A case study is presented of a winter depression over the British Isles in which extensive banded structure was observed within precipitation ahead of the surface warm front. Measurements of the mesoscale airflow and precipitation structure of the rainbands were made using a variety of radar techniques together with multiple radiosonde and aircraft observations. The measurements were made over the sea to avoid the confusing effects of topography. The dominant rainbands were oriented parallel to the surface cold front and were typically 100 km wide. They moved with a velocity faster than the underlying warm front. For the most part the bands were characterized by clusters of weak small‐scale convective cells due to the release of potential instability produced where tongues of relatively dry air of low θw in the middle troposphere overran low‐level moist air undergoing slantwise ascent above the warm frontal zone. Although there was the usual large‐scale, and thermally‐direct, circulation associated with the active warm front, the air which ascended as small‐scale convection within the rainbands entered a region of weak cold frontal baroclinicity, whereupon it participated in a thermally direct circulation of its own. This led to each rainband having a rearward‐sloping anvil cloud canopy characterized by ascending air with colder drier air descending beneath. Precipitation falling from the canopy evaporated within the underlying drier air thereby probably intensifying the descending branch of the circulation. Very large ageostrophic winds were measured in association with these circulations. The important ingredient responsible for the convective nature of the rainbands appears to have been the incursion of tongues of relatively dry air of low θw in the middle troposphere above the moist warm‐sector air in a region where the resulting instability could be realized by large‐scale ascent. Although the potential instability was very weak in the present case, the origin of the rainbands appears to have been similar to that of pre‐frontal squall lines. The intensity of the convection within rainbands depends on the stability but the very existence of any precipitation in the first place depends on other dynamical factors leading to widespread ascent.

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Ludlam 1963) produced a trail of damage across Wiltshire and Gloucestershire caused by violent winds and hailstones up to 3 inches in diameter. This paper gives an account of the storm as revealed by eye witnesses, press reports and radar records and, from an analysis of the prevailing meteorological conditions, discusses the factors likely to have caused its generation. THE STORMJuly 1967 was notable in many parts of England for periods of hot sunny weather with temperatures rising to near 30' C in some areas, but punctuated by violent rainstorms. As will be seen later, one such period of fine weather ended on 13 July with thunderstorms and rainfalls of over 3 inches in 12 hours in some places. In western areas of Wiltshire and neighbouring areas of Somerset and Gloucestershire the heatwave was broken by a violent hailstorm, with hailstones over z inches in diameter and, in some areas, damaging winds. The main areas affected by the storm are shown in Fig I, showing 24-hour rainfall isopleths and locations of severe weather.Aircraft reports a t 1700 GMT showed rapidly developing cumulonimbus over the Somerset-Dorset border with tops at that time approaching 27 ooo ft. By the time the storm reached the Mendips it was producing hail up to half-an-inch in diameterThe main movement of the storm was towards the north-east. Fig. I . Associated weather phenomena

SUMMARYA technique is described for determining the three-dimensional field of vertical air motion in the vicinity of mobile fronts. Radar reflectors are dropped from an aircraft at a height of 5 km and followed by high precision tracking radars. By making assumptions regarding the movement and conservativeness of the frontal system it has been possible to use this technique to obtain horizontal winds over a volume approximately 150 x 100 x 4 km depth with a typical drop separation of 30 km. Using the equation of continuity, vertical velocities have been calculated with an uncertainty of f 1 -2 cm s-l. In a warm front investigated on 19 January 1969, close agreement was obtained between the rainfall observed at the surface and that computed from the vertical velocities assuming saturation of the rising air.

A diagnostic dynamical study is presented of the three-dimensional field of motion based on data from dropsondes released through the warm-frontal rainband situation of 18 January 1971, and is complementary to an earlier radar-synoptic study of this situation.Principal features of interest are: (i) The identification of two low level jets. The first (axis below lkm) lies just ahead of and parallel to the surface warm front. The second (axis near 2km) overrides the first at an angle of about 50" and may be a continuation of the cold front jet overriding the warm front just ahead of the point of occlusion. The flow in the lower jet appears to be supergeostrophic; in the upper jet slightly subgeostrophic.(ii) A distinct mesoscale perturbation of the synoptic scale wind field associated with the rainbands above the warm front. Large negative values of the vertical component of absolute vorticity were observed in the main rainband.(iii) The mesoscale fields of motion produce a marked ' peak ' in the spectrum of horizontal and vertical kinetic energy.These and other features are discussed in relation to the results of a model of frontogenesis by B. J. Hoskins. Possible causes of the circulation associated with the rainbands and the interaction with other scales of motion are considered. U1 approximate boundaries of the lower and upper sections of the main warm frontal zone