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Hail frequency, distribution and intensity in Northern Greece
Atmospheric Research 93 (2009) 526–533
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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Hail frequency, distribution and intensity in Northern Greece Michalis Sioutas a,⁎, Terence Meaden b, Jonathan D.C. Webb b a b
ELGA - Meteorological Applications Center, International Airport Macedonia, 55103 Thessaloniki, Greece Tornado and Storm Research Organisation, Oxford, UK
a r t i c l e
i n f o
Article history: Received 25 December 2007 Received in revised form 17 September 2008 Accepted 20 September 2008 Keywords: Hail Crop Damage Central Macedonia Greece
a b s t r a c t Hail numerical and physical parameters are examined for an assessment of frequency, spatial and temporal distribution and intensity of hailfalls in Central Macedonia periphery, Northern Greece. Data from weather stations of the Hellenic National Meteorological Service (HNMS) indicated a mean yearly point frequency varying between 1 and 2 hail days for Central Macedonia. Crop insurance data from the Hellenic National Agricultural Insurance Organization (ELGA) showed a mean regional frequency of 22.3 hail days for the total cultivated area of Central Macedonia, about 6,500 km2, for the warm season, from April to September. Hailpad data representing almost one third of the total cultivated area of Central Macedonia, indicated a regional average of 8 hail days and 45 hailpads recording hail for the warm season. Spatial analysis over the hailpad area revealed a large variability of hail occurrence, with local maxima at the higher altitude and close to mountain barrier areas and a decreasing frequency over areas closer to the sea. The size distribution of a sample of 22,000 hailstones measured by the hailpads, showed a great majority, of 85.7%, having diameters up to 11 mm. An intercomparison of hail impact energies according to crop damaging potential indicated Central Macedonia hailfalls as the less intense compared to those of areas of Northern America. Application of the revised TORRO hail intensity scale extending to classes from H0 to H10, permitted a classification of Central Macedonia hailfalls appropriate for comparisons and correspondence between areas and countries about the severity and damaging of hailfalls. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Hail is common, mainly during the spring and summertime, in many areas of the world. Regions of higher hail frequency around the world are the middle latitudes zone, including the eastern Rockies of North America from Alberta, Canada southwards to New Mexico, and a European zone extending from the northern Iberian Peninsula across central Europe through to the northern Balkans up to the Caucasus. In the Southern Hemisphere, a major hail zone includes the area of Mendoza in Argentina, Southern Africa and parts of Australia. The observed frequency and distribution of hail is generally characterized by an extreme variability, attributed primarily to the hail formation process, and also to the limited availability of hail information. Despite those limitations,
⁎ Corresponding author. Tel.: +30 2310 472953; fax: +30 2310 472205. E-mail address: [email protected] (M. Sioutas). 0169-8095/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2008.09.023
considerable efforts have been developed in the international hail research towards the establishment of hail climatologies in various spatial and time scales (Chagnon, 1977; Admirat et al., 1985; Fraile et al., 1999; Vinet, 2001; Webb et al., 2001; Giaiotti et al., 2003; Schuster et al., 2005). In Northern Greece, hail is a common phenomenon during the warm season of the year from April to September and can be a dangerous and damaging event, occasionally producing significant damage to crops (Sioutas, 1999; Sioutas and Flocas, 2003). Annual average crop hail loss over Central Macedonia, one of the largest and most intensively cultivated areas of Northern Greece, is roughly estimated at about 20 million Euros based on insurance data of the last decade. To alleviate crop losses, an airborne cloud seeding program for hail suppression, the so called Greek National Hail Suppression Program (GNHSP) was organized for the project Area 1 of Central Macedonia (Fig. 1) and has been operating since 1984 under the auspices of the National Agricultural Insurance Organization (ELGA) (Karacostas, 1984; Sioutas and Rudolph, 1998). Moreover, one
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Fig. 1. Altitude map of Central Macedonia, Northern Greece. The project Area 1, of the Greek National Hail Suppression Program (GNHSP), is depicted.
more project area (not shown) situated in Thessaly, central Greece was also included in the hail suppression program. Within the context of the GNHSP, a hailpad network was installed in the project Area 1 providing hail data for the evaluation of the hail suppression operations as well as for an adequate sampling of hailfalls (Sioutas, 1991; Rudolph et al., 1994). The challenges of this study are to present a more comprehensive picture of the hail occurrence in term of temporal frequency, spatial distribution and intensity assessment for the Central Macedonia periphery, region of Macedonia, Northern Greece. All the available data were examined, including three main sources: the Hellenic National Meteorological Service (HNMS) conventional stations data, the ELGA crop insurance data base and the GNHSP hailpad network data. This paper is organized as follows: firstly, a brief review of the existing hail databases in Greece is given, with emphasis on the available data for Central Macedonia. Next, a hail climatology overview for Central Macedonia is presented, based on the frequency and temporal distribution of hail days, the spatial distribution of hailfalls and the hailstone size distribution. An analysis of damage potential to crop is then undertaken based on a comparison between total impact energy of Central Macedonia hailfalls and those of areas of Northern America. A hail intensity classification assessment is also presented using impact energy ranges incorporated in the TORRO international hailstorm intensity scale (H-scale) (Webb et al., 1986; Tornado and Storm Research Organization, www.torro.uk.org). The ANELFA hail intensity scale (A-scale) was also used for comparison (Dessens et al., 2007). 2. Hail data 2.1. National meteorological service stations Hail observations were provided by the Hellenic National Meteorological Service (HNMS) for more than 70 conven-
tional meteorological stations from all over Greece, covering a period of about 60 years, 1930–90. These data refer to days of hail observed on the ground with no information about hailstone physical properties, size and concentration. In this context, they can only describe the “point” frequency of hail, considering that one manned station observation of hailfall can be representative for an area of about 102 to 103 m2. Based on the HNMS data, hail occurs with a higher frequency over the western areas of Greece and the coast of the Ionian Sea, where a point mean yearly maximum of 8 hail days appears. Hail days in the Greek peninsula generally increase from east to west and from the coastal to mountain areas. Opposite, in the Aegean Sea hail days increase from west to east (Sioutas, 1999). For Central Macedonia, HNMS data indicated a yearly average of about 1 to 2 point hail days in most locales, with hailfalls mostly occurring during the warm season of the year (1 April–30 September) and a decreasing frequency from the interior to coastal areas. However, given the sparse conventional meteorological stations network operated by the HNMS, not sufficient for the observation and recording of hailfalls, the figure of hail occurrence should be completed and studied with additional sources including insurance and hailpad network data. 2.2. Insurance crop loss Regarding sources of multi-decadal hail data, the hail insurance crop loss database of ELGA for a 26-year period (1976–01) and for the warm season of each year (1 April–30 September) was examined. Hail days as extracted from the insurance data, represent the total cultivated area of Central Macedonia roughly estimated at about 6500 km2. A “hail day” based on the insurance database is defined as the calendar local 24-hour period on which hail was observed in at least at one municipality of Central Macedonia and the crop damage was confirmed by insurance payments. These are crop loss hail days and do not include other data like time of occurrence, hail size or hailfall duration. Since all the
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agricultural areas are included in the national scale insurance coverage provided by ELGA, at least the potentially damaging hailfalls are recorded and systematically evaluated. In this context, insurance hail day data can be considered as provided by an almost “continuous” network of observations not available in the HNMS data and thus they can be used to determine the “regional” hail frequency for the examined area. The concept of regional hail frequency would be generally applied to an area of about 102 to 104 km2 (Long, 1980). Nevertheless, limitations of the insurance data include the fact that records of hail loss only occur when crops are in growing or mature stage when they are susceptible to damage. However, this limitation introduces a rather small error in the hail frequency estimations, since hail occurrence in Central Macedonia is common during the warm season (1 April–30 September), the period that most of the crops grow, mature and harvest and thus almost all possible damaging hail events are reported and evaluated. Moreover, based on the HNMS weather stations' data, only a limited number of hail events are reported during the cold season of the year (1 October–31 March) in Central Macedonia and generally in Northern Greece (Kotinis-Zambakas, 1989; Sioutas and Flocas, 2003). 2.3. Hailpad network Hailpads have been used in many projects around the world to identify areas of hailfalls and to sample hail data such as the size distribution and concentration of hailstones (Schleusener and Jennings, 1960; Dessens, 1986; Dalezios et al., 1991). Although they have many drawbacks, such as lack of time resolution of multiple hail events, physical problems due to unknown hailstone shapes and drags, saturation of the pads and representative problems over the installation area, they remain as one of the most reliable and economic tools, for providing high resolution and objective hail measurements (Morgan and Towery, 1975; Dessens et al., 1996; Giaiotti et al., 2001). The hailpad network of the GNHSP has been in operation since 1984, normally from 15 April to 30 September, in the project Area 1, occupying the western part of Central Macedonia (Fig. 1). A total of about 140 hailpads was arranged in an area of about 2,400 km2, resulting to an average linear spacing of about 4.1 km between sites. Hailpads consist of a square piece (29 × 29 cm) of styrofoam material (CWWallmate) and 25 mm thick, mounted in an aluminium bracket on the top of a post about 1.5 m above the ground. The hailpad exposure surface before installation was lightly painted with a white paint to prevent disintegration because of ultraviolet rays of sunlight. The principle of operation is simply that falling hail hits the exposed surface of the styrofoam leaving permanent dents. After post storm hailpad check and change, the hailpads found to have hail dents were catalogued, inked for improving contrast and then analyzed by using a digital scanning system or a digitizing tablet or by scanning the hailpads and using image software. The actual hailstone diameter calculated was based on dent minor axis by using the calibration equation. The hailpad calibration equation was developed for each type of styrofoam material used. The calibration technique was that first originated with Schleusener
and Jennings (1960) and developed by Lozowski and Strong (1978). The maximum error in the estimated hail parameters from hailpads can be large, due to implied assumptions concerning: (a) hail characteristics (sphericity, density, drag coefficient, etc.), (b) calibration technique limitations, (c) reading uncertainties on the hailpad (hailstones assumed not to scatter and bounce upon impact, hail dents which are not clearly distinguishable when they are overlapping, or from those marks made by other objects such as bird peeks, human hits, etc.), (d) other sources introducing errors, such as wind-driven hailstones. Nevertheless, errors tend to be minimized through sampling large numbers of hailstones, and therefore hailpads can be offered as the most up to date successful sampling scheme to employ in large areas at a reasonable cost and sufficient density for an objective recording of hailfalls (Long et al., 1980). The hailpad data primarily include the size distribution and concentration of hailstones from which the hail kinetic energy and other parameters can be calculated. Several hail parameters are derived by the hailpad network that can be considered either per hail day or hailpad, such us: number of hailpads recording hail, total number of hailstones, maximum and median hailstone diameter, percent hailpad area covered by hail dents, total hailstone mass, hail kinetic energy, etc. The hailpad data set for this study includes hail days, hailpad number recorded hail, hailstone size distribution and total kinetic energy. 3. Results 3.1. Temporal and spatial distribution Hail is a phenomenon characterized by an extreme variability to temporal and spatial scales. Fig. 2 is a plot of the seasonal hail day numbers by using the insurance hail data for 26 seasons (1 April to 30 September), 1976–01 and of hailpad data for 14 seasons between 1984 and 2001. According to both data sources, the number of hail days through the years varies considerably. Insurance data sample demonstrate a regional average seasonal number of 22.3 hail days for Central Macedonia. A maximum of 41 hail days was registered in 1983 while secondary maxima of 32 hail days were recorded in 1977, 1989 and 1991, with a minimum of 10 and 11 hail days in the years 1996 and 1997, respectively. A decrease in insurance hail day number was recorded in the period from 1993 up to 1998, a period of six consecutive years with less than average hail day numbers that can primarily be attributed to the dry and warm conditions which prevailed during those seasons and resulted in a decrease of thunderstorm activity. The distribution of hailpad derived hail days and the corresponding hailpads recording hail for the project Area 1, are also illustrated in Fig. 2, with a maximum of 15 hail days in 1988, against a minimum of 4 hail days in 1997. A maximum of 86 hailpads recorded hail in the 1999 operational period, while a minimum of 14 hailpads did in 1986. The hailpad data resulted in a regional average seasonal number of 8 hail days with a seasonal average of 45 hailpads recording hail and about 6 hailpads per hail day. Note that, there are no data for 1991 and for 1994–96 because in those years the GNHSP and the hailpad network were not operated.
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Fig. 2. Seasonal distribution of hail days, from insurance and hailpad data for Central Macedonia, within the period 1976–2001.
The resulting difference in average hail day number as extracted by the insurance information (22.3 hail days) and by the hailpad data (8 hail days) can be considered reasonable since: (a) insurance data represent the total cultivated area of Central Macedonia, an area almost three times larger than the hailpad area (project Area 1), and (b) insurance data correspond to a longer time period, 26 seasons within the period 1976–2001 versus 14 seasons of hailpad data within the period 1984–01 that also include any possible effect of cloud seeding (Rudolph et al., 1994). Fig. 3 shows the monthly distribution of hail days upon insurance (1976–01) and hailpad data (14 seasons within 1984–01), also including the seasonal hailpad number recording hail. Both data sources indicate that, during the season from April to September, May and June represent the months of greatest hail frequency. Based on the insurance database, the largest portion of seasonal hail days occurred in June (28.1%) and May (28.6%), followed by July (17.7%), August (13.2%) and April (9.7%), while the lowest hail day number is for September (2.7%). By using the hailpad data, again June and May are the highest hail frequency months. However, June has recorded
the highest incidences, both in hail day occurrence with a percentage of 29%, and in hailpad numbers hit with 41% of the totals. It is followed by May with 25% of total hail days and 25% of total hailpads. An average number of 18 hailpads recorded hail in June, while an average of 11 hailpads recorded hail in May. September is the lowest hail frequency (of the warm season) month with 5% of the total hail day number and 6% of the total hailpads recording hail. The total point hail occurrences at each hailpad site location, for a total of 17 hailpad operational seasons within the period 1984–04, are plotted in Fig. 4, where total hail occurrences above 4 are displayed. The northern and western parts of the hailpad network area (project Area 1, Fig. 1) generally exhibit greater values in total hail occurrences that are about double of those in the southern part. A maximum of 15 hailfalls is located in the north-west part, with a secondary maximum of 14 hailfalls in the middle of the northern part of the hailpad network area. It should be pointed out that the northern and northwest hailfall maxima of the hailpad area are located at higher altitude and closer to mountain barriers, areas typically expected to be hail prone regions. It indicates the importance of orography as a major contributor to the
Fig. 3. Monthly distribution of hail days from insurance and hailpad data and hailpad number recorded hail in Central Macedonia, within the period 1976–2001.
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Fig. 4. Total number of hailfalls at each hailpad site location for 17 seasons of the hailpad network operation, within the period 1984–2004.
development and intensification of the hail producing thunderstorms. In contrast, the south-eastern minima of hailfalls correspond to lower elevation areas closer to the sea (Gulf of Thermaikos, see also Fig. 1). 3.2. Hailstone size distribution Concerning hailstone sizes above 5 mm, a total of about 22,000 hailstones from a 9-year hail data sample (1984–93) recorded by the hailpad network were categorized in 2-mm diameter intervals and the results are illustrated in Fig. 5. The highest proportion of the hailstone sample, corresponding to a percentage of 38.1%, had diameters between 5 and 7 mm. This is followed by the hailstone category 7 to 9 mm with a percentage of 30.4%. It is worth noting that the great majority, of about 85.7% of the total hailstones, had sizes up to 11 mm.
By examining the largest size of hailstones, a percentage of 3.1% of the total hailstone number had diameters larger than 15 mm. Classification of the maximum hailstones recorded in individual hailpads upon conventional sizes, such as categorized by Pea, Grape, Walnut and Golfball, showed the Pea size hailstones (5–12 mm) as the most dominant category reaching 48.2% of the totals and Grape size (12–20 mm) as following with 44.3%, then Walnut (20–32 mm) with 7% and Golfball (32–52 mm) with 0.5% of the total sample of maximum hailstones. 3.3. Hail kinetic energy related to crop damage Crop damage is the main economic impact of hailfall to farmers in hail prone regions. However determining the
Fig. 5. Hailstone size distribution of 22,000 hailstones categorized in 2 mm intervals, from a hailpad data sample for the 9-year period 1984–93.
M. Sioutas et al. / Atmospheric Research 93 (2009) 526–533 Table 1 Hailfall intensity comparison based on the hail kinetic energy distributions for Central Macedonia (Greece), Central Alberta (Canada) and North Dakota and Illinois, (USA). Hail impact energy range (J m− 2)
Central Macedonia, Greece
Central Alberta, Canada
North Dakota, USA
Illinois, USA
% of the total hailpad number 0.1–50 50.1–100 100.1–146
80.5 9.9 2.1
67.5 13.8 6
– – –
– – –
0.1–146 146.1–292 292.1–450 450.1–1000 N 1000 Total number of hailpads examined
92.5 3.3 2.7 1.5 – 334
87.3 5.4 2 4.4 0.9 763
83 8.7 3.7 4 0.6 319
75.2 9.2 11.3 – 4.3 913
relationships between hailfall parameters and damage to crops is not an easy task, since a variety of factors are involved linked both to extreme variability of hail intensity and the complexity of crop susceptibility to hail damage. Among the various parameters, the total hail kinetic energy integrated over a hailfall event has been most closely related to crop damage and in this context it has been used as a primary measure of the effectiveness of hail prevention projects (Changnon, 1971; Strong and Lozowski, 1977). The total kinetic energy (TKE) for an arbitrary hailfall, in assumed absence of the wind, is defined as: TKE = 0:5
X
2
ð1Þ
mi vi
where, mi and vi are the mass and terminal fall speeds of individual hailstones that hit a horizontal unit area. It can also be expressed for individual hailstones in an analytical form: 2 −1
4
TKE = 0:35 g ρ ρα CD DH
ð2Þ
belong 80.5% of the Central Macedonia hailfall cases and 67.5% of the Alberta cases. Hailfalls with impact energy values from 50 up to 450 J m− 2 usually cause a moderate damage and represent about 18% of Central Macedonia hailfalls and 27.2% of the Alberta hailfalls. Hailfalls with impact energies higher than 450 J m− 2, are usually severe, cause much damage and may completely destroy the agricultural crop. This severest hailfall category is represented by a small percentage, of 1.2% of Central Macedonia hailstorms, which is the smallest in comparison to 4.3% for Illinois, 4.5% for North Dakota and 5.3% for Alberta. 3.4. Intensity classification of Northern Greece hailstorms The TORRO (H) international hailstorm intensity scale was first developed in 1986 and a revised scale was then
Table 2 TORRO (H) International Hailstorm intensity scale (minor revision Nov 2005, Sioutas et al., 2005; Webb et al., 1986).
H0 H1
Intensity category
Typical hail diameter (mm)⁎
Probable impact energy (J⁎ m− 2)
Typical damage impacts
True (hard) hail Potentially damaging
5–9 (pea)
0–20
10–15 (large pea, mothball) 16–20 (marble, grape) 21–30 (large marble, walnut)
N 20
No noticeable damage. Slight general damage to plants, crops. Significant damage to fruit, crops, vegetation.
31–40 (pigeon's egg, squash ball) 41–50 (golf ball, Pullet's egg)
N 500
H2
Damaging
H3
Severe
H4
q
H5
Destructive
H6
q
51–60 (hen's egg)
H7
q
H8
q
61–75 (tennis ball N cricket ball) 76–90 (large orange N small Soft ball)
H9
Super hailstorms
91–110 (Soft ball, grapefruit)
H10
q
N 110 (melon)
−2
where, gravity acceleration g = 981.3 cm s , hailstone density ρ = 0.89 g cm− 3, air density at surface ρα = 1.07 × 10−3 g cm− 3, drag coefficient CD = 0.6, and, DH = hailstone diameter. One major source of possible error is implied by wind driven hailstones, which result in a greater total kinetic energy because of the increased horizontal component of impact velocity. To eliminate the horizontal component in the estimated total kinetic energy, the measure of the minor axis of hailstone dents is taken into account during the hailpad analysis assuming it would be equal to the axis of the same hailstone falling with no wind. For a comparison with similar research findings, categories of hail impact energy values (kinetic energy per unit area) are examined, as measured by the hailpad networks of Central Macedonia (Greece), Alberta, Canada) (Strong and Lozowski, 1977), North Dakota (USA) (Hagen and Butchbaker, 1967), and Illinois (USA) (Changnon and Towery, 1972) and the results are given in the Table 1. According to hail impact energy related to crop damage levels as proposed by Strong and Lozowski (1977), hailfalls of total impact energy up to 50 J m− 2 usually cause nil or small damage. In this category
531
N 100
N 300
N 800
Severe damage to fruit and crops, damage to glass and plastic structures, paint and wood scored Widespread glass damage, vehicle bodywork damage. Wholesale destruction of glass, some damage to tiled roofs, significant risk of injuries Bodywork of grounded aircraft dented, brick walls pitted, widespread damage to tiled roofs Severe roof damage, risk of serious injuries. (Severest recorded in the British Isles) Severe damage to aircraft bodywork. Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open. q
⁎Approximate range (typical maximum size in bold), since other factors (e.g. number and density of hailstones, hail fall speed and surface wind speeds) affect severity. For non spheroidal hailstones, the diameter refers to the mean of the co-ordinates.
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Table 3 Comparison of hail intensity levels on the TORRO (H) scale with ANELFA (A) scale, based on impact energy of Central Macedonia, Northern Greece hailfalls. TORRO H-hail intensity scale
Northern Greece point hailfalls (%)
ANELFA A-hail intensity scale
Northern Greece point hailfalls (%)
H0 H1 H2, H2–3 H3, H3–4 H4, H5 H5–6 and above
62.9 27.5 5.4 3.0 1.2 –
A0 A1 A2 A3 A4 A5
69.5 20.9 7.2 2.4 – –
introduced in 2005 (Webb et al., 1986; Sioutas et al., 2005; see also http://www.torro.org.uk/TORRO/severeweather/hailscale.php). The characteristic damage in the British Isles associated with each increment from H0 to H10 is listed in Table 2, but may need to be slightly modified for other countries to reflect differences in building materials and types; e.g. whether roofing tiles are predominantly slate, shingle or concrete. The scale also has six broader categories, potentially useful for forecasting and analysis. Dessens et al. (2007), by using hailpad measurements in France, developed the ANELFA (A) scale for hailfall intensity with public use in mind, introducing a six class scale, from A0 to A5. The A-scale, with increments from 0 to 5, can be used alongside the H-scale since there is good correlation between the ranges (Table 3). A2 covers H2 storms and those borderline H2–3 events, similarly A3 covers H3 and also extends to borderline H3–4 events. By relating hail impact energy values, as extracted by hailpad measurements from the 9-year period 1984–93 for Central Macedonia, to the corresponding levels H0, H1, H2, etc. of the H-scale, the resulting categorization is displayed in Fig. 6. As it indicates, a great proportion of 62.9% of Northern Greece hailfalls (334 events in nine years) reach up to H0 intensity level, and 27.5% reach H1 level. Concerning the H2 level, this corresponds to 5.4% of the hailfalls, H3 to 3% and H4 to 1.2% of the hailfalls. There were no hailfalls classified in the
destructive categories from H5 up to H10, in the 9-year hailpad data sample examined for Central Macedonia. The A-hail intensity scale was also examined for a comparison with the H-scale, by relating hail impact energy distributions and the results are displayed in Table 3. Based on the A-scale, a majority of 69.5% of Central Macedonia hailfalls were classified at A0 intensity level, 20.9% at A1, 7.2% at A2 and 2.4% at A3, with no hailfalls classified at A4 and A5 intensity levels. As a first comparison of the resulting categorizations (Table 3), the Central Macedonia hailfalls lay to a total of 5 intensity levels (H0 to H4) upon the H-scale, compared to a total of 4 levels upon the A-scale (A0 to A3). The differences between H and A scales mainly lie in the ranging of impact energy values; however the resulting differences in hailfall intensity scaling seem greater for the weak and moderate hailfalls. 4. Summary and conclusions Hail occurrence in Greece displays a considerable spatial and temporal variability. A yearly maximum average of 8 hail days is located in the centre-west based on the HNMS conventional weather stations data. In Northern Greece, hail is a spring and summer phenomenon with a mean yearly point frequency varied between 1 and 2 hail days evident from HNMS data and a trend for decreasing hail occurrence from the interior to coastal areas. For Central Macedonia, Northern Greece, by using the insurance hail data of ELGA, a regional average of 22.3 hail days is estimated for an area of about 6500 km2 and for the warm period of the year (1 April to 30 September). The insurance hail crop loss data are proved to be valuable in determining hail days and hail areas and after an appropriate quality control they can be used as a hail data source for regional scale climatic studies. The best available information on hail temporal and spatial patterns and on physical parameters for the study of the fine scale structure of hailfalls in Central Macedonia is derived from the hailpad data. A mean number of 8 hail days was recorded by the hailpad network seasonally over an area of about 2400 km2. June is the month of highest hail frequency with an average of 18 hailpads recording hail,
Fig. 6. Hail impact energy classification of Central Macedonia hailfalls based on the TORRO Hailstorm Intensity Scale.
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followed by May with 11 hailpads, results indicating a much larger extent of hailfalls in June compared to May. A large spatial variability of hailfalls is evident over the hailpad network area across short distances, revealing a strong correlation with the topographical factor, since the maxima of hail occurrence are located at higher elevation areas close to the lee sides of mountain barriers. In contrast, the southeastern minima of hailfalls correspond to low elevation places with extremely infrequent hailfalls along the coastal areas. Concerning hail size distributions based on a sample of about 22,000 hailstones measured by hailpads, the vast majority of the hailstones (85.7%) were found having sizes up to 11 mm. By classifying to conventional categories the maximum hailstones recorded in the individual hailpads, the great majority found to correspond to Pea size (48.2%) and Grape size (44.3%). Establishing correlations between hail intensity and crop damage is primary based on comparative studies between hail parameters and the measurement of the physical damage to crop. Analysis of hail kinetic energy distributions as an indicator of the hailfall intensity related to damage potential, showed a greater proportion of light hailfalls occurring in Central Macedonia compared to areas of North America. The greatest majority of Central Macedonia hailfalls may cause up to moderate damage to crop and only a small percentage but comparable to cases of North America hailfalls may causes much damage or completely destroy the crop. Hail intensity scales need to be developed and applied for an objective evaluation and assessment of hailfall severity and damage potential to crop, property and infrastructure. The TORRO hail intensity scale (H-scale) was used to assess Central Macedonia hailfall intensity and damage potential, by using hail impact energy values as derived from the GNHSP hailpad measurements. The majority of hailfalls, at about 62.9%, reached up to level H0 of the H-intensity scale, while 27.5% reached H1 level. By using the ANELFA scale (A-scale) an amount of 69.5% of Central Macedonia hailfalls, is classified at A0 intensity level and 20.9% at A1 level. Acknowledgements The authors are grateful to all personnel who have worked for the hailpad network operations and also to the Hellenic National Agricultural Insurance Organisation (ELGA) and the Hellenic National Meteorological Service (HNMS) for providing hail data. References Admirat, P., Goyer, G.G., Wojtiw, L., Carte, E.A., Roos, D., Lozowski, E.P., 1985. A comparative study of hailstorms in Switzerland, Canada and South Africa. J. Climatol. 5, 35–51. Changnon Jr., S.A., 1971. Hailfall characteristics related to crop damage. J Appl. Meteor. 10, 270–274.
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Changnon Jr., S.A., 1977. The climatology of hail in North America. Hail: A review of hail science and hail suppression, 38. AMS, Meteor. Monogr., pp. 107–128. Changnon Jr., S.A., Towery, N.G., 1972. Studies of hail data in 1972. Final Report. Illinois State Water Survey, Urbana. 28pp. Dalezios, N.R., Sioutas, M.V., Karacostas, T.S., 1991. A systematic hailpad calibration procedure for operational hail suppression in Greece. Meteorol. Atmos. Phys. 45, 101–111. Dessens, J., 1986. Hail in southwestern France. Part I: Hailfall characteristics and hailstorm environment. J. Climate. Appl. Meteor. 25, 35–47. Dessens, J., Fraile, R., Sanchez, J.L., 1996. Determination of hailstone size distribution inside a thundercloud from hailpad measurements at the ground. Proc. 12th Intern. Conf. on Clouds and Precipitation, Zurich, Switzerland, pp. 244–246. Dessens, J., Berthet, J.L., Sanchez, J.L., 2007. A point hailfall classification based on hailpad measurements: The ANELFA scale. Atmos. Res. 83, 132–139. Fraile, R., Sanchez, J.L., de la Madrid, J.L., Castro, A., Marcos, J.L., 1999. Some results from the hailpad network in Leon (Spain): noteworthy correlations among hailfall parameters. Theor. Appl. Climatol. 64, 105–117. Giaiotti, D., Gianesini, E., Stel, F., 2001. Heuristic considerations pertaining to hailstone size distributions in the plain of Friuli-Venezia Giulia. Atmos. Res. 57, 269–288. Giaiotti, D., Nordio, S., Stel, F., 2003. The climatology of hail in the plain of Friuli Venezia Giulia. Atmos. Res. 67–68, 247–259. Hagen, J., Butchbaker, A.F., 1967. Climatology of hailstorms and evaluation of cloud seeding for hail suppression in South-western North Dakota. Proc. 5th Conf. on Severe Local Storms. Amer. Met. Soc., pp. 336–347. Karacostas, T.S., 1984. The Design of the Greek National Hail Suppression Program. 9th Conference on Weather Modification. AMS, Park City, Utah, USA, pp. 26–27. Kotinis-Zambakas, S.R., 1989. Average spatial patterns of hail days in Greece. J. of Climate Vol. 2, 508–511. Lozowski, E.P., Strong, G.S., 1978. On the calibration of hailpads. J. Appl. Meteor., 16, 17–34. Long, A.B., 1980. On estimating hailfall frequency and hailfall area. J. Appl. Meteor. 19, 1351–1362. Long, A.B., Matson, R.J., Crow, E.L., 1980. The hailpad: materials, data reduction and calibration. J. Appl. Meteor. 19, 1300–1313. Morgan, G.M., Towery, N.G., 1975. Small-scale variability of hailfalls and its significance for hail prevention experiments. J. Appl. Meteor. 14, 763–770. Rudolph, R.C., Sackiw, C.M., Riley, G.T., 1994. Statistical evaluation of the 1984–88 seeding experiment in Northern Greece. J. Weather Modif. 26, 53–60. Schleusener, R.A., Jennings, R.C., 1960. An energy method for relative estimates of hail intensity. Bull. Amer. Meteor. Soc. 41, 372–377. Schuster, S.S., Blong, R.J., Speer, M.S., 2005. A hail climatology of the greater Sydney area and New South Wales, Australia. Intern. J. of Climatol. 25, 1633–1650. Sioutas, M.V., 1991. Hailfall measurements by the Greek National Hail Suppression Project. Proceedings 2nd Yugoslav Conference on Weather Modification, 2–4 April 1991, Mavrovo, Yugoslavia, pp. 210–217. Sioutas, M.V., Rudolph, R.C., 1998. Key variables to access seeding operations on the Greek National Hail Suppression Program. J. Weather Modif. 30, 22–29. Sioutas, M.V., 1999. Contribution to the study of hailstorms in Central Macedonia. PhD Thesis, Aristotelian University of Thessaloniki, Greece, pp 310 (in Greek). Sioutas, M.V., Flocas, H.A., 2003. Hailstorms in Northern Greece: synoptic and thermodynamic environment. Theor. App. Climatol. 75, 189–202. Sioutas, M.V., Meaden, G.T., Webb, J.D.C., 2005. Hail Frequency and Intensity in Northern Greece. Presentation 5th EMS Annual Meeting –7th ECAM 2005, Utrecht, The Netherlands, 12–16 September. Strong, G.S., Lozowski, E.P., 1977. An Alberta study to objectively measure hailfall intensity. Atmosphere-Ocean 15, 33–53. Vinet, F., 2001. Climatology of hail in France. Atmos. Res. 56, 309–323. Webb, J.D.C., Elsom, D.M., Meaden, G.T., 1986. The TORRO Hailstorm intensity scale. J. of Meteorol. 11, 337–339. Webb, J.D.C., Elsom, D.M., Reynolds, D.J., 2001. Climatology of severe hailstorms in Great Britain. Atmos Res 56, 293–310.
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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Hail frequency, distribution and intensity in Northern Greece Michalis Sioutas a,⁎, Terence Meaden b, Jonathan D.C. Webb b a b
ELGA - Meteorological Applications Center, International Airport Macedonia, 55103 Thessaloniki, Greece Tornado and Storm Research Organisation, Oxford, UK
a r t i c l e
i n f o
Article history: Received 25 December 2007 Received in revised form 17 September 2008 Accepted 20 September 2008 Keywords: Hail Crop Damage Central Macedonia Greece
a b s t r a c t Hail numerical and physical parameters are examined for an assessment of frequency, spatial and temporal distribution and intensity of hailfalls in Central Macedonia periphery, Northern Greece. Data from weather stations of the Hellenic National Meteorological Service (HNMS) indicated a mean yearly point frequency varying between 1 and 2 hail days for Central Macedonia. Crop insurance data from the Hellenic National Agricultural Insurance Organization (ELGA) showed a mean regional frequency of 22.3 hail days for the total cultivated area of Central Macedonia, about 6,500 km2, for the warm season, from April to September. Hailpad data representing almost one third of the total cultivated area of Central Macedonia, indicated a regional average of 8 hail days and 45 hailpads recording hail for the warm season. Spatial analysis over the hailpad area revealed a large variability of hail occurrence, with local maxima at the higher altitude and close to mountain barrier areas and a decreasing frequency over areas closer to the sea. The size distribution of a sample of 22,000 hailstones measured by the hailpads, showed a great majority, of 85.7%, having diameters up to 11 mm. An intercomparison of hail impact energies according to crop damaging potential indicated Central Macedonia hailfalls as the less intense compared to those of areas of Northern America. Application of the revised TORRO hail intensity scale extending to classes from H0 to H10, permitted a classification of Central Macedonia hailfalls appropriate for comparisons and correspondence between areas and countries about the severity and damaging of hailfalls. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Hail is common, mainly during the spring and summertime, in many areas of the world. Regions of higher hail frequency around the world are the middle latitudes zone, including the eastern Rockies of North America from Alberta, Canada southwards to New Mexico, and a European zone extending from the northern Iberian Peninsula across central Europe through to the northern Balkans up to the Caucasus. In the Southern Hemisphere, a major hail zone includes the area of Mendoza in Argentina, Southern Africa and parts of Australia. The observed frequency and distribution of hail is generally characterized by an extreme variability, attributed primarily to the hail formation process, and also to the limited availability of hail information. Despite those limitations,
⁎ Corresponding author. Tel.: +30 2310 472953; fax: +30 2310 472205. E-mail address: [email protected] (M. Sioutas). 0169-8095/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2008.09.023
considerable efforts have been developed in the international hail research towards the establishment of hail climatologies in various spatial and time scales (Chagnon, 1977; Admirat et al., 1985; Fraile et al., 1999; Vinet, 2001; Webb et al., 2001; Giaiotti et al., 2003; Schuster et al., 2005). In Northern Greece, hail is a common phenomenon during the warm season of the year from April to September and can be a dangerous and damaging event, occasionally producing significant damage to crops (Sioutas, 1999; Sioutas and Flocas, 2003). Annual average crop hail loss over Central Macedonia, one of the largest and most intensively cultivated areas of Northern Greece, is roughly estimated at about 20 million Euros based on insurance data of the last decade. To alleviate crop losses, an airborne cloud seeding program for hail suppression, the so called Greek National Hail Suppression Program (GNHSP) was organized for the project Area 1 of Central Macedonia (Fig. 1) and has been operating since 1984 under the auspices of the National Agricultural Insurance Organization (ELGA) (Karacostas, 1984; Sioutas and Rudolph, 1998). Moreover, one
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Fig. 1. Altitude map of Central Macedonia, Northern Greece. The project Area 1, of the Greek National Hail Suppression Program (GNHSP), is depicted.
more project area (not shown) situated in Thessaly, central Greece was also included in the hail suppression program. Within the context of the GNHSP, a hailpad network was installed in the project Area 1 providing hail data for the evaluation of the hail suppression operations as well as for an adequate sampling of hailfalls (Sioutas, 1991; Rudolph et al., 1994). The challenges of this study are to present a more comprehensive picture of the hail occurrence in term of temporal frequency, spatial distribution and intensity assessment for the Central Macedonia periphery, region of Macedonia, Northern Greece. All the available data were examined, including three main sources: the Hellenic National Meteorological Service (HNMS) conventional stations data, the ELGA crop insurance data base and the GNHSP hailpad network data. This paper is organized as follows: firstly, a brief review of the existing hail databases in Greece is given, with emphasis on the available data for Central Macedonia. Next, a hail climatology overview for Central Macedonia is presented, based on the frequency and temporal distribution of hail days, the spatial distribution of hailfalls and the hailstone size distribution. An analysis of damage potential to crop is then undertaken based on a comparison between total impact energy of Central Macedonia hailfalls and those of areas of Northern America. A hail intensity classification assessment is also presented using impact energy ranges incorporated in the TORRO international hailstorm intensity scale (H-scale) (Webb et al., 1986; Tornado and Storm Research Organization, www.torro.uk.org). The ANELFA hail intensity scale (A-scale) was also used for comparison (Dessens et al., 2007). 2. Hail data 2.1. National meteorological service stations Hail observations were provided by the Hellenic National Meteorological Service (HNMS) for more than 70 conven-
tional meteorological stations from all over Greece, covering a period of about 60 years, 1930–90. These data refer to days of hail observed on the ground with no information about hailstone physical properties, size and concentration. In this context, they can only describe the “point” frequency of hail, considering that one manned station observation of hailfall can be representative for an area of about 102 to 103 m2. Based on the HNMS data, hail occurs with a higher frequency over the western areas of Greece and the coast of the Ionian Sea, where a point mean yearly maximum of 8 hail days appears. Hail days in the Greek peninsula generally increase from east to west and from the coastal to mountain areas. Opposite, in the Aegean Sea hail days increase from west to east (Sioutas, 1999). For Central Macedonia, HNMS data indicated a yearly average of about 1 to 2 point hail days in most locales, with hailfalls mostly occurring during the warm season of the year (1 April–30 September) and a decreasing frequency from the interior to coastal areas. However, given the sparse conventional meteorological stations network operated by the HNMS, not sufficient for the observation and recording of hailfalls, the figure of hail occurrence should be completed and studied with additional sources including insurance and hailpad network data. 2.2. Insurance crop loss Regarding sources of multi-decadal hail data, the hail insurance crop loss database of ELGA for a 26-year period (1976–01) and for the warm season of each year (1 April–30 September) was examined. Hail days as extracted from the insurance data, represent the total cultivated area of Central Macedonia roughly estimated at about 6500 km2. A “hail day” based on the insurance database is defined as the calendar local 24-hour period on which hail was observed in at least at one municipality of Central Macedonia and the crop damage was confirmed by insurance payments. These are crop loss hail days and do not include other data like time of occurrence, hail size or hailfall duration. Since all the
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agricultural areas are included in the national scale insurance coverage provided by ELGA, at least the potentially damaging hailfalls are recorded and systematically evaluated. In this context, insurance hail day data can be considered as provided by an almost “continuous” network of observations not available in the HNMS data and thus they can be used to determine the “regional” hail frequency for the examined area. The concept of regional hail frequency would be generally applied to an area of about 102 to 104 km2 (Long, 1980). Nevertheless, limitations of the insurance data include the fact that records of hail loss only occur when crops are in growing or mature stage when they are susceptible to damage. However, this limitation introduces a rather small error in the hail frequency estimations, since hail occurrence in Central Macedonia is common during the warm season (1 April–30 September), the period that most of the crops grow, mature and harvest and thus almost all possible damaging hail events are reported and evaluated. Moreover, based on the HNMS weather stations' data, only a limited number of hail events are reported during the cold season of the year (1 October–31 March) in Central Macedonia and generally in Northern Greece (Kotinis-Zambakas, 1989; Sioutas and Flocas, 2003). 2.3. Hailpad network Hailpads have been used in many projects around the world to identify areas of hailfalls and to sample hail data such as the size distribution and concentration of hailstones (Schleusener and Jennings, 1960; Dessens, 1986; Dalezios et al., 1991). Although they have many drawbacks, such as lack of time resolution of multiple hail events, physical problems due to unknown hailstone shapes and drags, saturation of the pads and representative problems over the installation area, they remain as one of the most reliable and economic tools, for providing high resolution and objective hail measurements (Morgan and Towery, 1975; Dessens et al., 1996; Giaiotti et al., 2001). The hailpad network of the GNHSP has been in operation since 1984, normally from 15 April to 30 September, in the project Area 1, occupying the western part of Central Macedonia (Fig. 1). A total of about 140 hailpads was arranged in an area of about 2,400 km2, resulting to an average linear spacing of about 4.1 km between sites. Hailpads consist of a square piece (29 × 29 cm) of styrofoam material (CWWallmate) and 25 mm thick, mounted in an aluminium bracket on the top of a post about 1.5 m above the ground. The hailpad exposure surface before installation was lightly painted with a white paint to prevent disintegration because of ultraviolet rays of sunlight. The principle of operation is simply that falling hail hits the exposed surface of the styrofoam leaving permanent dents. After post storm hailpad check and change, the hailpads found to have hail dents were catalogued, inked for improving contrast and then analyzed by using a digital scanning system or a digitizing tablet or by scanning the hailpads and using image software. The actual hailstone diameter calculated was based on dent minor axis by using the calibration equation. The hailpad calibration equation was developed for each type of styrofoam material used. The calibration technique was that first originated with Schleusener
and Jennings (1960) and developed by Lozowski and Strong (1978). The maximum error in the estimated hail parameters from hailpads can be large, due to implied assumptions concerning: (a) hail characteristics (sphericity, density, drag coefficient, etc.), (b) calibration technique limitations, (c) reading uncertainties on the hailpad (hailstones assumed not to scatter and bounce upon impact, hail dents which are not clearly distinguishable when they are overlapping, or from those marks made by other objects such as bird peeks, human hits, etc.), (d) other sources introducing errors, such as wind-driven hailstones. Nevertheless, errors tend to be minimized through sampling large numbers of hailstones, and therefore hailpads can be offered as the most up to date successful sampling scheme to employ in large areas at a reasonable cost and sufficient density for an objective recording of hailfalls (Long et al., 1980). The hailpad data primarily include the size distribution and concentration of hailstones from which the hail kinetic energy and other parameters can be calculated. Several hail parameters are derived by the hailpad network that can be considered either per hail day or hailpad, such us: number of hailpads recording hail, total number of hailstones, maximum and median hailstone diameter, percent hailpad area covered by hail dents, total hailstone mass, hail kinetic energy, etc. The hailpad data set for this study includes hail days, hailpad number recorded hail, hailstone size distribution and total kinetic energy. 3. Results 3.1. Temporal and spatial distribution Hail is a phenomenon characterized by an extreme variability to temporal and spatial scales. Fig. 2 is a plot of the seasonal hail day numbers by using the insurance hail data for 26 seasons (1 April to 30 September), 1976–01 and of hailpad data for 14 seasons between 1984 and 2001. According to both data sources, the number of hail days through the years varies considerably. Insurance data sample demonstrate a regional average seasonal number of 22.3 hail days for Central Macedonia. A maximum of 41 hail days was registered in 1983 while secondary maxima of 32 hail days were recorded in 1977, 1989 and 1991, with a minimum of 10 and 11 hail days in the years 1996 and 1997, respectively. A decrease in insurance hail day number was recorded in the period from 1993 up to 1998, a period of six consecutive years with less than average hail day numbers that can primarily be attributed to the dry and warm conditions which prevailed during those seasons and resulted in a decrease of thunderstorm activity. The distribution of hailpad derived hail days and the corresponding hailpads recording hail for the project Area 1, are also illustrated in Fig. 2, with a maximum of 15 hail days in 1988, against a minimum of 4 hail days in 1997. A maximum of 86 hailpads recorded hail in the 1999 operational period, while a minimum of 14 hailpads did in 1986. The hailpad data resulted in a regional average seasonal number of 8 hail days with a seasonal average of 45 hailpads recording hail and about 6 hailpads per hail day. Note that, there are no data for 1991 and for 1994–96 because in those years the GNHSP and the hailpad network were not operated.
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Fig. 2. Seasonal distribution of hail days, from insurance and hailpad data for Central Macedonia, within the period 1976–2001.
The resulting difference in average hail day number as extracted by the insurance information (22.3 hail days) and by the hailpad data (8 hail days) can be considered reasonable since: (a) insurance data represent the total cultivated area of Central Macedonia, an area almost three times larger than the hailpad area (project Area 1), and (b) insurance data correspond to a longer time period, 26 seasons within the period 1976–2001 versus 14 seasons of hailpad data within the period 1984–01 that also include any possible effect of cloud seeding (Rudolph et al., 1994). Fig. 3 shows the monthly distribution of hail days upon insurance (1976–01) and hailpad data (14 seasons within 1984–01), also including the seasonal hailpad number recording hail. Both data sources indicate that, during the season from April to September, May and June represent the months of greatest hail frequency. Based on the insurance database, the largest portion of seasonal hail days occurred in June (28.1%) and May (28.6%), followed by July (17.7%), August (13.2%) and April (9.7%), while the lowest hail day number is for September (2.7%). By using the hailpad data, again June and May are the highest hail frequency months. However, June has recorded
the highest incidences, both in hail day occurrence with a percentage of 29%, and in hailpad numbers hit with 41% of the totals. It is followed by May with 25% of total hail days and 25% of total hailpads. An average number of 18 hailpads recorded hail in June, while an average of 11 hailpads recorded hail in May. September is the lowest hail frequency (of the warm season) month with 5% of the total hail day number and 6% of the total hailpads recording hail. The total point hail occurrences at each hailpad site location, for a total of 17 hailpad operational seasons within the period 1984–04, are plotted in Fig. 4, where total hail occurrences above 4 are displayed. The northern and western parts of the hailpad network area (project Area 1, Fig. 1) generally exhibit greater values in total hail occurrences that are about double of those in the southern part. A maximum of 15 hailfalls is located in the north-west part, with a secondary maximum of 14 hailfalls in the middle of the northern part of the hailpad network area. It should be pointed out that the northern and northwest hailfall maxima of the hailpad area are located at higher altitude and closer to mountain barriers, areas typically expected to be hail prone regions. It indicates the importance of orography as a major contributor to the
Fig. 3. Monthly distribution of hail days from insurance and hailpad data and hailpad number recorded hail in Central Macedonia, within the period 1976–2001.
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Fig. 4. Total number of hailfalls at each hailpad site location for 17 seasons of the hailpad network operation, within the period 1984–2004.
development and intensification of the hail producing thunderstorms. In contrast, the south-eastern minima of hailfalls correspond to lower elevation areas closer to the sea (Gulf of Thermaikos, see also Fig. 1). 3.2. Hailstone size distribution Concerning hailstone sizes above 5 mm, a total of about 22,000 hailstones from a 9-year hail data sample (1984–93) recorded by the hailpad network were categorized in 2-mm diameter intervals and the results are illustrated in Fig. 5. The highest proportion of the hailstone sample, corresponding to a percentage of 38.1%, had diameters between 5 and 7 mm. This is followed by the hailstone category 7 to 9 mm with a percentage of 30.4%. It is worth noting that the great majority, of about 85.7% of the total hailstones, had sizes up to 11 mm.
By examining the largest size of hailstones, a percentage of 3.1% of the total hailstone number had diameters larger than 15 mm. Classification of the maximum hailstones recorded in individual hailpads upon conventional sizes, such as categorized by Pea, Grape, Walnut and Golfball, showed the Pea size hailstones (5–12 mm) as the most dominant category reaching 48.2% of the totals and Grape size (12–20 mm) as following with 44.3%, then Walnut (20–32 mm) with 7% and Golfball (32–52 mm) with 0.5% of the total sample of maximum hailstones. 3.3. Hail kinetic energy related to crop damage Crop damage is the main economic impact of hailfall to farmers in hail prone regions. However determining the
Fig. 5. Hailstone size distribution of 22,000 hailstones categorized in 2 mm intervals, from a hailpad data sample for the 9-year period 1984–93.
M. Sioutas et al. / Atmospheric Research 93 (2009) 526–533 Table 1 Hailfall intensity comparison based on the hail kinetic energy distributions for Central Macedonia (Greece), Central Alberta (Canada) and North Dakota and Illinois, (USA). Hail impact energy range (J m− 2)
Central Macedonia, Greece
Central Alberta, Canada
North Dakota, USA
Illinois, USA
% of the total hailpad number 0.1–50 50.1–100 100.1–146
80.5 9.9 2.1
67.5 13.8 6
– – –
– – –
0.1–146 146.1–292 292.1–450 450.1–1000 N 1000 Total number of hailpads examined
92.5 3.3 2.7 1.5 – 334
87.3 5.4 2 4.4 0.9 763
83 8.7 3.7 4 0.6 319
75.2 9.2 11.3 – 4.3 913
relationships between hailfall parameters and damage to crops is not an easy task, since a variety of factors are involved linked both to extreme variability of hail intensity and the complexity of crop susceptibility to hail damage. Among the various parameters, the total hail kinetic energy integrated over a hailfall event has been most closely related to crop damage and in this context it has been used as a primary measure of the effectiveness of hail prevention projects (Changnon, 1971; Strong and Lozowski, 1977). The total kinetic energy (TKE) for an arbitrary hailfall, in assumed absence of the wind, is defined as: TKE = 0:5
X
2
ð1Þ
mi vi
where, mi and vi are the mass and terminal fall speeds of individual hailstones that hit a horizontal unit area. It can also be expressed for individual hailstones in an analytical form: 2 −1
4
TKE = 0:35 g ρ ρα CD DH
ð2Þ
belong 80.5% of the Central Macedonia hailfall cases and 67.5% of the Alberta cases. Hailfalls with impact energy values from 50 up to 450 J m− 2 usually cause a moderate damage and represent about 18% of Central Macedonia hailfalls and 27.2% of the Alberta hailfalls. Hailfalls with impact energies higher than 450 J m− 2, are usually severe, cause much damage and may completely destroy the agricultural crop. This severest hailfall category is represented by a small percentage, of 1.2% of Central Macedonia hailstorms, which is the smallest in comparison to 4.3% for Illinois, 4.5% for North Dakota and 5.3% for Alberta. 3.4. Intensity classification of Northern Greece hailstorms The TORRO (H) international hailstorm intensity scale was first developed in 1986 and a revised scale was then
Table 2 TORRO (H) International Hailstorm intensity scale (minor revision Nov 2005, Sioutas et al., 2005; Webb et al., 1986).
H0 H1
Intensity category
Typical hail diameter (mm)⁎
Probable impact energy (J⁎ m− 2)
Typical damage impacts
True (hard) hail Potentially damaging
5–9 (pea)
0–20
10–15 (large pea, mothball) 16–20 (marble, grape) 21–30 (large marble, walnut)
N 20
No noticeable damage. Slight general damage to plants, crops. Significant damage to fruit, crops, vegetation.
31–40 (pigeon's egg, squash ball) 41–50 (golf ball, Pullet's egg)
N 500
H2
Damaging
H3
Severe
H4
q
H5
Destructive
H6
q
51–60 (hen's egg)
H7
q
H8
q
61–75 (tennis ball N cricket ball) 76–90 (large orange N small Soft ball)
H9
Super hailstorms
91–110 (Soft ball, grapefruit)
H10
q
N 110 (melon)
−2
where, gravity acceleration g = 981.3 cm s , hailstone density ρ = 0.89 g cm− 3, air density at surface ρα = 1.07 × 10−3 g cm− 3, drag coefficient CD = 0.6, and, DH = hailstone diameter. One major source of possible error is implied by wind driven hailstones, which result in a greater total kinetic energy because of the increased horizontal component of impact velocity. To eliminate the horizontal component in the estimated total kinetic energy, the measure of the minor axis of hailstone dents is taken into account during the hailpad analysis assuming it would be equal to the axis of the same hailstone falling with no wind. For a comparison with similar research findings, categories of hail impact energy values (kinetic energy per unit area) are examined, as measured by the hailpad networks of Central Macedonia (Greece), Alberta, Canada) (Strong and Lozowski, 1977), North Dakota (USA) (Hagen and Butchbaker, 1967), and Illinois (USA) (Changnon and Towery, 1972) and the results are given in the Table 1. According to hail impact energy related to crop damage levels as proposed by Strong and Lozowski (1977), hailfalls of total impact energy up to 50 J m− 2 usually cause nil or small damage. In this category
531
N 100
N 300
N 800
Severe damage to fruit and crops, damage to glass and plastic structures, paint and wood scored Widespread glass damage, vehicle bodywork damage. Wholesale destruction of glass, some damage to tiled roofs, significant risk of injuries Bodywork of grounded aircraft dented, brick walls pitted, widespread damage to tiled roofs Severe roof damage, risk of serious injuries. (Severest recorded in the British Isles) Severe damage to aircraft bodywork. Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open. q
⁎Approximate range (typical maximum size in bold), since other factors (e.g. number and density of hailstones, hail fall speed and surface wind speeds) affect severity. For non spheroidal hailstones, the diameter refers to the mean of the co-ordinates.
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Table 3 Comparison of hail intensity levels on the TORRO (H) scale with ANELFA (A) scale, based on impact energy of Central Macedonia, Northern Greece hailfalls. TORRO H-hail intensity scale
Northern Greece point hailfalls (%)
ANELFA A-hail intensity scale
Northern Greece point hailfalls (%)
H0 H1 H2, H2–3 H3, H3–4 H4, H5 H5–6 and above
62.9 27.5 5.4 3.0 1.2 –
A0 A1 A2 A3 A4 A5
69.5 20.9 7.2 2.4 – –
introduced in 2005 (Webb et al., 1986; Sioutas et al., 2005; see also http://www.torro.org.uk/TORRO/severeweather/hailscale.php). The characteristic damage in the British Isles associated with each increment from H0 to H10 is listed in Table 2, but may need to be slightly modified for other countries to reflect differences in building materials and types; e.g. whether roofing tiles are predominantly slate, shingle or concrete. The scale also has six broader categories, potentially useful for forecasting and analysis. Dessens et al. (2007), by using hailpad measurements in France, developed the ANELFA (A) scale for hailfall intensity with public use in mind, introducing a six class scale, from A0 to A5. The A-scale, with increments from 0 to 5, can be used alongside the H-scale since there is good correlation between the ranges (Table 3). A2 covers H2 storms and those borderline H2–3 events, similarly A3 covers H3 and also extends to borderline H3–4 events. By relating hail impact energy values, as extracted by hailpad measurements from the 9-year period 1984–93 for Central Macedonia, to the corresponding levels H0, H1, H2, etc. of the H-scale, the resulting categorization is displayed in Fig. 6. As it indicates, a great proportion of 62.9% of Northern Greece hailfalls (334 events in nine years) reach up to H0 intensity level, and 27.5% reach H1 level. Concerning the H2 level, this corresponds to 5.4% of the hailfalls, H3 to 3% and H4 to 1.2% of the hailfalls. There were no hailfalls classified in the
destructive categories from H5 up to H10, in the 9-year hailpad data sample examined for Central Macedonia. The A-hail intensity scale was also examined for a comparison with the H-scale, by relating hail impact energy distributions and the results are displayed in Table 3. Based on the A-scale, a majority of 69.5% of Central Macedonia hailfalls were classified at A0 intensity level, 20.9% at A1, 7.2% at A2 and 2.4% at A3, with no hailfalls classified at A4 and A5 intensity levels. As a first comparison of the resulting categorizations (Table 3), the Central Macedonia hailfalls lay to a total of 5 intensity levels (H0 to H4) upon the H-scale, compared to a total of 4 levels upon the A-scale (A0 to A3). The differences between H and A scales mainly lie in the ranging of impact energy values; however the resulting differences in hailfall intensity scaling seem greater for the weak and moderate hailfalls. 4. Summary and conclusions Hail occurrence in Greece displays a considerable spatial and temporal variability. A yearly maximum average of 8 hail days is located in the centre-west based on the HNMS conventional weather stations data. In Northern Greece, hail is a spring and summer phenomenon with a mean yearly point frequency varied between 1 and 2 hail days evident from HNMS data and a trend for decreasing hail occurrence from the interior to coastal areas. For Central Macedonia, Northern Greece, by using the insurance hail data of ELGA, a regional average of 22.3 hail days is estimated for an area of about 6500 km2 and for the warm period of the year (1 April to 30 September). The insurance hail crop loss data are proved to be valuable in determining hail days and hail areas and after an appropriate quality control they can be used as a hail data source for regional scale climatic studies. The best available information on hail temporal and spatial patterns and on physical parameters for the study of the fine scale structure of hailfalls in Central Macedonia is derived from the hailpad data. A mean number of 8 hail days was recorded by the hailpad network seasonally over an area of about 2400 km2. June is the month of highest hail frequency with an average of 18 hailpads recording hail,
Fig. 6. Hail impact energy classification of Central Macedonia hailfalls based on the TORRO Hailstorm Intensity Scale.
M. Sioutas et al. / Atmospheric Research 93 (2009) 526–533
followed by May with 11 hailpads, results indicating a much larger extent of hailfalls in June compared to May. A large spatial variability of hailfalls is evident over the hailpad network area across short distances, revealing a strong correlation with the topographical factor, since the maxima of hail occurrence are located at higher elevation areas close to the lee sides of mountain barriers. In contrast, the southeastern minima of hailfalls correspond to low elevation places with extremely infrequent hailfalls along the coastal areas. Concerning hail size distributions based on a sample of about 22,000 hailstones measured by hailpads, the vast majority of the hailstones (85.7%) were found having sizes up to 11 mm. By classifying to conventional categories the maximum hailstones recorded in the individual hailpads, the great majority found to correspond to Pea size (48.2%) and Grape size (44.3%). Establishing correlations between hail intensity and crop damage is primary based on comparative studies between hail parameters and the measurement of the physical damage to crop. Analysis of hail kinetic energy distributions as an indicator of the hailfall intensity related to damage potential, showed a greater proportion of light hailfalls occurring in Central Macedonia compared to areas of North America. The greatest majority of Central Macedonia hailfalls may cause up to moderate damage to crop and only a small percentage but comparable to cases of North America hailfalls may causes much damage or completely destroy the crop. Hail intensity scales need to be developed and applied for an objective evaluation and assessment of hailfall severity and damage potential to crop, property and infrastructure. The TORRO hail intensity scale (H-scale) was used to assess Central Macedonia hailfall intensity and damage potential, by using hail impact energy values as derived from the GNHSP hailpad measurements. The majority of hailfalls, at about 62.9%, reached up to level H0 of the H-intensity scale, while 27.5% reached H1 level. By using the ANELFA scale (A-scale) an amount of 69.5% of Central Macedonia hailfalls, is classified at A0 intensity level and 20.9% at A1 level. Acknowledgements The authors are grateful to all personnel who have worked for the hailpad network operations and also to the Hellenic National Agricultural Insurance Organisation (ELGA) and the Hellenic National Meteorological Service (HNMS) for providing hail data. References Admirat, P., Goyer, G.G., Wojtiw, L., Carte, E.A., Roos, D., Lozowski, E.P., 1985. A comparative study of hailstorms in Switzerland, Canada and South Africa. J. Climatol. 5, 35–51. Changnon Jr., S.A., 1971. Hailfall characteristics related to crop damage. J Appl. Meteor. 10, 270–274.
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