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.

SIJMMARYA case study investigating the relationship of ice phase microphysical structure to differential reflectivity is presented. The distribution of ice crystal types in light precipitation is discussed and shown to agree with a theoretical model of an individual precipitation fallstreak. The particle characteristics were found to reflect sorting by size and type associated with fall through weak wind shear as well as growth in regimes of varying temperature and humidity within the fallstreak.The observed differential reflectivities were compared with the in situ particle measurements and the association of high values with planar crystal structures demonstrated. The particle size distribution was found to have a marked influence on radar returns, particularly through the obscuring effects of large particles of indefinite shape. This effect gave rise to relative variations of differential reflectivity which were found to differ from the predictions of monodisperse models. In particular large dendritic crystals of low density were found in this case to be associated with stronger signatures than denser plate-like crystals which from theoretical studies might be expected to produce the greatest differential reflectivity. Because of these complicating factors under stratiform conditions it was concluded that differential reflectivity data must be interpreted with considerable caution in the absence of in situ measurements or other supporting data.

SUMMARYWe study 20 cases in which 60-hour Numerical Weather Prediction (NWP) of specific synoptic events have been markedly improved by observations made during the final 15 hours prior to the forecast initial time. For each case, the observing systems which played major r6les in improving the accuracy of the forecast mean-sea-level pressure (m.s.1.p.) field are identified using a series of Observation System Experiments. The relative utility of each observation type is then assessed according to the frequency, over all cases, with which it delivers a major beneficial impact.The method of multiple case-study employed has the advantage that observation value is assessed according to the benefit delivered to forecasts of events (e.g. cyclogenesis) which are of prime importance to operational meteorology. The method, therefore, provides a useful complement to the conventional statistical approach which typically yields an 'average' benefit, calculated for all regimes (both active and 'quiet') occurring over a given period within a specific geographical region. The cases are selected from the most significant data-impact events detected in routine operational forecasts of m.s.l.p., for Europe and North America, during the period September 1993 to December 1995.Results indicate that, for weather systems developing over North America, radiosonde and aircraft reports contribute major forecast benefits most often, confirming their key r6le in the North American network. These two observing systems contribute with similar frequency, suggesting that the effectiveness of the less abundant radiosonde reports is boosted significantly by their profile format. Surface data and cloud-track winds are the next most frequent contributors.For weather systems developing over the North Pacific and North Atlantic, aircraft winds contribute forecast benefits most frequently, and by a wide margin. Conventional surface data also play a key r6le.Comparison of the utility of wind and temperature data suggests that, on average, the benefit of wind profiles is somewhat greater than that of temperature profiles, and that, in levels above -400 hPa, wind data are considerably more valuable than temperature data.The results give insights which can be used to guide the rationalization of existing networks within the northern hemisphere mid latitudes, and the following recommendations are made with a view to improving NWP over the European area.0 Acquire more aircraft data over North Atlantic routes and over Europe (including reports during climb and descent). However, caution must be exercised if aircraft data are used to replace profile information from radiosondes.0 Deploy more surface observations (e.g. drifting buoys) and shipborne radiosonde ascents over the North Atlantic.0 Make more use of pattern-tracking techniques for deriving 'clear-air' wind data from satellite water-vapour imagery.Continue investigation of the effectiveness of observations 'targeted' on objectively defined regions where model errors are predicted to grow most rapidly.

Results of some calculations are presented which show that the washout of droplets in a low‐level, orographically produced cloud by raindrops falling from a higher‐level cloud (formed by large‐scale ascent) can augment the rainfall rate by several mm h−1 over a hill a few hundred metres high. The dependence of this enhancement upon different meteorological parameters is discussed and calculated values are compared with some observed rainfall rates over south Wales. It is considered that this mechanism is most effective in enhancing warm‐sector precipitation over hills.

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