ISSN 1024�8560, Atmospheric and Oceanic Optics, 2011, Vol. 24, No. 3, pp. 231–234. © Pleiades Publishing, Ltd., 2011.

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1 MOTIVATION AND OBJECTIVES

A motivation for this study is the quality control ofABC derived with elastic backscatter lidars. In thelidar networks such control is carried by numericalexercises and lidar inter�comparisons campaigns [1,2]. Although well established, such procedures sufferfrom limitations. The numerical tests address only theprocessing algorithm. The intercomparison cam�paigns are expensive since they require to move thetested lidars to a common site. I.e. there is no self�con�sistent method for ABC quality control during thebackscatter lidar operation at the home site.

Another motivation is the importance of VR for airtraffic at airports [3]. Although this problem isaddressed by backscatter lidars since a long time [3],there are still open questions. As the measurementsshall be at slant�path, eye�safety regulations apply.Eye�safe wavelength probing means that the VR valueshall be re�evaluated for the visible wavelength range.One solution may be lidar measurements in directionin which the eye�safety requirements may be relaxed(e.g., vertical) or at some distance from the airports. Insuch case, it is necessary to demonstrate the consis�tency between the extinction in vertical direction andslant�path VR.

The above motivations determine the objective inthis study: to demonstrate the consistency between thebackscatter lidar determined extinction coefficientsand ABL top altitude, with the VR to reference targets(objects) at horizontal and slant path direction.

1 The article is published in the original.

LIDAR AND SITE

The lidar measurements are performed in Neuchâ�tel, Switzerland, 47.002°N, 6.955°E, 487 m above sealevel (asl). The backscatter lidar used in this study isbased on an instrument, initially developed for air�borne operation [4, 5]. Its adaptation for ground�based operation and respective results were alreadyreported elsewhere [2, 6, 7]. The performances of themain lidar subsystems are summarized in Table 1.

The ABC is derived with the classical Fernald’sinversion procedure [8, 9]. The values for the molecu�lar backscatter coefficient are obtained from meteoro�logical radiosonding at Payerne Aerological Observa�tory, situated. 20 km from Neuchâtel. The referenceaerosol and molecular EBR (lidar ratios) are respec�tively 50 [10] and 8π/3 [9].

The position of the lidar site provides lines�of�sights to several reference topographic objects in SwissPlateau and the Alpine ridge, that is, at direction southfrom Neuchâtel. This gives the opportunity to estimateVR visually by distinguishing the respective referencetarget (object) from its background [3, 11]. Table 2presents a list of the reference topographic targets(objects) selected for this study. In most of the cases,the visibility and the cloud presence were also docu�mented camera images, courtesy the Cantonal Policeof Neuchâtel.

The VR, RV, and the mean total extinction coeffi�cient σmean along the line�of�sight are linked viaKoschmieder equation [3, 11]. This relation is validfor wavelengths around 550 nm, and hence also for thelidar wavelength

REMOTE SENSING OF ATMOSPHERE, HYDROSPHERE, AND UNDERLYING SURFACE

Consistency between Backscatter Lidar Products and Visibility Range1

V. Miteva and R. Mattheyb

a CSEM, Rue de l’Observatoire 58, Neuchâtel, CH–2000 Switzerlandb University of Neuchatel, Dep. of Science, Neuchâtel, CH–2000 Switzerland

Received September 3, 2010

Abstract—We present a consistency of the following values: the aerosol backscatter coefficient (ABC) andtop of Atmospheric Boundary Layer (ABL), derived from backscatter lidar measurements from one side, andthe visually determined Visibility Range (VR) from the other. The VR is determined towards long�range ref�erence topographic targets in horizontal or slant path, while the lidar measurement is performed in vertical.The mean extinction coefficient along line�of�sights to reference topographic objects is calculated from thelidar derived back�scatter coefficient with model aerosol extinction�to�backscatter ratio (EBR), when nec�essary, taking into account the ABL top. The mean extinction coefficient along the line�of�sight to the refer�ence target is also determined from the VR via Koschmieder equation. The correlation coefficient betweenthe two data sets is R2 = 0.86 for all data points and R2 = 0.91 when selecting out the points with possible VRsystematic error at the farthest reference target.

DOI: 10.1134/S1024856011030122

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ATMOSPHERIC AND OCEANIC OPTICS Vol. 24 No. 3 2011

MITEV, MATTHEY

RV = 3.912/σmean. (1)

This study is performed along the following steps:—Lidar measurements with integration time of

1 hour; visual evaluation of RV.

—Determination of the ABL top by gradientmethod [12] and the profile of ABC profile [8, 9] fromthe lidar signal.

—Evaluation of the total extinction coefficients(molecular plus aerosol one) from the lidar deter�mined ABC and molecular backscatter coefficientsand the respective reference lidar ratios.

—Determination of average values for the totalextinction coefficient in ABL (σABL) and lower freetroposphere till 4000 m (σTropo).

—Evaluation of the mean extinction coefficientalong the line of sight to the reference object. Here weassume horizontal homogeneity of the atmosphere.For objects below ABL top, we consider σmean = σABL;for objects above ABL top, we consider:

(2)

In (2) RT is the distance to the reference target(object), R1 and R2 are the parts of RT below and abovethe lidar determined ABL top, where R1 + R2 = RT.

σmean

σABL

RT

���������R1σTropo

RT

������������R2.+=

RESULTS

A total of 44 daytime lidar measurements wereselected for this study, where the selection criterion isthe stabile synoptic situations, justifying the horizontalhomogeneity of the atmosphere. Figure 1 presents twoexamples from the data set, for 11 March 2007 and20 February 2007: respectively the lidar obtained ABCand the calculated total extinction coefficients pro�files, and images in south direction. As it is seen, highvalues of the ABC are obtained during March 11, a daywith low visibility range as illustrated by the image.Note that only the opposite shore of the lake is visible,while none of the objects further. This is opposite forFebruary 20, when high visibility occurs, allowing tosee the Alps ridge, with lidar measurements showingABC value lower by an order of magnitude.

The lidar determined σmean and visually determinedRV for the selected data set are presented in Fig. 2. Inaddition to the objects given in Table 2, it was possibleto insert a point with RV ~ 5 km, corresponding to acase thee opposite shore of the lake was not seen, but aboat in the middle of it was, what allowed RV determi�nation. The solid line in Fig. 2 presents theKoschmieder equation.

In our opinion, the main error source in suchapproach is in the visual RV estimation: a statisticalerror for all reference and a systematic under�estima�tion of RV for the most remote object. In Fig. 2 there isa group of five points with σmean below 3 × 10–4 m–1.These values are associated with RV = 95 km, deter�mined from the farthest reference target. This group ofσmean values fits better with Koschmieder equation ifwe assume RV ~ 150 km rather than 95 km. This is con�firmed by linear regression between σmean obtainedfrom the lidar measurements and σmean obtained fromvisibility range via Koschmieder equation. Assumingthat RV = 95 km for σmean below 3 × 10–4 m–1, weobtain R2 = 0.86. In case we assume RV = 150 km foromean below 3 × 10–4 m–1, we obtain R2 = 0.91. Thereason for under–estimation is the fact that RV couldsupersede 95 km, but a lack of farther references pre�vents such estimate.

Table 2. Reference topographic objects

No Object Range Altitude, asl Note

1 Opposite shore of lake Neuchâtel

8–12 km, by azimuth ~470 m http://fr.wikipedia.org/wiki/Lac_de_Neuch%C3%A2tel

2 Mt Vully 15–20 km, by azimuth 653 m http://fr.wikipedia.org/wiki/Mont_Vully

3 Mt Stockhorn 55 km 2190 m http://en.wikipedia.org/wiki/Stockhorn

4 Wildstrubel 75 km 3243 m http://en.wikipedia.org/wiki/Wildstrubel

5 Mt Jungfrau 95 km 4158 m http://en.wikipedia.org/wiki/Jungfrau

Table 1. Specifications of the micro�pulse backscatter lidar

Laser/Wavelength Micro�pulse/532 nm

Average power 18–20 mW

Polarization Linear

Beam divergence 0.25 mrad, full angle

Pulse repetition rate 5–6 kHz

Telescope type/aperture Kepler type/50 mm

Field of view 0.5 mrad, full angle

Interference filter: FWHM/Transmission

0.25 nm/38%

Range of full lidar overlap 400 m

Detection Type/Detectors Photon counting/PMTs

Range resolution and single measurement duration

30 m/6 s

ATMOSPHERIC AND OCEANIC OPTICS Vol. 24 No. 3 2011

CONSISTENCY BETWEEN BACKSCATTER LIDAR PRODUCTS 233

Neuchâtel, 20/20/07, 09:30–10:30 Mal 1–532 nmA

ltit

ude,

km

· ag

l

Backscatter coefficient, 1/m · sr

Extinction coefficient, 1/m

3

2

1

0 1 2 3 4 5 × 10–6

Neuchâtel, 11/03/07, 16:25–17:25 Mal 1–532 nm

Backscatter coefficient, 1/m · sr

3

2

1

0 5 10 15 × 10–7

Neuchâtel, 20/20/07, 09:30–10:30 Mal 1–532 nm

Alt

itud

e, k

m ·

agl

0 0.5 1.0 1.5 2.0 2.5 × 10–4

3.5

3.0

2.5

2.0

1.5

1.0

0.5

3

2

1

0 1 2 3 4 × 10–6

Neuchâtel, 11/03/07, 16:25–17:25 Mal 1–532 nm

Extinction coefficient, 1/m

Sud Tue Feb 20 11:56:16 2007 Sud Sun Mar 11 18:15:34 2007

Fig. 1. Left column presents the case on 20 February 2007. Upper panel: solid—aerosol backscatter coefficient; dashed—molec�ular backscatter coefficient. Middle panel: solid—aerosol extinction coefficient; dashed—molecular extinction coefficient.Lower panel: A photo in direction Alps (South) taken during the measurement. The right column presents the same values andphoto, but for 11 March 2007. The labels on the panels show the day and time in UTC.

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MITEV, MATTHEY

CONCLUSION

We presented dataset selected in stable atmosphericsituations that demonstrates the consistency betweenthe ABC and ABL top determined from backscatterlidar measurements from one side, and the visuallydetermined VR from the other side. This consistencyindicates to a possibility to use VR for quality controlof the lidar determined ABC, with the advantage ofavailability at the lidar home site.

The demonstrated consistency also indicates thatbackscatter lidar measurements at vertical directionmay provide monitoring of the slant path visibility atairports, in this way relaxing the eye�safety require�ments. A subject of a further study will be to identifythe number of points necessary for adequate VR eval�uation.

ACKNOWLEDGMENTS

This work is performed in the frame of the ECproject EARLINET–ASOS (RICA�025991). Grati�tude is due to Payerne Aerological Station ofMeteoswiss for the radiosonde profiles and to thePolice of Neuchâtel for the camera images. The con�sultations in visibility range determination from MmeJoanna Baniewicz are greatly appreciated.

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1.E–051.E+04

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MeasurementsKoschmieder’s equation

Visibility range, m

1.E–04

1.E+051.E+03

Fig. 2. Points are presenting the mean extinction as derivedfrom the lidar measurements (vertical axis) and the esti�mated visibility range during the respective measurement(horizontal axis). The solid line shows Koschmieder’sequation.