Torbjörn Gustavsson

Development of Temperature Patterns during Clear Nights

Abstract This paper examines nocturnal air temperature structure development in complex terrain. Clear nights were studied in order to compare the cooling rate in different topographical areas with a variety of land cover. It was found that large variations in temperature developed over a very short time period after sunset and that in situ cooling was the dominant factor causing this in valleys and other wind-sheltered locations. Shelter can be provided both from valley sides and from nearby trees with the main effect being to reduce the vertical mixing. The nighttime increase in temperature difference between valley bottoms and nearby reference areas was interpreted to be due to cold airflows. This was also shown by the increasing lateral extension of cold air accumulating in valleys. This development was found only in open valley locations. Sheltered areas cooled at a much faster rate than exposed sites during early evening. Further cooling did not increase the lateral extension of cold air or result i...

Thermal mapping – a technique for road climatological studies

The present study deals with the thermal mapping technique in a road climatological perspective. The temperature mapping technique is analysed in relation to instrument set-ups, data collecting and possible sources of errors in converting measured radiation to surface temperature values. Furthermore, analyses of thermal mapping recordings are discussed. Factors that give rise to a varying temperature pattern are presented (i.e. road-bed material, radiation and advection), and also how the thermal mapping technique can be used to determine the relative importance of these factors. The conclusion of the study is that the technique is very useful but it is very important to be aware of the limits and different source of errors involved.

Infrared thermography in applied road climatological studies

Abstract The aim of this paper is to describehow infrared thermography is used to analyse variation in road surface temperature during clear calm nights. Having knowledge of surface temperature variations along road stretches makes it possible to give an accurate survey of the risk of local slipperiness. The infrared equipment used is an Agema 870-system arranged on various kinds of platforms, stationary, car- and helicopter-mounted. Several factors are of importance in causing surface temperature variations, such as screening from the sun, variation in traffic intensity and construction materials of the road. This article reviews the influence of these factors on road surface temperature and how various platforms could be used to conduct detailed recordings. Sun screening by road rock cuts causes shadow patterns resulting in surface temperatures which are low when compared with sun exposed areas. Height and orientation of the screening object determine the intensity of the surface temperature difference....

Variation in road surface temperature due to topography and wind

SummaryThe present study involves a discussion of nocturnal temperature variation due to topography and prevailing wind speed. It is part of the ongoing project “Applied climatology for increased traffic safety and road maintenance”, which aims at a development of a local climatological model applicable to winter road conditions. The temperature recordings which are used are from sensors in the Swedish Road Weather Information System in the county of Skaraborg.During clear and calm nights, local air temperature differences have been related to different topographical environments. The main factors resulting in large temperature differences are the effect of wind shelter, stagnation or production of cold air, and of cold air advection.Variation in air temperatures during clear and windy nights has also been studied. At wind speeds higher than 4–5 m/s, the temperature variation is low. With decreasing wind speed the variation in temperature increases, but only for sensor sites which have some wind shelter by topography or vegetation.The air temperature variation which develops during clear nights affects the road surface temperature. In this paper, the connection between road surface temperature differences and air temperature variation is discussed. A linear relationship between the two variables exists.

MODELLING OF LOCAL CLIMATE FOR PREDICTION OF ROAD SLIPPERINESS

Recent research concerning the Swedish Road Weather Information System (VVIS) is discussed along with a method for determining temperature variations attributable to topography during different weather situations. Topographical areas are classified according to factors determining temperature differences. The investigation concentrates on areas where great temperature differences occur during different weather situations, producing a risk of local road slipperiness. Geographical factors analyzed and discussed include valleys, height differences, shadow areas, bridges, and regional climate. Cold air pool intensities are shown to be influenced by weather but a variation between different valleys also occurs owing to valley width area of cold air production, wind shelter, and ratio between height and width of valley slopes. During neutral or unstable lapse rate conditions, the lowest road surface temperature is closely related to the air temperature and decreases with increasing altitude above sea level. Screened areas have a lower surface temperature than areas exposed to sun. Factors such as bridge size and type also affect road surface temperature on bridges. These different factors are integrated into a local climatological model. (Author/TRRL)

The impact of screening on road surface temperature

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 and the preservation of the screening effect after sunset is observed. The effect after sunset can be described by: RSTdiff = A − B h + C h2 − D h3 − E h4 + F h5, 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. Copyright

Local temperature differences in relation to weather parameters

The objective of this paper is to focus on the influence of clouds and wind on air and road surface temperature variations between different types of local climate environments. The study area covers 160×130 km2 and includes 35 field stations in the Swedish Road Weather Information System (RWIS) and two synoptic weather stations. By combining data from the two sources, the spatial and temporal variations in air and road surface temperature have been analysed. In the first part of this paper the theoretical influence of different weather parameters is determined. In the empirical part of the study, a validation of the theoretical result is assessed using temperature and weather data from the study area. The results show that it is possible to calculate the temperature variations in relation to topographical siting and different weather factors. During day-time conditions, the effect of screening from the sun has a significant influence on the road surface temperature, even with cloudiness amounting to 4–6 octas, provided that the solar elevation is high. During night-time, the potential for pooling of cold air is determined by cloud cover and wind speed. When cloudy situations prevail during night-time, neutral stability is dominant resulting in a decrease with increasing altitude for both air and surface temperatures. Road surface temperatures, however, have a lower correlation with altitude than air temperature. The variation in surface temperature decreases with altitude is also larger and has a more even distribution than the air temperature decrease with altitude. Wind speed was not an important factor for the variation in surface temperature decrease with altitude, but insolation from the sun during day-time is one parameter to consider. Copyright

Road Climate in Cities:A Study of the Stockholm Area, South-East Sweden

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. Copyright

Temperature differences in the air layer close to a road surface

In this study, profiles of temperature and humidity (<250 cm above the road and 5 m into the surroundings) have been used to examine the development of temperature differences in the air layer close to the road. Temperature, humidity and wind profiles were measured, together with net radiation and observations of road surface state, at a test site at Road 45, Surte, Sweden. Measured temperature differences were compared with present weather, preceding weather, surface status, wind direction and other parameters thought to be important for the development of temperature differences. The results showed that large temperature differences (1–3°C between 250 cm and 10 cm above the road) occurred when there was a high risk of slipperiness caused by hoarfrost, snow or ice on the road. The temperature differences between different levels were associated with the exchange of humidity and temperature between the air layer and the road surface. The 10 cm level reflected the surface processes well. Higher levels were influenced by the surroundings because of turbulence and advection. This study emphasises the need for measurements to be taken at a height and place that reflects the processes at the road surface. Copyright

The applicability of similarity theory to a road surface

The existing models for predicting road surface temperature are often based on surface energy balance which includes contributions from sensible and latent heat exchanges between the atmosphere and road surface. The similarity theory is usually applied to compute the atmospheric turbulent heat fluxes. However, the theory only applies in horizontally homogeneous situations, which is certainly not the case for road conditions. An investigation is carried out to see how much measured profiles of mean wind and temperature from a road meteorological station deviate from the profiles given by similarity theory. The difference between the data and the theory is revealed by comparing the observed gradients with the theoretical ones. The sign of the vertical gradients of all data and the relationships between two vertical gradients at different levels under near-neutral conditions are examined. It is found that the measured profiles are all systematically different from the theoretical ones. The deviation is discussed within the context of the possible existence of an internal boundary layer. Moreover, detailed temperature profiles were measured to study the development of temperatures in the first 2.5 m. The results show that the sign of the temperature gradient reverses in a layer between 0.8 and 1.4 m and that a large change in temperature occurs in this layer; these effects were probably caused by local advection. Copyright