SUMMARYThe process of convection is described in terms of a unit or proton of convection -the bubble. A rising bubble of warm air sheds its outer skin steadily into a disturbed wake until it becomes exhausted completely or spreads out at a stable layer. The wake is a region where the ascent of further bubbles is favoured. The wakes of small bubbles close to the ground are aggregated into larger bubbles, which are more dilute, up to a level where they begin to penetrate hitherto undisturbed air, and then they waste away as they ascend further.The air above a bubble is lifted (and cooled) as the bubble approaches and then drains down the outside, the air close to the bubble being mixed into the wake. The wake of a clear bubble is buoyant but that of a cloudy bubble may sink if it is sufficiently chilled by dilution with surrounding clear air.The drag on a bubble is estimated from observations on rising cumulus towers, and a linear relation between buoyancy and limiting velocity is proposed. From this the horizontal velocity of the bubble relative to the surrounding air is deduced to be about the same as the vertical velocity when it ascends through strong shear.A bubble is a compact stable configuration for a rising element. Near the ground the heat is transported by small bubbles, which are less efficient, and therefore require a greater lapse rate, than the larger ones which can operate higher up. The process of aggregation is again renewed within large clouds so that the bubbles found at the top may be more dilute than if they had ascended directly from the base. Clouds growing in a shearing current will grow into the shear.
SUMMARYExisting theory of the stability of a stably stratified fluid containing a strong vertical shear suggests that unstable waves may develop when the curvature of the velocity profile changes sign and the Richardson number is somewhere less than &. Some observations are described which show the properties of atmospheric billow clouds formed in travelling amplifying waves (transverse to the shear vector), on occasions when these conditions appear to be met. Static instability seems to arise in parts of the wave-pattern where layers are inverted, and to cause a convective overturning, which may halt the wave development. The most pronounced waves occur in the upper troposphere in association with jet streams, in layers of strong wind shear which are usually dry. They probably only rarely produce clouds, and may more frequently be responsible for the clear-air turbulence encountered by aircraft. The associated relative air velocities occur over a range of scales : up to about 1 km in the convective regions, and up to the several km associated with the billow wave-lengths, with magnitudes of up to 10 m sec-' or more. THE WAVE-MOTIONS WHICH PRODUCE BILLOW CLOUDSFollowing the work of Helmholtz and others it is well-known that waves may develop on a horizontal surface separating fluid layers of different density and velocity. These waves tend to concentrate the vorticity into lines which lie normal to the shear vector, and therefore usually across the flow.Wegener, and later Haurwitz (see, e.g. Haurwitz 1941) presumed that similar waves are responsible for the formation of regularly spaced rolls of clouds called billow clouds (which commonly occur in the middle or low troposphere, and are recognized in the International Cloud Classification as the variety undulatus), and compared some observed and calculated wave-lengths. The latter were obtained for a system of two infinitely deep layers, in each of which the density and velocity is constant, separated by an interface which is a surface of discontinuity. It was presumed that this system reasonably represented the atmospheric situation in which the discontinuity is replaced by a transition layer shallow compared with the wave-lengths to be inferred. It was assumed that the waves moved with a velocity intermediate between the velocities on either side of the discontinuity, and it then appeared that there was an upper limit to the wave-length of unstable waves; when this limiting wave-length was calculated for the changes of wind and temperature observed at inversions associated with observed clouds, it was found to be close to the actual wave-length (Haurwitz 1941, p. 287; the correspondence with that of the wave of maximum amplification-rate would have been significantly less satisfactory). The theory cannot in any simple manner be extended to cover more realistic systems in which the fluid density and velocity have continuous distributions. However, Lord Rayleigh (see, e.g., Schlichting 1955, p. 322) showed that in a fluid of uniform density instability can arise o...
The technique of relative-flow analysis on isentropic surfaces is used to examine the large-("synoptic"-) scale situations associated with selected severe local storms near southern England and over the mid-western U.S.A. (including the Horsham, Wokingham, and Geary storms whose behaviour has been described in several previous publications). The storms occur ahead of major troughs, in the vicinity of confluence-lines (usually recognised a8 cold fronts over western Europe but as "dry-lines'' over the U.S.A.), where an increase of wind with height favours the organisation and intensification of cumulonimbus convection. Extreme instability arises where small-scale convection is confined to a lowermost 1 or 2 km (leading to an abnormally high wetbulb potential temperature) beneath a plume of very warm air lying downwind of an extensive arid plateau (Spain or Mexico). The instability is released where the (backed) low-level flow eventually reaches the edge of the restraining plume aloft. It appears that the occurrence of severe local storms demands a peculiarly favourable combination of geographical features and atmospheric flow-pattern.
The technique of relative-flow analysis on isentropic surfaces is used to examine the large-("synoptic"-) scale situations associated with selected severe local storms near southern England and over the mid-western U.S.A. (including the Horsham, Wokingham, and Geary storms whose behaviour has been described in several previous publications). The storms occur ahead of major troughs, in the vicinity of confluence-lines (usually recognised a8 cold fronts over western Europe but as "dry-lines'' over the U.S.A.), where an increase of wind with height favours the organisation and intensification of cumulonimbus convection. Extreme instability arises where small-scale convection is confined to a lowermost 1 or 2 km (leading to an abnormally high wetbulb potential temperature) beneath a plume of very warm air lying downwind of an extensive arid plateau (Spain or Mexico). The instability is released where the (backed) low-level flow eventually reaches the edge of the restraining plume aloft. It appears that the occurrence of severe local storms demands a peculiarly favourable combination of geographical features and atmospheric flow-pattern.
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