suburban site using acoustic sounding

A COMPARISON OF THE STRUCTURE OF THE AT~OS~~ERI~ BOUNDARY LAYERS IN CENTRAL LONDON AND A RURAL/SUBURBAN SITE USING ACOUSTIC SOUNDING A. M. SPANTON and

M. L. WILLIAMS

Warren Spring Laboratory, Department of Trade and industry, Stevenage, Herts, U.K.

Ahstrnct-Comparisons are presented of acoustic sounder (SODAR) records from a site in the centre of a major urban area, London, and a rural/suburban site at Warren Spring Laboratory, Stevenage, some 50 hm to the north. The records form a continuous data set for the 2-year period from April 1981 to March 1983. The data are examined for differed due to the urban influence on the lower boundary layer and variations on diurnal and seasonal timescales are discussed. In general it is found that the mixing height in London is between SOand 100 m higher than that at Stevenage. The usefulness of SODAR in analysing and interpreting hourly variations in pollutant concentrations is illustrated in the context of a period of elevated 03 concentrations measured at the two sites in question.

1. INTRODUCIION There is a mntinuing interest in urban air pollution issues and in the features of urban areas which can affect the dispersion of air pollutants. In order to understand the transport and diffusion of pollutants in urban areas it is necessary not only to describe these features q~tat~vely but also to quantify the effects wherever possible. The two important characteristics of large urban areas which affect the turbulent boundary layer are the increased surface roughness arising from large buildings, and the so-called urban “heatisland” effect whereby the urban boundary layer is warmed by Pirect heat output from space heating and other sources, and by the absorption of solar radiation and sub~uent re-emission as sensible heat by the building mass. In broad terms it may be expected therefore that, compared with more exposed rural areas, urban areas may experience fewer low-lying inversions for example, and that the urban boundary layer may be generally more unstable than in nearby rural areas. This qualitative feature is well-known and urban dispersion models for example often take ac~5unt of tbe e&et by not empioymg stabe FasquiR categ5ries or by shifting the associated rural stability towards the unstable end of the spectrum. This paper sets out to quantify some of the features of the urban boundary layer at heights up to 1000 m using acoustic sounder (SODAR) records obtained concurrently over a continuous t-year period in Central London and at a rural/suburban site some SOkm to the north at Warren Spring Laborat5r-y (WSL) in Stevenage. The acoustic radar is an ideal instrument f5r observ-

ing differences between the boundary

layers at two sites. It has the advantage of providing a continuous record of the structure of the atmosphere, in contrast to routine radiosonde ascents, which usually only provide data at 12-h intervals. Although not providing qua& tative data d&&y, the charts produced can be used to give reasonably accurate estimates of the mixing height at the sites concerned. The SODAR work described in this paper is an extensi5n of earlier WSL work in this area (Ma&ran, 1979; Maughan er ol., 1982). The location of Stevenage is shown in Fig. 1. The London site was 30 m above the ground in an area of, on average, buildings of similar height, while the Stevenage site was at ground Iev& in an open iield on the southwestern extremity of the town, - 1OOm south of buildings of - IOm height, with an open aspect to the south and west. The data for the 2 years

Fig. 1. Map showing position of Stevenage and London

0 Crown copyright 1987.

SODAR sites.

212

A. M. SPANTON and

April 1981-March 1983 have been analysed and the differences between the frequency distributions of various types of echoes and mixing heights at the two sites are presented. Comparisons are made between the summer and winter periods at both sites as well as the differences between the 2 years, 1981/2 and 1982/3, studied.

2. CLASSIFICATION

SCHEME

FOR SODAR ECHOES

Both instruments, Aerovironment model 300 systems, used in this study were operated in the monostatic mode, with a 100 ms tone burst of 1.6 kHz, repeating every 18 s, with a peak power of 100 W and a maximum range of 1 km. The height resolution and hence effective dead space is thus - 20 m, i.e. no information on temperature structure can be obtained for heights of - 20 m above the level of the instrument. The data obtained from the acoustic sounders are displayed on a facsimile chart. In order to make comparisons between the two sites, the patterns on the charts for each site were manually coded for each hour during the period April 1981-March 1983. The scheme used was that developed by Maughan (1979), with the addition of a category (99) for unknown echo patterns and the omission of the category for convective-type echoes during the night, so that no distinction was made between convection during the day and convective-type echoes at night. The amended classification scheme is detailed in Table 1. The determination of mixing heights from the coded data is a fairly straightforward procedure and was described in detail by Maughan et al. (1982). The term “mixing height” is used here, as in Maughan et al., to denote the level of a potential barrier to the dispersion of pollutants at the interface between stable and less stable air. In the case of a ground-based stable layer, the mixing height is taken to be the height from the ground to the top of the Layer.There has been a suggestion that monostatic SODAR may not be able to measure the

M. L. WILLIAMS

full depth of such an inversion layer where the turbulence may be suppressed or reduced (Bacci et al., 1984). However, earlier comparisons of SODAR derived inversion heights with those measured by temperature profiles and LIDAR give generally good agreement (Coulter, 1979; Von Gogh and Zib, 1978). A value of 30 m was added to all the heights calculated from the London data to allow for the height of the instrument above the ground. The amount of usable data obtained from both sites was high, with the London instrument operating for 95 ‘;b of the period and the WSL instrument for 90%. Of these data only 4 “/ of the hours were given code 99 (unknown echo pattern) at London, and 11 “;, at WSL. 3. FREQUENCY

DLSTRIIUTION

OF SODAR CATEGORlES

Table 2 gives the trequency of each category for the two sites throughout the period studied (April 1981-March 1983) and for the summer (AprilSeptember) and winter (October-March) periods. The main features to be noted from these data are the high frequency of convective-type echoes, both with and without an elevated layer, and the complete absence of ground-based stable layers, at the London site. These are both direct results of the heat island effect of the urban area. The differences between the summer and winter periods are more marked at the WSL site; the urban area has the effect of reducing the differences in the echo patterns between the two seasons. Several interesting observations can be made by investigating the diurnal variation in the SODAR records. Figures 2-5 show the percentage of each SODAR category occurring at each hour of the day for the two summer and the two winter periods at the two sites. Looking tirstly at the differences between the summer and winter results, one immediately obvious and predictable feature is the greater occurrence of convective-type echoes during the summer. As ex-

Table 1. Classification scheme for SODAR echoes Explanation

Code

Ground-based inversion only, to a height of xx0 m; thus 125 is ground-based inversion to a height of 250 m ZXXOO Ground-based inversion to height of xx0 m with multiple layering ahove Ground-based inversion to height of xx0 m with single layer above, height of base of 2xxYY elevated layer is yy0 m 3 Convection alone Convection plus elevated Layers,the height of base of which is xx0 m 4xx 5xx Single elevated layer only, height of base is xx0 m Multiple-layered systeeno height of base Layerdiscernible;these were often very near6cHl lxx

surface systems 6xx 75

9 99

Multiple-layered system, where height of base of the lowest layer is xx0 m No echo, chart operating; usually found near sunrise/sunset and indicative of a well-mixed adiabaticLayer No echo due to wind/rain, etc., interference Sounder inoperative Unknown

A comparison of the structure of the atmospheric boundary layers

213

petted not only is the duration of convection category longer in summer but the frequency of occurrence is much greater, with convection being observed around noon on more than 80% of the days considered at the London site, compared to about 50% in winter. The corresponding figures for the WSL site are ‘2175 and 30x, reflecting the lower frequency of convective echoes at the more rural site. The category denoting convection plus elevated layers can occur at all hours of the day in winter. During the summer the incoming solar radiation is sufficient to disperse the stable layers that have formed overnight, or at least to lift them to a height exceeding the range of the SODAR, SO that the frequency of occurrence of this category (4xx) in summer reaches a maximum around sunrise and then decreases to less than 5% by 1200-1300 h. However, during the winter the incoming solar radiation may not always be sufficient to erode the layer completely and so the category is observed with a higher frequency during the late morning and early afternoon. This effect is most noticeable at the London site, where the percentage of occasions with convective-type echoes below an elevated inversion remains at or about _ 20% throughout the day, with a peak around 080&0900 h of over 30%. The same effect is observed with the other categories indicating the presence of one or more layers (i.e. 2~x00, 2xxyy, 5xx, 600 and 6xx). That is, during the winter there is a higher proportion of days when layers are either present throughout the day or at least persist for a longer period before being eroded. Category 7, indicating a well-mixed adiabatic layer, is most often observed around sunrise and sunset. A comparison of the figures indicates that this category is most common in summer around sunset at the WSL site. This category is not observed so often at the London site, since in the urban area heat received during the daytime is absorbed by the buildings, roads and pavementsand stored to bere-emitted after sunset, thus increasing the period over which convective-type echoes are observed and, by increasing the instability of the atmosphere, reducing the number of occasions when a well-mixed adiabatic layer devoid of any heat fiux occurs. Turning now to the differences between the distributions of the categories at the two sites, and looking firstly at the summer results, the difference in duration of convection again stands out, with the effect more pronounced in 1981 than 1982. A measure of the duration of the occasions when convective echoes only were present is the width of the curve for this category at half its maximum height. At the urban site this duration is 11.5 h in 1981 and 9.0 h in 1982, whereas for the more rural site the corresponding figures are 7.0 and 7.5 h, respectively. It shouid be remembered that the differences between the 2 years studied should be treated with caution because of the year-to-year variations in meteorological conditions and also because of the somewhat subjective nature of the coding system. At the WSL site no incidences of convective echoes

214

A.

M. SPANTON~~~ M. L.

WILLIAM>

-

Convection only Cafwectlon beneath etevoted l”vewO” l l l Ground based wersions (wth and wthwt elevated cnversions) x-x-x Slngte elevated wwers~on ‘-.- ~ttlpte toyers w No echo .--’

B ;;e

loo

Landan

t

GMT

Fig. 2. SODAR categories-summer

at night were observed, whereas at the London site they occur throughout the night, both with and without etevated Iayers. This is again a result of the effect, noted above, of the large thermal capacity of the buildings and roads which leads to the re-emission, during the night, of heat stored during the day. For the prediction of concentrations of air pollution, the phenomenon of fumigation is important. When radiational cooling of the surface causes the formation of a stable layer overnight, pollution emitted into the layer can be trapped, causing increased ground-level concentrations, and equally, emissions from higher level sources may be prevented from reaching the ground. However, at sunrise, when the ground is heated by the incoming solar radiation, the lower levels of the atmosphere are heated and convective currents result in an unstable region below the layer. As the intensity of the solar energy received increases, so the stable layer is eroded and lifted. Fumigation occurs

1981

when the layer reaches such a height that pollutants above the layer become entrained and, because of the high degree of turbulence in the unstable region, are brought to the ground, resulting in enhanced ground level concentrations. The SODAR records can be used to indicate when fumigation may occur and the category 4xx, convection plus elevated layer, is indicative of the necessary conditions. It is not surprising, therefore, that this category occurs with the highest frequency around sunrise and then at lower frequencies during the day. Ground-based layers at the WSL site only occur during the evening and early morning; in the middle of the day any layers that are present are kept aloft as a result of convective activity. The index defined above as a measure of the duration of convective echoes has values of 10 and 8 h for the two winter periods at the London site, and 5 and 6 h at the WSL site. Again the convective echoes are observed

A comparison of the structure of the atmospheric boundary layers

.--.

Convection Convection

only beneath

215

elevated

inversIon l l l

Ground

based

(with ond mversions)

100

60

60

I

*x-x

Smgle

.-.-

Muttlple

M

No echo

mbefs~ofi3

without

elevated

elevated Inversion

toyen

London

c

’

0

2

4

6

6

IO

12

14

16

I8

20

22

GMT

Fig. 3. SODAR caiegories-winter

throughout

the day in London,

but only occur between

sunrise and sunset at Stevenage.

4.

FREQUENCY DISTRIBUTION OF MIXING HEIGHTS

Mixing heights were determined from the SODAR data as described in section 2. The overall distributions of mixing heights in the summer and winter periods at the two sites have been compared, as well as investigating ihe differences in diurnal variation of mixing height between the urban and rural sites. During the coding procedure the heights of stable layers, etc., are estimated to the nearest dm, in the following analyses the data have been collected together into categories of SOm width. Figure 6 shows the frequency distribution of these categories for the summer and winter periods being considered. It can be seen that there is basically very little difference between the shape of the distributions at

1981-1982.

each site in summer and winter, the main variation being the slightly higher percentage of lower mixing heights in winter. The occasions when no mixing height was discernible from the SODAR charts have been split into two categories and are shown at the top of the diagram. The first category includes those hours for which no other category was applicable (i.e. code 99) and those hours for which the instrument was not functioning (i.e. code 9). The second group consists of those hours when a mixing height of > 1OOOm (i.e. greater than the operating range of the SODAR) was assigned, that is hours when convective-type echoes occurred, but without a stable layer above (code 3), when no echo was visible (code 7) and when rain or wind interference (code 75) masked any possible echo from a stable layer: Heavy rain causes the echo to be obliterated completely, while high wind speeds mask any echo from turbulence due to temperature fluctuations, since in these cases mechanical turbulence dominates.

A. M. SPANTONZ& M. L. WILLIAMS

Canvectiononly s-“’ ccwectian beneath elevated loo t

WSL

l

invwston * * Gmund based inwstons (with and withcut &voted

inverstons) w-x Single elevated inversion 1-*s-o

z$

too

I-

Pibitipte tayers No echo

London

GMT Fig. 4. SODAR categories-summer

Temperature inhomogeneities cause scattering of the sound through 180”, while velocity inhomogeneities (mechanical turbulence)cause scattering in other directions only. The monostatic SODAR transmits and receives sound from the same point and is designed to receive only sound reflected by 180”, hence in high wind speeds no echo is received. No mixing heights < SOm were observed at the London site in this work, due partly to the location of the SODAR 30 m above ground level, plus the fact that no signal is received by the instrument from the first 20m of the atmosphere, since during this time the circuitry is switching between the transmit and receive modes. It should also be noted that no ground-based’ layers (strictly layers with a base at the height of the instrument) were observed in London (see section 3). The effect of the urban area can be seen in the height

1982.

of the most frequent category. For the WSL site this occurs at 50-1OOm in both summer and winter, whereas for the London site it is between 100 and 200 m in summer and 100-150 m in winter. A further indication of the higher mixing heights observed in London can be seen in Fig. 7 which shows the distribution of mixing heights observed at the London site for a given mixing height at WSL, again with the mixing heights grouped into SOm intervals. In this case we are concerned only with those hours for which a mixing height, < 1000 m, could be determined simultaneously at both sites and not just with the overall percentages during the period as in the above analysis. The block corresponding to the same category being observed at both sites has been shaded in each histogram. Above about 3OOm the distributions become less distinct, due to the small size of the sample

A comparison of the structure of the atmospheric boundary layers

217

-

Conmction only Convection bsneath elevated inversion l l l Ground based imerslons (with and without elevated mversions) **-ll Single elevated mversion .-.- Multiple layers No echo .--.

60

London

0

0

2

4

6

6

IO

12

Fig. 5. SODAR categories-winter

available in these categories during the 2 years. The size of the sample, N h, is shown in the top right-hand corner of each histogram; above 400 m in summer and 450 m in winter the samples become too small to give any significant result. It is evident, especially during the summer period, that the most frequent situation is an increase in mixing height of w SO-100 m between Stevenage and London. However, there is also a significant probability of a lower mixing height being observed in London, particularly during the winter and at heights (at WSL) above 100-150 m. Leading on from the diurnal variation in mixing height, it is possible to look at the annual variation as well. Figures 8 and 9 show the annual and diurnal variations in the form of contours of equal mixing height. As in the previous discussion, median values

14

16

16

20

22

1982-1983.

have been used to givea mixing height for each hour for each month. Also shown on these diagrams are the times of sunrise and sunset during the year. These curves follow closely the shape of the contours, showing the dependence of mixing height on the amount of incoming solar radiation. A comparison between the two figures shows again the tendency for higher mixing heights at the London site. The area enclosed by the U> 1000 m”contour is larger for the urban site and even in winter there is still a period of about 3 h in the early afternoon when on average no mixing height is discernible below 1000 m in London, whereas in Stevenage in December the average mixing height remains below 1000 m throughout the day. Similarly, at the Stevenage site it is possible to define a 100 m contour both in the early morning and late evening, but in London the lowest definable contour is 200 m.

218

A. M. SPANTON and M. L. Summers M,xq

Wmters

Codes 9/99

1

p-4

,“‘““:

WILLIAMS

k;L(///////////‘///d1::::::“/,

,,,,

,

] ( $,gd

95oIOq 900

9501

850

9001

800

ewj

750

BOOI

700

7501

650

7001

600

650 P

k22lLondon WSL

Fig. 6. Distribution of mixing heights at WSi and London, t981-1983.

5. DETAILED

ANALYSIS OF A PERIOD POLLUTION

OF ELEVATED

LEVELS

SODAR records can be used effectively in the analysis of periods of high pollution levels. The continuous nature of the data, in contrast to, say, twice daily radiosonde ascent data, can provide information on the vertical structure of the atmosphere throughout the period required. In this section, these data are used, together with pollution measurements from WSL sites in London and Stevenage, to analyse a period of elevated ozone concentrations. 5.1. An ozone episode 6-9 July 1982 5.1.1. The synoptic situation. A westerly airflow affected the British Isles at the beginning of July with depressions moving east to the north of Scotland. An area of higber pressure brought warmer weather to the south on the 6th with a weak front causing showery

weather in some regions. At the London Weather Centre and at Wyton in Cambridgeshire, some 50 km N of the WSL site, wind speeds were around 4 m s- ’ and mainly west or southwest in direction. On the 7th and 8th an anticyclone moving east near southern England gave long sunny periods and temperatures up to 28°C on the 8that the London Weather Centre. As this anticyclone moved east the winds backed towards the east by midday on the 8th. and by the 9th the anticyclone had given way to a cold front in the southwest with thundery showers occurring in several regions and reaching London by about 1800 h. 51.2. Interpretation of SODAR charts and pollutant measurements. The 0, measurements at WSL and at the WSL Central London Laboratory (CLL), some 3-4 km from the London SODAR site, are shown, together with meteoroLog&l conditions (wind speed and temperature) from the London Weather Centre, in Fig. 11. Also shown are the mixing heights derived

219

A comparison of the structure of the atmospheric boundary layers Winters

WSL Mixing height

Summers

C

a 400 -

450

,[fi

‘lo

250-300

“F&,_

‘36

200-250

“J__

138

Mixing height tm)

Fig. 7. Frequency distributions of mixing heights observed at London for given mixing heights at WSL 1981-1983. Shaded block indicates equal mixing heights observed in London and Stevenage (N = size of samples).

from the SODAR charts. The SODAR record for the London site is shown in Fig. 12. The 0, concentrations measured at WSL showed hourly maxima of _ 9 pphm on 8 and 9 July suggesting that photochemical mechanisms were responsible for the elevated 0, concentrations. At the Central London site, 0, maxima were _ 5 pphm on 8th and 9th and it is likely that, given the east wind directions, these London concentrations represent local NO sources, primarily from traffic, reducing the “rural” 0, concentrations which were likely to be of the order of those measured at WSL. The SODAR chart record, together with the wind

speed and temperature data, allows the analysis and interpretation of the broad features of the diurnal pattern of the 0, concentrations. The SODAR data capture at WSL was not complete and is not shown here but the pattern of events on each day at the London site was similar, with an overnight stable layer persisting until it was lifted and eroded by convection from below. Convection then continued until the late evening, when another stable layer formed and the cycle was repeated. The 0, concentrations at both sites followed a similar pattern, although the concentrations at WSL were somewhat higher than those at the London site on the 8 and 9 July. At night,

220

A. M.

SPANTON

and M. L.

WILLIAMS

sunflse

6

6

16

16

24

AMJJASONDJFM

Month

Month

Fig. 8. WSL-annual and diurnal variation median mixing height (m) 1981-1983.

of

below the stable layer which was very low or groundbased at Stevenage and elevated at typically 20&300 m at the Central London site, the 0, concentrations were low. In these conditions it is the sinks for 0, (reaction with NO and dry deposition) that determine concentrations. The reaction with NO is likely to be the dominant sink. This was probably the case during the early hours of the 6th. 7th and 8th at the Central London Laboratory, with NO concentrations of - 2, - 4 and - 4 pphm, respectively, and on the 7th and 8th at WSL, with NO concentrations of - 6 and - 9 pphm. On the 6th at WSL the concentrations of the two pollutants were approximately equal. The diurnal pattern of 0, concentrations at both sites therefore followed the classical pattern of low overnight concentrations with 0, destroyed primarily by NO emissions in urban areas and to some extent by dry deposition in the low stable layer. Following erosion and destruction of this layer by the daytime convective activity, 0, is brought to ground level from above the overnight inversion and, together with photochw production upwind and to some extent locally, 0, concentrations increase during the daytime. With the formation of a stable layer during the following evening, 0, decreases once more as the NO and dry deposition sinks determine the concentrations.

Fig. 9. London-annual and diurnal variation of median mixing height (m) 1981-1983.

However an extremely interesting exception to this “normal” pattern occurred in the early hours of 9 July, when peaks of - 7 and - 5 pphm were observed at the Central London and Stevenage sites, respectively between midnight and 0200 h. The duration of these peaks was several hours, despite the presence of a stable layer at - 100 m in Central London and a very low (- 3&50 m) layer at Stevenage. Similar features were observed during the 1976 0, episodes (Apling et al., 1977) and may have been caused by transport of 0, produced earlier in the day in more distant regions within, or at least just below, the stable layer, perhaps by the “nocturnal jet”, although more data from upwind sites and vertical concentration profiles would be needed to confirm this. It should also be noted that NO concentrations at this time were low at both sites (- 2 pphm at CLL, - 1 pphm at WSL). The SODAR record for the London site in Fig 12 shows no evidence of any breakdown in the persistent stable layer at - 200 mat this tune. This suggests that the 0, observed at 0000-0200 on 8/9 July has been advected to the measuring points having been formed upwind, probably during the previous afternoon. It is not the intention of the present papa to investigate in detail the transport mechanism leading to this nighttime 0, peak, simply to illustrate the role which

A comparison of the structure of the atmospheric boundary layers

6 July

7 July

8 Juty

London weather

centre

+ London weather

centre

9 July

Fig. 10. Ozone measurements and meteorological data for 6-9 July 1982.

SODAR can play in the interpretation elevated pollution concentrations. 6.

of periods of

CONCLUSIONS

Acoustic sounder (SODAR) data from sites in London and Stevenage have been analysed and compared over a 2-year period from April 1981 to March 1983, in an attempt to compare the respective characteristics of an urban and rural/suburban boundary layer. The records have been coded on an hourly basis and the frequency of occurrence of various categories examined. Differences in the frequency distribution

during the winter periods and the summer periods have been investigated as well as inter-site variations. The frequency distribution of hourly mixing heights obtained from the SODAR charts was also investigated. An analysis of the mixing heights observed in London and Stevenage showed that, when a mixing height less than 1000 m could be measured, the most frequently occurring heights in summer were between 100 and 200 m in London and SO-100 m in Stevenage. The corresponding heights for winter were lW-150 m for London and 50-100 m for Stevenage. OveraIl, the most frequent situation was an increase in mixing height of - 5&100 m between the rural/suburban

222

A. M. SPANTON and

M. L. WILLIAMS

-!

i

A

comparison of the structure of the atmospheric boundary layers

Stevenage site to the urban London site. However, an investigation of the frequency distribution of mixing heights occurring at the London site for a given mixing height at the Stevenage site indicated that it is not uncommon for the mixing height in Stevenage to exceed that in London, especially when low mixing heights occur in winter. The period over which convective-type echoes occur was also found to be significantly increased at the London site compared with the Stevenage site. Finally, the use ofSODAR in interpreting variations in hourly co~ntrations of pollutants during episodes has been illustrated for a period of elevated 0, concentration in summer. work was supported by the Department of the Environment as part of its research programme. The views expressed are the authors’own and do not necessarily represent those of the Departments of Environment or Trade and Industry. Acknowledgements-This

223

REFERENCES

Apling A. J., Sullivan E. J., Will~ms M. L., B&J D. J., Bernard R. E., Derwent R. G., Eggleton A. E. J., Hampton L. and Wailer R. E. (1977) Ozone concentrations in South-East England during the summer of 1976. Nature, Loti. 269, 569-573. Bacci P., Giraud C., Longhetto A. and Richiardone R. (1984) Acoustic sounding of land and sea breezes. Boundary-Layer Met. 28, 187-192. Coulter R. (1979) A comparison of three methods for measuring mixing-layer he&t. J. uppl. Met. l&1495-f499. Mau~han R. A. (1979) Freauencv of uotential contributions by&jor sources to’ground le;eI concentrations of SO, in the Forth Valley, Scotland: an application of acoustic sounding. Atmospheric Environment 13, 1697-1706. Maughan R. A., Spanton A. M. and Williams M. L. (1982) An analysis of the frequency distribution of SODAR derived mixing heights classified by atmospheric stability. Atmospheric

Environment

16, 1209-1218.

Von Gogh R. G. and Zib P. (1978) Comparison of simultaneous tethered balloon and monostatic acoustic sounder records of the statically stable lower atmosphere. f. appl. Met.

17, 34-39.