John Hallett

Production of secondary ice particles during the riming process

ICE crystals play a vital part in the formation of precipitation, particularly outside the tropics, so it is important to know the concentrations of ice particles in natural clouds and to attempt to predict them from measurements made, for example in laboratory cloud chambers, at the same temperature.

A comprehensive habit diagram for atmospheric ice crystals : Confirmation from the laboratory, AIRS II, and other field studies

Abstract Recent laboratory experiments and in situ observations have produced results in broad agreement with respect to ice crystal habits in the atmosphere. These studies reveal that the ice crystal habit at −20°C is platelike, extending to −40°C, and not columnar as indicated in many habit diagrams found in atmospheric science journals and texts. These diagrams were typically derived decades ago from laboratory studies, some with inherent habit bias, or from combinations of laboratory and in situ observations at the ground, observations that often did not account for habit modification by precipitation from overlying clouds of varying temperatures. Habit predictions from these diagrams often disagreed with in situ observations at temperatures below −20°C. More recent laboratory and in situ studies have achieved a consensus on atmospheric ice crystal habits that differs from the traditional habit diagrams. These newer results can now be combined to give a comprehensive description of ice crystal habits ...

Ice particle habits in Arctic clouds

Ice crystals in atmospheric clouds have shapes, which affect their density, terminal fall velocity, growth rate and radiative properties. In calculations for climate change predictions, weather forecasting of precipitation, and remote sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are assumed. Using new technology imaging instrumentation with a resolution of 2.3 µm, recent observations in Arctic clouds have shown that such pristine habits only describe approximately 3% of the particles. The measurements were made from an aircraft during April 1998 and cover a temperature range of 0 °C to −45°C. Boundary layer, multi-layer and cirrus clouds were examined. The commonly observed irregularly shaped particles either consisted of faceted polycrystalline particles or sublimating (solid to vapor) ice particles with smooth curving sides and edges. Since climate warming is now predicted to be largest in the Arctic, and cloud properties significantly affect the radiation balance, it will be necessary to consider the effects of non-pristine ice particle habits in such calculations and predictions.

Growth Rates and Habits of Ice Crystals between −20° and −70°C

Abstract A laboratory study of ice crystal growth characteristics at temperatures between −20° and −70°C has been performed at ice supersaturations and pressures comparable with those in the atmosphere using a horizontal static diffusion chamber. Maximum dimension, projected area, and volume growth rates, in addition to habit frequency, have been measured for individual habit types as functions of temperature, ice supersaturation, and air pressure. It was found that from −20° to −40°C and at ice supersaturations in excess of 2%, the most frequent habits observed were platelike polycrystals and plates, the complexity of forms increasing with increasing supersaturation. Columns appear with low frequency in this temperature range for all supersaturations. At low ice supersaturation (1%–2%), the habit consists of thick plates, compact polycrystals, and occasional short columns and is the region with the highest frequency of pristine crystals capable of producing halos. Just colder than −40°C, there is a marke...

Observations of the Distribution of Ice in Hurricanes

Abstract Observations of the type and distribution 0°C isotherm in three Atlantic hurricanes are presented. Supercooled drops, graupel, columns and aggregated snowflakes were observed. The supercooled drops were found only in convective updrafts stronger than 5 m s−1, but not all updrafts > 5 m s−1 contained appreciable liquid. Graupel was found in all updrafts at temperatures < −2°C, and small columns were sometimes found in downdrafts. Nonconvective rainbands contained 15–30 L−1 of snow composed of columns and what appeared to be large aggregates. Other stratiform regions contained 1–15 L−1 of medium and large aggregates; columns were occasionally found there also but only within about 15 km of convection. Hurricane convection is almost completely glaciated at the −5°C level. It is suggested that the ice particles observed at 6.0 km inside the convection result primarily from downward mixing on both sides of the eyewall updraft of ice formed in the convective areas at higher, colder levels. The ice in t...

Ice nucleation processes in upper tropospheric wave‐clouds observed during SUCCESS

We have compared in situ measurements near the leading-edges of wave-clouds observed during the SUCCESS experiment with numerical simulations. Observations of high supersaturations with respect to ice (>50%) near the leading edge of a very cold wave cloud (T <−60°C) are approximately consistent with recent theoretical and laboratory studies suggesting that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4 cm−3) are consistent with the number densities calculated in our model. In the warmer wave-cloud (T ≃−37°C) relatively large ice number densities were observed (20–40 cm−3). Our model calculations suggest that these large number densities are probably caused by activation of sulfate aerosols into liquid droplets followed by subsequent homogeneous freezing. If moderate numbers of effective heterogeneous freezing nuclei (0.5–1 cm−3) had been present in either of these clouds, then the number densities of ice crystals and the peak relative humidities should have been lower than the observed values.

Ice Crystal Concentration in Cumulus Clouds: Influence of the Drop Spectrum

Secondary ice crystals are thrown off when supercooled cloud drops are captured and freeze on a moving target in a cloud at —5�C. The rate of production of these ice crystals is proportional to the rate of accretion of drops of the diameter ≥24 micrometers.

The Temperature Dependence of the Viscosity of Supercooled Water

The viscosity of supercooled water has been measured, using a capillary flow technique, down to -23.8 °C. The large supercooling was obtained by removing nearly all suspended particles from the water by an ion exchange column. These results agree with those obtained by White and Twining in 1914, who obtained results down to -9.3 °C.

Electrification of the Hurricane

Abstract A survey of reports of electrical activity in hurricanes and typhoons from flight notes and personal experience (18 years, >230 eyewall penetrations for R. A. Black; ∼20 years for J. Hallett, plus that of others at the Hurricane Research Division), and perusal of flight notes dating from 1980, show that lightning in and within 100 km or so of the eyewall is usually sparse. However, occasionally, significant electrical activity (>one flash per minute) occurs in or near the eyewall. National Oceanic and Atmospheric Administration WP-3D aircraft penetrations through a number of storms relate the lightning occurrence to strong vertical velocity (>10 m s−1) and the presence of supercooled liquid cloud droplets extending to temperatures below −20°C. Specific measurements of cloud properties during eyewall penetrations show that the supercooled cloud water content increases with upward velocities >∼5.0 m s−1, as does the presence of large (>2 mm) supercooled drops. Measurements at temperatures >−13°C sh...

Experimental Studies of the Crystallization of Supercooled Water

Abstract The crystal fabric of ice formed by the freezing of supercooled water was examined in polarized light. Drops of radius 0.1 cm were frozen by homogeneous nucleation at −33C, by various foreign particles in suspension, or by impaction on a plane single crystal ice substrate. Bulk water, volume about 1.0 cm3, was frozen by the insertion of a single crystal of ice. Between 0 and −5C crystals invariably grew with a single orientation, identical with the nucleating crystal. With decrease of temperature increasing numbers of crystals with new orientations appeared, there being several hundred per drop when nucleation took place at −33C. Drops impacting on a substrate with vertical ‘c’ axis froze with horizontal ‘c’ axis in the temperature interval −5 to −15C. When the substrate was heated to 0C, drops always took the substrate orientation, even when supercooled to −22C. Measurements of the growth rate component parallel to the basal plane of dendrites growing in water at supercooling (ΔT) down to 20C fo...