indicates little about the fluid motion, and all cyclone-systems can be represented as waves we believe this nomenclature to be misleading.

WEATHER

These smaller scale motions are energetically rather trivial except that, just because the air converges and diverges over very much shorter horizontal scales than for the longer waves, the vertical velocities in them are larger than those in the long waves. Thus almost all are accompanied by very heavy rainfall (or, as with polar lows, snowfall). Again even fairly casual observation verifies that heavy, unexpected rain is often associated with a secondary trough that is barely detectable on a conventional analysis. This poses problems for numerical weather prediction because the resolution is usually chosen so as to resolve the longer waves and the shorter, more weather-intensive waves are poorly represented. A remedy might be to use a three-layer type of model in which the lower two layers were about one kilometre apart, so as to resolve the short shallow waves, with the third layer near the tropopause to represent the long waves. So far as I know this has never been tried as a scheme of numerical weather prediction.

REFERENCES

REITER, E. R. Topics in dynamical meteorology: 5. The spectrum of atmospheric motions. Weather, 33, pp. 328-334

WHITE, A. A. 1978 Topics in dynamical meteorology: 7. Atmospheric energetics (1) and (2). Weather, 33, pp. 408-416 and 446457

1978

EARLY WARM SPELLS-A CASE STUDY: 2 MARCH 1977

By I. T. LYALL Newark, Nottinghamshire

N an earlier article (Lyall 1971), occurrences of exceptional warmth in early I spring werc noted. Occasional maxima as high as 18OC have occurred before the end of February, with 25OC having been reached by late March. Such early warmth, it was noted, was usually associated with already very mild Tropical Maritime air being intensively warmed. This happened when abundant sunshine occurred and heating was confined to a shallow layer beneath a low-lying subsidence inversion. More occasionally, very high tem- peratures could be accounted for by an ‘anticyclonic’ fijhn which resulted in air from perhaps the one kilometre level, with a high potential temperature, being brought adiabatically to the surface in the lee of high ground.

A recent occurrence of early warmth was on 2 March 1977. Several areas in southern England reached 18-19OC on that day and at Exeter 2OoC was reported: this is the earliest date for such a temperature in Britain. This article sets out to examine various features of that day.

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DISTRIBUTION OF MAXIMA

High temperatures were reached in two main areas (Fig. 1). In south- west England, apart from Exeter, 19OC was also reported from many other stations including Ilfracombe, Torquay and Hemyock in Devon and Penzance and St Mawgan in Cornwall. Many of these are on the coast. The other main warm area was in the south-east where 18OC was much more widely reached in an area extending from Surrey and north Kent to Northamptonshire. At Rickmansworth (Hertfordshire), 196OC was recorded. Channel resorts were cold (e.g. 8OC maximum at Brighton) but the north Kent and Essex coasts were warm.

Fig. Ua). Location map of places mentioned in the text, B Brighton; CA Camborne; CR Crawley; CU Culdrose; E Exeter; HE Hemyock; HU Hum; I Ilfracombe; MB Mount Batten; P Penzance; R Rickmansworth; SM St Mawgan; T Torquay. (b) Distribution of maximum temperature, 2 March 1977

SYNOPTIC ANALYSIS

The synoptic situation on 2 March 1977 is illustrated by the 18 GMT surface pressure chart shown in Fig. 2. For several days, an anticyclone had been centred near the British Isles, moving south over the North Sea on 28 February and becoming established over France on 1 March. As the anti- cyclone became centred over France, a weak warm front moved north-east over England and Wales and was followed by a very mild, humid airflow. During the following night, minima in southern England were 8-9OC and at 06 GMT on the 2nd, dew-points were 7-9OC. This was accompanied by exten- sive low cloud, fog on southern coasts and drizzle in the Midlands. By noon all low cloud had cleared. Variable upper cloud was more persistent, and from the north Midlands northward more extensive. 2 March was not a particu- larly sunny day: of the stations included in the Dairy Weather Report (ex- cluding Guernsey), only Hurn and Mount Batten reported more than five hours sunshine. Despite this, very high maxima were recorded, with some remarkable combinations, such as 19OC with only 3.6 hours sunshine at Penzance.

Figs. 3 and 4 illustrate relevant radiosonde data. At 11 GMT on 2 March at Crawley (Fig. 3) there was a well-marked anticyclonic inversion, with its base as low as 980 mb. Whilst air below the inversion was quite moist (dew- point depression at loo0 mb was only 2.6 deg C), above it was very dry. (The lower layers at Crawley had shown stability through the anticyclonic spell

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since 28 February). At Camborne (Fig. 4), an inversion at 23 GMT on 1 March had broken down 12 hours later, but conditions were isothermal in the layer up to 910 mb. The lowest layers there were much drier than at Crawley (dew-point depression at 1000 mb was 7 deg C).

Fig. 2. Surface synoptic chart, 18 CMT, 2 March 1977

Fig. 3. Tephigrarn for lower troposphere, Crawley, 1 1 GMT, 2 March 1977. Solid line - dry-bulb temperature, dashed line - dew-point temperature

I 1 I I 1 5-c 10’ 15 .

Fig. 4. Temperature sounding, Camborne 0

x Solid line - dry-bulb temperature, dashed line - dew-point temperature

23 GMT 1 March 1977 1 1 GMT 2 March 1977

From the radiosonde data, an analysis has been made of the isentropic flow (at 1 1 GMT on 2 March) at a potential temperature of 26OC. Fig. 5 shows marked descent over Wales and most of the southern half of England but gentle ascent over south-west England. This is consistent with persistence of stability and a low-level inversion at Crawley (Fig. 3) and their breakdown at Camborne (Fig. 4).

AIRMASS ANALYSIS

It has been shown (Lyall 1971) that extreme early warmth is associated with Tropical Maritime air and in fact Pedgley (1962) states that Tropical Continental air ‘does not affect Britain in winter . . . (or) . . . it merely resembles a mild form of Polar Continental air on reaching Britain’. It has already been noted that there were differences between the two main areas of warmth on 2 March 1977. The surface layers were much drier in the south- west compared with the south-east, and this seems incompatible with maritime air moving from the west.

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To examine more closely the nature of the airmasses involved, a trajectory analysis was carried out. On a broad scale, air had been moving northward from North Africa since about 22 February and the red rain in western Scotland on 5 March was due to the transport of Saharan

Fig. 5 . Isentropic flow-analysis for potential temperature 26°C. Continuous lines are isobars on the e=26"C surface. Air flowing adiabatically maintains its value of 6 and if moving to a higher pressure is descending (and vice versa). Broken lines are isolines of vertical motion in m hr-' (see George 1972)

Fig. 6. Trajectory analysis of air reaching southern England on 2 March 1977 (at 12 GMT). Continuous lines denote trajectories drawn for 6-hour intervals; broken lines those drawn for 24-hour intervals

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dust. Fig. 6 shows trajectories of air which was over southern England at noon on 2 March. These trajectories were constructed for six-hourly intervals back to 06 GMT on 1 March, then back to 12 GMT on 28 February and then for 24-hour intervals. They should therefore be treated with some caution. How- ever, it does seem fairly certain that trajectories from both south-west and south-east England can be traced back to southern Spain. I t is probable that the air over south-east England on 2 March at noon left southern Spain on 27 February and that over the south-west left on 28 February. Air reaching southern Spain on 27 February was undoubtedly of Atlantic origin and retained its moisture subsequently. By contrast, that reaching southern Spain on 28 February had been over North Africa since at least 22 February. On the basis of its relative dryness and its probable trajectory it seems reasonable to classify the air over south-west England on 2 March as Tropical Continental.

CAUSES OF THE HIGH TEMPERATURES

In southern Britain, temperatures at 06 GMT were already at least 8OC and locally as high as ll-12OC. In the south-west, the highest maxima were localised and occurred in the absence of a low-level inversion. However, con- ditions in the lower layers were nearly isothermal (Fig. 4) which would favour the occurrence of an 'anticyclonic' fohn in the lee of high ground. The tem- peratures in the isothermal layer were near 15OC and thus, air crossing Dartmoor and subsequently descending 500 m on the lee-side would be adiabatically warmed to 2OOC- just the value reached at Exeter (with 19OC on much of the coastal strip including Torquay).

In the south-east the air was Tropical Maritime with a low-level inversion layer at a height of no more than 250 m. With strong heating concentrated into the layer below this height, the early low cloud soon dispersed and despite rather low sunshine totals, temperatures reached 18-1 9OC, a daytime rise of 10 deg C. The radiation balance of the lowest levels is now examined.

SURFACE ENERGY BUDGET

Consider the layer below the inversion '- at a nominal height of 250 m. There is no certainty about the overall warming throughout the morning and early afternoon through the layer but a figure of 7-5 deg C is a reasonable estimate. Over one square metre (m") the volume of air below the inversion is 2 5 X lo5 litres - at 10°C at 1000 mb this weighs 310 kg. The heat capacity of air is very near to 1 kJ kg-' degC'. To produce the estimated warming of the air column over one square metre under the inversion would require 2300 kJ m-z of heat transference from the surface. (Heat required to warm air =mass (310 kg)Xtemperature rise (7-5 deg C)Xheat capacity (1 kJ kg" deg C-I)=2300 kJ).

It is also assumed that the time of temperature maximum was around 1530 GMT. Hourly radiation data at Bracknell indicate a total global radiation on a horizontal surface up to 1530 GMT of 8350 kJ m-*. From this must be

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subtracted various quantities (many of them best estimates) to evaluate the heat available to warm the atmosphere.

(a) A certain amount would be conducted into the soil - probably about

(b) Heat would be lost by infra-red radiation. Based on the rate of 500 kJ m-2.

temperature fall from 16-24 GMT at Gatwick, this is estimated as 2500 kJ m-2. Evapotranspiration. Data from Corsham (Wiltshire) indicate an evaporation of 1.6 kg water m-2, in the 24 hours up to 09 GMT on 3 March. Assuming wind, cloud and humidity conditions to be similar to those further east, a value of evaporation from temperature minimum (assumed 06 GMT) up to 1530 GMT is taken as 1 kg m-', requiring latent heat of 2500 kJm-'.

The albedo must also be taken into account: an average figure of 15 per

These values give the energy available for atmospheric warming as: 085 X 8350- (5OO+2500+2500)e 1600

This is approximately 70 per cent of that required. In view of the findings that there is typically a downward flux of heat of this proportion (i.e. 30 per cent) through an anticyclonic inversion (Cattle and Weston 1975), a hypothesis of radiative warming of a low-level sub-inversion layer, combined with downward flux through the inversion, to account for this occasion of early spring warmth, seems to be correct.

cent is assumed reasonable.

CONCLUSION

High temperatures occurred over southern England on 2 March 1977, despite limited sunshine. The temperature in the south-east can be related to the restriction of heating to a shallow sub-inversion layer of Tropical Mari- time air. In the south-west high maxima occurred more locally, possibly as a result of fohn warming. The airmass involved seems to have been of North African origin.

ACKNOWLEDGMENTS

Data used in this article were drawn from the Daily .Weather Report, the Daily Telegraph and the monthly bulletin of the Climatological Observers Link. Special data were supplied on request by the Meteorological Office (hourly radiation values for Bracknell, by Met. 0. 1) and by Mr K. 0. Mortimore (evaporation data for Corsham).

REFERENCES Budget studies of heat flux profiles in the convective

WESTON, K. J. boundary layer over land. Quart. 3. R . Met.

The snowfalls o f 4 March 1970. Weather, 27, pp. 96-

Early warm spells since 1957. Weafher, 26, pp. 46-54. A Short Course in Elementary Meteorology. Met. 0.

CATTLE, H. and

GEORGE, D. J.

LYALL, I. T. PEWLEY, D. E.

1975

1972

1971 1962

SOC., 101, p . 353-363

110

707, H.M.S.O. London

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