A HEAVY RAINFALL IN THE EAST MIDLANDS

By I. T. LYALL

Newark, Nottinghamshire

O N the night of 8-9 September 1972, a heavy rainstorm hit the east Midlandsarea. In part of north Leicestershire in the 24 hours ending at 09 GMT on 9

September, over 60 mm rain fell. Finch (1972) shows that for Leicester such afall within two consecutive rain-days has occurred only once in the period 1956to 1971 inclusive. Fig. 1 shows the distribution of rainfall over much of Notting­hamshire, Derbyshire and Leicestershire and parts of neighbouring counties.Table 1 shows the 24-hour totals for stations contributing to the ClimateSummary for the east Midlands compiled by Mr D. S. B. Fellowes. The locationsof these stations are shown in Fig. 1. A large number of other stations reportmonthly totals and these were used to assess the distribution of what was themajor part of the month's rainfall (the remainder showing a very even distribu­tion over the area).

Newarke

-Finningley

<;'0

__so,

~ "~

\-, \:x~r; \~,"'fA \\J10ljSheffield

eBUX}on/ Chesterfield~Chal5worthe - e -Warsop

(\. _ \ : ~ramPton. ., " " \ , ' -Wingerworth

\ \t, -Ashover\ '10 \.'.

\'\. Ooft!:J)Matlock

Bel per- -Watnalle/I'

-Morley ~Mackworth- -Derby/

./ eLittleover~O I

_NewtownLinford

I

Fig. 1. Distribution of rainfall in east Midlands for 24-hours ended 09 GMT, 9 September1972 (in mm). Broken line - approximate position of 1000 ft contour

28g

TABLE I. 24-hour rain total at east Midland stations

Station

Repton SchoolMackworthLittleoverDerbvMorleyBelperMatlockAshoverWingerworthChesterfieldBramptonChatsworth HouseBuxtonSheffieldNewtown LinfordSutton BoningtonWatnallWarsopN~wark

Finningley

THE SYNOPTIC SlTUATIO:\

Rainfall (mm)

46'0

4°'947'74 2 '2

38 '0

38 '738'944'144'24°'44['431. 624'736 '1

60'154'043'047'047. 8

35'°

Altitude (ft AMSL)

18029°23°160350.lOO

65°58038029°

1100

4101010

520390

[6037°15°5°3°

Fig. 2 shows the surface chart for 06 GMT on 9 September. A small butintense depression of 994 mb was centred just off the coast of Yorkshire. It haddeveloped in the south-west approaches late on the 8th and moved north-eastovernight with an open warm sector and a trailing occlusion to another oldlow further to the south-west. The main centre of low pressure was almoststationary at about 69°N, 2°W.

At noon on the 8th no Daily Weather Report station reported heavy rainas present weather, but by IS GMT Thomey Island, Scilly and Culdrose weredoing so. At midnight moderate to heavy rain was widespread over Wales,central and southern England. By 06 GMT on the 9th, the rain was beginning toslacken off and was falling east of the broken line in Fig. 2. The rain area wouldappear to have moved with the low and was not associated with any particularfront, In terms of the treatment of heavy rains by Finch (1972), this case wouldappear to fit his categories' FD ' (frontal depression passing within 200 milesof station) and' MTD '. This latter refers to a marked temperature differenceacross the front, the occurrence of which is clearly borne out in Fig. 2.

RAIN IN THE EAST MlDLAN DS

This depression produced some of its heaviest rain in the east Midlands.It is interesting to note that in Finch's categories the types FD and MTD arethose most frequently responsible for a heavy rainfall at Leicester. The con­ditions also fit a forecasting' rule' developed by Parrey (1972) at Watnall.He investigated the data of the Trent River Authority for 1960-69. A heavyfall was defined as one of over 25 mm in one rainfall day, occurring at at least10 per cent of the Authority's stations, A heavy fall is to be expected when the

Fig. 2. Surface synoptic chart for 06 GMT,

9 September 1972 based on DailyWeather Report.Broken line - western boundary of mainrain area

following developments are likely within the 24-hour forecast period:-(i) a depression or wave-tip to pass into or across the area bounded by

latitudes 50° to 55ON and longitudes 0° to SoW,(ii) the lowest pressure at Watnall to be 1006 mb or lower,(iii) the 24-hour pressure tendency to be negative or not more than I mb

positive,(iv) the dew-point in the warm air to be at least 4 deg C above normal.All these conditions were fulfilled after 09 GMT on 8 September 1972.

(The dew-point data, taken from the Daily Weather Report, related to midnight,when values as high as 16°C were noted.)

The rainfall distribution suggests the north-east progression of the mainrain belt. So mm fell at many stations south-east of a line through Repton andWatnall. The heavier falls of 40 mm in the extreme north-west of the map(Fig. I) are no doubt of orographic origin.

Some smaller-scale features are of interest, notably the rain-shadows onthe east side of the Peak District and in areas south of Sheffield and the areashowing a fall in excess of 60 mm between Newtown Linford and Sutton Boning­ton. During the period of rain the wind was initially easterly, changing to north.westerly. The heaviest fall appears to have been around 03 GMT on 9 September;about that time the depression centre passed close by and the wind changed.The rain-shadows are probably a reflection of the heaviest falls in north Derby­shire and adjacent areas occurring after the wind had changed to north-westerly.The rain maximum is fairly local and is most probably associated with theChamwood Forest area just south of Newtown Linford. At the time, this partof the area would either have been in the warm air or this at least would nothave been far above the surface. Soundings at Crawley at 23 GMT on the 8th,

291

showed high humidity and a lapse up to about 700 mb close to the saturatedadiabatic with a small increase with height in dew-point depression. If similarconditions applied further north, then even the modest orographic ascent dueto the Charnwood hills (900 ft high) combined with the general active upliftcould easily have released latent instability and intensified the rainfall.

THE UPPER ATMOSPHERE

It has been seen that the synoptic situation which gave the heavy rainfallfits well into the classification schemes of Finch (1972) and Parry (1972), butsurface data alone give only an incomplete picture. Even at 18 GMT on 8 Sep­tember there was relatively little temperature contrast across the front whichwas rather diffuse and cannot be readily identified even at 23 GMT in the sound­ings from stations just on the cold side. At 12 GMT there was little to suggest thata depression would move into the area specified by Parrey nor that the overallpressure tendency would be negative.

8880--_"r

9000-----

E

Fig..). 300 mb contour chart at 23 GMT,

H September 1972. Heights in metres.Thick line represents jet stream.E: approximate area of jet entry.X: low centre at surface

At 23 GMT on 8th the then incipient low was favourably placed near theright entrance of a strong upper jet (Fig. 3), and the direction and speed of theupper winds suggest a north-east movement and a possible rapid deepening ofthe low. This rapid deepening would of course make the Iowan active rain­producing system, as a result of uplift of warm, very moist air. At the same time,examination of winds in the lower 500 mb shows that there was warm advectionin the warm air and cold advection in the cold air. The warm advection wouldmake vigorous ascent in the warm air even more likely, and the increased baro­clinity in the low combined with its deepening would further enhance its rain­potential.

The relative advections would also sharpen the frontal system appreciablyand indeed, by 06 GMT on the 9th, the temperature difference was noticeably

2<)2

increased (Finch's MTD type) and in the soundings at II GMT on the 9th, thefrontal layer was clearly defined at Crawley, Hemsby and Camborne.

CONCLUSION

Exceptionally heavy rain fell in the east Midlands on the night of 8-9September 1972. It occurred within a rapidly developing small low, and wasassociated as much with the low centre as with any front. In addition there wasmarked warm advection in the warm air and these factors combined to givemarked uplift of the warm air which was also substantially accentuated byorographic effects.

Several forecasting rules could have been applied in prospect but othersonly in retrospect, unless advection and upper wind data are also applied. Thissuggests that these rules are not in themselves an adequate guide but nonethe­less a very useful supplement to more' routine' data.

REFERENCES

FINCH. C. R.

PARREY. G. E.

1972

1972

Some heavy rainfalls in Great Britain, 1956-1971.Weather, 27. pp. 364-377

Personal communication

RADAR AND PHOTO STUDIES OF ALBERTAHAILSTORMS

By C. WARNER

University of Birmingham

WHILE current research points towards the use of radar for measuringprecipitation, and for short-term forecasting of storms, weather radars

have been used hitherto chiefly for examining structural features of weatherevents, as described for instance by Browning (1971, 1972). In Canada, aweather radar has been part of the Alberta Hail Studies Project since 1956,when the Stormy Weather Group at McGill University, under ProfessorsMarshall, Hitschfeld and Douglas, began a study of the storms which annuallycause a loss of crops to a value near £10 million. From a headquarters atPenhold, halfway between Calgary and Edmonton, hailstorms are observed asthey move generally eastwards from breeding grounds to the west, in the foot­hills of the Rocky Mountains. Early work was concentrated upon radar dataand farmers' hail reports (Douglas and Hitschfeld 1959), and recently experi­mental seeding of hailstorms from aircraft has been undertaken (Summers,Mather and Tredennick 1972). In this paper structural details of two hailstormsare examined, from different points of view.

EQUIPMENT

In 1967 an AN FPS/502 radar, adapted by the National Research Councilof Canada, was put into operation in Alberta (Chisholm 1968). Radiating atwavelength 10 em, it has a conical beam of width about 1'1°. Data are gathered

293