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Jean Dessens

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Jean Dessens

Abstract

A nonrandomized weather modification project, hail prevention by seeding from the ground, has been run since 1952 in a large area of southwestern France. From the beginning of the experiment, the parameter proposed to measure the seeding efficiency in the area covered by AgI ground generators was the loss-to-risk ratio derived from insurance data. The analysis of the trend of this parameter in three parts of France which constitute a target, a buffer and a control area, brings to light a relative decrease of the damage during the last years in the protected area. A new statistical test for detecting a shift in precipitation series, applied after a log-transformation to the loss-to-risk ratio series, indicates a decrease significant at the 0.01 level in the damage due to hail during the period 1965–1982 in the protected area, while no significant change has been observed in the buffer area. Since there has been a large increase in the number of generators, and, above all, the setting up of better equipment since 1965, seeding is a reasonable explanation for the hail decrease.

A double-ratio calculation with the target and control data gives a value of 41% for the decrease of the damage in the seeded area. Within this area, the global result is strengthened by the positive departmental correlation between the number of seeding stations per unit area and the hail decrease. The benefit-to-cost ratio of the project appears to be about 24.

The hypothesis of a seeding effect leads to the following main physical implications: 1) The seeding effect is only perceptible in the area where the generators are distributed and not downwind of this area; this is in keeping with the observation that the ice-forming nucleus concentration is only locally increased over the seeded area. 2) At least 65% of the hail situations in southwestern France are related to cold fronts; the decrease of the hail damage corroborates the results of the Argentinian hail suppression project where a beneficial influence of AgI ground seeding was found to be significant under cold front situations.

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Jean Dessens

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Jean Dessens

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Jean Dessens

Abstract

The large-scale hail prevention program operated by the Association Nationale d’Etude et de Lutte contre les Fléaux Atmosphériques in southwestern France combines the seeding of hail clouds by a network of silver iodide ground generators with a survey of hailfalls by a network of hailpads. Using the joint data from the two networks, a physical method has been developed to measure the change in hailfall severity in the seeded hailstorms. The method associates the number of hailstones larger than 0.7 cm in a point hailfall at the ground (the “hailstone number”) to the amount of seeding material released in the area where the storm was developing just before hailstone growth. The fundamental but not proven hypothesis employed is that a simple negative correlation should exist between the two parameters. During the years 1988–95, 630 point hailfalls were recorded on 53 hail days with seeding. The method indicates that the hailstone number is basically responsive to the amount of seeding material released 80 min before the time of the point hailfall in a 13-km radius area centered on the place where the storm was developing. The decrease in the hailstone number is in linear relation with the seeding amount—the more heavily seeded hailfalls decrease by 42%. The results are based on significant but weak correlations that have to be strengthened with a larger sample of hailfalls. However, they agree with former evaluations by insurance data of the same hail prevention program. The method gives a model that can be used to arrange the generator networks according to the movement of the storms.

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Jean Dessens

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In the context of an experiment of hail prevention in the Aquitaine region, 12 869 reports of damaging hail have been compiled during 29 years in an area of 88 980 km2. These observations, together with crop insurance data, have led to a unique hail climatology.

The data presented in this paper concern the geographical distribution of hail damage, the yearly, 5-day and hourly frequencies of hailfall, and the distributions of hailstone size and of hailfall duration. Most of these data are well explained by the fact that hail in the surveyed area is the result of almost any rather severe thunderstorm: large hail, however, is produced by a few isolated long-lived hailstorms traveling downwind of the central part of the Pyrenees with the strong upper level winds. Study of the mean characteristics of 30 of the most severe storms which have damaged the Aquitaine in the last three decades leads to the following description: a typical long-traveling hailstorm moves at 15 m s−1 for 1.5 h, dropping a hail strip 86 km long and 6 km wide. The direction of propagation is from the southwest, with an angular deviation of 28° to the right of the mean tropospheric wind. This wind is characterized by an increase in velocity up to 10 km (mean maximum: 32.6 m s−1) without any change in direction above 3 km. In some circumstances, these long-traveling hailstorms produce only hailspots along their path, although the convective and wind conditions are the same as for the major hailstorms.

The insurance data complete the observers reports because they take into account the severity of the hailfalls and also because they give the economical impact of the hail. A decrease in the mean annual percentage of loss is observed during the last two decades in Aquitaine. The significance of this change will be discussed in Part II which is related to a hail prevention project in the same region.

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Jean Dessens Jr.

Abstract

observations of waterspouts and tornadoes suggest some influence of surface roughness on these phenomena. In a laboratory model of a tornado-like vortex, this influence is measured in terms of tangential and vertical velocity profiles above the ground boundary layer. The roughness induces an increase of the core updraft, and the effect on the tangential velocity profile is such as to increase the radial Reynolds number.

In the atmosphere, geometrical similarity suggests that passing over forests or towns could greatly affect tornadoes, increasing the core diameter and mean updraft, but suddenly decreasing the explosive effects and wind speeds.

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Jean Dessens and John T. Snow

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In the period 1680–1988, 107 significant tornadoes in the Fujita scale categories F2–F5 have occurred in France. These include 49 such events in the historical period 1680–1959, and 58 events in the modem period 1960–1988. Estimates of the temporal and spatial climatological distributions of significant tornadoes in France have been developed that suggest

  1. June and August are the months with the greatest number of such tornadoes;

  2. 1600–1700 UTC is the interval in which occurrence is most likely, with a secondary maximum in likelihood between 1800 and 1900 UTC;

  3. the northwestern quarter of the country is the region where a significant tornado is most likely to occur. A second, much smaller area with several observations is evident in the far south-center portion of France, near the Mediterranean coast;

  4. two significant tornadoes can be expected in France each year;

  5. the mean area stricken by such a tornado is about 4 km2;

  6. France has a mean risk probability of a significant tornado occurring at a point of about 1.5 × 10−5 per year, a value some 15 times lower than the Great Plains of the United States.

During the cold season November–March, tornadoes are most frequently observed in northwest France. During the warm season April–October, they axe most frequently observed in the interior of the country.

An examination of the meteorological situations associated with 21 cases suggests that there is a distinctive synoptic pattern for each season. Analyses of tornado-producing situations show that French tornadoes usually occur where an unstable surface layer of modified air that originated in the Mediterranean is overlaid at midlevels by maritime air originating over the Atlantic. The instability in the surface layer develops while the air is in southern France. Such situations are characterized by high degrees of conditional instability between surface and midlevels in the troposphere as reflected in a steep lapse of wet-bulb potential temperature. However, case studies of individual events suggest that for a tornado to occur, the instability of the surface layer must be further enhanced by local warming and moistening The appearance of a secondary low-pressure center on or near a cold front advancing from the west is an additional favorable condition for the onset of severe weather.

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Christopher R. Church, John T. Snow, and Jean Dessens

Observations of vortices of various types produced in a large thermal plume are described. The apparatus used to generate the plume is the Météotron, an array of 105 fuel oil burners with a total heat output of approximately 1000 MW. Three types of vortices have been observed: 1) large counter-rotating rolls in the downstream plume, 2) intense small-scale vortices resembling very strong dust devils seen at the surface on the downwind side of the plume, and 3) very large columnar vortices produced when the lower portion of the plume goes into rotation as a whole. Three mechanisms leading to the concentration of vorticity necessary to produce these vortex types are discussed. These include tilting and stretching of horizontal vorticity present in the environmental wind field, generation of vorticity within the plume by the action of buoyancy and drag forces, and convergence of preexisting background vorticity from the environment. It is concluded, based on these observations and physical considerations, that the generation of vortices of moderate intensity is to be expected in large plumes, be their source a forest fire or an industrial operation.

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