Stability classification by acoustic remote sensing

Atmospheric Research, 20 (1986) 165--172

165

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

STABILITY CLASSIFICATION BY ACOUSTIC REMOTE SENSING

P. T H O M A S

Kernforschungszentrum Karlsruhe GmbH, lnstitut fur Meteorologie und Klimaforschung, Postfach 3640, D-7500 Karlsruhe 1 (Federal Republic of Germany) (Accepted for publication May 1, 1986)

ABSTRACT Thomas, P., 1986. Stability classification by acoustic remote sensing. Atmos. Res., 20: 165--172.

T w o different Doppler acoustic sounders have been operated at the Kernforschungszentrum Karlsruhe (KfK) since 1982. It has been investigated whether meteorological data from these sounders can be used for dispersion modeling and monitoring in the environment of pollutant-emitting plants. Data from the sounders and from a 200 m high meteorological tower have been sampled continuously for intercomparison. T w o schemes of stability cla.ssificationare presented. They are based on 30-rain mean values of the following meteorological data measured by the acoustic sounders: (a) standard deviation Ow of the vertical wind speed and horizontal wind speed u, at a height of 100 m; and (b) standard deviation ~r~ of the vertical wind direction at a height of 100 m and vertical profile of the backscattered amplitude Aw. The class limits applied in these schemes are determined by "statistical equivalence" with a standard classification scheme. This standard scheme is based on o~, measured by a vector vane at the 100 m level of the tower. Statistical equivalence in this context means that the frequency distributions of the classes are approximately equal at the same site and during the same period. The reliability of these schemes is investigated and compared to the standard scheme by correlation analysis. Finally, the schemes are compared with other c o m m o n l y applied classification methods. RI~SUMI~ Deux sondeurs acoustiques du type Doppler ont ~t~ en service au Kernforschungszentrum Karlsruhe depuis 1982. On a ~tudi~ si les donn~es m~t~orologiques fournies par ces sondeurs peuvent ~tre utilis~es dans des modules de dispersion ainsi que pour la surveillance des dispersions darts l'environnement des usines ~manant des polluants. Les donn~es fournies par les sondeurs et les donn~es en provenance d'une tour m~t~orolo gique de 200 m de hauteur ont ~t~ continuellement ~chantillonn~es a fin d'intercomparaison. Deux schemas d'attribution des categories de stabilit~ sont pr~sent~s. Ils sont bas~s sur des moyennes de 30 minutes des donn~es suivantes, mesur6es par les sondeurs acoustiques: (a) ~cart-type o w de h vitesse verticale et de h vitesse horizontale u, du vent 100 m d'altitude; (b) ~cart-type ff~ de la direction verticale du vent a 100 m d'altitude et profil vertical de l'amplitude r~trodiffus~e Aw. Les limites des categories employees dans ces seh~mas sont d~termin~es par des "~qui-

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166 valences s t a t i s t i q u e s " fi l'aide d ' u n s c h e m a s t a n d a r d de classification. Ce s c h e m a s t a n d a r d repose sur la valeur a~ mesur~e par u n e g i r o u e t t e ~ trois d i m e n s i o n s ~ 100 m d ' a l t i t u d e . L ' ~ q u i v a l e n c e s t a t i s t i q u e d a n s ce c o n t e x t e signifie l'~galit~ a p p r o x i m a t i v e des d i s t r i b u t i o n s e n f r ~ q u e n c e des categories p o u r le m ~ m e site et p e n d a n t la m ~ m e p~riode. La fiabilit~ de ces s c h e m a s est ~tudi~e et c o m p a r ~ e aux schemas s t a n d a r d m o y e n n a n t une analyse de c o r r e l a t i o n . P o u r finir, les s c h e m a s s o n t c o m p a r e s avec d ' a u t r e s m ~ t h o d e s de classification c o u r a m m e n t e m p l o y e e s . INTRODUCTION

In the Federal Republic of Germany, modern nuclear power plants have to be equipped with a measuring station furnishing, at any given time, the meteorological information that is necessary to estimate the dose equivalent in the environment of the plant due to radioactive releases. The basic meteorological information includes the wind speed and direction at the height of the stack and the atmospheric stability class. In this context, the KfK has been charged with testing two Doppler acoustic sounders (SODARs), available on the marked, to determine their technical performance, their relative suitability, and the quality of the meteorological data as compared with tower data. The SODARs tested were the DS 108, manufactured by the German c o m p a n y Rosenhagen, and the AO, manufactured b y the French c o m p a n y Remtech. The DS 108 and the AO were operated in 1982 and 1983, respectively. Both SODARs were located about 200 m north of the meteorological tower of the KfK, in the NW-corner of a meadow. The meadow is rectangular and a b o u t 100 m X 60 m in size. It is surrounded by a forest of pines about 20 m in height. The tower has a height of 200 m, its instrumentation and data acquisition have been described by von Holleuffer-Kypke et al. {1984). The results of the test have been published by Thomas et al. (1983) and by yon Holleuffer-Kypke et al. (1985). However, the stability classification was n o t investigated, or only briefly, in these publications. ATMOSPHERIC STABILITY CLASSES

Atmospheric stability classes have been defined by Pasquill (1974) and denoted by the letters A through F. The atmospheric turbulence decreases from A (very instable stratification) via D (neutral stratification) to F (very stable stratification). The stability classes are c o m m o n l y determined b y different methods using measured meteorological parameters which are strongly influenced by atmospheric turbulence, or vice versa. The methods and meteorological parameters used at KfK are, among others: standard deviation ov of the vertical wind direction measured at 100 m AGL by a vector vane (Fig.l); standard deviation oe of the horizontal wind direction, measured at 100 m AGL by a vector vane and a two~limensional vane; temperature difference A T between

167

Fig.1. V e c t o r vane.

30 m and 100 m AGL and horizontal wind speed u at 40 m AGL; radiation balance b and horizontal wind speed u at 40 m AGL. These methods were compared and checked for reliability by Nester (1980). The comparison of the different methods of determination of stability classes is based on the statistical equivalence:

Different methods furnish approximately equal frequency distributions of the stability classes at the same site, and during the same time interval. The comparison is based in every case on the o~-method, as the standard deviation of the vertical wind direction is considered to be influenced most directly by atmospheric turbulence.

168 STABILITY C L A S S I F I C A T I O N BASED ON S O D A R D A T A

Stability classes are determined from 30-min mean-values of the standard deviation ow of the vertical wind speed, measured at 100 m AGL, horizontal wind speed u, measured at 100 m AGL, and backscattered amplitude Aw, measured by the vertically pointing antenna up to 300 m AGL. The standard deviation o+ of the vertical wind direction, not measured directly by a SODAR, is calculated to be approximately: O~ = arctan (Ow/U) When using o~/SODAR for stability classification, the agreement with the o~/vector vane method is only poor. Therefore, the following two methods have been developed.

Classification by u- and aw-intervals The horizontal wind speed u is divided into intervals of 1 m/s. Within each interval, the classification is performed by ow, in accordance with Fig.2. The class limits of Ow within each u-interval are determined by the statistical equivalence, as described on p. 167: within each u/SODAR-interval, the frequency distribution of classes is the same, independent of whether they are determined by ~ / v e c t o r vane or by Ow/SODAR. The class limits of a~/vector vane are compiled in Table I. The results of the comparison between the a¢/vector vane m e t h o d and the u, Ow/SODAR m e t h o d of classification are compiled in Table II. This table contains the following information.

T¸

0

20

40

~)0

80

100

120 |~0

O'w{Cm/$) Fig.2. Classification b y h o r i z o n t a l w i n d s p e e d u and s t a n d a r d deviation Ow o f vertical wind s p e e d m e a s u r e d at 100 m AGL.

169

TABLE I Class limits of the standard deviation o~ of the vertical wind direction (time intervals: DS 108 Dec. 1981--May 1982; AO Dec. 1982--Nov. 1983) Class F

E

D

C

B

A

o~ (vector vane) a~ ( D S I08)

1.5° 4.55 °

3.3° --

7.0° 7.22 °

10.5 ° 12.03 °

14.5 ° 18.85 °

Qg

-2.80 ° --

1.63 --

-4.97 ° --

-8.10 ° --

-13.0 ° --

o~ (AO) Qg

TABLE

1.59

II

Correlation in % of stability classes determined by a¢ of a vector vane (V), and u, a w of the S O D A R A O (S) (time interval: Dec. 1982--Nov. 1983) S

~

C

D

E

Sum

A

B

F

A B C D E F

0.66 0.93 0.47 0.20 0.02 0.02

0.96 2.49 2.17 0.93 0.11 0.04

0.44 2.20 5.63 6.12 0.79 0.24

0.13 0.98 6.24 27.66 6.46 2.57

0.05 0.11 0.53 6.59 8.51 4.60

0.02 0.07 0.32 2.59 4.45 3.71

2.26 6.77 15.37 44.09 20.33 11.17

Sum

2.29

6.70

15.42

44.05

20.40

11.15

100.00

(1) The last line and column indicate the frequency of each class in the time interval considered as classified by av/vector vane and u, ~rw/SODAR, respectively. In order to fulfil the requirement of statistical equivalence, corresponding figures must be approximately identical. (2) The figure 6.24, e.g., at the intersection of column D and line C, shows the frequency in percent, at which o¢/vector vane indicates class D, and u, a w/SODAR indicates class C. (3) The figures in the diagonal demonstrate the frequency in percent with which both methods indicate the same class. (4) A great distance from the diagonal corresponds to a great difference in classes determined by both methods.

Classification by ov and the profile of Aw The vertical profile o f the relation:

Q(H) = Aw (H)/Aw (H + 20 m) is calculated (Fig.3) and its maximum, Qm~, is determined. Then the following preclassification is performed:

170 ~0L,

~:'e

I

i

Aw(HI

~:,:

~

~l

~I:~:

,c~(,

in m W

I

3e~

i /

_____D ~o

i

Qg

i 3= o

:,7

, 1 ('

. is

, ,o

;: 5

Q ( H )= A w ( H ) / A w ( H * 2 0 m )

Fig.3. Classification by A w and a~.

Qmax < Qg : classes are A, B, C or D, Qmax ~> Qg : classes are E or F. The limit Qg is determined by statistical equivalence. The sums of the frequencies of classes A, B, C and D or classes E and F are the same, independently of whether they axe determined by av/vector vane or by Aw/SODAR. This means that instable up to neutral stratification, on the one hand, and stable stratification, on the other, are distinguished by Qg. The following distinction between A, B, C and D and E and F, respectively, is made by d¢/SODAR. The limits of a¢/SODAR are determined by statistical equivalence. They are compiled together with the limit Qg in Table I. The results of the comparison between the av/vector vane and the Aw, d~/SODAR methods are compiled in Table III. TABLE III Correlation in % of stability classes determined by a¢ of a vector vane (V), and A w, o~ of the SODAR DS 108 (S) (time interval: Dec. 1981--May 1982) S ~

A

B

A B C D E F

1.04 1.45 0.09 0.03 0 0

0.77 3.66 2.70 0.24 0.18 0

Sum

2.61

7.54

C

D

E

F

Sum

0.31 1.79 8.30 5.93 0.96 0.18

0.16 0.50 5.70 26.54 6.22 1.76

0.04 0.15 0.61 6.87 10.06 3.97

0.03 0 0.07 1.26 4.34 4.09

2.36 7.55 17.46 40.87 21.76 10:00

17.46

40.90

21.69

9.81

t00.00

171 RESULTS

In Table IV, the two classification methods based on SODAR data are compared to other methods commonly used (also by KfK). The comparison is based in every case on the o¢/vector vane method. The figures corresponding to the methods o0, AT/u, and b/u have been published by Nester (1980). They refer to a time interval of 5 years between 1973 and 1977. The time intervals during which the SODAR data were sampled are indicated in Table IV. T A B L E IV Correlation coefficient and frequency of the same and different classes when different methods are applied, compared to the a¢/vector vane method (time intervals: DS 108, Dec. 1981--May 1982; AO, Dec. 1982--Nov. 1983;a0, AT, b: 1973--1977) Method

Correlation coefficient

Same class

Class diff. 1 step

Class diff. ~ 2 steps

u, o w DS 108 u, o"w AO

0.62 0.64

45 49

43 41

12 10

Aw, a~ DS 108 A w, a¢ AO

0.73 0.55

54 46

40 41

6 13

a0 AT/u b/u

0.88 0.82 0.68

72 59 47

27 37 41

1 4 12

(%)

(%)

(%)

In calculating the correlation coefficients, the figures 1 through 6 have been assigned each to the classes A through F. Before drawing conclusions from Table IV one should consider the following circumstances. (a) ao and the base parameter a~ are furnished by the same vector vane. (b) The meteorological parameters used in the a¢-, o'0-, AT~u-, and b/umethods are carefully checked and cross-checked against other simultaneously sampled data by the computerized data acquisition system and on a display by a meteorologist. During this procedure, they are partially discarded. A display control has not been performed with SODAR data. (c) The time interval during which data were sampled by DS 108 was only six months. The sounder was then slightly modified. After the modification, data sampling was continued for four months. After the modification, however, class limits changed and the figures corresponding to Table IV are slightly less favorable.

172 CONCLUSIONS

Considering Table IV, the following conclusions can be drawn. (1) If classification errors of one step are accepted, both methods and both sounders are working in a satisfactory manner. They are as good as the m e t h o d based on radiation balance and wind speed. (2) The u,aw-method is better suited to the AO sounder. The Aw,a~method is better suited for the DS 108 sounder. (3) The Aw ,a~-method applied to data from the DS 108 sounder is almost as good as the method based on temperature difference and wind speed.

REFERENCES Nester, K., 1980. Statistisch ~iquivalente Verfahren zur Bestimmung yon Ausbreitungskategorien. Seminar on Radioactive Releases and their Dispersion in the Atmosphere Following a Hypothetical Accident. Commission o f the European Communities, RisS. Pasquill, F., 1974. Atmospheric Diffusion. Wiley, New York, N.Y. Thomas, P., yon Holleuffer-Kypke, R. and Hiibschmann, W., 1983. Doppler acoustic sounding performance test. 2nd Int. Symp. on Acoustic Remote Sensing of the Atmosphere and Oceans, Rome. von Holteuffer-Kypke, R., Hiibschmann, W., Siiss, F. and Thomas, P., 1984. Meteorotogisches Informationssystem des Kernforschungszentrums Karlsruhe. AtomkernenergieKerntechnik, 44: 300--304. von Holleuffer-Kypke, R., Hiibschmann, W. and Thomas, P., 1985. Testbericht fiber das monostatische Doppler-SODAR B, KfK 3928. yon Holleuffer-Kypke, R., Hiibschmann, W. and Thomas, P., 1985. Testbericht fiber das monostatische Doppler-SODAR R, KfK 3929.