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Interaction between a natural snowfall and a cooling tower plume: An experimental study with a millimetric Doppler radar
ARintcd
r%ohnmmt
Vol.
21. No.
6. pp. 13754383.
ooo4-6981/87 Pergmon
1987.
in Orut Briuin.
13.00+0.00 Joumalr Ltd.
INTERACTION BETWEEN A NATURAL SNOWFALL AND A COOLING TOWER PLUME: AN EXPERIMENTAL STUDY WITH A MILLIMETRIC DOPPLER RADAR BERNARD CAMPISTRON
Centrc de Recherches Atmospheriques, Obscrvatoire Midi-Pyretrees, Campistrous, 65300 Lannemezan, France
(First received 11 May 1986 and received for publication 24 November 1986) Abstract-A precipitation band, about 30 km long and 2 km wide, downwind of a nuclear power plant was observed with a millimetric Doppler radar. The three-dimensional radar analysis showed that this band resulted from the growth, by a seeder-feeder process, of a natural snowfall falling in the moist plume produced by the exhaust of the nuclear plant. At the exit from the 0.6 km deep plume, the mean snow precipitation rate was approximately enhanced by a factor of two. This corresponds to an extraction rate of water in the plume by snow scavengingof 600 kgs- ‘, that is about one third of the water injection rate into the atmosphere by the cooling towers.
Key word index: Plume impact, snowfall modification, seeder-feeder process, plume visualization, Doppler radar, nuclear power plant, cooling tower plume.
1. INTRODUCTION
2. EXPERIMENTAL
The cooling towers of power plants inject in the atmosphere large quantities of heat and moisture; the potential meteorological effects have been studied experimentally and theoretically (for example: Hodin, 1982; Hanna and Gifford, 1975; Huff, 1972). Several snowfalls downwind of power plants were observed by Otts (1976) and Kramer et al. (1976). They suggested that these snowfalls were induced by the cloudy plumes of the cooling towers. In particular, the observations of Kramer et al., supported by in-sifu aircraft measurements, showed without a doubt the formation of snow in a plume extending to 1.6 km altitude and - 18” C. In this case however the ice nuclei, generally in largeconcentration (Parungo ef al., 1978) in the smoke produced by coal-fired power plants, certainly contributed heavily to the formation of ice particles. To explain these results, Koenig (1981) developed a numerical model which accounts for the existence of such a phenomenon and gives the meteorological conditions for the formation of snowfalls in wet plumes. The experimental study discussed here presents a radar observation of a snowband located downwind of the cooling towers of a nuclear plant. The radar observations, producing a fine scale three-dimensional description of the phenomenon, permit the identification of the origin of the precipitation and of the meteorological impact of the wastes from the nuclear plant.
CONDITIONS
In February 1983, a field project was launched by the Direction des Etudes et Recherches of E.D.F. (Electriciti de France) at the nuclear power plant of Dampierre-en-Burly. The objective of this 3-week field project was to study the local environmental impact of the heat and humidity wastes of the plant. The experimental system comprised principally an aircraft equipped for microphysic and dynamic measurements, a lidar, a rawindsonde station and the millimetric Doppler radar RABELAIS of the Observatoire MidiPyrtnees (O.M.P.). The latter three systems were located 4.5 km to the east of the nuclear plant (Fig. 1). The nuclear plant of Dampierre-en-Burly comprises four reactors each producing a thermal energy of 2785 megawatts yielding a net electrical energy of 900 megawatts. The cooling of the reactors is performed with water from the Loire river which is then injected
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l
NUCLEAR
\ NORTH
COOLING
Ikm
TOWER
Fig. I. Dampierre-cn-Burly experimental area.
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BERNARDCAMPISTRON
into the atmosphere as water vapour and condensed water forming a cloudy plume at the exit of the four cooling towers. The average water drop diameter, following filtering, is less than 20 pm. The water output of each tower corresponds to a flow rate of 0.5 m3 s- I. The height of each tower is 165 m, the base diameter is 135 m and the top diameter is 85 m. They are located in two pairs about 800 m apart. The main contribution to this study is the RABELAIS radar whose characteristics are the following: a wavelength of 8.6 mm, a peak power of 75 K W, a pulse length of 90m, and a beamwidth of 0.4”. The minimum detectable signal corresponds to a reflectivity factor of - 20 dBZ at a distance of IO km. The mean Doppler velocity is computed with the Pulse Pair Processor technique; the use of a double Pulse Repetition Frequency of 3125 and 2688 Hz permits the removal of velocity ambiguities over a range of +40ms-‘. The accuracy of the measurements of the Doppler velocity and of the reflectivity is 0.15 ms- ’ and 1 dBZ, respectively. The radar was calibrated with targets of known back-scattering cross-sections.
3.
day t * - 2’C)and weak precipitations of intermittent snow of small crystals. A vertical temperature and humidity sounding was launched from the field site at 1316 GMT. In particular, it shows that the air is saturated up to 1 km height which is favorable to the formation of a cloudy plume. However, since this sounding did not produce any data between 1.1 and 2.2 km height, the Trappes (located 120 km to the N of the field site) sounding of 1200 GMT is shown on Fig. 2a. This sounding reveals a moist potential instability in the lower layers up to about 1.6 km at the top of a temperature inversion. The wind distrjbution as a function of height shown on Fig. 2b was obtained with the VVP method (Waldteufel and Corbin, 1979) from the Doppler velocities measured with the radar at 0824 GMT. The figure shows a wind velocity maximum of 8 m s- ’ located at 0.3 km; above, the velocity decreases with height to a minimum of 3 m s-’ at 2.5 km height. Also, (4
GENERAL DESCRlPTtON OF THE CASE STUDY
Ofall the observations made during the field project, those of 9 February 1983, presented here, correspond to the clearest plume-environment interaction. The data were collected over more than I h by the RABELAIS radar. (a) The radar data The analysis of this interaction is based on a two volume scan of the radar. The first observation was obtained at 0824 GMT with panoramic sweeps of 38 km radius around the radar formed by a series of PPI scans between 1.5”and 18” by steps of 1” elevation angle; the radial resolution is 300 m and the azimuthal resolution is 0.74”. The second data set, acquired at 0844 GMT, consists of a series of RHI scans at 2” azimuth interval including the location of the plume over a radial distance of24 km and with a resolution of 150 m. The elevation sector extends from 0” to 27” in steps of 0.25”. The parameters measured by the radar, reflectivity and Doppler quantities, are inte~olated in a regular cartesian grid which permits the reconstitution of the data fields in horizontal and vertical planes. The value at a grid point is computed from a bilinear interp olation of the closest eight points radar data located on four radials with different elevation and azimuth angles (Mohr and Vaughan, 1979). This method has the advantage of preserving a resolution identical to that of tho radar acquiring the data. (b) Meteorological conditions The meteorological situation on 9 February 1983 is characterized by a moderate and cold NE flow giving rise to negative surface temperatures during the whole
5
(b)
--o-- DIRECTION 350° dD loo 20” 300 &$ I I I I I II
-e-
VELOCITY(ms-1)
Fig. 2. (a) Temperature (T) and dew point Cr,) sounding obtained on Y February 19X3 at I200 GMT from the Trappes station IOeated 120km north of Dampierre-en-Burly power plant. (b) Verticaldistribution of the wind velocity and direction derived from the Doppicr radar vciocitieson 9 Fchruary 1983a\ 0824 GMT
Interaction between a natural snowfall and a cooling tower plume the wind shear increases above the top of the temperainversion at 1.6 km height as the wind direction shifts rapidly from NNE to N at 2.5 km.
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(4
ture
LA.
(c) Local distribution of precipitations The echo patterns, observed at 0824 GMT with the RABELAIS radar, are shown in Fig. 3 as horizontal planes at various heights. The figure shows that the nuclear plant is nearly totally embedded in precipitations which cover almost entirely the area scanned by the radar over a radius of 38 km. In these reflectivity fields, Fig. 3a shows very clearly at 0.5 km height a narrow band of precipitation (boxed on the figure) located downwind from the nuclear plant, extending about 30 km in a direction parallel to that of the wind and converging on the nuclear plant. This precipitation band, related to the cooling towers, is also apparent on the horizontal reflectivity plane at 1 km height (Fig. 3b). At this level, echoes have a tendency to align parallel to the wind. Of all these banded structures, one is much more interesting to study in details for this discussion; it is located along the AB axis, shown on Fig. 3b at a distance of 7 km, and parallel to the precipitation band associated with the cooling towers. This band AB, sufficiently displaced from the power plant and not downwind from it, cannot be explained by the cooling wastes. A vertical plane along this band AB is presented on Fig. 4a. It shows that this band AB was formed by acharacteristic snow trail falling from generating cells extending vertically from 2.5 km to 4.5 km. Figure 4b shows a vertical plane of the reflectivity field perpendicular to both band AB and to the precipitation band associated with the nuclear plant. These two structures are included in overall precipitation and they appear to be connected and of identical characteristics. They are only distinguishable because of the greater reflectivity, of the order of IO dBZ, for the nuclear plant precipitation band.
(W
4. PLUME TRAJECTORY
In order to understand the role played by the plume from the cooling towers, in the formation of the precipitation band observed downwind of the plant it is imperative to know the behaviour of the plume and in particular the height of its top. However this information, unfortunately, is not available; moreover, tests have shown that the plume of condensed water is usually not detectable with the RABELAIS radar at distances greater than 0.5-1.0 km from the cooling towers. To obtain the trajectory and the behaviour of the plume, the Doppler radar data were used. Indeed, the cloudy plume, resulting from the waste water, is also carrying an important quantity of thermal energy waste which is capable of disturbing the natural dynamic field and as a consequence, to appear in the Doppler velocity of the snow scatterers. In order to
REFLECTIVITY FACTOR -I?___0 3 6 12 27 :.:.... . dBZ ----SCAN BOUNDARY o RADAR A POWER PLANT Fig. 3. Reflectivity factor patterns in horizontal planes at 0.5 km (a)and 1km (b) height, on 9 February 1983 at 0824 GMT.
characterize these dynamic perturbations, to be used as tracers of plume behaviour, the quantity A V defined below, was calculated for each radial velocity measurement:
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BERNARDCAMPISTRON
(4
(a) -6
-4
I
1
-2 X(km)
I
I
1
09FEB83
-20
-10
0
lO(km)
0
_
20
-8-
(b)
HEIGHT: 0.6km
__-___ l-----l
, 5 TEMPERATURE INVERSION . __-__________-_-__---~-
LAYER
OSFEB83
z
08H24GMT
t
Y: -3 km ,---.
‘;l
-30
-20
-10
0
10
20fkm) 30
Fig. 4. ReRectivity factor patterns at 0824 GMT in vertical planes oriented along AB axis (a) and CD axis (b) shown in Fig. 3b. In Fig.4b, the vertical cross-sections corresponding to the nuclear plant precipitation band and to the band AB are boxed, respectively with a solid and a dashed line. The refiaztivity factor scale and the symbols are identical as in Fig. 3.
where V, + , and V, are the Doppler velocities data on consecutive measurements which are 300 m apart along a radial for the 0824 GMT radar acquisition. AV was interpolated in a cartesian grid. Characteristic planes, for the 0824 GMT radar acquisition, are shown on Fig. 5. On these planes, contours of A V equal or greater than 0.6 m s- ’ are only shown. The threshold of 0.6 m s- I corresponds to the maximum intensity of natural environmental dynamic perturbations. In the horizontal plane at 600 m, shown in Fig. 5a, the organization of the A V contours, stretching along the windfieid and downwind from the nuclear plant, accounts rather clearly for the origin of these perturbations. The maximum A V values, reaching 3 ms-‘, are found near the nuclear plant. The horizontal distribution ofthis parameter reveals that, in fact, there
-6
-8
-4
-2Xtkm)O
(‘)
1 I 1... I I 1 I I _______ :,‘____ -________
08H24GMT
’
c
’
-8
----
’ -6
0.6
.--.
’
’
’
-4
AV (ms-I) 1.2
-2Yfkm)O
m
> I.6
Fig. 5. Contours of the Doppler velocity perturbations AVfield at 0824 GMT. (a) In a horizontal plane at 0.6 km height; (b) in a vertical plane perpendicular to the plume; (c) in a vertical plane along the plume.
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Interaction between a natural snowfall and a cooling tower plume are two distinct dynamic plumes (A and B) each one axially aligned on each pair of cooling towers. The vertical plane, Fig. 5b, obtained perpendicularly to the wind at a distance of 1.5 km downwind from the nuclear plant shows that plume A reaches a height of 1 km whereas plume B reaches only a height of 0.6 km. This difference in height may be due to the fact that plume A is associated with two cooling towers which are aligned along the wind direction. As a result, the respective wastes of these towers have vertical effects more efficiently cumulated than in the case of plume B produced by two towers which are oriented transversally to the wind direction. Figure 5c presents the A V pattern in a vertical plane parallel to the mean wind and along the mean horizontal axis of plume A. The distribution of the dynamic perturbations in this plane describes the typical trajectory behaviour of a plume which rises in the atmosphere, curves under the entrainment of the horizontal wind and spreads around the 1.6 km level at the top of the temperature inversion. The plume reaches this blocking layer at a distance of about 3.5 km from the power plant. The mean ascent velocity of the effluents can thus be estimated to be approximately 3 ms-r when using the horizontal winds given in Fig. 2b. The dynamic perturbations A Vdecrease in the mean as the distance from the nuclear plant increases; at distances greater than 9 km, the A V values are smaller than 0.6 m s-t and blend with the natural perturbations. The dynamic impact of the plume is then weak and is no longer detectable by the radar, which does not necessarily mean that the plume is no longer present. The AV fields were also computed for the 0844 GMT radar acquisition (Campistron, 1985). The results, not presented here, show nearly identical features.
On such a fine scale, a certain heterogeneity of the band, shown as distinct cores of greater reflectivity, is readily apparent. The explanation of the formation of this precipitation band is given on Figs 6c and 6d which present the reflectivity patterns in vertical planes oriented longitudinally along the mean axis of the band and crossing the power plant. It appears that the precipitation band results mainly from a snow trail similar to
04 5.
PLUME-ENVIRONMENT
INTERACTION
-8
-4
0
PROCESS
Having established the three-dimensional position of the plume, at least near the source of ertluents, the potential impact of the wastes on the formation of the precipitation observed downwind of the nuclear plant can now be analyzed. To do so, Fig. 6 shows reflectivity fields at 0824 and 0844 GMT in typical horizontal and vertical planes obtained near the nuclear plant. On some of these planes, the location of the plume obtained from the results of the analysis described in the previous paragraph is schematized. However, at distances greater than 9 km from the nuclear plant, the plume location is obtained by extrapolation and is thus only approximate. (a) Origin of the precipirarion band Both radar reflectivity measurements acquired at 20 min intervals and presented in horizontal planes in Figs 6a and 6b show the narrow band of precipitation about 2-3 km wide downwind from the nuclear plant.
09 FE8 I33 08H44GMT
Fig. 6 (a)_(b).
I
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BERNARDCAMPISTRON
0 Yfkml
(4
-4 . . .. .
3
-*-*
6
-
tX>6
I2 15
XIkm)
0
REFLECTIVITY FACTOR (dBZ1
m>21 _.a_
-10
-20
4
Xikml
Y (km1 0
0
Fig. 6 (c)-{e).
that associated to band A3 in Fig. 4a. The lack of radar
data at large elevation angles prevents the localization of the origin of these snow precipitations. However, the principle of continuity and the comparison with band AB which is in close proximity, suggest that these precipitations originate in the layer of generating cells,
SCAN or ECHOES BOUNDARY
Fig. 6. Reflectivity factor fields observed with the RABELAIS radar at 0824 GMT (a,c,c)ond at 0844 GMT (b,d,f) in horizontal planes at 0.6 km height (a, b) and in vertical planes atong the plume (c.d) and perpendicular to the plume (e,f). The schematic position of the plume is derived from the velocity perturbations AV fields. Far distances greater than 9 km fram the cooling towers, the plume positions result from an extrapolation.
with bases at 2.5 km height, upwind of the nuclear plant. Moreover the figures show, considering the ~haviour of the plume trajectory, that these cells cannot result from an artificial triggering of convection produced by the wastes of the cooling towers. During their fall from the level of generation these pr~pi~t~o~ are blown by the horizontal wind across the volume filled by the cloudy plume. The heaviest precipitations reach the ground about 13 km from the nuclear plant. The slope of the trail of snow is a function of the wind distribution with altitude, of the fall velocity of the snow crystals and of the translation velocity of the generating cells. In the vertical plane at 6.5 km from the plant and ~~ndicular to the band, shown in Fig. 6e, the precipitation at a mean height of 1.3 km is 0.5 km deep with at least a 10 km across wind. This figure shows that the plume is embedded in the proportion.
(b) Verticaf distribution oJ’reflectiviries It then appears that the precipitation band originates from a natural process which, by coincidence, occurred in the vicinity of the nuclear plant. At this
Interaction between a natural snowfall and a cooling tower plume
stage in the analysis and in spite of a number of clues, the effects of the cooling tower e&tents on the observed phenomenon cannot be delineated with any certainty. In order to remove this uncertainty, the vertical distributions of the mean and maximum radar reflectivity factor in horizontal slices 100 m deep were calculated between 0.3 and 3.0 height. These calculations were performed in three distinct volumes; the first one, whose horizontal cross-section is shown in Fig. 3a, comprises the power plant precipitation band and consequently the plume. The other two volumes are control volumes which are: the total area analyzed by the radar within the 38 km range except the previous volume, and the volume comprising the precipitation of snow related to band AB of Fig. 4a. The results are presented in Fig. 7 for the radar observation of 0824 GMT. These distributions reveal a characteristic height (4 -
PLUME
---ENVIRONMENT
O-
REFLECTIVITY
FACTOR (dB2)
04 -
PLUME
O10
---SNOW
REFLECTIVITY
TRAIL
AB
FACTOR (dB2)
Fin. 7. Comparisons between vertical distributions of the maximum and mean nfleetivity factors at 0824 GMT calculated in a volume containing the plume and in control volumes comprising (a) the environment and (b) the snow trail AB shown in Fig. 4a.
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located at about 1.6 km, the level of the top of the temperature inversion, below which the maximum and mean reflectivity factors in the area of the plume are in most cases greater than those in the control volumes. Moreover, they show that, between approximately 1.6 and 1.0 km height, the maximum reflectivity factor in the area of the plume rises by about 10 dBZ. Below 1 km height, the distribution practically in the average no longer varies. The reflectivity values, on Fig. 7, are large since they correspond*to precipitation echoes and not to cloud echoes. (c) Interaction mechanism These distributions lead to the conclusion that the effect of the effluents of the cooling towers, most compatible with all radar observations, results from a seeder-feeder type mechanism. The light trail of natural snow crystals generated at higher altitude intensifies as it falls through the ‘feeder’ cloudy plume between 1 and 1.6 km height. In this interaction zone, 0.6 km thick, the precipitation seeds the plume with ice crystals which will grow by riming, aggregation or water vapor deposition. In the dynamically active parts of the plume, mainly in the ascending region, turbulent motions will improve the collection efficiency and updrafts will carry upward natural snow crystals which will fall back to ground after growth. Indeed, the microphysical processes and their potential contributions are varied and complex and depend on the dynamic and the thermodynamic state of the plume and on the type of the seeding particles. In addition to the main snow precipitation trail, still lighter precipitations feed the plume and explain the 30 km length of the precipitation band. On Fig. 7b, the distribution of the maximum reflectivity factor of the light snowfall AB of Fig. 4a shows that this precipitation, once formed, in the average no longer varies as it falls to the ground. A similar observation could presumably be made in the case of the snow trail downwind of the cooling towers if there had been no plume. Indeed, these two snowfalls have the same origin, have identical reflectivity profiles above 1.6 km height and are in close proximity, as shown in Fig. 4b. In conjunction with the seeder-feeder mechanism, it is possible, as shown by Koenig (1981) with a numerical simulation, that the plume generated snow precipitation directly. In this case study presented here, if this mechanism occurred at all, it must be negligible compared to the seeder-feeder growth process proposed above. Indeed, according to Koenig, the production of appreciable precipitation requires a plume temperature below - 13°C. However in our case (Fig. 2a) the minimum temperature in the plume is - 12.5”C. In addition, the two radar observations of Fig. 6 show that the precipitation band at 0844 GMT is smaller and of weaker reflectivity than that measured 20 min earlier. This decrease in activity in the band is well correlated with a general decrease in reflectivity in
1382
BERNARD CAMPISTRON
the environment and, in particular, in the snow trail above 1.5 km. This correlation confirms the strong link between the natural snow trail and the power plant precipitation band. If the snow directly produced by the plume had been the primary cause of the phenomenon observed, it should have compensated for the decrease in activity in the natural snow trail.
6. PLUME-ENVIRONMENT
INTERACTION: QUANTI-
TATIVE RESULTS
Quantitative results of the interaction between the plume and the snow trail can be obtained by using conversion equations relating the radar reflectivity factor to physical quantities. However, since the characteristics of the scatterers are not readily known, the results obtained are only estimates. The following conversion relations, derived by Sekhon and Srivastava (1970) for agglomerated snow crystals, were used. R -- 0034 Z”.4S .
and
wide. This rate corresponds to about one third of the 2OOOkg s- ’ of water injected in the atmosphere by the cooling towers. Locatelli et al. (1983) reported on a case of a natural seeder-feeder process forming in a 1 km thick stratocumulus; in their observation the mean precipitation rate at the entrance into the layer was 0.01 mm h _ ’ and between 0.03 and 0.11 mm h _ I at the exit. These results are in good agreement with those reported here obtained with mean interaction values. For the maximum interaction values, an estimate of the growth parameters of the falling particles can be obtained. It is assumed that the particles are spherical and grow in a continuous mode by accreting particles in the plume. The following equation, relating the diameter di at the entrance into to the diameter d, at the exit from the plume is derived according to Pruppacher and Klett (1978): d,=d,+---
Ah.E,M
2P
M = 0.0135 Z”.”
where the reflectivity factor Z is expressed in mm6 m - 3, the rate of precipitation R in mm h - ’ and the water content M in gmW3. The physical parameters of the snow trail at the entrance into and at the exit from the plume, obtained from the profiles of mean and maximum reflectivities shown in Fig. 7, are listed in Table 1. These results show that the precipitations observed are of rather weak intensity. AM and AR respectively correspond to the increase in the water content and in the rate of precipitation of the snow trail as it crosses the plume. The mean precipitation intensity is approximately enhanced by a factor of two and the maximum intensity by a factor of three in the interaction with the plume. As a consequence, the snow precipitation partly scavenges the plume by extracting from it a portion of its condensed water which would have otherwise been dispersed and diluted in the atmosphere. The mean quantity of water extracted s- ’ is proportional to the horizontal cross-section of the plume, to the fall velocity of the ice particles and to the mean AM. A scavenging rate of 600 kgs- ’ is obtained with a fall velocity of I ms-’ and a plume horizontal crosssection described by a rectangle 30 km long and 2 km
where Ah is the thickness of the plume (600 m), E is the collection efficiency, M the liquid water content of the plume and P the density of the falling particles. The latter three parameters are assumed constant. To simplify further, the precipitation particles are assumed monodispersed. The reflectivity factor Z then becomes Z = N d6, where N is the number of particles per cubic meter and d, the diameter in mm of melted particles. In this equation certain parameters are not known; consequently a proper choice of values compatible with the observed phenomenon must be made. Thus, using N = 500 and p = 0.25 gcme3 the equation yields di = 1 mm and d, = 1.5 mm which results in &. M = 0.42, giving for example, E = 0.6 and M 5 0.7 gmT3. Similarly with N = 5000 and P one obtained d, = 0.7 mm and = 0.25 gcme3 d, = 1.1 mm which yield E. M = 0.30 or for example E = 0.6 and M = 0.5 g m - ‘. These results are only strictly applicable to the region in which the plume spreads out under the temperature inversion where the vertical velocity of the air can be assumed negligible. In the updraft zone of the plume, which is also seeded, the growth of natural particles can be much greater. Indeed, their trajectory through the plume can be as long as 1500 m when they
Table 1. Physical quantities of the seeder-feeder interaction deduced from the radar reflectivity data Height (km) Entrance Exit AM km-‘) AR (mmh-‘)
1.6
1
Temperature (“Cl
Z (dBZ)
M (gm--‘)
-11.3 -9
4 8
0.02 0.03
Mean values
R (mmh-‘) 0.05 0.08 0.01 0.03
(d:Z)
M (gm-“)
1s 26
0.05 0.14
Maximum values
R (mmh-‘) 0.16 0.50 0.09 0.34
Interaction between a natural snowfall and a cooling tower plume are entrained to the top ofthe plume and then fall back another 600 m through the plume when they are no longer sustained by updrafts. As a whole, these results must be considered as estimates only. In particular the use of other conversion equations shows that uncertainties in the physical quantities computed may be as large as 100%.
7. CONCLUSION Huff in 1972 postulated that a possible consequence of wet wastes from cooling towers was an increase in natural snow precipitations falling in the wet plume through a seeder-feeder mechanism. The reality of such a phenomenon has now been experimentally demonstrated with the fine-se&e, thr~~imensional observations of a Doppler radar. We have moreover shown, in particular, that one of the results of this type
of interaction is that snow precipitation can partly cleanse the plume and bring back to ground the condensed water which normally would have been dispersed in the atmosphere. In addition to these microphysical results, the Doppler capabilities were shown able to define the plume behaviour by means of the dynamic perturbations caused by thermal effluents. The seeder-feeder mechanism plays a major role in the initiation and growth of precipitation in extratropical systems it appears as aresult of this study, that wet plumes can be, under certain conditions, unique laboratories for quantitative studies of such phenomena. Acknowledgemenr-This research was supported by a financial grant from the Direction des Etudes et Recherches
(Electrici de France). The statements and conclusion in this paper are those of the author and not necessarily those of the Direction des Etudes et Recherches. I gratefully acknowledge
1383
G. Despaux who operated the Doppler radar and performed the observations discussed in this article. REFERENCES Campistron II. (1985) Etude experimentale
par radar mettorologique Doppler de certains types d’organisations convectives dans Ies systemes frontaux. These de Doctorat d’Etat, Universite de Toulouse Scienas. Hanna S. R.and Gifford F. A. (1975) Meteorological effects of energy dissipation at large power parks. Bull. Am. Mar. Sot. 56, 1069-1076. Hodin A. (1982) Impact atmosph~rique des a&or&t&ants du site du Bugey i. Synthise des &ultats exp&rim&taux. Electricite de France, Direction des Etudes et Recherches. Huff F. A. (:1972) Potential augmentation of precipitation from cooling tower effluents. Bull. Am. Met. Sot. 53, 639-644. Koenig L. R. (198i) Anomalous snowfall caused by naturaldraft cooling towers. ~~mosp~e~ic ~~~iro~rne~f 15, 1117-1128. Kramer M. L., Seymour D. E., Smith M. E., Reeves R. W. and Frankenberg 7. T. (1976) Snowfall observation from natural-draft cooling tower plumes. Science 193, 1239-1241. Locatelli J. D., Hobbs P. V. and Biswas K. R. (1983) Precipitation from strat~umuIus affected by fallstreaks and artificial seeding. J. Climate uppl, Mer. 22, 1393-1403. Mohr C. G. and Vaughan R. L. (1979) An economical procedure for cartesian interpolation and display of reflectivity factor data in three-dimensional space. J. oppl. Met. 18, 661-670. Parungo F. P., Allee P. A. and Weickmann H. K. (1978) Snowfall induced by a power plant plume. Geophp. Res. Lert. 5, 515-517. Pruppacher H. R. and Klett J. D. (1978) Microphysics 01 Clouds and Precipirorion. D. Reidel, Dordrecht. Sekhon R. S. and Srivastava R. C. (1970) Snow size spectra and radar reflectivity. J. amox Sci. 27, 299-307. Otts R. E. (1976) Locally heavy snow downwind from cooling towers. NOAA Technical Memorandum, NWS ER-62. Waldteufel W.and Corbin H. (1979)On theanalysisofsingleDoppler radar data. J. appl. Met. 18, 532-542.
r%ohnmmt
Vol.
21. No.
6. pp. 13754383.
ooo4-6981/87 Pergmon
1987.
in Orut Briuin.
13.00+0.00 Joumalr Ltd.
INTERACTION BETWEEN A NATURAL SNOWFALL AND A COOLING TOWER PLUME: AN EXPERIMENTAL STUDY WITH A MILLIMETRIC DOPPLER RADAR BERNARD CAMPISTRON
Centrc de Recherches Atmospheriques, Obscrvatoire Midi-Pyretrees, Campistrous, 65300 Lannemezan, France
(First received 11 May 1986 and received for publication 24 November 1986) Abstract-A precipitation band, about 30 km long and 2 km wide, downwind of a nuclear power plant was observed with a millimetric Doppler radar. The three-dimensional radar analysis showed that this band resulted from the growth, by a seeder-feeder process, of a natural snowfall falling in the moist plume produced by the exhaust of the nuclear plant. At the exit from the 0.6 km deep plume, the mean snow precipitation rate was approximately enhanced by a factor of two. This corresponds to an extraction rate of water in the plume by snow scavengingof 600 kgs- ‘, that is about one third of the water injection rate into the atmosphere by the cooling towers.
Key word index: Plume impact, snowfall modification, seeder-feeder process, plume visualization, Doppler radar, nuclear power plant, cooling tower plume.
1. INTRODUCTION
2. EXPERIMENTAL
The cooling towers of power plants inject in the atmosphere large quantities of heat and moisture; the potential meteorological effects have been studied experimentally and theoretically (for example: Hodin, 1982; Hanna and Gifford, 1975; Huff, 1972). Several snowfalls downwind of power plants were observed by Otts (1976) and Kramer et al. (1976). They suggested that these snowfalls were induced by the cloudy plumes of the cooling towers. In particular, the observations of Kramer et al., supported by in-sifu aircraft measurements, showed without a doubt the formation of snow in a plume extending to 1.6 km altitude and - 18” C. In this case however the ice nuclei, generally in largeconcentration (Parungo ef al., 1978) in the smoke produced by coal-fired power plants, certainly contributed heavily to the formation of ice particles. To explain these results, Koenig (1981) developed a numerical model which accounts for the existence of such a phenomenon and gives the meteorological conditions for the formation of snowfalls in wet plumes. The experimental study discussed here presents a radar observation of a snowband located downwind of the cooling towers of a nuclear plant. The radar observations, producing a fine scale three-dimensional description of the phenomenon, permit the identification of the origin of the precipitation and of the meteorological impact of the wastes from the nuclear plant.
CONDITIONS
In February 1983, a field project was launched by the Direction des Etudes et Recherches of E.D.F. (Electriciti de France) at the nuclear power plant of Dampierre-en-Burly. The objective of this 3-week field project was to study the local environmental impact of the heat and humidity wastes of the plant. The experimental system comprised principally an aircraft equipped for microphysic and dynamic measurements, a lidar, a rawindsonde station and the millimetric Doppler radar RABELAIS of the Observatoire MidiPyrtnees (O.M.P.). The latter three systems were located 4.5 km to the east of the nuclear plant (Fig. 1). The nuclear plant of Dampierre-en-Burly comprises four reactors each producing a thermal energy of 2785 megawatts yielding a net electrical energy of 900 megawatts. The cooling of the reactors is performed with water from the Loire river which is then injected
1375
l
NUCLEAR
\ NORTH
COOLING
Ikm
TOWER
Fig. I. Dampierre-cn-Burly experimental area.
1376
BERNARDCAMPISTRON
into the atmosphere as water vapour and condensed water forming a cloudy plume at the exit of the four cooling towers. The average water drop diameter, following filtering, is less than 20 pm. The water output of each tower corresponds to a flow rate of 0.5 m3 s- I. The height of each tower is 165 m, the base diameter is 135 m and the top diameter is 85 m. They are located in two pairs about 800 m apart. The main contribution to this study is the RABELAIS radar whose characteristics are the following: a wavelength of 8.6 mm, a peak power of 75 K W, a pulse length of 90m, and a beamwidth of 0.4”. The minimum detectable signal corresponds to a reflectivity factor of - 20 dBZ at a distance of IO km. The mean Doppler velocity is computed with the Pulse Pair Processor technique; the use of a double Pulse Repetition Frequency of 3125 and 2688 Hz permits the removal of velocity ambiguities over a range of +40ms-‘. The accuracy of the measurements of the Doppler velocity and of the reflectivity is 0.15 ms- ’ and 1 dBZ, respectively. The radar was calibrated with targets of known back-scattering cross-sections.
3.
day t * - 2’C)and weak precipitations of intermittent snow of small crystals. A vertical temperature and humidity sounding was launched from the field site at 1316 GMT. In particular, it shows that the air is saturated up to 1 km height which is favorable to the formation of a cloudy plume. However, since this sounding did not produce any data between 1.1 and 2.2 km height, the Trappes (located 120 km to the N of the field site) sounding of 1200 GMT is shown on Fig. 2a. This sounding reveals a moist potential instability in the lower layers up to about 1.6 km at the top of a temperature inversion. The wind distrjbution as a function of height shown on Fig. 2b was obtained with the VVP method (Waldteufel and Corbin, 1979) from the Doppler velocities measured with the radar at 0824 GMT. The figure shows a wind velocity maximum of 8 m s- ’ located at 0.3 km; above, the velocity decreases with height to a minimum of 3 m s-’ at 2.5 km height. Also, (4
GENERAL DESCRlPTtON OF THE CASE STUDY
Ofall the observations made during the field project, those of 9 February 1983, presented here, correspond to the clearest plume-environment interaction. The data were collected over more than I h by the RABELAIS radar. (a) The radar data The analysis of this interaction is based on a two volume scan of the radar. The first observation was obtained at 0824 GMT with panoramic sweeps of 38 km radius around the radar formed by a series of PPI scans between 1.5”and 18” by steps of 1” elevation angle; the radial resolution is 300 m and the azimuthal resolution is 0.74”. The second data set, acquired at 0844 GMT, consists of a series of RHI scans at 2” azimuth interval including the location of the plume over a radial distance of24 km and with a resolution of 150 m. The elevation sector extends from 0” to 27” in steps of 0.25”. The parameters measured by the radar, reflectivity and Doppler quantities, are inte~olated in a regular cartesian grid which permits the reconstitution of the data fields in horizontal and vertical planes. The value at a grid point is computed from a bilinear interp olation of the closest eight points radar data located on four radials with different elevation and azimuth angles (Mohr and Vaughan, 1979). This method has the advantage of preserving a resolution identical to that of tho radar acquiring the data. (b) Meteorological conditions The meteorological situation on 9 February 1983 is characterized by a moderate and cold NE flow giving rise to negative surface temperatures during the whole
5
(b)
--o-- DIRECTION 350° dD loo 20” 300 &$ I I I I I II
-e-
VELOCITY(ms-1)
Fig. 2. (a) Temperature (T) and dew point Cr,) sounding obtained on Y February 19X3 at I200 GMT from the Trappes station IOeated 120km north of Dampierre-en-Burly power plant. (b) Verticaldistribution of the wind velocity and direction derived from the Doppicr radar vciocitieson 9 Fchruary 1983a\ 0824 GMT
Interaction between a natural snowfall and a cooling tower plume the wind shear increases above the top of the temperainversion at 1.6 km height as the wind direction shifts rapidly from NNE to N at 2.5 km.
1377
(4
ture
LA.
(c) Local distribution of precipitations The echo patterns, observed at 0824 GMT with the RABELAIS radar, are shown in Fig. 3 as horizontal planes at various heights. The figure shows that the nuclear plant is nearly totally embedded in precipitations which cover almost entirely the area scanned by the radar over a radius of 38 km. In these reflectivity fields, Fig. 3a shows very clearly at 0.5 km height a narrow band of precipitation (boxed on the figure) located downwind from the nuclear plant, extending about 30 km in a direction parallel to that of the wind and converging on the nuclear plant. This precipitation band, related to the cooling towers, is also apparent on the horizontal reflectivity plane at 1 km height (Fig. 3b). At this level, echoes have a tendency to align parallel to the wind. Of all these banded structures, one is much more interesting to study in details for this discussion; it is located along the AB axis, shown on Fig. 3b at a distance of 7 km, and parallel to the precipitation band associated with the cooling towers. This band AB, sufficiently displaced from the power plant and not downwind from it, cannot be explained by the cooling wastes. A vertical plane along this band AB is presented on Fig. 4a. It shows that this band AB was formed by acharacteristic snow trail falling from generating cells extending vertically from 2.5 km to 4.5 km. Figure 4b shows a vertical plane of the reflectivity field perpendicular to both band AB and to the precipitation band associated with the nuclear plant. These two structures are included in overall precipitation and they appear to be connected and of identical characteristics. They are only distinguishable because of the greater reflectivity, of the order of IO dBZ, for the nuclear plant precipitation band.
(W
4. PLUME TRAJECTORY
In order to understand the role played by the plume from the cooling towers, in the formation of the precipitation band observed downwind of the plant it is imperative to know the behaviour of the plume and in particular the height of its top. However this information, unfortunately, is not available; moreover, tests have shown that the plume of condensed water is usually not detectable with the RABELAIS radar at distances greater than 0.5-1.0 km from the cooling towers. To obtain the trajectory and the behaviour of the plume, the Doppler radar data were used. Indeed, the cloudy plume, resulting from the waste water, is also carrying an important quantity of thermal energy waste which is capable of disturbing the natural dynamic field and as a consequence, to appear in the Doppler velocity of the snow scatterers. In order to
REFLECTIVITY FACTOR -I?___0 3 6 12 27 :.:.... . dBZ ----SCAN BOUNDARY o RADAR A POWER PLANT Fig. 3. Reflectivity factor patterns in horizontal planes at 0.5 km (a)and 1km (b) height, on 9 February 1983 at 0824 GMT.
characterize these dynamic perturbations, to be used as tracers of plume behaviour, the quantity A V defined below, was calculated for each radial velocity measurement:
1378
BERNARDCAMPISTRON
(4
(a) -6
-4
I
1
-2 X(km)
I
I
1
09FEB83
-20
-10
0
lO(km)
0
_
20
-8-
(b)
HEIGHT: 0.6km
__-___ l-----l
, 5 TEMPERATURE INVERSION . __-__________-_-__---~-
LAYER
OSFEB83
z
08H24GMT
t
Y: -3 km ,---.
‘;l
-30
-20
-10
0
10
20fkm) 30
Fig. 4. ReRectivity factor patterns at 0824 GMT in vertical planes oriented along AB axis (a) and CD axis (b) shown in Fig. 3b. In Fig.4b, the vertical cross-sections corresponding to the nuclear plant precipitation band and to the band AB are boxed, respectively with a solid and a dashed line. The refiaztivity factor scale and the symbols are identical as in Fig. 3.
where V, + , and V, are the Doppler velocities data on consecutive measurements which are 300 m apart along a radial for the 0824 GMT radar acquisition. AV was interpolated in a cartesian grid. Characteristic planes, for the 0824 GMT radar acquisition, are shown on Fig. 5. On these planes, contours of A V equal or greater than 0.6 m s- ’ are only shown. The threshold of 0.6 m s- I corresponds to the maximum intensity of natural environmental dynamic perturbations. In the horizontal plane at 600 m, shown in Fig. 5a, the organization of the A V contours, stretching along the windfieid and downwind from the nuclear plant, accounts rather clearly for the origin of these perturbations. The maximum A V values, reaching 3 ms-‘, are found near the nuclear plant. The horizontal distribution ofthis parameter reveals that, in fact, there
-6
-8
-4
-2Xtkm)O
(‘)
1 I 1... I I 1 I I _______ :,‘____ -________
08H24GMT
’
c
’
-8
----
’ -6
0.6
.--.
’
’
’
-4
AV (ms-I) 1.2
-2Yfkm)O
m
> I.6
Fig. 5. Contours of the Doppler velocity perturbations AVfield at 0824 GMT. (a) In a horizontal plane at 0.6 km height; (b) in a vertical plane perpendicular to the plume; (c) in a vertical plane along the plume.
1379
Interaction between a natural snowfall and a cooling tower plume are two distinct dynamic plumes (A and B) each one axially aligned on each pair of cooling towers. The vertical plane, Fig. 5b, obtained perpendicularly to the wind at a distance of 1.5 km downwind from the nuclear plant shows that plume A reaches a height of 1 km whereas plume B reaches only a height of 0.6 km. This difference in height may be due to the fact that plume A is associated with two cooling towers which are aligned along the wind direction. As a result, the respective wastes of these towers have vertical effects more efficiently cumulated than in the case of plume B produced by two towers which are oriented transversally to the wind direction. Figure 5c presents the A V pattern in a vertical plane parallel to the mean wind and along the mean horizontal axis of plume A. The distribution of the dynamic perturbations in this plane describes the typical trajectory behaviour of a plume which rises in the atmosphere, curves under the entrainment of the horizontal wind and spreads around the 1.6 km level at the top of the temperature inversion. The plume reaches this blocking layer at a distance of about 3.5 km from the power plant. The mean ascent velocity of the effluents can thus be estimated to be approximately 3 ms-r when using the horizontal winds given in Fig. 2b. The dynamic perturbations A Vdecrease in the mean as the distance from the nuclear plant increases; at distances greater than 9 km, the A V values are smaller than 0.6 m s-t and blend with the natural perturbations. The dynamic impact of the plume is then weak and is no longer detectable by the radar, which does not necessarily mean that the plume is no longer present. The AV fields were also computed for the 0844 GMT radar acquisition (Campistron, 1985). The results, not presented here, show nearly identical features.
On such a fine scale, a certain heterogeneity of the band, shown as distinct cores of greater reflectivity, is readily apparent. The explanation of the formation of this precipitation band is given on Figs 6c and 6d which present the reflectivity patterns in vertical planes oriented longitudinally along the mean axis of the band and crossing the power plant. It appears that the precipitation band results mainly from a snow trail similar to
04 5.
PLUME-ENVIRONMENT
INTERACTION
-8
-4
0
PROCESS
Having established the three-dimensional position of the plume, at least near the source of ertluents, the potential impact of the wastes on the formation of the precipitation observed downwind of the nuclear plant can now be analyzed. To do so, Fig. 6 shows reflectivity fields at 0824 and 0844 GMT in typical horizontal and vertical planes obtained near the nuclear plant. On some of these planes, the location of the plume obtained from the results of the analysis described in the previous paragraph is schematized. However, at distances greater than 9 km from the nuclear plant, the plume location is obtained by extrapolation and is thus only approximate. (a) Origin of the precipirarion band Both radar reflectivity measurements acquired at 20 min intervals and presented in horizontal planes in Figs 6a and 6b show the narrow band of precipitation about 2-3 km wide downwind from the nuclear plant.
09 FE8 I33 08H44GMT
Fig. 6 (a)_(b).
I
1380
BERNARDCAMPISTRON
0 Yfkml
(4
-4 . . .. .
3
-*-*
6
-
tX>6
I2 15
XIkm)
0
REFLECTIVITY FACTOR (dBZ1
m>21 _.a_
-10
-20
4
Xikml
Y (km1 0
0
Fig. 6 (c)-{e).
that associated to band A3 in Fig. 4a. The lack of radar
data at large elevation angles prevents the localization of the origin of these snow precipitations. However, the principle of continuity and the comparison with band AB which is in close proximity, suggest that these precipitations originate in the layer of generating cells,
SCAN or ECHOES BOUNDARY
Fig. 6. Reflectivity factor fields observed with the RABELAIS radar at 0824 GMT (a,c,c)ond at 0844 GMT (b,d,f) in horizontal planes at 0.6 km height (a, b) and in vertical planes atong the plume (c.d) and perpendicular to the plume (e,f). The schematic position of the plume is derived from the velocity perturbations AV fields. Far distances greater than 9 km fram the cooling towers, the plume positions result from an extrapolation.
with bases at 2.5 km height, upwind of the nuclear plant. Moreover the figures show, considering the ~haviour of the plume trajectory, that these cells cannot result from an artificial triggering of convection produced by the wastes of the cooling towers. During their fall from the level of generation these pr~pi~t~o~ are blown by the horizontal wind across the volume filled by the cloudy plume. The heaviest precipitations reach the ground about 13 km from the nuclear plant. The slope of the trail of snow is a function of the wind distribution with altitude, of the fall velocity of the snow crystals and of the translation velocity of the generating cells. In the vertical plane at 6.5 km from the plant and ~~ndicular to the band, shown in Fig. 6e, the precipitation at a mean height of 1.3 km is 0.5 km deep with at least a 10 km across wind. This figure shows that the plume is embedded in the proportion.
(b) Verticaf distribution oJ’reflectiviries It then appears that the precipitation band originates from a natural process which, by coincidence, occurred in the vicinity of the nuclear plant. At this
Interaction between a natural snowfall and a cooling tower plume
stage in the analysis and in spite of a number of clues, the effects of the cooling tower e&tents on the observed phenomenon cannot be delineated with any certainty. In order to remove this uncertainty, the vertical distributions of the mean and maximum radar reflectivity factor in horizontal slices 100 m deep were calculated between 0.3 and 3.0 height. These calculations were performed in three distinct volumes; the first one, whose horizontal cross-section is shown in Fig. 3a, comprises the power plant precipitation band and consequently the plume. The other two volumes are control volumes which are: the total area analyzed by the radar within the 38 km range except the previous volume, and the volume comprising the precipitation of snow related to band AB of Fig. 4a. The results are presented in Fig. 7 for the radar observation of 0824 GMT. These distributions reveal a characteristic height (4 -
PLUME
---ENVIRONMENT
O-
REFLECTIVITY
FACTOR (dB2)
04 -
PLUME
O10
---SNOW
REFLECTIVITY
TRAIL
AB
FACTOR (dB2)
Fin. 7. Comparisons between vertical distributions of the maximum and mean nfleetivity factors at 0824 GMT calculated in a volume containing the plume and in control volumes comprising (a) the environment and (b) the snow trail AB shown in Fig. 4a.
1381
located at about 1.6 km, the level of the top of the temperature inversion, below which the maximum and mean reflectivity factors in the area of the plume are in most cases greater than those in the control volumes. Moreover, they show that, between approximately 1.6 and 1.0 km height, the maximum reflectivity factor in the area of the plume rises by about 10 dBZ. Below 1 km height, the distribution practically in the average no longer varies. The reflectivity values, on Fig. 7, are large since they correspond*to precipitation echoes and not to cloud echoes. (c) Interaction mechanism These distributions lead to the conclusion that the effect of the effluents of the cooling towers, most compatible with all radar observations, results from a seeder-feeder type mechanism. The light trail of natural snow crystals generated at higher altitude intensifies as it falls through the ‘feeder’ cloudy plume between 1 and 1.6 km height. In this interaction zone, 0.6 km thick, the precipitation seeds the plume with ice crystals which will grow by riming, aggregation or water vapor deposition. In the dynamically active parts of the plume, mainly in the ascending region, turbulent motions will improve the collection efficiency and updrafts will carry upward natural snow crystals which will fall back to ground after growth. Indeed, the microphysical processes and their potential contributions are varied and complex and depend on the dynamic and the thermodynamic state of the plume and on the type of the seeding particles. In addition to the main snow precipitation trail, still lighter precipitations feed the plume and explain the 30 km length of the precipitation band. On Fig. 7b, the distribution of the maximum reflectivity factor of the light snowfall AB of Fig. 4a shows that this precipitation, once formed, in the average no longer varies as it falls to the ground. A similar observation could presumably be made in the case of the snow trail downwind of the cooling towers if there had been no plume. Indeed, these two snowfalls have the same origin, have identical reflectivity profiles above 1.6 km height and are in close proximity, as shown in Fig. 4b. In conjunction with the seeder-feeder mechanism, it is possible, as shown by Koenig (1981) with a numerical simulation, that the plume generated snow precipitation directly. In this case study presented here, if this mechanism occurred at all, it must be negligible compared to the seeder-feeder growth process proposed above. Indeed, according to Koenig, the production of appreciable precipitation requires a plume temperature below - 13°C. However in our case (Fig. 2a) the minimum temperature in the plume is - 12.5”C. In addition, the two radar observations of Fig. 6 show that the precipitation band at 0844 GMT is smaller and of weaker reflectivity than that measured 20 min earlier. This decrease in activity in the band is well correlated with a general decrease in reflectivity in
1382
BERNARD CAMPISTRON
the environment and, in particular, in the snow trail above 1.5 km. This correlation confirms the strong link between the natural snow trail and the power plant precipitation band. If the snow directly produced by the plume had been the primary cause of the phenomenon observed, it should have compensated for the decrease in activity in the natural snow trail.
6. PLUME-ENVIRONMENT
INTERACTION: QUANTI-
TATIVE RESULTS
Quantitative results of the interaction between the plume and the snow trail can be obtained by using conversion equations relating the radar reflectivity factor to physical quantities. However, since the characteristics of the scatterers are not readily known, the results obtained are only estimates. The following conversion relations, derived by Sekhon and Srivastava (1970) for agglomerated snow crystals, were used. R -- 0034 Z”.4S .
and
wide. This rate corresponds to about one third of the 2OOOkg s- ’ of water injected in the atmosphere by the cooling towers. Locatelli et al. (1983) reported on a case of a natural seeder-feeder process forming in a 1 km thick stratocumulus; in their observation the mean precipitation rate at the entrance into the layer was 0.01 mm h _ ’ and between 0.03 and 0.11 mm h _ I at the exit. These results are in good agreement with those reported here obtained with mean interaction values. For the maximum interaction values, an estimate of the growth parameters of the falling particles can be obtained. It is assumed that the particles are spherical and grow in a continuous mode by accreting particles in the plume. The following equation, relating the diameter di at the entrance into to the diameter d, at the exit from the plume is derived according to Pruppacher and Klett (1978): d,=d,+---
Ah.E,M
2P
M = 0.0135 Z”.”
where the reflectivity factor Z is expressed in mm6 m - 3, the rate of precipitation R in mm h - ’ and the water content M in gmW3. The physical parameters of the snow trail at the entrance into and at the exit from the plume, obtained from the profiles of mean and maximum reflectivities shown in Fig. 7, are listed in Table 1. These results show that the precipitations observed are of rather weak intensity. AM and AR respectively correspond to the increase in the water content and in the rate of precipitation of the snow trail as it crosses the plume. The mean precipitation intensity is approximately enhanced by a factor of two and the maximum intensity by a factor of three in the interaction with the plume. As a consequence, the snow precipitation partly scavenges the plume by extracting from it a portion of its condensed water which would have otherwise been dispersed and diluted in the atmosphere. The mean quantity of water extracted s- ’ is proportional to the horizontal cross-section of the plume, to the fall velocity of the ice particles and to the mean AM. A scavenging rate of 600 kgs- ’ is obtained with a fall velocity of I ms-’ and a plume horizontal crosssection described by a rectangle 30 km long and 2 km
where Ah is the thickness of the plume (600 m), E is the collection efficiency, M the liquid water content of the plume and P the density of the falling particles. The latter three parameters are assumed constant. To simplify further, the precipitation particles are assumed monodispersed. The reflectivity factor Z then becomes Z = N d6, where N is the number of particles per cubic meter and d, the diameter in mm of melted particles. In this equation certain parameters are not known; consequently a proper choice of values compatible with the observed phenomenon must be made. Thus, using N = 500 and p = 0.25 gcme3 the equation yields di = 1 mm and d, = 1.5 mm which results in &. M = 0.42, giving for example, E = 0.6 and M 5 0.7 gmT3. Similarly with N = 5000 and P one obtained d, = 0.7 mm and = 0.25 gcme3 d, = 1.1 mm which yield E. M = 0.30 or for example E = 0.6 and M = 0.5 g m - ‘. These results are only strictly applicable to the region in which the plume spreads out under the temperature inversion where the vertical velocity of the air can be assumed negligible. In the updraft zone of the plume, which is also seeded, the growth of natural particles can be much greater. Indeed, their trajectory through the plume can be as long as 1500 m when they
Table 1. Physical quantities of the seeder-feeder interaction deduced from the radar reflectivity data Height (km) Entrance Exit AM km-‘) AR (mmh-‘)
1.6
1
Temperature (“Cl
Z (dBZ)
M (gm--‘)
-11.3 -9
4 8
0.02 0.03
Mean values
R (mmh-‘) 0.05 0.08 0.01 0.03
(d:Z)
M (gm-“)
1s 26
0.05 0.14
Maximum values
R (mmh-‘) 0.16 0.50 0.09 0.34
Interaction between a natural snowfall and a cooling tower plume are entrained to the top ofthe plume and then fall back another 600 m through the plume when they are no longer sustained by updrafts. As a whole, these results must be considered as estimates only. In particular the use of other conversion equations shows that uncertainties in the physical quantities computed may be as large as 100%.
7. CONCLUSION Huff in 1972 postulated that a possible consequence of wet wastes from cooling towers was an increase in natural snow precipitations falling in the wet plume through a seeder-feeder mechanism. The reality of such a phenomenon has now been experimentally demonstrated with the fine-se&e, thr~~imensional observations of a Doppler radar. We have moreover shown, in particular, that one of the results of this type
of interaction is that snow precipitation can partly cleanse the plume and bring back to ground the condensed water which normally would have been dispersed in the atmosphere. In addition to these microphysical results, the Doppler capabilities were shown able to define the plume behaviour by means of the dynamic perturbations caused by thermal effluents. The seeder-feeder mechanism plays a major role in the initiation and growth of precipitation in extratropical systems it appears as aresult of this study, that wet plumes can be, under certain conditions, unique laboratories for quantitative studies of such phenomena. Acknowledgemenr-This research was supported by a financial grant from the Direction des Etudes et Recherches
(Electrici de France). The statements and conclusion in this paper are those of the author and not necessarily those of the Direction des Etudes et Recherches. I gratefully acknowledge
1383
G. Despaux who operated the Doppler radar and performed the observations discussed in this article. REFERENCES Campistron II. (1985) Etude experimentale
par radar mettorologique Doppler de certains types d’organisations convectives dans Ies systemes frontaux. These de Doctorat d’Etat, Universite de Toulouse Scienas. Hanna S. R.and Gifford F. A. (1975) Meteorological effects of energy dissipation at large power parks. Bull. Am. Mar. Sot. 56, 1069-1076. Hodin A. (1982) Impact atmosph~rique des a&or&t&ants du site du Bugey i. Synthise des &ultats exp&rim&taux. Electricite de France, Direction des Etudes et Recherches. Huff F. A. (:1972) Potential augmentation of precipitation from cooling tower effluents. Bull. Am. Met. Sot. 53, 639-644. Koenig L. R. (198i) Anomalous snowfall caused by naturaldraft cooling towers. ~~mosp~e~ic ~~~iro~rne~f 15, 1117-1128. Kramer M. L., Seymour D. E., Smith M. E., Reeves R. W. and Frankenberg 7. T. (1976) Snowfall observation from natural-draft cooling tower plumes. Science 193, 1239-1241. Locatelli J. D., Hobbs P. V. and Biswas K. R. (1983) Precipitation from strat~umuIus affected by fallstreaks and artificial seeding. J. Climate uppl, Mer. 22, 1393-1403. Mohr C. G. and Vaughan R. L. (1979) An economical procedure for cartesian interpolation and display of reflectivity factor data in three-dimensional space. J. oppl. Met. 18, 661-670. Parungo F. P., Allee P. A. and Weickmann H. K. (1978) Snowfall induced by a power plant plume. Geophp. Res. Lert. 5, 515-517. Pruppacher H. R. and Klett J. D. (1978) Microphysics 01 Clouds and Precipirorion. D. Reidel, Dordrecht. Sekhon R. S. and Srivastava R. C. (1970) Snow size spectra and radar reflectivity. J. amox Sci. 27, 299-307. Otts R. E. (1976) Locally heavy snow downwind from cooling towers. NOAA Technical Memorandum, NWS ER-62. Waldteufel W.and Corbin H. (1979)On theanalysisofsingleDoppler radar data. J. appl. Met. 18, 532-542.