Ice and Mixed Phase Clouds
At temperatures below 0C cloud particles may be composed of either supercooled liquid water or ice. The processes involved in ice particle formation are more complicated and less well understood than for water droplet formation. When a cloud rises through the freezing level droplets do not instantaneously freeze, in fact, a pure water droplet may continue to exist as a supercooled liquid drop down to temperatures approaching -40C. However, ice particles do form at much warmer temperatures than -40C via a number of ice nucleation and multiplication processes, so it is not unusual to observe fully glaciated clouds at temperatures as warm as -10C. Ice particles may form either by freezing (a liquid droplet freezes to create an ice particle) or deposition (where material is deposited directly from the vapour phase to create an ice particle). In each case ice particle production may occur homogeneously (only water molecules involved) or heterogeneously (other solid material assists the process). In the atmosphere ice particles may be formed by three processes: homogeneous freezing; heterogeneous freezing and heterogeneous deposition. Homogeneous deposition does not occur under atmospheric conditions.
Homogeneous freezing is the process by which a supercooled liquid drop freezes without the assistance of an ice nuclei. Homogeneous freezing becomes statistically more likely as temperature decreases such that below -39C all drops will freeze. The temperature at which homogeneous freezing occurs is affected by the presence of dissolved material in the droplet, especially when the droplet is a highly concentrated solution such as is the case for haze droplets.
Heterogeneous freezing is the process by which a supercooled liquid drop freezes with the assistance of a solid aerosol particle which is able to act as an ice nuclei. Heterogeneous freezing is thought to operate via several different modes: immersion nucleation, here a solid particle within an existing drop acts as a nuclei for ice formation and the droplet freezes; condensation nucleation, here water vapour condenses onto a solid particle to form a droplet, the particle then acts as an immersion nuclei; contact nucleation, here a solid particle is in collision with an existing droplet and initiates freezing of that drop.
Heterogeneous deposition is the process by which water vapour is deposited onto an ice nuclei and takes on a crystalline form directly without first being in the liquid phase. While this is considered to be theoretically possible in the atmosphere, there is no clear experimental evidence that this is an important atmospheric process.
The exact properties required for an aerosol particle to act as an ice nuclei are not fully known and may well be different for each of the modes above. Also it is likely that the temperature at which each nucleation mode will become active is different even for the same type of particle. This is an area of considerable research effort in laboratory ice cloud studies. In general it is believed that ice nuclei are quite different to droplet nuclei, while droplet nuclei are soluble, ice nuclei are generally insoluble. Factors such as size shape and crystal structure are believed to be important for ice nuclei. Aerosol such as mineral particles (e.g. desert dust etc.), soot and certain bacteria have been observed to act as ice nuclei and these all have very different characteristics. Additionally droplets with certain alcohol monolayer coatings have been found to freeze at much warmer temperatures than would be expected for homogeneous nucleation. Measurements of ice nuclei concentrations in the atmosphere are rather difficult, but there is evidence that only a small fraction of the aerosol population are able to act as ice nuclei and that this fraction increases with increasing particle size. This fraction is much lower than the fraction of particles which are able to act as droplet nuclei, with typical ice nuclei concentrations being around 1-10/litre.
Measurements of ice particle concentrations in cloud are often several orders of magnitude higher than ice nuclei concentrations. This is because there are several ice multiplication or secondary ice production mechanisms whereby more ice particles are produced from existing ice crystals in the cloud. Mechanisms which are currently thought to be important are mechanical fracturing of evaporating crystals; fragmentation of large drops during freezing and splinter formation during riming of ice crystals. Riming occurs when a liquid droplet is in collision with an existing ice crystal and freezes instantly, becoming part of the crystal. This last process is generally referred to as the Hallett-Mossop process. The ice splinters produced by these processes may initiate the freezing of additional drops by contact nucleation, or may grow into larger crystals by vapour deposition.
Once crystals are nucleated they may grow rapidly by vapour diffusion, especially in the presence of super cooled water, as it is energetically more favourable for water to exist as ice rather than liquid at such temperatures. Thus ice crystals grow at the expense of droplets which evaporate; this is known as the Bergeron process. Under these conditions a 15µm ice particle may grow at a rate of up to 1µm/s.
Ice crystals have a basic hexagonally symmetrical structure, but may grow into a number of different habits depending on the conditions (temperature, pressure and supersaturation) under which it has grown. Common habits found in clouds in the atmosphere include hexagonal plates, hollow and solid hexagonal columns, needles, stellar and branched habits. Often growth will begin independently from multiple points on the nuclei resulting in more complex structures which may be composed of several crystals of the same habit, or even mixtures of several habits. Additionally aggregate crystals may form as particles collide with each other within the cloud, and in mixed phase clouds particles often become rimed by collision with droplets. Many laboratory studies have been carried out over the years to determine which growth conditions produce which type of crystal, thus it is possible to infer the growth conditions which have been present within a cloud by examining the type of crystals present. Some common crystal habits are listed in the table below, illustrated with examples of the habit observed in the atmosphere using the Cloud Particle Imager. Some information is given about conditions of crystal formation, though studies are still ongoing to fully determine the conditions under which different crystals are formed, and there are still some differences between lab experiments carried out in this field. In addition to the mostly regular habits shown in the table, in many cases atmospheric clouds contain a very significant number of completely irregular ice particles of all sizes.
Different crystal habits all interact with radiation differently to each other, and differently to water drops in warm clouds, thus radiative properties of ice clouds are dependant on the habit of crystals present. Regular habits also scatter differently depending on their orientation relative to incident light, if crystal orientation is random as may be expected then such effects cancel out. However under certain conditions crystals have been observed to all take on the same orientation, with a significant impact on the optical properties of the cloud. Some spectacular optical effects may be produced by ice clouds composed of orientated regular crystals.
Table of Ice Crystal Habits.
Plates are formed at a wide range of temperatures warmer than -40C and at a wide range supersaturations. Plate thickness is also dependant on temperature and supersaturation and at temperatures between -20 and -30C, close to water saturation skeletal plates are observed. The thin plates in this figure probably formed at temperatures between -10 and -20 close to water saturation.
Columns are formed at colder temperatures than plates generally below -40C at a range of supersaturations. As with plates, column aspect ratio is influenced by temperature and supersaturation. Additionally columns may be solid, hollow, or scroll like. This image shows a variety of column types.
Rosettes are formed at temperatures below about -50C, often at quite high supersaturation (typically greater than water saturation) and are often found in frontal or aged anvil cirrus cloud. It is thought that these crystals grow from supercooled drops (or possibly haze droplets) which have undergone homogeneous freezing.
Crystal with sector like branches
These crystals are typically formed at temperatures around -15C at supersaturations greater than those at which plates form, but less than those at which broad branched crystals (above) form. There is however a range of temperatures and supersaturations between about -13 and -17C at which these crystals may form.
Riming occurs in mixed phase clouds when water droplets are in collision with ice crystals, the majority of riming occurs at temperatures between -5 and -25C. The collision causes the droplet to freeze instantaneously maintaining an almost spherical shape. The figure shows crystals with just a few rime particle attached, where the original crystal shape is clearly distinguishable, and others where there is so much rime that it is impossible to determine the original crystal habit.
Formed from assemblages of plates and plate like habits. These crystals have been observed in anvil cloud produced by tropical convection, and in the laboratory under conditions of high electric field strengths. It is thought that charging of ice crystals in strong electric fields may favour the edge-edge chain arrangement of these crystals, which would not be expected to occur frequently in aggregates formed by random collisions.