The 0resund Experiment A Nordic Mesoscale Dispersion Experiment Over a Land-Water-Land Area

Transcription

1 The 0resund ExperimentA Nordic Mesoscale Dispersion Experiment Over a Land-Water-Land Area Abstract The atmospheric dispersion process and the modifications in the wind field across the 20-km-wide 0resund strait between Denmark and Sweden were studied in this project. The meteorological observational network extended over an 80-km-wide cross section through the 0resund. The dispersion process was investigated by carrying out SF tracer experiments, and the air trajectories were determined by tetroon flights. Emphasis was placed on the periods when the water surface was colder than the surrounding land, which in the experimental period typically occurred during daytime. Although the measuring program was intensified during the day, when tracer releases were carried out, a fairly large proportion of the meteorological measurements were carried out on a regular day-andnight basis throughout the whole experimental period. This yielded data for a large variety of meteorological situations. All data from the experiment will be made available for general use. 1. Introduction During and 1984 an experiment was carried out with the purpose of investigating the nature of the turbulence and atmospheric dispersion over a region with a relatively cold water and warm land surface. The goal was to obtain a data set that can be used to verify mesoscale models in a coastal environment. The campaign was carried out over the 0resund, the strait between Denmark and Sweden. The strait also gave name to the experiment. Emphasis was placed on measuring the meteorological parameters that control the atmospheric dispersion process. For this purpose use was made of a large proportion of the available meso-and micrometeorological instrumentation in the Nordic countries in addition to considerable contributions from non-nordic countries. The atmospheric dispersion and the advection process in the area were investigated by carrying out tracer experiments and by tetroon flights. The tracer measurements comprised time-averaged and instantaneous concentrations. From a scientific point of view the 0resund region is well suited for such an investigation because the coastline on both sides of the strait is rather straight, the two coastlines are almost parallel, and the land area is nearly flat. The width of the strait is about 20 km. The land east of 0resund is rather level, hilly farmland reaching a height of about 100 m (5-10 km) inland. On the western side of 0resund lies Copenhagen with its suburbs; this area is very flat but has high surface roughness due to the urban character. During the experimental period the water-surface temperature of the 0resund was a few degrees (C) less than the land temperature during daytime. The measuring campaign took place during the period 15 to During 4-10 a special intensive measuring program was carried out. The campaign was a joint effort of 15 institutions located 1985 American Meteorological Society Bulletin American Meteorological Society Sven-Erik Gryning Meteorology Section Ris0 National Laboratory DK-4000 Roskilde, Denmark in the Nordic countries (Denmark, Finland, Norway, and Sweden), and in Belgium, Germany, and in the Netherlands. In this paper the experiment will be outlined; scientific results will be presented later by the individual groups participating in the experiment. 2. Experimental design In the meteorological measuring program information was obtained about the flow field along an 80-km-long traverse passing through the 0resund. In the tracer experiments SF was released close to the upwind coastline and sampled in the 0resund, at the opposite coastline, and farther inland. All indications of time in the paper are given in Central European Time (CET). a. Meteorological measurements Figure 1 shows the locations where the meteorological measurements were carried out. The locations were chosen in order to document the large spatial variety of meteorological conditions: over land upwind from the coast, at the coast, in the 0resund, at the downwind coast, and inland downwind FIG. 1. The geographical location where the meteorological measurements were carried out. The numbers refer to 1) Ris0, 2) Sjaelsmark, 3) Aved^re, 4), 5) Charlottenlund, ) Margretheholm, 7) Kastrup Airport, 8) Middelgrunden, 9) Saltholm, 10), 11) Furulund, 12) Maglarp and 13) Borlunda. Jaegersborg is located 4 km northwest of position 5. Some of the cities in the area are shown. 1403

2 Vol., No. 11, November from the 0resund. A large variety of meteorological parameters were measured during the experiment; they are listed below. 1) Three-component Doppler sodars were used to measure the wind speed and direction in the lowest several hundred meters of the atmosphere. Locations 4, 7, 8, 10, 11, and 13 in Fig. 1 were equipped with a sodar. The sampling time was 10 min at positions 7, 11, 13 and 20 min at positions 4, 8, 10. 2) Radiosondes were launched from Jaegersborg (situated 4 km northwest of position 5) and from position 13 every six hours during the entire period and every three hours during the intensive measuring period as well as during tracer experiments. During tracer releases, minisondes (pressure, temperature, and wetbulb temperature) were launched from a fishing boat from various positions in the 0resund. 3) Microwave radiometer for GHz were used to estimate temperature profiles up to the tropospause every 20 min during cloudless conditions. Profiles were derived with support from a priori data from previous radiosondes. The microwave radiometer was installed at position 13. 4) Turbulence measurements were carried out at three levels in the 100-m mast at position 10 (hot wire, temperature, and humidity) during most of the campaign and at a 115-m height at position 4 (cup, vane, and vertical propeller; three components of the windvelocity fluctuations) during tracer releases. The signals from the instruments at both positions were digitized with a sampling rate of 1 Hz. 5) Profiles of wind and temperature were measured at tall masts (100 m) at positions 1, 10, 12; at small masts(~15 m) at positions 2,3,5,,10,and ll;andat two masts, one at the east and one at the west coast, both very close to the water, at position 9. In addition, humidity, pressure, and radiation (net and global) were measured at position 13 and a few other positions. ) The wind profile throughout the lowest 2 km of the atmosphere was determined from pilot-balloon flights. The balloons were launched from position 10. Information regarding temperature and humidity in the lowest 300 m of the atmosphere was obtained from tethered balloons. These measurements were carried out at positions 5 and 10. 7) During the intensive measuring period, the research vessel Aranda determined wind velocity, wind direction, temperature, and humidity in addition to the temperature of the water surface. Aranda moved between five positions in the 0resund, making six consecutive 10 min-averaged measurements at each position. Every three hours water waves were measured by a wave rider, positioned 7 km west of position 10. These measurements gave information about the significant wave height and the matching frequency of the water waves. 8) An airplane participated in four of the tracer experiments. The airplane performed east-west traverses at heights between 50 m and 300 m making continuous measurements of dissipation and temperature. One of the purposes of these measurements was to determine the height of the internal boundary layers due to the water and land discontinuities. The airplane program also included a south-north traverse over the 0resund at a constant-pressure surface in order to determine the baroclinicity in the area. 9) Satellite images from NOAA-7 and NOAA-8 were recorded twice a day (10 CET and CET) throughout the entire campaign. These images give information on the skin-surface temperature of the 0resund under cloudless conditions. 10) A special study of the transformation on the microscale of the air as it is advected from sea to land took place at position 10. Two masts, each with a dense profile instrumentation, were used, one near the coastline and the other 200 m inland. In addition, a large number of very small masts were put up. During selected periods flux measurements were carried out with a sonic anemometer. Additional parameters such as net radiation, surface temperature, and water temperature, etc. were recorded as well. b. Dispersion and advection The atmospheric dispersion was investigated by carrying out SF (sulphurhexafluoride) tracer experiments. The tracer was released at a 95-m height from the meteorological mast at position 10 during easterly winds and at a 115-m height from the mast at position 4 during westerly winds. A typical tracer-experiment release started at 0800 CET. The sampling then started at 1100 CET and stopped at 1200 CET. The institutions that took part in the tracer measurements intercompared SF standards during the campaign. Instantaneous tracer concentrations were measured using a van, an airplane, and a boat. The van, equipped with a gas chromatograph, drove around in the area searching for the tracer plume in the period between the start of the tracer release and the start of the sampling, thus guiding the setup of FIG. 2. Measured wind speeds at 100-m and 10-m heights through a cross section of the experimental area. The measurements were performed during tracer sampling on 5. The wind was from the east. The 10-m wind speed at is estimated from the wind profile between 50 m and 125 m as measured by the acoustic sounder.

3 Bulletin American Meteorological Society TABLE Characteristic meteorological conditions and times for the tracer experiments. Tracer Experiments Experiment (1984) Place of Release Start of ReleaseStart of SamplingStop of Sampling (GMT) Meteorology During Tracer Sampling Wind at at 115-m Height speed (ms- ) mean direction Wind at at 95-m Height speed (m-s ) mean direction Global Radiation at (W-m ) * Standard deviation of direction. the tracer-sampling units. At the start of the time-averaged tracer sampling, the van stopped and began measuring instantaneous tracer concentrations once per minute at a position in the middle of the plume. The airplane performed traverses in the middle of the 0resund perpendicular to the plume while measuring consecutive 1/2-s-averaged tracer concentrations. The traverses were made at typical heights of 50,100,150, and 200 m. In one of the experiments tracer concentrations were also measured from a fishing boat, making traverses in the 0resund. Time-averaged tracer concentrations were measured at ground level by automatic tracer-sampling units. The consecutive tracer samples by the various groups were taken in a way such that the total averaging time was one hour. The tracer-sampling units were positioned in an arc close to the water front, i.e., at positions where the plume was not influenced by the land, and in an arc about 2 km inland such that the tracer plume had already been influenced by the land. In some of the experiments more than two arcs were set up. The positions of the tracer-sampling units were separated by 1.5 degrees as seen from the release point. The advection in the area was investigated during the intensive phase with tetroon flights. A tetroon is a balloon designed to fly on an isopycnic surface after release. The tetroons were tracked by radar. They were launched near and were flown at a height of about 50 m. are constructed from measurements by sodar, radiosondes, and from tall and small masts. The easterly wind passes over rather smooth farmland east of at a speed of the order of 10 m s. When passing over the 0resund, the wind at 100 m gradually speeds up, reaching a maximum near the coast. Over urban Copenhagen a substantial decrease in the wind speed can be observed. The wind speed at 10 m has a pronouncedly different structure. Over the 0resund the wind speed first gradually increases, reaches a maximum near the eastern shore, and then begins to decrease. The decrease continues to the opposite shore. Over Copenhagen the 10-m wind remains nearly unchanged. The decrease over the 0resund is likely to be caused by the formation of the stable layer above the cold water surface. Over the level farmland west of Copenhagen the wind speed again increases. Figure 3 shows simultaneously measured temperatures in the cross section. The temperature at the 10-m level is seen to drop more than 3 C over the 0resund, but it raises very quickly to the previous level over the land downwind from the 0resund. The air is slightly unstable upwind from the 0resund, then it gradually turns stable as it passes over the cold water surface, reaching the most stable level at the downwind coastline. _1 3. The experiments A total of nine tracer experiments were carried out during the campaign. Two of the experiments belong to the intensive phase. Table 1 gives a few characteristic features of the meteorological conditions during the tracer releases. The periods of operation of the various instruments throughout the entire campaign can be found in Gryning (1985). Among the nine tracer experiments the one on 5 has already attracted attention. It was the last experiment during the intensive phase and by far the most densely instrumented with meteorological sensors as well as tracer samplers. The wind speed at 100-m and 10-m heights through a cross section of the experimental area is shown in Fig. 2. The curves FIG. 3. Potential temperature at 10 m and potential temperature difference between 10 m and 2-m heights through a cross section of the experimental area. The measurements were performed during tracer sampling on 5. The wind was from the east.

4 140 FIG. 4. Vertical profiles of potential temperature (dotted line), temperature (solid line), and dew-point temperature (dashed line) up to a 1000-m height, measured by a minisonde during the tracer experiment on 5. The sonde was launched from a position in the middle of 0resund between and Charlottenlund. This stable stratification is very quickly turned into an unstable one downwind from the 0resund. Figure 4 shows a sounding from the middle of the 0resund with the minisonde system. A stable layer of 50-m height can be observed together with an overlying inversion at 700 m. This stable layer near the ground is observed in all of the minisonde launches over the 0resund on that day, but not in the radiosondes from Jaegersborg and Borlunda. This is in accordance with Fig. 3. The development of the internal boundary layers caused by the temperature and roughness differences between land and sea surfaces are clearly seen in the measurements of the dissipation that were carried out by the airplane. Figure 5 shows the dissipation at a 270-m height over the 0resund region. The gradual lowering of the turbulence as the air passes the 0resund is clearly seen. The internal boundary layer from Copenhagen intersects the flight path a few kilometers inland, making itself known by a more than doubling of the dissipation. The change is seen to occur rather abruptly. Figure shows one-hour-averaged tracer concentrations, which in this experiment were measured in four arcs. The closest arc was 21 km and the most distant 30 km from the source. During the experiment the wind direction gradually FIG. 5. Turbulence measured by an airplane in a traverse at a 270-m height over the 0resund region on Turbulence was measured by a dissipation-rate sensor. The wind was easterly. Vol., No. 11, November 1985 turned the tracer plume towards the north, causing the steep increase in concentration on the northern fringe of the concentration distribution. The vertical tracer-concentration distribution, Fig. 7, was determined in the 0resund 11 km from. This was done by the airplane that performed traverses perpendicular to the plume while measuring consecutive 1/2-s-averaged tracer concentrations at heights of 50, 100, 150 and 200 m. Tracer concentrations were simultaneously measured from a boat that made traverses at the same distance from the release point as the airplane. The crosswind-integrated concentration profile, Fig. 7, shows a characteristic maximum at about a 50-m height. This elevated maximum is probably associated with the ground-level inversion that was observed above the 0resund water surface and with the downward vertical wind velocity due to the increase in the wind speed over the water. The trajectories of the tetroon-flights on 5 are shown in Fig. 8. The tetroons were launched from a point close to and were typically flown at a level of 700 m. The tetroon flights indicated a directional shear in the wind between 100 m and 700 m of roughly 10 degrees. Only a few of the measured data have been used in the illustrations of the experiment on 5. It is apparent from Table 1 that experiments have been carried out under a wide range of meteorological conditions. The data set, therefore, will be well suited to evaluate mesoscale models in a coastal environment. 4. Framework of the 0resund project The planning of the project was carried out by institutions from the Nordic countries with NORDFORSK (Nordic Cooperative Organization for Applied Research) acting as project coordinator. The following institutions from the Nordic countries participated in the measuring campaign: from Denmarkthe Air Pollution Laboratory of the National Agency of Environmental Protection, Danish Meteorological Institute, and Ris0 National Laboratory; from Finland Finnish Meteorological Institute, the Institute of Marine Research, and Department of Geophysics at Helsinki University; from NorwayNorwegian Institute for Air Re- FIG. 7. Vertical profile of crosswind-integrated tracer concentrations, determined from near-instantaneous tracer-concentration measurements carried out by an airplane and a boat. The traverses were made perpendicular to the plume 11 km from the release point at. From the experiment on 5.

5 Bulletin American Meteorological Society 1407 FIG.. Measured one-hour-averaged tracer-concentration profiles for the experiment on 5. The distance from the tracer-release point (, position 10) is indicated on the figure. Tracer-release rate was.2 g's - 1. search; from SwedenChalmers University of Technology, Department of Meteorology at Uppsala University, Studsvik Energiteknik AB, Swedish Meteorological and Hydrological Institute, Swedish Military Weather Service, and Swedish National Defence Research Institute. The following institutions from non-nordic countries participated in the measur- FIG. 8. Trajectories of the tetroon flights on Flight number 1 started at 0738 CET, mean flight level was 500 m; flight number 2 started at 0905 CET, mean flight level was 700 m; and flight number 3 started at 1035 CET, mean flight level was 750 m. ing campaign: from BelgiumS.C.K./C.E.N. Nuclear Energy Research Center; from GermanyKernforschungszentrurrt Karlsruhe; and from the NetherlandsFree University of Amsterdam. Acknowledgments. The support from all the scientists and groups that participated in the experiment is deeply acknowledged. The never-failing interest of Mari-Mai Lagus, NORDFORSK, is fully appreciated. The project received assistance from the Danish Post and Telecommunication, Directorate of Civil Aviation in Denmark and Sweden, Danish Air Force, Gentofte Kommune,and nuclear power plant. The experiment was made possible by grants or support from: in BelgiumS.C.K./C.E.N. Nuclear Energy Research Center; in DenmarkDanish Technical Research Council, Ministry of Energy, and National Agency of Environmental Protection; in FinlandFinnish Meteorological Institute, Institute of Marine Research, and Department of Geophysics at Helsinki University; in the NetherlandsFree University of Amsterdam; in NorwayNorwegian Institute for Air Research; and in SwedenNational Institute of Radiation Protection, Ringhals nuclear power plant, Studsvik Energiteknik AB, Swedish Board for Space Research, and Swedish Environmental Protection Board. Further support to the project was received from NORDFORSK and the Commission of the European Communities. References Gryning, S. E., 1985: Description of a recent Nordic mesoscale dispersion experiment over a land-water-land area (the 0resund experiment). Proc. Seminar on the Results of the Indirect Action Research Programme, Safety of Thermal Water Reactors ( ), Commission of the European Communities, Brussels,