Winter indexes are used in Road Climatology to describe the main characteristics of the climate in an area and relate to the amount of maintenance activity needed. Index calculations can show the severity of winter in a specific area by comparing different climate parameters, or compare the climate between different years or seasons.
Objective of IRWIN is to develop an improved winter road index using historical observations from the Road Weather Information System (RWIS) networks in Sweden and Finland. Index can be used as an assessment aid in present-day as well as future climate scenarios.
Project starts with summarising the present state-of-the-art of global climate models, emission and climate scenarios, effects of climate change on the road network, winter indexes, and the key factors and calculation methods in constructing those. A database of possible future road condition scenarios is developed using archived historic RWIS-data, and combining those with widely accepted climate change scenarios. Observations from the Swedish and Finnish RWIS stations will be collected from national Road Administrations from as long time span as has been archived with homogeneous contents and quality. Target is to get 10 years of observations of air and surface road temperature, moisture, wind speed, rainfall amount and precipitation type.
In the next step, the raw archived observational data will be processed into regular interval time series. The Finnish and Swedish data will be reformatted to similar data format. Climate downscaling is performed to establish the climate database. The phase-space method to select the historical analogue days matching the future days will be developed.
The second phase will develop and test a winter index technique to evaluate such things as the spatial variations of winter maintenance needs as well as the cost/benefit of various winter maintenance strategies. With the help of the databases and index calculations, other types of weather related events, such as strong winds, heavy precipitation, flooding, freezing and thawing, can be mapped and assessed. With well-defined interfaces, the new system can be easily adapted to other countries outside the project test areas in Sweden and Finland. Possibilities of using road weather data from other countries will be investigated.
IRWIN project was successful in collecting large enough database from the Swedish and Finnish road weather information systems and their archived observations. The quality-controlled database was useful for reliable climate downscaling operations, which were utilising well-established global climate models. Maintenance actions were also collected from the six climatologically different study areas for winter index calculations.
The results showed that according to well-established climate models, temperature increase would be largest in the northern areas in Sweden and Finland in coming decades. Same areas will experience a larger amount of events when there is a shift from plus to minus degrees (index 9), and therefore these areas need more maintenance due to slipperiness caused by this shift. Only the region in southwestern Sweden will in the future have fewer days when temperature shifts from plus to minus degrees due to a warmer climate of that area.
The future will bring more rainy days on a cold surface in the north due to the milder climate and more rainy days in the north instead of precipitation falling as snow. The northern areas will also experience more slipperiness due to frost days when the surface temperature is low and the dew point is larger than the surface temperature. These frost days will occur less frequent in the future in the more southerly areas, due to fewer days with minus degrees.
Higher temperatures will also result in precipitation falling as rain instead of snow, which can be seen as a large decrease of ploughing events (index 1 to 6). Index 1 showed a positive change in the more northerly regions but indices with higher winds showed a decrease in almost all regions. Historically, regions S1 and S2 had more events of index 5 which might be caused by the higher winds in these areas. These two regions however show a quite large decrease in these events in the future. Northern Sweden and southwestern Finland, which have moderate winds today, will in the future experience an increase in index 5.
The need for maintenance operations will change in the different regions as the climate changes. A warmer climate can both mean more needs for salting due to more slippery roads but at the same time less ploughing events when precipitation falls as rain instead of snow.
The index developed in this study has shown to be a useful tool for future maintenance operations. It can give valuable information to stakeholders as to where and when measures need to be taken. Similar assessments could be done relatively easily in other countries too, if enough road weather information was archived and available. Taken into account the seriousness of climate change and its implications, it is highly recommended that Road Owners if not yet do so, start operational archival and quality control of all road weather observations in their countries