Eight detailed case studies are summarized to clarify the structure and mechanism of orographically enhanced frontal rain over hills of modest height. The observations were obtained as part of a field project in south Wales in which data from a 3-dimensionally scanning radar were combined with autographic raingauge data. The results show that the generation of orographic rain is consistent with Bergeron's seederfeeder mechanism, according to which raindrops from upper-level (seeder) clouds wash out small droplets within low-level (feeder) clouds formed over the hills. It is demonstrated that the orographic enhancement is strongly influenced by the low-level wind speed. The largest enhancement of rainfall occurred in association with strong winds, and also high relative humidity, below 2 km. The radar showed that over 80% of the enhancement occurred in the lowest 1.5 km above the hills. It also showed that the periods of enhanced rainfall were associated with the passage of pre-existing areas Df precipitation. The precise value of the upwind rainfall rate was rather unimportant in influencing the orographic increment provided the rainfall rate upwind exceeded about 0.5 mm h-', These findings are compared with the results of theoretical calculations based upon the washout model of Bader and Roach. SP9 NIVX 3IHdVtl9OXO 3 0 SNOILVAtl3S80* The radar was a mobile Plessey 43s with the following characteristics: frequency 2860 GHz, antenna diameter 3.7 m, polarization vertical, pulse length 1.5 ps, pulse repetition frequency 275 s-', peak power 650 kW. minimum detectable signal 110 dB m.

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Some of the largest falls of orographic rain in the western parts of the British Isles are associated with wintertime cold fronts. This paper contains foul case studies of wet cold fronts which were all characterized by south-westerly prefrontal low-level jets but were widely different in other respects. The distribution of rainfall in England and Wales is analysed using a network of autographic raingauges, specially augmented near the coast and over the hills of south Wales, and the orographic effects areexplained using data fromseries of hourly rawinsonde ascents fromasingle station. The rain is considered in three distinct regions: pre-frontal, the surface cold front, and post-frontal. Behind the front orographic effects were found to be well defined but rather slight. At the surface front orographic effects were negligible, heivy rain tending to occur regardless of topography. Ahead of the front, orographic effects varied from small to very large, depending on the existence of a moist low-level feeder cloud and seeding particles. The occurrence of heavy orographic rain ahead of the cold front is favoured by the presence of the low-level jet, since this ensures the replenishment of a high liquid water content in the feeder cloud despite its rapid depletion by washout. Low-level ascent, or at least the absence of descent, in the general airflow upwind of the hills is also needed if the feeder cloud is to have a high liquid water content. The required seeding particles in some cases originate from melted ice crystals grown aloft; alternatively they may be generated within the low-level cloud itself even in the absence of the ice phase.

SUMMARYFrequent digital cloud imagery from Meteosat and precipitation patterns from radar have been analysed together with surface observations and conventional and satellite soundings, to develop a conceptual model of a small mesoscale convective system. The system, observed to the south-west of England, had many of the features previously found in the United States, where the term mesoscale convective system or complex is applied to a cluster of thunderstorms that generates a common cirrus shield of mesoscale dimensions. A mesoscale region of fairly uniform, essentially stratiform, rain developed beneath the cirrus shield downwind of the active convection. Thunder was widespread throughout the precipitation area which was also characterized by a surface mesohigh. The mesoscale convective system, and another similar one before it, lasted for about half a day each, during which time they produced large falls of rain over an extensive area. The mesoscale convective systems developed in a region of locally strong baroclinicity on the eastern flank of a cut-off low. They happened to form over the sea and were decoupled from the surface by a cool easterly boundary layer flow. Their development could be traced to the onset of advection from France of warm moist air with a wet-bulb potential temperature in excess of 18°C at 850mb. The arrival of this air coincided with the intensification of the SE-ly flow around the approaching cut-off low.