Coalescence and Secondary Ice Development in Cumulus Congestus Clouds

Understanding ice development in Cumulus Congestus (CuCg) clouds, which are ubiquitous globally, is critical for improving our knowledge of cloud physics, cloud resolution and climate prediction models. Results presented here are representative of data collected in 1,008 penetrations of moderate to strong updrafts in CuCg clouds by five research aircraft in six geographic locations. The results show that CuCg with warm (> ∼20°C) cloud base temperatures, such as in tropical marine environments, experience a strong collision-coalescence process. Development of coalescence is also correlated with drop effective radius > ∼12 to 14 µm in diameter. Increasing the cloud-base drop concentration with diameters from 15 to 35 µm and decreasing the drop concentration < 15 µm appears to enhance coalescence. While the boundary-layer aerosol population is not a determinate factor in development of coalescence in tropical marine environments, its impact on coalescence is not yet fully determined. Some supercooled large drops generated via coalescence fracture when freezing, producing a secondary ice process (SIP) with production of copious small ice particles that naturally seed the cloud. The SIP produces an avalanche effect, freezing the majority of supercooled liquid water before fresh updrafts reach the −16°C level. Conversely, CuCg with cloud base temperatures ≤ ∼8°C develop significant concentrations of ice particles at colder temperatures, so that small supercooled water drops are lofted to higher elevations before freezing. Recirculation of ice in downdrafts at the edges of updrafts appears to be the primary mechanism for development of precipitation in CuCg with colder cloud base temperatures.

The Mount Everest plume in winter

Mt. Everest’s summit pyramid is the highest obstacle on earth to the wintertime jet-stream winds. Downwind, in its wake, a visible plume often forms. The meteorology and composition of the plume are unknown. Accordingly, we observed real-time images from a geosynchronous meteorological satellite from November 2020 through March 2021 to identify plumes and collect the corresponding meteorological data. We used the data with a basic meteorological model to show the plumes formed when sufficiently moist air was drawn into the wake. We conclude the plumes were composed initially of either cloud droplets or ice particles depending on the temperature. One plume was observed to glaciate downwind. Thus, Everest plumes may be a source of snowfall formed insitu. The plumes, however, were not composed of resuspended snow.

Mass of different snow crystal shapes derived from fall speed measurements

Meteorological forecast and climate models require good knowledge of the microphysical properties of hydrometeors and the atmospheric snow and ice crystals in clouds, for instance, their size, cross-sectional area, shape, mass, and fall speed. Especially shape is an important parameter in that it strongly affects the scattering properties of ice particles and consequently their response to remote sensing techniques. The fall speed and mass of ice particles are other important parameters for both numerical forecast models and the representation of snow and ice clouds in climate models. In the case of fall speed, it is responsible for the rate of removal of ice from these models. The particle mass is a key quantity that connects the cloud microphysical properties to radiative properties. Using an empirical relationship between the dimensionless Reynolds and Best numbers, fall speed and mass can be derived from each other if particle size and cross-sectional area are also known. In this study, ground-based in situ measurements of snow particle microphysical properties are used to analyse mass as a function of shape and the other properties particle size, cross-sectional area, and fall speed. The measurements for this study were done in Kiruna, Sweden, during snowfall seasons of 2014 to 2019 and using the ground-based in situ Dual Ice Crystal Imager (D-ICI) instrument, which takes high-resolution side- and top-view images of natural hydrometeors. From these images, particle size (maximum dimension), cross-sectional area, and fall speed of individual particles are determined. The particles are shape-classified according to the scheme presented in our previous study, in which particles sort into 15 different shape groups depending on their shape and morphology. Particle masses of individual ice particles are estimated from measured particle size, cross-sectional area, and fall speed. The selected dataset covers sizes from about 0.1 to 3.2 mm, fall speeds from 0.1 to 1.6 m s−1, and masses from 0.2 to 450 µg. In our previous study, the fall speed relationships between particle size and cross-sectional area were studied. In this study, the same dataset is used to determine the particle mass, and consequently, the mass relationships between particle size, cross-sectional area, and fall speed are studied for these 15 shape groups. Furthermore, the mass relationships presented in this study are compared with the previous studies. For certain crystal habits, in particular columnar shapes, the maximum dimension is unsuitable for determining Reynolds number. Using a selection of columns, for which the simple geometry allows the verification of an empirical Best-number-to-Reynolds-number relationship, we show that Reynolds number and fall speed are more closely related to the diameter of the basal facet than the maximum dimension. The agreement with the empirical relationship is further improved using a modified Best number, a function of an area ratio based on the falling particle seen in the vertical direction.

Secondary ice production during the break-up of freezing water drops on impact with ice particles

We provide the first dedicated laboratory study of collisions of supercooled water drops with ice particles as a secondary ice production mechanism. We experimentally investigated collisions of supercooled water drops (∼ 5 mm in diameter) with ice particles of a similar size (∼ 6 mm in diameter) placed on a glass slide at temperatures >-12 ∘C. Our results showed that secondary drops were generated during both the spreading and retraction phase of the supercooled water drop impact. The secondary drops generated during the spreading phase were emitted too fast to quantify. However, quantification of the secondary drops generated during the retraction phase with diameters >0.1 mm showed that 5–10 secondary drops formed per collision, with approximately 30 % of the secondary drops freezing over a temperature range between −4 and −12 ∘C. Our results suggest that this secondary ice production mechanism may be significant for ice formation in atmospheric clouds containing large supercooled drops and ice particles.

No evidence for a productive Hallett-Mossop process so far

&lt;p&gt;Mixed-phase clouds are essential elements in Earth&amp;#8217;s weather and climate system. Atmospheric observation of mixed-phase clouds occasionally demonstrated a strong discrepancy between the observed ice particle and ice nucleating particle number concentration of one to four orders of magnitude at modest supercooling [1-3, 5, 7]. Different secondary ice production (SIP) mechanisms have been hypothesized which can increase the total ice particle number concentration by multiplication of primary ice particles and hence might explain the observed discrepancy [2, 4, 6].&lt;/p&gt; &lt;p&gt;In this study we focus on SIP as a result of droplet-ice collisions, commonly known as Hallett-Mossop [9] or rime-splintering process. Our main objectives are (i) to quantify secondary ice particles and (ii) to learn more about the underlying physics. Therefore, we develop a new experimental set-up (Ice Droplets splintEring on FreezIng eXperiment, IDEFIX) in which quasi-monodisperse supercooled droplets collide with a fixed ice particle. IDEFIX is designed to simulate atmospheric relevant conditions such as temperature, humidity, impact velocities and collision rates. The riming process is observed with high-speed video microscopy and an infrared measuring system. Further, the produced secondary ice particles are counted via impaction on a supercooled sugar solution. Preliminary results from a first measurement campaign suggest that we observed single SIP events but did not found evidence for a productive Hallett-Mossop process so far.&amp;#160; We plan to continue with rime-splintering experiment in order to gain better statistics and to expand the parameter space (e.g., droplet size distribution).&lt;/p&gt; &lt;p&gt;[1] Crosier, J., et al. 2011, DOI: 10.5194/acp-11-257-2011.&lt;/p&gt; &lt;p&gt;[2] Field, P.R., et al. 2016, DOI: 10.1175/amsmonographs-d-16-0014.1.&lt;/p&gt; &lt;p&gt;[3] Hogan, R.J., et al. 2002, DOI: 10.1256/003590002321042054.&lt;/p&gt; &lt;p&gt;[4] Korolev, A. and T. Leisner 2020, DOI: 10.5194/acp-20-11767-2020.&lt;/p&gt; &lt;p&gt;[5] Mossop, S.C. 1985, DOI: 10.1175/1520-0477(1985)066&lt;0264:toacoi&gt;2.0.co;2.&lt;/p&gt; &lt;p&gt;[6] Sotiropoulou, G., et al. 2020, DOI: 10.5194/acp-20-1301-2020.&lt;/p&gt; &lt;p&gt;[7] Taylor, J.W., et al. 2016, DOI: 10.5194/acp-16-799-2016.&lt;/p&gt;

Properties of Cirrus Clouds over the European Arctic (Ny-Ålesund, Svalbard)

Cirrus is the only cloud type capable of inducing daytime cooling or heating at the top of the atmosphere (TOA) and the sign of its radiative effect highly depends on its optical depth. However, the investigation of its geometrical and optical properties over the Arctic is limited. In this work the long-term properties of cirrus clouds are explored for the first time over an Arctic site (Ny-Ålesund, Svalbard) using lidar and radiosonde measurements from 2011 to 2020. The optical properties were quality assured, taking into account the effects of specular reflections and multiple-scattering. Cirrus clouds were generally associated with colder and calmer wind conditions compared to the 2011–2020 climatology. However, the dependence of cirrus properties on temperature and wind speed was not strong. Even though the seasonal cycle was not pronounced, the winter-time cirrus appeared under lower temperatures and stronger wind conditions. Moreover, in winter, geometrically- and optically-thicker cirrus were found and their ice particles tended to be more spherical. The majority of cirrus was associated with westerly flow and westerly cirrus tended to be geometrically-thicker. Overall, optically-thinner layers tended to comprise smaller and less spherical ice crystals, most likely due to reduced water vapor deposition on the particle surface. Compared to lower latitudes, the cirrus layers over Ny-Ålesund were more absorbing in the visible spectral region and they consisted of more spherical ice particles.