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Optics & Laser Technology 39 (2007) 306–312

www.elsevier.com/locate/optlastec

The Durban atmospheric LIDAR

A. Moorgawaa,�, H. Bencherifb, M.M. Michaelisa, J. Porteneuvec, S. Malingaa

aSchool of Pure and Applied Physics, University of KwaZulu-Natal, Durban 4041, South AfricabLaboratoire de Physique de l’Atmosphere, UMR-CNRS 8105, Universite de La Reunion, BP 7151, St-Denis, Reunion Island, France

cService d’Aeronomie du CNRS, BP 3, 91371 Verrieres-le-Buisson, France

Received 2 June 2004; received in revised form 23 January 2005; accepted 25 July 2005

Available online 28 September 2005

Abstract

A brief description of the Durban atmospheric LIDAR (acronym for light detection and ranging) system for the measurement of

vertical temperature profiles is presented. In its original configuration, a 10Hz-laser was used as the transmitter for the LIDAR. The

10Hz-laser has now been replaced by a 30Hz-laser delivering five times more power. Both lasers have been used separately to sample

the atmosphere above Durban. A comparative analysis of the backscattered signals obtained separately from each laser shows that the

30Hz-laser has a much greater stratospheric range. The wavelength emitted for both lasers is 532 nm.

A comparison of the average monthly LIDAR temperature profiles has been computed between 20 and 60 km. The LIDAR

temperature profiles have been compared with the South African Weather Service (SAWS) radiosonde temperature measurement for the

lower stratosphere, between 20 and 27 km. The agreement between the two measurements is good in the lower stratosphere where SAWS

radiosondes overlap with LIDAR. A comparison of the LIDAR and SAGE II (stratospheric aerosol and gas experiment) aerosol

measurements has also been carried out.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: LIDAR; Aerosol; Stratosphere

1. Introduction

We report on vertical temperature profiles of theatmosphere obtained with two lasers used to sample theatmosphere above Durban. The performance of the lightdetection and ranging (LIDAR) system with a 10Hzneodymium:yttrium aluminium garnet (Nd:YAG) laser iscompared to that with a recently installed 30Hz Nd:YAGlaser. The lasers show promising application for measuringthe temperature and relative density of the atmosphereover Southern Africa, particularly the 30Hz-laser whichhas a greater range. The article is divided into four parts: adescription of the LIDAR instrumentation, a comparisonof the backscattered signals and the atmospheric tempera-ture profiles obtained with each laser, a comparison of theaverage monthly temperature profiles as derived fromLIDAR, from co-located South African Weather Service

e front matter r 2005 Elsevier Ltd. All rights reserved.

tlastec.2005.07.014

ing author. Tertiary Education Commission, Reduit,

: +230 4678796/4678800; fax: +230 4676579.

ess: moorgawaa@intnet.mu (A. Moorgawa).

(SAWS) radiosondes and from the CIRA-86 model, andfinally a comparison of LIDAR/SAGE II (stratosphericaerosol and gas experiment) aerosol measurements.

2. Atmospheric LIDAR principle and instrumentation

The atmospheric LIDAR principle consists of transmit-ting a laser beam vertically into the sky, which interactswith air molecules and aerosols (particles whose size variesbetween 0.1 and 1.0 mm) in the troposphere and strato-sphere. The backscattered photons are collected byparabolic mirrors and transmitted to photomultiplierdetectors. The acquisition of data is carried out in thephoton counting mode. The return signal is integrated togenerate a count vs. altitude profile. The LIDAR is usuallyrun at night and only under a clear sky. Daylight (evenafter filtering) affects the photomultipliers and most clouds(except cirrus clouds) absorb so much of the laser beamthat the return signal from the upper atmosphere isnegligible.

ARTICLE IN PRESSA. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312 307

Fig. 1 is a schematic representation of the Durbanatmospheric LIDAR. The laser is a pulsed Nd:YAG. Thefundamental wavelength, lo ¼ 1064 nm, is frequencydoubled using a potassium dihydrogen phosphate (KDP)crystal. Since its installation in April 1999 [1–2], theDurban atmospheric LIDAR operated with a Spectra-Physics Nd:YAG laser at a repetition rate of 10Hzdelivering an average power of 3W. In May 2002, the10Hz laser was replaced with a more powerful Nd:YAGdelivering at least 5 times more power and operating at afrequency of 30Hz. Table 1 compares the characteristics ofthe two lasers.

The emission wavelength le has been selected so that itdoes not correspond to any transition characteristic of anyconstituent of the atmosphere (absorption or resonance).The dichroic mirror used at the emission point of the laserseparates the second harmonic beam from the fundamental(Fig. 1).

Laser light is transmitted into the atmosphere afterpassing through the dichroic mirror and a Galileantelescope. The latter expands the beam 10 times andsimultaneously reduces its divergence by the same amountthus increasing the backscattered intensity from a givenobserved scattering volume.

The LIDAR system at Durban operates with twoacquisition channels, a high altitude channel (channel A)and a low altitude channel (channel B). Table 1 summarisesthe characteristics of the transmitter and receiver systems.Backscattered photons from high altitude are received by

Fig. 1. Schematic diagram of the Rayleigh-Mie LIDAR as implemented

at Durban, South Africa.

Table 1

Characteristics of the transmitter and receiver systems of the Durban LIDAR

Transmitters

Spectra physics GCR-150 Spectra physics GCR

Emitted wavelength: le ¼ 532nm Emitted wavelength:

Repetition rate: 10Hz Repetition rate: 30H

Energy per pulse: 300mJ Energy per pulse: 50

Pulse width: 6–7 ns Pulse width: 5–7 ns

Beam divergence (FWHM): 0.50mrad Beam divergence (F

two parabolic mirrors. The mirrors are held inside two longtubes, which shield them from luminous interference. Theoptical fibres located at the focus of each mirror carry thebackscattered photons from the telescopes to the detectionbox, through channel A (Fig 1). Mounted bistatically, asmaller mirror is used to receive backscattered photonsfrom the lower layers of the atmosphere (channel B). Thephotons are collected at the focus of the telescope andtransmitted by an optical fibre to the detection box, whichcontains an interference filter, a collimator and a photo-multiplier (PM). Fig 1 shows the LIDAR system used,including all main subsystems: emission source, receptiontelescopes, detection/counting module and storage unit.Each channel has an interference filter centred on le ¼

532 nm with a narrow bandwidth (Dl ¼ 1 nm) and isplaced between the arrival point of the optical fibre and thecollimator.The high and low altitude signals detected by the PMs are

amplified by pre-amplifiers in direct contact with the PMs(Hamamatsu R1477 S). The main characteristics of the PMsare summarised in Table 2. Moreover, in order to reduce thesaturation generated by the initial burst received by the PMof channel A from the lower atmospheric layers, anelectronic shutter is used. It inhibits the photoelectronacceleration until the return signal is below saturation level.The principle consists of applying an inverted voltage on twodynodes of the PM during a period of time of about 50ms.Furthermore, the PM of channel A is contained in a

Peltier-effect cooling cell (model C 659-S) which reducesthe noise due to dark current by lowering the cathodetemperature and maintaining it between �15 and �20 1C.The Peltier cooling system has a built-in water-coolingsystem.Note that because of its small receiver telescope, no

electronic shutter is applied to channel B.Fig 2 presents the channel A return signal recorded on

June 08 1999 with the electronic shutter set at 60 ms.The pre-amplified signals from the PMs are processed by

an electronic module of the acquisition system. Theelectronic module has a 100MHz pass-band and anintegration time of 1 ms per bin, for a total range of 1024bins. Hence the vertical resolution of the LIDAR is 150mand can be degraded by grouping the bins. The data arestored per channel (channel A and B) by a computer incounts per microsecond.

Receivers

-3 Newtonian telescopes

le ¼ 532nm Channel A: 2 parabolic mirrors

z Diameter of each mirror: 445mm

0mJ Focal length: 2000mm

Channel B: 1 parabolic mirror

WHM): 0.50mrad Diameter: 200mm

Focal length: 1000mm

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Table 2

Characteristics of the photomultiplier of the Durban LIDAR

Type Hamamatsu R 1477 S

Maximum voltage 1000V

Gain 107

Quantum efficiency at 532 nm 17%

Cathode sensitivity 72.9mA/W

Rising time 2.2 ns

Transit time 22 ns

Anode dark current Typical 2 nA, max 5 nA

10-4 10-2 100 1020

20

40

60

80

100

120

Number of photons (/shot/µs)

Alti

tude

(km

)

signalnoisesignal - noise

Fig. 2. Plot of the LIDAR raw data for 08 June 1999 where the electronic

shutters were set at 60ms. The total noise (discontinuous line) is

parametrised as a parabolic curve over the altitude range 100–150km.

The useful signal (continuous line) is that from which the total noise is

subtracted.

A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312308

The LIDAR return signal is due primarily to molecules(Rayleigh scattering) and aerosols (Mie scattering). Thechange of the curve slope, on a logarithmic scale, allowsone to locate the height at which the noise is higher thanthe atmospheric signal. In fact, the most significant noise islocated in the upper part of the LIDAR return signal. Thisis due to the statistical error incurred during photoncounting coupled with the sky background noise, whichincreases with increasing altitude.

3. Results

Plots a and b in Fig 3 show the analogue output from thePM vs. time in microsecond per shot of the laser forchannel A and B, respectively obtained with the 10Hz-laseras transmitter.

The peaks on the right are due to return from cirrusclouds. The peak at about 55 ms, which corresponds to avertical height of 55�10�6�3�108

2m ¼ 8:25 km. This low-level

return serves as a good reference for the optimisation of the

LIDAR signal during the optical alignment procedure. Thesmall peaks on the far left of the signal (Fig 3a) are due tothe attenuating effect of the electronic shutter on the returnsignal in the presence of low clouds.Figs 4(a) and (b) show a comparison of channel A (sig A)

and channel B (sig B) return signals obtained with the 10and 30Hz lasers, respectively. Both lasers were run for thesame period, viz. five and a half hours. The first profile(Fig 4a) was averaged over 198,000 laser shots of the10Hz-laser, while the second profile (Fig 4b) was averagedover 594,000 laser shots of the 30Hz-laser.Knowing that the statistical error on the signal S is

inversely proportional to the square root of the receivedphoton number NðDS=S / 1

� ffiffiffiffiffiNpÞ, the use of the 30Hz-

laser is expected to improve the height range of theLIDAR. In fact, the height at which channel A signalbecomes noisy appears near 75 km for the 30Hz-laser(Fig 4b), while it appears 10 km below for the 10Hz-laser(Fig 4a).

3.1. Comparison of LIDAR/CIRA-86 temperature profiles

The COSPAR International Reference Atmosphere(CIRA)-1986 climatological model is a compilation ofexperimental and theoretical data. It is based mainly onnadir infrared soundings from the selective chopper radio-meter (SCR) experiment on board NIMBUS-6 from 1973to 1974 and from the pressure modulated radiometer(PMR) experiment on NIMBUS-7 from 1975 to 1978.These two experiments yield temperature profiles between20 and 80 km [3]. Together with tropospheric measure-ments carried out between 1958 and 1973 [4], they giveclimatological monthly averages between 0 and 80 km witha global average from 801S to 581N.The comparison of the LIDAR temperature profiles

obtained from the signals of Figs 4a and 4b is shown inplots (a) and (b) of Fig 5. The temperature profile iscalculated by assuming that the scattering from the laserbeam is of Rayleigh type (molecular scattering) and theatmosphere behaves as an ideal gas in hydrostaticequilibrium [5]. A full description of the temperatureretrieval method from the LIDAR data has been givenpreviously [1,2,5]. As can be seen in Fig 5b, the temperatureprofile obtained with the 30Hz-laser has a maximumheight of 72 km compared to that obtained with the 10Hz-laser, which has a maximum height of 62 km.The difference between the two experiments is in the

repetition rate of the laser: plot 5a shows the temperatureprofile as derived from channel A with the 10Hz-laser,while plot 5b is retrieved from channel A with the 30Hz-laser. These correspond to temperature profiles calculatedon the basis of 198,000 and 594,000 laser shots, respec-tively, during the same period of observation (five and halfhours). Actually, as discussed in the previous subsection,the obtained difference in the maximum heights oftemperature profiles (72 and 62 km, respectively) is due tothe statistical error decrease with increasing laser shots

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Fig. 3. Real-time backscattered signal as displayed on the oscilloscope for (a) channel A (high altitude), and (b) channel B (low altitude). The y-axis is the

analogue output from the photomultiplier and the x-axis is the time in microseconds.

0

20

40

60

80

100

120

Alti

tude

(km

)

sigA

noiseA

sigA - noiseA

sigB

noiseB

sigB - noiseB

combined signal

10-5 1000

20

40

60

80

100

120

Number of photons (/shot/µs)10-5 100

Number of photons (/shot/µs)

Alti

tude

(km

)

sigA

noiseA

sigA - noiseA

sigB

noiseB

sigB - noiseB

combined signal

10Hz-laser 30Hz-laser

(a) (b)

Fig. 4. Comparison of backscattered LIDAR signals for channel A and B obtained with (a) 10Hz laser and (b) 30Hz laser. Both signals were integrated

over the same period: five and a half hours. The horizontal arrows indicate the height where the return signal from channel A becomes noisy.

A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312 309

(inversely proportional to the square root of the number ofused samples).

The maximum height is calculated to be that at which thesignal to noise ratio of the LIDAR output reaches 10%.

3.2. Comparison of LIDAR/SAWS monthly average

temperature profiles

The SAWS launches twice a day (01.00 a.m. and 12.30p.m. South African local time) balloon radiosonde Vaisala

RS80 at the Durban International airport which is about11 km SSW of the University of KwaZulu-Natal, Durban.Thus the LIDAR station and the radiosonde site are quasi-co-located. The radiosonde data consists of severalparameters such as: pressure, temperature, relative humid-ity, dew point temperature, altitude, wind direction andwind speed. For our purpose, we have worked with thetemperature vs. altitude for the early morning data (01.00a.m.) as this corresponds closely to the time of the runningof the LIDAR.

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180 200 220 240 260 280 30020

25

30

35

40

45

50

55

60

65

70

75

Temperature (K)

Alti

tude

(K

m)

LIDAR

CIRA-86 model

LIDAR uncertainty

180 200 220 240 260 280 30020

25

30

35

40

45

50

55

60

65

70

75

Temperature (K)

Alti

tude

(K

m)

LIDAR

CIRA-86 model

LIDAR uncertainty

(a) (b)

Fig. 5. Comparison of the LIDAR/CIRA-86 temperature profiles for 25 May 1999 obtained with (a) 10Hz laser and (b) 30Hz laser. Both profiles were

obtained after five and a half hours of LIDAR acquisition.

A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312310

Fig. 6(a) is a plot of the LIDAR temperature and theSAWS radiosonde temperature measurements for 25 May1999. Plots (b), (c) and (d) in Fig. 6 are the comparisons ofthe average monthly LIDAR temperature with the averagemonthly SAWS radiosonde temperature and the CIRA-86model temperatures. Those comparisons are made for themonths of May, June and August 1999. As can be seen thetwo profiles are easily joinable. As shown in the plots, inthe overlap zone LIDAR and SAWS profiles are in quitegood agreement and show relatively similar temperatureprofiles. The discrepancies do not exceed 5K(DT ¼ |TLI-

DAR�TSAWS|p5K, where T denotes temperature), whichcan be considered as reasonable, taking into account thepossible influence of stratospheric aerosols (aerosol scatter-ing overestimates the LIDAR return and thus under-estimates temperature values), and the great variability ofthe temperature in the winter stratosphere. The measure-ment comparisons are limited to the lower stratosphere(between 20 and 28 km) as the balloon usually burstsaround 29–30 km.

As for comparison between LIDAR and CIRA-86monthly temperature profiles, plots (b), (c) and (d) inFig 6 indicate a good agreement in the upper stratosphereand in the lower mesosphere. The wave-like structuresappearing above 40 km in the LIDAR temperature profilemay be related to vertical propagation of gravity waves(GW) and planetary waves (PW).

GW and PW are mainly generated in the troposphere.They propagate upward into the stratosphere duringwinter in westerly winds [6]. Due to the exponentialdecreases of atmospheric density with height, PW andGW disturbance amplitudes are expected to increase inwinter due to the reversal of the zonal wind in thestratosphere [7–8].

3.3. Comparison of LIDAR/SAGE II aerosol measurements

SAGE II is an instrument launched aboard the satelliteERBS (Earth Radiation Budget Satellite) flown by NASAwhich is used to monitor stratospheric gases and aerosolconcentration [9]. The SAGE II instrument scans the sunvertically during both spacecraft sunrise and sunset. Valuesof atmospheric extinction through the limb path aremeasured from the top of the atmosphere to cloud topsor the earth’s horizon. The transmission data are thenaveraged into 1 km vertical increments and inverted toyield profiles of aerosol extinction at the wavelengths 1.02,0.525, 0.453 and 0.385 mm.The LIDAR aerosol extinction profile at 532 nm has

been compared with that of SAGE II at 525 nm for June 111999 (Fig. 7). The agreement between the two profiles isquite reasonable. The extinction coefficients derived fromLIDAR are lower compared to SAGE II values in the23–30 km altitude range for two reasons:

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Fig. 6. (a) Comparison of LIDAR and SAWS temperature profiles obtained over Durban on 25 May 1999. The LIDAR profile is framed by the

temperature-retrieved values at �s, the total uncertainty. (b), (c) and (d) show average monthly LIDAR and SAWS temperature profiles for May (over 11

profiles), June (over 15 profiles) and August (over 11 profiles), 1999. The corresponding CIRA-86 monthly climatological profiles are superposed (see

legend). LIDAR and SAWS monthly profiles are framed by profiles taking into account the corresponding standard deviations.

A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312 311

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Fig. 7. Comparison of aerosol extinction profiles obtained from LIDAR

and SAGE II for 11 June 1999.

A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312312

(i)

the measurement technique of SAGE II which scansthe atmosphere between the earth and the sunhorizontally with a resolution of 1 km at 525 nmwavelength is different from that of the LIDAR whichsamples the atmosphere vertically over a given locationwith a resolution of 150m at 532 nm wavelength;

(ii)

the aerosols show large variability in the lower strato-sphere above Durban (a subtropical site) as suggestedby Bencherif et al. [10].

For height above 30 km the SAGE II extinction profileoverlaps closely with that of the LIDAR indicating that thetwo experiments are representative of the extinction profilefor the latitude of Durban.

4. Conclusion

A comparison of the performance of a 10Hz-laser and a30Hz-laser used to sample the atmosphere above Durbanshows that the 30Hz-laser is a better instrument formeasuring the temperature in the middle atmosphere.Depending on the atmospheric weather conditions (clearsky), the 30Hz-laser can probe the atmosphere up to analtitude of 74 km, giving vertical temperature profiles. Theoptimum height reached by the laser depends on goodweather conditions: free from haze and low humidity. Suchconditions are prevalent mostly during winter.

The average monthly LIDAR temperature profile agreesquite well with the average monthly SAWS temperatureprofile and CIRA-86 model. The large oscillations of theLIDAR temperature profiles in the upper stratosphere canbe attributed to the vertical propagation of planetary

waves and gravity waves, which originate in the tropo-sphere [11–14]. The comparison of the aerosol extinctionprofile obtained with the Durban LIDAR and the satellitedata (SAGE II) shows that the two profiles agree quite wellin the stratosphere.In the future, we envisage adding a Raman channel to

the existing LIDAR system in order to measure simulta-neously temperature and aerosol extinction in the tropo-sphere and lower stratosphere.

Acknowledgements

This work was supported by the French Ministry ofForeign Affairs and Co-operation and CNRS, by theNational Research Foundation (NRF) of South Africa andby the Regional Council of Reunion Island.We would like to thank the Physics workshop at the

University of KwaZulu-Natal, for the proper maintenanceof the LIDAR.

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