LUIS GARCÍA TEVENET Ciudad: Buñol (Valencia)

beginning of 1985 and now represents the most extensive and technologically advanced system of its kind in Europe. Experimental work had been in progress for several years, with a mobile radar installation in North Wales in the early 1970s. By 1980, four weather-radar installations were producing precipitation data in real time and, since then, two more have been added to the network in the UK and one has been installed in Eire. It is envisaged that eventually a network of radars will provide a coverage of the entire country (Figure 2.1).

From the outset, weather radar in England and Wales has been seen primarily as an aid to flood forecasting, and groupings of water authorities, the Meteorological Office and the Ministry of Agriculture have been involved in providing funding for radar installations. In Scotland, progress in the installation of weather radar has been slower than in the rest of the UK, since the potential benefits for flood alleviation are perceived as smaller although, significantly, road transport has been identified in a recent report as potentially benefiting.

Weather radar identifies and quantifies areas of precipitation detected within the radar field. Maximum radar range is 210km, although frequently precipitation is not detected beyond the 100km limit. A central network computer at the Meteorological Office

Weather radar and the highway engineer

Figure 2.1 Weather radar sites operational, under construction and planned in the British Isles in 1992.

3 1 combines data from all the radars into one ‘composite’ image which is updated every 15min and transmitted to the user via dedicated British Telecom lines. The visual display of the current pattern of precipitation over the UK employs eight stepped intensities, each represented by a different colour. The colour-coded pixels making up the composite image give a resolution equal to a 5km square. Precipitation type is not readily discernible from the weather-radar display system, and various sources of error, such as spurious generation of apparent precipitation, may contribute to an incorrect estimate of probable rainfall; it is therefore advisable to use the radar in conjunction with a consultancy service provided by a Weather Centre. To monitor the progress of precipitation patterns through time, it is important to be able to store and retrieve historic radar images, and this facility has been developed and recently much refined. At any time stored images can be replayed in sequence, allowing the user to monitor the development and movement of specific precipitation configurations. There is also the potential to incorporate a floppy-disk unit in the storage hardware, which considerably enhances the total data-storage capability.

While the possibility of making radar imagery widely available through teletext and viewdata services has been discussed, this seems unlikely to take place in the foreseeable future in the UK, and there are no plans to offer a weather radar service in this form. Consequently, if the highway engineer wishes to make use of weather radar, the only option is to purchase the weather-radar imagery and the microcomputer equipment required to display it. Costs are high, since the investment in providing the system is large and the major customers, the water authorities, have shown that the market will tolerate a high price for the service. The radar facility’s ability to pay for itself, in terms of winter maintenance and other cost savings over specific periods, has not yet been fully demonstrated. Weather radar could help to reduce road traffic accidents by providing local information on precipitation which could be used to prevent the development of hazardous road surface conditions.

Research at University College, Swansea has identified a number of situations where cost savings could be expected as a result of having direct access to weather-radar information. These include two common weather scenarios:

(a) On cold, showery winter nights, individual showers can be tracked.

Wetted roads may freeze over once skies clear after showers, producing locally hazardous early morning road conditions. During such weather conditions, the showers which are generated over the warm seas around the UK drift across coasts and inland before dying out. Traditional forecasts cannot provide the fine detail of shower tracks and local authorities have little choice but to salt an

Weather radar and the highway engineer

The winter maintenance of highways 3 2

entire road network, much of which may remain dry and thus not be subject to an ice hazard.

(b) Following heavy snowfall, information on the time of cessation of the snowfall and the likelihood of recurrence can optimize clearing operations and prevent clearance work being undone by new snowfalls. Traffic management, particularly on motorways, could be enhanced by the use of weather radar for the determination of advised speeds on illuminated matrix signals.

If the benefits of radar imagery are to be fully achieved, then working practices may have to be reconsidered. Clearly, 24–hour working by personnel trained in the interpretation of weather radar-imagery is essential and, as in the case of Devon, this suggests a centralized control room or county headquarters making decisions. This would be prefer-able to decentralized decision-making. There may be a case for groups of neighbouring counties’ sharing the cost of such a facility. A further important issue is whether radar imagery can provide precipitation warnings quickly enough to ensure the effectiveness of operations carried out by users. The question of warning provision has been the subject of recent research at Swansea. Results suggest that composite weather-radar imagery could provide visual warnings of precipitation of up to three hours on about 85% of occasions and this should be sufficient to ensure the effectiveness of such winter maintenance operations as precautionary salting and the call-out of snow-ploughs (Perry et al. 1986).

2.3 WEATHER INFORMATION FOR THE DRIVER