There is a severe risk of road icing and slipperiness when rain or sleet falls on a frozen road surface. Traffic can be severely affected, with a higher frequency of road accidents. These conditions usually occur during warm frontal passages when cold weather and low road surface temperatures are replaced by warmer air and precipitation. Two case studies from southern Sweden show the spatial and temporal distribution of rain or sleet on a frozen road surface. The influence of regional-scale characteristics is demonstrated. Air temperature governs the spatial distribution when temperatures around -10 °C precede the event. Weather stations in an open environment experience slippery conditions first. If temperatures are just below zero before the front arrives, road surface temperature becomes the crucial factor. Weather stations in locations screened by forest or road cuts are the first to register this event. The methodology presented in this paper can be used to increase knowledge of the spatial and temporal patterns of road slipperiness in an area. As a result, forecasts and maintenance activities can be improved and there is a better understanding of the relationship between synoptic events and road slipperiness.

This study examines the influence of urban and meteorological factors on the surface air temperature field of the medium-sized city of Szeged, Hungary, using mobile and stationary measurements under different weather conditions between March and August 1999. This city, with a population of about 160 000, is situated on a low, flat flood plain. Efforts have been concentrated on investigating the maximum development of the urban heat island (UHI). Tasks include the determination of the spatial distribution of seasonal mean maximum UHI intensity and modelling of existing conditions in the period being studied. Multiple correlation and regression analyses are used to examine the effects of urban parameters (land-use characteristics and distance from the city centre determined in a grid network) and of meteorological parameters (wind speed, temperature) on thermal conditions in the study area. The results indicate isotherms increasing in regular concentric shapes from the suburbs towards the inner urban areas where the mean maximum UHI intensity reaches more than 3 °C in the studied periods. A strong relationship exists between urban thermal excess and distance, as well as built-up ratio. In contrast, meteorological conditions do not have any significant effect on the UHI intensity at the time of its maximum development.

In order to decide whether or not a weather service supplier is giving good value for money we need to monitor the quality of the forecasts and the use that is made of the forecasts to estimate their value. A number of verification statistics are examined to measure the quality of forecasts - including Miss Rate, False Alarm Rate, the Peirce Skill Score and the Odds Ratio Skill Score - and a means of testing the significance of these values is presented. In order to assess the economic value of the forecasts a value index is suggested that takes into account the cost-loss ratio and forecast errors. It is suggested that a combination of these quality and value statistics could be used by weather forecast customers to choose the best forecast provider and to set limits for performance related contracts.

In this study, thermal mapping along the road and recordings from field stations in the Swedish Road Weather Information System (RWIS) are analysed. The purpose is to explore how long it takes for air and road surface temperatures to adjust from clear to overcast conditions after a warm front arrival, and to see if there are any variations in reaction time between the two temperatures. The study also focuses on whether different sitings affect the adjustment time when the warm front arrives and if the time of arrival is important for the resulting temperature variations. The results show that weather preceding the front, temperature difference when the front arrives, arrival time of the front, wind speed and the occurrence of precipitation all have an effect on the adjustment time. Air temperature reacts faster to a weather change than the road surface temperature. During the evening the road surface temperature can still be affected by previous shading for up to four hours after a front arrival. When the total cloud cover increases above 4 oktas before noon there are no or small lags in reaction time between air and road surface temperature, since there is insufficient time to establish shading effects.

A recent requirement charged to the Aviation Weather Center (AWC) is to produce significant weather charts for aviation users with, among other forecast products, forecast locations of significant fronts. To increase forecaster productivity, the AWC decided to evaluate the possibility that fronts should be first drawn objectively. Hewson (1998) describes the basic technique which uses a variation on the Renard & Clarke (1965) frontal locator function to find the fronts. The AWC had to overcome many problems in implementing Hewson's techniques. This paper illuminates the problems and describes the AWC solutions. As a result of the AWC's success, objective frontal analyses and forecasts are now a reality, and the productivity of forecasters increased.

This study considers night-time urban air and road surface temperature variations under different synoptic weather conditions. Five years of winter weather data from a single weather station were classified into groups with similar air-mass characteristics. Using a two-stage process of principal components analysis followed by cluster analysis, the resulting clusters were used to sort paired air and road-surface temperature observations recorded at 2100hr from the West Midlands road weather information network into days with similar meteorological controls. The six resulting clusters have good between-group variation and feature a dominant wind direction. The results show that the difference between air and road surface temperatures is significant except during strong westerly conditions. Days featuring clear and anticyclonic weather showed the greatest between-site variation and evidence of the urban heat island effect. Under these conditions, road surface temperatures were warmer than the air - the opposite of what one would normally expect in rural areas. Those days with wet and windy weather were found to have the least between-site variation, excepting that due to altitude. The presence of a high vapour flux in the urban environment literally dampens the temperature response.

This paper presents a general review of ensemble forecasting techniques, with a focus on short-range and tropical cyclone predictions. The basic ideas and terminology of ensemble forecasting are introduced, and using four measures to evaluate an ensemble (ensemble mean forecast, consistency, spread versus skill, and inclusiveness), various potential utilities (e.g. dynamical probabilistic forecasts) are illustrated. Since the perturbation methodologies designed for medium-range forecasts of mid-latitude synoptic-scale systems - singular vectors, bred modes, and so on - are already quite mature, they are only briefly described here. The general problems encountered in applying ensemble forecasting techniques to short-range and tropical cyclone forecasts are diagnosed, and some recent studies on these topics reviewed. In general, the perturbation methodologies used for short-range ensembles to date can have a skill comparable to or slightly higher than their corresponding high-resolution control forecast. However, the complicated problem of the relationship between initial condition errors and model deficiencies persists. A similar situation also applies to ensembles designed for tropical cyclone forecasting. An additional difficulty is the different error characteristics encountered in the tropics, mainly the result of the strong convection in the area and the mutual interaction with the ocean. Studies from several research groups use quite different perturbation methodologies, but the results are encouraging. Most of them performed ensemble forecasting of tropical cyclone motion, but extensions to tropical cyclone intensity forecasts are also being developed.

In this paper the position and intensity of the long waves around South America are related to the precipitation in Argentina. For this purpose indices of zonal and meridional circulation as well as of the curvature of geopotential height at 1000 and 500 hPa were selected. The daily 12 UTC analyses from the European Centre for Medium Range Forecast for 1980 to 1988 were used, averaged over at least five days. A strong positive (negative) anomaly of the meridional index at 1000 and 500 hPa is associated with a negative (positive) anomaly of precipitation. Positive (negative) anomalies of the zonal index at both levels are related to negative (positive) deviations of rainfall. Positive (cyclonic) anomalies of the curvature at 500 and 1000 hPa correspond to enhanced precipitation over the central-eastern part of the country, while over the central-western part the correspondence is of opposite sign. Once methods for forecasting long wave behaviour are available, including for the region around South America, the results may be used to produce medium-range and long-range forecasts of precipitation anomalies over Argentina.

A mesoscale diagnostic wind model, based on mass conservation, is utilised to develop a wind climatology for the southern Appennines in Italy, with the aim of identifying areas suitable for wind energy production plants. The model includes the kinematic effects of the high ground, anabatic and katabatic flows along mountain slopes as well as blocking effects. The climatological analysis carried out appears quite effective in selecting favourable areas where a more detailed investigation based on boundary layer models and observational field studies may lead to the final specification of locations suitable for mountain wind farms.

The need for innovative snow and ice control techniques and processes has continued to grow as national and European road networks have developed substantially over recent decades. The demand for improvement, including the sophistication of the techniques and technology used, continues to be driven by the increasing need for safe and efficient national and international road freight and passenger transport and by the environmental and other policies affecting highways (roads and bridges).

The European Commission project ‘COST Action 344: Improvements to snow and ice control on European roads and bridges’ started in April 1999 as a three-year project, with participation from 18 European countries - Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Hungary, Iceland, Ireland, Norway, Poland, Romania, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

The project's aims are to:

1) review existing international practices,

2) define snow and ice control requirements in different European climatic regions,

3) specify ‘Best Practice’ in different European climatic regions,

4) develop guidelines for the integration of specified snow and ice control methods into network level road management and maintenance systems,

5) recommend improvements to driver information and traffic management systems, and

6) recommend future research projects.

This COST Action will promote the exploitation of technological advances in the application and distribution of snow and ice control measures, with a view to providing significant environmental and safety benefits and lower operational costs. Millions of ECUs could be saved through lower operational costs and a reduction in adverse effects on the highway infrastructure and the environment. For road users, more effective management of winter operations could lead to a reduction in traffic delays and accidents. For practitioners, implementation of ‘Best Practice’ should enhance standards and lead to ‘best value’ being achieved. The implementation of best value could provide the means to measure the performance of the winter maintenance service within various road administrations. For widespread dissemination of knowledge, interim results of the COST Action are being made available to European and national policy-makers, regional planners, engineers, road and vehicle operators, industry and academia. The internet, a CD-ROM, email, handbooks and events such as workshops, conferences and seminars are also being used to target a wider audience of practitioners and researchers.

The annual amount of salt used to prevent slippery conditions on roads was estimated with the help of winter monthly mean air temperatures. Air temperature is measured reliably at almost all meteorological stations and the use of air temperature for the estimation of road maintenance conditions is a tempting alternative compared with the use of more complicated indices calculated using meteorological observations available at only a small number of stations. It was found that in Finnish climatological conditions, warmer than normal weather in November and March and colder than normal weather in December, January and February reduces the need for salting. Warm mid-winter months mean slippery conditions and, consequently, an abundant use of salt. The temperature explained about 60% of the annual variation of salt use and thus gave surprisingly good estimates for the salting amounts.

This study measured the amount of hoar-frost on the road surface with the help of copper plates. The amounts were compared to data collected at a station in the Road Weather information System (RWiS) together with additional measurements of the temperature and humidity profile above the road surface. The data were used to specify the weather parameters when hoar-frost forms on the road surface and relate the measured amount of hoar-frost and resulting friction to measured weather parameters. The aim was also to see whether there was an increased possibility of predicting the amount of hoar-frost with additional measurements of the temperature and humidity profiles. The results of this study confirm that the amount of hoar-frost deposited on the road surface increases with increasing average wind speed, increasing maximum difference between dew point (Td) and road surface temperature (Tr) and increased duration when Tr < Td, measured at the RWiS station. However, the difference between Td and Tr failed to indicate hoar-frost on several occasions due to the location and poor accuracy of the measuring equipment. Measurements of temperature and humidity in a profile up to 2.5 m above the road surface showed that the air layer up to 0.4 m reflected conditions at the road surface whereas the surrounding areas influenced the measurements at higher levels. A forward multiple regression performed on the data showed that measurements of temperature and humidity at lower levels improved predictions of deposition of hoar-frost on the road surface. Measurements of friction on the road surface when hoar-frost was present showed that friction decreased with increasing difference between Td and Tr and it also increased with the duration of Tr < Td. There was no correlation between the amount of hoar-frost and friction at the road surface. The friction is probably influenced not only by the amount of hoar-frost but also by the structure of the hoar-frost, which has not been determined in this study.

The difference between air and road surface temperature in urban and rural areas is an important consideration when modelling the road climate. In this study the effect of the urban heat island in the Stockholm area on road climate is examined. Factors such as distance from the city centre, traffic and topography are analysed in order to assess their impact on the spatial variation of road and air temperature.

The performance of the European Centre for Medium-Range Forecasts model in forecasting the tracks of tropical cyclones in the South China Sea and parts of the western North Pacific for the years 1991-8 was assessed using 2.5° resolution data transmitted operationally over the WMO Global Telecommunication System. Minimum mean sea level pressure centres were used to identify and track the positions of tropical cyclones, and in 1998 the positions of the centre of maximum 850 hPa relative vorticity were also used. The model's performance was compared with that of subjective forecasts made by the Hong Kong Observatory and with no-skill forecasts as represented by a climatology-persistence method in the 1990s. Significant improvements in the model forecasting of tropical cyclone tracks were identified towards the end of that period. The model mean forecast errors for 24-hour, 48-hour and 72-hour positions were 215 km, 340 km and 453 km in the early 1990s (1991-3) respectively, compared with 155 km, 244 km and 292 km in the late 1990s (1996-8).

Two methods for estimating ten-day rainfall totals from Meteosat infra-red imagery were compared for the April-June 1996 ‘long rains’ of Kenya in an area covering the eastern highlands and the Tana and Athi river basins. One of these (the Bristol ‘B4’ method) was then used for rainfall estimation for the whole of Kenya, for November 1996 and the other, the TAMSAT ‘Cold Cloud Duration’ (CCD) method was used to estimate rainfall for the whole of Kenya for November 1997 to April 1998. April-June 1996 was an unusual season with very few large rainstorms. For this comparison period the B4 method gave better estimates of actual rainfall than the TAMSAT method because it used a variable cold cloud threshold temperature and ongoing calibration against rain gauge data. Comparison of ten-day CCD totals with rainfall for the 1997-1998 period indicated that using the TAMSAT method gave best rainfall estimates for the arid and semi-arid areas of eastern and northern Kenya and for months other than the main rainy season months of November and April. Both methods could be used successfully to identify periods with well below or well above average rainfall even over highland areas, and they are therefore useful for providing food security early warnings.

An approach to evaluating probability forecasts for dichotomous events, based on their economic value over all possible cost/loss ratio decision problems, is proposed. The resulting Value Score (VS) curve shows non-dimensionalised relative economic value as a function of the cost/loss ratios for different decision-makers, over their full meaningful range. The VS curve is similar in terms of computational mechanics and graphical display to the Relative Operating Characteristic (ROC) curve, but the ROC curve is shown to be insensitive to either conditional or unconditional biases and thus to reflect potential rather than actual skill. The possibility of collapsing the VS curve into a single scalar score is addressed, and it is shown that the results can depend very strongly on the assumed distribution of cost/loss ratios in the community of forecast users.

The feasibility of using the proposed relationship between model-generated upper-tropospheric potential vorticity (PV) fields and Meteosat water vapour images in forecast initialisation has been investigated by a number of case studies involving model forecasts of North Atlantic cyclones. The objective of each case study was to reduce the errors in the forecast track and depth of a cyclone by re-initialising the forecast via upper-level PV adjustment and PV inversion. The methodology has employed the PV-water vapour relationship to detect mismatches between PV fields and water vapour images, and has attempted to correct these mismatches by manual adjustment of the PV fields. These PV adjustments have been converted to wind and temperature increments by means of PV inversion and used to initialise the model fields in a way that preserves dynamical balance. Five case studies with small initialisation errors were carried out. The results of the case studies show some partial positive impacts on forecasts of cyclonic development, but overall the results do not demonstrate that the use of the PV-water vapour relation for forecast initialisation is a practical proposition for operational use.

Clear air turbulence (CAT) at high altitude remains a hazard to aviation which can result in passenger injury and aircraft damage. Two limited surveys of CAT events over South Africa, 1993-1995 (inclusive) and 1998, are used to illustrate the most likely synoptic conditions under which CAT can be expected. A case study of CAT associated with an upper-air trough and a mountain wave is presented. The study also evaluates the effectiveness of the Ellrod Turbulence Index (ETI) derived from model data provided by the UK Met. Office. A forecast of ETI derived from the Global Spectral Model of the United States National Center for Environmental Prediction (NCEP) is also reviewed.

Of the full set of observations carried out at weather stations, the ‘present weather’ observation is one of the most difficult to automate. Initially, elements such as rain and fog were automated. However, during the past ten years a new group of sensors, aimed at identifying the type and intensity of precipitation, has emerged. These sensors have come to be called Present Weather Sensors (PWSs). Most of the early tests and intercomparisons have been limited to liquid precipitation. Not much attention has been given to investigating the reasons for disagreement between the primary reference, the professional meteorological observer, and the automated observations. This paper presents an analysis of an intercomparison between observer and one type of PWS (Vaisala FD12P), in conditions of both liquid and frozen precipitation. An interaction model is introduced as a new and improved tool for the quality control of PWSs.

Atmospheric dispersion models are now being used by some local authorities in the United Kingdom as part of their management of urban air quality. Local authorities are also involved in using air pollution model output in environmental impact assessments for industrial projects, assessing the impact of road improvement or traffic management schemes and environmental health studies. The study, based in Northampton, UK, is concerned with the limitations of the meteorological data available to local authorities. The results indicate that while there is no major problem in the case of long-term average pollution concentration, meteorological data limitations pose problems when modelling individual events, particularly on occasions when higher pollution levels are expected.