The variation of road-surface temperatures in Devon, UK during cold and occluded front passage

Transcription

1 Meteorol. Appl. 6, (1999) The variation of road-surface temperatures in Devon, UK during cold and occluded front passage N L H Wood and R T Clark*, Institute of Marine Studies, University of Plymouth, Plymouth PL4 8AA, UK *Now at the Met. Office, Bracknell, UK Road-surface cooling during three contrasting frontal passages was examined for various sites with widely varying exposure across Devon. Delays in road cooling were dependent on exposure, topography, thermal inertia, precipitation and surface wetness. Scattered cloud and cloud bands complicate the spatial variability, but for winter maintenance, worst-case scenarios of complete cloud clearance produce important guidelines for prioritising salting routes. 1. Introduction The forecasting of road-surface temperatures relies on an accurate synoptic-scale weather forecast for input to a surface energy balance model, a variety of which have been developed in recent years (Thornes & Shao, 1991a). This paper highlights the complexity of synoptic situations facing the operational forecaster of roadsurface temperatures. The effect of cloud and its input to these models has been investigated by McLean & Wood (1995). The interaction of the frontal scale with the microscale of individual sites is dependent on topography, height, exposure and surface condition, so there is a need to understand how the synoptic-scale changes in the weather affect the road-surface temperatures. In the winter maintenance context, pre-salting crews need to know the most vulnerable roads for icing and to this extent thermal mapping is a help (Shao et al., 1997). The influence of sudden changes in air temperature on road-surface temperature was discussed in Gustavsson & Bogren (1990), when they researched slipperiness of roads due to warm air advection in Sweden. In southern Britain icy roads are more likely to result from the abrupt arrival of a cold air mass with associated clearing skies. The timing of this is crucial and for the 5/6 March 1997 occasion described in this paper, road-surface temperatures do not fall below 0 C. Depressions arriving in the UK are often welloccluded, so a relatively simple case of cloud clearance is not the norm. This is illustrated by the situation on 23/24 January The synoptic-scale changes associated with cold or occluded front passage and leading to a variation in road-surface temperature are: Change in wind speed and direction Precipitation The energy balance at the road surface is the result of the interaction of these changes with the specific road characteristics: Sky view Wind exposure Thermal properties Valley or hill Traffic density The weather ahead of the cold front in Devon during winter is usually mild and cloudy with drizzle around the coast and hills. The cloudy conditions prevent solar heating of the surface and hence sub-surface daytime storage of heat. In this air mass the temperature rarely falls below 5 C and often remains around 10 C even at night. Therefore, no freezing occurs in these conditions. As a cold or occluded front approaches, the wind usually strengthens and rain often falls, except where large-scale sinking of air in the frontal region suppresses vertical instability. Behind the front there is often a sharp drop in air temperature and a marked decrease in cloud cover. Showers and broken cloud may occur behind the cold front, a result of instability after a long track over relatively warm sea. With clearing skies at night, however, radiative cooling overland dominates, creating a surface inversion. This reduces the surface wind speed, effectively decoupling the surface layer from the synoptic-scale flow. A complete clearance behind a cold front is rare, and in reality the road cooling at a particular site may be delayed by the following factors: Changes in cloud cover Advection of cold air Thermal inertia Cloud cover 111

2 N L H Wood and R T Clark Sky view factor Variation in wind speed Precipitation Surface becoming dry The variety of site conditions is expected to lead to a remarkable variation in road-surface temperature. Nocturnal temperature variation at an open site results from an energy balance between net longwave radiative cooling and the counter-effect of turbulent mixing of the air near the surface. At a sheltered or closed site the turbulent mixing is suppressed but radiative cooling is reduced owing to a limited sky view. Wetness is a key factor, resulting not only from rain and snow but also from run-on and spray. In cases of high relative humidity at sheltered sites, evaporative cooling may be slow. Road construction material may be an additional influence, but Thornes & Shao (1991b) showed during a sensitivity analysis that a 20% variation in road construction material had a very small effect on the minimum road-surface temperature, producing a maximum difference of only 0.4 C. The sites in this study are all constructed with rolled asphalt surfaces. Their locations and general conditions of exposure are shown in Figure 1 and Table Case Studies /24 January 1996 Figure 2 illustrates the spiral cloud signature of the maturing depression, which advected cold continental air and gave a partial but temporary cloud clearance in the late evening. A surface depression slowly moved into Devon from the south-east with an associated occluded front, whilst a blocking high-pressure system Figure 1. Location of Devon road weather stations used in this study. existed over eastern Europe and Scandinavia, as shown in Figure 3. Table 2 shows the variation in road and air cooling at sites across Devon in response to partial clearance of cloud, advection of cold air and precipitation. Generally, air cooling was most rapid in valley locations such as Pridhamsleigh, Marley Head, Kingsteignton and Culver Bottom. After a delay, these sites showed most rapid surface cooling. Cloud and precipitation began to return to Devon from the southeast after 0130 GMT, so rapid surface cooling was consequently suppressed and surface freezing occurred late: after 0530 GMT on 24 January or not at all. Table 1. Classification of road-weather stations Station Altitude (metres asl) Exposure Topography Beaford Moor 166 Open Hilltop Bratton Down 320 Open On high ground Culver Bottom 91 Closed Deep narrow valley Haldon Hill 219 Partly closed Hilltop Kingsteignton 24 Open Wide river valley Marley Head 143 Partly closed In cutting Pridhamsleigh 43 Closed In cutting Roborough 143 Open Flat Sourton Cross 277 Open Hilltop Stopgate Cross 253 Open Hilltop Three Horse Shoes 169 Open Hilltop Wellparks 42 Partly closed Wide valley West Country Inn 198 Partly closed South-facing slope Whiddon Down 238 Open Hilltop 112

3 Road-surface temperature at cold and occluded fronts (a) (b) (c) (a) (b) Figure 2. NOAA infrared imagery from (a) 0143, (b) 0324 and (c) 0819 GMT on 24 January 1996 (courtesy of the University of Dundee). Variable cloud and wind gave variable precipitation and wetness, which in turn produced variable cooling. Figure 4 shows the reaction of the road-surface temperature to ±0.5 C and a 15-minute resolution at the mid-devon site of Sourton Cross. The wind speed increased to 12 ms 1 at midnight as the occlusion passed, which increased turbulent mixing and dried the surface. With the exception of Kingsteignton, surface freezing only occurred at the elevated stations. This is Figure 3. Surface synoptic charts for 1200 GMT on (a) 23 January and (b) 24 January 1996 (from Weather Log, Royal Meteorological Society). 113

4 N L H Wood and R T Clark Figure 4. Data at Sourton Cross on 23/24 January Table 2. Time delays after cloud clearance and temperature decreases after 2200 GMT on 23 January 1996 Station Time of partial Time delays Temperature decrease rate Surface freezing cloud clearance (±10 min) ( C/hour) onset time (GMT ±10 min) for road temp. to fall (GMT) after air temp. falls Air temp. Surface temp. Kingsteignton Three Horse Shoes none Marley Head not known Stopgate Cross Pridhamsleigh none Culver Bottom none Roborough none Whiddon Down Wellparks none Beaford Moor Bratton Down already <0 C West Country Inn Notes: (a) Cloud clearance time is the time when cloud cover estimated from satellite imagery became less than 3 oktas. (b) No known frost onset time at Marley Head is available owing to road-surface sensor failure at 0644 GMT. in accordance with McLean & Wood (1993), who showed that the lowest minimum road-surface temperatures on such windy occasions are recorded at elevated locations. In general terms, a depression over the English Channel can cause significant snowfall during mid-winter (as occurred over Devon on 17 December 1997) if its northern edge is likely to produce precipitation. Cold air advection from the east or north-east further increases the possibility of frozen precipitation. The surface and air temperatures during this type of scenario are not unusually cold; indeed air temperatures were above freezing point at most locations at 0000 GMT on 24 January November 1996 The variation of road-surface temperature was examined over a three-day period during the passage of a deep occluding depression. Figure 5 shows the surface synoptic charts for this period. Frontal passage took place in the early hours of the 19th, with 12 mm of rain falling in Plymouth between 2100 and 0600 GMT. Figure 6, for Haldon Hill, shows the surface temperature variation to ±0.1 C with three maxima and three minima. Solar heating led to the daytime maxima and is seen to be suppressed by cloud and wetness on the 19th. It is remarkable how the afternoon and evening surface cooling was halted by the arrival of rain and increase in wind speed. The road-surface temperature

5 Road-surface temperature at cold and occluded fronts rose by 4 C and suggests that evaporative cooling did not play a part until the rain stopped. Broken cloud and scattered showers during the day gave a variable trace in surface temperature and served to hinder surface freezing the following night despite clearing skies. Wind speed increased overnight, enabling turbulent mixing to counteract surface radiative cooling. This contrasts with the evening of the 20th, when clear skies and decreasing wind allowed rapid surface cooling. Figure 7, for Sourton Cross, shows a very similar roadsurface temperature variation over the three days to that at Haldon Hill. The rain period on the night of the 18th/19th again resulted in a rise in surface temperature, although somewhat less at 3 C owing to its higher elevation and more exposed position, which lead to higher wind speeds March 1997 High pressure over continental Europe, shown in Figure 8, gave contrasting conditions to the previous case in terms of frontal cloud and rain associated with the eastward advance of a depression across the Norwegian Sea. Devon experienced total cloud with occasional light rain during most of 5 March before the passage of the cold front at approximately 2000 GMT. Almost complete cloud clearance and a fall in wind speed occurred behind this front, so this presented a worst-case scenario. Figure 9, for Sourton Cross, shows a rapid cooling of the road surface that was halted by the arrival of cloud at midnight. As in the previous case study, a rise in surface temperature occurred when rain fell, on this occasion light rain starting at 0200 GMT. Evaporative and radiative cooling could proceed after this rain, and the road-surface temperature fell to a minimum of 2.5 C around dawn. This smaller-scale feature behind the cold front clearly prevented freezing conditions and presents a complication for the forecaster concerning what initially appeared to be a relatively simple case. High pressure and cloud-free conditions predominated during daylight hours on 6 March allowing solar warming of the surface to 22 C and wind speed increases with convective activity. A warm front arrived in the evening, which prevented cooling the following night. 3. Conclusion Figure 5. Surface synoptic charts for 1200 GMT on (a) 18 November, (b) 19 November and (c) 20 November 1996 (from Weather Log, Royal Meteorological Society). The response of the road after frontal passage on the synoptic scale has been shown to be far from simple. Cold air associated with complete cloud clearance would be expected to give the most rapid response. However, not all roads have a complete sky view, and precipitation and local topography can play a part in the cooling of the road. Devon has highly varying topography that encompasses a complex interaction between the synoptic and local scales. On partly cloudy 115

6 N L H Wood and R T Clark Figure 6. Data at Haldon Hill, November Figure 7. Data at Sourton Cross, November nights, radiative cooling and surface wetness across the region is variable. The winter maintenance engineer on these occasions is best advised to keep a close eye on the cooling curves in the first instance, and the satellite cloud imagery where available (see McLean & Wood, 1995). Gustavsson (1991) concluded that large air temperature changes due to warm air advection influenced 116 the road surface temperatures at stations where temperatures were initially lowest, e.g. valleys. It should be noted, however, that the case of warm air advection is simpler, since spatial variation in cloud cover can be less of a problem at a warm front. A wind speed above approximately 5 ms 1 at 3 m in Devon means that roadsurface temperatures are lower in elevated positions

7 Road-surface temperature at cold and occluded fronts Figure 8. Surface synoptic charts for 1200 GMT on (a) 5 March, (b) 6 March and (c) 7 March 1997 (from Weather Log, Royal Meteorological Society). compared with lowland sites (McLean & Wood, 1993). For lighter wind conditions the opposite is true. In simple terms, it is these cold sites which are most likely to cool below 0 C, but with a typical increase in wind speed at frontal passage, this is not always the case. Acknowledgement The authors wish to thank the Environment Department of Devon County Council for the road weather data and partial funding. Figure 9. Data at Sourton Cross, 5 7 March

8 N L H Wood and R T Clark References Gustavsson, T. (1991). Analyses of local climatological factors controlling risk of road slipperiness during warm-air advections. Int. J. Climatol., 11: Gustavsson, T. & Bogren, J. (1990). Road slipperiness during warm-air advections. Meteorol. Mag., 119: McLean P. J. & Wood N. L. H. (1993). Coastal influence on winter road surface temperatures in the county of Devon, UK. Transportation Research Record, No. 1387, , National Research Council, Washington DC. McLean, P. J. & Wood, N. L. H. (1995). The use of sequential satellite images in forecasting road-surface temperatures. Meteorol. Appl., 2: Shao, J., Swanson, J. C., Patterson, R., Lister, P. J. & McDonald, A. N. (1997). Variation of winter road-surface temperature due to topography and application of thermal mapping. Meteorol. Appl., 4: Thornes, J. E. & Shao, J. (1991a). A comparison of UK road ice prediction models. Meteorol. Mag., 120: Thornes, J. E. & Shao, J. (1991b). Spectral analysis and sensitivity tests for a numerical road surface temperature prediction model. Meteorol. Mag., 120: