fÖhn conditions in the lee of the sarek mountains, sweden

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FOHN CONDITIONS IN THE LEE OF THE SAREK MOUNTAINS, SWEDEN By I. Y. ASHWELL University of Alberta, Calgary and F. G. HANNELL McMaster University, Ontario URING August and September of 1959 members of the British Schools D Exploring Society’s Expedition to Arctic Sweden, based at A in Fig. I, carried out certain glaciological and micrometeorological projects. In the course of these, it seemed that the area near A was affected by Fohn winds, and it has been suggested that these may have important effects on the melting of ice and snow. This effect probably reaches its maximum at some altitude above sea-level, in the remoter parts of the mountains, and it has been considered useful to analyse some of the results of the Expedition’s work, since niost of the weather reporting stations operated for the Swedish Hydrological and Meteorological Institute in this area are at lower levels in the valleys. The main meteorological station of the project was located at A (Fig. z), in the mouth of a drainage channel from the large upland basin of the small Paktesjokk stream. The station, at an altitude of 846 m, was equipped with the normal screened thermometers and a thermograph, together with an anemometer of cup-counter pattern and a sensitive wind-vane, and a raingauge. Due to the confined position of the station the wind direction was rarely representative of anything other than down-valley flow. Observations at this station were made at the main daytime synoptic hours from 11 August to 7 September 1959. Fig. I. The synoptic situation at 06 GMT 20 August 1959 showing Bod0 on the Norwegian coast and Station A inland, both lying in the warm sector of an Atlantic depression 51

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FOHN CONDITIONS IN THE LEE OF THE SAREK MOUNTAINS, SWEDEN By I. Y. ASHWELL University of Alberta, Calgary

and F. G. HANNELL McMaster University, Ontario

URING August and September of 1959 members of the British Schools D Exploring Society’s Expedition to Arctic Sweden, based at A in Fig. I,

carried out certain glaciological and micrometeorological projects. In the course of these, it seemed that the area near A was affected by Fohn winds, and it has been suggested that these may have important effects on the melting of ice and snow.

This effect probably reaches its maximum at some altitude above sea-level, in the remoter parts of the mountains, and it has been considered useful to analyse some of the results of the Expedition’s work, since niost of the weather reporting stations operated for the Swedish Hydrological and Meteorological Institute in this area are a t lower levels in the valleys.

The main meteorological station of the project was located at A (Fig. z) , in the mouth of a drainage channel from the large upland basin of the small Paktesjokk stream. The station, a t an altitude of 846 m, was equipped with the normal screened thermometers and a thermograph, together with an anemometer of cup-counter pattern and a sensitive wind-vane, and a raingauge. Due to the confined position of the station the wind direction was rarely representative of anything other than down-valley flow. Observations at this station were made at the main daytime synoptic hours from 11 August to 7 September 1959.

Fig. I. The synoptic situation at 06 GMT 20 August 1959 showing Bod0 on the Norwegian coast and Station A inland, both lying in the warm sector of an Atlantic depression

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1;ig 2. The location of Station A and the subsidiary meteorological stations in Sorth Sweden.

The main purpose of the project was to study conditions of air drainage from the Paktesjokk basin, and for this purpose a line of 12 stations was established across the outlet from the basin, with a further three stations run- ning up the valley to point 15 (Fig. 2 ) . Cup counter anemometers at stations I, 12 and 15 were read each morning and evening. When conditions were still and clear, five parties of two members each made carefully timed and simul- taneous traverses between stations so that the whole line of stations was covered a t regular intervals. Readings were made of temperature and humidity with aspirated psychrometers, and of wind direction and speed, whilst grass- minimum thermometers located at certain of the stations were read a t each traverse. The whole of the bottom of the valley was occupied by a large snow- patch, and it is most likely that many of the readings a t station A, lower down thc valley, were affected by this snow, especially in the daytime, when melting of the snow added to the humidity of the air. To illustrate this effect, some figures are shown in Table I, which compares readings taken on the slopes abo1.e the snow-patch with those at Station A. Unfortunately the readings at the out-stations were taken by Assman psychrometer, and those a t Station A were read from the screened thermometers, so that the comparison is not esact. However, there would seem to be a significant decrease in humidity on the slol)es above the snow-patch.

Snowpatches indicated by horizontal hatching

Fig. 3. Variations of meteorological elements in late August and early September 1959 at stations operated by the British Schools Exploring Society

The variations of several elements in late August and early September are shown in Fig. 3. It will be noticed that there are two periods of high temperatures, the first 19-23 August and the second from 6 September to the end of observations. During both the periods the vapour pressure was comparatively low, and daytime relative humidity fell to below 60 per cent on each occasion.

The full line on the curves of windspeed shows the daily average of wind- speed at Station 15, which, being in the Paktesjokk basin itself, was much better exposed than Station A. With these values are shown those of the gradient- wind and direction (dashed lines), calculated from the 850 mb charts by the Swedish Meteorological and Hydrological Institute. Only during a blizzard from the north were the two windspeeds in any way comparable. This is not the result of local topography, since the high ground lay to the north of Station 15, and it would be expected that differences would be greatest with northerly winds. However, differences are in fact greatest with winds between west and south-west, that is, when the flow is over the Sarek Mountains, lying in this direction. I t therefore seems possible that the low observed wind speeds at Station 15 are the result of turbulence and waves aloft, resulting from air flow over these mountains. Further support to this point is given by the frequent occurrence of multiple-lenticular wave clouds over the area (Figs. 5 and 6, page

The difference in wind speeds is most marked during times of highest temperatures, and further evidence of a Fohn effect can be found by tracing the movement of air from Bodo, on the Norwegian coast, some 200 km to the west

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55).

Potential Temperature Deg C

I'ig. 4. l'ephiyrani showing trajectory of air from Bod0 (B) t o station X (A) . At the level o f A the theoretical dry-bulb, 1 3 . 4 T , is almost identical with the actual value, but the theoretical (a) and actual (b) wet-bulb temperatures are different for reasons discussed in thc text

of A, under such conditions. Fig. I shows that on 20 August a t 06 GMT both Bod0 and Station A lay in the warm sector of a depression, under a westerly flow of air. Assuming a starting point for air movement a t Bodo, the trajectory of the air can be traced theoretically on a tephigram (Fig. 4).

From Bodo (level B) the air is assumed to rise to a height of 830 nib, representing the mean summit-level of the Sarek Mountains (highest points about 2,000 m). Descent from that level to that of Station A (A) should result in dry-bulb tcniperature 13.1"C, wet-bulb 9.h"C ((a) on Fig. 4). -4ssuming a 3 hr period for the air to cross the mountains to A, the actual temperature \.slues a t this time arc dry-bulb 13.3"C and wet-bulb 1o.0"C ((b) on Fig. 1). 1 lie discrcy)anc!. between the theoretical wc~t-l,ulb (a) and the actual \.slue (b) may be an effect o€ the snow-patch above Station A, shown above in Table T.

I t would seem, therefore, that Fiilin effects do occur in this area. Thanks are due to the Swedish Meteorological and Hydrological Institute

and the Norwegian Meteorological Institute for providing information. Also to the Meteorological Office for the loan of instruments. This work would have been impossible but for the assistance of members of the Expedition, who carried out accurate observation in all weathers from calm to blizzard.

~.

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AI,TOCI~MIyLI.S LENTICIYLARIS. stoke, Hampshire

A s1:nset view taken at Mattingley, near Basing-

Photografih by F. J . Smith This pleasing effect of snow on bushes was seen at Mablethorpe, Lincoln-

On the previous day there had been frequent snow SNOW SCENE. shire, on the morning of 3 March 1965. showers in an unstable north-easterly airstream coming from the North Sea

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Fig. 3

SOME CLOUD PHOTOGRAPHS TAKEN IN JAN MAYEN During the summer of 1963 an expedition from Imperial College, London,

visited the island of Jan Mayen in the Greenland Sea. The sketch map shows that the island may be divided topographically into two parts connected by a narrow, low-lying neck of land. South Jan Mayen consists of a line of dissected volcanic hills reaching a maximum elevation of 769 metres. North Jan Mayen is dominated by one of the largest volcanic cones in the world, the Beerenberg, whose highest point, Haakon VII Topp, is 2,277 metres above sea level.

As part of the meteorological programme, a photographic record of cloud forms was taken and some of the more interesting pictures are shown here. An attempt has been made to interpret the cloud forms in the light of the tem- perature and wind structure of the atmosphere shown by upper-air soundings made at Jan Mayen Meteorological Station.

Looking north-west from High Camp. The structure of the cloud is revealed by light and shade, with the sun shining on the underside of the medium cloud. Above 4,000 metres, which is close to the estimated height of the cloud base, the atmosphere was quite moist with a lapse rate near to, and in one layer exceeding, the saturated adiabatic value. Below 4,000 feet the atmosphere was dry with a near isothermal lapse rate for

Fig. I, 13 July at 2305 GMT.