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The use of weather radar in assessing deposition of radioactivity from chernobyl across England and Wales
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Atmospheric Enuironmenr Vol. 22, No. 9. pp. 189S-1900,19t38. Printed in Great Britain.
THE USE OF WEATHER RADAR IN ASSESSING DEPOSITION OF RADIOACTIVITY FROM CHERNOBYL ACROSS ENGLAND AND WALES H. M. APSIMON and K. L. SIMMS* Air Pollution Group, Mechanical Engineering Department, Imperial College, London, SW7 2AZ, U.K.
C. G. COLLIER Meteorological Office, Bracknell, Berkshire, RG 12 2SZ, U.K. (First received 28 Augusr 1987 and received for publicarion 14 March 1988) Abstract-Deposition of radionuclides from the Chernobyl accident depended critically on patterns of precipitation intercepting the material. This paper describes the use of the RAINPATCH model to calculate wet deposition of 13’Cs over England and Wales. This puff-based model makes direct use of precipitation data measured by weather radar to determine the scavengingof airborne material. The detailed spatial and temporal resolution of when and where material was scavenged provides good agreement with meaaurements. Since all the data used could potentially have been available at the time, such methods could usefully be applied in real time in the event of any future accident releasing such radionuclides. Key word index: Wet deposition, weather radar, Chernobyl, nuclear accidents, r3’Cs.
I. INTRODUCTION
The Chernobyl accident has emphasized the importante of wet deposition of radioactivity for the consequences of such a release. The difficult& of accounting for the patchy and intermittent nature of precipitation intercepting a cloud of radioactivity from a nuclear accident, and the shortcomings of assessment techniques used hitherto in nuclear risk analysis, have been discussed in previous Papers (ApSimon, 1986; ApSimon and Simms, 1986). Here it was pointed out that the problems of obtaining detailed and accurate information on precipitation could be solved by modem weather radar networks. The RAINPATCH model had been developed at Imperial College to exploit this data in assessing patterns of deposited contamination for hypothetical accidents. Following the accident, the weather radar data from the U.K. Meteorological Office have been assembled in conjunction with monitoring information supplied by the C.E.G.B. and used with the RAINPATCH model to simulate the passage of the Chernobyl cloud across England and Wales, and the associated deposition. This paper describes the results obtained, and illustrates the good agreement with the monitoring survey carried out by the Institute for Terrestrial Ecology. Since all the data used could in principle have been available in real time at the time of the
*Present address: Central Electricity Research Laboratories, Kelvin Avenue Leatherhead Surrey KT22 7SE, U.K.
accident, this demonstrates what a powerful tool such analysis could be in any future accident situation, should it occur, In the sections below the weather radar network is described and the data made available by the C.E.G.B. detailed. The meteorological situation, and simulation of the passage of the cloud using this data, are explained in sections 4 and 5. Section 6 shows the results obtained, and the agreement with measurements. Suggestions for future development within the context of emergency procedures for nuclear accidents within Europe are given in the conclusions.
2. THE WEATHER RADAR NEl’WORK OF THE UY. METEOROLOGICAL OFFICE
At present five weather radar operated by the U.K. Meteorological Office provide continuous monitoring over England and Wales (see Fig l), and there are plans to extend this network to give coverage over the whole of the U.K. (Ryder and Collier, 1988). The network provides valuable service to water authorities and others concerned with rainfall accumulation, flooding and storm damage. This paper illustrates its use in assessment of nuclear accident consequences. Each weather radar has a maximum range of 210 km. The radar signal is reflected back by rain droplets, and the strength of the reflected signal is related to the intensity of the precipitation. Difficulties can arise from the intersection of the radar beam with hills and other objects and also from the refraction of
1895
H. M. APSIMON
1896
vAt June 1964 75 km rodiur the radius of 2OOkm represents the extreme theoretical
range of a
-4: &
weather radar
et al.
due to the stratification of the atmosphere. Real-time adjustments are therefore made using data from a small number of telemetery raingauges to provide continuous calibration. The radars scan at four elevations every 5 min, and yield estimated precipitation for a grid with 2-km resolution within 75 km of the radar site and a 5 x 5 km grid beyond. The data obtained are relayed in real time to the Meteorological Office at Bracknell where a composite picture over the country is produced. The weather radar thus already provides a very detailed picture of rainfall over England and Wales, with a much finer resolution than the traditional hourly raingauge network. Typically the latter are placed40 km apart, which is large compared with the dimensions of intense rain cells. Moreover, the raingauge data are not routinely assembled and usable for maybe several weeks, whereas the weather radar data are available within a few minutes of the rainfall being observed. the beam
3. PASSAGE OF THE CHERNOBYL
CLOUD
ACROSS
THE U.K.
Fig. 1. The locations of the five U.K. weather radar systems installed by June 1984, and the identical system at Shannon in the Republic of Ireland. The outer circles show the 210-km maximum ranges of the radars and the inner circles give the 75km range within which it is possible to obtain quantitative estimates of intensity. The box gives the region covered by the composite data grid.
The Chernobyl accident happened at 1.23 local time on 26 April 1986 (21.0&23.04l GMT on 25 April), and the release continued with varying intensity for 10 days. Trajectory analysis (ApSimon et al., 1987) shows how just a portion of this release, lasting 2-3 h at the most, passed over the U.K. arriving early on 2 May (Fig. 2). This is important because it led to a relatively sharp peak in concentration advancing across the
Fig. 2. Trajectories from Chernobyl. Originating at 12.00 GMT on (A) 25, (B) 26, (C) 27, (D) 28 and (E) 29 April 1986. The dots are separated by periods of 3 h.
Assessing deposition of radioactivity from Chernobyl
1897
U.K., and to achieve a correct picture of the wet deposition it is necessary to assess correctly where and when this steep ripple of radioactivity encountered rain. Elsewhere, in Norway for example, where the material passed over continuously for a more pro-
longed period, there is a relatively good correlation between deposition of ’ 37Cs and the total rainfall over the 2 days of 28 and 29 April (Saltbones, personal communication). In the U.K. there is no corresponding agreement with total rainfall during 24 May when the cloud passed over, because of the much shorter duration of the peak-see Fig. 3. It was therefore important to resolve exactly when this sharp peak of contamination passed over England and Wales. Here data from C.E.G.B. nuclear power stations have proved most valuable. C.E.G.B. stations have monitoring systems consisting of an array of detectors surrounding the reactor site and capable of detecting any slight increase of radioactivity. The most sensitive of these (GEMS-Guardian Environmental Monitoring System) were capable of detecting arrival of the Chernobyl cloud quite clearly. Data were obtained for five stations, including three stations instrumented in this way, registering the time of passage of the peak concentrations. The sharpness of the peak, in which air concentrations rose and fell by an order of magnitude within just a few hours, is clearly illustrated in Fig. 3. The data provided a good indication of the time of arrival and duration of passage of the main part of the radioactivity at a band of locations across the country. Trajectory analysis (using the same techniques which gave the results illustrated in Fig. 2) was then used to give the time of passage of the peak across the rest of England and Wales. The results are consistent with observations elsewhere, and show the arrival of the cloud from the continent early on Friday 2 May and its departure across the Irish Sea by late afternoon of that day-see Fig. 4.
4. THE METEOROLOGICAL SITUATION
The meteorological situation over 1-3 May, when the main pass of radioactivity over the U.K. occurred, is illustrated in Fig. 5. The material was caught up in a frontal system with a centre of low pressure just to the southwest of the British Isles. The air motion was highly three-dimensional, with rapid updraughts associated with cells of intense precipitation; and an overall sloping ascent of warm air feeding upwards ahead of the cold front [see Browning (1985) for the structure of frontal systems]. The trajectories shown in Fig. 4 and used in this analysis were obtained by straightforward quasi-geostrophic methods, and it would be interesting to compare them with more complex ones obtained from windfields more representative of the frontal structure. Nevertheless, the two-dimensional trajectories applied are very easily and quickly calculated from standard
3
4
6
8
Fig. 3. Particulate concentration in air at Sellafield during May 1986.
Fig. 4. Measured Gamma data sites and RAINPATCH trajectories. All times GMT on 2 May 1986.
meteorological observations, and their use seems to be justified in this situation by the broad agreement between the results obtained and observations of deposited radioactivity.
5. USE OF THE RAINPATCH MODEL
The RAINPATCH model simulates a radioactive cloud as an assembly of puffs. Each puff follows an independent trajectory in a series of time steps, registering the precipitation encountered from the weather radar data and representing differential depletion across the puff as material is deposited. Simultaneously turbulent redistribution and spreading of
H. M. APSIMON et al.
1898 12 CC GMT I MAY 198ti
1200GMT
2 MAY 1966
1200 GMT 3 MAY 1986
Fig. 5. Synoptic
material are allowed for. A simple wash-out model is used for the removal in precipitation, as though the whole depth of the cloud is uniformly scavenged. The wash-out coefficient is varied with rainfall rate. In this paper results are given for 137Cs as the most important long-term nuclide using the empirical relation. A= lo-4JO.8s-’ has been used (J being the rainfall rate in mm h- ‘). This commonly used wash-out treatment is a gross simplifaction of the complex dynamic, physical and chemical processes involved in the removal of pollutants within the below precipitating cloud.
charts.
More complex models are also under development to study pollutant remova in different types of storms, and it is hoped to apply them to the analysis of the Chernobyl cloud over the U.K., but they would not be suitable for immediate use within an emergency situation with limited data and time constraints. Dry deposition can also be included. The deposition of 13’Cs is, however, dominated by the wet removal processes but for r311 dry deposition is of importance too. In simulating the Chernobyl cloud, an assembly of seven puffs in a row, moving along calculated trajectories synchronized to arrive at the locations in Fig. 4
Assessing deposition of radioactivity from Chernobyl
1899
at the observed times, was used to represent the advance of the main body of radioactivity across England and Wales. The quantity of “‘Cs reaching the U.K. was taken to be a 3-h segment of the material released during 26 April, for which figures were provided by the Soviet Authorities (U.S.S.R., 1986). With allowances made for losses en route, the 13’Cs calculated to have reached the U.K. was 8.4 x lOI4 Bq. This was divided between the seven puffs used to model the ridge. 6. ~TI~ATED
PATFERN OF DEPOSCTION OF ‘=Cs OVER ENGLAND AND WALES
The resulting estimated pattern of wet deposition using the RAINPATCH model and the weather radar data is shown in Fig. 6. It clearly indicates the highest levels of deposition over the northern half of Wales, the western coast of Cumbria and the Isle of Man, all of which experienced heavy rain during the passage of the cloud. Two periods of heavy rain which contributed significantly to the deposition are shown in Figs 7 and 8. Patches at lower levels also occur over the north coast of East Anglia and across central England. This may be contrasted with the total accumulated rainfall over the U.K. on 2 and 3 May, showing a quite differing distribution, with heavier rain over south Wales (Fig. 9), for example, which did not coincide with the passage of radioactivity. The calculated pattern of wet deposition may be compared with the pattern deduced from measurements by the Institute of Terrestrial Ecology (Horrill, personal communication) and shown in Fig. IO. The calculated results are in general higher since the quantity of ‘37Cs reaching the U.K. has probably been overestimated and the measured results are for grass
Fig. 7. Total rain recorded by the weather radar for the hour from 13.00 GMT 2 May 1986. Contour levels are 1mm and 7 mm.
Fig. 8. Total rain recorded by the weather radar for the hour from 16.00 GMT 2 May 1986. Contour levels are 1 mm and 7 mm.
and do not include any material washed off the grass into the soil. However, the agreement on the pattern of the highest areas of contamination within England and Wales is remarkably good. Unfortunately, comparison cannot be extended to Scotland since there is not yet weather radar coverage there. It will be interesting to see, as more caesium measurements are made, whether the agreement improves in areas where current measurements are fairly sparse. In this sense, the results may be useful to indicate areas where further measurements could profitably be made. 7. SUMMARY AND CONCLUSIONS
Fig. 6. Wet deposition results from the RAINPATCH model. Contour levels are z 5000 Bqm-’ (closehatched), - 1ooOBq rn-’ (hatched) and 100 Bq me2.
This paper has illustrated how well the pattern of Cs deposited in rainfall over England and Wales can be
1900
H. M. APSIMON et al.
deduced using simple computer models from weather radar data on precipitation and observations of passage of the cloud at a few stations. Since all the data could be potentially available in any future accident, the analysis illustrates how this would be a powerful technique to assist those responsible in any future accident, should it occur. It is felt that if such maps of estimated deposition had been available in the U.K. immediately after passage of the cloud they would greatly have assisted in monitoring and assessment by directing attention to the worst affected areas. Since Cs from Chernobyl will still be detectable for some time in soils, and studies of its transfer through ecological systems are being intensively pursued, it is hoped these relatively detailed results may be of use to these programmes of research. On a European scale, there are weather radars in several countries, including Finland (where the application to deposition from Chernobyl has also been demonstrated) (Rantalainen, personal communication), Sweden, The Netherlands, France, Switzerland and W. Germany. Exchange of these data in near realtime is being developed through the COST 73 programme of the European Community (Collier et al., 1988). At the U.K. Meteorological Office the FRONTIERS scheme (Conway and Browning, 1988) is being developed, which will enable radar and satellite data to be combined to infer rainfall patterns. Thus there is considerable scope within Europe for applying these techniques should they be necessary.
Fig. 9. Total rain recorded by the weather radar for the period 09.00 GMT 2 May 1986 to 09.00 GMT 3 May 1986. Contour levels are 2 mm and 10 mm.
Acknowledgements-This work was supported by the Nuclear installations Inspectorate. We are grateful to F. B. Smith and K. A. Browning of the Meteorological Office, to H. Macdonald of the C.E.G.B., to D. Horrill of the Institute of Terrestrial Ecology and to the other members of the Air Pollution Group at Imperial College for their assistance.
REFERENCES
Fig. 10. Measured 13’Cs on grass in Bq m-*. Produced the Institute for Terrestrial Ecology.
by
ApSimon H. M. (1986) The use of computers in emergency situations and choice of dispersion model. Proc. IAEA Symp., IAEA-SM-280112, pp 213-223. ApSimon H. M. and Simms K. L. (1986) Estimating the effects of rain and snow on potential reactor accident consequences. Nucl. Energy 25, 235-242. ApSimon H. M., Wilson J. J. N., Guirguis S. and Stott P. S. (1987) Assessment of the Chernobyl release in the immediate aftermath. Nucl. Energy 26, 295-301. Browning K. A. (1985) Conceptual models of precipitation systems. Met. Mag. 114, 293-319. Collier C. G., Fair C. A. and Newsome D. H. (1988) Weather radar meteorology in Western Europe--the COST-73 project. Bull. Am. met. Sot. (submitted). Conway B. J. and Browning K. A. (1988) Weather forecasting using interactive analysis of radar and satellite imagery. Phil. Trans. R. Sot. (in press). Ryder P. and Collier C. G. (1988) Future developments of the U.K. Weather Radar Network. Weather Radar and Flood Forecasting (edited by Collings V. K.). John Wiley, Chichester (in press). U.S.S.R. State Committee on the Utilization of Atomic Energy (1986). The accident at the Chernobyl Nuclear Power Plant and its consequences. Proc. IAEA Exports Meeting, 25-29 August 1986, Vienna.
Atmospheric Enuironmenr Vol. 22, No. 9. pp. 189S-1900,19t38. Printed in Great Britain.
THE USE OF WEATHER RADAR IN ASSESSING DEPOSITION OF RADIOACTIVITY FROM CHERNOBYL ACROSS ENGLAND AND WALES H. M. APSIMON and K. L. SIMMS* Air Pollution Group, Mechanical Engineering Department, Imperial College, London, SW7 2AZ, U.K.
C. G. COLLIER Meteorological Office, Bracknell, Berkshire, RG 12 2SZ, U.K. (First received 28 Augusr 1987 and received for publicarion 14 March 1988) Abstract-Deposition of radionuclides from the Chernobyl accident depended critically on patterns of precipitation intercepting the material. This paper describes the use of the RAINPATCH model to calculate wet deposition of 13’Cs over England and Wales. This puff-based model makes direct use of precipitation data measured by weather radar to determine the scavengingof airborne material. The detailed spatial and temporal resolution of when and where material was scavenged provides good agreement with meaaurements. Since all the data used could potentially have been available at the time, such methods could usefully be applied in real time in the event of any future accident releasing such radionuclides. Key word index: Wet deposition, weather radar, Chernobyl, nuclear accidents, r3’Cs.
I. INTRODUCTION
The Chernobyl accident has emphasized the importante of wet deposition of radioactivity for the consequences of such a release. The difficult& of accounting for the patchy and intermittent nature of precipitation intercepting a cloud of radioactivity from a nuclear accident, and the shortcomings of assessment techniques used hitherto in nuclear risk analysis, have been discussed in previous Papers (ApSimon, 1986; ApSimon and Simms, 1986). Here it was pointed out that the problems of obtaining detailed and accurate information on precipitation could be solved by modem weather radar networks. The RAINPATCH model had been developed at Imperial College to exploit this data in assessing patterns of deposited contamination for hypothetical accidents. Following the accident, the weather radar data from the U.K. Meteorological Office have been assembled in conjunction with monitoring information supplied by the C.E.G.B. and used with the RAINPATCH model to simulate the passage of the Chernobyl cloud across England and Wales, and the associated deposition. This paper describes the results obtained, and illustrates the good agreement with the monitoring survey carried out by the Institute for Terrestrial Ecology. Since all the data used could in principle have been available in real time at the time of the
*Present address: Central Electricity Research Laboratories, Kelvin Avenue Leatherhead Surrey KT22 7SE, U.K.
accident, this demonstrates what a powerful tool such analysis could be in any future accident situation, should it occur, In the sections below the weather radar network is described and the data made available by the C.E.G.B. detailed. The meteorological situation, and simulation of the passage of the cloud using this data, are explained in sections 4 and 5. Section 6 shows the results obtained, and the agreement with measurements. Suggestions for future development within the context of emergency procedures for nuclear accidents within Europe are given in the conclusions.
2. THE WEATHER RADAR NEl’WORK OF THE UY. METEOROLOGICAL OFFICE
At present five weather radar operated by the U.K. Meteorological Office provide continuous monitoring over England and Wales (see Fig l), and there are plans to extend this network to give coverage over the whole of the U.K. (Ryder and Collier, 1988). The network provides valuable service to water authorities and others concerned with rainfall accumulation, flooding and storm damage. This paper illustrates its use in assessment of nuclear accident consequences. Each weather radar has a maximum range of 210 km. The radar signal is reflected back by rain droplets, and the strength of the reflected signal is related to the intensity of the precipitation. Difficulties can arise from the intersection of the radar beam with hills and other objects and also from the refraction of
1895
H. M. APSIMON
1896
vAt June 1964 75 km rodiur the radius of 2OOkm represents the extreme theoretical
range of a
-4: &
weather radar
et al.
due to the stratification of the atmosphere. Real-time adjustments are therefore made using data from a small number of telemetery raingauges to provide continuous calibration. The radars scan at four elevations every 5 min, and yield estimated precipitation for a grid with 2-km resolution within 75 km of the radar site and a 5 x 5 km grid beyond. The data obtained are relayed in real time to the Meteorological Office at Bracknell where a composite picture over the country is produced. The weather radar thus already provides a very detailed picture of rainfall over England and Wales, with a much finer resolution than the traditional hourly raingauge network. Typically the latter are placed40 km apart, which is large compared with the dimensions of intense rain cells. Moreover, the raingauge data are not routinely assembled and usable for maybe several weeks, whereas the weather radar data are available within a few minutes of the rainfall being observed. the beam
3. PASSAGE OF THE CHERNOBYL
CLOUD
ACROSS
THE U.K.
Fig. 1. The locations of the five U.K. weather radar systems installed by June 1984, and the identical system at Shannon in the Republic of Ireland. The outer circles show the 210-km maximum ranges of the radars and the inner circles give the 75km range within which it is possible to obtain quantitative estimates of intensity. The box gives the region covered by the composite data grid.
The Chernobyl accident happened at 1.23 local time on 26 April 1986 (21.0&23.04l GMT on 25 April), and the release continued with varying intensity for 10 days. Trajectory analysis (ApSimon et al., 1987) shows how just a portion of this release, lasting 2-3 h at the most, passed over the U.K. arriving early on 2 May (Fig. 2). This is important because it led to a relatively sharp peak in concentration advancing across the
Fig. 2. Trajectories from Chernobyl. Originating at 12.00 GMT on (A) 25, (B) 26, (C) 27, (D) 28 and (E) 29 April 1986. The dots are separated by periods of 3 h.
Assessing deposition of radioactivity from Chernobyl
1897
U.K., and to achieve a correct picture of the wet deposition it is necessary to assess correctly where and when this steep ripple of radioactivity encountered rain. Elsewhere, in Norway for example, where the material passed over continuously for a more pro-
longed period, there is a relatively good correlation between deposition of ’ 37Cs and the total rainfall over the 2 days of 28 and 29 April (Saltbones, personal communication). In the U.K. there is no corresponding agreement with total rainfall during 24 May when the cloud passed over, because of the much shorter duration of the peak-see Fig. 3. It was therefore important to resolve exactly when this sharp peak of contamination passed over England and Wales. Here data from C.E.G.B. nuclear power stations have proved most valuable. C.E.G.B. stations have monitoring systems consisting of an array of detectors surrounding the reactor site and capable of detecting any slight increase of radioactivity. The most sensitive of these (GEMS-Guardian Environmental Monitoring System) were capable of detecting arrival of the Chernobyl cloud quite clearly. Data were obtained for five stations, including three stations instrumented in this way, registering the time of passage of the peak concentrations. The sharpness of the peak, in which air concentrations rose and fell by an order of magnitude within just a few hours, is clearly illustrated in Fig. 3. The data provided a good indication of the time of arrival and duration of passage of the main part of the radioactivity at a band of locations across the country. Trajectory analysis (using the same techniques which gave the results illustrated in Fig. 2) was then used to give the time of passage of the peak across the rest of England and Wales. The results are consistent with observations elsewhere, and show the arrival of the cloud from the continent early on Friday 2 May and its departure across the Irish Sea by late afternoon of that day-see Fig. 4.
4. THE METEOROLOGICAL SITUATION
The meteorological situation over 1-3 May, when the main pass of radioactivity over the U.K. occurred, is illustrated in Fig. 5. The material was caught up in a frontal system with a centre of low pressure just to the southwest of the British Isles. The air motion was highly three-dimensional, with rapid updraughts associated with cells of intense precipitation; and an overall sloping ascent of warm air feeding upwards ahead of the cold front [see Browning (1985) for the structure of frontal systems]. The trajectories shown in Fig. 4 and used in this analysis were obtained by straightforward quasi-geostrophic methods, and it would be interesting to compare them with more complex ones obtained from windfields more representative of the frontal structure. Nevertheless, the two-dimensional trajectories applied are very easily and quickly calculated from standard
3
4
6
8
Fig. 3. Particulate concentration in air at Sellafield during May 1986.
Fig. 4. Measured Gamma data sites and RAINPATCH trajectories. All times GMT on 2 May 1986.
meteorological observations, and their use seems to be justified in this situation by the broad agreement between the results obtained and observations of deposited radioactivity.
5. USE OF THE RAINPATCH MODEL
The RAINPATCH model simulates a radioactive cloud as an assembly of puffs. Each puff follows an independent trajectory in a series of time steps, registering the precipitation encountered from the weather radar data and representing differential depletion across the puff as material is deposited. Simultaneously turbulent redistribution and spreading of
H. M. APSIMON et al.
1898 12 CC GMT I MAY 198ti
1200GMT
2 MAY 1966
1200 GMT 3 MAY 1986
Fig. 5. Synoptic
material are allowed for. A simple wash-out model is used for the removal in precipitation, as though the whole depth of the cloud is uniformly scavenged. The wash-out coefficient is varied with rainfall rate. In this paper results are given for 137Cs as the most important long-term nuclide using the empirical relation. A= lo-4JO.8s-’ has been used (J being the rainfall rate in mm h- ‘). This commonly used wash-out treatment is a gross simplifaction of the complex dynamic, physical and chemical processes involved in the removal of pollutants within the below precipitating cloud.
charts.
More complex models are also under development to study pollutant remova in different types of storms, and it is hoped to apply them to the analysis of the Chernobyl cloud over the U.K., but they would not be suitable for immediate use within an emergency situation with limited data and time constraints. Dry deposition can also be included. The deposition of 13’Cs is, however, dominated by the wet removal processes but for r311 dry deposition is of importance too. In simulating the Chernobyl cloud, an assembly of seven puffs in a row, moving along calculated trajectories synchronized to arrive at the locations in Fig. 4
Assessing deposition of radioactivity from Chernobyl
1899
at the observed times, was used to represent the advance of the main body of radioactivity across England and Wales. The quantity of “‘Cs reaching the U.K. was taken to be a 3-h segment of the material released during 26 April, for which figures were provided by the Soviet Authorities (U.S.S.R., 1986). With allowances made for losses en route, the 13’Cs calculated to have reached the U.K. was 8.4 x lOI4 Bq. This was divided between the seven puffs used to model the ridge. 6. ~TI~ATED
PATFERN OF DEPOSCTION OF ‘=Cs OVER ENGLAND AND WALES
The resulting estimated pattern of wet deposition using the RAINPATCH model and the weather radar data is shown in Fig. 6. It clearly indicates the highest levels of deposition over the northern half of Wales, the western coast of Cumbria and the Isle of Man, all of which experienced heavy rain during the passage of the cloud. Two periods of heavy rain which contributed significantly to the deposition are shown in Figs 7 and 8. Patches at lower levels also occur over the north coast of East Anglia and across central England. This may be contrasted with the total accumulated rainfall over the U.K. on 2 and 3 May, showing a quite differing distribution, with heavier rain over south Wales (Fig. 9), for example, which did not coincide with the passage of radioactivity. The calculated pattern of wet deposition may be compared with the pattern deduced from measurements by the Institute of Terrestrial Ecology (Horrill, personal communication) and shown in Fig. IO. The calculated results are in general higher since the quantity of ‘37Cs reaching the U.K. has probably been overestimated and the measured results are for grass
Fig. 7. Total rain recorded by the weather radar for the hour from 13.00 GMT 2 May 1986. Contour levels are 1mm and 7 mm.
Fig. 8. Total rain recorded by the weather radar for the hour from 16.00 GMT 2 May 1986. Contour levels are 1 mm and 7 mm.
and do not include any material washed off the grass into the soil. However, the agreement on the pattern of the highest areas of contamination within England and Wales is remarkably good. Unfortunately, comparison cannot be extended to Scotland since there is not yet weather radar coverage there. It will be interesting to see, as more caesium measurements are made, whether the agreement improves in areas where current measurements are fairly sparse. In this sense, the results may be useful to indicate areas where further measurements could profitably be made. 7. SUMMARY AND CONCLUSIONS
Fig. 6. Wet deposition results from the RAINPATCH model. Contour levels are z 5000 Bqm-’ (closehatched), - 1ooOBq rn-’ (hatched) and 100 Bq me2.
This paper has illustrated how well the pattern of Cs deposited in rainfall over England and Wales can be
1900
H. M. APSIMON et al.
deduced using simple computer models from weather radar data on precipitation and observations of passage of the cloud at a few stations. Since all the data could be potentially available in any future accident, the analysis illustrates how this would be a powerful technique to assist those responsible in any future accident, should it occur. It is felt that if such maps of estimated deposition had been available in the U.K. immediately after passage of the cloud they would greatly have assisted in monitoring and assessment by directing attention to the worst affected areas. Since Cs from Chernobyl will still be detectable for some time in soils, and studies of its transfer through ecological systems are being intensively pursued, it is hoped these relatively detailed results may be of use to these programmes of research. On a European scale, there are weather radars in several countries, including Finland (where the application to deposition from Chernobyl has also been demonstrated) (Rantalainen, personal communication), Sweden, The Netherlands, France, Switzerland and W. Germany. Exchange of these data in near realtime is being developed through the COST 73 programme of the European Community (Collier et al., 1988). At the U.K. Meteorological Office the FRONTIERS scheme (Conway and Browning, 1988) is being developed, which will enable radar and satellite data to be combined to infer rainfall patterns. Thus there is considerable scope within Europe for applying these techniques should they be necessary.
Fig. 9. Total rain recorded by the weather radar for the period 09.00 GMT 2 May 1986 to 09.00 GMT 3 May 1986. Contour levels are 2 mm and 10 mm.
Acknowledgements-This work was supported by the Nuclear installations Inspectorate. We are grateful to F. B. Smith and K. A. Browning of the Meteorological Office, to H. Macdonald of the C.E.G.B., to D. Horrill of the Institute of Terrestrial Ecology and to the other members of the Air Pollution Group at Imperial College for their assistance.
REFERENCES
Fig. 10. Measured 13’Cs on grass in Bq m-*. Produced the Institute for Terrestrial Ecology.
by
ApSimon H. M. (1986) The use of computers in emergency situations and choice of dispersion model. Proc. IAEA Symp., IAEA-SM-280112, pp 213-223. ApSimon H. M. and Simms K. L. (1986) Estimating the effects of rain and snow on potential reactor accident consequences. Nucl. Energy 25, 235-242. ApSimon H. M., Wilson J. J. N., Guirguis S. and Stott P. S. (1987) Assessment of the Chernobyl release in the immediate aftermath. Nucl. Energy 26, 295-301. Browning K. A. (1985) Conceptual models of precipitation systems. Met. Mag. 114, 293-319. Collier C. G., Fair C. A. and Newsome D. H. (1988) Weather radar meteorology in Western Europe--the COST-73 project. Bull. Am. met. Sot. (submitted). Conway B. J. and Browning K. A. (1988) Weather forecasting using interactive analysis of radar and satellite imagery. Phil. Trans. R. Sot. (in press). Ryder P. and Collier C. G. (1988) Future developments of the U.K. Weather Radar Network. Weather Radar and Flood Forecasting (edited by Collings V. K.). John Wiley, Chichester (in press). U.S.S.R. State Committee on the Utilization of Atomic Energy (1986). The accident at the Chernobyl Nuclear Power Plant and its consequences. Proc. IAEA Exports Meeting, 25-29 August 1986, Vienna.