551.543 : 551.553.532.53

Short-period variations in surface pressure and wind

By I. J. W. POTHECARY Meteorological Ofice, Dunstable

(Manuscript received 28 September 1953, in revised form 5 May 1954)

SUMMARY Periodic variations in surface pressure and wind velocity recorded by autographic instruments on

5 July 1952 are related to wave motion along a horizontal temperature discontinuity. The wave motion is considered to have been set up when the outflow of cold air from an intense outbreak of thunderstorms over the western English Channel temporarily blocked the north-easterly airstream beneath an inversion over southern England and the Midlands.

The oscillation was propagated upstream of the lower current and across the upper south-easterly current for over 250 mi, with an amplitude approaching 200 m in the earlier stages.

1. INTRODUCTION In the early hours of the morning of 5 July 1952 several stations in southern England

recorded an unusual oscillation of wind velocity, and many stations recorded an oscillation of pressure.

There was little apparent change of weather associated with this disturbance, although 24 hours later a thundery outbreak, which was severe in the west of the country, moved northward from the English Channel and was responsible for a marked pressure jump resulting from the normal thunderstorm processes.

An analysis of pressure and wind records, from a number of widely scattered stations in southern England and the Midlands, showed that the oscillation moved north-east against the lower airstream for over 250 mi, at a speed which slowly decreased from an initial value of over 20 m/sec as the amplitude of the effects became smaller.

From the available evidence it is suggested that the effects shown on the autographic records were due to an oscillation set up in an inversion at about 830 mb. A warm layer was overlying a north-easterly airstream and formed the lower boundary of an upper south-easterly current.

It is possible that the oscillation was set up when the lower airstream was temporarily blocked by the outflow of cold air from an intense outbreak of thunderstorms over the western English Channel.

2. SYNOPTIC SITUATION

During 2 July a cold front, associated with a small depression moving north-east along the Norwegian coast, crossed the British Isles and moved into France and the Low Countries, setting off thundery activity over the Continent.

Figure 1. Surface chart for 0001 GMT, 5 July 1952.

395

396 I. J. W. POTHECARY

By 12 hr, 3 July, the front had become slow moving, and later became retrograde over northern France. Meanwhile an anticyclone moved quickly north-east across the British Isles towards Scandinavia, where it was centred on 5 July.

Shallow thundery lows had dominated the weather over the whole of France since the beginning of the month, and by 24 hr, 4 July, thundery outbreaks had extended across the whole of the Bay of Biscay and the southern half of the English Channel, to the south of the warm front, which, by then, was moving slowly northward.

An extensive area of severe thunderstorms was set off during the night of 4-5 July in the vicinity of the Channel Islands, and it was this thundery area which moved north- ward across the British Isles during G July, with severe thunderstorms in the west.

3. THE TEMPERATURE AND WIND STRUCTURE OVER SOUTHERN AND CENTRAL ENGLAND

During the night of 4-5 July surface winds were north-easterly, becoming easterly over Devon, Cornwall and west Wales. Surface windspeeds varied from 5 to 20 kt according to the exposure of the station.

The Camborne, Larkhill, Hemsby and Liverpool radiosonde ascents at 03 GMT,

5 July, showed that a horizontal warm layer, some 150 mb thick, extended over southern England and the Midlands, between 950 and 800 mb. At Larkhill, and to a lesser extent at Camborne, the warm air was present in two distinct layers; one at 950 mb and the other at 830 mb.

The lower layer resulted from night cooling at the surface after a day of convection on 4 July, which gave rise to an approximately dry adiabatic lapse rate below the warm layer already in existence at 830 mb.

The origin of the upper warm layer lay in the intense heating during the previous three days over the Continent and, on 1 July, over southern England, with surface temperatures in excess of 90"F, which produced a layer of convectively mixed air from the surface to 700 mb, to the south of the cold front.

HUMIMTY MIXING RATIO IN GM/KG OF SATURATED AIR

'4 -TEM?ERCTURL

t DEW POINT

Figure 2. Larkhill radiosonde ascent, 03 CMT, 5 July 1952.

PRESSURE AND WIND OSCILLATIONS 397

On 2 July the south-eastward movement of the cold front brought cooler air across the British Isles. During 4 July the wind above 900 mb over southern England veered from north-easterly to south-easterly as the front became retrograde, and a layer of warm air spread northward over southern England and the Midlands.

At 03 hr, 5 July, over the Midlands and south-east England, the wind was east-north- east up to 850 mb, with a mean speed of about 20 kt through the layer, veering to south-east with a marked decrease in speed through the warm air above this level. Although the evidence does not indicate the existence of sharp discontinuities, the rate of change of velocity through the inversion is large so that the conditions approximate to an idealized model of the atmosphere in which a discontinuity of velocity coincides with a discontinuity of density. This model is inferred as the lack of wind data from Larkhill at 03 hr pre- cludes any more definite statement of the wind structure.

4. THE EFFECT OF THUNDERSTORMS OVER THE WESTERN ENGLISH CHANNEL

Between 18 GMT and 22 GMT, 4 July, a severe outbreak of thunderstorms occurred over the Channel Islands and the western English Channel. Lightning was reported at midnight to the south-east of Plymouth and south-west of Calshot. A thunderstorm was reported at Portland Bill. Severe thunderstorms were affecting Brittany and the Cherbourg peninsula. '

In an area of thunderstorms of this magnitude the outflow of cold air at the surface is considerable and will cause a sudden rise of pressure. This feature was recorded at many places in southern England and the Midlands as thunderstorms moved northward during the morning of 6 July.

It is suggested that a sudden and considerable outflow of cold air from the thunder- storm area abruptly blocked the easterly flow beneath the warm layer somewhere over

I 6 JULY 12 W T

I 5 JULY

1030

I 1 OSCILLATION T m L R - u*lcc

OSCILLATION 1

I I OSCILLATION' TUUCOCIUTORM SUffiC

Figure 3. Barograms, 4 July-6 July 1952. Figure 4. Hourly isochrones of the pressure (a) Farnborough. (b) Abingdon. (c) Croydon. disturbance, 5 July 1952.

398 I. J. W. POTHECARY

the English Channel, between Guernsey and the English coast ; setting up an oscillation in the discontinuity. This oscillation was then propagated upstream of the lower flow and across the upper flow as a wavelike disturbance of sufficient magnitude to cause the surface pressure and wind variations recorded at many stations.

After the initial outburst of cold air the easterly flow was re-established over the Channel, relieving the temporary blockage of the lower flow on the coast.

5 . PRESSURE EFFECTS ASSOCIATED WITH THE DISTURBANCE

Barograms were examined for 35 stations in England and Wales, south of the Humber. A distinct pressure oscillation was shown on 29 records, those which showed no effect being in extreme south-east England, and in Cornwall where the trace was extremely unsteady throughout the night. No effects were recorded in Wales, or north-east of a line from the Mersey to the Wash.

The time of the start of the pressure oscillation was determined for as many stations as possible from the barograms, and the anemograms where these were available. This timing was subject to an error of about five minutes, assuming that the records had been correctly time marked.

The times were plotted and hourly isochrones drawn which showed that the source of the disturbance was over the western English Channel, and,that the disturbance moved north-eastward with decreasing speed and a decrease in the amplitude of its effects.

In each case the pressure oscillation began with a sudden rise followed by an oscillatory decay to the original pressure which lasted for about 90 minutes (Fig. 3). The traces are extended through the morning of 6 July to show the effect of the outflow of cold air at the surface as thunderstorms moved over or near the stations. No thundery activity occurred over England away from the South Coast during the night 4-5 July.

6. WIND VELOCITY EFFECTS OF THE DISTURBANCE

Dines pressure-tube anemograms were examined for 11 widely-spaced stations. Nine No of the records showed a marked effect associated with the oscillation of pressure.

effects could be traced at St. Merryn in Cornwall, or at Aberporth in Wales.

I I I I I SPEED I I I I I SPEED

10 I0

KNOTS KNOTS

0 0 0 I 2 3 4 CMT I a 3 4 5

300

DIR

2 0 0

DECS

I00

n v

(a) Farnborough. (b) Abingdon.

Figure 5. Anemograms 5 July 1952.

AAAAnj 0

(c) Croydon.

PRESSURE AND WIND OSCILLATIONS 399

The three individual records with the most marked effects are described below. The other records show that the surface flow was reversed for a time at Calshot, Hampshire, and at Henstridge, Dorset, although a marked periodicity appeared in the speed at Calshot. At Boscombe Down and Thorney Island the surface flow was temporarily blocked, but at places further away from the coast a periodicity in both speed and direction was imposed on the mean flow. There is some evidence of a periodicity as far away as Bramcote, Warwickshire.

(a) Farnborough, Hampshire

The Farnborough anemogram (Fig. ja) was first affected at 0130 GMT when the direction veered from 070" through south-west to 070" in 18 min, while the speed rose to a sharp maximum of 13 kt. The direction then oscillated about north for four periods with amplitudes of 180, 130, 110 and 100 degrees before finally settling down to the original speed and direction at 03 GMT.

Apart from the first 13-kt gust there was only one other pronounced maximum of speed, which was a gust of 1 2 kt when the wind backed to a turning point at 270" in the first period. The speed remained remarkably steady at 5 kt from 0200 GMT until 0230 GMT

when the original speed of about 8 kt was resumed.

(b) Abingdon, Berkshire

The direction periodicity and the first sudden veer through a complete circle (Fig. 5b) are similar to that recorded at Farnborough (Fig. 5a). A further turn through 360" occurred under almost calm conditions before the periodicity became established.

The speed maxima coincided with the periodic swing through the original direction of north-east, falling almost to calm for any other direction.

In the direction trace six full periods oscillating about north are clearly shown, with successive direction amplitudes of 170, 160, 110, 80, 70 and 50 degrees. The period of the oscillations slowly increased from 15 to 20 min.

(c) Croydoti, Surrey

At 0215 GMT, from steady conditions with a speed of 8 kt from a direction of 030",

During the following 90 min there was a steady periodic variation in direction and

The speed maxima of about 10 kt coincided with the periodic swing through the

the direction backed to 290" and the speed fell off to less than 2 kt.

speed with a periodic time of about 20 min (Fig. 5c).

original direction.

7. DISCUSSION

The reversal of the surface flow at Chlshot, combined with a periodic oscillation of the speed, shows that the cold air flowing outward from the thunderstorm area cannot have been far to the south-west.

At Boscombe Down, 30 mi to the north-west of Calshot, the surface flow was tem- porarily stopped by the downstream block. A periodic oscillation of wind superimposed on a steady flow was nearly realized at Thorney Island, 20 mi east of Calshot, although a prolonged gust to a maximum speed of 28 kt, from a direction across the flow beneath the inversion, indicated that some effect other than a purely periodic oscillation was present, at least for the first 15 min.

400 I. J. W. POTHECARY

A periodicity superimposed on the surface flow was well established at Farnborough, Abingdon and Croydon, and it is over this area, at least 50 mi from the estimated closest approach of the cold air, that the behaviour of the discontinuity is discussed.

The Larkhill radiosonde ascent at 03 GMT, 5 July, which was representative of the air over this area, showed two well-marked inversions (Fig. 2). Unfortunately no winds were available from that ascent to show which inversion coincided with the boundary between the upper and lower currents. An objective estimate of the mean velocities of the upper and lower airstreams over the area was made assuming that the upper inversion could be considered coincident with the wind discontinuity.

8. ANALYSIS The problem may be treated simply as an analysis of the behaviour of the discontinuity

between two superimposed streams in an incompressible fluid, with the lower stream, of depth h, density p , and velocity U flowing over a rigid surface. The upper stream is assumed to be unlimited in depth.

The following relations between variations of wind speed and pressure at the surface are obtained (Goldie 1925) :

Aps = (U cos p - V)2 . * (1)

A U s = ( V c 0 ~ p - V ) . f (2)

h a

where Aps is the amplitude of the surface variation of pressure, AU, the amplitude of the surface variation of wind speed, a the amplitude of the wave motion of the discontinuity, j3 the angle between the direction of the lower airstream and the wave motion, and V is the velocity of the wave motion.

It is emphasized that the assumption of incompressibility, and the concentration of all the stability at the inversion, while giving equations that can easily be handled and applied, is a great over-simplification of the atmosphere, but in the present case a com- parison between the theoretically deduced speed of the lower airstream and an objective estimate of the actual value showed an interesting measure of agreement.

The theoretically deduced amplitude of the wave motion of the discontinuity is therefore not likely to be greatly in error.

The following values were taken as common to Farnborough, Abingdon and Croydon: h = 1,650 m, U = 10 m/sec, = 145", and p = 1.13 x lo3 g/m3.

Table 1 gives for Farnborough, Abingdon and Croydon, the values observed for various parameters and then deduced from Eqs. (1) and (2).

TABLE 1.

( 4 Farnborough

Deduced from autographic records Velocity of the wave motion, V (m/sec)

Wave-length, X (km) 13.3

13 1,020 Period of oscillation of wind direction, T (sec)

ZAP, (mb) 1 ZAU, (kt) 8

(U cos fl - V), (m/sec) - 21

(b) Abingdon

16 750

12.0 1.4

10

- 24

( 4 Croydon

11 1,200

13.2 1 9

- 19

Deduced from Eqs. (1) and (2) (V cos /3 - V), (m/sec) - 22 - 24 - 19

a ( 4 162 177 201

PRESSURE AND WIND OSCILLATIONS 401

9. CONCLUSION

The model used is a rough approximation to the actual condition of the atmosphere but the quantitative results show that it is by no means invalid in spite of its simplicity.

It indicates oscillations of amplitude of the order of 200 m, or about one-eighth of the depth of the lower layer, which could lead to the formation, and quick dispersal, of a stratocumulus type of layer cloud, provided that the air is near saturation at the level of the discontinuity.

This condition was present at Liverpool at 03 GMT, 5 July, and in view of the similarity of all the English radiosonde ascents at that time, the condition was probably widespread over a large area. At Filton the following cloud changes were reported during the early hours of 5 July : ' Between 0001 GMT and 0100 GMT low cloud increased from nil to six oktas, while a light NE wind veered to ESE. By 0200 GMT the wind had backed to north- east again, while the low cloud decreased to nil.'

The wave motion is estimated to have reached Filton by 0030 GMT so that the cloud reported was almost certainly associated with the oscillation.

The amplitude of the wave motion demonstrates its capacity as a means of trans- ferring energy from small-scale weather systems quickly over large distances, irrespective of the wind field.

Oscillations on this scale may have some bearing on the variability of frontal rain and the case analyzed is a particular example of a situation which is very similar to a warm front. If latent instability is present in the warm air it is possible for a similar process of energy transfer to be responsible for rain breaking out, or intensifying, in an otherwise unpredictable manner.

Ac KNOWLE DGMENT

This paper is published with the permission of the Director of the Meteorological Office.

REFERENCE

Goldie, A. H. R. 1925 Quart. /. R. Met. Soc., S1, p. 239.