◽  
Cloud Processing ◽  
Mixed Phase Clouds

Abstract. This modelling study explores the availability of mineral dust particles as ice nuclei for interactions with ice, mixed-phase and liquid water clouds, also tracking the particles' history of cloud-processing. We performed 61 320 one-week forward trajectory calculations originating near the surface of major dust emitting regions in Africa and Asia using high-resolution meteorological analysis fields for the year 2007. Without explicitly modelling dust emission and deposition processes, dust-bearing trajectories were assumed to be those coinciding with known dust emission seasons. We found that dust emissions from Asian deserts lead to a higher potential for interactions with high clouds, despite being the climatologically much smaller dust emission source. This is due to Asian regions experiencing significantly more ascent than African regions, with strongest ascent in the Asian Taklimakan desert at ~25%, ~40% and 10% of trajectories ascending to 300 hPa in spring, summer and fall, respectively. The specific humidity at each trajectory's starting point was transported in a Lagrangian manner and relative humidities with respect to water and ice were calculated in 6-h steps downstream, allowing us to estimate the formation of liquid, mixed-phase and ice clouds. Practically none of the simulated air parcels reached regions where homogeneous ice nucleation can take place (T≲−40 °C) along trajectories that have not experienced water saturation first. By far the largest fraction of cloud forming trajectories entered conditions of mixed-phase clouds, where mineral dust will potentially exert the biggest influence. The majority of trajectories also passed through regions supersaturated with respect to ice but subsaturated with respect to water, where "warm" (T≳−40 °C) ice clouds may form prior to supercooled water or mixed-phase clouds. The importance of "warm" ice clouds and the general influence of dust in the mixed-phase cloud region are highly uncertain due to considerable scatter in recent laboratory data from ice nucleation experiments, which we briefly review in this work. For "classical" cirrus-forming temperatures, our results show that only mineral dust IN that underwent mixed-phase cloud-processing previously are likely to be relevant, and, therefore, we recommend further systematic studies of immersion mode ice nucleation on mineral dust suspended in atmospherically relevant coatings.

scholarly journals The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2

2018 ◽  
Vol 18 (12) ◽  
pp. 8807-8828 ◽  
Author(s):  
Ulrike Lohmann ◽  
David Neubauer
Keyword(s):  
Climate Sensitivity ◽  
Liquid Water ◽  
Climate Model ◽  
Supercooled Liquid ◽  
Cloud Feedback ◽  
Mixed Phase ◽  
Shortwave Radiation ◽  
Ice Clouds ◽  
Cloud Optical Depth ◽  
Mixed Phase Clouds

Abstract. How clouds change in a warmer climate remains one of the largest uncertainties for the equilibrium climate sensitivity (ECS). While a large spread in the cloud feedback arises from low-level clouds, it was recently shown that mixed-phase clouds are also important for ECS. If mixed-phase clouds in the current climate contain too few supercooled cloud droplets, too much ice will change to liquid water in a warmer climate. As shown by Tan et al. (2016), this overestimates the negative cloud-phase feedback and underestimates ECS in the CAM global climate model (GCM). Here we use the newest version of the ECHAM6-HAM2 GCM to investigate the importance of mixed-phase and ice clouds for ECS. Although we also considerably underestimate the fraction of supercooled liquid water globally in the reference version of the ECHAM6-HAM2 GCM, we do not obtain increases in ECS in simulations with more supercooled liquid water in the present-day climate, different from the findings by Tan et al. (2016). We hypothesize that it is not the global supercooled liquid water fraction that matters, but only how well low- and mid-level mixed-phase clouds with cloud-top temperatures in the mixed-phase temperature range between 0 and −35 ∘C that are not shielded by higher-lying ice clouds are simulated. These occur most frequently in midlatitudes, in particular over the Southern Ocean where they determine the amount of absorbed shortwave radiation. In ECHAM6-HAM2 the amount of absorbed shortwave radiation over the Southern Ocean is only significantly overestimated if all clouds below 0 ∘C consist exclusively of ice. Only in this simulation is ECS significantly smaller than in all other simulations and the cloud optical depth feedback is the dominant cloud feedback. In all other simulations, the cloud optical depth feedback is weak and changes in cloud feedbacks associated with cloud amount and cloud-top pressure dominate the overall cloud feedback. However, apart from the simulation with only ice below 0 ∘C, differences in the overall cloud feedback are not translated into differences in ECS in our model. This insensitivity to the cloud feedback in our model is explained with compensating effects in the clear sky.

scholarly journals The potential influence of Asian and African mineral dust on ice, mixed-phase and liquid water clouds

2010 ◽  
Vol 10 (18) ◽  
pp. 8649-8667 ◽  
Author(s):  
A. Wiacek ◽  
T. Peter ◽  
U. Lohmann
Keyword(s):  
Liquid Water ◽  
Mineral Dust ◽  
Dust Emission ◽  
Ice Nucleation ◽  
Transport Processes ◽  
Mixed Phase ◽  
Ice Clouds ◽  
Cloud Processing ◽  
Ice Nuclei ◽  
Mixed Phase Clouds

Abstract. This modelling study explores the availability of mineral dust particles as ice nuclei for interactions with ice, mixed-phase and liquid water clouds, also tracking the particles' history of cloud-processing. We performed 61 320 one-week forward trajectory calculations originating near the surface of major dust emitting regions in Africa and Asia using high-resolution meteorological analysis fields for the year 2007. Dust-bearing trajectories were assumed to be those coinciding with known dust emission seasons, without explicitly modelling dust emission and deposition processes. We found that dust emissions from Asian deserts lead to a higher potential for interactions with high ice clouds, despite being the climatologically much smaller dust emission source. This is due to Asian regions experiencing significantly more ascent than African regions, with strongest ascent in the Asian Taklimakan desert at ~25%, ~40% and 10% of trajectories ascending to 300 hPa in spring, summer and fall, respectively. The specific humidity at each trajectory's starting point was transported in a Lagrangian manner and relative humidities with respect to water and ice were calculated in 6-h steps downstream, allowing us to estimate the formation of liquid, mixed-phase and ice clouds. Downstream of the investigated dust sources, practically none of the simulated air parcels reached conditions of homogeneous ice nucleation (T≲−40 °C) along trajectories that have not experienced water saturation first. By far the largest fraction of cloud forming trajectories entered conditions of mixed-phase clouds, where mineral dust will potentially exert the biggest influence. The majority of trajectories also passed through atmospheric regions supersaturated with respect to ice but subsaturated with respect to water, where so-called "warm ice clouds" (T≳−40 °C) theoretically may form prior to supercooled water or mixed-phase clouds. The importance of "warm ice clouds" and the general influence of dust in the mixed-phase cloud region are highly uncertain due to both a considerable scatter in recent laboratory data from ice nucleation experiments, which we briefly review in this work, and due to uncertainties in sub-grid scale vertical transport processes unresolved by the present trajectory analysis. For "classical" cirrus-forming temperatures (T≲−40 °C), our results show that only mineral dust ice nuclei that underwent mixed-phase cloud-processing, most likely acquiring coatings of organic or inorganic material, are likely to be relevant. While the potential paucity of deposition ice nuclei shown in this work dimishes the possibility of deposition nucleation, the absence of liquid water droplets at T≲−40 °C makes the less explored contact freezing mechanism (involving droplet collisions with bare ice nuclei) highly inefficient. These factors together indicate the necessity of further systematic studies of immersion mode ice nucleation on mineral dust suspended in atmospherically relevant coatings.

scholarly journals Cloud phase identification of low-level Arctic clouds from airborne spectral radiation measurements: test of three approaches

2008 ◽  
Vol 8 (4) ◽  
pp. 15901-15939 ◽  
Author(s):  
A. Ehrlich ◽  
E. Bierwirth ◽  
M. Wendisch ◽  
J.-F. Gayet ◽  
G. Mioche ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Mixed Phase ◽  
Ice Crystals ◽  
The Arctic ◽  
Pure Liquid ◽  
Phase Identification ◽  
Ice Clouds ◽  
Mixed Phase Clouds

Abstract. Boundary layer clouds were investigated with a complementary set of remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds that formed in a cold air outbreak over the open Greenland sea showed a variety in their thermodynamic state. Beside the predominant mixed-phase clouds pure liquid and ice clouds were observed. Utilizing the measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud were defined and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) characterized by ice and water absorption. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of cloud albedo and reflectance. Radiative transfer simulations showed that IS, IP and IA range between 5 to 80, 0 to 20 and 1 to 1.25, respectively, with lowest values indicating pure liquid water clouds and highest values pure ice clouds. IS and IP were found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. IA has the disadvantage that this index is mainly dominated by the uppermost cloud layer (τ<1.5). Typical boundary layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements showed a good agreement in identifying the dominant mixed-phase clouds and a pure ice cloud at the edge of the cloud field.

scholarly journals Precipitation regimes over central Greenland inferred from 5 years of ICECAPS observations

2017 ◽  
Author(s):  
Claire Pettersen ◽  
Ralf Bennartz ◽  
Aronne J. Merrelli ◽  
Matthew D. Shupe ◽  
David D. Turner ◽  
...  
Keyword(s):  
Liquid Water ◽  
Large Scale ◽  
Greenland Ice Sheet ◽  
Mixed Phase ◽  
Back Trajectory ◽  
Spectral Variation ◽  
Ice Clouds ◽  
Cloud Liquid Water ◽  
Precipitation Regimes ◽  
Mixed Phase Clouds

Abstract. A novel method for classifying Arctic precipitation using ground based remote sensors is presented. Using differences in the spectral variation of microwave absorption and scattering properties of cloud liquid water and ice, this method can distinguish between different types of snowfall events depending on the presence or absence of condensed liquid water in the clouds that generate the precipitation. The classification reveals two distinct, primary regimes of precipitation over the Greenland Ice Sheet (GIS): one originating from fully glaciated ice clouds and the other from mixed-phase clouds. Five years of co-located, multi-instrument data from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit (ICECAPS) are used to examine cloud and meteorological properties and patterns associated with each precipitation regime. The occurrence and accumulation of the precipitation regimes are identified and quantified. Cloud and precipitation observations from additional ICECAPS instruments illustrate distinct characteristics for each regime. Additionally, reanalysis products and back-trajectory analysis show different synoptic- scale forcings associated with each regime. Precipitation over the central GIS exhibits unique microphysical characteristics due to the high surface elevations as well as connections to specific large-scale flow patterns. Snowfall originating from the ice clouds is coupled to deep, frontal cloud systems advecting up and over the southeast Greenland coast to the central GIS. These events appear to be associated with individual storm systems generated by low pressure over Baffin Bay and Greenland lee cyclogenesis. Snowfall originating from mixed-phase clouds is shallower and has characteristics typical of supercooled cloud liquid water layers, and slowly propagates from the south and southwest Greenland along a quiescent flow above the GIS.

scholarly journals Statistics on clouds and their relation to thermodynamic conditions at Ny-Ålesund using ground-based sensor synergy

2018 ◽  
Author(s):  
Tatiana Nomokonova ◽  
Kerstin Ebell ◽  
Ulrich Löhnert ◽  
Marion Maturilli ◽  
Christoph Ritter ◽  
...  
Keyword(s):  
Microwave Radiometer ◽  
Single Layer ◽  
Mixed Phase ◽  
The Arctic ◽  
Time Data ◽  
Ice Clouds ◽  
Mean Values ◽  
Cloud Temperature ◽  
Thermodynamic Conditions ◽  
Mixed Phase Clouds

Abstract. The French–German Arctic Research Base AWIPEV at Ny-Ålesund, Svalbard, is an unique station for monitoring cloud related processes in the Arctic. For the first time, data from a set of ground-based instruments at AWIPEV observatory are analyzed to characterize the vertical structure of clouds. For this study, a 14-month dataset from Cloudnet combining observations from a ceilometer, a 94 GHz cloud radar and a microwave radiometer, is used. The total cloud occurrence of 81 %, with 44.8 % of multi-layer and 36 % of single-layer clouds was found. Among single-layer clouds the occurrence of liquid, ice and mixed-phase clouds was 6.4 %, 9 % and 20.6 %, respectively. It was found, that more than 90 % of single-layer liquid and mixed-phase clouds have LWP values lower than 100 and 200 g m2, respectively. Mean values of IWP for ice and mixed-phase clouds were found to be 273 and 164 g m2, respectively. The different types of single-layer clouds are also related to in-cloud temperature and relative humidity under which they occur. Statistics based on observations are compared to the ICON model output. Distinct differences in liquid phase occurrence in observations and the model at different environmental temperatures leading to higher occurrence of pure ice clouds and lower occurrence of mixed-phase clouds in the model at temperatures between −20° and −5 °C become evident. The analyzed dataset is useful for satellite validation and model evaluation.

scholarly journals Physical properties of High Arctic tropospheric particles during winter

2009 ◽  
Vol 9 (2) ◽  
pp. 7781-7823 ◽  
Author(s):  
L. Bourdages ◽  
T. J. Duck ◽  
G. Lesins ◽  
J. R. Drummond ◽  
E. W. Eloranta
Keyword(s):  
Boundary Layer ◽  
Inversion Layer ◽  
Lower Troposphere ◽  
High Arctic ◽  
Mixed Phase ◽  
Ice Crystals ◽  
Blowing Snow ◽  
Ice Clouds ◽  
Arctic Conditions ◽  
Mixed Phase Clouds

Abstract. A climatology of particle properties in the wintertime High Arctic troposphere is constructed using measurements from a lidar and cloud radar located at Eureka, Nunavut Territory (80° N, 86° W). Four different particle groupings are considered: aerosols, mixed-phase clouds, ice clouds and boundary-layer ice crystals. Two-dimensional histograms of occurrence probabilities against depolarization and radar/lidar colour ratio, as well as their vertical distributions, are presented. The largest ice crystals originate from mixed-phase clouds, whereas the smallest are topographic blowing snow residuals in the boundary layer. Ice cloud crystals have depolarization and size decreasing with height. The depolarization trend is associated with the large ice crystal sub-population. Small crystals depolarize more than large ones in ice clouds at a given altitude, and show constant modal depolarization with height. Ice clouds in the mid-troposphere are sometimes observed to precipitate to the ground. Water clouds are constrained to the lower troposphere and are associated with the surface inversion layer depth. Aerosols are most abundant near the ground and are frequently mixed with the other particle types. The data are used to construct a classification chart for particle scattering in wintertime Arctic conditions.

scholarly journals Precipitation regimes over central Greenland inferred from 5 years of ICECAPS observations

2018 ◽  
Vol 18 (7) ◽  
pp. 4715-4735 ◽  
Author(s):  
Claire Pettersen ◽  
Ralf Bennartz ◽  
Aronne J. Merrelli ◽  
Matthew D. Shupe ◽  
David D. Turner ◽  
...  
Keyword(s):  
Liquid Water ◽  
Large Scale ◽  
Greenland Ice Sheet ◽  
Mixed Phase ◽  
Back Trajectory ◽  
Spectral Variation ◽  
Ice Clouds ◽  
Cloud Liquid Water ◽  
Precipitation Regimes ◽  
Mixed Phase Clouds

Abstract. A novel method for classifying Arctic precipitation using ground based remote sensors is presented. Using differences in the spectral variation of microwave absorption and scattering properties of cloud liquid water and ice, this method can distinguish between different types of snowfall events depending on the presence or absence of condensed liquid water in the clouds that generate the precipitation. The classification reveals two distinct, primary regimes of precipitation over the Greenland Ice Sheet (GIS): one originating from fully glaciated ice clouds and the other from mixed-phase clouds. Five years of co-located, multi-instrument data from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit (ICECAPS) are used to examine cloud and meteorological properties and patterns associated with each precipitation regime. The occurrence and accumulation of the precipitation regimes are identified and quantified. Cloud and precipitation observations from additional ICECAPS instruments illustrate distinct characteristics for each regime. Additionally, reanalysis products and back-trajectory analysis show different synoptic-scale forcings associated with each regime. Precipitation over the central GIS exhibits unique microphysical characteristics due to the high surface elevations as well as connections to specific large-scale flow patterns. Snowfall originating from the ice clouds is coupled to deep, frontal cloud systems advecting up and over the southeast Greenland coast to the central GIS. These events appear to be associated with individual storm systems generated by low pressure over Baffin Bay and Greenland lee cyclogenesis. Snowfall originating from mixed-phase clouds is shallower and has characteristics typical of supercooled cloud liquid water layers, and slowly propagates from the south and southwest of Greenland along a quiescent flow above the GIS.

scholarly journals Properties of Arctic liquid and mixed phase clouds from ship-borne Cloudnet observations during ACSE 2014

2020 ◽  
Author(s):  
Peggy Achtert ◽  
Ewan J. O'Connor ◽  
Ian M. Brooks ◽  
Georgia Sotiropoulou ◽  
Matthew D. Shupe ◽  
...  
Keyword(s):  
Water Content ◽  
Liquid Water ◽  
Liquid Water Content ◽  
Mixed Phase ◽  
Cloud Base ◽  
Ice Clouds ◽  
Ice Water Content ◽  
Mixed Phase Clouds ◽  
Ice Water ◽  
Cloud Top Temperature

Abstract. This study presents Cloudnet retrievals of Arctic clouds from measurements conducted during a three-month research expedition along the Siberian shelf during summer and autumn 2014. During autumn, we find a strong reduction in the occurrence of liquid clouds and an increase for both mixed-phase and ice clouds at low levels compared to summer. About 80 % of all liquid clouds observed during the research cruise show a liquid water path below the infra-red black body limit of approximately 50 g m−2. The majority of mixed-phase and ice clouds had an ice water path below 20 g m−2. Cloud properties are analysed with respect to cloud-top temperature and boundary layer structure. Changes in these parameters have little effect on the geometric thickness of liquid clouds while mixed-phase clouds during warm-air advection events are generally thinner than when such events were absent. Cloud-top temperatures are very similar for all mixed-phase clouds. However, more cases of lower cloud-top temperature were observed in the absence of warm-air advection. Profiles of liquid and ice water content are normalised with respect to cloud base and height. For liquid water clouds, the liquid water content profile reveals a strong increase with height with a maximum within the upper quarter of the clouds followed by a sharp decrease towards cloud top. Liquid water content is lowest for clouds observed below an inversion during warm-air advection events. Most mixed-phase clouds show a liquid water content profile with a very similar shape to that of liquid clouds but with lower maximum values during warm-air advection. The normalised ice water content profiles in mixed-phase clouds look different from that of liquid water content. They show a wider range in maximum values with lowest ice water content for clouds below an inversion and highest values for clouds above or extending through an inversion. The ice water content profile generally peaks at a height below the peak in the liquid water content profile – usually in the centre of the cloud, sometimes closer to cloud base, likely due to particle sublimation as the crystals fall through the cloud.

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