Scientific Research and Essays Vol. 6(4), pp. 896-907, 18 February, 2011 Available online at http://www.academicjournals.org/SRE DOI: 10.5897/SRE10.920 ISSN 1992-2248 ©2011 Academic Journals

Full Length Research Paper

Development of Mie LIDAR system and initial cloud observations over Central Himalayan region

T. Bangia1,2*, A. Kumar1, R. Sagar1, P. K. Agarwal3 and S. K. Singh1

1Aryabhatta Research Institute of observational sciences (ARIES), Manora Peak, Nainital, Uttarakhand 263 129, India.

2Uttar Pradesh Technical University, (UPTU), Lucknow, Uttar Pradesh 226021, India.

3Ideal Institute of Management and Technology, Ghaziabad, Uttar Pradesh 201003, India.

Accepted 14 December, 2010

Aryabhatta Research Institute of Observational Sciences (ARIES) at Manora Peak (29° 22’ N, 79° 27’ E), Nainital is located in the Central Himalayan region at an altitude of ~ 1950 m. This free tropospheric site away from urban pollution has potential to study the dynamics of lower atmosphere. Taking advantage of its geographical location, a ground based monostatic pulsed Mie LIght Detection and Ranging (LIDAR) system was developed indigenously at the site and made operational from January, 2010. The present paper describes the system details including its optical analysis and initial observations of thin and thick clouds for 2 and 27 January, 2010 respectively. The cloud thickness, vertical position, aerosol extinction and optical depth values were deduced for understanding the cloud dynamics at this pristine site. The observational results were validated with moderate resolution Imaging Spectroradiometer (MODIS) satellite level 3.0 data available for the site. Key words: Mie, telescope, aerosol, extinction, cloud.

INTRODUCTION LIDAR is an active remote sensing system extensively being employed for studying earth’s atmosphere in terms of aerosol distribution (Barnaba and Gobbi, 2001), extinction and back-scattering coefficients (Fernald et al., 1972), optical depths (Chen et al., 2009), temperature (Hauchecorne and Chanin, 1980) etc. It is a versatile tool for boundary layer (Kunkel et al., 1977) and cloud studies (Reagan et al., 1989) required for estimating Earth’s radiation budgets (Beyerle et al., 2001). LIDAR system probes high energy Light Amplification by Stimulated Emission of Radiation (LASER) beam into the atmos-phere and collects back-scattered photons by means of a telescope. Photons are detected and measured using the detection system. The detection system employs highly sensitive light detectors like Avalanche Photo Diodes (APD) or Photo Multiplier Tube (PMT) that convert an incident photon into an electrical current pulse which is then amplified and filtered using a pulse discriminator.

Discriminator output is measured in terms of photon counts as a function of range or time using the Multi

*Corresponding author. E-mail: bangia@aries.res.in.

Channel Scaler (MCS) card and acquisition software. The total signal recorded by MCS across the averaging period of interest is stored in a computer. Analysis of signal intensity versus return time allows for range resolved deduction of atmospheric parameters.

A continuous monitoring of atmosphere from various geographical locations is required to understand the local and global effects on climate due to anthropogenic and natural activities. Ground based Mie LIDAR systems for atmospheric studies are operational in southern, western and northern regions of India (Bhavani et al., 2006; Jayaraman et al., 1995; Arya et al., 2007). Himalayan region in India is known to be very dynamic in nature and influenced by western disturbances (Hatwar et al., 2005). Taking the benefit of its geographical location an indi-genously designed monostatic pulsed, Mie LIDAR system was developed and made operational at ARIES from January, 2010. System was developed using optimized optics, mechanics and advanced electronics for data acquisition and processing. Optical design analysis for existing 380 mm Cassegrain telescope and back end detection optics was carried out in ZEMAX- EE simulation package to optimize system parameters. The data analysis software was developed in matrix laboratory

Bangia et al. 897

Figure 1. Block diagram of Mie LIDAR system at ARIES.

(MATLAB) environment. Clouds play an important role in weather and climate.

They regulate the radiative balance of the earth-atmosphere system by reflecting the incoming solar radiation and absorbing the outgoing infrared radiation. The net cooling/warming by the clouds is the sum of these two processes, and it depends on the type and amount of clouds present (Ramanathan et al., 1989). The long term observations of clouds over this mid latitude and high altitude observing site can contribute signi-ficantly in understanding Earth’s radiation budget, global climate change and turbulence. An initial attempt in this direction was made using the thin and thick cloud observations of 2nd and 27th January, 2010, respectively with the developed Mie LIDAR System. MATERIALS AND METHODS

System description

Mie LIDAR system at ARIES (Figure 1) consists of three major subsystems namely transmitter, receiver and Detector and Data Acquisition System (DDAS) (Liu et al., 2007). The transmitter (Figure 2a) consists of a Q-switched, Nd: YAG pulsed LASER operating at 532 nm with pulse energy of ~ 750 mJ (Table 1). Observer can set various parameters such as pulse energy,

wavelength, Q-switch delay, pulse repetition rate etc. from the console panel on LASER control unit. Control unit regulates the system temperature and quality of water used for cooling the flash lamps and LASER rods. Safety interlocks are provided to shutdown the system in case of any malfunction or deviation. An external water-cooled chiller is employed for cooling the LASER unit. A 9 mm diameter green visible LASER beam is expanded using 10× beam expander to reduce the beam divergence from 0.5 to < 0.1 mrad. Reduction in the beam divergence allows a narrow Field Of View (FOV) for the receiving telescope, thereby reducing background noise.

The entire transmission system is mounted on an optical breadboard having vibration dampeners and height adjustment provision. Expanded LASER beam of 90 mm diameter is passed through a window of the wall separating transmitter and receiver rooms and made to fall on a thick hard coated flat steering mirror of 250 mm diameter. Mirror oriented at 45° to LASER beam axis makes it parallel to telescope axis by directing it vertically up in to the atmosphere through an opening provided in the roof (Figure 2b). Mirror is provided with precise azimuth, elevation and tilting controls using an indigenously designed and manufactured mirror mount to align the beam parallel to the axis of receiving telescope. LASER beam path is shielded with a 150 mm diameter tube to reduce random scattering and ensure eye safety of observers.

The receiver consists of a Cassegrain telescope (Figure 2c) having primary and secondary mirrors of diameter 380 and 123 mm, respectively (Table 2). The telescope tube assembly is mounted vertically on a table of 1 × 1 m fabricated in - house using 10 mm thick mild steel sheet and standard angles. Mounting table is supported on height adjustment screws having maximum

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Figure 2. Snapshots of Mie LIDAR sub-systems. (a) LASER source and beam expander acting as transmitter; (b) Transmission of LASER beam; (c) Cassegrain telescope; (d) Instrumentation rack, and (e) Roll-off roof.

Table 1. Specifications of transmitter system.

Mie LIDAR transmitter system

LASER (Pulsed ) Nd: YAG (Quanta make)

Wavelength 532 nm

Pulse frequency 30 Hz

Pulse width 7 ns

Pulse energy 750 mJ

Beam expander 10 x

Beam divergence (after expansion) 0.1 mrad

movement of ± 25 mm from base. Screws are mounted on vibration dampeners to minimize the transfer of vibrations from ground to optics during observations. Mounting table has a gap of ~ 1 m from ground to accommodate back-end optics and detection electronics. Back-end optics consists of two collimating lenses (F/2) and a narrow-band (~ 1.0 nm) interference filter having transmission of ~ 60 %. The selection and position of different optical elements of back-end optics with the receiver telescope was ascertained through ZEMAX analysis. Collimating lenses concentrate the entire

FOV of telescope within the active diameter of PMT. A field limiting iris was used to restrict the FOV to 6 mrad. The backscattered light was band - limited using interference filter, thereby increasing the signal to noise ratio (SNR) by cutting off the sky background. The mechanical axis of PMT was aligned with optical axis of telescope using a plumb line and a low-power He - Ne LASER beam.

The DDAS comprises of PMT, cooling unit, power supply unit, pulse discriminator and MCS package (Table 3). The PMT has a 52 mm diameter, end window photomultiplier with infra-red sensitive

Bangia et al. 899

Table 2. Specifications of receiver system.

Mie LIDAR receiver system

Telescope Classical cassegrain

Primary mirror diameter 380 mm

Primary mirror focal length 1905 mm

Primary mirror focal ratio f/5

Secondary mirror diameter 123 mm

Secondary mirror focal length 829 mm

Telescope focal ratio f/15

Field of view 6 mrad

Table 3. Specifications of detector and data acquisition system.

Mie LIDAR detector and data acquisition system

Detector PMT (Electron tubes)

Active diameter of cathode 9 mm

Quantum efficiency at 532 nm 15%

Sensitivity 105 A/lumen

Single e- rise time and FWHM 2 and 3 ns

Single e- transit time 45 ns

Cooling Thermoelectric

Pulse discriminator-input range 0 to -150 mV

Jitter < 20 ps FWHM

MCS counting rate (max) 150 MHz

Dwell time (selectable) From 100 ns to 1300 s per channel

S20 photocathode, electrostatically reduced to 9 mm diameter, and 14 Be Cu dynodes of linear focused design. It is cooled to ~ -20°C before observations to minimize dark current. The operating PMT voltage is set at -1860 V, corresponding to a gain of ~ 10

7 within its

plateau region. Image of back-scattered LASER beam from different heights is formed on an active area of PMT through back-end optics. Output of PMT is then fed to pulse discriminator having an amplifier of 1 GHz, optimized for pulse widths from 250 ps to 5 ns. The pulse discriminator is essentially a high-speed comparator (Argall and Sica, 2002) whose state changes when signal from the

PMT exceeds a pre-set level (discriminator level). Pulse discriminator is tuned for reception of very weak signals at a threshold of ~ - 10 mV. The discriminated output is fed to MCS card mounted in the Peripheral Component Interconnect (PCI) slot of computer arranged with power supply and cooling unit in an instrumentation rack (Figure 2d). MCS records the counting rate of events as a function of time. When a scan starts, it starts counting the input events in first range bin of its digital memory and at the end of a pre-selected dwell time it advances to next range bin and so on.

A motorized roll-off-roof was designed and installed as a unique facility for automated LIDAR observations at night (Figure 2e). It was constructed using standard angles, galvanized iron corrugated sheets, cast iron wheels and steel beams (acting as rails). A worm wheel drive with 1 horse power, 3Φ motor was grouted on the roof of LIDAR building for automated operation. Optical analysis

The optical design optimization and analysis were carried out by using ZEMAX-EE simulation package. The optical layout, encircled

energy, spot diagrams, variation of RMS wavefront error and distortion were simulated by taking the optical parameters of system (Boyarchuk et al., 2008). The result depicts that rays passing through the hole of primary mirror after reflection from secondary mirror are focused on to the focal plane of PMT through collimating lenses and a narrow band interference filter at 532 nm (Figure 3).

Encircled energy, f is the fraction of total energy,

( )I r contained within a circle of a specified radius r, centered on

the chief ray reference point. This fraction is always between 0 and 1 and is defined as:

0

0

( )

( )

r

I r dr

f

I r dr

∞=∫

∫

(1)

A variation on encircled energy with fields computes the radial coordinate at which a specified fraction is achieved. It was concluded from the plot that 80% of the encircled energy lies in 55 micron radius (Figure 4). Spot diagram for three fields at 0.1, 0.0 and -0.1° were analyzed. Root-mean-square (RMS) spot radius which is the distance between each ray and the reference point is squared averaged over all the rays and then square root is taken. RMS spot radius gives a rough idea of spread of the rays, since it depends upon every ray. GEO radius is radius of circle centered at the reference point enclosing all the rays. Spot diagram shows the RMS spot size as 33.385, 35.678 and 33.385 micron and GEO radius as 67.333, 55.103 and 67.333 micron for three fields at 0.1, 0.0 and - 0.1°, respectively (Figure 5).

Variation of RMS wavefront error in waves with field was

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45 mm

304.8 mm 1352.55 mm

50 mm

Figure 3. Optical layout of the system.

Figure 4. Variation of encircled energy of the system with field.

determined and found to vary marginally from 0.13 to 0.12 with the variation in field from 0.0 to 0.1° (Figure 6a).

RMS wave front error is used to calculate Strehl ratio, a measure for the optical quality of telescopes. The variation of Strehl ratio was determined as about 0.51 to 0.58 with the variation in field from 0.0

to 0.1° (Figure 6b). Distortion whether tanF θ or Fθ , measures

the deviation between the reference and actual image height and selects the best f-value to minimize the distortion, without the restriction that the proportion of linearity be defined by the focal length of the system. It was determined as 0.169 mm (Figure 6c).

RESULTS LASER pulses were fired at a Pulse Repetition Frequency (PRF) of 30 Hz with pulse energy ~750 mJ at 532 nm. The received intensity, ( , )I rλ in terms of photon

counts was calculated up to ~ 15 km height with ~ 0.3 km resolution using standard LIDAR equation (Measures,

1984) given as:

2 ( , ') '

2

0( , ) ( ) ( , ) ( )

r

ro

r dr

I r I r CO r r e

σ λ

λ λ β λ−

−∫

= (2)

Where 0 ( )I λ , is transmitted LASER intensity in terms of

photon counts at 532 nm, C is system constant, O(r) is

overlap factor, ( , )rβ λ and ( , )rσ λ are back-scattering

and extinction coefficients, respectively and r is the height. The system constant, C is given by:

2

rA cC

η τ= (3)

where η is efficiency of receiver system, rA is receiving

Bangia et al. 901

Figure 5. Spot diagram for the variation in fields.

S

trehl ra

tio R

MS

w

ave fr

ont

err

or

in

waves

+Y field in degrees

+Y field in degrees

Figure 6. (a) Variation of RMS wavefront error in waves with field: (b) Variation of Strehl ratio with field, and (c) distortion curve

telescope area, τ is pulse width of transmitted LASER

beam and c is the speed of light. ( )O r remains zero in

non-overlap region of LIDAR and typically reaches 1 in

complete overlap region. ( )O r vs. r was plotted by

considering LASER divergence of 0.1 mrad, telescope FOV of 6 mrad and separation ~1 m with 0° inclination angle between LASER and telescope (Figure 7). From the plot, it is seen that LASER beam enters into the entire FOV of telescope from a distance of ~ 300 m.

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Ov

erl

ap

fac

tor

Figure 7. Overlap factor vs height plot.

Above 15 km, the signal was weak, merely at the noise level due to the appearance of clouds. Clouds were pre-venting the LASER beam from propagating upward any further, resulting in only noise being observed. Observa-tions from the system are normally carried out for 256 range bins with a temporal resolution of 2 µs corres-ponding to height up to 76.8 km. A typical sample profile up to this height has been plotted (Figure 8). The signal represents the average photon count profiles for ~ 2 h duration. The mean background noise level is estimated from the signal returns above 30 km, from where the count level was stable and constant over the integration period. The obtained mean is subtracted from the signals upto 15 km. The back-scattering and extinction coeffi-cients are contributed by both aerosols and molecules and expressed as:

( , ) ( , ) ( , )a m

r r rσ λ σ λ σ λ= + (4)

and

( , ) ( , ) ( , )a m

r r rβ λ β λ β λ= + (5)

where, subscript (a) and (m) indicate aerosols and molecules, respectively.

Molecular contributions were calculated by taking data from CIRA 1986 standard atmosphere model for our site

(Rees et al., 1990). The constants assumed weres

N

(molecular number density) as 2.54743 × 1025

m-3

and

γ (depolarization factor) as 0.035 (Sagar et al., 2004).

Molecular backscatter coefficient ( )m

rβ was estimated

by considering the theoretical molecular LIDAR ratio

/m m m

S σ β= as 8 / 3π sr, under the condition of zero

molecular absorption. Various techniques to solve Equation (2) have been employed (Fernald, 1972, 1984; Klett, 1981, 1985, 1986; Sasano and Nakane, 1984). For our present analysis, we have adopted Fernald method,

by assuming the aerosol LIDAR ratio, /a a a

S σ β= as

constant that is, 30 over the entire range (Takamura and Sasano, 1987). Mathematically, Fernald method can be expressed as:

(6)

where ( )X r is range normalized signal given by

2( , )I r rλ and cr is the reference height.

Since, the outgoing LASER pulse and the telescope FOV are in full overlap at ~ 0.3 km, so the inversion is computed starting at an initial height from 0.3 km. The atmospheric parameters were extracted from cloud observations on 2nd January, 2010 during 18:50 to 19:30 h (IST) between 9.3 to 10.8 km and on 27th January, 2010 during 18:40 to 19:20 hrs (IST) between 5.4 to 8.4 km. The mean vertical profiles of log-range adjusted

( )exp[ 2( )] ( )

( ) ( )( )

2 ( ) exp[ 2( ) ( ') ']( ) ( )

c

c c

r

a m m

r

a mr r

ca a m m

a c m c r r

X r S S r dr

r rX r

S X r S S r dr drr r

β

β β

ββ β

− −

+ =

− − − +

∫

∫ ∫

Bangia et al. 903

P

ho

ton

co

un

t (l

og

sc

ale

)

Figure 8. Typical sample profiles up to 76.8 km for ~ 2 h continuous operation.

(a) (b) Figure 9. Time and height variation plot for log-range adjusted power S(r) of: (a) 2nd January, 2010 during 18:50 to 19:30 h (IST), and (b) 27th January, 2010 during 18:40 to 19:20 h (IST).

power, ( ) log( ( ))S r X r= (Klett, 1981) for 2nd and

27th January, 2010, respectively were plotted (Figures 9a and b). The figures clearly indicate the presence of clouds on these two days.

The mean vertical profiles of aerosol extinction coefficients and aerosol optical depth (AOD) for the observations of 2nd January, 2010 between 18:50 to 19:30 h (IST) and of 27th January, 2010, between 18:40 to 19:20 h (IST), respectively were plotted (Figure 10 and 11). The aerosol extinction decreases sharply with increasing height (Figures 10a and b). The mean value of aerosol

extinction was ~ 0.003 km-1

between 0.3 to 9 km for 2nd January and ~ 0.02 km

-1 from 0.3 to 5.1 km for 27

th

January, 2010. The mean AOD values obtained after integrating the extinction profiles from 0.3 to 15 km were 0.04 and 0.22 for the observations of 2nd January, 2010 and 27th January, 2010, respectively (Figure 11a and b). The AOD results were compared with the MODIS Level 3.0 data aboard the Aqua (Figure 12a and b) and Terra satellites (Figure 13a and b) at 550 nm for our location and found to be in good agreement (Chan, 2009). AOD of cloud ( AOD

cloud) is an important parameter in radiative

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Aerosol extinction, σσσσ (km-1

) Aerosol extinction, σσσσ (km-1

)

(a) (b) Figure 10. Mean Aerosol extinction profile of: (a) 2nd January, 2010 during 18:50 to 19:30 hrs (IST), and (b) 27th January, 2010 during18:40 to 19:20 h (IST)

.

(AOD) (AOD)

(a) (b) Figure 11. Mean AOD profile of: (a) 2nd January, 2010 during 18:50 to 19:30 h (IST), and (b) 27th January, 2010 during 18:40 to 19:20 h (IST).

transfer studies and is obtained from LIDAR data using the equation:

AOD ( )cloud

r drσ= ∫top

base

h

h

(7)

where, ( )rσ is the aerosol extinction within the cloud,

which extends from cloud base (hbase) to cloud top (htop) (Sassen et al., 1989). The optical depth of clouds was calculated as 0.0084 (over 9.3 to 10.8 km) and 0.1137 (over 5.4 to 8.4 km) for 2nd and 27th January, 2010, respectively (Figures 14a and b). It indicated the existence of thin cloud on 2nd January and thick cloud on 27th January, 2010.

DISCUSSION The mean AOD values obtained by LIDAR after integrating the extinction profiles from 0.3 to 15 km were 0.04 and 0.22 for the observations of 2nd January, 2010 and 27th January, 2010, respectively. Thin clouds with mean AOD value of 0.0084 on 2nd January, 2010 were found at high altitude from 9.3 to 10.8 km. Clouds at high altitudes can absorb the long wave outgoing radiation from the Earth’s surface while reflect the short wave incoming solar radiation (Baker, 1997). Thick clouds with mean AOD value of 0.1137 on 27th January, 2010 were found from 5.4 to 8.4 km. The temperature of the lower atmosphere depends on heat radiated from the Earth’s surface and therefore decreases with increasing distance from the Earth’s surface. Heating tails off gradually below

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(a) (b) Figure 12. AOD results from MODIS Level 3.0 data aboard the aqua satellite at 550 nm of: (a) 2nd January, 2010, and (b) 27th January, 2010.

(a) (b) Figure 13. AOD results from MODIS Level 3.0 data aboard the Terra satellite at 550 nm of: (a) 2 January, 2010, and (b) 27 January, 2010.

the cloud top for thick clouds, however for thin clouds it parallels the clear-sky heating profile (Lieberman et al., 2003). Western disturbances in the Himalayan region causing widespread snowfall have been studied during January, 2002 (Hatwar et al., 2005). On the basis of previous studies on clouds, we can expect that the cloud observed on 2nd January, 2010 may contain pure ice crystals, while that on 27th January, 2010 may contain supercooled water droplets or ice crystals or a mixture of both (Dowling and Radke, 1990). Temporal variations in cloud properties and their association with the meteoro-logical parameters can be ascertained with the statistical cloud studies. The long-range transport of aerosols influences the climate and affects the environment (Ramanathan et al., 2001).

Earlier studies on typical orography of the Himalayas for monsoon predictions (Webster et al., 1998) and recent studies on enhancement in the aerosol layer (Srivastava et al., 2010) make it imperative to explore this region through LIDAR studies. Conclusions The indigenously developed monostatic pulsed Mie LIDAR system at ARIES, Manora Peak is operational to study aerosol concentration, optical depth, cloud pro-perties etc. from this Central Himalayan site for a better understanding of atmospheric dynamics. Mie LIDAR optical system analysis was carried out to optimize the

906 Sci. Res. Essays

(a) (b) Figure 14. Mean AOD profile of cloud for: (a) 2nd January, 2010 from 9.3 to 10.8 km during 18:50 to 19:30 h (IST), and (b) 27th January, 2010 from 5.4 to 8.4 km during 18:40 to 19:20 h (IST).

system parameters. Results obtained from the cloud observations of thin and thick clouds on 2

nd and 27th

January, 2010 are presented and discussed. The results were validated from the MODIS level 3.0 data aboard the Aqua and Terra satellites for the observing site. The observations from this system at the pristine north Indian location in central Himalayan region will provide ground based data to enhance the understanding of atmospheric exchange processes and climatology in this highly dynamic region. ACKNOWLEDGEMENTS We thank S. Bhattacharjee, A. Omar, D. V. Phanikumar, A. Reddy and P. Pant for providing their valuable support. We also acknowledge the MODIS mission scientists and associated NASA personnel for the production of the data used in the present research effort. T. B. acknowledges the encouragement and support from the RDC committee of UPTU, Lucknow. This project is funded by Department of Science and Technology, Government of India. REFERENCES Argall P, Sica R (2002). LIDAR in Encyclopedia of Imaging Science and

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