- Email: [email protected]
The Durban atmospheric LIDAR
ARTICLE IN PRESS
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 a
School of Pure and Applied Physics, University of KwaZulu-Natal, Durban 4041, South Africa Laboratoire de Physique de l’Atmosphe`re, UMR-CNRS 8105, Universite´ de La Re´union, BP 7151, St-Denis, Reunion Island, France c Service d’Ae´ronomie du CNRS, BP 3, 91371 Verrie`res-le-Buisson, France
b
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 10 Hz-laser was used as the transmitter for the LIDAR. The 10 Hz-laser has now been replaced by a 30 Hz-laser delivering five times more power.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 30 Hz-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 the atmosphere obtained with two lasers used to sample the atmosphere above Durban. The performance of the light detection and ranging (LIDAR) system with a 10 Hz neodymium:yttrium aluminium garnet (Nd:YAG) laser is compared to that with a recently installed 30 Hz Nd:YAG laser. The lasers show promising application for measuring the temperature and relative density of the atmosphere over Southern Africa, particularly the 30 Hz-laser which has a greater range. The article is divided into four parts: a description of the LIDAR instrumentation, a comparison of the backscattered signals and the atmospheric temperature profiles obtained with each laser, a comparison of the average monthly temperature profiles as derived from LIDAR, from co-located South African Weather Service Corresponding author. Tertiary Education Commission, Re´duit, Mauritius. Tel.: +230 4678796/4678800; fax: +230 4676579. E-mail address: [email protected] (A. Moorgawa).
0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.07.014
(SAWS) radiosondes and from the CIRA-86 model, and finally a comparison of LIDAR/SAGE II (stratospheric aerosol and gas experiment) aerosol measurements.
2. Atmospheric LIDAR principle and instrumentation The atmospheric LIDAR principle consists of transmitting a laser beam vertically into the sky, which interacts with air molecules and aerosols (particles whose size varies between 0.1 and 1.0 mm) in the troposphere and stratosphere. The backscattered photons are collected by parabolic mirrors and transmitted to photomultiplier detectors. The acquisition of data is carried out in the photon counting mode. The return signal is integrated to generate a count vs. altitude profile. The LIDAR is usually run at night and only under a clear sky. Daylight (even after filtering) affects the photomultipliers and most clouds (except cirrus clouds) absorb so much of the laser beam that the return signal from the upper atmosphere is negligible.
ARTICLE IN PRESS A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312
Fig. 1 is a schematic representation of the Durban atmospheric LIDAR. The laser is a pulsed Nd:YAG. The fundamental wavelength, lo ¼ 1064 nm, is frequency doubled using a potassium dihydrogen phosphate (KDP) crystal. Since its installation in April 1999 [1–2], the Durban atmospheric LIDAR operated with a SpectraPhysics Nd:YAG laser at a repetition rate of 10 Hz delivering an average power of 3 W. In May 2002, the 10 Hz laser was replaced with a more powerful Nd:YAG delivering at least 5 times more power and operating at a frequency of 30 Hz. Table 1 compares the characteristics of the two lasers. The emission wavelength le has been selected so that it does not correspond to any transition characteristic of any constituent of the atmosphere (absorption or resonance). The dichroic mirror used at the emission point of the laser separates the second harmonic beam from the fundamental (Fig. 1). Laser light is transmitted into the atmosphere after passing through the dichroic mirror and a Galilean telescope. The latter expands the beam 10 times and simultaneously reduces its divergence by the same amount thus increasing the backscattered intensity from a given observed scattering volume. The LIDAR system at Durban operates with two acquisition channels, a high altitude channel (channel A) and a low altitude channel (channel B). Table 1 summarises the 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.
307
two parabolic mirrors. The mirrors are held inside two long tubes, which shield them from luminous interference. The optical fibres located at the focus of each mirror carry the backscattered photons from the telescopes to the detection box, through channel A (Fig 1). Mounted bistatically, a smaller mirror is used to receive backscattered photons from the lower layers of the atmosphere (channel B). The photons are collected at the focus of the telescope and transmitted by an optical fibre to the detection box, which contains an interference filter, a collimator and a photomultiplier (PM). Fig 1 shows the LIDAR system used, including all main subsystems: emission source, reception telescopes, 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 is placed between the arrival point of the optical fibre and the collimator. 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 PMs are summarised in Table 2. Moreover, in order to reduce the saturation generated by the initial burst received by the PM of channel A from the lower atmospheric layers, an electronic shutter is used. It inhibits the photoelectron acceleration until the return signal is below saturation level. The principle consists of applying an inverted voltage on two dynodes of the PM during a period of time of about 50 ms. Furthermore, the PM of channel A is contained in a Peltier-effect cooling cell (model C 659-S) which reduces the noise due to dark current by lowering the cathode temperature and maintaining it between 15 and 20 1C. The Peltier cooling system has a built-in water-cooling system. 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. The electronic module has a 100 MHz pass-band and an integration time of 1 ms per bin, for a total range of 1024 bins. Hence the vertical resolution of the LIDAR is 150 m and can be degraded by grouping the bins. The data are stored per channel (channel A and B) by a computer in counts per microsecond.
Table 1 Characteristics of the transmitter and receiver systems of the Durban LIDAR Transmitters Spectra physics GCR-150 Emitted wavelength: le ¼ 532 nm Repetition rate: 10 Hz Energy per pulse: 300 mJ Pulse width: 6–7 ns Beam divergence (FWHM): 0.50 mrad
Receivers Spectra physics GCR-3 Emitted wavelength: le ¼ 532 nm Repetition rate: 30 Hz Energy per pulse: 500 mJ Pulse width: 5–7 ns Beam divergence (FWHM): 0.50 mrad
Newtonian telescopes Channel A: 2 parabolic mirrors Diameter of each mirror: 445 mm Focal length: 2000 mm Channel B: 1 parabolic mirror Diameter: 200 mm Focal length: 1000 mm
ARTICLE IN PRESS A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312
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Table 2 Characteristics of the photomultiplier of the Durban LIDAR Type
Hamamatsu R 1477 S
Maximum voltage Gain Quantum efficiency at 532 nm Cathode sensitivity Rising time Transit time Anode dark current
1000 V 107 17% 72.9 mA/W 2.2 ns 22 ns Typical 2 nA, max 5 nA
120 signal noise signal - noise
100
Altitude (km)
80
3.1. Comparison of LIDAR/CIRA-86 temperature profiles
60
40
20
0
LIDAR signal during the optical alignment procedure. The small peaks on the far left of the signal (Fig 3a) are due to the attenuating effect of the electronic shutter on the return signal 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 10 and 30 Hz lasers, respectively. Both lasers were run for the same period, viz. five and a half hours. The first profile (Fig 4a) was averaged over 198,000 laser shots of the 10 Hz-laser, while the second profile (Fig 4b) was averaged over 594,000 laser shots of the 30 Hz-laser. Knowing that the statistical error on the signal S is inversely proportional to thesquare pffiffiffiffiffi root of the received photon number NðDS=S / 1 N Þ, the use of the 30 Hzlaser is expected to improve the height range of the LIDAR. In fact, the height at which channel A signal becomes noisy appears near 75 km for the 30 Hz-laser (Fig 4b), while it appears 10 km below for the 10 Hz-laser (Fig 4a).
10-4
10-2
100
102
Number of photons (/shot/µs) Fig. 2. Plot of the LIDAR raw data for 08 June 1999 where the electronic shutters were set at 60 ms. The total noise (discontinuous line) is parametrised as a parabolic curve over the altitude range 100–150 km. The useful signal (continuous line) is that from which the total noise is subtracted.
The LIDAR return signal is due primarily to molecules (Rayleigh scattering) and aerosols (Mie scattering). The change of the curve slope, on a logarithmic scale, allows one to locate the height at which the noise is higher than the atmospheric signal. In fact, the most significant noise is located in the upper part of the LIDAR return signal. This is due to the statistical error incurred during photon counting coupled with the sky background noise, which increases with increasing altitude. 3. Results Plots a and b in Fig 3 show the analogue output from the PM vs. time in microsecond per shot of the laser for channel A and B, respectively obtained with the 10 Hz-laser as transmitter. The peaks on the right are due to return from cirrus clouds. The peak at about 55 ms, which corresponds to a 6 8 vertical height of 5510 2310 m ¼ 8:25 km. This low-level return serves as a good reference for the optimisation of the
The COSPAR International Reference Atmosphere (CIRA)-1986 climatological model is a compilation of experimental and theoretical data. It is based mainly on nadir infrared soundings from the selective chopper radiometer (SCR) experiment on board NIMBUS-6 from 1973 to 1974 and from the pressure modulated radiometer (PMR) experiment on NIMBUS-7 from 1975 to 1978. These two experiments yield temperature profiles between 20 and 80 km [3]. Together with tropospheric measurements carried out between 1958 and 1973 [4], they give climatological monthly averages between 0 and 80 km with a global average from 801S to 581N. The comparison of the LIDAR temperature profiles obtained from the signals of Figs 4a and 4b is shown in plots (a) and (b) of Fig 5. The temperature profile is calculated by assuming that the scattering from the laser beam is of Rayleigh type (molecular scattering) and the atmosphere behaves as an ideal gas in hydrostatic equilibrium [5]. A full description of the temperature retrieval method from the LIDAR data has been given previously [1,2,5]. As can be seen in Fig 5b, the temperature profile obtained with the 30 Hz-laser has a maximum height of 72 km compared to that obtained with the 10 Hzlaser, 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 temperature profile as derived from channel A with the 10 Hz-laser, while plot 5b is retrieved from channel A with the 30 Hzlaser. These correspond to temperature profiles calculated on the basis of 198,000 and 594,000 laser shots, respectively, during the same period of observation (five and half hours). Actually, as discussed in the previous subsection, the obtained difference in the maximum heights of temperature profiles (72 and 62 km, respectively) is due to the 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.
120
120 sigA
sigA
noiseA
noiseA
sigA - noiseA 100
noiseB
sigB - noiseB
sigB - noiseB
10Hz-laser
60
30Hz-laser
40
40
20
20
0
(a)
0
100
Number of photons (/shot/µs)
combined signal
80
Altitude (km)
60
10-5
sigB
noiseB
combined signal
80
Altitude (km)
sigA - noiseA 100
sigB
(b)
10-5
100
Number of photons (/shot/µs)
Fig. 4. Comparison of backscattered LIDAR signals for channel A and B obtained with (a) 10 Hz laser and (b) 30 Hz 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.
(inversely proportional to the square root of the number of used samples). The maximum height is calculated to be that at which the signal 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 12.30 p.m. South African local time) balloon radiosonde Vaisala
RS80 at the Durban International airport which is about 11 km SSW of the University of KwaZulu-Natal, Durban. Thus the LIDAR station and the radiosonde site are quasico-located. The radiosonde data consists of several parameters such as: pressure, temperature, relative humidity, dew point temperature, altitude, wind direction and wind speed. For our purpose, we have worked with the temperature vs. altitude for the early morning data (01.00 a.m.) as this corresponds closely to the time of the running of the LIDAR.
ARTICLE IN PRESS A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312 75
75
70
70
65
65
60
60
55
55
Altitude (Km)
Altitude (Km)
310
50 45
50 45
40
40
35
35
30
30 LIDAR
LIDAR
CIRA-86 model
25
CIRA-86 model
25
LIDAR uncertainty 20 180
(a)
200
220
240
260
280
LIDAR uncertainty 20 180
300
Temperature (K)
(b)
200
220
240
260
280
300
Temperature (K)
Fig. 5. Comparison of the LIDAR/CIRA-86 temperature profiles for 25 May 1999 obtained with (a) 10 Hz laser and (b) 30 Hz laser. Both profiles were obtained after five and a half hours of LIDAR acquisition.
Fig. 6(a) is a plot of the LIDAR temperature and the SAWS radiosonde temperature measurements for 25 May 1999. Plots (b), (c) and (d) in Fig. 6 are the comparisons of the average monthly LIDAR temperature with the average monthly SAWS radiosonde temperature and the CIRA-86 model temperatures. Those comparisons are made for the months of May, June and August 1999. As can be seen the two profiles are easily joinable. As shown in the plots, in the overlap zone LIDAR and SAWS profiles are in quite good agreement and show relatively similar temperature profiles. The discrepancies do not exceed 5 K(DT ¼ |TLIDARTSAWS|p5 K, where T denotes temperature), which can be considered as reasonable, taking into account the possible influence of stratospheric aerosols (aerosol scattering overestimates the LIDAR return and thus underestimates temperature values), and the great variability of the temperature in the winter stratosphere. The measurement comparisons are limited to the lower stratosphere (between 20 and 28 km) as the balloon usually bursts around 29–30 km. As for comparison between LIDAR and CIRA-86 monthly temperature profiles, plots (b), (c) and (d) in Fig 6 indicate a good agreement in the upper stratosphere and in the lower mesosphere. The wave-like structures appearing above 40 km in the LIDAR temperature profile may 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 during winter in westerly winds [6]. Due to the exponential decreases of atmospheric density with height, PW and GW disturbance amplitudes are expected to increase in winter due to the reversal of the zonal wind in the stratosphere [7–8]. 3.3. Comparison of LIDAR/SAGE II aerosol measurements SAGE II is an instrument launched aboard the satellite ERBS (Earth Radiation Budget Satellite) flown by NASA which is used to monitor stratospheric gases and aerosol concentration [9]. The SAGE II instrument scans the sun vertically during both spacecraft sunrise and sunset. Values of atmospheric extinction through the limb path are measured from the top of the atmosphere to cloud tops or the earth’s horizon. The transmission data are then averaged into 1 km vertical increments and inverted to yield 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 11 1999 (Fig. 7). The agreement between the two profiles is quite reasonable. The extinction coefficients derived from LIDAR are lower compared to SAGE II values in the 23–30 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.
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waves and gravity waves, which originate in the troposphere [11–14]. The comparison of the aerosol extinction profile obtained with the Durban LIDAR and the satellite data (SAGE II) shows that the two profiles agree quite well in the stratosphere. In the future, we envisage adding a Raman channel to the existing LIDAR system in order to measure simultaneously temperature and aerosol extinction in the troposphere and lower stratosphere. Acknowledgements This work was supported by the French Ministry of Foreign Affairs and Co-operation and CNRS, by the National Research Foundation (NRF) of South Africa and by the Regional Council of Reunion Island. We would like to thank the Physics workshop at the University of KwaZulu-Natal, for the proper maintenance of the LIDAR. References Fig. 7. Comparison of aerosol extinction profiles obtained from LIDAR and SAGE II for 11 June 1999.
(i) the measurement technique of SAGE II which scans the atmosphere between the earth and the sun horizontally with a resolution of 1 km at 525 nm wavelength is different from that of the LIDAR which samples the atmosphere vertically over a given location with a resolution of 150 m at 532 nm wavelength; (ii) the aerosols show large variability in the lower stratosphere above Durban (a subtropical site) as suggested by Bencherif et al. [10]. For height above 30 km the SAGE II extinction profile overlaps closely with that of the LIDAR indicating that the two experiments are representative of the extinction profile for the latitude of Durban. 4. Conclusion A comparison of the performance of a 10 Hz-laser and a 30 Hz-laser used to sample the atmosphere above Durban shows that the 30 Hz-laser is a better instrument for measuring the temperature in the middle atmosphere. Depending on the atmospheric weather conditions (clear sky), the 30 Hz-laser can probe the atmosphere up to an altitude of 74 km, giving vertical temperature profiles. The optimum height reached by the laser depends on good weather conditions: free from haze and low humidity. Such conditions are prevalent mostly during winter. The average monthly LIDAR temperature profile agrees quite well with the average monthly SAWS temperature profile and CIRA-86 model. The large oscillations of the LIDAR temperature profiles in the upper stratosphere can be attributed to the vertical propagation of planetary
[1] Bencherif H, Moorgawa A, Morel B, Michaelis MM, Leveau J, Hauchecorne A, et al. First validation of stratospheric temperature profiles obtained by a Rayleigh LIDAR over Durban, South Africa. S Afr J Sci 2000;96:487–92. [2] Moorgawa, A., Michaelis, M.M., Diab, R D., Anderson, J., Kuppen, M., Prause, A.R., et al. (2000). South African atmospheric LIDAR. In Proceedings of the High-Power Laser Ablation III. Proc SPIE, 4065, 324–337. [3] Barnett JJ, Corney M, Middle atmosphere reference model derived from satellite data. In: Labitzke K, Barnett J, Edwards B, editors. Handbook for map. IL: Urbana; 1985. p. 47–85. [4] Oort AH, Global atmospheric circulation statistics 1958–1983, NOAA professional paper 1983; 14:180–5. [5] Hauchecorne A, Chanin ML. Density and temperature profiles obtained by LIDAR systems between 35 and 70 km. Geophys Res Lett 1980;7:565–8. [6] Salby ML. Fundamentals of atmospheric physics, San Diego, CA, USA. Int Geophys Ser 1996;61:627. [7] Schoeberl MR. Stratospheric warmings:observations and theory. Geophys Space Phys 1978;16:521–38. [8] Labitzke K. Stratospheric mesospheric midwinter disturbances. J Geophys Res 1981;86:9665–78. [9] Chu WP, McCormick MP, Lenoble J, Brogniez C, Pruvost P. SAGE II inversion algorithm. J Geophys Res 1989;94:8339–52. [10] Bencherif H, Portafaix T, Baray JL, Morel B, Baldy S, Leveau J, et al. LIDAR observations of lower stratospheric aerosols over South Africa linked to large scale transport across the southern subtropical barrier. J Atmos Sol Terr Phys 2003;65:707–15. [11] Hauchecorne A, Chanin ML. A mid-latitude ground-based LIDAR study of stratospheric warmings and planetary wave propagation. J Geophys Res 1982;88:3843–9. [12] Jenkins DB, Wareing DP, Thomas L, Vaughan G. Upper stratospheric and mesospheric temperatures derived from LIDAR observations at Aberystwyth. J Atmos Terr Phys 1987;49:287–98. [13] Wilson R, Hauchecorne A, Chanin ML. Gravity spectra in the middle atmosphere as observed by Rayleigh LIDAR. Geophys Res Lett 1990;17:1585–8. [14] Marenco F, di Sarra A, Cacciani M, Fiocco G, Fua D. Thermal structure of the winter middle atmosphere observed by LIDAR at Thule, Greenland during 1993–1994. J Atmos Sol Terr Phys 1997;59: 2151–8.
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 a
School of Pure and Applied Physics, University of KwaZulu-Natal, Durban 4041, South Africa Laboratoire de Physique de l’Atmosphe`re, UMR-CNRS 8105, Universite´ de La Re´union, BP 7151, St-Denis, Reunion Island, France c Service d’Ae´ronomie du CNRS, BP 3, 91371 Verrie`res-le-Buisson, France
b
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 10 Hz-laser was used as the transmitter for the LIDAR. The 10 Hz-laser has now been replaced by a 30 Hz-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 30 Hz-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 the atmosphere obtained with two lasers used to sample the atmosphere above Durban. The performance of the light detection and ranging (LIDAR) system with a 10 Hz neodymium:yttrium aluminium garnet (Nd:YAG) laser is compared to that with a recently installed 30 Hz Nd:YAG laser. The lasers show promising application for measuring the temperature and relative density of the atmosphere over Southern Africa, particularly the 30 Hz-laser which has a greater range. The article is divided into four parts: a description of the LIDAR instrumentation, a comparison of the backscattered signals and the atmospheric temperature profiles obtained with each laser, a comparison of the average monthly temperature profiles as derived from LIDAR, from co-located South African Weather Service Corresponding author. Tertiary Education Commission, Re´duit, Mauritius. Tel.: +230 4678796/4678800; fax: +230 4676579. E-mail address: [email protected] (A. Moorgawa).
0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.07.014
(SAWS) radiosondes and from the CIRA-86 model, and finally a comparison of LIDAR/SAGE II (stratospheric aerosol and gas experiment) aerosol measurements.
2. Atmospheric LIDAR principle and instrumentation The atmospheric LIDAR principle consists of transmitting a laser beam vertically into the sky, which interacts with air molecules and aerosols (particles whose size varies between 0.1 and 1.0 mm) in the troposphere and stratosphere. The backscattered photons are collected by parabolic mirrors and transmitted to photomultiplier detectors. The acquisition of data is carried out in the photon counting mode. The return signal is integrated to generate a count vs. altitude profile. The LIDAR is usually run at night and only under a clear sky. Daylight (even after filtering) affects the photomultipliers and most clouds (except cirrus clouds) absorb so much of the laser beam that the return signal from the upper atmosphere is negligible.
ARTICLE IN PRESS A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312
Fig. 1 is a schematic representation of the Durban atmospheric LIDAR. The laser is a pulsed Nd:YAG. The fundamental wavelength, lo ¼ 1064 nm, is frequency doubled using a potassium dihydrogen phosphate (KDP) crystal. Since its installation in April 1999 [1–2], the Durban atmospheric LIDAR operated with a SpectraPhysics Nd:YAG laser at a repetition rate of 10 Hz delivering an average power of 3 W. In May 2002, the 10 Hz laser was replaced with a more powerful Nd:YAG delivering at least 5 times more power and operating at a frequency of 30 Hz. Table 1 compares the characteristics of the two lasers. The emission wavelength le has been selected so that it does not correspond to any transition characteristic of any constituent of the atmosphere (absorption or resonance). The dichroic mirror used at the emission point of the laser separates the second harmonic beam from the fundamental (Fig. 1). Laser light is transmitted into the atmosphere after passing through the dichroic mirror and a Galilean telescope. The latter expands the beam 10 times and simultaneously reduces its divergence by the same amount thus increasing the backscattered intensity from a given observed scattering volume. The LIDAR system at Durban operates with two acquisition channels, a high altitude channel (channel A) and a low altitude channel (channel B). Table 1 summarises the 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.
307
two parabolic mirrors. The mirrors are held inside two long tubes, which shield them from luminous interference. The optical fibres located at the focus of each mirror carry the backscattered photons from the telescopes to the detection box, through channel A (Fig 1). Mounted bistatically, a smaller mirror is used to receive backscattered photons from the lower layers of the atmosphere (channel B). The photons are collected at the focus of the telescope and transmitted by an optical fibre to the detection box, which contains an interference filter, a collimator and a photomultiplier (PM). Fig 1 shows the LIDAR system used, including all main subsystems: emission source, reception telescopes, 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 is placed between the arrival point of the optical fibre and the collimator. 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 PMs are summarised in Table 2. Moreover, in order to reduce the saturation generated by the initial burst received by the PM of channel A from the lower atmospheric layers, an electronic shutter is used. It inhibits the photoelectron acceleration until the return signal is below saturation level. The principle consists of applying an inverted voltage on two dynodes of the PM during a period of time of about 50 ms. Furthermore, the PM of channel A is contained in a Peltier-effect cooling cell (model C 659-S) which reduces the noise due to dark current by lowering the cathode temperature and maintaining it between 15 and 20 1C. The Peltier cooling system has a built-in water-cooling system. 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. The electronic module has a 100 MHz pass-band and an integration time of 1 ms per bin, for a total range of 1024 bins. Hence the vertical resolution of the LIDAR is 150 m and can be degraded by grouping the bins. The data are stored per channel (channel A and B) by a computer in counts per microsecond.
Table 1 Characteristics of the transmitter and receiver systems of the Durban LIDAR Transmitters Spectra physics GCR-150 Emitted wavelength: le ¼ 532 nm Repetition rate: 10 Hz Energy per pulse: 300 mJ Pulse width: 6–7 ns Beam divergence (FWHM): 0.50 mrad
Receivers Spectra physics GCR-3 Emitted wavelength: le ¼ 532 nm Repetition rate: 30 Hz Energy per pulse: 500 mJ Pulse width: 5–7 ns Beam divergence (FWHM): 0.50 mrad
Newtonian telescopes Channel A: 2 parabolic mirrors Diameter of each mirror: 445 mm Focal length: 2000 mm Channel B: 1 parabolic mirror Diameter: 200 mm Focal length: 1000 mm
ARTICLE IN PRESS A. Moorgawa et al. / Optics & Laser Technology 39 (2007) 306–312
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Table 2 Characteristics of the photomultiplier of the Durban LIDAR Type
Hamamatsu R 1477 S
Maximum voltage Gain Quantum efficiency at 532 nm Cathode sensitivity Rising time Transit time Anode dark current
1000 V 107 17% 72.9 mA/W 2.2 ns 22 ns Typical 2 nA, max 5 nA
120 signal noise signal - noise
100
Altitude (km)
80
3.1. Comparison of LIDAR/CIRA-86 temperature profiles
60
40
20
0
LIDAR signal during the optical alignment procedure. The small peaks on the far left of the signal (Fig 3a) are due to the attenuating effect of the electronic shutter on the return signal 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 10 and 30 Hz lasers, respectively. Both lasers were run for the same period, viz. five and a half hours. The first profile (Fig 4a) was averaged over 198,000 laser shots of the 10 Hz-laser, while the second profile (Fig 4b) was averaged over 594,000 laser shots of the 30 Hz-laser. Knowing that the statistical error on the signal S is inversely proportional to thesquare pffiffiffiffiffi root of the received photon number NðDS=S / 1 N Þ, the use of the 30 Hzlaser is expected to improve the height range of the LIDAR. In fact, the height at which channel A signal becomes noisy appears near 75 km for the 30 Hz-laser (Fig 4b), while it appears 10 km below for the 10 Hz-laser (Fig 4a).
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The LIDAR return signal is due primarily to molecules (Rayleigh scattering) and aerosols (Mie scattering). The change of the curve slope, on a logarithmic scale, allows one to locate the height at which the noise is higher than the atmospheric signal. In fact, the most significant noise is located in the upper part of the LIDAR return signal. This is due to the statistical error incurred during photon counting coupled with the sky background noise, which increases with increasing altitude. 3. Results Plots a and b in Fig 3 show the analogue output from the PM vs. time in microsecond per shot of the laser for channel A and B, respectively obtained with the 10 Hz-laser as transmitter. The peaks on the right are due to return from cirrus clouds. The peak at about 55 ms, which corresponds to a 6 8 vertical height of 5510 2310 m ¼ 8:25 km. This low-level return serves as a good reference for the optimisation of the
The COSPAR International Reference Atmosphere (CIRA)-1986 climatological model is a compilation of experimental and theoretical data. It is based mainly on nadir infrared soundings from the selective chopper radiometer (SCR) experiment on board NIMBUS-6 from 1973 to 1974 and from the pressure modulated radiometer (PMR) experiment on NIMBUS-7 from 1975 to 1978. These two experiments yield temperature profiles between 20 and 80 km [3]. Together with tropospheric measurements carried out between 1958 and 1973 [4], they give climatological monthly averages between 0 and 80 km with a global average from 801S to 581N. The comparison of the LIDAR temperature profiles obtained from the signals of Figs 4a and 4b is shown in plots (a) and (b) of Fig 5. The temperature profile is calculated by assuming that the scattering from the laser beam is of Rayleigh type (molecular scattering) and the atmosphere behaves as an ideal gas in hydrostatic equilibrium [5]. A full description of the temperature retrieval method from the LIDAR data has been given previously [1,2,5]. As can be seen in Fig 5b, the temperature profile obtained with the 30 Hz-laser has a maximum height of 72 km compared to that obtained with the 10 Hzlaser, 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 temperature profile as derived from channel A with the 10 Hz-laser, while plot 5b is retrieved from channel A with the 30 Hzlaser. These correspond to temperature profiles calculated on the basis of 198,000 and 594,000 laser shots, respectively, during the same period of observation (five and half hours). Actually, as discussed in the previous subsection, the obtained difference in the maximum heights of temperature profiles (72 and 62 km, respectively) is due to the 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.
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(inversely proportional to the square root of the number of used samples). The maximum height is calculated to be that at which the signal 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 12.30 p.m. South African local time) balloon radiosonde Vaisala
RS80 at the Durban International airport which is about 11 km SSW of the University of KwaZulu-Natal, Durban. Thus the LIDAR station and the radiosonde site are quasico-located. The radiosonde data consists of several parameters such as: pressure, temperature, relative humidity, dew point temperature, altitude, wind direction and wind speed. For our purpose, we have worked with the temperature vs. altitude for the early morning data (01.00 a.m.) as this corresponds closely to the time of the running of the LIDAR.
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Fig. 6(a) is a plot of the LIDAR temperature and the SAWS radiosonde temperature measurements for 25 May 1999. Plots (b), (c) and (d) in Fig. 6 are the comparisons of the average monthly LIDAR temperature with the average monthly SAWS radiosonde temperature and the CIRA-86 model temperatures. Those comparisons are made for the months of May, June and August 1999. As can be seen the two profiles are easily joinable. As shown in the plots, in the overlap zone LIDAR and SAWS profiles are in quite good agreement and show relatively similar temperature profiles. The discrepancies do not exceed 5 K(DT ¼ |TLIDARTSAWS|p5 K, where T denotes temperature), which can be considered as reasonable, taking into account the possible influence of stratospheric aerosols (aerosol scattering overestimates the LIDAR return and thus underestimates temperature values), and the great variability of the temperature in the winter stratosphere. The measurement comparisons are limited to the lower stratosphere (between 20 and 28 km) as the balloon usually bursts around 29–30 km. As for comparison between LIDAR and CIRA-86 monthly temperature profiles, plots (b), (c) and (d) in Fig 6 indicate a good agreement in the upper stratosphere and in the lower mesosphere. The wave-like structures appearing above 40 km in the LIDAR temperature profile may 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 during winter in westerly winds [6]. Due to the exponential decreases of atmospheric density with height, PW and GW disturbance amplitudes are expected to increase in winter due to the reversal of the zonal wind in the stratosphere [7–8]. 3.3. Comparison of LIDAR/SAGE II aerosol measurements SAGE II is an instrument launched aboard the satellite ERBS (Earth Radiation Budget Satellite) flown by NASA which is used to monitor stratospheric gases and aerosol concentration [9]. The SAGE II instrument scans the sun vertically during both spacecraft sunrise and sunset. Values of atmospheric extinction through the limb path are measured from the top of the atmosphere to cloud tops or the earth’s horizon. The transmission data are then averaged into 1 km vertical increments and inverted to yield 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 11 1999 (Fig. 7). The agreement between the two profiles is quite reasonable. The extinction coefficients derived from LIDAR are lower compared to SAGE II values in the 23–30 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.
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waves and gravity waves, which originate in the troposphere [11–14]. The comparison of the aerosol extinction profile obtained with the Durban LIDAR and the satellite data (SAGE II) shows that the two profiles agree quite well in the stratosphere. In the future, we envisage adding a Raman channel to the existing LIDAR system in order to measure simultaneously temperature and aerosol extinction in the troposphere and lower stratosphere. Acknowledgements This work was supported by the French Ministry of Foreign Affairs and Co-operation and CNRS, by the National Research Foundation (NRF) of South Africa and by the Regional Council of Reunion Island. We would like to thank the Physics workshop at the University of KwaZulu-Natal, for the proper maintenance of the LIDAR. References Fig. 7. Comparison of aerosol extinction profiles obtained from LIDAR and SAGE II for 11 June 1999.
(i) the measurement technique of SAGE II which scans the atmosphere between the earth and the sun horizontally with a resolution of 1 km at 525 nm wavelength is different from that of the LIDAR which samples the atmosphere vertically over a given location with a resolution of 150 m at 532 nm wavelength; (ii) the aerosols show large variability in the lower stratosphere above Durban (a subtropical site) as suggested by Bencherif et al. [10]. For height above 30 km the SAGE II extinction profile overlaps closely with that of the LIDAR indicating that the two experiments are representative of the extinction profile for the latitude of Durban. 4. Conclusion A comparison of the performance of a 10 Hz-laser and a 30 Hz-laser used to sample the atmosphere above Durban shows that the 30 Hz-laser is a better instrument for measuring the temperature in the middle atmosphere. Depending on the atmospheric weather conditions (clear sky), the 30 Hz-laser can probe the atmosphere up to an altitude of 74 km, giving vertical temperature profiles. The optimum height reached by the laser depends on good weather conditions: free from haze and low humidity. Such conditions are prevalent mostly during winter. The average monthly LIDAR temperature profile agrees quite well with the average monthly SAWS temperature profile and CIRA-86 model. The large oscillations of the LIDAR temperature profiles in the upper stratosphere can be attributed to the vertical propagation of planetary
[1] Bencherif H, Moorgawa A, Morel B, Michaelis MM, Leveau J, Hauchecorne A, et al. First validation of stratospheric temperature profiles obtained by a Rayleigh LIDAR over Durban, South Africa. S Afr J Sci 2000;96:487–92. [2] Moorgawa, A., Michaelis, M.M., Diab, R D., Anderson, J., Kuppen, M., Prause, A.R., et al. (2000). South African atmospheric LIDAR. In Proceedings of the High-Power Laser Ablation III. Proc SPIE, 4065, 324–337. [3] Barnett JJ, Corney M, Middle atmosphere reference model derived from satellite data. In: Labitzke K, Barnett J, Edwards B, editors. Handbook for map. IL: Urbana; 1985. p. 47–85. [4] Oort AH, Global atmospheric circulation statistics 1958–1983, NOAA professional paper 1983; 14:180–5. [5] Hauchecorne A, Chanin ML. Density and temperature profiles obtained by LIDAR systems between 35 and 70 km. Geophys Res Lett 1980;7:565–8. [6] Salby ML. Fundamentals of atmospheric physics, San Diego, CA, USA. Int Geophys Ser 1996;61:627. [7] Schoeberl MR. Stratospheric warmings:observations and theory. Geophys Space Phys 1978;16:521–38. [8] Labitzke K. Stratospheric mesospheric midwinter disturbances. J Geophys Res 1981;86:9665–78. [9] Chu WP, McCormick MP, Lenoble J, Brogniez C, Pruvost P. SAGE II inversion algorithm. J Geophys Res 1989;94:8339–52. [10] Bencherif H, Portafaix T, Baray JL, Morel B, Baldy S, Leveau J, et al. LIDAR observations of lower stratospheric aerosols over South Africa linked to large scale transport across the southern subtropical barrier. J Atmos Sol Terr Phys 2003;65:707–15. [11] Hauchecorne A, Chanin ML. A mid-latitude ground-based LIDAR study of stratospheric warmings and planetary wave propagation. J Geophys Res 1982;88:3843–9. [12] Jenkins DB, Wareing DP, Thomas L, Vaughan G. Upper stratospheric and mesospheric temperatures derived from LIDAR observations at Aberystwyth. J Atmos Terr Phys 1987;49:287–98. [13] Wilson R, Hauchecorne A, Chanin ML. Gravity spectra in the middle atmosphere as observed by Rayleigh LIDAR. Geophys Res Lett 1990;17:1585–8. [14] Marenco F, di Sarra A, Cacciani M, Fiocco G, Fua D. Thermal structure of the winter middle atmosphere observed by LIDAR at Thule, Greenland during 1993–1994. J Atmos Sol Terr Phys 1997;59: 2151–8.