airborne co_2 coherent lidar for measurements of atmospheric aerosol and cloud backscatter

Airborne CO2 coherent lidar for measurementsof atmospheric aerosol and cloud backscatter

Robert T. Menzies and David M. Tratt

An airborne CO2 coherent lidar has been developed and flown on over 30 flights of the NASA DC-8research aircraft to obtain aerosol and cloud backscatter and extinction data at a wavelength near 9,um. Designed to operate in either zenith- or nadir-directed modes, the lidar can be used to measurevertical profiles of backscatter throughout the vertical extent of the troposphere and the lowerstratosphere. Backscatter measurements in absolute units are obtained through a hard-target calibra-tion methodology. The use of coherent detection results in high sensitivity and narrow field of view, thelatter property greatly reducing multiple-scattering effects. Aerosol backscatter profile intercompari-sons with other airborne and ground-based CO2 lidars were conducted during instrument checkout flightsover the NASA Ames Research Center before extended depolyment over the Pacific Ocean. Selectedresults from data taken during the flights over the Pacific Ocean are presented, emphasizing intercompari-sons with backscatter profile data obtained at 1.06 Rm with a NASA Goddard Space Flight CenterNd:YAG lidar on the same flights.

1. Introduction

Performance analyses of various embodiments of anEarth-orbiting Doppler lidar for global troposphericwind field measurements require intensive study ofthe global variability and climatology of atmosphericaerosol and cloud backscatter and extinction coeffi-cients. An airborne backscatter lidar has been devel-oped at the Jet Propulsion Laboratory (JPL) formultiple flights on the NASA Ames Research CenterDC-8 research aircraft and for measurement of thevertical profiles of atmospheric backscatter through-out the vertical extent of the troposphere and thelower stratosphere, with emphasis on coverage of awide range of latitudes. The 9.11-[Lm wavelengththat can be achieved with the transversely excitedatmosphere (TEA) CO2 laser by the use of the 0-18isotope has been considered the primary wavelengthin various assessments and design studies of anEarth-orbiting Doppler lidar, beginning with earlyNational Oceanic and Atmospheric Administration(NOAA) studies' and including the NASA LaserAtmospheric Wind Sounder (LAWS) instrument con-cept.2 The use of the rare-isotope laser line elimi-

The authors are with the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, Pasadena, California 91109.

Received 13 July 1993; revised manuscript received 25 January1994.

0003-6935/94/245698-14$06.00/0.© 1994 Optical Society of America.

nates the losses that are due to atmospheric CO2absorption, an important advantage for an instru-ment in a power-limited spacecraft environment.Modeling studies of the wavelength dependence ofaerosol backscatter for various aerosol compositionsalso indicated the possibility of backscatter enhance-ment in the 9.11-im region for certain compositions(ammonium sulfate, various silicate minerals) thatare expected in the troposphere.3 4 For the airbornebackscatter lidar flights, the 9.25-pm wavelength ofthe standard 2C 1602 laser 9R(24) line was chosen asa feasible alternative for which relatively high lasertransmitter efficiency could be obtained in a muchless costly laser medium and for which the aerosoland cloud backscatter characteristics closely resemblethose at 9.11 [Lm. Experience with ground-basedlidar operation at both the 9.25 and the 10.6-vLmwavelengths had verified the existence of backscatterenhancement at the shorter wavelength.5 Althoughatmospheric CO2 absorption losses are significantover path lengths of several kilometers, absorptionlosses can be taken into account with sufficientaccuracy to reduce the error in retrieval of backscat-ter coefficients to an acceptable level.

In this paper we provide a description of theAirborne Backscatter Lidar (ABL) instrument and ofthe field use of a hard-target calibration methodology,which was necessary to convert the flight data intoquantitative, calibrated profiles of the backscattercoefficient. Below, we present examples of data ob-

5698 APPLIED OPTICS / Vol. 33, No. 24 / 20 August 1994

tained when the ABL instrument was flown on theDC-8 during the Global Backscatter Experiment(GLOBE) missions over the Pacific Ocean during late1989 and mid 1990. The DC-8 carried a suite ofinstruments on these missions, which was dedicatedto measurements of aerosol properties. These in-cluded a Marshall Space Flight Center (MSFC) fo-cused cw CO2 laser backscatter instrument thatcollected data at aircraft altitude at 9.11 and 10.6-Ipm 6 and a Goddard Space Flight Center (GSFC)pulsed lidar that used a Nd:YAG transmitter, withthree-wavelength (0.53, 1.06, and 1.54-im) capabil-ity.7 8 The JPL and the GSFC groups have engagedin numerous aerosol and cirrus backscatter intercom-parison studies recently in an attempt to understandbetter the wavelength dependences of backscatterfrom various aerosol and cirrus cloud observations.These studies have been motivated in large part bythe interest in performance studies of Doppler lidarsat shorter infrared wavelengths corresponding tosolid-state laser transitions. Examples of multiwave-length intercomparison profiles are included.

2. Instrument Description

In Section 2 we describe the construction of and alsothe optical and control subsystems that comprise theABL. The ABL instrument was designed to providecalibrated aerosol and cloud backscatter data over awide dynamic range, with emphasis on the measure-ment of atmospheric backscatter in relatively cleanregions of the troposphere at a CO2 laser wavelengthnear 9 pLm. At the beginning of the design phase itwas known from ground-based CO2 lidar measure-

ments of backscatter in marine air masses from thePacific Ocean9 that backscatter coefficients ranged aslow as 10-"1 m-l sr-'. It was decided that the ABLshould have a backscatter sensitivity level of 10-11m-1 sr-' at a range of a few kilometers, whichmandated coherent detection and an injection-seededTEA CO2 laser transmitter. Papers describing mod-eled backscatter at CO2 laser wavelengths based onaerosol microphysical data from clean air over thePacific Ocean first appeared in early 1988; in thesepapers it was concluded that the backscatter coeffi-cients could be as low as 10-11 m-1 sr-' a significantpercentage of the time.1 0" 1 Actual CO2 lidar back-scatter measurements above Mauna Loa in Hawaiiduring the Fall of 1988 indicated frequent cases ofvery low backscatter in the upper troposphere.12These investigations confirmed the need for high-sensitivity airborne lidar measurements.

The ABL instrument optical layout and controlsystem schematics are depicted in Figs. 1 and 2,respectively. The major ABL subsystems are de-scribed below.

A. Transmitter Laser

The ABL transmitter laser is a commercial pulsedTEA CO2 unit constrained to operate in a single modeby means of injection seeding with a signal providedby a stable cw source.13"14 The transmitter is oper-ated with unstable resonator optics for high energyextraction and is grating tuned to the 9R(24) transi-tion at 9.25 tim to take advantage of spectral reso-nances found with the materials that generally consti-tute atmospheric aerosols derived from natural

\- AIRCRAFT WALL

I He-Ne --0 -- -- - ----------- --

0(1) Beam-Steering Optics (3) Pulse Shape Monitor (5) Offset Photomixer (7) Shutter

(2) Beam Expansion/Contraction (4) MCT Photoconductor (6) Signal Photomixer (8) LO Power MonitorLens Pair

Fig. 1. JPL ABL optical layout schematic. Inj, injection; LO, local oscillator; MCT, mercury cadmium telluride.

20 August 1994 / Vol. 33, No. 24 / APPLIED OPTICS 5699

Fig. 2. Block diagram of major elements and laser frequency control loops. I, in-phase; Q, quadrature; MOD., modulation; PLT,piezoelectric transducer; VCO, voltage-controlled oscillator; H.V., high-voltage; CAMAC, computer automated measurement and control;

O.G.E, optogalvanic; CKT., circuit.

sources.3-5 The transmitted pulse energy is 1 J,with a pulse duration of -1 Ls. Table 1 summarizesthe operational parameters of this laser. Additionalsteps were taken to make this unit rugged, in prepara-tion for flight; the grating-mount subassembly wassubstantially rebuilt, and the base panel by augmen-tation with aluminum braces.

The unstable resonator output coupler was de-signed, based on empty cavity theory, to avoid theparasitic transverse-mode competition that had beenthe subject of previous investigation with the JPLground-based lidar transmitter,' 5 while simulta-neously providing near-collimated output commensu-rate with the input acceptance range of the transmittelescope. It became evident following experimentalstudy that the optical activity of the TEA CO2 gainmedium was significantly affecting the mode param-

Table 1. Laser Parameters and Performance Specifications

WavelengthPulse lengthPulse energyPulse envelopeTEA moduleCavity lengthCavity magnificationGrating (master in aluminum)Output coupler (Ge meniscus)

S1

S2

9.25 l. m (9R24)1 ls1.1J35 mm x 25 mmLumonics 8201.05 m2.0Plane, 135 rulings mm-1

-8-m radius of curvature, anti-reflection with 85% R centralregion (10 mm x 15 mm)

16-m radius of curvature, anti-reflection

eters; consequently the coupler optic required substan-tial refiguring before a sufficiently collimated beamcould be obtained from the laser. Multiple trans-verse-mode oscillation remained a problem, but wecould ameliorate this problem by operating the cavityin a slightly misaligned state.15

B. Transmit-Receive Optics

The transmitter beam is first directed through a 5 xall-reflective off-axis Dall-Kirkham expander beforeprojection of the beam into the atmosphere. Afterthe transmitted beam passes through the expander, ithas a 15-cm cross section. Backscattered radia-tion from the atmosphere is collected with matchedcompression optics, optically combined with the localoscillator (LO) and focused onto a cooled mercurycadmium telluride (MCT) photomixer (operating at77 K) to generate the heterodyne signal. The fo-cused beams are f/10. The photomixer is backbi-ased to maximize the high-frequency responsivity atthe voltage-current characteristic operating pointwhen the LO is incident. The frequency responserolls off above 100 MHz.

The choice of bistatic transceiver geometry that isevident here sacrifices receiver area (and thereforesignal-to-noise ratio) and inherent resistance to trans-mit-receive misalignment (both mechanical and tur-bulence induced) for the sake of (a) reduced risk ofdetector damage as a result of strong near-field lidarreturns should the aircraft unexpectedly enter acloud and (b) reduced susceptibility to near-field datadistortion effects that are due to finite pulse length.16The lidar itself is capable of viewing both above and


below the aircraft through a 450 plane mirror thatselects either the upward (zenith) or the downward(nadir) viewing mode. Optical egress from the air-craft is accomplished through two (one zenith, onenadir) 35-cm-diameter double antireflection-coatedgermanium pressure windows mounted just insidethe aircraft fuselage.

C. Receiver and Pulse Diagnostics

The difference frequency heterodyne output from thesignal photomixer feeds directly to a cascaded ampli-fier combination comprising a 20-dB low-noise firststage followed by an 80-dB linear amplifier withvoltage-controllable gain capability, both with a 10-MHz bandwidth at a center frequency of 30 MHz.The IF output from the high-gain stage is thenprocessed with a complex demodulator. (Before thesecond GLOBE mission in the Spring of 1990 a linearvideo detection scheme had been used.'7 The use ofa linear video detector, in which the output voltage isproportional to the input RF voltage, i.e., the squareroot of the received lidar signal power, improves thereceiver dynamic range capability but introducescomplications and potential biases in the retrieval ofbackscatter coefficients.'8) The complex demodula-tor splits the signal into in-phase and quadraturecomponents that are down converted to the base bandby mixing them with a 30-MHz reference oscillator.The in-phase and the quadrature outputs of thecomplex demodulator are each boosted a further 20dB, low-pass filtered to eliminate higher-order har-monics, separately digitized, and individually loggedonto high-density magnetic cartridge tape on a pulse-by-pulse basis. This acquisition strategy ensuresthat all frequency information is retained for subse-quent analysis during the data-validation stage, whilein addition permitting selective post facto pulse aver-aging based on considerations of atmospheric homoge-neity (e.g., exclusion of isolated cloud features fromthe aggregate backscatter profile). The only disad-vantage of this approach is that the operational pulserepetition frequency is currently limited by the datasystem to 5 Hz.

Several measures were taken to obviate leakage oftransmitter-induced electromagnetic interference andextraneous RF interference into the signal chain.These include shielded enclosures for the lidar signalphotomixer and RF amplification stages, use of self-contained rechargeable battery packs to power theamplifiers and provide bias current to the photo-mixer, and copper-clad semirigid coaxial cablethroughout the receiver chain, including the connec-tion between the photomixer Dewar and the shieldedenclosure for the bias source and amplifiers.

The lidar signal is digitized with 12-bit resolutionand at a rate of 10 megasamples/s, with a recordlength of 2048 (i.e., the total available measurementrange is > 30 km). To preserve the phase relation-ship between the in-phase and the quadrature rec-ords (and therefore maximize signal-to-noise ratioperformance) the two digitizer channels must be

synchronously clocked. The outgoing laser pulse issampled with a high-speed, room-temperature MCTphotoconductive detector, which thus furnishes thetrigger signal for the digitizers. The signal outputfrom this detector is also displayed on a widebandoscilloscope for real-time mode-purity monitoring.

Although they were not available during the fieldmeasurement campaigns described in this paper,future deployments of the ABL will incorporate en-hanced pulse diagnostics, which have already under-gone extensive ground tests in the laboratory environ-ment at JPL. First, a portion of the output pulsemonitor signal is analyzed with a multimode discrimi-nator circuit that alerts the system operator with anaudible alarm when a multimode pulse is detected.'9

Second, a -20-dB directional coupler feeds part of theIF output from the high-gain amplification stage to afrequency counter, which displays the time-averagedlidar return mixing frequency on a calibrated, color-coded light-emitting diode bar graph display,20 thusproviding a quick-look readout of the pulse-to-pulseIF frequency.

D. Laser Operation Maintenance Control Loops

The choice of a coherent (heterodyne) receiver configu-ration offers optimum sensitivity (and therefore maxi-mum measurement range) but imposes relativelystringent tolerances on the transmitter laser fre-quency stability because of the need to maintain acalibrated instrumental responsivity. Because of thisfact, and the adverse conditions of the aircraft environ-ment (vibration, cabin pressure excursions, in-flightturbulence, aircraft maneuvering, etc.), closed-loopfrequency stabilization of the cw and the pulsed lasersbecomes essential. Hence lidar operation in flight isgoverned with three interdependent feedback loops,as described below. (See also Fig. 2.)

The cw injection seeding, or master oscillator (MO),laser is a commercial waveguide model that is gratingtuned and stabilized at the 9R(24) line center bymeans of the optogalvanic technique.2 ' Injectionseeding of the transmitter laser is accomplishedthrough the unstable resonator output optic in themanner illustrated in Fig. 1. A portion of the injec-tion-oscillator output is first directed through thebeam-expanding optics and weakly focused through a2-mm-diameter hole (bored at the appropriate angle)in a silicon flat with a highly reflecting surface on theexit face. This portion of the oscillator serves as aspatial filter and also effectively attenuates the smallportion of the transmitter pulse that couples backthrough the optics along the injection-seeding path.(The fraction of the transmitter pulse energy thatcouples back to the injection-oscillator output coupleris less than 10-5, which is equivalent to maximumpower levels of 5-10 W, which is well below damagelevel but is enough to perturb the laser dynamicsseriously. The injection oscillator typically recoverswithin 15 Rus.) The weakly diverging injection beamis then directed through a 7-mm-diameter hole in thefolding mirror immediately in front of the TEA laser


output coupler and then through the output couplerinto the transmitter cavity. The output coupler iswedged, which eliminates direct feedback of the firstsurface-reflected component. Fabry-Perot fringesare impressed on the portion of the injected beamthat is reflected by the transmitter laser cavity whenthe cavity length is tuned with a piezoelectric tuningelement. The fraction of the reflected componentthat couples through the central hole in the foldingmirror is largely intercepted by the reflective surfaceof the spatial-filtering flat and focused onto a cooled(77 K) MCT photoconductive detector, which is la-beled (4) in Fig. 1. A selected fringe is monitored onthis detector and used as a metric for closed-loopsingle-mode operation of the transmitter (see Fig. 2).

The cw LO is a commercial open resonator CO2laser (although experience on the first missions flownin 1989 made it necessary to rebuild the innerstructure of this unit entirely to provide for greatermechanical stability). A portion of the LO output ismixed with the output from the MO (Fig. 1), and theensuing heterodyne beat signal is processed with anear-Gaussian bandpass filter (25 < fnom/MHz • 35),the output from which is then used as a lock-in sourceto feedback stabilize the LO operating frequency at 30MHz from the frequency of the MO (and therefore thefrequency of the transmitter laser). This ensuresthat the nominal center frequency of the detectedlidar signal is 30 MHz, to which all the receiver RFelectronics are necessarily tuned. The receiver band-width is 10 MHz. (The choice of an offset hetero-dyne mixing frequency guarantees that 1/f noise isrelegated to negligible levels in the receiver.)

Closed-loop compensation of aircraft-attitude-induced Doppler shift was considered but was ulti-mately deemed unnecessary. Typically aircraft pitchangles during cruise are no greater than 2 andpresent no problem with regard to detection of thereturn signal. However, during occasional pro-longed aircraft ascent or descent the pitch angle couldreach or exceed ±5°, which corresponds to Doppler-induced frequency shifts of up to ±4 MHz at the200-m/s nominal aircraft velocity. Although shiftsof this magnitude are commensurate with the limitsof our 10-MHz receiver bandwidth, provision wasnevertheless made for manual compensation of moreextreme aircraft attitude effects up to pitch angles of25°. (The maximum conceivable pitch angle wouldactually be of the order of 45°, corresponding to thezero forward thrust flight mode of the aircraft.However, in this circumstance it is unlikely thatdata-acquisition operations would be accorded highpriority.) We compensated for aircraft-attitude ef-fects by mixing the LO-MO difference frequencysignal with the signal from a voltage-controlled oscil-lator, which was tunable over the range 40-80 MHz.Adjustment of the voltage-controlled oscillator thenensures that the difference frequency between thevoltage-controlled oscillator and the lidar heterodynesignal can be maintained at the 30-MHz fixed filtercenter frequency (Fig. 2).

E. Lidar Support Structure

The subframe that supports the optics and the trans-mitter laser is custom designed and fabricated from ahigh-strength 6061-T6 extruded aluminum alloy boxsection welded into a stiff structure. The somewhatlarge physical dimensions of the optical surface resultin numerous lengthy optical lever arms that areimplicitly vulnerable to the detrimental influence ofexternal physical disturbance mechanisms. Pro-jected mechanical flexures in the DC-8 aircraft vibra-tion and thermal environment were calculated withMSC/NASTRAN analysis to verify that the system de-sign limited optical misalignment to the requiredtolerances. All optics on the table are mounted incustom-designed lockable adjusting mounts. A pho-tograph of the entire frame is shown in Fig. 3. Ascan be seen from Fig. 3, the table is supported by awelded A-frame structure at the forward end and by alow-format flight electronics rack at the aft (opticaloutput) end. Both the A-frame and the electronicsrack affix directly to the aircraft seat-mounting rails,so it was necessary to decouple the optical table fromits support structure. This function was performedwith a system of neoprene-lined cylindrical journalsconfigured in a mutually orthogonal pattern, whichprovided the necessary degree of mechanical isolationof the table from the DC-8 airframe while simulta-neously preventing system rotation and migration inthe horizontal and vertical planes.

The large dimensional envelope of the systemmeant that the optical table had to be disassembledfor shipping as well as uploading to and downloadingfrom the DC-8. Consequently the table was de-signed in two portions that mate together at roughlythe center of the structure. The mating flanges arefitted with captive locating pins, which ensures thatdisassembly or reassembly will result in maximummisalignments of the order of micrometers. Thismeans that virtually all critical alignment can be

Fig. 3. Pictorial view of the JPL ABL instrument. In flight theoptical table and the transmitter beam path are completely en-closed by rigid shrouding, which does not appear in this photo-graph.


performed in the more benign laboratory environ-ment at JPL, with only minimal adjustments re-quired after shipping and subsequent installationaboard the DC-8.

Before its deployment on the first GLOBE PacificOcean circumnavigation in the Fall of 1989, the ABLwas subjected to a 1-month instrumental checkoutinterval to assess system ruggedness and alignmentstability in a flight environment. During this phasethe instrument experienced approximately 30 takeoff-landing cycles with no measureable loss of transmit-receive coalignment and with calibration fluctuationsnot exceeding 15%. By the end of the Spring 1990GLOBE field campaign, the ABL had flown in excessof 300 flight hours and undergone over 70 takeoff-landing cycles, with only minimal alignment degrada-tion observed among the critical optical paths. Thislevel of performance is an essential attribute for sucha system when it is deployed to remote sites, whereresources are often limited and opportunities forinstrument recalibration are impractical or (morecommonly) nonexistent.

3. Hard-Target Calibration Methodology

The calibration approach used with the ABL instru-ment is identical in philosophy to the approachformulated several years ago for application to theJPL ground-based aerosol backscatter lidar.18,22 Insummary, the lidar transmitter' beam is made toimpinge on a planar target that is oriented at 450 inthe s plane to the incident beam and that is located ata sufficient range (- 1 km) for significant overlapbetween the transmit beam and the receiver field ofview. Figure 4 shows the computed range depen-dence of the three-dimensional transmit-receive over-lap integral for the ABL. This characteristic wasgenerated in fashion analogous to that previouslydescribed for the JPL ground-based lidar23 and soshould not be confused with the range-dependentcoherent lidar receiving efficiencies presented else-where.24 Note that the nominal target position indi-

0.7

0.6

0

00

0.5

0.4

0.3

0.2

0.1

00 5 10 15 20

RANGE (km)

Fig. 4. Range-dependent three-dimensional (3-D) transmit-receive overlap integral of the JPL ABL. The broken line at the1-km range denotes the nominal location of the flame-sprayedaluminum target during field calibration operations.

cated is not too far removed from the plateau condi-tion, in contrast to the situation that prevails for theJPL ground-based backscatter lidar.23 Dictated as itwas by operational restrictions in force at the NASAAmes Research Center, the 1-km target locationmight be thought to be closer inside the Rayleighrange (- 1.9 km) than we are comfortable with forour 15-cm-diameter transmitter beam. However,previous analysis indicates that only negligible erroris incurred by the assumption of Fraunhofer propaga-tion at such short range.23

The choice of material for the field target is predi-cated on the desirability of stable, high-reflectanceproperties with minimal spectral variability in theCO2 laser region that vary relatively slowly withincidence angle. The resistance of the material toenvironmentally induced changes in these propertiesis also of paramount importance. Past studies haveshown that flame-sprayed aluminum offers manyadvantages in this regard25,26; consequently we fabri-cated our 2 m x 2 m field target from this material.The 450 incidence angle is selected to avoid the broadspecular enhancement of the reflectance that is char-acteristic of diffusely reflecting metal surfaces and torender more accurate traceability to reference mea-surements of the near-Lambertian sulfur target thatwas used as a transfer standard.25 26

The lidar transmitter beam was coboresighted withthe expanded output from a low-power He-Ne laser(see Fig. 1) to provide a visible surrogate for theinfrared beam as an alignment aid for sighting thelidar beam onto the calibration target. Typically, weverified the correct positioning of the IR beam on thetarget by observation of audible emissions from theinteraction of the beam with highly absorbing fabricplaced temporarily in the beam path. Visual targetacquisition obviously necessitated removal of thegermanium pressure window, but the window wasreinstalled before the actual calibration measurement.A large relay mirror was positioned on the taxiwaybeneath the optical port to direct the lidar beamhorizontally toward the target site (ensuring that inso doing the plane of polarization was not inadvert-ently rotated; i.e., the outgoing beam axis must beparallel to the aircraft fuselage).

When the lidar beam falls completely on the target,and the receiver gain is appropriately reduced toavoid saturation of the return signal, it is thenpossible to acquire data that are representative of thelidar system response to a known stimulus. Wecalibrate the optical attenuation along the line ofsight by operating the lidar at the regular gain settingwith the target removed and by computing atmo-spheric extinction by applying the slope method.5' 23

These operations rely to a great extent on the exis-tence of a stable, well-mixed, and homogeneous atmo-spheric boundary layer. However, the conditionsoften prevailing at field calibration sites (i.e., busyairfields and a variety of weather conditions) fre-quently tended to militate against stability of themixed layer, so that although in theory the calibra-


tion exercise as described might appear straightfor-ward, in practice it often proved time consuming.(The molecular contribution to the extinction, whichis normally the predominant contribution, can bemodeled with temperature, humidity, and barometricpressure sensor data. This provides a good firstapproximation to the total extinction.)

4. ABL Field Deployments

The JPL ABL instrument has to date been used inthree extended field campaigns aboard the NASADC-8 research aircraft. The first of these was essen-tially a prolonged instrumental shakedown under-taken as a flight of opportunity in conjunction withthe JPL Airborne Synthetic Aperture Radar. Thismission (officially designated as SAR/DARPA/ESA'89) took place over several sites in continental Eu-rope, the United Kingdom, Greenland, and NorthAmerica. However, the ABL instrument was oper-ated only during the predeployment engineeringflights and the trans-Atlantic and transcontinentalU.S. legs; the instrument was not operated for theremainder of the mission to conserve componentlifetime in anticipation of the GLOBE '89 mission,which was to take place soon thereafter. Althoughonly a representative data set was acquired (a fewexamples of which were reproduced and discussed inRef. 27), the duration and the number of these flights(> 30 when nonexperimenter engineering and profi-ciency exercises are included) provided valuable verifi-cation of the shock and vibration resilience of theABL instrument when it was operated in an aircraftenvironment. Table 2 lists the flights in question,along with their associated ABL operations. Theintercomparison with the ground-based NOAA WavePropagation Laboratory (WPL) CO2 lidar during anoverflight of Boulder, Colorado, on the transcontinen-tal flight (No. 6) is discussed in Section 5.

The two GLOBE Pacific Ocean survey missionswere designed primarily to provide aerosol backscat-ter data from a large area that was expected tocontain background regions of pristine troposphericair, with emphasis on coverage of a wide range oflatitudes. The major motivation was to provide thesedata to support LAWS performance assessments and

Table 2. SAR/DARPA/ESA '89 Summary Mission Profile and ABLActivitiesa

FlightNo. Region Investigated ABL Operations

1 N. California Instrument engineering flight2 California-Baja Instrument engineering flight

3-5 N/A ABL inactive6 Trans-U.S. Intercomparison with NOAA WPL7 Trans-Atlantic Aerosol profiles over ocean

8-19 N/A ABL inactive20 Trans-Atlantic Aerosol profiles over ocean21 Northeastern U.S. Cloud backscatter over land22 Trans-U.S. Aerosol profiles over land

23-27 N/A ABL inactive

aMission window 20 July 1989-15 September 1989.

Table 3. GLOBE '89 Summary Mission Profile and ABL Activitiesa


1 N. California (coastal) Instrument engineering flight2 N. California (land) Instrument engineering flight3 N. California (land) Instrument engineering flight4 Transit to Hawaii Aerosol profiles over ocean5 Hawaii local Instrument intercomparisons6 Transit to Samoa Aerosol profiles over ocean7 Transit to New Aerosol profiles over ocean

Zealand8 Transit to Melbourne, Aerosol profiles over ocean

Australia, excursionto Antarctic Circle

9 Melbourne local Instrument intercomparisons10 Melbourne local Instrument intercomparisons11 Trans-Australia Aerosol profiles over arid desert12 Transit to Japan Tropical oceanic profiles13 Japan local Instrument intercomparisons14 Japan local Instrument intercomparisons15 Japan local SAGE II intercept16 Transit to Alaska Aerosol profiles over ocean17 Alaska local Sea-ice reflectance18 Transit to California Aerosol profiles over ocean19 Boulder Colorado Intercomparison with NOAAWPL

aMission window 20 October 1989-8 December 1989.

instrument development. The NASA MSFC EarthScience and Applications Division was responsible forthe mission design and for coordinating the opera-tions of the instrument investigator teams. Althoughin mission planning the highest priority was given toaerosol measurements, emphasis in the postmissiondata analysis activities has also been given to cloudbackscatter. The GLOBE I and GLOBE II missionswere scheduled to take place nominally 6 monthsapart (in Fall 1989 and Spring 1990, respectively; seeTables 3 and 4 for mission profiles) to gain someinsight into the influence of seasonal factors in deter-

Table 4. GLOBE '90 Summary Mission Profile and ABL Activitiesa


1 N. California (coastal) Instrument engineering flight2 N. California (coastal) Instrument engineering flight3 Transit to Alaska Oceanic aerosols-sea-ice4 Transit to Hawaii Aerosol profiles over ocean5 Hawaii local Instrument intercomparisons6 Transit to Samoa Aerosol profiles over ocean7 Transit to Tahiti Aerosol profiles over ocean8 Transit to New Zealand Aerosol profiles over ocean9 New Zealand local Instrument intercomparisons

10 Transit to Melbourne, Aerosol profiles over oceanAustralia, excursionto Antarctic Circle

11 Trans-Australia Aerosol profiles over arid desert12 Transit to Japan Tropical oceanic profiles13 Japan local Instrument intercomparisons14 Transit to Hawaii Aerosol profiles over ocean15 Transit to California Aerosol profiles over ocean

aMission window 1 May 1990-5 June 1990.


mining atmospheric aerosol loading over the PacificOcean. In these missions a complementary suite ofinstruments, which included lidars operating at vari-ous wavelengths from the visible to the mid-IR, aswell as a number of in situ particle sampling-analysissystems, was used. In addition, the DC-8 platformitself is permanently equipped with numerous sen-sors that provide a comprehensive time-framed meteo-rological and navigational data set to the investiga-tors on board. The most relevant of these sensorsare included with the complement of GLOBE-specificinstruments listed in Table 5.

Several checkout flights took place before the firsttransit flight in each of the two GLOBE missions.In addition to determining the operational status ofthe instruments during these flights, we analyzeddata for assessment of calibration methodologies andfor intercomparison exercises. For the purpose oflidar intercomparison, the NOAA WPL CO2 lidar28

was transported to the NASA Ames Research Centerand operated during DC-8 overflights. An exampleof CO2 lidar intercomparison data obtained duringone of the checkout flights before the Spring 1990deployment over the Pacific Ocean is discussed inSection 5.

GLOBE transit flight segments were primarilydedicated to the acquisition of aerosol and cloudmeasurements under whatever conditions were deter-mined as optimal on a real-time basis by the indi-vidual experimenter groups. So-called local flights(beginning and terminating at the same land base)were conducted in a considerably less free-form man-ner, as these flights were regarded as scheduledopportunities for data intercomparison and valida-tion, as well as instrument health checks. Fre-quently, such flights would also be routed to overflyland sites where lidar or aerosol sampling stationswere operating, thus providing further opportunities

Table 5. DC-8 GLOBE Instrument Payload

OperatingWavelength

Instrument (>.m) Institution

CO2 lidar (nadir-zenith) 9.25 JPLNd:YAG lidar (nadir- 0.53,1.06,1.54 GSFC

zenith)cw CO2 lidars (horizon- 9.11, 10.59 MSFC

tal)Optical particle counter 0.633 Ames Research CenterPreconditioned optical 0.633 University of Hawaii,

particle counter HonoluluFilter-impactor system - Georgia Institute of

TechnologyIntegrating nephelom- 0.45, 0.55, 0.7 NOAA Global Moni-

eter (GLOBE '89 only) toring of ClimateChange

DC-8 IR temperature 8-12 Ames Research Centerradiometer

Nadir-zenith-port video - Ames Research Centerscene recorders

DC-8 radar altimeter - Ames Research Center

for enhancing the overall science return from themissions. Several intercomparisons between the JPLand the GSFC lidar data sets obtained during theGLOBE '90 transit and local flights have been made,and representative examples are included in Sec-tion 6.

5. CO2 Lidar Intercomparisons

The overflight of the NOAA WPL CO2 lidar atBoulder, Colorado, on 3 August 1989 provided avaluable intercomparison opportunity. The atmo-sphere directly above Boulder was clear, with awell-defined boundary layer. In the altitude regionof overlap, it was evident from the first look at thedata that the vertical structure of the aerosol backscat-ter profile obtained with the JPL ABL instrument ina nadir-viewing mode from a flight altitude of 6 kmand the profile obtained by NOAA from the groundwere in good agreement. The rather high backscat-ter levels within the thick boundary layer on thatoccasion resulted in saturation of the NOAA receiver.However, after a series of gain measurements weremade in the high-signal regime and compensation fornear-range receiver saturation was included, the back-scatter levels obtained with the two lidars agreed towithin 1.5 dB throughout the data record, affirmingthe validity of the hard-target calibration techniquesused by each group independently.

Before each of the GLOBE Pacific Ocean missionsit was agreed that the NOAA WPL lidar would betransported to the NASA Ames Research Center sothat it could be used in lidar intercomparisons, whichwould be an important element of data-validationexercises for the JPL ABL instrument and the NASAMSFC cw laser backscatter instrument (as mentionedin Section 1). The hope was that the lidar intercom-parisons could be made over a large dynamic range ofreturn signal intensity, in which we would fullyexercise and monitor the lidar performance, recogniz-ing that atmospheric conditions would dictate theextent to which these goals would be realized. Eachinstrument group conducted individual hard-targetcalibrations. The NOAA and the JPL lidar intercom-parisons were conducted while both lidars were onthe ground, as well as during checkout flights whenthe DC-8 overflew the location of the NOAA lidar.Sequential overflights of the NOAA lidar site atseveral altitudes also permitted intercomparisons withthe MSFC instrument data (which were obtained ataircraft altitude).

An important outcome of the intercomparisonsmade during the checkout flights in the Fall of 1989was the discovery of JPL lidar receiver nonlinearity atlow signal levels, which were relevant to measure-ments in clean atmospheric conditions or at longranges from the lidar. (The free tropospheric airabove the Moffett Field location was generally veryclean during this period of time.) At this time theJPL lidar receiver contained a video detector toconvert the heterodyne signal down to the base band,as stated in Section 2. It was determined that whenconditions made it necessary to average pulses exten-


sively to extract the lidar backscatter signal from thenoise, the receiver responsivity was nonlinear to adegree that made it necessary to add corrections inthe data-processing software. The nonlinear behav-ior was not detected in the hard-target calibrationsbecause the experimental arrangement was such thatthe backscatter from the remotely deployed targetproduced midrange signal levels. (The hard-targetcalibration exercise was not meant to test linearityover a wide dynamic range.) The nonlinear behaviorwas not noticed in the prior Boulder overflight inter-comparison either, because the backscatter signallevels from the atmosphere at that time were largeenough to be above the region of significant nonlinear-ity. Past experience with the laboratory coherentCO2 lidar at JPL had provided indications of video

'detector nonlinearities, but they were less significantbecause there was much less reliance on pulse averag-ing to increase the signal-to-noise ratio because of thecomparatively low operational pulse repetition fre-quencies and larger energy-aperture product. Inretrospect, these intercomparisons, which were under-taken during a variety of atmospheric conditions,served a very useful purpose.

During the interval between the two GLOBE mis-sions the video detector in the JPL ABL receiver wasreplaced with a complex demodulator, resulting inmuch improved linearity at the low signal levelscharacteristic of backscatter from clean upper tropo-spheric air masses. The hardware modification was

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considered to be a preferable alternative to softwarecompensation. The retrieval of quantitative back-scatter coefficients when a linear video detector (forwhich the output voltage is proportional to the squareroot of the lidar received optical power) is used issusceptible to biases when the detector is outside itsdynamic range of linearity, and the use of averagecorrection factors in software that are appropriate fora particular pulse-averaging condition can only beapproximate.

Before the Spring 1990 GLOBE mission a secondseries of CO2 lidar intercomparisons was conducted inthe environs of the NASA Ames Research Center.Again the lidars were independently calibrated withremotely deployed hard targets. One checkout flightwas dedicated to Ames overpasses at various altitudesto facilitate intercomparisons. The MSFC laser back-scatter instrument data were collected during ascentsand descents as well as during level flight in thevicinity of Moffett Field. In this manner, verticalprofiles of aerosol backscatter could be produced byeach of the three instruments: the airborne MSFCand JPL instruments and the ground-based NOAAinstrument. The particular day of this flight fur-nished almost ideal circumstances for such an inter-comparison. Rather high aerosol concentrationswere observed, even at high altitudes, and prominentand stable aerosol layering was noted over more than50 km of flight track in the north-south heading, ascan be seen in the JPL lidar data plot shown in Fig. 5.

GLOBE 90 FLT 90-02-02 DAY: 124 4 4 MAY 1990MOFFErr FIELD (LOCAL)FILES 13-16, 18-19 TIMES: 18:37-18:42, 18:54-18:56 UTCALTITUDE: FL400 (AMES OVERPASSES)

Fig. 5. Cross section of the logarithm of the aerosol backscatter coefficient (in inverse meters per steradian) measured at the 9.25-pm lidarwavelength in the vicinity of NASA Ames Research Center, Moffett Field, California. The vertical line denotes the separation between thefirst overpass on a north-south leg and a second overpass on a (shorter) south-north leg, both at a flight level of 40,000 ft. (12 km) pressurealtitude.


7

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DISTANCE FROM NOAA WPL LIDAR SITE (km)

:

Figure 5 comprises two successive Moffett Fieldoverpasses at an aircraft pressure altitude of 12 kmon two successive legs: a north-south leg beginning45 km north of Moffett Field, followed within a fewminutes by a shorter south-north leg. These datacorrespond to approximately 600-m horizontal resolu-tion and 150-m vertical resolution. The scale on theleft-hand side of Fig. 5 ranges over 3 orders ofmagnitude in f3, the backscatter coefficient expressedin units of inverse meters per steradian. Figure 6 isa composite data set showing the profiles obtainedwith the JPL and the NOAA lidars when the aircraftpassed over Moffett Field at a 13-km altitude alongwith the MSFC data obtained during the various leveloverpasses as well as during the ascents and descents.We obtained the JPL profile by pulse averaging thedata over a 30-s integration time, corresponding to a7-km section of flight track including the NOAA WPLlidar site, during the first overpass. The verticalstructure observations made with the three instru-ments were consistent. No attempt was made toaccount for aerosol extinction in either the JPL or theNOAA WPL backscatter profile retrievals, althoughwith such high aerosol loading the aerosol opticaldepth through the lower 10 km of the atmospheremay have been as high as 0.2. Considering thedifficulty of making intercomparisons of airborne andground-based instrument data, which have spatialand temporal averaging regimes that are fundamen-tally different, the overall intercomparison was en-couraging.

6. Overview of Data from GLOBE Pacific Ocean SurveyMissions

Many of the data from both GLOBE missions havebeen processed and converted into vertical profiles ofatmospheric backscatter in calibrated absolute units,using two horizontal resolution modes, each withvertical resolution of 150 m. The horizontal resolu-tion of the higher resolutioin mode is typically in the

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_ MENZIES, et al JPLJPL Noise Level

--- JONES et al. MSFC (9.1 gim)\MNEPOST et al. WPL (9.28 r)\

""I1111, , ,-.mrr.""

04-05-9018:42 UTC9.25 r (JPL)

- I I ,, -14 -13 -12 -11 .10 -9 .8 -7 -6 .5 .4

LOG BACKSCATTER (m-1 sr -1)

Fig. 6. Aerosol backscatter profile intercomparisons over MoffettField, California, 4 May 1990. The monotonic dashed curve onthe left-hand side (JPL noise level) is the noise-equivalent-backscatter level for the JPL lidar, for a 30-s integration time.Hatched curve, Jones et al. 17; Short-dashed curve, Post et al. 17

range of 500 m, depending on the aircraft cruisespeed. False color display plotting of the data in thehigher resolution mode permits the recognition ofboth vertical layering and the extent of horizontalhomogeneity at a given altitude, which can then beused for judging the relevance of spatial averagingover larger scales. The lower-resolution mode isused for the high sensitivity necessary to extractbackscatter coefficients in the relatively clean regionsof the troposphere. The integration time used forthe lower-resolution mode is longer than the time forthe higher mode by a factor of 40 (resulting in typicalhorizontal resolution of 20 km), which results in theability to measure backscatter levels in the vicinity of10-11 m-1 sr-' at ranges of a few kilometers from theaircraft. These data files have been sorted intolatitude bins (of 15° width for GLOBE '89 data and10° width for GLOBE '90 data) to produce latitude-altitude cross-section overviews for each mission.In this procedure all the data from a given latitudebin are fitted to a lognormal distribution at eachaltitude by a data dropout compensation process asdescribed by Post and Cupp.12 Extensive cloud-recognition algorithms are used in this procedure toseparate the influence of either optically thin oroptically thick clouds from the aerosol and to elimi-nate data that are uncalibrated because of propaga-tion through clouds of significant optical thickness.

We can summarize several observations based onthe processed GLOBE data, for both aerosol andcloud studies.

Aerosol Observations(a) A large variability was observed in the plan-

etary boundary layer over the Pacific Ocean, both ingeometric thickness and in aerosol backscatter level.The backscatter coefficients varied from 3 x 10-9 to10-7 m- sr-' in boundary layers that were thickenough (> 300 m) to be observable.

(b) An extended high-altitude haze layer was ob-served during both missions over the northwesternPacific Ocean basin, with a gradual transition to cleanupper tropospheric conditions occurring during theflight from Japan to Hawaii in the Spring 1990mission. The layer observed in late November 1989decayed significantly during a 6-day period of observa-tions in the vicinity of Japan. The top of the high-altitude extended layer observed in Spring 1990reached 10 km. The major component of theseaerosols was surface-derived refractory material,29

very likely from the Asian continent.(c) Observations of aerosol backscatter from mid-

and upper-tropospheric air indicated variability overa range of 4 orders of magnitude, with the high end ofthe range representing the air masses over the north-west Pacific Ocean, in the general vicinity of Japan,where 13 levels as high as 5 x 10-8 m-1 sr-' wereobserved in dust layers. The observations in thesouthern hemisphere indicated variability, although


""I .1...11

the maximum observed levels were approximately10-9 m-' sr-'.

Cloud Observations(a) Cirrus clouds were observed frequently in the

upper troposphere. Although the typical backscatter-to-extinction ratio for ice particles is much less at 9.25jim than in the mid-visible,30 3' the lidar observationswere sentitive enough to detect tenuous layers of thincirrus clouds for which the estimated extinctioncoefficients were as low as 10-6 m-1. These layerswere sub-visible for viewing angles near the zenith orthe nadir but were discernible when viewed throughthe cabin windows at zenith angles near 900.

(b) Lidar penetration often occurred not onlythrough the cirrus and other clouds of small tomoderate optical thickness but also through occa-sional porous regions in stratocumulus clouds, inwhich cases the cloud base altitudes could be identi-fied. (The 9-,im wavelength and the narrow receiverfield of view render multiple-scattering effects insig-nificant in all cases for which the backscattered signalis above the receiver noise level.32 Consequently thecloud base altitude can be determined unambigu-ously.)

(c) In cases of clouds of small to moderate opticalthickness the lidar data can be used to determine thecloud optical thickness, relying on the backgroundsea-surface reflectance signals to provide a calcula-tion of the double-pass attenuation through thecloud.32 For the liquid water clouds, the total liquidwater path can be determined, because at wave-lengths in the 10-,um region the approximately linearrelationship between the volume extinction coeffi-cient and the liquid water content is independent ofthe cloud microphysics.3 334

7. JPL-GSFC Lidar Intercomparisons from the GLOBE'90 Mission

The GLOBE missions provided a unique opportunityto study the wavelength dependence of aerosol andcloud backscatter at multiple wavelengths, from themidvisible 532-nm doubled Nd:YAG laser wavelengthto the 9.25-jim CO2 laser wavelength. Both the JPLand the GFSC lidar groups exercised extensive calibra-tion procedures to provide backscatter coefficients inabsolute units at each lidar wavelength. (Ref. 8contains a description of the GSFC airborne lidar, thelidar calibration techniques used for the GLOBEmissions, and data examples, including comparisonswith Mie calculations of aerosol backscatter based onaerosol microphysical data collected with in situinstruments on the DC-8.) For each lidar the point-ing direction was either the nadir or the zenith butnot both simultaneously. Although there were exten-sive periods during each flight in which the lidarswere pointing in opposite directions to observe fea-tures of interest both above and below the aircraftaltitude, the lidars were observing in the same direc-tion approximately half the time. Both groups haveprocessed most of the flight data in high-resolution

quick-look format (corresponding to spatial resolu-tion between 200 and 500 m along the flight track),and these data products have been used to selectcertain time intervals for further study. Studies todate have concentrated on intercomparisons of1.06- and 9.25-jim data, primarily by the use of the500-m-resolution data segments for cirrus cloud inter-comparisons and high-sensitivity data segments (cor-responding to longer integration times and 20-kmhorizontal resolution) for aerosol intercomparisions.It is evident from comparison of these data sets that ahigh degree of correlation in the spatial structuresexists, even when observing variability on the scale of500 m at ranges of over 12 km when the lidar ispointed toward the nadir. This evidence supportsthe conclusion that the lidar pointing directionsdiffered by less than 3°.

A fundamental distinction between the atmo-spheric backscatter data characteristics at these twowavelengths is that the aerosol backscatter is ob-served as an enhancement above the molecular (Ray-leigh) backscatter at 1.06 im, whereas the Rayleighbackscatter is below the receiver noise level at 9.25jim. Thus the atmospheric density profile must becarefully calculated to extract the aerosol contribu-tion at 1.06 jim, especially in the low-,3 regime whenthe backscattering ratio is much less than unity.The receiver noise for the 9.25-jim lidar can beconverted to an effective noise-equivalent backscat-ter, and in fact a 2 cr (corresponding to twice the rmsnoise level) curve is included on all 9.25-jLm backscat-ter profile plots.

Selected 1.06- and 9.25-jim aerosol backscatterprofile intercomparisons are shown in Figs. 7-9.These are examples for which a well-defined verticalstructure existed, thus providing benchmarks for thespatial coalignment of the two lidars as well as a widedynamic range of backscatter for purposes of compari-son. Figures 7-9 make manifest the high degree of

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Backscatter Coefficicnt (I /m sr)

Aerosol backscatter profiles at the GSFC 1.06-,um lidarwavelength (dashed curve) and the JPL lidar 9.25-pum wavelength(solid curve), over the Pacific Ocean at 20° N latitude, 137' Elongitude, during the time interval 05:11-05:13 UTC, 31 May1990. (The dotted curve is the JPL lidar noise-equivalent-backscatter coefficient. Discontinuities in this quantity indicatewhere the cloud-filtering procedure has been used.)


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Backscatter Cocfficicnt (1/m sr)

Fig. 8. Aerosol backscatter profiles at the 1.06- and 9.25-pum lidarwavelengths over the Pacific Ocean at 30° N latitude, 138° Elongitude, 31 May 1990. The time intervals for the profiles are06:31-06:33 UTC (GSFC lidar) and 06:34-06:36 UTC (JPL lidar).

correlation between the backscatter profiles at thesetwo wavelengths. The profiles shown in Figs. 7-9have been cloud filtered by using the high-resolutionlidar data to identify time-series profile segmentscontaining clouds and by selectively removing thoseportions of the profile segments affected by the cloudsbefore averaging. The clouds observed in these se-lected time frames were scattered cumulus clouds,either near the top of the boundary layer or atmidlevel altitudes.

The profiles shown in Figs. 7 and 8 correspond tothe observation of a well-defined elevated aerosollayer, which grew to a thickness of several kilometersas the aircraft proceeded north between 15° and 300 Nlatitude on a flight between Darwin, Australia, andTokyo, Japan. Near 200 N (see Fig. 7) this layerextended from 4.5 to 7 km altitude. Below theelevated layer the aerosol density was much reduced,until the boundary layer was reached. Further tothe north the elevated aerosol layer was observed atprogressively higher altitudes; at 300 N (Fig. 8) the

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Backscatter Coefficient (I /m sr)

Fig. 9. Aerosol backscatter profiles at the 1.06- and 9.25-pum lidarwavelengths over the Pacific Ocean at 230 N latitude, 162° Wlongitude, 04:56-04:58 UTC, 4 June 1990.

profiles indicated that the elevated layer had a sharplower boundary near 6 km and a more diffuse upperboundary, decaying with increasing altitude above 9km but still evident at 10 km. By this time, multipledistinct aerosol layers had appeared between 2 and 5km, with the high-resolution lidar data showing somevariability in the vertical structure of these layers.(The differences in vertical structure between the twolidar profiles in the 2-5-km region can be ascribed to a2-min temporal offset between the profiles, whichcorresponds to a 25-km offset along the flight track.The lidars were viewing in opposite directions duringthis part of the flight, with a nadir-zenith modeswitch occurring for each lidar in the 06:33-06:34UTC time period.) The lidar data indicated that themarine boundary layer height was 500-750 m in theregions relevant to Figs. 7 and 8. Above the bound-ary layer, at altitudes for which the backscatter wasabove the noise threshold for the ABL instrument,the ratio of 1(1.06 jim)/P(9.25 im) is in a narrowrange of 12-18 over a dynamic range of nearly 2orders of magnitude, with the ratio increasing tovalues of 40-50 in the lower backscatter regionsabove or near the top of the elevated layer. It islikely that the refractive-index resonances of theAsian dust component of this aerosol enhance thebackscatter at 9.25 jim by a factor of 2 to 3 comparedwith the 1.06-im backscatter (the real part of therefractive index being in the range 1.7-1.9 at 9.25jim, compared with 1.5 at 1.06 Lm).4 3 5-37 Withinthe boundary layer the backscatter ratio is in therange of 50-80. Assuming that the larger boundary-layer aerosol particles most responsible for the lidarbackscatter are hygroscopic sea salt coated withwater,38 39 with a modeled refractive index of 1.30 +0.04i at 9.25 jim and 1.35 + 0.OOi at 1.06 jim, we canshow that the refractive-index contribution to thebackscattering efficiency is nearly the same at the twowavelengths. The observed boundary-layer backscat-ter coefficients at 9.25 jim are well within the rangecalculated by Gerber38 for a 9.11-jim wavelength,based on the aerosol model developed by Gathman3 9

but are somewhat lower than the mean backscattervalue, indicating low to moderate surface wind speeds.

The profile shown in Fig. 9, corresponding to anobservation near the end of the flight between Tokyoand Hawaii, is indicative of high aerosol content inthe lower troposphere, with transitions to loweraerosol content occurring above 5 km altitude.Aerosol layering was evident throughout this flight,with the aerosol backscatter coefficient at 9.25 imbeing predominantly above 10-9 m-' sr-I in the lower5 km of the troposphere. Asian dust is again a majorcontributor to the aerosol above the boundary layer.In Fig. 9 the 1 ratio is near 20 throughout the altituderange above 1 km, whereas the corresponding dy-namic range of 13 values covers 2 orders of magnitude.The ratio increases to values in the 50-80 rangewithin the boundary layer.

In summary, for the cases of moderate to high dustloading in the free troposphere, nearly identical verti-


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cal structure was observed at the JPL lidar 9.25-jimand GSFC lidar 1.06-jim wavelengths. The 13 ratioswithin and just above the boundary layer were consis-tently larger than in the dust-laden free troposphere.Further intercomparisons in other atmospheric re-gimes are in progress.

8. Summary and Conclusions

The JPL ABL has been used successfully aboard theNASA DC-8 aircraft to make absolute measurementsof aerosol and cloud backscatter coefficients, as wellas sea-surface reflectances, with a calibration method-ology that is based on well-characterized target mate-rials. Determination of cloud opacities by the use ofsea-surface reflectance also appears to be feasibleunder certain conditions.3 2 The use of coherentdetection has enabled us to obtain quantitative mea-surements over a dynamic range of 7 orders ofmagnitude, from the low aerosol backscatter coeffi-cients in the clean troposphere and lower strato-sphere to the high backscatter from specularly en-hanced cirrus clouds and the sea-surface reflectance.The mechanical and thermal design of the ABLinstrument proved sufficient to maintain optical align-ment tolerances in the aircraft environment. Strayoptical scattering and electromagnetic interferenceproved to be the most difficult problems encounteredin the flights, although these effects were amelioratedin an evolutionary manner.

Intercomparison exercises with othei7 lidar instru-ments have proved essential in lending credibility tothe calibration methodology and the resulting data.Multiwavelength intercomparisons, which are ongo-ing, will provide valuable data for airborne andEarth-orbiting lidar performance analyses, includingspecifically the NASA (LAWS). Intercomparisonswill also provide unique insight into the transport ofaerosols and the radiative effects of aerosols and thinclouds. Future joint lidar flights with calibratedbackscatter measurement capability at additional in-frared wavelengths will provide further informationregarding aerosol and cloud properties.

The authors are grateful to James Spinhirne andcolleagues at NASA Goddard Space Flight Center,Madison Post of the NOAA Wave Propagation Labo-ratory, and William Jones and colleagues at theNASA Marshall Space Flight Center for providingdata to us for the purpose of describing multiwave-length lidar intercomparison activities.

The construction and deployment of the ABL aredue to the efforts of numerous persons: C. Esproles,S. Dermenjian, A. Brothers, M. Shumate, and D.Haner at the Jet Propulsion Laboratory (JPL); P.Flamant at the Centre National de la RechercheScientifique (CNRS) Laboratoire de Meteorologie Dy-namique; G. Ancellet at the CNRS Service d'A6-ronomie; the staff of the Optical Corp. of America(formerly Perkin-Elmer Applied Optics Operations),Garden Grove, California; and the personnel of NASAAmes Research Center Medium Altitude Missions

Branch, whose cooperation contributed in large mea-sure to the success of the ABL.

This work was carried out by the JPL, CaliforniaInstitute of Technology, under contract with NASA.

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airborne co_2 coherent lidar for measurements of atmospheric aerosol and cloud backscatter

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