ESSA II SEES MOUNTAIN WAVES OVER BRITAIN By G. A. CORBY Meteorological Oflce, Bracknell

N the morning of 6 September 1966 an intense depression arrived near the 0 Faeroes, having moved rapidly north-eastwards from a position in mid- Atlantic on the previous day. A deep westerly current swept across the country behind the fronts, with wind speeds exceeding IOO kt in the upper troposphere over the north of Britain and reaching 150 kt at Stornoway near the axis of the jet stream. At Shanwell (eastern Scotland) at midday the cold air extended to about goo mb and was overlain by a very stable layer reaching 600 mb, above which the lapse rate was approximately wet adiabatic to the tropopause. We can assert with confidence that under these conditions well marked mountain waves were bound to occur and nowadays the conditions favouring mountain lee waves are sufficiently familiar that it seems unnecessary to provide further justification for the assertion in this short article.

Numerous aircraft reports of standing waves were received from pilots who flew over Scotland or Northern Ireland or who flew up from the south. These indicated strong wave activity at various heights up to about 5 km. One pilot estimated that the vertical currents in waves near Prestwick about midday reached 10 m sec-’ (2,000 ft per min), an extremely high value for waves over Britain, whilst another pilot flying between Belfast and Glasgow at about the same time lost 40 k t in airspeed, presumably whilst endeavouring to maintain height. A forecaster at Abbotsinch reported that ‘he could not remember a day when he received so many reports of widespread standing wave activity’.

Warnings of mountain waves and/or turbulence were issued at various times from late on 5 September onwards for both the Scottish and Preston Flight Information Regions. This is an encouraging state of affairs for, bearing in mind that not much more than a decade ago most forecasters had little knowledge of mountain waves, it indicates that they must now possess a valuable familiarity with the phenomenon.

Progress in satellite meteorology has been rapid since the launching of the first TIROS in 1960 and currently the cloud pictures taken by ESSA 11 launched early in 1966 are becoming an everyday forecasting tool a t the Meteorological Office at Bracknell, thanks to the success of the facilities for automatic picture transmission. The large picture in the centre pull-out shows three overlapping pictures taken by ESSA 11 during orbit 2408. The central portion which includes the British Isles was photographed at 1003 GMT on 6 September 1966. Southern England appears black in the picture and the coasts of East Anglia, Kent and the Channel coast are clearly visible. Extending across Northern Ireland and Scotland is a remarkable sequence of cloud bands lying approximately N-S, and these are doubtless wave clouds. The most intense waves appear from the picture to have been somewhat east of Prestwick, which is consistent with the aircraft report of considerable vertical currents in that area. The significance

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of lee wave clouds as seen on satellite pictures has been discussed by Fritz (1965). It is possible to obtain a fairly accurate wavelength from the picture by

measuring over several waves and a value of 22 - 3 km was found in this way. It is interesting to note that over Southern Ireland and also to the lee of the Welsh mountains and the Pennines the waves have a much shorter wavelength, indeed about half that over Scotland. It is well known that the wavelength is roughly linearly proportional to the mean tropospheric wind speed and this reduction in wavelength is consistent with the falling off southwards of the upper winds over the country.

Some years ago when it was not easy to obtain positive evidence of mountain waves, a study was undertaken to ascertain how successfully waves could be detected from variations in the rate of ascent of radiosondes. The technique and some results were described by Corby (1957). The current procedures a t British upper air stations are now partly automated and this enables such an exercise to be carried out more precisely than previously because human errors in the readings and timing are largely eliminated. The original records from the midday sounding made on 6 September from Shanwell were therefore analysed to see whether they showed evidence of waves.

I I 2 3 4 5 6 7 8 9 1 0 1 1

APPROX HEIGHT OF BALLOON kM)

Fig. I . Radiosonde ascent from Shanwell 12 GMT 6 September 1966. Graph shows ratc of ascent of sonde in m sec-1 as a function of time

The results are shown on Fig. I which gives the rate of ascent of the balloon in m sec-' during the first 30 min of the sounding, as deduced from the rates of change of pressure on the sonde over half-minute intervals, due allowance being made for density variations. Fig. I provides additional evidence of the presence of well-marked waves to the lee of the mountains of Scotland, the waves having a substantial amplitude in mid-troposphere. The vertical

currents found exceed one third of the mean rate of ascent of the balloon 6) to 7 m sec-I) and therefore their reality is beyond doubt. Various estimates of the dominant wavelength can be made from Fig. I and these yield values around 25 km which is a little longer than that measured from the satellite picture. However, the two sources of information differ in time by about 2 hours, and a small erosion of the stable layer, due to the cold air at the rear of the depression becoming deeper in that interval, could easily account for the increase in wavelength.

In this short note we haw merely focussed attention on one small area of one satellite picture. For those interested in learning more of the fascinating art of interpreting such pictures, the World Mcteorological Organization has recently issued Technical Note No. 75 which is an excellent account of the subject, supported by more than a hundred well-reproduced examples of satellite pictures, including several of wave clouds in various parts of the world.

REFERENCES CORBY, G . A.

FRITZ. S.

1957 A preliminary study of atmospheric waves using radio- sonde data. Quart. J., Roy. M e t . SOC., 83 (355). pp. 49-60

1965 The significance of mountam lee waves as seen from satellite pictures. J. A p p . M e f . , 4 ( I ) , pp. 31-37

ESSA I1 SATELLITE PICTURES The following centre pages show a selection of satellite pictures as received

from ESSA 11 at the Meteorological Officc, Bracknell. The large centre picture is a composite of three consecutive pictures 'taken at about 10 GMT on 6 September 1966 when ESSA 11 was on polar orbit number 2408. The position of the cross in the centre of each picture is known from prior calculations based on knowledge of the satellite's orbit. Using a simple optical device the lines of latitude and longitude are projected on to the picture and traced by hand. The positions of the various cloud masses may then be determined. The values of the lines of latitude and longitude on this composite picture have not been reproduced, but the lines are drawn at intervals of 5 degrees and may be identified from the following positions of the central cross. From top to bottom 73-3 N, 15.2 E; 57-3 N, 09.5 W; 39.4 N, 19-7 W. The time interval between the pictures is approximately 6 minutes. It is instructive to compare the positions and orientation of the cloud masses with the appropriate synoptic chart in the Weather Log for September which was published with last month's Weather.

The two other satellite pictures were received early in October and the appropriate synoptic charts will be found in the current Weather Log. The larger is a composite of two pictures and shows a very well marked cloud edge associated with a developing cold frontal zone. The smaller picture was taken at 0904 GMT on 9 October. The coast of North Africa is clearly seen in the lower half of the picture and Gibraltar just above the centre.

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