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Roland List

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Roland List

Abstract

By setting up and solving general equations for the heat balance and the material transfer between a spherical hailstone and its environment it is possible to show how the different variables such as air temperature, air pressure, liquid water content of the air (in the form of drops), hailstone diameter, speed of fall of the hailstone, its surface temperature and growth rate are interdependent. At the same time growth zones can he delimited within which accretion is accompanied by evaporation or sublimation of H20, or where an increase of mass by sublimation exceeds the amount of accretion. From the growth conditions it is possible to classify the resulting ice deposits on a physical basis.

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Roland List

Abstract

Modeling by Gillesple and List established the evolution to equilibrium of raindrop number distributions in one-dimensional shaft models. Donaldson and List et al. (LDS) then demonstrated mathematically that equilibrium distributionsƒ for raindrop number concentration N consist of a product of the rainfall rate R and a shape function Δ (which is independent of Donaldson et al. and LDS) showed that such spectra contain three at diameters of ∼0.3, 0.8 and 1.8 mm, whereas Brown obtained only the two smaller ones.

For equilibrium distributions, the present shows that the mass concentration M and the radar reflectivity Z are also proportional to the rainfall rate R, with Z=C z R and that N,M, R and Z are mutually proportional. Calculation provided C z=742μm3 (mm h−1)−1, which is suggested to be a universal constant for steady tropical rain.

Data from a tropical rain experiment, carried out jointly by the Malaysian Meteorological Service and the University of Toronto, confirmed the equilibrium peaks at diameters of ∼0.3 (estimate), 0.9 and 1.9 mm. Due to basic deficiencies of the disdrometer only study state events could be used to assess radar reflectivities. The four available cases of steady rain consist of 112 one-minute spectra with different types and origins of MID (warm clouds, clouds containing ice particles, convective or stratiform clouds). As in the model, the radar reflectivity Z as calculated from measured drop spectra, was found to be proportional to rainfall rate.

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Roland List

Abstract

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Roland List

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Roland List

The ever-increasing severe economic damage imposed on national and world wide economies by severe weather, the need for sufficient and safe water resources for an increasing world population, and the threat of adverse climate change led to this critical assessment of the state-of-the-art of weather modification (WM) and to a proposal of a road map for the future.

Special attention is given to rain enhancement because it is further developed than snowpack augmentation, hail suppression, tornado and hurricane modification, and other weather-related disaster control ideas. The question of what makes a rain enhancement experiment acceptable to the scientific community is answered by the World Meteorological Organization's (WMO) criteria, which address statistical evaluation, the measurement of rain, the understanding of nature's precipitation processes with the underlying physics and dynamics of clouds and cloud systems, and the transferability of experiment design. These criteria are no longer specific enough or satisfactory and will have to be reconsidered.

An actual WM experiment also involves a variety of techniques and technologies, aspects that need to be complemented by numerical modeling of clouds and cloud responses to seeding. Modeling also allows assessment of the extra-area effects, that is, detrimental effects of precipitation on adjacent areas. Assimilation models may be giving better estimates of the rain at the ground because they can integrate restricted information from radar and rain gauges with mesoscale meteorological and remote sensing, as well as hydrological, data. However, massive improvements in computer capacity are required to handle these problems.

Weather modification has been progressing very slowly in the past because of the enormity of the problem and the fact that the precipitation process is far from being understood. Considering that rain increases are attempted within a range of 10%–20%, the lack of knowledge at corresponding accuracy is particularly evident in the fields of cloud physics, cloud and cloud systems dynamics, weather forecasting, numerical modeling, and measuring technology.

Benefits of new intensive studies of precipitation processes will not be limited to WM; they are also vital to improving weather forecasting and climate change modeling. There is one additional aspect of WM; WM can also be used to test newly developed precipitation physics and models by studying if the clouds react to seeding in the predicted manner.

This article is a wake-up call to put more intellectual and financial resources into the exploration and modification of the precipitation processes in all their forms. All these points lead to the suggestion of an outline of a national precipitation research and weather modification program.

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Roland List

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Liquid water skins on spongy deposits of hailstones that grow while gyrating in a wind tunnel environment, have been routinely observed to be supercooled at the water skin–air interface to as low as −5°C and more. The average water skin thickness (up to 1 mm) in the main growth region is calculated on the basis of the molecular conduction of the latent heat of freezing from the spongy substrate at the base of the water skin to its surface. The heat transfer is gradient-driven and relates directly to the speed of ice accretion on the hailstone. An extrapolation of an equation for the ice growth speed in supercooled bulk water suggests a supercooling of the ice–water interface of the order of −0.3°C.

The physical picture emerging is that of an ice sponge from which a fragile dendrite mesh grows into the water skin with a very homogeneous front and an advance speed that is controlled by the diffusion of heat (heat conduction) away from the ice front. Combining all results, it can be categorically stated that all surface points of growing hailstones have temeperatures below the freezing point of water.

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Roland List

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Features and performance of a pressure controlled vertical icing tunnel are described which allow the investigation of the formation of single hailstones in simulated cloud conditions. It is expected that new facts and a better understanding of the physics of hail formation can be found by the use of these quite unique facilities.

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Roland List

Abstract

In conformity with experimental results the heat transfer of gyrating spherical hailstones is divided into two parts. One takes place over a normally wet but supercooled equatorial region of limited roughness, whiles the other occurs over a rough, dry polar zone which is at a substantially lower temperature than the equatorial region. The sensitivity of this complex heat transfer is studied by establishing the response of the heat transfer to changes of 15 individual parameters relative to the results of a single hailstone growth experiment.

The propagation of errors in individually setting the icing conditions of a laboratory experiment is somewhat different from the sensitivity study. Both sensitivity and error hierarchies are given and conclusions are made about measurement accuracies. The measurements for the case at hand suggest that treating hailstones as smooth, nonrotating particles underestimates the heat transfer by a factor as high as two. For the treatment of a whole dataset containing many hailstone growth experiments, roughness may no longer be considered as a single-valued quantity since it seems to vary with hailstone latitude and icing conditions.

The tool used, the “spreadsheet,” is ideal to assess the complex heat budget. It serves to indicate potential problems before large experimental series are launched.

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Roland List

Abstract

No abstract available.

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