Ice Production Process Research Articles

A modelling study of ice‐spectrum modes in deep frontal clouds

AbstractResults from a one‐dimensional mixed‐layer model with explicit microphysics and a discrete ice‐spectrum were compared to in situ data provided by the Met Office C‐130 aircraft. Bimodal ice‐spectra were observed. The first mode was found at diameters near the lower limit of the instrumentation resolution (25 µm) at a roughly constant concentration. The second mode was found initially at diameters exceeding 200 µm but it shifted to larger sizes and smaller peak concentrations as depths below the cloud top increased.The objectives of the paper are threefold: validation of the model with real cloud data; an analysis of parameters controlling the rate of aggregation, and to show that a secondary ice‐production process is needed to maintain the first mode and produce a second mode which is sensible.A reasonable first mode could only be maintained by producing a constant supply of small ice particles at a rate of at least 10 m−3s−1. Primary nucleation was unable to supply this rate of ice‐particle production at temperatures above −20°C. In the model, it was achieved by invoking a secondary ice‐particle production process, such as the break‐up of evaporating ice crystals. The rate of production required is in broad agreement with laboratory investigations of secondary ice‐particle production during evaporation.The relative importance of the processes of aggregation, deposition and sedimentation to the production of the bimodal spectra is also studied. While it is possible to grow ice particles to large sizes by deposition alone, the number concentration observed in these clouds produces so much competition for water vapour that deposition growth is very limited. The aggregation process is the strongest mechanism for the production of the larger ice‐particles, which then precipitate. Consequently, its strength controls the rate of small ice‐particle production needed, and so is mainly responsible for the second mode. Copyright © Royal Meteorological Society, 2003. P. R. Field's contribution is Crown copyright.

A laboratory study on supercooling and frazil ice production processes in winter coastal polynyas

The rates of frazil ice production and salt flux in wind‐generated open water were estimated on the basis of results from laboratory experiments that model coastal polynyas in a severe winter. The measurements were made under various conditions of wind speed, air temperature, and water salinity. The rate of ice production increased with increasing wind speed and water salinity, and with decreasing air temperature. The high rates of ice production are attributed to the vigorous production of underwater frazil crystals. Salt fluxes due to frazil production were also much greater than those of sheet ice growing vertically under calm atmospheric conditions. The important factor that governs such a process of high ice production lies in supercooling and underwater ice production. In the case of common seawater of salinities larger than 25‰, supercooled water that formed on the surface sinks into the underlying seawater at its freezing point by a density instability as well as by a wind‐forced convection. The sinking of the supercooled water and the resulting production of numerous frazil crystals under the water lead to the maintenance of open water for a long period. Thus large heat fluxes occur through the open water that is continuously exposed to cold air, producing a large amount of the underwater ice at high rates. This mechanism for ice production in open waters is driven by a strong, cold wind and is enhanced by increasing water salinity.