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FRANCO PRODI

Hailfall Characteristics with Altitude

Reprinted (rom the

Proceedings oJ the 13th International Meeling OR Alpine Meleorology Saint - Vincent, 17-19 September 1974

RIVISTA ITALIANA DI GEOFISICA E SCIENZE AFFINI

roto l (1975), pp. 105 -IOi

Ha i Ifa Il Characteristics 1vil h Altitude

FRANCO PRODI (*)

A depende-nce of hail frequency on altitude is an almost obvious expectation, considering the proacess of melting of hailstones while falling through the werm layers of thc atmosphere. However this ex­pectation is far from being SUppOl t!d by ex~ensive

observations of h lilfalls over mountains: this is mostly due to the difficulties which one faces in establishing networks of observers or instruments in hardly ac­cessible areas. On the other si de comparisons of hai I statistics of flat region at different altitude and far from each other are not pos~ible, as hail c1imatology depends strongly on the geohydrographic characte­ristics of each area. For instance we could not com­pare data of the Po valley in Italy (- 50-200 m.s.l.) with data of the Great Planes o€. the Unites States ( ~ 1300 m.s.l.) as the two regions differ markedly in many features at the synoptic scale besides the altitude.

There are good economica I reasons to investigate this aspect of hail c1imatology. On some alpine valleys valuable cultivations a'e possible for favorable local climatic conditions: in the Adige valley the wind that the farmers name the Garda-Iake wind makes it pos­sible to rise grape up to the top of the valley and in many lateral valleys. A markedly higher frequency of hail with altitude would be an important factor to be considered in predicting the amount of final product and defining the maximum altitude of pro­fitable cultivations.

The reasons for the meteorologist to study the subject are no less urging as it involves aspeets of storm and precipitation dynarnics besides the melt­ing of hailstones. In faet to study the hailstorms over a mountain area essentially three different simplify­ing hypotesis can be made:

- The structure and the evolution of the storm is practically unaffeored by the underlying orographic system; it has almost the same characteristics (vertical development, velocity, Iifetime of cells and orienta­tion of the hailstreaks at the ground) it would have on a flat area. Under this assumption the effect of the mountain would be to intercept precipitations at various altitudes.

- The upper structure and generai evolution of the storm is practically unaffected, while the below c10ud circulation and the lower tropospheric wind field are strongly modified by the orography. Under this assumption preferred orientations of the hails­wath and many effects of physical geography would be expected at the precipitation scale at the ground.

(') Istituto di Fisica dell'Atmosfera (CN R), Via Val­doncga l la, 37100 Verona.

2

- The mountain affect the whole storm life, in promating instability and generating the hailcloud, in organizing its internaI circuhtion, determining its path and consequently its precipitation pattern. There is alsa the possibility that this happens only in case 01: local storms and not of storms connected with cold j:roonts.

Reliable and exhaustive data about hailfalls at the ground are needed to evaluate these hypotesis. The Trento province is a suitatl! area for investigaùon of storms aver mountains: the Adige valley is aimesi north-Sauth oriented and communicates with the other side of the Alps with the pass of lowest altitude (1375 m.s.I.), which is preferred in lower troposphere air mass movements. Aside the Adige valley there is a set of variously oriented valleys and mountains exceeding 3000 m.s.!.; the Po valley is c10se enough to allaw comparison of storms motion in similar synopt-ic conditions.

Surface data about hailfalls have been provided by the Stazione Sperimentale Agraria Regionale through a dense network of trained hail observers (74 in the 1973 and 226 in the 1974 hail seasons). The observers have been organized mostly along the valleys and this requires a modification of thc hai I parameters nor­mally used in flat regions. In the present investiga­tion which airns to evidence the hailfall characteristics with altitude, the observers have been separated in c1asses of 200 m a1t;tude interval, starting from the 100 m level up to 1600 m leve!. The hail index for a given a1titude interval is the ratio of the number of hail reports to the number of observers within the interval.

Dealing with hai! in a mountain area i t is necessary a1so to specify that we exclude those hydrometeors which rnight be confused with hail, that is snow pel­lets (or soft hai!) and ice pellets, which rnay be pro­duced by a variety of supercoo1ed clouds. For this reason data have been considered from summer months only (lune, Juiy and August) when the zero level is high enough to rule out the possibility of rnisjudgment.

The main result is shown in Table I by the beh a­viour of the hail index with a1titude during the t\\ o hail seasons. From tbe lowest altitude interval (with values of 0.2-0.4 typical of tbe Po valley) the hail index increases a1most continuously to 0.7-1.8 at the 1000 m leve!. Due to the scanty number of observers at the levels higher than 1000 m there is a great va­riability in the corresponding hai! index, which rea­ches however very high values. The tendency tO Iugher hail indices with altitude is more pronounced

,

TABLI! l: Hai! index in ,ht alrirude "1It.r fJQI during tht J973 and 1974 seasons ,

=---- - -- - - ~ -

- 1'9H

Ahit.ude inre,evo} (m) 1°°-3°° 301-5°0 50 1-700 7°'1-9°0 9°1-1100 1101- 13°0 13°1-15°0 -

Hai! index 0 . 18 0 . 83 0.78 2.6 1.8 I 4

No, of obseevees 22 12 18 7 6 6 I

1974

Hail index 0·39 0.67

~ 0.66 0·7 4

No, of observers 76 SI 62 26 IO I

n 1973 than in 1974 hail season ; this is confirmed by he fact that 17 out of the 21 hail days ha'Ve been àe­ected only O1.1er 5°0 m level in 1973 while 8 out of 8 in 1974.

First the increase of hailfall frequency with altitude

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Fig, 1 - Size discn'bucion oj hailsrones obserfJed ar che ground (al and calculared in cloud on rhe basi, 0/ SU1.AKVELIDZE and ATLAS graphs, for a temperature gradienc o[ 0,7 oCI 1 00 m

and zero level ar 3500 m (bl.

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should be discussed in terms of the effect of melting in the various intervals . In prindple one coulà de­signe a statistica I investigation on the evolution of hailstone size distributions due to melting and test the observed size disrributions at various levels with the aim of evidenoing if the results are interpretable with melting alone or if other meohanisms are in­volveà. Unfortunately the uncerrainry in evaluating the melting parameters seems to discourage from such an effort. In fact melting of hailstones is a rathe·r complicateà process depending on hailstone characte­ristics such as the terminai velo city , affeored by size, density anà shape, and on the rhermodynamic con­ditions of au below the cloud. In practice graphs construored on a theoretical basis are available that enable the effecr of melting to be direcrly determined for given values of the temperature gradienrs and dif­ferenr heights of the 0° C level (ATLAS et al., 1960; SULAKVELIDZE et al., 1965). An example of the com­putation on the basis of those graphs is given (Fig. I) from a hailfall in a loe a :ion not far from the area under investigation (PRODI & WIRTH, 1973). We observe that a remarkable difference in size distributions results using Sulakvelidze or Atlas graphs.

For comparison with data from the Trento net­work the observations from 12 meteorological sta­tions over A1ps and Norrhem Apennines have been considered, derived from the Annuari di Statistiche Meteorologiche, in the 1960 to 1971 hail seasons. The observations which probably include groupels and ice pellets, have been related to the altirude of the stations (Fig. 2). The altirude range is different from the Trento network ; however, for equal levels there is a good correspondence between the two groups of èata. The data from the meteorological stations over 1;00 m.s.l. show a steep increase of hail frequency; similar behaviour has been observed by LAUSCHER (1973) in Salzburg province.

CARTE & BASSON (1970), which organized a vety

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Fig. 2 - Number 01 hail days (including groupels and ice pellers) vs altr'rude derived by observarion Irom 12 mOUnlain starions Dver A/ps and NOTrhern Appennines in the 1960 lo

1971 hai! seasons.

go od network of observers of hail events in South Africa, ma de a comparison of hail frequency in two 50 square miles areas, one dose to }ohannesburg (1675 m.s.l.) and the other dose to Pretoria (1370 m.s.l.) having the same demity of observers (Fig. 3).

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Fig. 3 - Number Dj hail days over lWO SO sq.mi. areas in Johannesburg and Prelon"Q during yeors Irom 1962 10 1969.

(CARTE & BASSON, 1970).

Both areas had more or less the same number of mild storms and the same average number of storms producing hailstones larger than 3 cm; however Johannesburg experienced more storms with hai I·

stones of medium size (I to 3 cm) than Pretoria during every one of the seven years considered. This result seems explanable by melting, as for the largest hai)· stones melting causes an almost negligible diameter rcduction within the 300 m altitude interval, while hailstones of the media n class replace at the lower level those of smallest size which melt in between. The physiography 'and the altitude ranges of the regions are different and the interpretation cannot be imme­diately transferred to the alpine area. In the Trento province the high frequency of mild storms at the upper levels and the absence of storms during morn­ing suggest that besides to the effect of melting the increase of hail frequency with altitude may be at­tributed to a lar.ger number of local storms induced by mountains. This suggestions has to be supported by combined mesos~ale invest igations of mountain storms.

REFERENCES

(1) Annuari dj statistiche meteorologiche, J STA T, Roma. - (') D. ATLAS ec al. 1960: Radar scatter by large hai/o Quart. J. Roy. Me!. Soc. 86,468 - (') A. E. CARTE & L L. BASSON, 1970: Hail in the Pretoria- Wirwarersrand area, 1962-1969. CSIR Res. Rept. 293,1-28. - (') O. FERRARI Relazione 1973. Stazione Sperimencale Agraria Forestale Regionale. S. Michele all ' Adige (TN). - (') F. LAUSCHER : 1973 : Hagel im Lande Salzburg. Wetter und Lebel, Jahr­gang 25, '973, 234-239. - (') F . PRODI & E. WIRTH, '973 : Mesoscale and microphysical investigacion of an isolared hailsrorm. Riv. It. di Geof. XXII, N. 3/4, p. 165. - (') G. K. SULAKVELlDZE et al., 1965: Obrazovanie oblakov i metodi vozgeislvia no gradavie processù. GidrometOizdat, Leningrad.

Riassunto - La variazione della frequenza di grandinate con la quota è stata studiata analizzando dati provenienti da una rete di osservatori di grandine in un'area alpina durante due stagioni grandinigene. Come indice si è con­siderato il rappono fra notizie di grandine pervenute e numero di osservatori esistenti per ogni intervallo di 200

m di altezza. Si è osservatO un incremento dell'indice con la quota, leggero al di sotto dei 1000 m e molto più sensibile a quote superiori. Il risultato è di·scusso in ter­mini dell'effetto della fusione dei chicchi negli strati caldi della troposfera e della diversa struttura e dinamica dei temporali grandinigeni su aree alpine. Viene inoltre com­piuto un confronto con altri dati provenienti da di verse regioni geoidrografiche.

Summary - Hailfall frequency with altitude has been investigated OD the basis of data from a dense network of hail observer5' in aD alpine area during tWO hail seasons. As a parameter) tbe ratio of hai! repons and tbe Dumber of observers in any 200 m altitude iot:rval has been COD­

sidered. Hai! frequency increases with alti tu de, gently below 1000 m and notably at bigher e.levations. The re­sult is discussed in terms or the process of melting of hailstones through the warm layers of the troposphere and tbe different strucrure and dynamics of hailstorms over mountains. A comparisoD is ma de with data from other regions.

UNIONE TIPOGRAFICA, MILANO