The effect of screening on road surface temperatures is analysed using data from the Swedish road weather information system (RWIS) together with data from thermal mapping. The study deals with the analysis of temperature variations caused by shading of the road surface during clear daytime conditions and focuses on the relation between solar elevation and magnitude of road surface temperature differences between screened and sun-exposed areas. Also included is an analysis of how the temperature differences during the day affect the establishment of temperature variations after sunset and the time it takes after sunset for temperature differences caused by shading to decline. The results show that the magnitude of road surface temperature differences between sun exposed and screened sites (RSTdiff) that develop during clear day conditions can be attributed to solar elevation (β) and can be expressed by the equation: RSTdiff=−2.7+0.46(β). A relationship between the daily maximum temperature difference andthe preservation of the screening effect after sunset is observed. The effect after sunset can be described by: RSTdiff=A−Bh+Ch2−Dh3−Eh4+Fh5, where h is the time in hours after sunset and A to F are constants related to the time of the year at the actual site.

A comparison of atmospheric transport and dispersion calculations over complex terrain was investigated using a mobile profiling system (MPS) versus standard meteorological balloons. Meteorological and sulfur hexafluoride (SF6) concentration data were collected and used to evaluate the performance of a transport and diffusion model coupled with a mass consistency wind field model. Meteorological data were collected throughout April 1995, and parts of August 1995. Both meteorological and concentration data were measured in December 1995. Once the models were validated, the comparison of performance with different upper-air data were accomplished. The models used included the SCIPUFF (Second-order Closure Integrated Puff) transport and diffusion model and the MINERVE mass consistency wind model. Evaluation of the models was focused primarily on their effectiveness as a short-term (one to four hours) predictive tool. These studies showed how the combination of weather and transport models could be used to help direct emergency response following a hazardous material release. The models were used in tandem to direct the deployment of mobile sensors intended to intercept and measure tracer clouds. The MINERVE model was validated for the specific terrain of interest using April 1995 data. The capability of SCIPUFF driven by realistic three-dimensional wind fields generated by MINERVE is demonstrated using data collected in December 1995.

The aim of data-impact studies at the UK Met. Office is to investigate how observations affect the accuracy of model forecasts. Results from such experiments provide useful evidence on which to base the design of observational networks. This project, using a case study approach, investigated the relative benefit of different observation types within The Met. Office's Mesoscale Model domain on forecasts of three-hourly precipitation accumulation over the UK up to 12 hours ahead. The method used assesses the impact of assimilating single observation types, or a limited combination of types, where impact is measured against a control forecast obtained after a dummy assimilation using no observations. In experiments for 13 case studies, the observation types that most frequently provided a beneficial impact when presented alone to the assimilation were sonde data, surface data and data from the Moisture Observation Processing System (MOPS).

Convective precipitation events in northern Italy during 1996 and 1997 are analysed using two infrared-based geosynchronous satellite rainfall estimation methods to verify the level of applicability of the techniques for operational applications in the area, their quantitative results, and relative performances. The Negri–Adler–Wetzel (NAW) and the convective stratiform technique (CST) are applied to METEOSAT's thermal infrared (IR) data. C-band radar reflectivity fields detail the vertical and horizontal structure of the cloud systems, and radar rainfall data are retrieved. Satellite rain areas are checked against simultaneous radar rainfall retrievals through a contingency analysis procedure. A semi-quantitative analysis is presented. Positive brightness temperature differences between water vapour and thermal IR channels are also examined and related to the storms' development stage and rainrate. Results show that NAW and CST perform reasonably in delimiting rain areas during active convection and care should be used in the initial and final development stage when statistical parameters lose most of their significance. NAW tends to overestimate rainfall while CST approaches more closely radar measurements. Most common errors arise from considering only portions of the storm, contamination from cold non-precipitating cloud, and merging of two or more cloud masses of independent origin. Operational applications, though not completely quantitative, are also possible, including positive values of the difference between water vapour and IR brightness temperature.

The Otaki Precipitation Estimation by Radar (OPERA) programme was designed to investigate the processes that lead to enhancement of rainfall over the Tararua ranges of New Zealand. These ranges rise to 1500 m above the coastal plain and enhancement of rainfall by windflow over these hills leads to annual hill-top rainfall of over four times that upwind. The OPERA experimental campaigns aimed to characterise the enhancement processes by analysing data collected from a transect of high-resolution rain gauges and a locally deployed, high-resolution radar, supported by scanning radar and satellite observations. Measurements made during these experiments showed that orographic enhancement led to hill-top accumulations often twice that upwind, and up to as much as a factor of seven in one case. The data suggest that the most frequent occurring enhancement mechanism was triggered convection. This mechanism leads to an increase in rainfall over the hills of around a factor of two, primarily through an increase inthe duration of rain. Seeder/feeder-type enhancement occurs less frequently but leads to larger enhancements.

Precipitation enhancement over a low coastal hill in winter is demonstrated for particular associations of synoptically determined onshore airflow and local geography encountered over south-eastern Newfoundland. Four such cases, involving a mixture of pre-warm-frontal precipitation types at surface temperatures just below freezing, are analysed using comparative surface gauge records from sites at the coast and hill summit and detailed volume scan data from a Doppler radar. Whilst precipitation at the hill summit and upwind coast was of similar overall duration in each case, the surface rates on the hilltop exceeded those at the coast by 1.0-4.0 mm h-1 during different stages of the events. Analysis of the Doppler reflectivity patterns reveals that intensities are especially enhanced near the windward hill crest, supporting the observed association of greatest enhancement with a strong local upslope wind component exceeding 20 m s-1. In the majority of these cases the enhancement is maintained primarily by the ‘seeder-feeder’ mechanism, which appears to be accelerated during precipitation transitions and with the surface warm front 120-150 km distant; however, a preliminary phase of enhancement due to topographically induced uplift of stable pre-frontal air is also recognised in one case.

The risk of dam failure in India, coupled with better analysis of extreme storm events, has led to the estimation of revised one-day probable maximum precipitation (PMP). For some water conservation projects where the catchment area is very large, there is a need to have estimates of PMP for durations of both two and three days. These are provided in this paper. It was found that the two-day PMP over the Indian region varied from 100 cm over the central peninsula to over 290 cm in eastern India. For a duration of three days the corresponding values are 120 cm and 370 cm. For water management projects which include impounding reservoirs, an estimate has been made of the probable maximum flood (PMF) based on the new PMP estimates. The results of the design flood estimates were compared with existing estimates at the same sites and in most cases were found to be considerably higher. A comparison of the revised estimates of PMF and recorded floods in India shows that the trend of the rates of runoff in relation to catchment area differ by an average of 38% for an area of 1000 km2 and 46% for an area of 10 000 km2. These results show that some dams in India may be unsafe in the event of the probable maximum flood.

The diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water-vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower-stratospheric and higher-tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms.

This paper presents a prediction system for regional crop growth in Sweden, recently set up at SMHI (Swedish Meteorological and Hydrological Institute). The system includes a state-of-the-art crop growth model, WOFOST (WOrld FOod STudies) and inputs from meteorological mesoscale analysis. The simulated crops are spring barley, spring rape, oats and winter wheat, and the period of investigation is 1985-98. The simulated water-limited grain yield is used as a predictor in the yield prediction procedure. The technological time trend describing the yearly increase of the production level is accounted for as well. Yield prediction based on crop growth modelling is justified since the ability to forecast the yield is higher compared to that using the technological time trend alone. The prediction errors are of the order of 8 to 16%, with the lowest errors for winter wheat and spring barley.

A method for classifying different types of slipperiness on roads in Sweden is described. Using this method it is possible to survey road conditions in different areas and between different years to optimise winter road maintenance. Winter road maintenance in Sweden is generally undertaken by the national road administration to improve winter-time road conditions, thereby keeping up the traffic flow and decreasing the accident rate. As a number of different types of slipperiness may develop on roads in winter, each due to a specific set of meteorological variables, maintenance work can be a complicated task. With the proposed classification method it becomes easier for the winter maintenance personnel to analyse information on road conditions and survey the distribution of road slipperiness in a region. The classification is performed with an expert system using meteorological data from the Swedish Road Weather Information System. The road condition is classified as good or as one out of ten different types of slipperiness on roads. Road conditions during three different winter periods are analysed. The results show that variations in climate produce substantial differences in annual road condition characteristics. The output from the expert system classifying road slipperiness is compared with recorded winter road maintenance reports. Maintenance action took place on 49% of all occasions when road conditions were classified as slippery.

This paper illustrates a key lesson related to most uses of long-range climate forecast information, namely that effective weather-related decision-making requires understanding and integration of weather information with other, often complex factors. Northern Illinois University's heating plant manager and staff meteorologist, along with a group of meteorology students, worked together to assess different types of available information that could be used in an autumn natural gas purchasing decision. Weather information assessed included the impact of ENSO events on winters in northern Illinois and the Climate Prediction Center's (CPC) long-range climate outlooks. Non-weather factors, such as the cost and available supplies of natural gas prior to the heating season, contribute to the complexity of the natural gas purchase decision. A decision tree was developed and it incorporated three parts: (a) natural gas supply levels, (b) the CPC long-lead climate outlooks for the region, and (c) an ENSO model developed for DeKalb. The results were used to decide in autumn whether to lock in a price or ride the market each winter. The decision tree was tested for the period 1995-99, and returned a cost-effective decision in three of the four winters.

Hailfall is one of the most dangerous and damaging phenomena associated with severe thunderstorms. Its remote identification is still under investigation, especially by means of polarimetric radar data. The need to use polarimetric measurements is a limit to operational implementation of the techniques developed up till now. The routine sampling of radar allows the computation of the differential hail signal HDR (Aydin et al., 1986) every 15 minutes. The assessment of the HDR is performed by comparison with the ground truth from a hailpad network. The spatial resolution of this network is quite coarse (one pad per 16 km2) for the collection of all the hail shafts, as they generally have a small horizontal extent and are short lived. Such bounds have been considered in the analysis of four large hailfalls recorded in 1997. This study reveals also the limitation on radar hail detection arising from the frequent occurrence of strong differential attenuation. Nevertheless, to identify regions where a sharp rain-hail transition occurs, polarimetric measurements work much better than algorithms based only on horizontal reflectivity. Results show that use of HDR gives a high probability of correct hail detection as well as an acceptable number of false alarms.

Heavy rain events constitute a major operational problem in the Mediterranean area. The present work tries to contribute to a better understanding of this problem by considering the possible indirect influence of surface depressions in the triggering and location of the heavy rain. The idea is that any surface low contributes to the organisation of the low-level flow. This then affects the creation/intensification of a low-level feeding current of warm and wet air that can sustain heavy rain if there is convective instability. Two approaches have been used: (a) a numerical and synoptic approach, Part 1, and (b) a statistical approach, Part 2. In Part 1, the differences between a control run and a numerical experiment without the Iberian mountains for the 'MAP Piedmont flood event' are discussed. Among other effects, the Iberian orography seems to intensify a secondary low near Liguria as well as increase the mixing ratio convergence and the rainfall intensity near it. A few other cases of heavy rain are also briefly reviewed in order to identify the presence of a Mediterranean depression in the vicinity.

During 1994/95 a UK Overseas Development Administration (now Department for International Development - DFID) funded project was undertaken to predict ozone amount over Punta Arenas in southern Chile. A low-cost satellite receiver was installed to receive the digital data stream from the NOAA series of weather satellites. The ozone channel of the TIROS Operational Vertical Sounder instrument on the NOAA satellites was used to obtain maps of stratospheric ozone for the area around Punta Arenas in near real time. Also, a simple model was developed to predict the amount of column ozone from the forecast 100 hPa temperatures obtained from the UK Meteorological Office. These techniques used together made it possible to issue timely warnings to the general public of Punta Arenas before the Antarctic ozone hole moved across southern South America during the Austral spring.

The coastal climates of France have been studied using temperature and precipitation data from 1961 to 1990. It is found that the coastlines of the French Atlantic (and adjacent seas) and Mediterranean show many similar climatic features, but the coastal orientation and nearby topography cause some local climatic peculiarities. The relatively flat topography near the coasts of the North Sea, English Channel and the Atlantic result in a fairly homogeneous coastal climate. In these coastal regions there are days of frost each year, but the precipitation amount is usually not high, especially in summer on the Atlantic coast. On the other hand, the mountainous areas of Provence and the Riviera, which are adjacent to the Mediterranean, cause a juxtaposition of varied coastal climates. Large temperature and rainfall differences are observed, especially between the capes and the heads of gulfs, but also between the narrow and wide gulfs. During the warm months, some sea breezes cause coastal thermal peculiarities, which depend on the orientation of the prevailing flow.

A cold-air comma cloud crossed southern Britain early on 18 February 1997 bringing strong gusts and convective precipitation. Development of the comma cloud occurred in association with a positive upper tropospheric potential vorticity (PV) anomaly. A mesoscale analysis of the comma cloud was carried out using special three-hourly soundings from UK radiosonde stations together with data from rainfall radar, satellite, NWP model output and a wind-profiling radar. The analysis reveals that the comma cloud tail was of ana-form whilst the polar cold front (behind which the comma cloud developed) was kata-form. Further analysis indicates that the polar cold ?w front and the comma cloud front were tending to become one entity at the surface and can thus be considered an example of a kata-cold front undergoing a partial transition to more of an ana-form locally in the region of an upper tropospheric positive PV anomaly.

Topographic exposure is a variable which represents the degree of shelter afforded to a location. A detailed knowledge of exposure is useful for a wide variety of applications, ranging from the dynamics of drifting snow to the optimal location of wind-farms. The degree of exposure can be calculated by considering the relative heights and distances of the horizon surrounding the point of study. Numerous approaches have been developed to measure this useful variable, from the early qualitative studies of wind-shaped trees to Pyatts's quantitative TOPEX assessment. However, these studies have been superseded by the use of computers and in particular the application of digital terrain models. These provide a fast office-based solution, virtually eliminating the need for fieldwork.

A climatological study of the occurrence of freezing precipitation at ground level is presented. It is based on reports of freezing precipitation in SYNOP messages from the three winters covering the years 1995-1998. The geographical domain covered is Western and Central Europe, extending northwards to Scandinavia and southwards to North Africa. The types of freezing precipitation selected are freezing rain, freezing drizzle and ice pellets. The frequency of occurrence of each of these types of freezing precipitation was computed. The results show that freezing precipitation is not an uncommon phenomenon in Europe, with the combined frequency of the various types of freezing precipitation (either drizzle, rain or ice pellets) being 0.5 to 1% of all SYNOP messages that include an observation of the current weather in Central Europe. The surface air temperature intervals associated with freezing precipitation have been studied. An attempt was then made to determine the mode of formation of freezing precipitation (either by melting of snow crystals in a layer of positive temperatures or by a process of coalescence or diffusional growth) by studying the thermodynamic soundings associated with freezing precipitation events.

The structure and track of monsoon depressions over India during the summer monsoon have been simulated using a double-nested limited-area numerical weather prediction model. Four distinct cases of monsoon depressions that formed over the Bay of Bengal and adjoining areas have been studied. Initial conditions for the simulations are from either the European Centre for Medium Range Weather Forecasts, Reading, UK or the National Centre for Medium Range Weather Forecasting, New Delhi, India. The model is integrated for up to 48 hours for each case and the results are compared with verification fields. Forecasts of mean sea level pressure and low-level wind indicate that the location of the centres of the depressions and their track could be predicted satisfactorily even though the magnitude of the central pressure is slightly too high. Temperature forecasts show close agreement with the verification analyses and the distribution of precipitation is well simulated. The vertical cross-sections of temperature and wind forecasts show the correct vertical structure. RMS errors of the mean sea level pressure, wind and temperature indicate that the model could simulate the large-scale fields reasonably well. RMS errors of the tracks of the depressions confirm the fact that the high-resolution nested grid model can predict the tracks of the depressions with reasonable accuracy.

Errors and uncertainty in radar estimates of precipitation result both from errors in the basic measurement of reflectivity and from attempts to relate this to the precipitation falling at the ground. If radar data are to be used to their full potential, it is essential that effective measures are taken to mitigate these problems. The automatic processing of radar data that forms part of the UK Met. Office's Nimrod system addresses a number of specific sources of error. These include the identification and removal of spurious echoes resulting from anomalous propagation of the radar beam, errors resulting from variations in the vertical profile of reflectivity and radar sensitivity errors. Routine verification of the surface precipitation estimates has been undertaken, largely through comparison with rain gauge observations, over a range of timescales, which has allowed the benefits of the quality control and correction processes to be quantified. Although the improvement derived varies according to the dominant synoptic situation, an average reduction in the root-mean-square difference between gauge and radar data of 30% can be achieved.