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J-P. Chalon
,
J. C. Famkhauser
, and
P. J. Eccles

Abstract

The detailed structure and evolution of radar echoes observed in a multicellular hailstorm are analysed. General environmental conditions, overall radar echo development, and precipitation measurements are briefly discussed, but the analysis is mainly concerned with a particular event which was thoroughly observed by several different field facilities of the National Hall Research Experiment. This hailstorm, which evolved in a systematic and periodic manner, is the subject of four companion papers appearing in this issue.

Overall storm characteristics are found to compare closely to earlier descriptions of multicell hailstorms occurring in the High Plains. The motion of the main system was to the right of the mean wind vector in the cloud layer. Cell velocity was along but less than the wind in the mid–troposhere. Propagation by new cell growth in a preferred location with respect to existing radar echoes dominated the motion of the overall system. Study of the formation and evolution of individual cells showed that discrete new echoes formed near the altitude of 7 km MSL (−12°C) on the storm's right forward flank 5 to 10 km ahead of existing echo components at approximately 15 min intervals. Each grew rapidly in intensity and height and by moving more slowly than the overall echo complex soon became the main storm component. Average lifetime of individual cells, including the period from visually perceptible turrets, to &ldquo:first echo”, to echo decay, was 45 min. Thus, as many as three cells were found to coexist in varying stages of development. The ascent rate of visual cloud turrets and the history of maximum radar reflectivity of individual cells after the appearance of first echo indicate that the longest in–cloud residence time available for particle growth to the largest observed hail sill (1.5 cm diameter) was between 30 and 35 min.

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J. P. Chalon
,
G. Jaubert
,
J. P. Lafore
, and
F. Roux

Abstract

Durirg the night of 23/24 June 1981, new Korhogo, Ivory Coast, a squall line passed over the instrumented area of the COPT 81 experiment. Observations were obtained with a dual-Doppler radar system, a sounding station and 22 automatic meteorological surface stations. Data from these instruments and from satellite pictures were analyzed to depict the kinematic and thermodynamic structure of the squall line. Composite analysis techniques were used to obtain a vertical cross section of the reflectivity structure and of the wind field relative to the line. The redistributions of air, moisture and thermodynamic energy by the convection wet calculated through averaged two-dimensional wind fields from a dual-Doppler radar system. The method also allowed the evaluation of the exchanges that were occurring between the convective and the stratiform regions.

This squall line had many similarities with tropical squall lines previously described by others. The leading convective part, composed of intense updrafts and downdrafts, and the trailing part, containing weak mesoscale updraft and downdraft, were separated by a reflectivity trough. A notable feature of this line was the presence of a leading anvil induced by intense easterly environmental winds in the upper troposphere. Observations of the evolution of the system at different scales indicated that the mesoalpha-scale (following the classification of Orlanski) and the mosobeta-scale patterns combined to allow the system to have optimum conditions for maximum strength and a maximum lifetime.

A rear-to-front flow was found at midlevels in the stratiform region. The flow sloped downward to the surface and took on the characteristics of a density current in the forward half of the squall lice. Entering the convective region, this flow was supplied with cold air by the convective downdrafts and played an important role in forcing upward the less dense monsoon flow entering at the leading edge.

Calculations of mass, moisture and energy transports showed the importance of the transfers between the convective and the stratiform regions. Particularly large quantities of condensate and energy were transferred from the convective region toward the anvils and made important contributions to the precipitation budget in the stratiform region, while large quantities of water vapor and latent heat energy were transferred from the stratiform region toward the convective region through the rear-to-front flow. Diabatic heating resulting from condensation in the convective region was also evaluated.

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K.A. Browning
,
J.C. Frankhauser
,
J.-P. Chalon
,
P.J. Eccles
,
R.G. Strauch
,
F.H. Merrem
,
D.J. Musil
,
E.L. May
, and
W.R. Sand

Abstract

A model of an evolving hailstorm is synthesized from data presented in four related papers in this issue. The storm model, which is applicable to a class of ordinary multicell hailstorms and similar to earlier models derived by workers in South Dakota and Alberta, is discussed in terms of the growth of hail and its implications for hail suppression. Hail is grown in time–evolving updrafts that begin as discrete new clouds on the flank of the storm. Low concentrations of embryos develop rapidly within each of these clouds. The embryos subsequently grow into small hailstones while suspended near or above, the −20°C level as each new cloud grows and becomes the main updraft. Recycling is not a feature of this model as it is in supercell models. To improve the chance of silver iodide seeding being effective in suppressing the growth of hall in multicell storms, it is proposed that the seeding should be carried out not in the main updraft as is often the practice, but, rather, in the regions of weaker updraft associated with the early stages of developing clouds an the flank of the storm.

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Alain Joly
,
Dave Jorgensen
,
Melvyn A. Shapiro
,
Alan Thorpe
,
Pierre Bessemoulin
,
Keith A. Browning
,
Jean-Pierre Cammas
,
Jean-Pierre Chalon
,
Sidney A. Clough
,
Kerry A. Emanuel
,
Laurence Eymard
,
Robert Gall
,
Peter H. Hildebrand
,
Rolf H. Langland
,
Yvon Lemaître
,
Peter Lynch
,
James A. Moore
,
P. Ola G. Persson
,
Chris Snyder
, and
Roger M. Wakimoto

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as well as the number of drifting buoys. These facilities will be activated during multiple-day intensive observing periods in order to observe the same meteorological systems at several stages of their life cycle. A central archive will be developed in quasi-real time in Toulouse, France, thus allowing data to be made widely available to the scientific community.

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