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.

The aim is to evaluate the use of infrared satellite precipitation estimates for nowcasting purposes in the context of a real-time flood-warning scheme. A radar-based calibration technique is described which is applied to the Negri–Adler–Wetzel scheme. This procedure employs radar data over a defined calibration area to estimate, for each satellite image, actual rain-rates to be used in the Negri–Adler–Wetzel scheme. Calibrated satellite estimates obtained from this procedure can be used to diagnose areas of precipitation beyond radar range, thus allowing an extension of precipitation nowcasting lead time. Calibrated estimates are compared with radar rainfall measurements and results are discussed for various sizes of integration area. Calibration reduces consistently both bias and variance of the error of the original Negri–Adler–Wetzel estimates, even for integration areas as small as 2000 km2. This indicates the capabilities of the new technique for nowcasting purposes over medium-sized river basins.

Insured weather losses in the US reached record highs in the early 1990s, leading to major concerns in the insurance industry about the causes, including the possibility of climate change due to global warming. Several studies addressing the interpretation of the record high losses used historical insurance data sets, those for crop-hail losses and others based on the catastrophic events to the property insurance industry, both covering the 1949–1995 period. The past loss values were adjusted by insurance experts for shifting coverage, inflation, and evolving construction practices. The resulting adjusted values of crop loss and costly catastrophes to property both showed similar distributions for 1949–1995, with losses being high in the 1950s and again in the early 1990s. This distribution was found to have a weak relationship with extra-tropical cyclone activity in the US, but most of the recent increase in weather losses to insured property was found to be related to shifting societal factors that have put ever more property at risk in storm-prone areas, including coastal areas and large metropolitan areas. The low property and crop storm losses of the 1966–1985 period had created an incorrect perception of the weather risk in the insurance industry, which did not understand nor appreciate the existence of the decadal-scale fluctuations that exist in climate conditions.

The correlation between Meteosat water vapour (WV) data and positive anomalies of potential vorticity (PV) is investigated using a case study involving cyclogenesis in the Alpine region and a north-west African depression. Correlation analyses are performed at different isobaric surfaces in the middle and upper troposphere for two latitudinal belts (25–35° N – ‘southern’, and 35–52.5°N – ‘northern’). A significant correlation is obtained at 500 hPa (correlation coefficient range: 0.58–0.64). Linear regression models are fitted to the samples from the northern and southern belts. On the basis of these results, the use of WV data as a tool for detection of errors in model-derived PV fields is discussed. It is concluded that, though some of the discrepancies between PV fields and WV images could be due to errors in the model analysis or prognosis, some mismatches are possibly due to the variable sensitivity of the WV channel because of the changes in pressure over the commonly used PV fields. It is suggested that the comparison of WV data with the field of positive PV anomalies at 500 hPa might be another approach to help solve the problem of verifying and validating numerical output.

A diagnostic study of two cases of severe thunderstorms in Catalonia (the north-eastern region of Spain) is presented. The first occurred on 24 August 1993. A squall line, with bow echo configuration in the radar images, crossed Catalonia from west to east. Strong winds, large hail up to 7 cm in diameter, and heavy rain producing floods were observed. The meteorological situation was characterized by an upper-level trough approaching Spain from the north-west. This upper trough developed a low, which displaced the warm, dry air located over the Iberian Peninsula towards the east and north-east. The warm, dry air overran the cold, humid Mediterranean air producing a strong inversion (LID) capped by an elevated mixed layer over the western Mediterranean and also over the Ebro valley. Hand-drawn analyses permitted the identification of a secondary trough; a combination of this trough and a jet streak located over Spain was responsible for the mesoscale lifting mechanism that was able to break the LID and maintain the deep convection. The second case occurred on 31 August 1994. A tornado developed over south Catalonia and large hail, up to 5 cm in diameter, also fell. The meteorological situation was also characterised by an upper-level trough advancing from the west towards Spain. Accurate hand-drawn analyses identified a small thermal low over Catalonia leading to strong convergence at low levels. Over the same area, two jet streaks overlapped their left exit and right entrance zones, producing an enhancement of the ascending motion forced by the low-level convergence. However, the typical environment for supercell development was not present over Catalonia. The tornado was probably produced by a non-supercell thunderstorm.

A method to predict extreme roof snow loads is described. The procedure utilizes a combination of recent climate data (antecedent snow water equivalent information), probabilistic precipitation amount forecasts and historical extreme snow water equivalent data. Use of the procedure is illustrated using precipitation amount probabilities derived directly from a statistical (MOS) guidance product and using conditional climatological probabilities, both with a lead time of 36 hours. The procedure is applied across a portion of the north-eastern United States where extreme snow loads during January 1996 resulted in widespread roof damage, injuries, and a death. Based on the 90th percentiles of the forecast precipitation amount distributions, the procedure accurately identified the areas in which greater than 100-year return interval snow loads were observed.

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.

Road-surface cooling during three contrasting frontal passages was examined for various sites with widely varying exposure across Devon. Delays in road cooling were dependent on exposure, topography, thermal inertia, precipitation and surface wetness. Scattered cloud and cloud bands complicate the spatial variability, but for winter maintenance, worst-case scenarios of complete cloud clearance produce important guidelines for prioritising salting routes.

In this study an attempt is made to investigate the upper-tropospheric downstream development over north-west Europe, which leads to surface cyclogenesis in the central Mediterranean. A case study is analysed to demonstrate that the upper-tropospheric downstream development could be closely related to the upper-tropospheric frontogenesis that appears upon the north-eastern flank of a blocking high. The frontogenesis is characterised by a jet streak within a strongly baroclinic zone and a tropopause folding associated with cold stratospheric air intrusion into the troposphere. According to this interpretation, the eddy ageostrophic divergence of eddy geopotential fluxes (dispersion and spreading of eddy kinetic energy), other than friction dissipation and barotropic conversion to the mean flow, is mainly responsible for the loss of kinetic energy from a decaying depression of synoptic scale that has passed the mature stage. This dispersed eddy kinetic energy accumulates in the vicinity of the aforementioned jet streak where it is transferred downstream and further triggers the generation or rejuvenation of a new disturbance.

Accurate prediction of thunderstorms during the pre-monsoon season (April-June) in India is essential for human activities such as construction, aviation and agriculture. Two objective forecasting methods are developed using data from May and June for 1985–89. The developed methods are tested with independent data sets of the recent years, namely May and June for the years 1994 and 1995. The first method is based on a graphical technique. Fifteen different types of stability index are used in combinations of different pairs. It is found that Showalter index versus Totals total index and Jefferson's modified index versus George index can cluster cases of occurrence of thunderstorms mixed with a few cases of non-occurrence along a zone. The zones are demarcated and further sub-zones are created for clarity. The probability of occurrence/non-occurrence of thunderstorms in each sub-zone is then calculated. The second approach uses a multiple regression method to predict the occurrence/non-occurrence of thunderstorms. A total of 274 potential predictors are subjected to stepwise screening and nine significant predictors are selected to formulate a multiple regression equation that gives the forecast in probabilistic terms. Out of the two methods tested, it is found that the multiple regression method gives consistently better results with developmental as well as independent data sets; it is a potential method for operational use.

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.

Methods to calculate extreme wind speeds are described and reviewed, including ‘classical’ methods based on the generalized extreme value (GEV) distribution and the generalized Pareto distribution (GPD), and approaches designed specifically to deal with short data sets. The emphasis is very much on the needs of users who seek an accurate method to derive extreme wind speeds but are not fully conversant with up-to-date developments in this complex subject area. First, ‘standard’ methods are reviewed: annual maxima, independent storms, r-largest extremes with the GEV distribution, and peak-over-threshold extremes with the GPD. Techniques for calculating the distribution parameters and quantiles are described. There follows a discussion of the factors which must be considered in order to fulfil the criterion that the data should be independent and identically distributed, and in order to minimize standard errors. It is commonplace in studies of extreme wind speeds that the time series available for analysis are very short. Finally, therefore, the paper deals with techniques applicable to data sets as short as two years, including simulation modelling and methods based on the parameters of the parent distribution.

The NERC radar facility located near Aberystwyth (52.4° N, 4.1° W) was established for remote sensing of the structure and dynamics of the mesosphere, stratosphere and troposphere; hence the name MST radar. The facility records scatter and reflection from metre-scale irregularities in refractive index produced by atmospheric waves and turbulence. The measurement of the three-dimensional wind field at tropospheric and lower stratospheric heights represents the primary capability of the facility and is expected to be particularly useful in short-range local weather forecasting operations. At the same time, the temporal and height resolution achievable lend themselves to studies of smaller-scale, time-varying phenomena, such as waves and turbulence. Particular attention has been paid to the characteristics and vertical propagation of mountain waves and inertia-gravity waves, and estimates made of the vertical flux of horizontal momentum associated with such waves, known to play a significant role in the large-scale atmospheric circulation. The turbulence associated with the dissipation of these waves, and also generated in the vicinity of jet streams, has also been examined. Quasi-specular reflection of radar waves associated with changes in temperature gradients has helped monitor the height and sharpness of the tropopause and also the structure of frontal zones. The technical capabilities of the facility and the continuity of data provided by its all-weather and extended operation have been shown to be well suited to both mesoscale meteorological and research applications.

A theoretical model of flow over a change in surface roughness shows that given an average geostrophic wind of between 8 and 13 m s−1 and near-neutral conditions the difference in equilibrium wind speed at 50 m height over roughness of 0.1 m and over the sea surface is about 20% or 2 m s−1. However, in non-neutral conditions the difference between on- and off-shore wind speeds is affected by changes in stability conditions. Using data from an off-shore monitoring project it is shown that flow over sea is typically associated with smaller stability corrections to the logarithmic wind speed profile owing to an increase in the number of near-neutral and unstable observations compared to flow over land. The presence of land is still detectable after flow of 1–2 km off-shore but its effect on stability and therefore the wind speed profile is reduced. The differences between stability climates on- and off-shore are not simple to elucidate since a number of factors such as the orientation of the coastline with regard to the prevailing wind direction, water depth and latitude are probably also important.

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 increasing accuracy of data from meteorological satellites coupled with the scarcity of tropical raingauge data and the need for information regarding the fundamental tropical seasons (dry and wet), has led to the development of procedures for climatic analysis that incorporate satellite products. A previously designed procedure based on satellite outgoing longwave radiation and raingauge data is modified to characterise each calendar month of a neo-tropical region (Guri, 7.5° N, 62.5° W, 2.5° lat, 2.5° long, Jan 1974 – Dec 1996, Venezuela). The intensities (categories) of the two fundamental tropical seasons are explored by applying a further four-category procedure. Observed runs of categories lasting four, five and six years, not expected in the 23-year period (1974–1996), indicate that there is a degree of persistence. This persistence could provide industry with information that will allow improved annual planning.

This paper deals with ‘mushroom-shaped’ patterns that may be observed in water vapour (WV) imagery. Its associated conceptual model has been adapted from the ‘T’ configuration. For this model some remarkable properties of WV imagery have been used: the spatial and temporal continuity of atmospheric structures, the more detailed upper-level circulation features and the possibility of observing and monitoring relationships between them. Using this model, and from the kinematic viewpoint, it is possible to infer some features such as deformation zones, rotation centres (vorticity centres) and relative maxima of wind in the upper-middle-level flows. Some examples are presented to show the usefulness of this conceptual model for the interpretation of WV imagery, the diagnosis of upper-middle-level flow and the evaluation of the numerical model.

Precipitation forecasting experiments have been carried out with HIRLAM, a numerical weather prediction model. The model has been run with three different horizontal gridlengths: 22, 11 and 5.5 km. An attempt has been made to estimate the appropriate magnitude of the horizontal diffusion, which is used to control small-scale noise in the model, by looking at kinetic energy spectra. It is shown that, for the higher resolutions, smoothing the orography gives smoother precipitation patterns. The small-scale precipitation resulting from runs with the original orography has negligible extra skill compared to the smoothed orography runs. The results show that the model is able to forecast good precipitation amounts, even with 22 km gridlengths. No significant improvements occur when the higher horizontal resolutions are used. Experiments have been performed using the tendencies of the physical parameterisations computed on a coarser grid than that of the dynamics. The resulting precipitation patterns are very similar and this indicates a more economical way of integrating the model.

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 comparative performance of the WASP and MS-Micro/3 airflow models and of the DAMS scoring system in calculating the wind climate in complex forested terrain has been examined. An analysis was carried out of predicted and observed wind speeds collected over 18 months at six monitoring sites in mountainous country in the Cowal Peninsula, western Scotland. Both airflow models and the DAMS system produced variable results: the airflow models were most accurate on exposed hill tops whereas DAMS tended to be more accurate in valleys and lower slopes. Taken as a whole this study showed that the DAMS scoring system performed as well as the other airflow models considered.