cesen~1

agriCuLTURA

AGROMETEOROLOGY

EDITED BY

FRANCO PROGI FEDERICA ROSSI

GRAZIELLA CRISTOPERI

Edizioni del Comune di Cesena

cesen~'1 1

agriCulTURA

AGROMETEOROLOGY

EDITED BY

Franco Prodi Dipartimento di Fisica, Università di Fierrara e Dirett0re dell'Istitut0 FISBAT, C.N.R., Bologna

Federica Rossi Ricercatore del Centro di Studio per la Tecnica Frutticola, C.N.R., Bologna

Graziella Cristoferi Direttore del Centro di Studio per la Tecnica Frutticola, C.N.R., Bologna

2nd International Cesena Agricultura" Conference

Cesena, 8-9 October 1987

Editrice C0mpositori Bologna Via Stalingrado 97/2 1>' 051(327837-327811

Copyright © Comune di Cesena 1987

AD rights reserved. No part of t~is book may be reproduced in any form without permission in writing from the publisher.

Contents Indice

Invited speakers - Re/atori ad invito

Opening addresses - 'Presentazioni

Nowcasting, satellite meteorology and remote sensmg

IX

XI

Land surface temperature from satellite data 3 H. GrassI

Autornated recogni~ion of doudtypes from satellites and its application to

nowcasting 27 E. Litjas

A weather inforrnation system for agri culture in Sweden 47 E. Liljas

Hail problem: present status of research and prevention

The need for weather and climate modification R. List

Hai! research and hail suppression A. Waldvogel

Laboratory hai! studies F. li'rodi

lcing frost Environmental and physiological factors that influence cold hardiness and

frost resistance in perennial crops l.A. Flore, G.S. Howell Jr.

Evapotranspiration and irrigation

Climatological determination of irrigation requirements B. Primault

Agrometeorological crop monitoring and forecasting

G.F. Popov

61

87

11l

139

153

496 CONTENTS

Irrigation scheduling and plant water use 189 IR.J. Reginato

Mete0ro10gy in crop pr0tectioA

Analyzing one effects of clima te and weadher on pests 203 A.P. Gutierrez

Microclimate and arthropod phenologies: the leaf miner Phyllonorycter blancardella F. (Lep .) as an example 225 J. Baumgiirtner, M. Severini

Agrometeorology and plant protection 245

D. Rijks

Meteorology and plant disease 255 M.J. Jeger

Disease €omorol by cornjDuterized systerns 277 P. Mandrieli

Productivity forecast and environmental factors In crop growth The importance of climate and microclimate to yield and guality in horti-cultural crops 287 A.N. Lakso

Agrometeorological models of agri culture productivity for land planning 299 G. Maracchi, F. Miglietta

Sirnulation of eneFgy amd mass exchamge witnin l'he soil- JD lan~-atmosphere sys~em anci tne pFogress of agmmeneorology 47) LG. G ingoi, O.ID. Sirotenko

Posters

The introduction of Nowcasting system development in Chima Liar,g Bi-Jun, Wang Ang-Sheng

Combined satellite and radar image processing in meteorologica l Now­

casting F. Prodi, V. Levizzani, 1'. Georgiadis

Forecast of frost duration A.M. Pandolfi, M. Pitzalis, M. Nardi

'Fhe influence of envir@mmental and ':JUltural factoFs om sJDDimg fros~ semsi­oivity of aJDple fil0wers V. R0ssi , P. Bat~ilani

331

335

339

345

CONTENTS

Tissue response of pear to cold injury N. Filiti, M. Rizzetto

The preliminary study om annificial and maturaI hailstones F. Prodi, Wang Ang-Sheng

The hail problem and some studies of hailstorms in China Wang Ang-Sheng

Research of the Italo-J ugoslavian common hail prevention system D. Vento

Evapotranspiration and infiltration in a hydrological mode! on a regional scale F. Mantovani, M. Menziani, S. Pugnaghi, M.R. Rivasi, R. Santangelo

A method to calculate water-deficit probabilities M. Nardi, A. Ballado,i , A.M. Pand0lfi

Preset evapotFams]:iliraoioms andI irrigation of orcnaFdIs P. Mannini, R. Genovesi

Vegetative ana Hopping performance influenced by evapotranspiration and irrigation P. Mannini

Effect of different water deficits on fruit and vegetative growth of appIe trees o. Failla, S.M. Cocucci, C. Poma Treccani

Phenology and vegetative behaviour of soylJeam as affected IDy water avail­ablhty in two different s0ils R. Giovanardi, P. Ceccen

The climatic data base: a 1!lsefl1!l1 instrumen~ for agriculture planming A. Milella, P. Deidda, M.R. Filigheddu, D. Spano, G. Bianco

Radiant energy spectral distribution in fwit tree canopy E. Baldini, F. Rossi

Towards a «chemical climate»: need for an assessment of economical impact of atmospheric ozone on crop species in Italy G. Lorenzini

Control of photosynthesis umder sah stress iJil nhe ollve F. Loreto, G. Bongi

fhe eXJii>omential -p01ymornial !lilodel of yield forecas~i!ilg ~or spring wheat amd potat®es L. KuchaF

497

351

357

361

365

367

371

375

381

387

389

397

401

409

411

4t5

498 CONTENTS

Bioclimatological relationships of chestnut in the province of Cosenza 417 LM. Pinnola, M.C. LOFenzett,i, A. Baltadori

Climate-pheI'lology Felationships on vinegrape cv «Albana» (iV. vini/era L.) 425 C. LmFieri, S. TUHi, P. Volpelli, S. Poni, E , Magnanini, A, Zanott,i

Effect of temperatuFe on the earliness of peach filDwering 435 A. Baltadori, I.M, Pinnola

Weather risk assessment for crop planning 441 M, Pitzalis. A,M. Pandolfi, M.C. Lorenzetti , M. Nardi, LM , Pinnola

Risk assessment of thermal conditions for agricultural planning 445 M.C. Lorenzetti, M. Pitzalis, A, Baltadori

Influence of bioclimate on lemon croppmg m the peninsula of Sorrento and Amalfi 455 N. Pilone

The GpeFatiGI'lal acnivities of rhe regiomal meteorological service Gf Emilia-Romagna (SMR): agromet<wrolGgical aspects 459 R. 13arbieni, L. Botarelli, V. Marletto, F. Nuccionti, A, Salsi, F, Zino"i

Operational activities of the regional \1l1leteorological seFvice of Emilia-Ro-magna (SMR): meteorological aspects 463 C .. çacciamani, G , Galliani, G. Lenzi, F. Nucciotti , S, Nanni, T. Paccagnella. G . Simo-0I1li

Climatic dusters in the regio n of Friuli-Venezia Giulia M. Geschia, E. Toppano, D, Cauz

471

INT. CONF. ON AGROMETEOROLOGY, CesellO 1987 F. Prodi, F. Rom~ G. Cristo/eri eds. Fondazione Cesena Agriculluro Pubi.

Hail research and hall suppression

A. WALDVOGEL

Atmospheric Physù;s ETH, Ziirich, Switzerland

Summary

The discrepancy between opinions of pe0ple doing hai! research or hai! sL\ppression is dis­cussed. It is shown, nloat there are severa! fundamental problems of the phenomenon ha il, which complicate a elear answer about the effect of a given hail suppression method. Two of these problems are the detection and the exact measurement of hail by remote sensing. An overview about radar techniques with simple but accurate solutions to overcome these two problems are discussed in detail. In a further chapter, there is a detailed description of the conduct and evaluation of the latest international, randornized hail suppression experiment, made in Centra! Switzerland from 1977-1982. The last chapter discusses possibilities, how to integrate the two diverging opinions about recent hail suppression methods.

Introduction

Ihe scientific weather modification is about 40 years old now. It began wi~h the sensational discovery of Vincent J. Schaefer (1946) , then working at the GeneraI Eiectric Research Laboratory, who found that ice crystals could be produced in a cloud €ONsisting of supercooled cloud droplets only by dropping dry ice into it. Ihe explanatiom. for tnis result is, that dry ice with a temperature of - 78.5 °C causes water droplets to freeze sp€mtaneously. As Soon as a cloud contains both water droplets and ice crystals, the ice cry.stals grow very fast because the supercooled cloud dIOplets eV'<Iporate am.d the free water molecule are bound by the ice crystals. Ibis behaviour is called Berge­ronrFim.deisen processo It was well known, but on1y the discovery of Schaefer made it possible to initiate the precipitation process artificially.

Only some momths later, Bem.ard Vonnegut discovered that silver iodide bad tbe same effect as dry ice. Ime advantage of silver iodide, compared with dry ice is its simp1e application amd the relatively high nucleation temper­ature which is at -4°C. With ome gram of a pyrotechnical mixture of silver iodide it is possible to produce a tremendous large mumber of ice nuclei

88 A. W ALDVOGEL

(1014). Such pyrotechnical mixtures are used in rockets, flares or ground geft­

erators and it is dear, that the smoke produced by such systems can be dis· persed very easily into clouds. The microphysical explanation for the effect of silver iodide is its structure of the crystals which show a hexagonal habit very similar to the one of ice. The existence of crystals of silver iodide in a supercooled doud therefore forces the droplets to evaporate and the result is the formauon of precipitation.

It did not take a long time, until it was tried, by using silver iodide as an artiflclal Ice. nudei, to dissolve supercooled fogs, to initiate and augment snow and ramfall and to suppress hail. Many sensational results of large ef· fects of th dif" wea. er mo . lCatlon programs have been reported at the beginning. ~oday the sltuatlon IS more difficult: The scientific community thinks thal I ere IS no proof of any large doud seeding effect with the exception of su. percooled fogs For th h il . h . . . I . . e a suppreSSlOn t e sltuatlOn IS even more comp ex as an mcrease of hail d t din . A I (1977) A I ue o see g operatlons cannot be exduded, see t· aS

h L ~ arge number of weather modifiers and doud seeders on the

ot er l'lanu are convinced th t IT f h h ti Ib f

'· a enects o weat er modification exist and t al le cost ene It ratlos of d" ' .

effect for a small' see mg operatlons are very low, which means a b.lg How l'S 't 'bl pt1

hce, see for mstance Burtsev (1985). The question now IS:

1 pOSSI e t at there . h l . d operation althou h h d' IS suc a arge controversy between SClence an

11'" g f t e Iscovery of Vonnegut is 40 years old? e pu~pose Gl thls pap . " '" .

uation one e h er IS, .to gIVe some mSlght mto the complex SII· ncounters w en deali . h h . . .

pia in how it is possible tha l ng Wlt weat er modlflCatlOn, and to ex· cloud seeding op .' t C ear answers m respect of the true effect of

erauons are stili '. Th '. . ternational randomiz d h il mlssmg. e dlscusslon of the latest, m· ities one has today t: d a pr~enuon program further shows the possibil­Sl!!ch an experiment h etec~ an measure hahl with remote sensing, and how standing of the hail f as to . e evaluated sciemtifically to gain a better unden·

ormatlon processo

The hail problem

. The discrepancy mentioned abGlve . flrst group has its orig' . h IS due to two gro~ps of prob~ems. The l d h In In t e natur f il c o~ s, w ereas the second e Gl convective douds, especially ha

whlch have been developed grohup lcan be named as measuring techniques,

q ' . m t e ast t h Uantltatlve measurement . en years only to such an extent t al r. S are posslble Th f' - rorecastIng hail, . e Irst group can be spIit into: occurrence of hail ,

HAlL RESEAROH AND HAlL SUPPRESSION

and the second group mainly contains the problems of: - detection of hail, - measurement of hail.

89

Forecasting hail in a quantitative manner would mean that it is possible to predict the exteJilt of a hail cell induding the Jilumber and size of the hail­stones, the celi movement, the shape of the celi, the duration of the hailfali and rainfali, the maximum rates of precipitation etc. It is dear that we are far away from having solved ali these problems today. And this is also true for the much less sophisticated qualitative aspect of the problem. If one had bhe answers to the questions mentioned above, it would not be difficult to test any method proposed for hail prevention. Because in such a case one would have to take just one hail doud, apply the hail suppression technique and compare the treated hail celi with what one would have measured if no treatment would have been used at ali.

Because there is no such prediction available nowadays, there are three interpretations to comment a given hail prevention operation:

1) no effect, 2) more hail due to the influence, 3) less hail due to the influence. It is dear that ali three interpretations can be given with "good" argu­

ments always and whatever has happened. Because there is such a necessity of proof, there is no way to decide which argument represents the truth. As a conseql!lence, most discussions about operational hail suppressions culmi­nate and end in the statements: One believes or one do es not. A scientific answer to a given hail prevention method can only be achieved, by compari­ng a large number of seeded and natural hail douds which have been ob­served and measured in a given region during a weli defined time interval. It is ifolilportant that the selection Imlode to treat the hail celis is made purely ob­jective and decided long before any artificial influence starts. Such a proce­dure is calied: A randomized experiment.

Hailfali is a rare evento This is especialiy true for sensationaliy large hail­storms of which one is still talking many years later. Although most precip­ltaUon falling during the summer is produced via the graupel mechanism, there are OJiliy a few perceJilt of ali convective celis which have hai1stones fali­mg onto the ground. Gonsiclering a distinct convective celi, the probability is only about 10%, that this celi develops into a hail celi. This fact can be used by op~;ational hail suppression companies, because their method was "suc­cessful m most cases. And if there is some hail on the ground, there is al­ways the argument: The damage would have been worse if no operation wouJ.d have beeJil made. It has beeJil disclilssed above, that until now there is

90 A. WALDVOGEL

no way to solve tbis problem scientifically, except with a. randomized experi­ment. From a practical and economical pomt of vlew It IS _ very Important therefore, to be able to separate hai! cells from harmless ram cells ear1y ID

the cell's lifetime which means, a technique to detect hai! cells is needed. This is rusClllssed in the following section.

Hai! detection

The detection and measurement of hai! by radar has been of interest since the beginnings of radar meteorology. Especially forecasters of severe weather and scientists working in hai! prevention operations had a funda­mental need for sl'Ich a techniGjue. ANd indeed, one of tne first criteria used in the positive identification of dangerous hai! clouds early in the !ife time of convective cells was developed by Soviet scientists working in hai! preven­tion. The criterion relies on the use of the geometrica! properties of a doud which were originally measured using optical devices rather than radar.

More sophisticated methods for hai! deteouion have been developed since i1.960, using ten centimeter radars (Geotis, 1963), multiwavelength radars (Al­Ias and Ludlam, 1961) and more recently radars capable of determining the polanzatlon ratio of the scattering hydrometeors see the articles of Seliga and Bringi (1976) and Barge (1972). With the ex~eption of one investigation by Eccles (1979) and the studies discussed in section 4, however, no quanti­tatlve results of the comparison between radar and ground network derived hailfall d.ata have been published. One reason for this may have been the du­ftcrury ID operating sirnultaneously a very complex radar system and a dense and thus tlme-consuming ground network during a whole season to gather

~a~a from a few hailfall events only. The investigations of hailfalls discussed e 0fuli on the other hand, could take advantage of an existing dense and

care fY surveyed hai!pad network. It contained 335 ground stations in an area o 1340 krn2 iR C I S - I d d' l aft . . entra wltzer an and was a1ways serviced imme lat~ Y

h~rha hPodsslble hai! clO\lld appeared in the test area. The hailpad servlce w IC a a crew of ten wa - d" - . I do-. d h . s mcorpotate Wlthm an mternatlona, ran ~~fe ai! suppression program which started in 1977 (Federer et al., 1978-

'fh The history ~f h~i! cri~eria in the Soviet UFliom is more than 20 years olcl. ere are sever CrItena ID use and these differ in the mumber or the type of

HAlL RESEARCH AND HAlL SUPPRESSION 91

parameters involved. The criterion -which is employed in the hai! suppression program mentioned above, was developed in collaboration with Soviet scien­tists to take account of our climatology, see Federer et al. (1978-79). It is a slight modificatiom of the Moldavian hail criterion and needs the following parameters: The radar measured (10 dBZ) height of the cloud top (Hv), the corresponding temperature (T v), the height of the maximum reflectiviry (HZM) , the corresponding temperature (TZM) , the value of (~) and the ratio of the heights between the cold and the warm regions of me cloud (hj h +). A schema tic picture of a cloud defining the above mentioned parameters is given in Fig. L The parameters have to be continually updated during the whole lifetime of an existing celi and for each new celi at short time intervals (1-2 minutes) . The values of these parameters are then transformed with the help of empirical tables to determine hai! probabilities P = P (H v, Tv, H ZM,

TZM' ZM' h j h+). In practice, the procedure is much less complicated: Qnly three parameters (Hv, H ZM' ZM) have to be frequently remeasured in real time since the other parameters are not dependent on individuai cells but can be detiermmed from the thermodynamic strucuure of the sl!Irrounding air. The complex hai! probabiliry P can then be obtained in real time with the aid of a small computer_ A detai!ed description of the tables and the analog

visible contour

10 dBZ contour

30dBZ h-

HFigure 1 - A scheme 01 a hall celi. The poral1leters used lor the criterion are 011 the right side. H B: cloud base, V-H0~h H -H -h -) o a- +'

92 A. W ALDVOGEL

computer is given in a report by the Cloud Physics Group ETH (1976). A celi is defined as a hail celi if the fo11owing conditions are met:

p > 50% ZM > 45 dBZ HZM > Ho

To test the validity of this criterion (Eq. 1), a comparison was made be· tween radar data of 195 hail ce11s and their corresponding track on the hail· pad ground network, see Waldvogel et al. (1979). The results are summarized in Table 1. As can be seen from Table 1, criterion 1 is safe: Alt hail ce11s are

Table 1 . The 195 cells tested far hail Occurrence with criterion 1. Hail celi means the criterion il met and hail is found on the ground. A failure means, the criterion is not met but hail is foul/d.

The last Golumn gives the hypothetical number of rockets needed to seed the cells

Celi type Mail Criterion 1 Number of Rockets yes no yes no cells "needed"

weak rain celi x x 124 O strong rain celi x X 38 77 hail celi x x 33 165 failures x x O O

totaI 195 242

detected. H0wever the . f hi' h . d" . 11 . ' pnce or t s IS t at In a dltlOn 38 strong ram ce s, WhlCh never pnodl!lced hai!l 6!il the grond, also fulfi!ll the criterion and tl1us are mterpreted as hail ee11s by the radar. T'his property of the criterion cal!lS' ~ha waste of seedmg material of 32%, shown in the last column of Table 1.

e redason for the nonlinear relation between the number of ce11s and rock· ets IS ue to the fact th h h il 11 h . at t e a ce s (33) last at least twice as long as t e strong ram ce11s (38) d h .

A furthe . an t us reqUire more seeding material.

h h . r compatIson of radar and hailpad data of the 195 ce11s showed t at t e cntenon (1) ca b l d b . . . . . (" n e rep ace y a slffipler yet slightly more efflclent cntenon cntenon 2):

H 45 > Ho + 1.4 km ~2)

This means that if the hei ht f h g o t e 45 d!BZ C0!iltom is 1[ol0re tnaro. 1.4 km

HAlL RESEARCH AND HAlL SUPPRESSION 93

above the zero degree Celsius leve!, the ce11 is a «hall cel1>. The ratio be­tween hail ce11s and strong rain ce11s ameliorates to 33:28 and the waste of seeding material is reduced to 27% . This discovery is an important result of the pFesent investigations: Criterio n (2) is not only simpler but also surpris­ingJ;y sÌFnllar to the originai ' NHRE criterion used in the declaration of a hall day iN northeast Colorado, see Foote and Knight (1979), and to the one used in a hail prevention operation in South Africa (Mather, 1976) . This result was unexpected in view of the different hall climatologies at these two areas but can be explained easlly from the cloud physics (Waldvogel et aL., 1979) which are supported from in situ airplane measurements. The same results found at different places and with different methods give further confidence m the generai validity of the hall detection criterion.

Hai! measurement

l!n ?rder to detect hail ce11s from an ensemble of clouds it is important to r~cogro.lze the dangerous ce11s at a very early stage in their deve!opment. With :0 e ~easl!lrement of the hailfall by radar, on tbe otber hand, one would lle

o tamf

ahs accurate a measurement as possible of the damage or the amount o ail on the gl'O d Th t h the whole hailstorm A f un. . ere ore, t e radar data are integrated for lished b G . . lrst qualitatlve approach to this problem was ub-

th f· y I eotlS (1963), whereas Waldvogel et al. (1978a 1978b) p d e lrst so utIon f h '. ,presente

timeter radar Theor

t e guantltatlve measurement of hailfalls with a ten cen-

t~e Z-E relat;onshi:~I~~~~t:ha;~to~le~s v.:hic~ have to be solved are: 1) (E) and the radar reflectivity (Z) 2) ~ o t e kmetlc energy of the hallstones a!ilO 3) the separation of rad ' h t ~ scattermg behavlOur of the hailstones s\!lmmary of the s01utions f ar

h ec o~l ue to ra~n from tbose due to hail. A

or t e pro ems 1)-3) IS dlscussed in the following.

Z·È relations

A straightforward I . tained by measurin so utl0n for the re!ationship between È a .

:~~~:is ~hat E an~ ~:r~a~;~:~ ~~~~ ~~~i~a~ions. Tre advan~~g; ~; ~~~ tered in Sth ue to differences in sampling sizes o le samp e and this elirninates

e case of a' r ocations which spectra have b compatlson between radar and h il are encoun-eqll\Ppecl Withee~ ~easured at the centre of severe hl ; pad data. Hailstome

astone and raindro", ali storms wlth vehicles t" spectrometers (Waldvogel

et al.,

94 A. \Xl ALDVOGEL

101 101.--------------'--~

• July 6. 1973

6 June 27. 1975 •

i É

1 l lune 27. 1975 b

• July 11,1975 •

[in~~l [m~51 • JuIW 11. 1975 b

• May 19. 1976

Figll,e 2 - The Z-E "lations l'om lou, diffe,ml hallstorms, meas . ured al six measuring sites.

. f m four severe hailstorms 1978a). The analysis of 175 hailstone spectra . ro Z È relations, as €an shows an unexpected agreernent for the four dlfferent. _ be seen in Fig. 2. lt leads to the generally valid relatlons:

È = 5 . 10-6 . 20.84 (3)

. l d Z' ( 6/}) This is base on d the as-where E is given in Uoules/m s) an ID mm m . The rela-sumptions that the hailstones are spheres afold Rayleigh scatterer~. hailpad tion in Eq. (3) will be used in the following to transform radar mto data and vice versa.

Hailstone scattering

. d still is the The scattering of electrornagaetic waves by hailstones was an . al in-subject of many theoretical and experimental investigations. Of lpecl etur

e terest is the scattering behaviour due to the kind (.dry or wet) an s~: these of tne sl!lrfaee, one snape of the hailstones etc. A diScussl0n of some. tions problems can be found in Ecdes (1979) or Ulrich (1978). The m'V.estr \\l'et

show a complex scattering behaviour. E),Ten in the case of slrnp e

HAlL RESEARCH AND HALL SUPPRESSION 95

rin cannot be rdated easily to the sim­spheres, for instance, the .backsca~~ a tailstorm hailstones with many differ-pie Rayleigfu law. But addlt1onally, d Therefore to overcome thls prob-h and surfaces are encountere . , ent s apes d . l the following Ansatz was ma e. em,

f Z (x, y, t) dx dy dt = k,L Ze (x, y, t) dx dy dt sionn

(4)

The parameter Z is the radar measured equivalent radarBreflectivity . e. h fl .. factor and k is a constant. y compar-

whereas Z IS the Rayle.lg re ecdtlvlty d h il ad with radar measurements it

ing hailspectrometer Wlth radar ata an a p l t h h il-was found that the eonstant k is equal to one (k = 1), at east or eavy a falls, see Waldvogel et al. (1978b). This means that a sImple rdauon between Z and Z can be used: ,

Z = Z e

for quantities which are integrals in time or space.

(5)

Rain/hail separation

The separation of rain from hail echos is in theory at least not possible with a singl

e wave length radar. However, a simple solution was found for

heavy hailfalls, Waldvogel et al. (1978b). It is the so called "cutting method", w~ich neglects radar reflectivities equal and less than 60 dBZ because these are " due to rain only". Reflectivities greater than 60 dBZ are "due to hail on­ly". It is dear that this simplification, found empirically, has also to be un­derstood in an integrated sense. In other words that it is possible to have re­Uectivities greater than 60 dBZ which are due to rain only and less than 60 dBZ fo

r hail at single

points, but this effect is averaged out when the time or area lIltegrals of hailfalls are considered.

A second, slightly more sophisticated method is the so-called "gradual meth d" l ' . il .

o . t IS sun ar to the "cuttmg method" except that the transition be­twu een "raio" and "hail" reflectivities is graduaI rather than abrupto Radar re-ectlVltles from the in/h il . b d d E b . ra a regio n etween 55 an 65 BZ are accounted bo

r

. y the use of a weighting function: The weighting for the 56 dBZ COntri-Utlon 18 !G°lc of it t l . b E 20"lc T ° . s rue va ue glVen y q. (3), whereas that for 57 dBZ is C00}° etc. he welghtmg of the reflectivities of 65 dBZ and abo

ve is 100% A

• paD1S0Fl of the tw h d h h h . . o met o s s ows t at t e dlfference between time and

96

50

:lo :lo G 1:l7:l

2001-212 1H oT ; IMIN

55- ~

-'!S -'iO -3S

50

:lo:lo ~ 1:l7:l

2001-2121H oT; IMIN o; o SOKM EGR JM-2 I-

lO ...... . 1 00 ~

-, - "lO -)

-s -1 - 10 O

ol STANCE EAST (KMl

-30 -2 - 1 -1

olSTANCE EAST (KMl O

A. WALDVOGEL

30

25

20

1S ~

I lO >-

o: <Sl Z

5 w u z a:

0>-Cf)

D

-5

lO

5

30

25

20

15 ~

I lO >-

o: <Sl Z

5 w Wl z a:

0>-Cf)

D

-5

lO

5

Figure 3 . Radar data of a hailfall on 9.9.79 '. . be lower figllre sholO' the isolill" of th k' . The upper figllre gw" the 55 dBZ contollrs every mlllute. T

e lIIella ellergy far l, lO, 1O@ 11111', the graduai method wa' Il,,d.

97 HAlL RESEARCH AND HAlL SUPPRESSION

area integrated values of E (x, y, t) is small for heavy ha ilfalls , but for me.di­um and light hailfalls the graduaI method seems t? be supenor to the cuttmg method as it takes into account smaller refleCtlVltleS as well.

An example

Fig. 3 comprises two diagrams for a hailfall òn September 9, 1979. The upper diagram gives the contours of the 55 dBZ radar reflectivity. These contours are plotted for every minute from 2001 until 2121. During this time the storm moved from West to East. The radar site is indicated with a cross (+) having the coordinates (0,0) . The lower diagram reproduces the corre­sponding radar data derived isolines of the kinetic energy for the values 1, 10 and 100 ]1m2 The graduaI method was used for this plot and therefore the isoline of l J/m2 agrees very well with the envelope of the 55 dBZ contours.

An example for the agreement between hailpad and radar data is present­ed in Fig. 4. lt shows on the left the hailpad sites, their identifications, the kin~tic energy value in J/m2

, the areas represented by the hailpads and the IS0nnes of 30, 100 and 300 J/m2

. On the right the radar data derived isolines of the kinetic energy for 10, 30, 100 and 300 j/m2 are depicted. The resolu­tlon of the measuring points is 1/3 km compared to about 2 km for the hail-

• I l I

F' o 2 3 'km ,gli" 4 . The hailf, /I f

pad sii" with the a O lune 1, 1978. The moSi important h l d d of E(Z) are PkJtte;"er~b)vallsues (l /m') and the areas re;resen;~:~ ' t~ta har~ g'd'ven in (a), showing the hail-

m · a o !]1m'). y . e at pa s. The radar derived isolines

98 A. WAWVOGEb

pads. Therefor~ it ~S cle~i th;~o:h:nf::h~~sct::r~:d t~~tFe~~~s a~;oe~rsth~ much gr~:::r T~: t\~o t~e and area integrated kinetic energy values, hhailpad . ell' l'n this ex ampIe . The value from the hailpads IS 11.2 Giga owever, agree w . . Joules and ff0JiIl the radar it is 9.0 Giga Joules.

Hail suppression

lt has been discussed above, that an objective and scientifically sou~d ~_ vestigation of a given hail suppression technique or method can o~ y e achieved with a randomized experiment. Additionally, there IS a need or an objective and quantitative measure of hai! parameters such as mass, e~err etc. lt is dear, that damage alone is a very poor ~arameter because 0al tt0~ large variability such a quantlty has for a glven hai!storm and a natur . pography with different kinds of agrioultural areas, forests, lakes etc. The~: ternatiGlro.al randomlzed nai! suppressloro. expenment (Gwssversuch IV) il filledl all the requirements mentioned above to test the most promlsmg ha prevention technique.

Main result 01 Grossversuch IV

A large amount of effort and preparation has been invested in the plan­ning of Grossversuch IV. lt was tried to define the final test before any data have been evaluated, so that any possible bias from the test itself on the out­come of the l!ilain result could be avoided. lro. addition one searched for nhe most powerful evaluation menhod us.iB.g predictor and diseriminant functions, the most appropriate test, aro. exaotly measuuable test variable, and so on. The whole procedure has beelil published well .iB. advance of the evaluation phase of Grossversuch IV (see Federer et at., 1978-79). .

The experiment itself lasted from 1977-1982. A total number of 216 hail cells have been investigated. The seeding operation has been applied on 94 out of the 216 hail cells, and a total number of about 500 large Oblako rock­ets have been used. Fig. 5 gives an example of the seeding operations for one hai! celi, whlch occurred on 6 August 1978. The top figure shows the test ar­ea and the. 45 dBZ comours of the treated celi every 10 minutes. Also shown are the traJectones of the 17 rockets shot imo the hai! cell. The b0ttom figure glves the lsolmes of the kinetic energy of the hail celi derived from hai!pad data. It can be seen, that the kinetic energy is deereasing durirog the seeding

HAll RESEARCH AND HAlL SUPPRESSION

GEWITTERZELLE VOtJ1 6. AUGUST 1978

/ /

tls 1640 /

PPI , 45 dBZ KONTUREN

IMPFUNG MIT 17 RAKETEN

, O 10 km

" F 22

23 o G

21 o o H 20 o o o

17 18 19 0. o :~@OO o : :0 o o

16 0 0 °._ C\ 000

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o o o o o 0, o o

o o o y o o 0 0 o o

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0°00/°0° o o o o o 0À o o o ~~~..,·~...::·~O o o o o o o cv o RAD6'R

o 0° 0 °0 o o o o o / o

% o o o o o o ~ o ~ZERN o o o ./"""

" , . ,. ,. ... ,. ,. I, o o 00 ~;.;--.. o -,o o o ,/J /"'" o o o .. I o o

C l io"· 4L.-- -- - .. 1..0_"V ' .. .. / "T .... .... t I o o

o' .... .. .... /. .. ... NAPF".. .. .. '" o I /' ""I/,,"

e: ++ ... / .... .... : .............. j./ F, + + / .. .... .../': ,

r 4.... ........ ... / .. , G : +/........ .... .... • .. .. 0 J H] / + + +.... .. .. r -- -

s--.. ...... + / I 'L· ......... + + + + .... + .. / . , L -- , , .. + / , _J

1+ "t ....... / ++, L _____ ~_, ~ ,

o

" HAGELDETEKTORENNETZE

o

'it+ L + +' , ~. : L _ ________ 'tj

~ 3 4 5 6 7 8 9 lO Il 12 13 14 IS 16 17

• DETEKTOR VOM HAGEL GETROFFEN KONTUREN IN J/M2

99

p

Figure 5 - The seeded celi /rom 6 Augusi 1978. The 45 dBZ COlllours in zi.lelvals of 10 minute. are ShOLOII, " are the trajectorie! ollhe rockets l ired illto the celi. Two 01 the 15 trajeclories have to be counled Iwice b:

caUse

two rockets have been / ired into the same dÙ·ection . Thc figure 011 the bottom gives the isolines 01 I e kllretlc ener~ measured by the hailpads for O, 20 and 50 ]/m'.

100

r

.20 ->-u C QJ -~ t:r QJ '-"- .10 -

QJ

'-

-

O. O

A. WALDVOGEL

D NO-SEED (122 cells in 43 days)

~ SEED ( 94 cells in 33 days )

r

2 4

l''' med R R med no-seed seed

r~ 8 10

R = l n ( E GR + 1 )

Figllre 6 - His/ogram 01 the respollse variable R=III(EoR+ l) lor seed a!ld Ilo-seed cells. R is /he ari/hmetic meall; med is the medùm. EGR is the global killetic energy ilJ 106 joules.

operations. From the subjects discussed above however it should be clean, that one can~ot take any conclusions from this ' single cas~ but the total num­ber of ceIls mvestlgated has to be considered.

A very simple evaluation of Grossversucn IV is givero in Fig. 6. lt shows the nelatlve .nreql!lency of kinetic energies of seeded and unseeded hail cells. 'l'he mspectlon of Fig. 6 shows, that the mean as well as the median of the kmetlc energy values of the treated hail cells are larger than the correspond­mg values of the natUFal cases. 'l'his result which dearly points towavds a negablve seedmg effect is cOlilfirmed by the confirmatory test using radar as weIl as hailpad data Both l' · · b . ' . . . Il . ' . . eva uatJons can e summanzed as: 'l'here IS no sta-~stJc~ y tg:ifIcant difference between seeded and unseeded hail celis. For eta s o t e test procedure, see Fedeveu et al. (1986).

HAlL RESEARCH AND HAlL SUPPRESSION 101

Arnon the many additional exploratory tests made of the Grossversuch IV data a~d also published in Federer et al. (1986), there are some (8 out of 110) which have an indication of a seeding effect (P < 0.05). It IS mterestmg. to mention that two bave a favorable wbereas six have an unfavorable seedmg effect. It is weli known, that statisticaliy significant results can appear due to the multiplicity effect , which means that some out of a number of tests turn out significant by pure chance. 'l'he largest amount of exploratory tests, how­ever, do es not have any seeding effect at ali_ 'l'hese facts should. ?e kept m mind when considering single exploratory tests of weather modlflcatlon ex­periments.

Seeding operations

Altbough tbe main result of Grossversuch IV is known, it is interesting to see if tnere are any seeding effects at al!1, when a stratification of the ha,j] celis against intensity of hailfali is made. Before applying such a stratiflcatlon, It seems to be important to give some generaI comment about such a proce­dure. 'l'here are two possibilities for seeded statistical units: One is the as­sUmptioll that the seeding procedure has no effe et at ali. The alternative is nhat some reaction takes pIace after seeding. In the first case there is no problem to make a stratification of the celis in smali and large ones. In the second case, however, it is an impossible task, because some large seeded hail celis are large due to the seedlllg (unfavorable effect) and therefore would belong to the group with smali hail celis. Other large seeded hail celis did not react to the treatment alld therefore realiy are large ones. The third possibility is tbat the seeding effect was a favorable one and then the origl­nally large hail celi has to be found in the ensemble with smali celis. It is clear that similar possibilities can be developed for smali seeded hail celis. At the present time it is not possible to separate hail celis which behave in the complex manner described above.

Altlb.0l!lgh a stratification of seeded hail celis according to their intensity is an unPossible task, an investigation has been made to see, whether "large" cells bave a lower seeding coverage compared to the smali ones. The detailed descnptlon of tbis ana1ysis is given by Schiesser and WaJdvogel (1985). The :~m resuit is: Large hail celis have a better ~eeding coverage than smali

s .. At a flrst glance thls looks like a contradlctory result, however, when cons1dering the operational and logistical problems of a seeding operation it r;~~ìclear that a large and thus 10ng lasting celi (see Waldvogel and Schmid,

(an be treated @asier. This is illustrated in Fig. 7, where the seeding

--~-

102

> O u

o w w Cf)

1. O -

--

. 8 -

-

. [) ---

.4 -

--

. 2 ---

{i) •

O

A. \Y,IALDVOGEL

XXX~II: X

\ . • Y!-

! i xX><x"

x • • • • • II>< •

x Xx x x •

x x x x Xx

x

x xx x x x

x • x

x x

x x

• x

x

x

I ""1' 'l'' '' l '''' I'I' 'l' ' '" I I ''l' ", 1 1 "' 1 " " 1 , I " I "

50 100

DURAT I ON [M I N ) . ' "{ Il . 19 larger storms havillg Figure 7 - Scotter diagrom o! the. seedil1g coverage agamsI dura/fOl/ oJ Ihe haI ce .

ki"etic energie! (EGR) > 1 Gtga JOllle.

. . f h h il li A t tal number of 94 coverage is plotted agamst the duratlon o t e a ce. o li h ing ki-seeded hai! celis have been studied, 19 out of the 94 are large ce s ~v ith a netic energies larger than 1 Giga joule. Tme large celis ~re represel1lt~at : tl!Je black dot in FIg. 7. lLookmg at thls flgllre, one unmedlately sees, smali ones: average the large celis have a better seeding coverage than the a e larger About 85% of nhe large cells (16 out of 19) have a seedmg cover 3

0

;' The than 50% and for about 70% (13 out of 19) it is even larger than 7 9:' eed-average seeding coverage for the 19 large celis is 0.69 and for ali the s ed hai! cells it is 0.58. . o f the

Considering that the 19 long lasting hai! celis contnbute to 96 y~ o that total kinetic energy of all the seeded hail celis, and furtber consldenrg the these cells had a good seeding coverage (> 70%), it is difficult to exp am sa outcome of ~he Grossversuch IV experiment with the excuse tro.at there wa bad seeding coverage. . . alit is

Anonher and even more detai!ed approaeh to value the see(\,l,rng qu Y

l!

Il l'

I

HAlL RESEARCH AND HAlL SUPPRESSION 103

k ts) Such an evaluation has bee~l by consiclering single seeding e~entse;71~8~). 'The main idea of tbis study IS publishecl by Waldvogel and Sc I~~sies of the hai! celi measured dunng two the comparison of the kmetlc ene g I ~t (10 minutes) represents the Urne time intervals !ltb and tHe

· ~he mte~: M bis the time interval (10 minutes) immecdiately before the seedmg, wh . ' be measured. It is dear that during wbich the reaction of the(dseledm) dg cman

g which the seeding material is

. cl" I !lt e ay ur l A there is a tbir tune mterva . di . f bail suppression take p ace . cl h . hyslca reactlons o dispersed an t e mlerop . . F' 8

.. f tbi idea IS shown m 19. . d'-schematlc plcture o S . l ' . the hypothesis of the see mg oper-

The justiflcatLOn of thls eva uatlon IS t ent a hail celi has under-ation wbich states: latev~r e h h il duction wili be reduced a _

Wl d velopment or trea m f

gone before a dist0ct seedl~g ev~n\t t~e :ee~g materia! of the rocket n

ter a glven delay tune durmg w ~ d' ration should have a smalier could act. It is dear that a success see

h mg ope d with the one Eb mea­

energy value E. measured durmg !lt. w en compare . d . smed during !ltb. And it is also dear that the dlfference D defme as .

D = E. - Eb (6)

should be negative in such a case. The data of 202 seeding events for the 19 large hai! cells mentioned above are given in FIg .. 9 m ~ stem and Ieaf .dIa­gram, showing the logarithms of negative and posItive drfferences as defmed in Eq. (6). A randomization test of location testing the hypothesis that no difference would be found between the two averages showed a non-slgmf­icant two-tailed P-value of 0.18. Other time-intervais for the delay time ~td and for the seeding effect (!lt.) have been tested in the same way but none af the calculations showed any significant difference.

Va1'iability IJf hailfatls

Another important point when considering the outcome of a weather madification experiment is the specific c1in1atology of the test area. One could suggest for instance, that there is a big difference between the hail di­matology of CentraI Switzerland and the one of other countries. Unfortunate­!y no values about the natural variability of hail mass or hail kinetic energy of hailfalls measured by hailpads are availabie from areas in the Soviet Union. Therefo

re we compare the hail climatology of the Grossversuch IV area with

those

of other COllntries having such information. Measurements from un­seeded hai! cells are considered only to exclude any influence of a possibie seedlmg effect.

104 A. WALDVOGEL

Knight et al. (1979) published DCC's (Cumulative percent of hail - cumu­lative percent of number of hail days) for NE Colorado, Alberta, Kenya and South Africa. Fig. lO shows these DDC's together Wlth the curve for the Grossversuch IV data. It can be dearly seen that the curve (Grossversuch IV) is very dose to the Alberta Clirve. The one from N~ Colorado is also ve­ry dose up tG a cmmulatlve percent of hai! mass of 80 }lo. The l~rgest dlffer­ence is found between the curves from GV IV and Kenya, whlch could be due to the method (kg made-tea lost) used to derive a DDC (Kenya).

The comparison of the curves shows that, although there are large dimat­ological differences between the different areas, there is an astonishing agree­ment between the different hail climatologies. Considering now the agree­rnent between the curves it seems highly speculative to assume that the dif­ferent areas of hail prevention in the Soviet Union with different climatol­ogies should ali show a drastically different behaviour in corresponding DCC's . .

Conclusions and discussion

The complexity of the evaluation and interpretation of hai! suppression prograrns has been discussed. It has been shown, that this complexity is the

>- RQCKET n \.:J a: w Z

-~ w

u I-w z :.::: -' <{ .... :r: ....

" u.... o X ;:)

-' u....

tn TlME Figllre 8 . The lime euollilioll o[ Ihe flllx o[ Ih h ' . . 6.'b: be[ore seedillg 6., . delay l ' A .tI e ari kmellc energy o[ a <eeded hail celi. Time in/eruals are

• d· UNe, utc: e"ect 0/ seeding.

HAlL RESEARCH AND HAlL SUPPRESSION 105

main reason that, although the scientific weather modification is 40 years old now there is stili a large discrepancy between the opinions about the effect of h~i! suppression of people doing hail research or operational hai! suppres­sion.

There are several fundamental points of the phenomenon hail per se wnich complicate an obiective evaluation of a hail suppression program, such as:

- The difficulty to forecast the development of hail celis. - The large variability of hai! celis. - The frequency of hai!. - The measurement of hai! damage. Because of ali these facts, only a randomized experiment can guarantee

an obiective evaluation of a given hai! suppression technique. The evaluation of a hai! prevention program or an experiment which is embedded in such a project will have a bias towards a favorable seeding effect, if it is evaluated by people ernvolved in any kind with the main puupose of the proiect per definitionern. 'These psychological "effects " exist, even if the evaluation has been plarmed as objective as possible.

The results of the two la test hail suppression experiments in Northeast Colorado (Foote and Knight, 1979) and in Centrai Switzerland (Federer et al., 1986) lead to the same conclusion: There is no effect in respect of a hai!

negative di fferences (N) pos i tive di f ferences (P)

5 98765 O 1346 7 4

20 666 5541 1 0789 4 10 9988766665554432 1111 2 11789 5 99865532 10 3 11 234446666777777778899 23 24 99999888876664443333 1110 4 001122333444667779 19 18 9999988666554333322 5 1112567888999 21 13 65553333322222 111 1000 6 00011112223333445555566777 26 l 7 00

2 2 8 ì07 l n ( I 06 11 + l. O ) 95 = 4. 22 l n ( 061 + l. O) = 4.54 rat io N/ P = 0 .93

P = 0.1 8

Figure 9 S (. . lem and lea[ dia ra f h . IISlllg E,J [or 202 seedù'g e;e,,;' o l eh logan/hms oj negative and positive d'flerenees 01 kinetic ene'gJl

s an t e tu Ile tntervals 6.tb= 6.td= 6.t

e= lO min.

106

w L:J <! L <! o V1 V1 <! L

-' <! :x: u... o f-Z w u Cl:: w Cl.

w > f-<! -' ::::> L ::::> u

100

80

60

40

20

NE COLORADO, USA HAlL MASS

ALBERTA, CANADA LOSS/RISK RATIO

NELSPRUIT, SOUTH AFRICA

A. WALDVOGEL

... REGISTERED CROP DAMAGED T.O 100 ...

KENYA, AFRICA kg MADE - TEA LOST

0--0 SWITZERLAND GROSSVERSUCH IV HAlL MASS

°0~~~2LO~--~40-' ~--~60---L--8LO--~~100

CUMULATIVE PERCENT OF NUMBER OF HAlL DAYS

Figure 10 . Comparisoll 01 the double amJUlative CUrves (cumulative percent o/ hai/-cumulative percclIl 01 IIl1mber of hail days) of NE Colorado, Alberta, Kenya and South Africa with the otle attoined in Switzerlolld.

reduction when using the rnost prornising techniques of hail suppression, If there IS an effect at all, there is an increase of hail due to the seeding, how· ever, this staternent is not statistically significant. One can ask the following questlons now: Are these main results restricted to the areas in which the rwo expertments took piace, or are tmey generally valid? Strict!y scientificaliy s~eakmg, the ftrst question has to be answered with yes and the second one Wlth no,

This conclusion is supported h . d' . . ' l sure· . w en consi ermg In Sltu alrp ane rnea ~en~, made m Centrai Switzerland with the armoured T-28 airplane of the

out Dakota School of Mines and Technelogy (Waldvogel et al., 1987),

HAlL RESEARCH AND HAlL SUPPRESSION 107

which showed a complete absence of big supercooled raindrops. The conse­quence of this finding is, that the hail growth process is via.a graupel. mecha­nism which cannot be influenced by any reasonable seedmg techDlque. In othe; regions it could well be, that different hail growth mechanisms exist. A further ptoblern with the T-28 measurements is, that these studies have been made a posteriori to the Grossversuch IV and only in a small number of moderate hailstorms, Thus a generalization of this finding is not correct.

Considering different hail climatologies from very different regions, how­ever, it seerns, that there is a worldwide behaviour of hailstorms, wbich is generally valid and which shows only very little variation. If tbis finding is an argument to generalize the main results of recent hail suppression experi­ments, cannot be answered at the present time. The situation in the field of hai! prevention thus can be summarized as follows: There are two experi­ments which c1early deny the possibility of a favorable seeding effect. There are on the other hand a large number of operational hail prevention projects which show large positive seeding effects. Although some evaluation tech­niques of tmese programs are questionable, it is not justifiabie EO simply reject ali these observations, The answer to the problem of hail suppression can on­ly be gained with further high quality experiments, which do not only de­crease the uncertainty of the subject, but give a lot of knowledge about the fundamental mechanisrns oE hail formation.

References

AnAs D. and LunLAM F.H. , 1961. Multi-wavelength radar reflectivity of hailstorms. Quart. ]. Roy, Mete. or., 87, 523.534.

BATLAS

D., 1977. The paradox of hail suppression. Scienee 195 139-145. ARGE B L 1972 Hail d t ' . h l ' ' di '. '

r . M . . e eC~lOn Wlt a po anzatlon verslty radar. Stormy weather gr0Up. Scientific re-B po t W'7l, McGill Unlv. , Montreal, Canada.

UR~SEV LI., 1985, Crop protection against hail damage in the USSR. Preprints, Fourth WMO Sci. Conf. C on Weathe, Modification, H onolulu, 61}.623.

LOUD PH"ICS GROUP ETH 1976 P . k G . E'd H el .' . l'O)e t rossversuch IV, Berlcht iiber 1975. Wiss. Mitteilung Nr. 75 E I g. ag KommlSsion ETH Ziirich '

CCLES p.]. , 1979 Com' f ,. . tadars IEEE T paflSons.o remote measurements by smgle . and dual . wavelength meteorological

FE . rans. Geosclences Electronics, GEA, 205-217. DERER B. , W ~L"VOGBL A S W H

].F" 1978.79. PIan ., OHMlD " AMPBL F. , ROSINI E. , V ENIJ'O ]i). , AnMI",,1J Jl>. and MEZEIX IV. PUI'e a d A l f~ the SWISS randomlzed hail suppresslOn experlment. Design of Grossversucb

FEDERER B~' pp . eophys., 117, 548·571. ., ALDVOGEL A SeHMID W Se H H H

W., BADER ] MEZEIX " ., H~ESSER . ., AMPEL F ., SCHWElNGRUBER M " STAHEL 1986 Main " I f G].F., DORAS N., D AUBIGNY G. , DERMEGREDITCHIAN G .nd VENTO D

FOOTE B'G dreKsu

ts o rossvetsuch IV. ]. Climate Appl. Meteor. 25 917.957' ., . . an NIGHT C A 1979 R ul ' . ' '. .

Colorado. Part 1 ] A . /' . es ts of a randomlzed had suppresSion experiment in Northeast GEO~IS S G 196 . . . pp . Meteor., 18, 1526·1537.

. ". 3. Some <adar measurements of hailstor.ms. ]. Appl. Meteor., 2, 270-275.

108 A. WALDVOGEL

K C A F G Band SUMMERS P.W., 1979. Resul" of a randomizecl hail suppression experi· NIGHT . ., OOTE .' . . . h f h . I

. N h C I do Part IX· Overail dlscusslon and summary In r e context o p ySlca re· ment In ort east o ora . . . ,earch. ]. Appl. Meteor., 18, 1629-1639. . .

MATHER G.H., TREDDENICK D. and PARSONS R. , 1976. An observed relationshlp between the helght of the 45 dBZ contours in storm profiles and surface hail reports. ]. Appl. Meteor., 15, U36-1340.

SCHLESSER H.H .• nd \"(IADDVOGEL A. , 1985 . The seeding coverage of hail ceils in Grossversuch IV. Bre· p,inlS, Fourth WMO Sci. ConE. on Weather Modification, Honolulu, 547-552.

SEblGA T.A. and BRINGI V.N., 1976. Potential use of radar dIfferenllal relleouvIty measuremenlS ar ortho-gonal polarization far measuring precipitation. ]. Appl. Meteor., 15, 69-75. . .

ULRICH C.W., 1978. Relationships of equivalent reflectivity factor IO the verucal Iluxes of mass and krneuc energy of hail. ]. Appl. Meteor., 17, 1803-1808. .

WALDVOGEL A. , SCHMlD \"(I. and FEDERER B., 1978a. The kinetic energy of hailiails. Par! 1: HailslOne speetra. ]. App. Meteor_, 17, 1515-1520.

WALDVOGEL A., FEDERE' B. SCHMID \"(I. and MEZEIX J.F. , 1978b. The kinetic energy of hailfalls. Parto Il: Radar and hailpads. ]. Appl. Meteor., 17, 1680-1693.

WALDVOGEL A., FEDERER B. and GRlMM P., 1979. Criteria for rhe detection of hail ceils. ]. Appl. Meteor., 18, 1521-1525.

\"(I ALDVOGEL A. and SOHMlD E., 1982. The kineric enecgy of hailfails. Pan III: Sampling errors inferred from radar dara. J Appl. Meteor., 21, 1228-1238.

WALDVOGEL A. and SCHIESSER H.H., 1985 . Time evolurion of seeded hail cells in Grossversuch IV. Pre· prinrs, Fourth WMO Sci. Conf. on Weather Modification, Honolulu, 541-546.

WALDVOGEL A., KLEIN L. , MUSlL D. and SMImH P.L., 1987. Characreristics of radar-iclentified big drop zones in Swiss hailstorms. 1. Climate Appl. Meteor., 26.

Ricerca e soppressione della grandine

Nella prima parte della relazione si illustrano e si discutono i motivi della grande discordanza di opinioni fra studiosi che si occupano della ricerca e della soppressione . della grandine. Si possono distinguere due categorie: pre­VISIOne e venftcarsl della grandine da un lato' rilevazione e misurazione della grandine dall' altro. La previsione è impossibile ed è il motivo dell' esistenza della sopracltata discordanza. Il verificarsi della grandine è il fenomeno più CntlCO che per la sperun' t' d' . d ' _. h' de . ~ en aZione I una tecOlca I soppreSSIOne, nc le espenmcmtl della <durata di almenlil alcuni anni, affincmé i risultati degli espe­nmentl possano avere valore statisticamente significativo_ . ~ ~na pa~te Successiva della relazione vengono discusse tecniche sempli-

illCI, e a tre plU sOhsticate, di rilevazione della grandine mediante radar. Viene ustrata lO dettaglio un li h

Z cl ' d . cl' a semp ce procedura che utilizza un radar a lung ez-a on a UOica I 3 cm ill " . d

' e vengono ustrat1 I nsultati delle prove con otte con questa procedura Ed' curezza l . II d" una proce ura che consente di individuare con st-

e partlce e I grand' Il . . L . . d Il me ne e prune fasi della loro formaZIOne- a misurazione e agra d' .

che in questo n lne illvlene trattata in un' altra parte della relazione; an-caso VieNe list t · 'li a

una lunghezza d'o d d' lO ra a una tecnica mediante radar che utl zz n a l cm Le' '. ff 'd o-no messe a conDrOnt . d : . misuraZIOni e ettuate coro. i!l ra ar veng

o con I atl rilevati da una Dittissima rete di «naiJpads» a

HAlL RESEARCH AND HAIL SUPPRESSION 109

terra. Per le grandinate di grande e piccola entità, si nota un 'eccellente con­cordanza fra i dati rilevati con il radar e quelli misurati con «hailpad», consi­derando l'enorme variabilità del fenomeno grandine in relazione alle penden­ze o ai valori globali di energia o di massa .

Nell'ultima parte della relazione si prendono in esame i problemi e le possibilità di sperimentazione di una determinata tecnica di soppressione del­la grandine, sottolineando che questo tipo di sperimentazione dovrebbe ba­sarsi su prove randomizzate. La maggior parte dei parametri misurati, la pro­va statistica finale, la procedura randomizzata, la conduzione dell'esperimen­to e molti altri parametri debbono essere definiti con notevole anticipo ri­spetto all 'inizio dell'esperimento. Inoltre, la stessa operazione di seminagione dovrebbe essere basata su un ragionevole concetto «cloudphysical» e la tecni­ca di seminagione dovrebbe essere la più promettente, tale da consentire di applicare l'agente di seminagione all'epoca e nel luogo più indicati ed alla ne­cessaria concentrazione_ Vengono riferiti i risultati della sperimentazione di una tecnica di soppressione della gFandine e di analisi esplorative e discussa l'importan,,,a di tali esperimenti. '