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INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 9,309-319 (1989) 551.578.45:551.555.6:551.589.1(495)

SUMMER SNOWFALLS OVER THE MOUNT OLYMPUS AREA H. S . SAHSAMANOGLOU

Department of Meteorology and Climatology. Aristotelian University of Thessaloniki. Thessaloniki, Greece

Received 30 Murch I988 Revised 23 June 1988

ABSTRACT

The present paper is a study of summer snowfalls over the Mount Olympus range (altitude > 2500 m) in northern Greece. This occurs about three times every summer and is, of course, linked to the atmospheric circulation over Europe. At least two days before snowfall, a deep trough, reaching almost to the northern end of the western Mediterranean, causes a strong southward invasion of cold air. This cold air-mass movement results in cyclogenesis near northern Italy. A depression is formed which moves to the east, causing bad weather over northern Greece and a snowfall over the higher part of Mount Olympus. The freezing of air masses over the mountain area is so strong that the dry-bulb freezing level found at 640 hPa two days before snowfall, reaches 710 hPa immediately after snowfall.

KEY WORDS Summer snow Snow day

INTRODUCTION

Snowfall over Greece during summer is an extremely rare occurrence. An exception to this is the snowfall noted every summer over the Mount Olympus area, particularly on the higher summits, i.e. those exceeding 2500 m altitude. Such snowfall is accompanied by bad weather over the rest of Greece; this results from a specific atmospheric circulation over Europe, while over the Mount Olympus area there is a particular dynamic and thermodynamic situation.

The present study is based on data collected at the meteorological stations of the Mount Olympus area, that is: Keoa (altitude 1750 m), Iartio (altitude 2380 m) and Eko (altitude 2817 m). Data has also been obtained from some other stations surrounding the Mount Olympus area, to a mean distance of 100-150 km; these are the stations of Thessaloniki airport (altitude 5 m), Larissa (altitude 75 m) and Florina (altitude 660 m) (Figure 1).

For a better understanding of conditions accompanying the phenomenon, a thermodynamic study of the air masses over Mount Olympus is possible with the help of the upper air radiosonde data from Thessaloniki airport (a distance of 80 km from Mount Olympus).

The method of anomalies has been used in an attempt to follow the evolution of atmospheric circulation over Greece during the two days before the phenomenon, on the day of its appearance, and the day following. In particular, we have studied the anomalies of the 500 hPa isobaric surface, of the thickness of the 1000/500 hPa layer and of the mean sea level (MSL) pressure. We have used for this study, data provided by the British Meteorological Service. The period covered is the ten years from 1963 to 1972.

These snowfalls and the bad weather accompanying them are due to the passage of a depression over northern Greece or the Balkans which resulted from cyclogenesis occurring somewhere in the region including Italy, the Adriatic Sea, the Ionian Sea, and the southern Balkans. This cyclogenesis is due to a concentration of relative vorticity. This is caused by horizontal advection of cold air masses in the middle troposphere, which start near the region of the British Isles and Scandinavia and reach to the central Mediterranean. To these same depressions Metaxas (1 974) has attributed the low temperature observed

0899-841 8/89/030309-11$05.50 0 1989 by the Royal Meteorological Society

310 H. S. SAHSAMANOGLOU

Turkey Mt. OlympUS

Figure 1. Map of northern Greece

occasionally in July and August in Greece. This system will not always cause the appearance of snowfall over Greece in summer and it is different to some atmospheric circulation types favouring winter snowfall in Greece (see Prezerakos and Angouridakis (1979, 1984) for winter snowfall in Athens and Lioki-Livada- Tselepidaki (1 979) for winter snowfall over Greece).

2. STATISTICAL ESTIMATION OF SUMMER SNOWFALLS

Summer snowfalls over Mount Olympus, particularly on summits exceeding 2500 m altitude, are a remarkably steady occurrence every year: almost always three times every summer (July, August, September). By studying the data collected at the Mount Olympus meteorological stations (particularly those from Eko, which has the highest altitude) we can see that summer snowfalls present a maximum frequency of appearance during the first ten-day period of September, and a minimum frequency during the last ten-day period of July. Half of the summer snowfalls occur in the interval from 20 August to 20 September. The interval from 20 July to 20 August has the lowest occurrence of the phenomenon (see Table I).

The summer snowfalls have a duration varying from some minutes to several hours. Snow remains on the ground for some hours, exceptionally for some days, and its thickness may reach 10 cm. Usually, snowfall is followed by rain which, of course, hastens thawing.

The geographical position of Mount Olympus (latitude 40"N), its short distance from the sea (approximately 10 km), and the temperatures prevailing in the area (Sahsamanoglou, 1978), do not favour the

Table I. Frequency (per cent) of appearance of summer snowfalls over the Mount Olympus area (1963-1972)

1-10 11-20 21-30 (31)

July 15.6 9.4 0 August 3.1 9.4 15.6 September 18.8 15.6 12.5

MOUNT OLYMPUS SUMMER SNOWFALL 311

appearance of snow during summer. Therefore, when it does appear we have to look for exceptional conditions and values in the thermodynamic situation over the mountain area as well as in the atmospheric circulation over middle latitudes in general. A change in weather over the whole of northern Greece is apparent nearly two days before snowfall. There is an overcast sky with rain and some storms, and especially a marked fall in temperature preceding the snowfall (Tables I1 and 111). This fall in temperature amounts to 5°C to 6°C in mountain meteorological stations and 4°C to 5°C at low-level meteorological stations.

3. THE SYNOPTIC SITUATION DURING SNOWFALL DAYS

Snowfall on Mount Olympus in summer has to be attributed to an atmospheric circulation with special characteristics. In this attempt to study the pressure systems that are responsible for the appearance of the phenomenon, mean charts are prepared for the 500 hPa surface, for the 1000/500 hPa layer as well as for the mean sea-level pressure (Figures 2-5). These were examined for the day of snowfall (D-day) as well as for the two preceding days (D- 1 and D-2) and the day following (D+ 1). On these same charts, the isopleths of their respective anomalies have been traced. The isopleths for heights and thickness have been traced with a 60 gpm interval, those of their anomalies at 20 gpm. Isobars have been traced with a 4 hPa interval and their anomalies at 2 hPa. Areas where anomalies are important at 0.05 level of significance are shaded with horizontal lines, those where anomalies were significant at the 0.01 level are shaded with vertical lines.

At least two days before snowfall the zonal circulation over the North-east Atlantic, at the 500 hPa level is transformed to meridional with the formation of a trough over central Europe (Figure 2A) which reaches down to the western Mediterranean. Similar conditions favour the winter snowfalls in northern Greece (Makrogiannis and Sahsamanoglou, 1981).

Table 11. Air temperature ("C) at 12.00 GMT before and after snowfall day (1963-1972)

Day

Meteorological station - 3 -2 - 1 D + 1 + 2 + 3

Eko (2817 m) 7.8 7.3 5.1 1.8 2.3 4.2 5.3 Iatrio (2380m) 11.9 11.6 10.1 6.5 6.3 7.8 10.0 Keoa (1750 m) 16.7 17.0 15.0 11.5 10.7 12.4 13.8 Larissa (75 m) 31.4 31.1 30.1 25.3 260 27.5 28.0

Table 111. Maximum and minimum air temperature ("C) at 12.00 GMT before and after snowfall day (1963-1972)

Meteorological station

Florina (660 m) Max. Min.

Larissa (75 m) Max. Min.

Thessaloniki Airport (5 m) Max. Min.

- 3 -2 - 1 D + 1 + 2 $ 3

28.0 28.0 26.0 23.0 22.0 24.5 25.0 13.5 13.0 13.5 12.5 10.5 10.5 11.5

32.0 32.5 31.0 28.5 28.0 29.5 29.5 16.5 16.0 16.5 16.5 14.0 15.0 15.0

31.0 30.5 29.5 27.0 26.5 28.0 28.5 17.0 17.0 17.5 17.5 163 15.0 16.0


Figure 2. Mean synoptic maps of D - 2 day


Figure 3. Mean synoptic maps of D- 1 day


Figure 4. Mean synoptic maps of D day


Figure 5. Mean synoptic maps of D + 1 day


An examination of charts of the mean 500 hPa level indicates that during the following days this trough is displaced to the east (at an average of 5" day-') and deepens at the same time (Figures 3A and 4A). In this way, on the day of snowfall it is situated over the whole of the Greek area. As a result there is a marked fall in the height at the 500 hPa level, an indication of a strong, cold invasion. At the same time, a field of positive anomalies forms over the North-east Atlantic at the 500 hPa level, which, one day before the snowfall, reach values that at certain points exceed 80 gpm. During the following days this field diminishes and at the same time is displaced slowly to the north-east (Figures 2A, 3A, 4A, and 5A). This field of positive anomalies is an indication of the presence of a new anticyclone at the surface situated a little to the east of the centre of positive anomalies. The suggestion that this is really a new anticyclone and not an extension of the Atlantic anticyclone is supported by the fact that no marked increase in pressure appears in the North Atlantic High. As can be seen from the mean surface charts, which will be analysed later, this anticyclone is not stationary but moves to the north-east.

Over the area where the trough appears, a negative anomaly field is formed, which grows stronger, reaching its maximum absolute values the day snow appears; with values exceeding 80 gpm at many points. These values are double the values given by Metaxas (1974) as monthly values for July and August. The consequence of a large negative anomaly field is the displacement towards Greece of very cold, polar air masses, which when they come in contact with the warmer air masses over Greece make available large amounts of potential energy.

From the mean 1000/500 hPa thickness charts (Figures 2B, 3B, 4B, and 5B) it can be seen that before snow appears, to the east of British Isles there is the formation of a thermal anticyclone, which the day following the snowfall begins moving to the north-east. The presence of this thermal anticyclone over this area is indicated by a field of positive anomalies over the same area which reach values higher than 20 gpm (Figures 2B and 4B). On the same charts, over central Europe there is an extended field of strong negative anomalies which is displaced to the south-east, getting stronger at the same time. By the day of the snowfall, it covers the whole of the Greek area and its values exceed 80 gpm. This negative anomaly field indicates the presence of a large amount of available potential energy which results in cyclogenesis over northern Italy or the Adriatic Sea two days before snowfall. The flow of cold air reinforces this depression which follows the movement of the trough and in this way, two days after its formation, its fronts are active over Greece. As the displacement of this depression is relatively rapid, it cannot be followed on the mean surface charts (Figures 2C, 3C, 4C, and 5C).

On these mean surface charts the appearance of an anticyclone can be noted to the east of the British Isles with a mean central pressure of 1020 hPa and a field of positive anomalies which are statistically highly significant, exceeding 6 hPa. As indicated before, the anticyclone and the anomaly field moves to the north- east becoming less active. On the mean surface charts it can also be seen that the Pakistan Low appears weakened over the eastern Mediterranean Sea and the Middle East area two days before the snowfall.

4. ATMOSPHERIC STRUCTURE OVER NORTHERN GREECE DURING THE SUMMER SNOWFALLS

The preceding synoptic analysis has shown that the temperature of air masses over northern Greece and the Balkans presents an important decrease on the day snow appears because cold air masses have arrived over the Greek area. This fall in temperature begins at least two days before the snowfall and extends to the whole of the lower troposphere. This decrease is particularly important at the 850 hPa level, exceeding 6°C (Table IV). Another remarkable fact is the displacement of the 0°C level from 626 hPa (on D-3 day) to 710 hPa (on D-day). This means that on the snowfall day the 0°C temperature level reaches approximately the altitude of the Eko meteorological station (Table V).

The thickness of the lower troposphere everywhere shows a continuous reduction, beginning almost two days before snowfall, being completed on the day of snowfall. The reduction is greatest in the lower levels, that is the layer 1000 to 850 hPa (Table VI). The values of the thickness of the 1000/850 hPa layer during summer snowfalls are higher by 50 to 100 gpm than values given by other workers for snowfalls in general; see Boyden


Table IV. Dry bulb temperature and wet bulb potential temperature at 12.00 GMT (1963-1972)

Day: D - 2 Day: D- 1 Day: D Day: D+ 1 Level @Pa) TYC) 6,(K) TCC) O,(K) T C C ) 6 J K ) T("C) 6,(K)

400 - 24.0 - 25.5 -27.1 - 26.7 500 -12.8 314 -12.5 314 -15.8 311 -14.5 312 700 3.8 302 2 5 302 0.2 301 0.2 300 850 16.2 298 15.0 297 9.6 294 10-6 294

lo00 27.2 294 25.7 293 24.2 292 242 292

Table V. The 0°C level at 12.00 GMT (1963-1972)

Day D-4 D-3 D-2 D-1 D D + l D + 2 D + 3 Level @Pa) 629 626 639 660 710 693 655 655

Table VI. Thickness (gpm) at 12.00 GMT (1963-1972)

Day

D-2 D-1 D D + 1

1o00/500 Max. Mean Min.

700/500 Max. Mean Min.

8501700 Max. Mean Min.

lo00/850 Max. Mean Min.

58 10 5681 5610 2770 2655 2620 1650 1612 1590 1580 1423 1390

5760 +61 5659

5520 2690

+38 2649 2600 1620

+16 1604 1570 1500

+48 1406 1360

5720 +59 5588

5500 2680

+23 2619 2590 1620

+16 1588 1560 1410

+31 1376 1320

5790 +60 5621 +77

5480 2690

+23 2632 +37 2550 1630

+I5 1590 +21 1560 1430

1360 +28 1390 +20

(1964), Lioki-Livada-Tselepidaki (1979), Prezerakos (1984), Sahsamanoglou and Makrogiannis (1978). Differences are even more important for the thickness of the 1000/500 hPa layer, as compared to those given by the same workers as well as by Murray (1952) for winter snowfalls, exceeding 300 gpm.

As can seen from Table IV, the atmosphere is stable, in general aO,/az >O, before and after the day of snowfall.

In the 1000/850 hPa layer, the wind turns clockwise with height (warm advection) with the exception of D - 1 day when the wind turns counterclockwise (cold advection). In the 850/400 hPa layer, the wind turns counterclockwise with the exception of D - 1 day when the wind turns clockwise. Finally, in the 400/300 hPa layer, the wind turns counterclockwise in the days before snowfall and clockwise on D and D + 1 day (Table VII).


Table VII. Wind direction and speed (knots) at 12.00 GMT (1963-1972)

D-2 D-1 D D + l Level @Pa) Direction Speed Direction Speed Direction Speed Direction Speed

300 269 31.3 259 49.9 252 41.7 306 29-5 400 27 1 19.4 263 41.8 248 38.6 303 21-4 500 272 162 25 1 3 1.0 253 28.1 300 208 700 289 6.3 247 12.5 282 12.2 325 12.6 850 31 1 3.9 238 6.0 310 9.2 339 13.5

lo00 289 4.8 293 4.8 315 7.8 33 1 11.8

5. CONCLUSIONS

Summer snowfalls on the Mount Olympus area are observed usually at altitudes exceeding 2500 m. Their frequency is approximately three times every summer. The period 20 July to 10 August usually has the smallest number of snowfall days.

At least two days before the snowfall, air temperature at northern Greek meteorological stations near sea level, as well as at the mountain meteorological stations on Mount Olympus show an important fall. The maximum temperature falls by 4°C to 5°C on average, that is, it reaches values almost equal to (T,,, ) - 2a.

These snowfalls and the bad weather accompanying them are due to the passage of a depression over northern Greece or the Balkans which resulted from cyclogenesis over northern Italy, the Adriatic sea, the Ionian sea, and the southern Balkans. This is caused by a horizontal advection of cold air masses which start near the British Isles or from Scandinavia and reach to the central Mediterranean. This system will not always cause the appearance of snowfalls over Greece in summer and it is different to the system causing winter snowfalls in Greece.

Summer snowfalls are observed when the values of positive anomalies at the 500 hPa level to the west of the British Isles and the values of negative anomalies at the same level over Greece reach above 80 gpm.

During snowfall days, the Pakistan Low appears reduced, at least over the eastern Mediterranean area. The important temperature falls in air masses observed at least two days before snowfall are extended with

height. The greatest reduction of temperature is at the 850 hPa level, amounting to more than 6°C on average. This results in a fall of the 0°C level from 626 hPa to 710 hPa.

During the days preceding the appearance of snow on Mount Olympus the atmospheric structure over the mountain is generally stable.

ACKNOWLEDGEMENTS

The author is indebted to B. D. Giles, Department of Geography, University of Birmingham, U. K. for going through the manuscript critically and offering valuable suggestions.

REFERENCES

Boyden, C. J. 1964. ‘A comparison of snow predictors’, Meteorol. Mag., 93, 353-365. Lioki-Livada-Tselepidaki, I. 1979. ‘Snowfall in Greece’, Ph.D. thesis, University of Athens, 209 pp. (in Greek). Makrogiannis, T. J. and Sahsamanoglou, H. S. 1981, ‘Anomalous atmospheric circulation favouring snowfall in N. Greece’, J . Meteorol.,

Metaxas, D. A. 1974. ‘Atmospheric circulation anomalies affecting the summer temperature in Greece’, Proceedings of the Seminar on

Murray, M. A. 1952. ‘Rain and snow in relation to the 1000/850 mb and 1000/500 rnb thickness and the freezing level’, Meteorol. Mag., 81,

6, 225-230.

Atmospheric Physics, September 1973, Athens, pp. 17-20.

5-8.


Prezerakos, N. G. 1984. ‘Atmospheric structure during snowfall over Athens’, Not. Meteorol. Seroice Athens Bull., 11 (in Greek). Prezerakos, N. G. and Angouridakis, V. E. 1979. ‘Characteristics of snowfalls in Athens’, Bull. H.M.S., 4, 12-32 (in Greek). Prezerakos, N. G. and Angouridakis, V. E. 1984. ‘Synoptic consideration of snowfall in Athens’, J . Climatol:, 4,269-285. Sahsamanoglou, H. S. 1978. ‘The mountain mass of Mt. Olympus as a heating source of the lower troposphere’, 15 l T A M Grindelwald

Sahsamanoglou, H. S. and Makrogiannis, T. J. 1978. ‘The form of precipitation in the area of N. Greece in relation to suitable indices’, Schweiz, 19-23 September 1978, pp. 243-246.

Bull H.M.S., 3, 11-20 (in Greek).

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