Contrail forecasting techniques used within the UK Meteorological Office have changed little since the early 1940s and there has been no formal verification of the results. Recent research has shown that old forecasting techniques may not be wholly applicable to modern aircraft that now use more efficient engines. In order to compare the performance of both the old and new forecasting techniques a validation trial was carried out over a nine-month period in which RAF pilots reported when and where contrails did and did not occur. These observations were compared with the predicted contrail levels from both the old and newer forecasting techniques. The results indicate that both techniques give a similar performance and that neither significantly out-perform a simple persistence prediction.

A continuously calibrated infrared (IR) geostationary satellite rainfall estimation technique (CCB4) is introduced, in the context of the Nile River Forecast System, an operational system for hydrological modelling and forecasting. The CCB4 incorporates near-real-time rain gauge data to continuously calibrate optimum IR rain/no-rain thresholds and daily rain rates on a daily time step. The ability of the CCB4 and two comparative techniques to estimate daily rainfall at the regional and pixel scales is assessed, using Meteosat IR imagery and gauge data from six wet season months covering three years. The CCB4 shows improved skill in identifying rain days and estimating daily rain amounts at a range of spatial scales, from regional to pixel scales. At the pixel scale, however, improved root mean square errors remain relatively high, ranging between 66% and 84% of the mean unconditional rain rate.

Unlike other currents along the subtropical west coasts of continents, the Leeuwin Current off Western Australia transports warm, fresh tropical waters poleward and is strong enough to overwhelm the upwelling tendencies of the coastal wind stress. The representation of this unusual and important current in the global eddy-permitting ¼° Parallel Ocean Climate Model (POCM) is examined, and implications for coastal fisheries and regional weather and climate discussed. Comparison of POCM fields with those from the Leeuwin Current Interdisciplinary Experiment for austral autumn and winter 1987 suggests that model Leeuwin Current is consistent with these observations in terms of its location and depth distribution, surface gradients and meridional variation in mixed layer depths but is a bit weaker, warmer and fresher. The model Leeuwin Undercurrent is similar in strength to that observed during autumn/winter 1987. Similar to satellite observations, the model Leeuwin Current displays a number of meander, eddy and offshoot features. The frequency distribution and length scales of the model features are generally consistent with estimates of these parameters derived from the satellite images. It is concluded that POCM output can be usefully applied to improve understanding of the Leeuwin Current and associated regional impacts.

The motion of thunderstorms in southern Germany was investigated. The thunderstorms were observed by a lightning position system during the summer months of the years 1992–1996. On average every second day thunderstorms were observed somewhere in southern Germany. In general thunderstorms approached from westerly and south-westerly directions. The average speed was 13 m s−1. No significant relation between the occurrence of thunderstorms and the large-scale synoptic pattern described by the Grosswetterlagen (large-scale weather pattern) was found. Thunderstorms were observed during almost all Grosswetterlagen. The reduction to eight weather patterns based on the low-level flow in southern Germany showed that thunderstorms are likely when the flow has a westerly (43%) or easterly direction (20%). Three distinct groups of different lighting patterns could be identified: stationary, moving thunderstorms and thunderstorm lines. The convective available potential energy (CAPE) and the wind shear were retrieved from radio soundings from München and Stuttgart. On average CAPE was 583 J kg−1 for stationary thunderstorms, 701 J kg−1 for moving thunderstorms and 876 J kg−1 for thunderstorm lines. The corresponding average bulk Richardson numbers were 37, 22 and 21. The steering level was found to be at about 6 km m.s.l. However, it should be noted that in most cases the soundings do not completely describe the local environment of thunderstorms, since radio soundings are only available twice a day.

The Cooperative Research Centre for Southern Hemisphere Meteorology in collaboration with the Bureau of Meteorology has developed an ultraviolet index (UVI) forecast system to support the Australian UVI forecast issued to the public by the Bureau of Meteorology. In an effort to alert the public to the possibility of harmful levels of ultraviolet radiation the UVI forecast has been issued daily as part of the meteorological weather report since spring 1996. The predicted UVI indicates the amount of ultraviolet radiation that will reach the Earth's surface at local noon next day. The forecast includes the effects of cloud cover. The global clear sky UVI is calculated using a two-stream radiative transfer model and a deterministic ozone forecast scheme. The system utilises meteorological fields from the global numerical weather forecast model of the Bureau of Meteorology. This paper describes the UVI forecast system, and its results are compared with observational data from the Australian Radiation Laboratory for several Australian cities for the period January–December 1997. Possible improvements are outlined since the UVI forecast system is the subject of ongoing development.

The synoptic situations associated with significant temperature decreases in Buenos Aires are analysed. Knowledge of these situations for the winter season is very useful for the community, especially when they are related to the consumption of natural gas, which is one of the most important sources of energy in Argentina. The rotated principal component method is used to classify the synoptic situations. The significant temperature decreases in winter during the period 1980–1988 occur in five modes at 1000 hPa and three modes at 500 hPa. The relationship between the two levels identifies two dominant patterns. The first has a strong high-pressure system with a northwest-southeast axis and a maximum strength near 80° W, 40° S; this pattern is related to south-southeast winds over Buenos Aires. The second type shows a high-pressure system at 105° W, 48° S and a minimum near 110° W, 30°–35° S; this kind of pattern is associated with persistent blocking situations in the South Pacific which result in persistent winds from the south over Buenos Aires. In addition, a relationship between such situations and the resulting increases in the demand for natural gas in residential areas is shown. This will help in the forecasting of such events in the future.

An automated contrail detection algorithm has been applied to AVHRR data to obtain the position of contrails. Nearly simultaneous data from the Europe Model of the Deutscher Wetterdienst have been used to find typical weather conditions associated with 742 persistent contrails. Further, a flow pattern analysis identifies typical regions where the occurrence of contrails was above average. These regions are in the upper atmosphere: (a) ahead of a surface warm front either in moist warm layers before the cirrus clouds arrive or more likely with the cirrus in a warm conveyor belt and (b) ahead of a surface cold front in rapidly moving cold air in the turbulent regions near a band of strong wind (though the speed is not necessarily as high as in a jet). Usually, the atmosphere is baroclinic in the contrail region. Most of the detected contrails occur in divergent flow patterns in the upper troposphere in slowly rising warm or locally turbulent cold air masses.

This paper describes the meteorological variables directly affecting precipitation in the southern region of South America. Results show the relationships between daily precipitation at Ezeiza, Buenos Aires, and 500 mb dynamic variables (e. g. geopotential heights, relative geostrophic vorticity and its advection, vorticity tendency and zonal and meridional geostrophic wind components) using regional synoptic analyses from the Servicio Meteorológico Nacional of Argentina for 1986. The distribution of significant biserial correlation coefficients reveals the preferred positions for the anomalous wind components which favour the occurrence of rainfall at Ezeiza. In the same way, spatial distributions and temporal lag relations of 500 mb relative vorticity anomalies in relation to precipitation show the positions and relative movement of centres of anomalously cyclonic and anticyclonic vorticity which increase the probability of precipitation at Buenos Aires. Vorticity advection does not yield higher correlations, at least in a daily temporal scale. The geographical distribution of temporal means and standard deviations of geopotential heights and zonal and meridional wind components at the 500 mb level are also shown.