Hail Particles Research Articles

A Comparison between Polarimetric Radar and Wind Profiler Observations of Precipitation in Tropical Showers

This paper describes the results of an experiment that combines the data from a 5-cm-wavelength polarimetric radar and multiple-frequency wind profilers to examine the polarimetric signatures associated with the microphysical structure of several relatively shallow thunderstorms and also to examine quantitative rainfall measurements made with the polarimetric radar. These shallow storms produce considerable amounts of centimeter-sized hail. The presence and size of this hail are deduced from the wind profiler data. The melting hail particles produce a distinctive polarimetric signature with large values of differential reflectivity ZDR and suppressed values of the correlation coefficient between the signals at horizontal and vertical polarization. Comparisons between the mass-weighted mean drop diameter and differential reflectivity have been performed and show reasonable agreement with theoretical expectations, although the observed ZDR are somewhat smaller than expected. This may be associated with the theoretical assumption of the Pruppacher–Beard oblateness relationship even though there is evidence to suggest that real raindrops may be less oblate on average in convective rain. Quantitative polarimetric rainfall estimators have been compared with rainfall rates derived from the profiler drop size distribution retrievals and show reasonably good agreement when reflectivity values are matched.

Three-Dimensional Representations of Hexagonal Ice Crystals and Hail Particles of Elliptical Cross Sections

Mathematical expressions based on earlier ideas of the author are given to describe the three-dimensional surfaces of hexagonal ice crystals and conical graupel and hail particles. In the former class, the expressions closely parallel the expression for prolate spheroid but the cross section is transformed into a hexagon by an expression designed previously. A special transformation containing a preset constant, ε, is implemented so that the three-dimensional, instead of two-dimensional, surface of the crystal is described. In the case of conical particles with elliptical cross sections, the equation that describes a conical body of revolution is modified to describe conical bodies with elliptical cross sections.

On the sensitivity of hail accretion rates in numerical modeling

Numerical models of convective clouds with a bulk-water parameterization scheme extend the hail spectrum unrealistically to zero size. Since some of the microphysical characteristics of clouds depend critically on the hail accretional growth, it is desirable to see how the accretion rates may be improved by considering only hail sized particles, equal or greater than 0.5 cm in diameter. An equation for hail accretion rates which includes only hail particles larger than 0.5 cm is derived. Calculations show that the new hail accretion rates may differ from their earlier values by a factor of more than 3. The new equation for hail accretion rates is tested in a 1-D time-dependent model. The numerical results show that the modeled cloud with the new formula is more effective to produce the amount of rainfall and hail in shorter time than in previous case. The result is in fair agreement with observations.

High-Resolution Passive Microwave Observations of Convective Systems over the Tropical Pacific Ocean

Abstract This paper presents high-resolution passive microwave measurements obtained in the western Pacific warm pool region. These measurements represent the most comprehensive such observations of convection over the tropical oceans to date, and were obtained from the Advanced Microwave Precipitation Radiometer (AMPR) aboard the NASA ER-2 during the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The AMPR measures linearly polarized radiation at 10.7, 19.35, 37.1, and 85.5 GHZ. Nadir brightness temperature scatterplots suggest that the three lower frequencies respond primarily to emission/absorption processes. Strong ice scattering is relatively rare, as absolute magnitudes of the ice-scattering signature do not approach those measured in strong convection over land. This is apparently related to the reported weaker updraft velocities over tropical oceans, which would create and suspend relatively smaller graupel or hail particles in the upper cloud. Observations withi...

Mechanism of charge transfer between colliding ice particles in thunderstorms

We derive a theoretical mechanism for the charge transfer between colliding ice and soft hail particles, based on current ideas of surface melting. According to this theory, each of the ice surfaces is covered with a film of quasi‐liquid water, whose thickness can be estimated as a function of temperature and local curvature. We show that during contact of two particles, fluid will flow from the thicker to the thinner layer, and we estimate the volume transferred in atmospherically relevant situations. We postulate a charge structure within the surface‐melted layer based on observations on similar systems and show that this leads to estimated charge transfers consistent in approximate magnitude and sign with trends observed in the laboratory.

Ice particle generation during evaporation

Simulated soft hail (graupel) particles are grown in a laboratory chamber and evaporated under controlled conditions of wind speed, temperature and undersaturation. Particle breakup is observed directly and recorded by video, and also inferred from abrupt changes of charge. A particle with a diameter of a few mm may give rise to several hundred particles under some conditions. Breakup occurs by irregular evaporation of the rime branches, which thin to less than 10 μm prior to fracture. Break up is also observed during evaporation of individual faceted vapor grown crystals. All ice particles are strong; the fracture strength is estimated to be at least ten times the bulk fracture strength of ice.

Electrostatic image theory for layered dielectric sphere

Static image theory for a homogeneous dielectric sphere, recently formulated by one of the authors, is generalised to the problem of a multilayer dielectric sphere in terms of a line-image charge. The imageline source is seen in this case to be in relation to the so-called static reflection coefficient that could be labelled as a finite Mellin transformation. This transformation is discussed and a Table of corresponding transforming functions is given. The result can be applied in a more complete quasistatic analysis of interactions of layered dielectric spherical particles in an artificial dielectric media. More closely, a two-layer sphere is considered with special cases of a thin layer, and that of a low contrast between the layers. Numerical results are calculated for the radiowave propagation problem of attenuation due to melting hail particles, assumed to be composed of a spherical ice core with a spherical water shell.

Macroscopic permittivity of dielectric mixtures with application to microwave attenuation of rain and hail

A theory is presented for calculating the macroscopic effective permittivity of mixtures that are composed of a background medium and inhomogeneous scatterers. The scatterers are two-component spheres, where both the spherical core and the spherical shell that forms the surface of the scatterer are homogeneous with a constant permittivity. The effective permittivity is calculated as the function of the constituent permittivities and their volume fractions. The analysis is quasistatic and therefore the results only apply to mixtures where the sizes of the scatterers are considerably smaller than the wavelength of the operating RF field. The results are applied to calculating the microwave attenuation of melting hail, at a frequency of 1 GHz and where the hydrometeors are assumed to be in the form of a spherical ice core with a spherical water coating shell. There is a peak in the attenuation of hail as it falls through a warm layer. The attenuation is greater with a thin water coating (the fraction of water from the hail particle is 0.09) than it is when the hail particle has completely melted. This resembles the same phenomenon of increased extinction which is observed in the radar backscattering from melting hail.

Differential radar scattering properties of model hail and mixed‐phase hydrometeors

The differential reflectivity (ZDR) radar signal contains information on the shape and alignment of ice phase hydrometeors. Under certain circumstances, then, this signal and reflectivity factor can be used to identify the presence of hail particles. Such interpretations require knowledge about the scattering properties of various types and shapes of hydrometeors, including ice, water‐coated ice, and graupel. These hydrometeors are examined for their differential scattering properties obtained from computations using the transition (T) matrix method of Waterman and an extension of this theory to two‐layered bodies. Differential radar cross sections together with circular depolarization ratios are presented, and several instances of possible hail detection by radar using ZDR signatures are illustrated.

A numerical study of thunderstorm electrification using a three dimensional model incorporating the ice phase

A numerical model of cumculonimbus allowing a simulation of the growth of the electric field is described. The cloud model is three dimensional and includes a representation of the ice phase. The charge generating mechanism is assumed to arise from the collision of rimed hail particles with smaller ice crystals. Two possibilities are considered: an inductive method, in which hail particles are polarized in the local electric field, and a non-inductive ice-ice method, where charge transfer is attributed to a difference in surface potential of the ice particles in contact. These mechanisms are compared for a test cloud displaying the rainfall characteristics of a typical, small maritime thunderstorm and their sensitivity to the assumed form of the hail size spectrum examined. A cloud of relatively small size is chosen to constitute a more severe criterion of the strength of the electrification mechanisms than is possible in large thunderstorms where weaker modes of charge transfer could also produce a lightning discharge. Hence the comparison of mechanisms presented is specifically for a small thunderstorm. The results indicate that the electric field can reach a breakdown threshold within half an hour of the appearance of precipitation with the inductive method, providing that the hail size spectrum represents relatively small particles and that the effect of multiple collisions of any ice crystal with more than one hail particle is discounted, i.e. when it is assumed that ice crystals are uncharged before impact. It is found that the non-inductive ice-ice mechanism can lead to lightning within a similar time providing that the product of ice particle concentration and charge separation per collision exceeds 7 pC per litre.

A numerical study of thunderstorm electrification using a three dimensional model incorporating the ice phase

AbstractA numerical model of cumculonimbus allowing a simulation of the growth of the electric field is described. The cloud model is three dimensional and includes a representation of the ice phase. The charge generating mechanism is assumed to arise from the collision of rimed hail particles with smaller ice crystals. Two possibilities are considered: an inductive method, in which hail particles are polarized in the local electric field, and a non‐inductive ice‐ice method, where charge transfer is attributed to a difference in surface potential of the ice particles in contact.These mechanisms are compared for a test cloud displaying the rainfall characteristics of a typical, small maritime thunderstorm and their sensitivity to the assumed form of the hail size spectrum examined. A cloud of relatively small size is chosen to constitute a more severe criterion of the strength of the electrification mechanisms than is possible in large thunderstorms where weaker modes of charge transfer could also produce a lightning discharge. Hence the comparison of mechanisms presented is specifically for a small thunderstorm. The results indicate that the electric field can reach a breakdown threshold within half an hour of the appearance of precipitation with the inductive method, providing that the hail size spectrum represents relatively small particles and that the effect of multiple collisions of any ice crystal with more than one hail particle is discounted, i.e. when it is assumed that ice crystals are uncharged before impact. It is found that the non‐inductive ice‐ice mechanism can lead to lightning within a similar time providing that the product of ice particle concentration and charge separation per collision exceeds 7 pC per litre.

Early Discoverers XXXI: Descartes’ Observations on the Amsterdam Snowfalls of 4, 5, 6 and 9 February 1635

In Les météores (1637), in the midst of a father far-fetched theory of their origin, Descartes incorporated a detailed description, from actual observation, of a variety or kinds of snow crystals and hail particles. These even include the tsuzumi crystals, described as “wheels joined two by two by an axle” or as “little crystal columns decorated at each end with a six-petalled rose”, and such exceptional forms as twelve-rayed and eight-rayed stars. The accompanying diagrams, which have been reproduced a number of times, are distinctly inferior to his verbal description.

Early Discoverers XXXI: Descartes’ Observations on the Amsterdam Snowfalls of 4, 5, 6 and 9 February 1635

In Les météores (1637), in the midst of a father far-fetched theory of their origin, Descartes incorporated a detailed description, from actual observation, of a variety or kinds of snow crystals and hail particles. These even include the tsuzumi crystals, described as “wheels joined two by two by an axle” or as “little crystal columns decorated at each end with a six-petalled rose”, and such exceptional forms as twelve-rayed and eight-rayed stars. The accompanying diagrams, which have been reproduced a number of times, are distinctly inferior to his verbal description.

Observations of Vertical Motions and Particle Sizes in a Thunderstorm

Abstract By means of a vertically pointing pulsed-doppler radar, observations were made of a growing thunderstorm moving overhead. An updraft having a maximum speed exceeding 19 m sec−1 was observed in the upper part of the cloud. The character of the vertical velocity distribution supports earlier speculations about the pattern of vertical velocity near the top of an updraft. Inferences are made of the characteristics of precipitation in this cloud. The analyses lead to the conclusion that there were hail particles in the cloud. Although maximum echo intensity occurred in regions where hail particles with diameters of about 1 cm were present, maximum backscatter of radar power came from particles about half this size.