The purpose of this paper is to show relationships among several hailfall parameters and degree of crop damage to those concerned about hail hazards in Japan. There are a few papers discussing this subject in Japan, but in those were used hail data obtained by interviews and questionaries from people living within and around hail damage areas. Such data are generally very inaccurate and not infrequently unbelievable values are reported as truly occurred. Even now physically unsound theories using unrealistic scales have been adopted even in an authorized handbook as a guide to hailfall characteristics vs. crop injury relationships. On account of establishment of hail observation network by the National Research Center for Disaster Prevention (NRCDP), and later increase of number of stations operated by local governments, it has become possible to obtain more accurate data in a portion of northwestern Kanto district where damaging hailstorm hit relatively frequently. However, the instruments used there are so called hail pads, the simplest kind hail-recorder.
The parameters examined in this paper are the maximum hailstone diameter
dmax, number of hailstones
N (for all sizes) and
N* (for
d>6.4mm) per unit area, amount of hail per unit area
M, impact energy per unit area
EI, and percent crop damage
D. We deduced hailstone diameters from dent diameters on aluminum foils. Discrete values are obtained. The value 6.4mm which happened to be equal to 0.25 inches is one of such a value. The percent crop damage is determined just after the hailstorm, and the values are quite rough. Although there were various kinds of crops in the area under consideration, the values of
D refer to wheats which were just before harvest. The case studied here occurred on 9 June 1975. On this day damaging hail occurred at many places scattered over the wide area in northern and eastern Honshu, the main island of Japan. Gunma prefecture experienced the most extensive damaging hail ever known. About a dozen of hailclouds were identified by the NRCDP's radar situated at Fujioka in the southern part of this prefecture. Detailed study of the hailstorm were made by the authors (Omoto, Yagi, and Seino, 1976). The damaging hail discussed in this paper was brought by one of a quasi-supercell type convective system. It formed near the southwestern edge of bandshaped multicellular intense convective system, but unlike to other previously formed cells this one moved more or less independent of the main echo. The storm accompanied extraordinary intense rain (20-30mm in 5 minutes or less) as well as severe hail and strong gusts. There appears to be a quasi-periodic change in the intensity of the storm as revealed by the distributions of various hailfall parameters (Fig. 8). However, no corresponding change was identified from the radar data.
Following statements are made as the summary of this preliminary study. (1) Degrees of crop injuries are relatively well correlated with following hailfall parameters: a) impact energy per unit area
EI, b) the amount of hail per unit area
M, c) the number of hailstones equal or larger than 6.4mm. (2) Total number of hailstones per unit area
N, and maximum hailstone diameter
dmax are less correlated with crop injuries compared with those given above. (3) Damage intensity is roughly proportional to Log
EI and
N* for 30≤
D≤95% (note: Fig. 10 showing this relationship is more or less illustrative and not given for practical use). (4) Hailstones smaller than 6.4mm in diameter give weak or no damage to crops unless the amount is extraordinarily great. (5) Duration of hailfall in the area was very short, i.e., even in severely damaged area it was less than 5 minutes.
(6) Hailfall amounts converted to rainwater were relatively small. Even in an area with severe
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