portable lidar system for atmospheric boundary layer measurements

Optical Engineering 45�7�, 076201 �July 2006�

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Portable lidar system for atmospheric boundarylayer measurements

Yellapragada Bhavani KumarGovernment of IndiaDepartment of SpaceNational MST Radar FacilityPost Box No. 123Tirupati, 517 502, IndiaE-mail: [email protected]

Abstract. A portable lidar system has been developed for monitoringthe evolution of the atmospheric boundary layer �ABL� using aerosols asthe tracers of atmospheric motion. This lidar system utilizes state-of-the-art technology with a high repetition rate, low pulse energy laser, andphoton-counting detection. The system is capable of continuous, autono-mous data acquisition in both daytime and nighttime. During daytime, theaerosol profiles measured with the lidar are found to have sufficient sig-nal strength to observe the structure of the ABL, which plays an impor-tant role in air pollution phenomena. The time series of the range-squared signal has been used for the interpretation of the ABLdynamics. © 2006 Society of Photo-Optical InstrumentationEngineers. �DOI: 10.1117/1.2221555�

Subject terms: aerosols; atmospheres; backscattering; diode lasers; laserapplications; lidar; neodium lasers.

Paper 050417RR received May 25, 2005; revised manuscript received Dec. 12,2005; accepted for publication Dec. 16, 2005; published online Jul. 11, 2006.

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1 Introduction

Recently there has been a growing demand for collectingdata on the depth of the atmospheric boundary layer �ABL�,which is of interest in several different areas such as me-teorology, pollutant studies, and global modeling. Correcttreatment of the physical processes in the ABL is of crucialimportance for weather forecasting and numerical simula-tions of climate change.

Over the last 30 years, lidar instruments have been de-veloped to study the structure of the atmosphere by meansof the elastic and inelastic scattering of light from constitu-ents of the atmosphere such as air molecules and aerosols.By using aerosols as the tracers of atmospheric dynamics,lidar can identify several parameters of the ABL such asboundary layer top and entrainment zone depth1–3 in realtime with high temporal and spatial resolutions. However,the traditional lidar instruments used so far during the lastthree decades have typically been high-energy pulse, lowrepetition rate systems. Recent advances in the solid-statelasers, detectors, and data acquisition systems have enabledthe development of a new generation lidar technology, amicropulse lidar4 �MPL� that employs a high pulse repeti-tion rate, low pulse power transmitter approach. A numberof papers have reported on the applications of MPL to en-vironment studies.5,6 The temporal evolution of the bound-ary layer height and boundary layer aerosol can also bestudied.6

Under an Indo-Japanese collaboration program, a back-scatter lidar system7 was set up at Gadanki, India in March1998 to study the tropical atmosphere. The basic lidar sys-tem configuration is such that it has been providing aerosolheight profiles covering the upper troposphere and lowerstratosphere. To fill the gap region in aerosol height pro-files, a project was taken up recently to develop a lidar

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ystem with a low pulse energy approach. The project wasunded by the Department of Space, Government of India.he low pulse energy approach in our method is similar to

hat in the MPL. However, the MPL system design is suchhat the same telescope is employed for both transmittingnd receiving the laser beam and always a small portion ofhe transmitted laser reflected inside the telescope oftenamages the detector. To overcome this difficulty, the laseread and transmit optics are mounted on the side of theeceive telescope. Because the laser pulse energy is low, theidar system was configured to use photon-counting detec-ion. But the disadvantage of photon-counting detectors isheir inherently small dynamic range.8 However, the use ofhoton-counting detection along with high repetition rateasers allows a wide dynamic range in the measured signal.y employing a minimal number of commercially availableomponents, the cost of construction of this lidar systemas been reduced considerably.

For the first time in India, a lidar system with high rep-tition rate, low pulse energy diode pumped neodymium:ttrium aluminum garnet �Nd:YAG� laser operating at32 nm has been developed and made operational at Na-ional MST Radar Facility �NMRF�, Gadanki for monitor-ng the evolution of ABL using aerosols as the tracers oftmospheric dynamics. The location of lidar site is Gadanki13.5°N, 79.2°E; 375 m mean sea level� situated close toirupati, a famous temple town, in the southern part ofndia. The purpose of this paper is to present the ability ofnewly developed, low pulse energy lidar system usage inBL measurements.

Instrument Setuphe newly developed portable lidar system has been in op-ration at a fixed location in a temperature controlled cubi-al. Figure 1 shows the schematic block diagram of theidar system. The lidar transmitter system is the second har-
onic output of a microchip �all-in-one laser cavity�
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Kumar: Portable lidar system for atmospheric boundary¼

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�Nd:YAG� laser �Laser-Compact model LCS-DTL-314QT�. It is a laser that is diode pumped and acousticswitched. The output pulse energy is 2 to 20 �J dependingon the repetition rate. The output pulse energy is set at10 �J at 2500 Hz repetition rate. The laser beam diameteris 0.4 mm and its divergence is less than 1.5 mrad. Thelaser beam was expanded to 3 mm in diameter and colli-mated to have beam divergence of about 200 �rad. Thelight output was linearly polarized with the degree of po-larization being greater than 99%. The laser beam is sentinto the atmosphere using two mirrors kept at 45-degangles. Figure 2 shows the characteristics of laser output.The output energy of laser decreases as a function of thepulse repetition rate.

A monoaxial configuration was employed in the lidarsystem. The laser backscattered light was received by a

Fig. 1 Schematic block diagram of portable lidar system.

Fig. 2 Characteristics of the diode laser pumped Nd:YAG laser.
XLaser pulse energy as function of repetition rate.


assegrainian telescope, whose diameter and F value were5 and cm 9, respectively. The geometrical form factor forcoaxial lidar having no apertures �other than the objective

ens or mirror of the telescope� or obstructions is unity,rovided the divergence angle of the laser beam is less thanhe opening angle of the telescope.9 Hence an iris �pinhole�iameter of 0.5 mm was used to obtain a receive field ofiew of about 400 �rad. A narrowband interference �IF�lter was positioned in front of the photomultiplier tubePMT�. An IF filter, whose center wavelength and band-idth were 532 nm and 0.5 nm, respectively, was used to

educe background light. A high gain PMT �Hamamatsu3234� was used as the photon detector. The pulse signals

rom PMT were passed through a discriminator �Phillipsodel 6908� and fed to a personal-computer-based multi-

hannel analyzer �EG&G Ortec model MCS- pci�. The in-trumental bin width was normally set at 200 ns, corre-ponding to a height resolution of 30 m. Usually a 300 000hot integrated photon-count profile constitutes a raw datarofile that corresponds to a time resolution of 120 sec. Theransmit and receive optics are housed in a closed box toaintain a clean, thermally stable, and dry environment.rief specifications of the system are given in Table 1.

Results and Discussionhe use of the lidar technique for boundary layer ranging

elies on the altitude resolved measurement of atmosphericackscatter intensity from outgoing laser radiation. Theackscatter intensity measured by lidar is proportional tohe signal backscattering by particles and air moleculesresent in the atmosphere. The expression that relates lasernergy output �Eo� and the backscattered signal P�r�, in thease of a coaxial lidar configuration, is given by9

�r� = Eokct

2

A

r2��r�T2�r� , �1�

here k is a constant function of intrinsic efficiencies of theidar system, ct /2 refers to the laser pulse length in thetmosphere �the factor 2 refers to the pulse round-trip time�nd A /r2 is the solid angle extended by the telescope mirrorf area A. The term ��r� is the volume backscatter coeffi-ient, which relates to the number of photons that would beackscattered when an atmospheric thickness is crossed byhe laser pulse length of � and is given in units of m−1 sr−1.he term T�r� refers to the transmissibility offered by thetmospheric path to the laser photons traveling from theround to a given distance r. In the above equation, theerm T�r� appears squared, because the laser photons haverst to travel to and then return from the distance r.

The measured signal intensity P�r�, given in terms ofhoton counts, corresponds to signal counts Ps�r� due to thetmospheric backscatter and counts PN due to sky back-round signal. It is represented as

�r� = PS�r� + PN. �2�

he lidar signal is usually a background signal correctednd transformed into a variable that removes the rangequare �1/r2� dependence10 X�r� or its logarithm S�r�.11

2
�r� = �P�r� − PN�r �3�
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�r� = ln X�r� �4�

he boundary layer �BL� height from lidar profiles is de-ned differently by numerous researchers. Sasano et al.12

dentified the BL height as the height where the signalackscatter begins to decrease from a relatively higheralue to lower region. Endlich et al.13 described BL heights the height at which a maximum negative gradient ofaser backscatter in vertical direction occurs. Flamant et al.3

lso mentioned the BL height as a zone of minimum in theertical gradient of the backscatter as defined by Endlich etl.13 A generally more reliable method than that givenbove involves the direct comparison of the backscatter sig-al with a fitted model Rayleigh backscattering profile. Theoundary layer height can then be defined as the first alti-ude point for which the measured backscatter profile ex-eeds the Rayleigh model profile by some fixed amount �.ollowing Melfi et al.,2 � is chosen to be 25%, although it

s noted that the boundary layer height retrieved by thisethod is not particularly sensitive to reasonable values of. Figure 3 shows this analysis on a typical trace. The lidarackscatter profiles recorded at 04:50 local time �LT� �dur-ng nighttime� and 08:51 LT �during daytime� on 6 January005 are shown in Figs. 3�a� and 3�b�, respectively. Theayleigh model is fitted between the indicated altitudes.

The height profile of the range-squared signal at 14:00

ig. 3 �a� A lidar backscatter profile recorded at 04:50 LT �duringighttime� on 6 January 2005. The Rayleigh model is fitted betweenhe indicated altitudes. �b� A lidar backscatter profile recorded at8:51 LT �during daytime� on 6 January 2005. The Rayleigh model

s fitted between the indicated altitudes.

Table 1 Portable lidar system specifications.

Specification

Laser

Type LD pumped Q switchedNd:YAG

Model LCS-DTL-314 QT,Laser-Compact, Russia

Output wavelength 532 nm

Output energy per pulse 2 to 20 �J

Repetition rate 0.2 to 10 KHz

Pulse duration � 10 ns

Beam size 0.4 mm

Polarization linear

Beam divergence � 1.5 mrad

Beam expander magnification 8X

Divergence of the expanded laserbeam

� 200 �rad

Receiver

Telescope geometry Cassegrainian

Diameter 15 cm

Telescope F ratio 9

Field of view � 400 �rad

IF filter bandwidth �full width at halfmaximum�

0.5 nm

Detector PMT, Hamamatsu R3234

PMT gain �typical� 2.5�107

Quantum efficiency � 10%

Data acquisition system

Type Single photon counting

Model EG&G Ortec, MCS-pci

Maximum counting rate 150 MHz

Dwell time selectable 100 ns to 1300 s

Number of channels selectable 65 536

Mechanical and environmental specifications of lidar housing

Cubical temperature � 22°C

Overall dimension lidar �Tx and Rxoptics�

30�60�100 cm

Weight of lidar �Tx and Rx optics� �25 kg

T �during daytime� on 8 December 2004 is shown in Fig.

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4, together with the profile of the atmospheric temperatureand relative humidity obtained by a simultaneous radio-sounding performed at the lidar site. The top of the ABL orthe mixed layer can be identified from the radiosonde, aswell as from the lidar profile. To demonstrate the perfor-mance of the lidar system, a continuous 48-h observationwas made over Gadanki site in January 2005 during theclear sky period. The variation of range corrected signalS�r� is shown in Fig. 5. Figures 5�a� and 5�b� represent thetime evolution of lidar range-squared signal S�r� from 0:00LT on 6 January to 0:00 LT on 8 January 2005 and com-pared along with the surface temperature measurements,respectively. Over land surfaces, the boundary layer has awell-defined structure that evolves with the diurnal cycle.14

Fig. 4 Comparison between the lidar range-squrelative humidity� performed at the same hour o

Fig. 5 Time evolution of range-squared signal SLT on 8 January 2005. �b� Temporal variation of
the above period.


ence, we discuss the observations in terms of internalublayers of the ABL, the major components of the BLtructure, such as the mixed layer, the residual layer, andhe stable or nocturnal boundary layer. From Fig. 5�a�, onean observe the mixed layer �ML� development on twoays from the lidar signal enhancement during 06:00 to5:00 LT. The ML or convective boundary layer �CBL� isormed by the convection that arises from solar heating ofhe earth’s surface and is associated with organized thermalransport due to highly developed vertical motion. The tur-ulence is essentially associated with the thermal transportrom the ground to the upper layers during daytime.14 On 6nd 7 January 2005, clouds were observed at the top of theBL during its growth phase �06:00 to 10:00 LT on 6 Janu-

ignal with radiosonde profiles �temperature andcember 2004 at 14:00 LT.

erved by lidar during 0:0 LT on 6 January to 0:0e temperatures measured over lidar site during

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ary and 10:00 to 14:00 LT on 7 January 2005�. During mostof the morning and afternoon, surface heating caused thelifting and mixing of the low level moist air with dry airaloft, leading to the development of cumulus clouds. On 6January 2005, the ML was observed reaching a maximumheight of 1.5 km at 15:00 LT. The growth of the BL on thisday was gradual. This may be due to a slow rise of groundtemperatures recorded. The depth of ML or CBL dependsmainly upon the diurnal variation of surface temperature.14

On 7 January 2005, the formation of ML was found to befaster than the corresponding one on 6 January 2005. Thismay be due to quicker rising of ground temperature thanthe previous day. On 7 January 2005, the ML reached apeak height of about 1.6 km at 15:00 LT. A thin residual-layer �RL�, the remnants of previous day’s ML, was alsoseen and eroded completely by the ML around 12:00 LT.

The RL is another ML basically formed by the daytimeturbulence and decays due to reduced surface heating aftersunset. On 6 and 7 January 2005, the RL appeared at aheight of 1.5 km initially and the later descended to 1 kmabout 20:00 LT. An elevation of RL is observed close to21:00 LT, which may be due to background dynamical con-ditions. The elevated RL is formed into a stratified structurelater and appeared to decay in strength with time. In theearly hours of the day, RL is generally represented as aneutrally stratified elevated layer, whose characteristics aregenerally observed to be initially those of the ML from theprevious day.14 Soon after the sunset, buoyant �eddy� pro-duction ceases and the atmosphere changes to a near neu-tral condition. This is a less turbulent RL containing theremnants of the daytime ML. In the late night hours, as thesurface gets cooled, the elevated eddies are directed down-ward. Due to the negative buoyancy, turbulent motions getdamped. This causes formation of a stable region �stratifiedlayer� near to the surface known as stable boundary layer�SBL� or nocturnal boundary layer �NBL�. In Figure 5�a�,the strong signal regions indicated by dark gray as a layeredstructure indicated by the SBL or NBL. On 6 and 7 January2005, the SBL is initially identified at about 500 m and iselevated gradually to about 800 m.

The portable lidar system has been successfully used tostudy the structure, dynamics, and the evolution of ABLusing aerosols as atmospheric tracers. Using the portablelidar system, the internal sublayers of ABL such as ML,RL, and SBL or NBL have been identified clearly over thetropical rural site Gadanki. This result shows the feasibilityof the low pulse energy lidar system application in ABLmeasurements.

4 ConclusionsA portable lidar system with low pulse energy approach hasbeen developed and successfully applied to study the struc-ture, dynamics, and the evolution of ABL using aerosols as
atmospheric tracers. a


cknowledgmentshe author would like to thank the National MST Radaracility, Department of Space, Government of India forunding the project “Boundary Layer Lidar” and also theecessary infrastructure support given for realization of theroject.

eferences

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2. S. H. Melfi, J. D. Spinhirne, S. H. Chou, and S. P. Palm, “Lidarobservations of vertically organized convection in the planetaryboundary layer over the ocean,” J. Clim. Appl. Meteorol. 24, 806–821 �1985�.

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comments,” Appl. Opt. 23, 652–653 �1984�.1. J. D. Klett, “Lidar inversion with variable backscatter to extinction

ratios,” Appl. Opt. 24, 1638–1643 �1985�.2. Y. Sasano, A. Shigematsu, H. Shimizu, N. Takeuchi, and M. Okkuda,

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Yellapragada Bhavani Kumar received hislicentiate in electronics and communicationengineering in 1986. He obtained a post-graduate degree in mathematics from Ku-vempu University, India. From 1992 topresent, he has been at the National MSTRadar Facility �NMRF�, Department ofSpace, Government of India, as a scientist/engineer with research interests in remotesensing and instrument development. Hehas successfully demonstrated the vibra-

ional Raman technique to NMRF lidar system in deriving atmo-pheric temperature and water vapor profiles. He has developed aesonance lidar system for atmospheric sodium measurements atMRF, Gadanki. Currently he is working on low pulse energy lidar
pplications for atmospheric aerosol and cloud research.
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portable lidar system for atmospheric boundary layer measurements

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