AIRBORNE BACKSCATTER LIDAR: LITE VALIDATION AND CO-LOCATEDGROTIND-BASED RADAR MEASUREMENTS DTIRING CLARE '98

Harald Flentje, Wolfgang Renger,German Aerospace Research Centre,

M. Quante,GKSS Research Centre,

1. INTRODUCTION

Clouds and aerosols play a major role in atmos-pheric radiative transfer as well as chemical andmicro-physical processes and therefore in earth'sclimate. The most powerful tools for space-borneoperational observation of cloud and aerosolparameters on a global scale and coverage arelidar and radar systems. Actually, only these ac-tive remote sensing techniques are capable ofproviding information from dark regions as e.g.the polar winter atmosphere and with high verti-cal and horizontal resolution.The macroscopic properties of clouds like alti-tude, boundaries, internal structure and predomi-nating phase can be retrieve*d from combinedradar and lidar data with much higher accuracythan from either instrument alone. Combinedcloud radar and lidar data has previously beenused to obtain information on cloud boundaries,e.g. Uttal er al. (1995), Weitkamp er al. (1999),Clothiaux et al. (1999), or cloud micro-physicalstructure (Intieri et al. (1993)), however of eitherice or water clouds. No systematic investigationson mixed phase clouds are available. Thus theexamples of this study show the potential of thecomplimentary information in radar and lidardata.That these capabilities are transferable to a spacedeployment of active remote sensing instruments,is confirmed by comparing space-borne and air-borne lidar profiles during the first Lidar In-spaceTechnology Experiment (LITE) in 1994. Theonly relevant constraints of the space-borne- ascompared to the airborne instrument turned out to

. Corresponding author address: Harald Flentje, Ger-

man Aerospace Research Center (DLR), Lidar Groupat Institute fbr Atmospheric Physics, D-82230 Wess-l ing, Germany; e-mai l : Harald.Flent je@dlr .de

Martin Wirth, and Gerhard EhretDLR, Oberpfaffenhofen, Germany

O. DanneGeesthacht, Germany

be the reduced resolution due to a larger observedarea and the enhanced contribution of multiplescattering to the signal affecting the distanceassignment and the derived optical depths. Aselected measurement of this experiment is dis-cussed in the 2no chapter.The 3'd section contains results of the CLARE,'98campaign from 5 - 23 October 1998 at Chilbol-ton, UK, where a ground based 95 GHz radar(GKSS) and an airborne 3 wavelengths lidar(DLR) were operated simultaneously. During theoverpasses of the aircraft over the radar sitenearly the same air-volumes were profiled.

2. LTTF' VALIDATION EXPERIMENT

The first Lidar In-space Technology Experi-ment (LITE) was operated on board the spaceshuttle in September 1994. The system wasdesigned to measure range-resolved back-scatter of laser pulses at 1064nm,, 532nm, and354nm wavelengths (cf. Figure 1, Table 1). Itwas able to detect cloud top heights cloudgeometrical and optical depths, as well asprofiles of backscatter and extinction coeffi-cients and boundary layer top heights Kiemleet al . (1997), Flentje et al. (2000), Flentje etal. (2000).During the shuttle-bolxe LITE experimentcorrelative lidar measurements were per-formed at many places. The European contri-bution ELITE was co-ordinated and partlyfunded by the Technical centre of the Euro-pean Space Agency ESA/ESTEC. DLR per-formed airborne backscatter Lidar measure-ments similar to, and correlated with over-passes of the shuttle-borne LITE instrument.

Peter FrancrsUK Meteorological Office, Met. Research Flight, Farnborough, Hampshire, UK

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Figure 1: Biaxial optical system and beampath of ALEX lidar. Left side: transmitter,right: receiver. System parameters accordingto Table l. Overlap of the laser beam and thereceiver field-of-view is achieved after adistance of ctbout I-L5 km.

Table l: ALEX system parantetersTransmitterWavelengths, Energy perpulse, detector type

Pulse lengthsBeam divergencePulse repetition rateTelescope radiusField of viewFocal leneth

The backscatter Lidar (ALEX) was operatedin downward looking mode on board theDLR meteorological research aircraft Falcon20 cruising at an altitude of typically 12 km.It makes use of a Nd:YAG laser emitting atl064nm. Frequency doubling and triplingprovides 532nm and 354nm channels. Thereceived 532nm signal is split into the twoperpendicularly polarised portions whichallows to calculate the depolansation of thelight. With a repetition rate of lOHz for typi-cal aircraft speed of 150m/s the raw dataresolution is about 15m horizontally and l2mvertically. However, to improve the S/N ratiothe compromise between signal noise andresolution leads to a respective processeddata resolution of some 100m/some 10mhorizontally/verlically. Four nighttime flightsin northern Germany and over the North Sea,

covering a total of seven overpasses of thespace shuttle have been carried out. ALEXmeasurements have been performed for the-ses overpasses on distances of about 300 km.However, this length was crossed by theshuttle in about 40 seconds, whereas the Fal-con aircraft took roughly 30 minutes. Theactual overpass corresponds to about themiddle of each leg, resulting in a maximumtime difference of 15 minutes between themeasurements from LITE and ALEX. trig. 2depicts the backscatter returns for both in-struments at 532nm wavelength for orbit 32.The agreement between the two cross sec-tions is striking. There is an extended cirro-stratus throughout the whole distance with atop height of about 8.4 km just below thetropopause, sometimes in two layers, thecloud base lying between 5 and 7 km. Thereare broken, thick clouds in several layers inseveral layers top heights between 0.5km and4 km. Mostly they are not thick enough tomask the ground return signal (at 0 km)

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Figure 2: Backscatter cross sections at 532nm wavelength for ALEX and LITE instru-ment.

Atmospherlc Bockscotterlng (Pl) ot 532 nmI l{g-94 Orblt 32

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The Rayleigh and Mie components of themean backscatter profile for the return at532nm is shown in Figure 3. The Rayleighcomponent is depicted by the straight line.The solid lines refer to an analysis includingmultiple scattering effects, while the brokenlines refer to conventional single scatteringanalysis. The shape of the cloud and thebackscatter values are reproduced well, al-though there are slight differences in thecloud bases (LITE thin line being about 300m lower). They can partly be attributed todifferent sampling volumes in a highly vari-able cirrus cloud of the two instruments,since the spot sizes differ between about 5 mfor ALEX to about 800 m for LITE. It is ob-vious that mean backscatter profi'les fromboth instruments agree very well.

( l /km.sr)

Figure 3: Backscatter profiles at 532 nm with( - ) and without (----) multiple scatteringeffects included. Thick: ALEX, thin : LITEinstrument

Optical depths of the cirrus and maximumMie backscatter coefficients both as functionof longitude are shown in Figure 4. Despitethe different horizontal resolution of theALEX (Ax - 5km) and LITE (Ax = 52kminstruments, the values agree reasonablewell. We have to note that optical densitiesderived from ALEX measurements are al-most independent of whether multiple scat-tering is calculated or not. This is a result ofthe 2-level clear air calibration applied forthe lidar inversion and of the small absolutelaser beam diameters in the sampling vol-ume. The latter is much larser in case of the

LITE instrument because of the large dis-tance between lidar and sampling volume.Therefore, optical densities retrieved fromLITE data are much more sensitive to multi-ple scattering effects. Comparing the twoinstruments, optical thickness of the ALEXinstruments come out to be about 30 Vohigher than those from the LITE instrument.

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Figure 4: Optical depths and maximum Mie-backscatter cofficient of ALEX (+----+) andLITE (A-A) instruments as function of lon-gitude

3. THE CLARE'98 CAMPAIGN

As during LITE, for the CLARE'98 cam-paign the 3 wavelengths lidar ALEX (cf.Figure 1) was operated in down-lookingmode onboard a FALCON 20 aircraft. Fur-ther the GKSS cloud radar MIRACLE(Danne and Quante (2000)) was used whichis currently taken for various studies of cloudproperties, e.g. Quante et al. (1996), Danne etal. (1999), Fujiyoshi et al. (1999).Meteorological situation: Throughout theFALCON-campaign the synoptic situation inwestern Europe was dominated by a pro-

TAU BETWEEN 4.8- 8,8 KM

Royleigh ond Mie Bockscotter (LON 6,9- 9.4)

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nounced westerly current in which shortwave disturbances in rapid succession af-fected South Britain. The associated frontspassed nearly daily and caused distinct air-mass transitions between the pre- and postfrontal flows. Thus overcast and changefulconditions prevailed with multi-level mixedphase clouds frequently occurring throughoutthe troposphere. Thus this period was espe-cially suitable to investigate the differencesbetween the airborne lidar- and the ground-based 95 GHz radar profiles within thehighly inhomogeneous mixed phase clouds.Due to their different attenuation and sensi-tivity to particle size and phase both instru-ments provide complementary information ofcloud boundaries and cloud structure. Casestudies from 2O and 2I October from the twoweek field campaign in marine air-massesover Southern UK are presented. For the in-terpretation in-situ measurements of instru-ments installed on board a Hercules C-130aircraft of the UKMO are also considered.

Table 2: Date, time, position, and appliedIidar ratio for selected overpasses of the Fal-con at the radar site. The position dffirenceis given in meters as indicated by the navi-gation system of the Falcon relative theground site co- ordinate s.Date

(lower panel of Figure 5) indicates, that thenarrow stratocumulus layers consisted ofmainly liquid (sphencal) particles.Radar reflectivity and lidar backscatter coef-ficient profiles for the overpass are directlyintercompared in Fig. 6. The altitudes aregiven with respect to sea level, their accuracyis estimated to be about +30 m. The lidarprofile is averaged over 1 s which coffe-sponds to a horizontal cloud scale of 220 m.The radar data has been averaged over 9 s inorder to match the spatial scale of the lidardata at an altitude of 4 km. These time-spaceconversions used the wind measurementsobtained by the Hercules. For the displayedtime segment the radar was operated in FFT-mode, therefore data is available only for therange window between 2.8 and 5.2 km.Comparison of the profiles in the overlappingheight band reveals a different behaviour forthe sensors. While the lidar detected two nar-row, well separated peaks, the radar profile ismuch broader and the main peak occurs at alower height. Also the relative amplitudes ofthe peak values differ. The lidar signalprobably experienced strong attenuationalong the in-cloud path. The thin layers didnot appear as a clearly distinguishable featurein the radar data.The differences between the profiles are mostprobably due to the different micro-physicalproperties of individual layers in the cloudregion. In Rayleigh approximation the radarsignal is proportional to D6, while the lidarsignal is proportional to about D2 , with Ddenoting the diameter of the scattering parti-cles. In addition the phase of the particlesplays a role. It is assumed that the extendedthin layers detected by the lidar consisted ofliquid water while the rest of the cloud wasdominated by ice crystals. As can be seen inFig. 5, the depolarisation of the lidar signalaround 14:20 UTC was very low in the threenarrow layers suggesting the existence ofspherical scatterers. Depolarisation ratios ofless than L0 %o were found. Linear depolari-sation ratios as measured by the radan onlyindicate the presence of crystals in the lowerpart of the cloudy region, but there is no evi-dence for a decreased LDR around the lowestlidar peak at 4 km.

20 Oct.9820 Oct.982l Oct.982l Oct.98

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On 20 October 98 starting already beforenoon a rapid formation of cimrs was ob-served above 8 km height due to the ap-proaching occluded cold front of lowVALERIE II. In a strong westerly current (=15 m/s) a nanow altostratus cloud layer in3.8 - 4.2km altitude appeared above a densestratocumulus cloud cover at the top of theboundary layer near 2 km. The altostratuslayer with backscatter ratios of more than1000 in the infrared channel blocked the lidarbeam nearly completely (see Figure 5). Thelow depolansation ratio of less than 57o

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Figure 5: Lidar backscatter ratio @ l064nm,532nm, depolarisation ratio at 532nm on 20Oct.98. 14:15 - 14:21 UT. Chilbolton is lo-cated near -1.43E.

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Figure 6: Vertical profiles of radar reflectiv-ity (red) and backscatter coffi @ 1064 nm(blacH at 14:20:29 on 20 Oct.98. Lidar datais averaged over I s (about 218 m), the radardata was averaged over 9 s to represent thesame spatial scale.

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This might be an indication for the coexis-tence of liquid water and ice at this heightlevel. This aspect is confirmed by in situ mt-cro-physical measurements made with probesinstalled on the Hercules C- 130, whichpassed Chilbolton at I4:I9:53 UTC at analtitude of 4 km. As shown by the liquid wa-ter content measured with the Johnson-Williams liquid water probe along a nearlyco-located leg of the Hercules, in parts of thecloud the liquid water content exceeded 0.1gm-'(Figure 9).

might indicate that they were detached fromthe rest of the cloud.

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Figure I0: Radar reflectivity and lidar back-scatter cofficient @ l064nm on 20 Oct. 98,14:32:53. The lidar data was averaged over1 s (about 212 m), the radar data was aver-aged over B s in order to match the spatialscale.

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Figure I 1: Five consecutive profiles ofequivalent radar reflectivity and lidar back-scatter cofficient at 1064 nm for the over-pass at 14:32:53 UTC on 20 October 1998.

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At 14:32:59 UTC the radar was profiling theentire troposphere and therefore a signalfrom the cimrs layer with reflectivitiesaround -30 dBZ. also appears. Again the twodominant narrow peaks in the lidar data be-tween 4 and 4.5 km are not obvious in theradar profile, which shows a much thickerlayer. The lidar received also signals fromthe ice dominated part of the cloud between4.5 and 5.2 km. For the mid-level layer thecloud top is placed almost at the same height(5.2 km) by the two instruments. Due to at-tenuation the base region, which extendeddown to about 2.5 krn, and its structure is notresolved by the lidar as shown in Figure 10.

Figure 1l compares 5 consecutive profiles(averaged horizontally over 212 m) of radarreflectivities and lidar backscatter coeffi-cients around the overpass time. It can beseen that the lowest lidar peaks, which areagain associated with a thin, presumably liq-uid layer, decrease in altitude, while the radarpeaks stay almost at a constant height. This

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On 21 October 1998 the cold front of theprevious day had passed by. Post frontal thecloud cover broke up and mid-level cloudsreaching from 4 km to 6.5 km in altitudepassed Chilbolton in a strong westerly flow.The top of the boundary layer around 1-1.5km was marked by low cumulus clouds. Bothcloud layers can be seen in the lidar back-scatter signal as well as in the time series ofradar reflectivities shown in Figure J and 13,respectively. Typical radar reflectivitiesranged between -I0 dBZ. and -25 dBZ".

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dBZe (HH) GI(SS W-band Radar

at the time of the Falcon overpasses around10:19 UT and 10:50 UT appears to be betterresolved by the lidar. Actually the upperband disappeared from the radar signalshortly before 10:19 UT, probably leavingquite small evaporating particles behind,which due to the much smaller wavelengthstill could be detected around 6.7 km heightby the lidar as shown in Figure L4. Themaximum radar reflectivities occur about 1km lower at 5.5 km and may be due to rela-tively few larger ,e.g. precipitating, particles,which only produce a small backscatter sig-nal. However the lidar beam does not pene-trate this cloud, thus its overall structure canonly be resolved by the synergetic informa-tion from both instruments.During the 10:49:54 UT overpass (see Fig-ures 7, 8) the cloud had become weaker andmore homogeneous. From the in-situ meas-urements of the Hercules it can be concludedthat by this time there were neither smallliquid (evaporating) nor large precipitating(ice-)particles. Thus cloud boundaries andinternal structure of radar and lidar agreequite well except for the smallest scales.

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Figure 14: Radar reflectivity and lidar back-scatter cofficient @ l064nm on 2l Oct. 98,l0: 19:04. The lidar data is averaged over I s(about 175 m), the radar data was averagedover 6 s in order to match the spatial scale.Red dashed lines mark the range window forthe radar.

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The mid-level cloud layer reveals a verticallydisrupted band like structure which however

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4. SUMMARY

Two comparisons of remote sensing instru-ments have been discussed in this study: Anairborne lidar towards a space-borne lidarand to a ground based radar.In the first case the different target-receiverdistance leads to a stronger affection of thespace-borne system by multiple scattering.The general structure of the clouds mostlyagreed, except for few cases where the cloudbase was washed out due to longer path-lengths of multiply scattered photons dis-turbing the distance determination. Due tothe larger observed volume the optical densi-ties of the clouds were determined by up to30Vo lower from the space-borne than fromthe airborne data.The comparison of ground based cloud radarand airborne lidar measurements clearly re-veals the enorrnous information gain by asynergetic use of both systems. Their largelydifferent wavelengths results in different sen-sitivities to particles size distributions andsignal attenuation. Different regions of theclouds were highlighted by the instrumentsand only their synergetic use revealed theentire structure of the mixed phase cloudswith liquid layers and fallstreaks below thecloud base.The agreement between lidar and radar tendsto increase with decreasing spatial displace-ment between the observed volumes (Table2), since inhomogeneities within the ob-served clouds occur on small (Figure 11).Thus, a closely co-located positioning of bothinstruments is essential.

REFERENCES

Clothiaux, E. E., T.P. Ackerman, G.G. Mace,K.P. Moran, R.T. Marchand, M.A. Miller, andB.E. Martner, "Objective Determination ofCloud Heights and Radar Reflectivities Using aCombination of Active Remote Sensors at theARM CART Site." Accepted for publication, J.Appl. Meteor., 1999.

Danne, O., M. Quante, D. Milferstiidt, H. Lemke,and E. Raschke, "Relationships betweenDoppler spectral moments within large-scale

cirro- and altostratus cloud fields observed by aground-based 95 GHz cloud radar." J. Appl.Meteor., 38, 175-1 89, L999.

Danne, O., and M. Quante, The GKSS 95-GHzCloud Radar during CLARE'9S: System Per-formance and Data Products, in Proceedings ofCLARE'98, to appear in 2000

Flende, H., C. Kiemle and W. Renger, Multiplescattering in mixed phase mid-level ice/waterclouds during CLARE'98 and implications forspace lidar, in Proceedings of CLARE'98, toappear in 2000.

Flende, H., C. Kiemle, V. WeiB, M. Wirth andW. Renger, The 3 wavelength lidar ALEX andother instrumentation on board the Falcon air-craft during CLARE'98, in Proceedings ofCLARE'9S, to appear in 2000.

Fujiyoshi, Y., M. Quante, O. Danne, and E.Raschke, "Properties of Deep Stratiform IceCloud Revealed by 95 GHz Cloud Radar - ACase Study." Contr. Atmos. Phys.,72, II3-I25,1999.

Intrieri, J. M., G.L. Stephens, W.L. Eberhard, andT. Uttal, "A method for determining cirruscloud particle sizes using a lidar/radar back-scatter technique". J. Appl. Meteor.,32, L074-1082, 1993.

Kiemle, C., G. Ehret, A. Giez, K. J. Davis, D. H.Lenschow and S. P. Oncley, Estimation ofboundary layer humidity fluxes and statisticsfrom airborne differential absorption lidar(DIAL), J. Geophys. Res., 102, D24, 29189-29203. 1997.

Quante, M., O. Danne, E. Raschke, I. PopSte-fania, and A. Pazmany, 'Observations of cloudstructure with a 3.2 rrun-wave radar. Proceed-ings 12th Interantional Conference on Cloudsand Precipitation, Zirich, Schweiz, Vol. I , 424-427, 1996.

Weitkamp, C., H. Flint, W. Lahmann, F.A.Theopold, O. Danne, M. Quante, and E.Raschke. "Simultaneous radar and lidar cloudmeasurements at Geesthacht (53.5" N, 10.5'E);' Phys. Chem. Earth (B),,24, 163-166, 1999.

Uttal, T., E.E. Clothiaux, T.P. Ackerman, J.M.Intrieri, and W.L. Eberhard, 'Cloud boundarystatistics during FIRE II'. J. Atmos. Scl., 52,427 6-4284, rgg5.

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