RECENT RESEARCH ON FRONTS By I. J. W. POTHECARY Meteorological Ofice, London

NE of the great advances in the development of the science of meteorology 0 was brought about by the work of Bjerknes and Solberg (1922) who introduced the polar-front theory of the formation of depressions. ’ The develop- ment of techniques for making meteorological observations in the upper air during the following years lent the weight of observational evidence to the theoretical picture, and frontal analysis became an important part of forecasting technique. In more recent years the development of the radiosonde and the increasing use of aircraft has extended our knowledge of the finer structure of the atmosphere, and a more complicated picture of frontal structure is beginning to emerge.

The standard ‘ text-book ’ cross-section through a warm front, based on Bjerknes’ original diagram, is illustrated in Fig. I. This description has been of exceptional value to the synoptic meteorologist but the increasing demand for finer detail has drawn attention to a greater complexity which is reflected in the difficulty of fore- casting frontal behaviour. The main feature of Bjerknes’ frontal picture was the single frontal surface separating warm and cold air and defining, at the warm front, the underside of the frontal cloud system. The first fact to emerge from more detailed observation was that the front is not a surface but a zone of rapid temperature transi- tion between air masses.

From 1950 to 1952 the Meteorological Research Flight, flying from Farn- borough, camed out a research programme which involved flights through 23 fronts of various types. The general flight path in relation to the fronts was similar to the particular flight illustrated in Figs. 2 and 3, although the pattern was varied according to the position of the front relative to Farnborough. Observations of temperature, frost-point and cloud structure were recorded every 500 feet on the climbs and descents and every half minute, or about two miles apart, in level flight.

The analysis of this first series of flights (Sawyer 1955) gave some valuable general information about fronts and showed, too, that individual fronts were often widely different in their structure. The total horizontal temperature

Fig. I . A ‘ classical ’ warm front

contrast between air masses averaged about 15'F and took place over about 600 miles ; on average a change of 9°F was concentrated in the frontal zone which had an average horizontal width of about 130 miles. Frontal zones less than 50 miles wide were unusual. The frontal zone is defined in the horizontal as that part of the region of transition between air masses over which the temperature changes more rapidly with distance than on either side, and in the vertical as the layer through which the decrease of temperature with height is less than either below or above.

Temperature cross-sections based on radiosonde observations and cross- sections drawn from the aircraft data gave the same distribution of temperature and similar positions for the boundaries of the frontal zones ; there was no evidence for temperature variations with a scale between the small random fluctuations found in any air mass and the broad features of a frontal zone. A second series of flights is undcr analysis but one example will suffice to show the complicated humidity-patterns that exist in frontal structure. A cross- section drawn from the aircraft observations of temperature made during a flight on 29 Kovember 1954, is shown in Fig. 2 and the corresponding frost-point depression cross-section is shown in Fig. 3.

,400

++ / -10

400

Fig. 2 . (Left) The cross-section of a warm front drawn from Meteorological Research Flight observations of temperature on 29 November 1954

+ Aircraft track - Isotherms in "F

Frontal zone boundaries The point from which the air was tracked back for 36 hours

___-- x

I;ig. 3. (Rzghl) The cross-section of a warm front drawn from Meteorological Research 1;Yight observations of frost point depression on 29 November 1954

t Aircraft track - Frost point depression in "I:

Vrontal zone boundaries x The point from which the air was tracked back for 36 hours.

-----

l'hc hatched area shows air with a humidity of less than 10 per cent

148

On almost every cross-section showing frost-point depression through a front there is some indication of a tongue of dry air extending downwards in the vicinity of the frontal zone and usually tilted in the direction of the slope of the front. The cross-sections of frost-point depression drawn from the aircraft data show that the humidity, especially near the frontal zone, has a structure which, although similar in its broad features, is much more complicated than the radiosonde information suggests. In the cross-section illustrated in Fig. 3, and on several other occasions, the frost-point depressions reach at least 60°F and are twice as large as those in the corresponding cross-section based on the radiosonde data. Relative humidities of less than 5 per cent occur in the middle of the frontal zone and down to below 10,000 feet! Despite this complication the temperature structure is clear cut and straightforward.

The presence of this very dry air in or near the frontal zone does, of course, have a pronounced influence on the frontal cloud structure. A study of the cloud observations on the flights shows that usually there is a greater slope to the forward edge of the frontal cloud-system than to the frontal zone, so that there may therefore be clear air between the rear of the frontal zone and the forward edge of the frontal cloud above about 10,000 ft.

The dryness of the air must result from considerable subsidence at some earlier stage in the formation of the frontal system. The dry air at a pressure level of 500 mb in the frontal zone of the warm front illustrated in Figs. 2 and 3 was tracked back for 36 hours, to an area where the tropopause was at about 400 mb and where the air was then at 435 mb (Fig. 4). It was from this area that the air was accelerated into the left entrance of a jet stream and subsided

-.-. -.-. Air Tmjcctory

- 5 O O m b m not 03h. 28/11E.

200 miles O-

r'

Fig. 4. The trajectory of the dry air observed at 500 mb in the frontal zone A o3h 28 November 1954 pressure 435 mb, temperature -36'F B 15h 28 November 1954 pressure 485 mb, temperature -23OF C 15h 29 November 1954 pressure 500 mb, temperature -18OF

50mb during 12 hours, but as it passed through the body of the jet stream during the following 24 hours a subsidence of only 15 mb occurred. The subsidence in the left entrance is the result of a flow of air to the left across the axis of the jet stream as the air is accelerated. There is a corresponding ascent in the right entrance. It is this circulation that is responsible for bringing air that is already dry down from the upper troposphere and lower stratosphere into the frontal zone a t an early stage in its formation. Later on in the history of the front the presence of this dry air may modify the activity and distribution of frontal precipitation.

These features which have been described show something of the new ideas on the structure of fronts. The efforts of the Meteorological Research Flight are resulting in the accumulation of a great deal of valuable data, the analysis of which will serve to substantiate or modify the development of theoretical models. I t is not beyond possibility that it may eventually be possible to predict frontal activity by electronic computation.

I am indebted to the Director of the Meteorological Office for permission to publish this article.

BJERKNES, J . , and SOLBERG, H.

SAWYER, J, S.

REFERENCES 1922 Life cycle of cvclones and the polar front theory

of atmospheric circulation. Geof. Pub., Oslo, Vol. 52, No. I The free atmosphere in the vicinity of fronts. Geophys. M e m . , London, Vol. 12, No. 96

1955

Fig. I . The field of motion in and ,wound a bubble. The arrows show the velocity of the particles not taking the turbulent mixing into xcoun t . The particles inside the bubble are moving faster than those outside and are moving into, and mixing with, the fluid in the Imth of the bubble. The bubbleis turned inside out several times, rach time being represented by a step in the paths of two average particles shown. The particle on the left appears close to the front and passes round to the rim where it becomes temporarily stationary. The particle on the right remains inside the bubble circulating round the core of the vortex ring (C). The ring grows in proportion to the size of the bubble, and the motion would appear t o be circulating round the core to an observer moving with the bubble. To a stationary observer it appears t o he circulating round the ring X on the rim of the bubble (see article on P U P 151)