Remote Sensing of Cirrus Radiative Parameters during …vnoel/pub/pub/Sauvage1999_MWR.pdf · 2005....

486 VOLUME 127M O N T H L Y W E A T H E R R E V I E W

q 1999 American Meteorological Society

Remote Sensing of Cirrus Radiative Parameters during EUCREX’94. Case Study of17 April 1994. Part I: Observations

LAURENT SAUVAGE,* HELENE CHEPFER,1 VINCENT TROUILLET,# PIERRE H. FLAMANT,*GERARD BROGNIEZ,1 JACQUES PELON,# AND FRANCK ALBERS@

* Laboratoire de Meteorologie Dynamique, Ecole Polytechnique, Palaiseau, France1 Laboratoire d’Optique Atmospherique, Universite des Sciences et Technologies de Lille,

Villeneuve d’Ascq, France# Service d’Aeronomie du CNRS, Universite Pierre et Marie Curie, Paris, France

@ GKSS, Institute of Atmospheric Physics, Geesthacht, Germany

(Manuscript received 21 July 1997, in final form 30 April 1998)

ABSTRACT

During the intensive European Cloud and Radiation Experiment 1994 (EUCREX’94) conducted off the coastof Brittany (France) over the Atlantic Ocean during April 1994, natural cirrus have been analyzed from in situand remote sensing measurements. The authors have particularly studied the case of 17 April 1994. For thisday a cirrus bank is described by a complete dataset, that is, classic airborne thermodynamical measurements,microphysical (forward scattering spectrometer probe) and OAP-2D2-C (optical array probe-cloud) probes man-ufactured by Particle Measuring System, and radiative (Barnes Precision Radiation Thermometer, Eppley pyr-anometers, and upward- and downward-looking pyrgeometers) measurements above and below the cloud. Morespecific airborne instruments were used such as upward backscatter lidar with polarization capabilities (LEAN-DRE) on board the Avion de Recherches Atmospheriques et Teledetection and the Polarization and Directionalityof the Earth’s Reflectances (POLDER) radiometer on board the Falcon for measurement of bidirectional andpolarized reflectances. The scene was also documented by NOAA-12/Advanced Very High Resolution Radiometerdata. However, the nonsphericity of cirrus ice crystals is clearly demonstrated by the lidar backscattering de-polarization ratio measurements (Dp 5 24%) and by the absence of any rainbow in POLDER bidirectionalreflectances. A specular reflection of the solar light observed on POLDER images indicates the presence ofhorizontally oriented ice particles in the cloud. All these optical properties will be studied in a companion paper(Part II) and compared with optical properties derived from microphysical models in order to evaluate theradiative impact of natural cirrus clouds.

1. Introduction

Cirrus and high-level clouds permanently cover near-ly 20% of the globe (Warren at al. 1988), and theirimpact on climate has been recognized for more than adecade (Liou 1986). An objective of the World ClimateProgramme is to improve our present knowledge of cir-rus clouds, in order to perform reliable prediction oftheir impact on the earth radiation budget. This goalrequires an appropriate description—in completeness,representativity, and accuracy—of their physical andoptical properties to be used in models. In order to il-lustrate the need in this area, it has been shown that theshape and size distribution of crystals and their orien-tation in space modify to a large extent the scattering

Corresponding author address: Dr. Gerard Brogniez, Laboratoired’Optique Atmospherique, Universite des Sciences et Technologiesde Lille, U.F.R. de Physique, 59655 Villeneuve d’Ascq, Cedex,France.E-mail: [email protected]

properties of cirrus clouds (Takano and Liou 1989;Brogniez et al. 1992) and, consequently, their radiativeproperties.

Recently, it has become an issue to document cirrusclouds in order to derive their physical and optical prop-erties that are needed to conduct process studies andparametrization and validation activities in the frame-work of atmospheric model applications. Until now, fewfield experiments combining in situ and radiative mea-surements have been conducted on natural cirrus. Pal-tridge and Platt (1981) derived empirical relations be-tween ice water content and radiative properties whereasFoot (1988) investigated the optical properties of cirruscloud from aircraft-based radiative and microphysicalmeasurements. The experimental problem is due to sev-eral factors such as a semitransparency of this type ofcloud, their high-altitude and large spatial inhomoge-neity, and a highly variable composition (i.e., ice crys-tals of different shapes and sizes, and possibly orien-tation). Part of the issue has been adressed during thecourse of projects conducted in the United States and

APRIL 1999 487S A U V A G E E T A L .

TABLE 1. Major field campaigns during the period 1987–95 as part of the two European programs: ICE and EUCREX.

Fieldcampaigns Date Geographical areas Main objectives

ICE’87ICE’89

Sep–Oct 1987Sep–Oct 1989

North SeaNorthern Germany and

North Sea

Frontal cirrusFrontal cirrus and contrails

CIRRUS’92Arktis’93EUCREX’93EUCREX’94

Sep–Oct 1992Feb–Mar 1993Sep–Oct 1993Apr 1994

Southern GermanyArctic (Svalbord)Northern Atlantic OceanAtlantic Ocean, France

Frontal and jet cirrusCirrus over ice and arctic seaLife cycle of convective cirrusRemote sensing of stratiform clouds

in Europe: the two First International Satellite and CloudClimatology Project Regional Experiments in 1986 and1991, respectively (see special issues of Monthly Weath-er Review, Vol. 118, No. 11, 1990 and the Journal ofthe Atmospheric Sciences, Vol. 52, No. 23, 1995), andthe International Cirrus Experiment (ICE’89) in 1989(Raschke et al. 1990). During the field campaigns insitu probes, passive (i.e., radiometers) and active (i.e.,lidars, radars) remote sensors, either airborne or groundbased, have been deployed to document different cirruscloud layers at the mesoscale while satellite data wereprovided with synoptic-scale observations.

Actually, a synergism of remote sensors is foreseenas a key issue for the retrieval of cirrus cloud charac-teristics that are relevant to radiative transfer and pa-rametrization studies. This prompted us to address thepotential synergism of a new radiometer, the Polariza-tion and Directionality of the Earth’s Reflectances (POL-DER), with a backscatter lidar during EUCREX’94, afield campaign that was held in Brittany, France, in April1994, in the framework of the European Cloud RegionalExperiment (EUCREX) project. POLDER is a multi-spectral and polarization radiometer. Two POLDER ra-diometers and the Lidar Embarque pour l’Etude desAerosols, Nuages, Dynamique, Rayonnement et Espe-ces minoritaires (LEANDRE) were installed on boardtwo aircraft.

In this paper (Part I) we report here on a well-doc-umented cirrus cloud case (17 April 1994) over the At-lantic Ocean off the Brittany coast. The cloud propertiesare derived from airborne instruments (radiometers, li-dar, in situ probes) and satellite data. Some of the find-ings using the remote sensors are validated by in situmeasurements. Section 2 presents an overview of EU-CREX’94. Section 3 deals with the measurement strat-egy and presents the synoptic conditions prevailing on17 April 1994. Section 4 presents the instruments andthe methodologies used to retrieve the cirrus cloud ra-diative parameters. Section 5 presents the whole datasetcollected on 17 April 1994. In a companion paper(Chepfer et al. 1999, hereafter Part II) this dataset isused to constrain both microphysical models and radi-ative transfer codes in order to derive the cirrus cloudoptical and radiative properties.

2. Overview of EUCREX’94

The European Cloud Radiation Experiment, a pro-gram focused on the earth radiation budget and climatechange, has been promoted by the European Communitystarting in 1991 through 1995 (Raschke et al. 1996).This activity took place in the framework of the WorldClimate Research Programme and Global Energy andWater Cycle Experiment (WCRP 1990). EUCREX is afollow on of the ICE experiment.

The cooperative field campaigns conducted in theframework of EUCREX are presented in Table 1. DuringEUCREX’94 different types of clouds, that is, cirrus,stratiform midlevel clouds, and stratocumulus, havebeen documented. Three instrumented research aircraftand a ground-based site (488259N, 48479W) were in-volved. The three aircraft were (i) a turboprop Fokker-27 called ARAT (Avion de Recherches Atmospheriqueet Teledetection), operated by the French Institut Na-tional des Sciences de l’Univers (INSU) and the InstitutGeographique National; (ii) a jet Falcon operated by theGerman Deutsche forschungsanstalt Luftund Raumfart(DLR); and (iii) a turboprop Merlin IV operated byMeteo-France. Table 2 presents the aircraft performanceand instrumental payloads. The three aircraft were basedat the Brest-Guipavas airfield (488279N, 48259W). Theinstrumented site was located at CROSS Corsen (Centrede Recherches, d’Observation, de Secours et de Sau-vetage or CC hereafter), 1 km away from the seashore.Several ground-based remote sensors, among them twolidars, have been deployed at CC.

The ARAT did participate in all measurements, thatis, low-, mid- and high-level clouds; the Falcon was notinvolved in measurements of low-level clouds; whilethe Merlin IV flew for measurements of midlevel stra-tocumulus clouds. A summary of missions undertakenduring EUCREX’94, with date, time (UTC), duration,location over the Atlantic Ocean, and objectives is pre-sented in Table 3. The time duration is the maximumflight duration among the three aircraft. All flight tracksstarted or ended at the ground-based site (CC) for thepurpose of intercomparison and validation. The lettersA, F, and M in Table 3 stand for ARAT, Falcon, andMerlin IV, respectively.


TABLE 2. Aircraft performance and instrumental payloads.

ARAT/Fokker-27 Falcon Merlin IV

Ceiling altitudeCruising airspeedEndurance

5500 m80–100 m s21

3–4 h

11000 m170–190 m s21

2.5–3 h

7000 m100–130 m s21

4–5 h

Instrumentation

Backscatter lidarPOLDERRadiometers

Eppley PyrgeometersEppley PyranometersBarnes PRT-5 (narrow FOV)

In situ probes (p, T, u)Microphysics probes

YesYes

YesYesYesYesNo

NoYes

YesYesNoYesYes

NoNo

YesYesYesYesYes

TABLE 3. Summary of the various missions conducted during EUCREX’94. All flight tracks started or ended at the Cross Corsen site (aninstrumented site). A, F, and M stand for ARAT (Fokker F-27, Falcon, and Merlin IV), respectively.

Date(Apr 1994) Mission Aircraft

Take-off (UTC)and duration

Flight trackorientation Sciences objectives

11 1 A, F, M 1330, 2 h NW Clear sky/intercomparison flight12 2 A, F, M 0900, 3 h NW Multilayer, stratiform clouds, Sc15 3 A, F, M 1015, 3 h NW Sc, Cu

17

45

A, FA, F

0730, 3 h1230, 3 h 20 min

NWS

Thin Ci onlyThick Ci on low clouds

18 6 A, F, M 1000, 3 h NW Homogeneous Sc layer19 7 A, F, M 0900, 2 h NW As/Sc and contrails20 8 M 1520, 1 h 30 min Local flight Sc21 9 A, F 0800, 2 h N Contrails

22

1011

A, FA, F

0630, 4 h1000, 3 h

SS

Ci on low cloudsCi only

232526

121314

A, MA, MA, M

0830, 2 h 20 mn0800, 3 h0800, 2 h

NWW

Broken ScThick AsMultilayer, Sc, and As

In the following sections (3–6) we report on the re-trieval of cirrus radiative properties using the resultsobtained during missions 4 and 5 on 17 April 1994.

3. Measurement strategy for the cirrus case on17 April 1994

On 17 April 1994, the meteorological charts show thepresence of high pressure centered over the northernAtlantic, extending to Ireland and partly over England.A low was located over southern France, extending fromthe east of Spain to northern Italy. On 16 April, the lowwas stronger over northern France. This system, in con-nection with another low over northern Iceland andSweden, resulted in a strong jet stream bringing polarair over northern Europe. The entrance of the jet streamwas located over southern England, northeast of Brit-tany. It may explain the formation of an extended cirruslayer over the Atlantic Ocean near the Brittany coast,as a result of the presence of an ageostrophic circulation.The wind speed at 300 hPa decreased from 40 m s21 to

15 m s21 on two consecutive days (16 and 17 April)over the English Channel, as the low moved southward.

Two aircraft, the ARAT and the Falcon (see section2 and Table 3), were involved in the measurements con-ducted on 17 April 1994 to document a single cirruslayer observed on satellite data [Meteosat and NationalOceanic and Atmospheric Administration-12/AdvancedVery High Resolution Radiometer (NOAA-12/AVHRR)]. Two back-to-back flights were conducted onthat day, mission 4 in the morning over northern Brit-tany and mission 5 in the afternoon over southern Brit-tany. The two flights took place in the cold sector ofthe front. The ARAT carrying the lidar and radiometersflew underneath the cirrus layer at a constant altitudeof 4.5 km all the time, whereas the Falcon overflew thecirrus layer first for POLDER measurements and thenmade several legs at different levels inside the cirruscloud for microphysics measurements using in situprobes (see Fig. 1).

The large-scale cloudiness as observed by NOAA-12/AVHRR (channel 4, 10.3–11.3 mm) at 0855 and 1620


FIG. 1. Synoptic scheme of flight: the Falcon overflew the cirrus,with the POLDER downward-looking radiometer and PMS FSSP andPMS OAP-2D2-C probes, while the ARAT underflew the same cirrusbank with lidar and upward-looking PRT-5. The two aircraft wereequipped with upward- and downward-looking pyranometers and pyr-geometers. The large-scale cloudiness was documented by NOAA-12/AVHRR data.

FIG. 2. (a) NOAA-12/AVHRR image obtained in channel 4 (10.3–11.3 mm) at 0855 UTC 17 April 1994 over the area of aicraft mea-surements. The track of the Falcon overflying the cirrus bank duringmission 4 is superimposed on the satellite image (straight solid line).(b) Same as (a) but for image obtained at 1620 UTC. The track ofthe Falcon overflying the cirrus bank during mission 5 is superim-posed on the satellite image (dashed line).

UTC is presented in Figs. 2a and 2b. The flight tracksfor missions 4 and 5 are displayed as a solid line anda dashed line, respectively. The parameters to be re-trieved are presented in Table 4.

The radiosonde temperature profiles at Brest-Guipa-vas at 0000 and 1200 UTC on 17 April 1994, and thoseobtained by the Falcon during ascent and descent forthe two missions, are in good agreement. The data re-corded by the Falcon are displayed in Fig. 3. The tro-popause height is at 10.5 km (with an uncertainty of0.3 km), the temperature is 2628C, and the mean lapserate in the upper troposphere is 26.98C/km.

Humidity soundings were performed with three dif-ferent humidity sensors on board the Falcon: a cryo-genic-dewpoint sensor, a Lyman-a sensor, and a Vaisalahumicap. The cryogenic-dewpoint sensor provided themost reliable humidity measurements. The Lyman-asensor gave unrealistically high values of humidity dueto an additional absorption by ozone, which has provendifficult to correct. The humicap sensor was unreliablewith erratic fluctuations of the measurements. Even so,the cryogenic-dewpoint sensor showed significant dis-agreement between humidity profiles recorded duringascent and descent, with a 3-h time difference on thesame flight, and 5 h between the two missions. Themeasurements show three moist layers in the vertical at3–5 km with 30%–50% maximum relative humiditywith respect to liquid water, 7–9 km with 40%–60%maximum relative humidity with respect to liquid water,and 10–11.5 km with low relative humidity (5%–20%with respect to liquid water, 10%–35% with respect toice).

Mission 4 (Fig. 4a) proceeded in three legs for theARAT—A1, A2, A3, along the same flight track at analtitude of about 4.6 km, and six legs—F1–F6, at dif-ferent altitudes along the same track for the Falcon.During mission 5 (Fig. 4b), the ARAT performed threelegs: A1–A3, at the same altitude along the same trackwhile the Falcon proceeded in eight legs: F1–F8. Thecloud boundaries retrieved by lidar (see section 4b) are


TABLE 4. Cirrus radiative parameters and thermodynamical param-eters measured by remote sensors and in situ probes on board thetwo aircraft (ARAT and Falcon).

Cloud andatmosphericparameters Instrument Carrier

Cloud height, geometricalthickness DH

Optical thickness at 532nm (dSW)

Vertical structure of ex-tinction (a) and back-scatter (b) coefficientsat 532 nm

Backscaterring phasefunction P(p) or back-scatter to extinction ra-tio (k) P(p) 5 4p k

Vertical structure of lineardepolarization ratio (D)

Backscatter Lidar ARAT

Bidirectional reflectances(r)

Polarized reflectance (rp)

POLDER Falcon

Downward (F ) and up-↓SW

ward shortwave (F )↑SW

fluxes

Pyranometer ARAT, Falcon

Downward (F ) and up-↓LW

ward longwave (F )↑LW

fluxes

Pyrgeometer ARAT

Down- and upward beamemittance (e)

Infrared optical thickness(dIR)

PRT-5 ARAT

Thermodynamics: temper-ature, pressure, and hu-midity profiles

In-situ probes Falcon

Microphysics: ice watercontent, particles size,concentration

2D-C, FSSP Falcon

FIG. 3. Atmospheric temperature profile obtained for the ascents ofthe Falcon during missions 4 and 5.

outlined in Figs. 4a and 4b. The characteristics of eachleg are presented in Table 5.

4. Instrumentation and retrieval of radiativeparameters

The two aircraft payloads (see Table 4) are comple-mentary for radiation and microphysics measurements.In this section we present the remote sensors and in situprobes, and the methodologies used to retrieve the cirruscloud parameters.

a. Radiometers

The ARAT and the Falcon were equipped with twosets of Eppley cleardome pyranometers and an Eppleypyrgeometer looking upward and downward (see Tables2 and 4). A Barnes precision radiation thermometer(PRT-5) looking upward was installed on board theARAT to measure infrared radiance (L) in the 8–14-mm

spectral region within a 35-mrad field of view. It cor-responds to a footprint of 100-m diameter at a 3-kmrange. All the radiometer data were recorded using a 1-Hz sampling frequency.

Upward and downward effective beam emittances «↑↓

are calculated using IR radiances L↑↓ (Allen 1971):↑↓ ↑↓L 2 L0↑↓e 5 , (1)↑↓ ↑↓L 2 L1 0

where L↓ is the downward IR radiance below the cirruscloud, which is measured by the PRT-5 on board theARAT. Here, L↑ is derived from NOAA-12/AVHRRdata; and are the IR radiances that would be↑↓ ↑↓L L0 1

measured if the cirrus cloud emittance is zero (i.e., clearsky) or unity (i.e., blackbody), respectively. The actualeffective beam emittances computed using Eq. (1) areslightly different than the true absorption beam emit-tances as they account for the infrared scattering con-tribution (Platt and Stephens 1980).

The downward beam emittance «↓ requires the knowl-edge of the clear-sky and blackbody radiances .↓ ↓L L0 1

Here, is measured in clear air area during the flight,↓L0

in the absence of cirrus cloud as indicated by the lidar.For , a theoretical value for is computed first using↓ ↓L L1 0

the LOWTRAN-7 code (Kneizys et al. 1988). Then,is computed by adding a hypothetical blackbody at↓L1

an altitude corresponding to the altitude of the maximumof the backscatter coefficient, which is determined bylidar (see section 4b). The temperature at this level istaken from the Falcon sounding. To compute the↓L0

input parameters are (i) the transmission and spectralcharacteristics of the interference filter, (ii) the temper-ature and humidity profiles recorded by the Falcon, and(iii) a vertical profile of ozone density corresponding to


a midlatitude atmospheric model (McClatchey et al.1971). Because the humidity measurements are less re-liable, they are adjusted in order to make the computedvalues for equal to the measurement. The adjustment↓L0

bears on 65% or less of the humidity value.In order to derive «↑, the upward IR radiance in clear-

sky corresponds to the maximum temperature of a↑L0

histogram plot of the NOAA-12/AVHRR channel 4 data.The effective infrared absorption optical thickness dIR

is linked to the effective beam emittance (Platt 1972):

dIR 5 2ln(1 2 «↑↓). (2)

The intrinsic radiometric noise corresponds to fluctua-tions of the order of 0.02 in dIR. The main uncertaintyin deriving dIR is the cirrus cloud temperature. The var-iability of the cloud geometrical thickness correspondsto a 5-K uncertainty, which in turn corresponds to anuncertainty of 0.03 in dIR. In comparison, the error re-sulting from the nonscattering cloud approximation isa few percent or less (Platt and Stephens 1980; Brogniezet al. 1992). The overall uncertainty of the effectivebeam emittance is about 15%. Figure 5 displays « re-trieved from PRT-5 and AVHRR data as a function oflatitude for mission 4. The AVHRR data at 0855 UTCare located along the ARAT flight track. The relativechange with latitude for the two datasets agree, whilethe absolute values show a significant disagreement. Itis not due to the difference in footprint (i.e., 100 m forPRT5 and 1 km for AVHRR) but to the temperatureused in the computation.

A proper calibration of radiometers is needed to de-rive the radiative fluxes. In response to this need anintercomparison flight involving the three aircraft wasconducted over the Atlantic Ocean on a clear day (mis-sion 1; see Table 3). The aircraft flew in close formationwithin tens of meters at a constant altitude of 1.6 km.The upward and downward shortwave fluxes re-↑↓FSW

corded by the pyranometers have been corrected for achange in solar zenith angle (us 5 44.428–45.818) duringthe flight. The results are presented in Table 6. The threedatasets are consistent in that the measurements fallwithin a common range.

The downward longwave fluxes recorded by the↓FLW

pyrgeometer on board the ARAT for two flight altitudesof 1.5 and 5.2 km are compared to theoretical valuesin Table 7. The experimental data are corrected accord-ing to Saunders et al. (1992). The theoretical values arecomputed using a narrowband radiative transfer code(Morcrette and Fouquart 1985) and a maritime aerosolmodel (WCP 1986). An aerosol optical thickness at 550nm of 0.138 between 0 and 5.2 km of altitude (and0.013 between 0 and 1.5 km) is used to fit the mea-surements with theoretical values.

A value of the plane albedo (a) of the sea–cirruscloud–atmosphere system is derived from the pyrgeo-meter data recorded by the Falcon (F) overflying thecirrus layer:

↑(F )SW Fa 5 . (3)↓(F )SW F

Figure 6 shows the plane albedo as a function of distancefor leg F3 of mission 4 and leg F1 of mission 5. Theresults are corrected for a mean solar zenith angle of us

5 558 for mission 4 and us 5 388 for mission 5, re-spectively.

b. Backscatter lidar

The backscatter lidar provides direct information oncirrus cloud structure (height, geometrical thickness),optical properties (profiles of extinction and backscattercoefficients), and parameters linked to the microphysicalcharacteristics of the cloud particle (backscatter-to-ex-tinction lidar ratio, depolarization ratio).

The lidar LEANDRE was installed on board theARAT (Pelon et al. 1990). The transmitter is an Nd-YAG laser emitting 60 mJ per pulse at 0.53 mm and100 mJ per pulse at 1.06 mm with a 10-Hz pulse rep-etition frequency. The two laser emissions are linearlypolarized. The laser beam divergence of 2.5 mrad givesa footprint of 7 m at 3 km. The optical power back-scattered off the atmosphere is collected by a receivertelescope (30-cm diameter or 0.08 m2 collecting area,3.5 mrad field-of-view full angle). Three detection chan-nels record two signals in parallel, at 0.53 mm withparallel (S\) and perpendicular (S⊥) polarizations and at1.06 mm. The lidar signals at 0.53 mm can be calibratedin absolute values in clean air corresponding to a lowaerosols burden (see the appendix). The vertical reso-lution along the line of sight is 15 m; it is set by thetransient digitizer sampling frequency (10 MHz).Twelve lidar shots in a row are averaged to increase thesignal-to-noise ratio (SNR).

The backscatter lidar can operate either in a near ze-nith or nadir viewing mode. It takes 1 min to switchthe looking configuration, so up- and downlooking mea-surements can be implemented on successive legs dur-ing the same flight. The uplooking measurements aremade at a fixed viewing angle of 208 toward the rightof the fly track and 108 toward the rear of the aircraft.

For EUCREX’94 the cirrus optical properties are de-rived using the two 0.53-mm channels (Spinhirne andHart 1990; Young 1995). Figures 7a and 7b display 2Dplots of the backscattering coefficient as a function ofdistance from CROSS Corsen for legs A1, A2, and A3of mission 4 and leg A2 of mission 5. The maximumvalue of the backscattering coefficient reaches 0.06km21 sr21 and 0.10 km21 sr21 during mission 4 and 5,respectively. The cirrus boundaries are indicated by ared dashed line; the altitude of the maximum back-scattering coefficient is indicated by a black solid line.

Figure 8 displays a scatterplot of dIR versus , dIRad532

is derived from data collected by the upward lookingPRT-5 radiometer on board the ARAT (see section 4a),and is the lidar apparent optical thickness not cor-ad532


FIG. 4a. Mission 4 proceeded in three legs for the ARAT at an altitude of 4.6 km, three legsover the cirrus at altitudes of 11.2 and 11.5 km, and three legs inside the cirrus at different altitudesfor the Falcon. All legs were flown along the same flight track. Legs of the ARAT are indicatedby letter A, while the legs of the Falcon are indicated by letter F. Diamonds indicate the beginningof each leg. Cirrus boundaries as measured by lidar on board the ARAT are also indicated (dots),as well as the location of the POLDER data analysis (D).

FIG. 4b. Same as Fig. 4a but for mission 5: three legs for the ARAT at a constant altitude of4.6 km, two legs over the cirrus at 10.8 km, and six legs at different altitudes inside the cirrusfor the Falcon.

rected for multiple scattering effects (Nicolas et al.1997). The best least-squares fit of the data is a straightline set to start at the origin. It shows that dIR/ øad532

0.99, whereas a factor of 2 is expected from theory for

large particles (i.e., geometrical approximation). Ac-cording to this result and previous studies (Nicolas etal. 1997), we correct the optical thickness by a multi-plicative factor of 2. The large scatter of data displayed


TABLE 5. Description of the legs of Falcon and ARAT for missions 4 and 5. Shown are times of beginning and end of measurements,corresponding distance from CC, and mean altitude.

Mission 4 Mission 5

F10834 → 0843 UTC

29.5 → 123.9 km11 200 m

F11303 → 1325 UTC

84.2 → 298.1 km10 650 m

A11257 → 1324 UTC215.8 → 110.2 km

4600 m

F20849 → 0857 UTC

81.9 → 1.4 km11 200 m

F21330 → 1340 UTC293.7 → 189.1 km

10 650 mA2

1328 → 1403 UTC93.5 → 318.9 km

4590 m

F30908 → 0914 UTC

48.9 → 118.8 km11 600 m

A10908 → 0926 UTC

1.4 → 104.4 km4580 m

F31346 → 1356 UTC228.7 → 327.6 km

9400 m

F40922 → 0936 UTC132.9 → 0.6 km

9420 mF4

1402 → 1406 UTC298.5 → 258.4 km

8500 m

F50941 → 0951 UTC

10.4 → 101.6 km8800 m

A20933 → 1000 UTC143.9 → 4.2 km

4600 mF5

1410 → 1414 UTC277.3 → 318.0 km

7900 mA3

1404 → 1414 UTC323.3 → 276.9 km

4620 m

F60955 → 1005 UTC

93.9 → 0.8 km8200 m

A31003 → 1018 UTC

1.6 → 93.3 km4560 m

F61421 → 1426 UTC296.4 → 252.5 km

7300 m

F71432 → 1435 UTC283.8 → 311.5 km

6690 m

F81441 → 1500 UTC288.4 → 135.3 km

6080 m

F 5 leg of Falcon flight; A 5 leg of ARAT flight.

TABLE 6. Intercomparison of pyranometer measurements (meanvalue and standard deviation) during mission 1 (see Table 3), whenthe three aircraft flew at an altitude of 1.5 km.

AircraftF , (s)↓

SW

(W m22)F , (s)↑

SW

(W m22)

ARATFalconMerlin IV

809 (3)809 (5)812 (6)

46.5 (0.7)46.5 (1.5)46.5 (0.5)

FIG. 5. Comparison between effective beam emittances derivedfrom AVHRR measurements (channel 4) and PRT-5 on board theARAT during leg A2.

in Fig. 8 is due to the different pointing direction forthe lidar and the PRT-5 (off by ø208) and samplingvolumes (the footprints are separated by 1 km at thealtitude of the cirrus cloud).

A profile of depolarization ratio for particles Dp isderived from a total depolarization ratio for particlesand molecules:

S (x, z)⊥D(x, z) 5 . (4)S (x, z)\

Then Eq. (4) is normalized to the molecular depolar-ization ratio Dm 5 2.8% (Young 1980) assuming cleanair above or below the cirrus cloud. Finally, Eq. (4) iscorrected for the molecular contribution Dm to deriveDp displayed in Figs. 9a and 9b. Here, Dp is plotted asa function of distance from CROSS Corsen for legs A1,A2, and A3 of mission 4 and leg A2 of mission 5. Themaximum value is 50%.

c. POLDER

Two POLDER downward-looking instruments wereoperated by the Laboratoire d’Optique Atmospherique(Deschamps et al. 1990) on board the Falcon and theARAT. POLDER measures the spectral radiances andpolarization characteristics of the solar light scatteredoff the clouds, the atmosphere, and the earth surface.The principle of measurement is presented in Fig. 10.The airborne instruments are a simplified version of themultichannel radiometer on board the Japanese Ad-


TABLE 7. Comparison of pyrgeometer measurements on board the ARAT with model results for clear air and maritime aerosols, at flightaltitudes of 1.5 and 5.2 km (see text).

Flightaltitude

(km)

F (W m22)↑LW

MeasurementsClear air

model

Maritimeaerosolsmodel

F (W m22)↓LW

MeasurementsClear air

model

Maritimeaerosolsmodel

1.55.2

345300

352315

345300

205150

165110

205140

FIG. 6. Plane albedo of the earth–atmosphere–cirrus system ob-tained from pyranometers on board the Falcon during leg F3 of mis-sion 4 and leg F1 of mission 5 as a function of the distance fromCROSS Corsen.

vanced Earth Observing System platform launched inAugust 1996 (Deschamps et al. 1994).

The two key components of POLDER are (i) a charge-coupled device (CCD) matrix (see Table 8) set in thefocal plane of a wide field-of-view telecentric optics(focal length of 3.565 mm) and (ii) a rotating wheelcarrying five narrowband filters, which transmit polar-ized light (Table 8). For POLDER on board the Falconthe field of view is 6528 along the aircraft track and6428 cross track. At 1 km the footprint is 2.5 km 31.8 km and the resolution (for one pixel) is 9 m 3 8 m.

The POLDER measurements presented hereafterwere taken in the presence of a single cirrus layer only

in the absence of low or midclouds underneath. Theinformation provided by POLDER is composed of two-dimensional normalized bidirectional total reflectancesri and two-dimensional normalized bidirectional polar-ized reflectances , where i stands for any of the fiveirp

channels. We have

ipLir 5 and (5a)

iE cosus s

ipLpir 5 , (5b)p iE cosus s

where Li and are the incoming total and polarizediLp

radiances, respectively; the solar irradiance at the topiE s

of the atmosphere; and us the solar zenith angle. Theuncertainty on reflectances is about 10%.

The present analysis on the cirrus cloud radiativeproperties is conducted using the 864-nm channel be-cause the molecular contribution to the signals is weakand there is no noticeable absorption. Two sequencesof POLDER measurements during mission 4 and 5 aredisplayed in Fig. 11, which presents the polar projectionof POLDER images recorded on homogeneous andthick cirrus clouds during leg F3 of mission 4 (Figs.11a and 11c) and leg F1 of mission 5 (Figs 11b and11d). The POLDER measurements are contemporarywith lidar measurements. They are correlated to thehighest values of the plane albedo (see Fig. 6), that is,at about 90 km and 160 km from CC, for F3 of mission4, and F1 of mission 5, respectively. Figures 11a and11c are an average of two images; Figs. 11b and 11dan average of three images. Figures 11a and 11b presentthe normalized bidirectional total reflectances [Eq. (5a)].The normalized polarized reflectances [Eq. (5b)] are pre-sented in Figs. 11c and 11d. Each concentric circle dealswith a constant zenith angle uy . The solar principal planeis along the 08–1808 axis, the azimuthal viewing anglewy is set from this axis, and 08 corresponds to the solardirection. The corresponding solar zenith angles andFalcon headings are us 5 558 and H 5 138 (Figs. 11aand 11c), and us 5 388 and H 5 2208 (Figs. 11b and11d), respectively.

The total reflectance reaches 65%, while the polarizedreflectance reaches only 2.5%. The lowest value for thetotal reflectance is on the order of 50% for mission 5(Fig. 11b) and 35% for mission 4 (Fig. 11a). In Figs.


FIG. 7a. Bidimensional values of the backscatter coefficient b(x, z)(in km21 sr21) for the three legs of mission 4 as measured by thelidar. The space variables x and z are the distance from CROSS Corsenand the altitude, respectively. Legs A1, A2, and A3 are plotted fromtop to bottom (see text). Color scales concern the values of b(x, z)between 0 and 0.056 km21 sr21. White parts of these color scalesindicate the maximum values reached by the backscatter coefficientduring the leg.

FIG. 7b. Same as Fig. 7a but for leg A2 of mission 5.

FIG. 8. Scatterplot of infrared optical thickness dIR derived fromthe PRT-5 radiometer as a function of the lidar apparent optical thick-ness . Only data corresponding to mission 4 are reported here.ad532

The slope of the least squares fit of data to a straight line is equalto unity.

11b and 11d, we notice the presence of a hot spot. Thishot spot occurs at a zenithal viewing angle uy 5 us 538.08, and it is located in the solar principal plane. Thisphenomena is persistent for several tens of kilometers;it is even stronger when the cirrus cloud becomes thick-er. According to this correlation it is not due to a seaglint effect but to a specular reflection of sunlight oncirrus cloud. Such a phenomenon cannot be observedin Figs. 11a and 11c; it is due to high values of the solarzenith angle in the morning, the POLDER configuration,and geometry of observation.

Figures 12a and 12b present reconstructed POLDERimages for the total reflectance. They are made of theonly central line perpendicular to the Falcon headingon successive POLDER images and are indicated bytwo orientations, that is, 938 and 2678 in Fig. 11a forleg F3 of mission 4, and 708 and 2508 in Fig. 11b forF1 of mission 5. The length of the track is 70 km (i.e.,about 700 successive images) for F3 (mission 4) and214 km (i.e., about 2140 images) for F1 (mission 5).The cross-track dimension is limited by the POLDERfield of view and range to the cirrus layer. It is 3.6 kmfor F3 (mission 4) and 3 km for leg F1 (mission 5) whenthe Falcon overflew at a height of 1.5 km and 1.3 km,respectively. The arrows in Figs. 12a and 12b indicatesthe location of the POLDER image presented in Fig.


FIG. 9a. Bidimensional values of the backscattering depolarizationratio D(x, z) (in %) obtained from the airborne lidar for the three legsof mission 4. The space variables x and z are the distance from CROSSCorsen and the altitude, respectively. From top to bottom are plottedlegs A1, A2, and A3 (see text). Color scales concern the values ofD(x, z) between 0% and 70%. The cirrus boundaries are indicated bya red dashed line; the black solid line indicates the altitude of themaximum backscattering coefficient.

FIG. 9b. Same as Fig. 9a but for leg A2 of mission 5.

FIG. 10. Synoptic scheme of POLDER measurements principle.The solar zenith angle is us. The particular viewing direction, char-acterized by the zenith viewing angle uy and by the relative viewingazimuth angle wy 2 ws, illuminates a particular pixel of the CCDmatrix.

11; they are correlated to high values of the total re-flectance.

d. Microphysics by in situ measurements

Two complementary PMS optical probes (from Par-ticle Measuring System Inc., Boulder, CO) on board theFalcon have been used to determine a size distributionof the particles: (i) a PMS forward scattering spectrom-eter probe (FSSP-100) assuming spherical particles withequivalent diameter ranging from 2 to 47 mm, and (ii)a PMS optical array probe (OAP-2D2-C), which pro-

vides two-dimensional images of particles with meansize from 50 to 800 mm.

Heymsfield et al. (1990) have addressed the problemof using FSSP for a retrieval of the microphysical char-acteristics of ice cloud particles. Gayet et al. (1996a)showed that FSSP information is pertinent when itagrees with OAP-2D2-C measurements in a commonsize range, which is the case in the present study. Thenominal size resolution of OAP-2D-2C is 25 mm for anairspeed of 100 m s21; the resolution is decreased to 50to 70 mm for a true airspeed in the range of 150–180m s21.

The size distribution is expressed in cross-sectionarea, which is obtained directly from the OAP-2D2-Cprobe after correction for the electronic response time(Albers 1989). The cross-section area is computed ac-cording to the approaches proposed by Knollenberg(1970), for the depth of field, and Heymsfield and Baum-gardner (1985), for the ‘‘entire in method.’’ A discussionon the relative accuracy of 2D-C probes and processingmethods is presented in Gayet et al. (1993).

Figure 13 presents the distribution of cross-sectionarea for legs F4, F5, and F6 of mission 4 (panel a), and


TABLE 8. Characteristics of the two airborne POLDER instrumentson board the Falcon and ARAT (Fokker 27), respectively.

POLDERon board

the ARAT

POLDERon board

the Falcon

Wavelength(nm)

443 with polarization570670——864 with polarization911

443 with polarization——763765864 with polarization911

CCD array(32 mm 327 mm) perpixel

384 3 288 pixels 288 3 242 pixels

legs F3, F4, and F5 of mission 5 (panel b). The icewater content (IWC) particle number density (N) andeffective cross-section area of particle (Aeff) in Table 9are calculated according to Gayet et al. (1996b). Theresults show a high variability of both N and IWC ondata taken on successive legs and two missions. Duringmission 4, N and IWC increased with decreasing heightinside the cirrus layer evidencing a settling process,whereas a strong vertical inhomogeneity prevailed dur-ing mission 5. The effective cross-section area Aeff ismore constant, considering that for the whole datasetthe mean value is ^Aeff& 5 1852 mm2 with a standarddeviation sA 5 356 mm2. For an equivalent sphere theycorrespond to a mean effective radius ^reff& 5 24 mm,and standard deviation sr 5 2 mm.

5. Cirrus properties

a. Cloud morphology

Histograms of cloud boundaries retrieved by lidar aredisplayed in Fig. 14. For mission 4 they show a cloudtop at a maximum altitude of 10.6 km, that is, for a95% cumulative probability of the observations and amean value of 9.9 km with a standard deviation of 0.5km (Fig. 14a1). The tropopause is located at 10.5 km(see section 3). The cloud base is located at 7.8 km fora 5% cumulative probability, and the mean value is 8.5km with a standard deviation of 0.6 km (Fig. 14a2).These boundaries agree well with the temperaturesoundings, which show a lower boundary for the uppermoist layer at 7 km (see section 3). The mean geomet-rical thickness retrieved by lidar is 1.3 km (50% cu-mulative probability of the observations) with a standarddeviation of 0.26 km (i.e., a 20% normalized standarddeviation). The layer height is lower to the west end byabout 1 km. The cirrus layer fills only one-half of theupper moist layer; even so, the mean geometrical thick-ness is half the distance between the upper and lowerboundaries (2.8 km). A similar analysis for mission 5

undertaken a few hours later shows the cloud top at 10.5km (Fig. 14b1) and cloud base at 6.8 km (Fig. 14b2).The results suggest the importance of mesoscale processinteraction with the background dynamics for the for-mation of cirrus clouds.

b. Microphysics

Crystal size and shape play a key role in scatteringprocesses in the context of radiative transfer problems.An assumption of spherical particles in cirrus clouds isfrequently made despite its limitation for interpretation.The relevant microphysical characteristics of ice crys-tals for the case study of 17 April 1994, that is, size,shape, and orientation, are presented hereafter.

1) SIZE

Microphysical in situ measurements (section 4d)show an effective radius of about 24 mm for equivalentspheres.

2) SHAPE

The measurements indicate the presence of nonspher-ical particles according to (i) the significant lidar de-polarization ratios measured by LEANDRE (Dp 5 20%–30%), and (ii) the absence of a rainbow at 1508 (scat-tering angle) in bidirectional reflectances measured byPOLDER (see Fig. 11), as a rainbow characterizes thepresence of spherical particles. Our results and conclu-sions are in agreement with previous studies that re-ported the existence of a large variety of crystal shapesin cirrus clouds (e.g., Krupp 1991; Miloshevich andHeymsfield 1997).

3) ORIENTATION

A possible preferred orientation of ice crystals hasbeen discussed in the past. It has been reported usinglidar measurements. This phenomonon has not been ob-served by in situ measurements. It cannot be observedby LEANDRE in the present case because the lidar ispointing at 208 from the zenith. We take advantage ofthe POLDER measurements to address this issue. Dur-ing the course of leg F1 of mission 5 the POLDER datashow a peak of reflectance in the solar principal planewhen the viewing zenith angle is equal to the solarzenith angle uy 5 us 5 388 (see Figs. 11b and 11d).This specular reflection of solar light (cirrus glint) is anindication of the presence of horizontally oriented par-ticles. However, it cannot be concluded that the samephenomenon did not occur during leg F3 of mission 4when the solar angle was us ø 558 because the POLDERobservation angle uy 5 6508 precludes such an obser-vation.


FIG. 11. Total reflectance image [(a) and (b)] and polarized reflectance image [(c) and (d)] at 864 nm of wavelengthof the cirrus bank overflown during leg F3 of mission 4 [(a) and (c)] and during leg F1 of mission 5 [(b) and (d)]. For(a) and (c), the solar zenith angle is us 5 55.38, whereas for images (b) and (d) us 5 38.08. The color scale is 35%–60% for total reflectances and 0%–2.5% for polarized reflectances. Each concentric circle represents a constant valueof zenithal viewing angle uy . The solar principal plane is indicated by the 08–1808 axis (08 corresponding to the solardirection). The azimuthal viewing angle wy is set from this axis.

c. Optical properties

1) BACKSCATTERING PHASE FUNCTION AND

DEPOLARIZATION RATIO

The probability density function of the backscatter-to-extinction lidar ratio k measured during mission 4 is

presented in Fig. 15. The mean value is 0.024 sr21 witha standard deviation of 0.006 sr21. It corresponds to abackscattering phase function P(p) 5 0.30 with a stan-dard deviation of 0.08. Figure 16 displays the proba-bility density function of the lidar depolarization ratiofor mission 4. The mean value is Dp 5 24% with a


FIG. 12. Images of cirrus banks, at 864 nm of wavelength, over-flown during (a) leg F3 of mission 4 and (b) leg F1 of mission 5.These images are constructed using the central cross line of the CCDmatrix [242 detectors corresponding to cross tracks of about (a) 3.6km and (b) 3 km, respectively]. Locations within the legs of the twoentire POLDER images of cirrus cloud total and polarized reflectancesare indicated by arrows.

FIG. 13. Global particle area distribution as determined by FSSP-100 and 2D-C probes during legs F4, F5, and F6 of mission 4 andlegs F3, F4, and F5 of mission 5.

TABLE 9. Mean values of ice water content (IWC), particle con-centration (N), and effective cross-section area of the particle sizedistribution obtained from in situ measurements of PMS FSSP andPMS OAP-2D2-C on board the Falcon during the legs of missions4 and 5 inside the cirrus cloud.

Mission

Leg

4

F4 F5 F6

5

F3 F4 F5

IWC (mg m23)N (l21)Aeff (mm2)

18403

1385

30569

2290

48813

2290

22432

1520

36639

1662

5115

1963

standard deviation of 4%. The values obtained for P(p)and Dp, which are characteristics of the size, shape, andorientation of particles, are compared to the results ofcalculations for nonspherical particles (see Part II).

2) OPTICAL THICKNESS

The probability density function of d532 during mis-sion 4, legs A1, A2, and A3 (see section 4b), displaysa negative exponential distribution (Fig. 17a). The meanoptical thickness is 0.49 with a standard deviation of0.48. The same histogram for the infrared optical thick-ness dIR (see section 4a) shows the same behavior (Fig.17b), and the mean value is 0.25 with a standard de-viation of 0.24. Similar results are obtained for mission 5.

3) ASYMMETRY FACTOR

The asymmetry factor g for ice crystals is essentialin the radiative transfer problem. A value for g can bederived using shortwave reflectances and infrared emit-tances according to Wielicki at al. (1990). The theoret-ical reflectances are computed using an adding–doublingcode (De Haan et al. 1986) with a clear-sky albedo ofabout 5%, an atmosphere made of a cirrus layer, andparticles with optical properties that can be describedby Henyey–Greenstein scattering functions. The direc-tional reflectances have been computed for g rangingbetween 0.5 and 0.9, and shortwave optical thicknessdSW between 0.1 and 15. The infrared optical thicknessdIR is calculated using the approximation of large par-ticles, and the cloud emittance is derived from Eq. (2).

Using this method, the cloud directional reflectances canbe plotted as a function of cloud emittance.

Following the method presented above, a comparisonwas made with the thicker and more homogeneous partof the cirrus layer observed during mission 4. In Fig.18 the normalized reflectances at nadir measured byPOLDER on board the Falcon (leg F3) are plotted as afunction of the beam emittances derived from the PRT-5 on board the ARAT (leg A2, see section 4a). Themean solar zenith angle is us 5 558. The results showa weak correlation of the experimental data. A nonco-incidence in footprint between the two radiometers(PRT-5 and POLDER) as discussed above may accountfor the discrepancy.


FIG. 14a. Histogram of the probability density of cloud boundariesaltitude from lidar measurements corresponding to mission 4: (a1)cloud-top altitude probability density, and (a2) cloud-base altitudeprobability density.

FIG. 14b. Same as Fig. 14a but for mission 5.

FIG. 15. Histogram of the probability density of backscatter-to-ex-tinction ratio k obtained during the legs A1, A2, and A3 of mission 4.

6. Discussion and conclusions

The various measurements collected on an extendedcirrus cloud on 17 April 1994 during EUCREX’94 con-stitutes an extremely valuable dataset for radiative trans-fer studies at the mesoscale. The dataset includes dif-ferent measurements: (i) in situ thermodynamical mea-surements (pressure, temperature, and humidity pro-files); (ii) in situ microphysics measurements; (iii)radiative measurements, for example, upward anddownward solar and infrared fluxes collected fromabove and below the cirrus cloud, and nadir infraredradiances collected from below the cloud; (iv) airbornelidar measurements, that is, backscattered profiles, lineardepolarization ratio, backscatter-to-extinction lidar ra-tio, and optical thickness; and (v) airborne POLDERbidirectional reflectances, and bidirectional polarized re-flectances.


FIG. 16. Histogram of the probability density of backscatteringdepolarization ratio obtained during the legs A1, A2, and A3 of mis-sion 4.

FIG. 17a. Histogram of the probability density of optical thicknessd532 given by the lidar for the three legs of mission 4.

FIG. 17b. Histogram of the probability density of infrared opticalthickness dIR given by the infrared radiometer for the three legs ofmission 4.

This dataset, collected during the case study on 17April, allows us to present a description as complete aspossible of a cirrus cloud at the mesoscale, regardingits radiative properties (i.e., albedo, transmission, op-tical thickness) and microphysical properties (i.e., crys-tals size). Additional information has been collected onthe way the scattering processes by ice crystals canmodify the state of polarization of visible light (i.e.,lidar depolarization ratio and POLDER bidirectional po-larized reflectances).

An analysis conducted on the dataset leads to thefollowing conclusions.

1) The cirrus cloud observed on 17 April 1994 isstrongly inhomogeneous and its spatial variabilityseems to be quasiperiodic at a scale of about 10 km.

2) The in situ microphysical measurements indicate anequivalent radius of 20 mm for the cirrus cloud par-ticles (under the crude assumption of spherical par-ticles), with no information on the shape of particles.The assumption of spherical particles that is cur-rently made is somewhat contradictory with the re-sult below but is not adressed in the present study,as it requires dedicated instrumentation not availableduring EUCREX’94.

3) Nonsphericity of cirrus cloud particles is suggestedby different observations: significant values of thelidar depolarization ratio (see Sassen 1991), and ab-sence of a rainbow in bidirectional polarized reflec-tances made by POLDER for scattering anglesaround 1508. This suggests a significant contributionof nonspherical particles (Goloub et al. 1994).

4) The presence of particles horizontally oriented is in-dicated by specular reflection in solar light collectedby POLDER (total reflectance and polarized reflec-tance). This phenomenon persists for some time andbecomes stronger when the cloud is optically thicker,

indicating clearly that the specular reflection is notdue to sea glint but must be attributed to the presenceof horizontally oriented crystals in the cirrus cloud.

In addition, a first analysis of the optical propertiesof ice crystals has been conducted using simulationsbased on Henyey–Greenstein scattering functions andthe present dataset. A comparison of the nadir reflec-tances measured by POLDER and the effective beamemittances derived from the PRT-5 radiometer, with dif-ferent simulation results, does not give satisfactoryagreement. This is attributed to the fact that the twoinstruments were not on board the same aircraft and donot have the same spatial resolution.

As a summary, this paper presents an extented datasetcollected during the EUCREX’94 campaign, and the


FIG. 18. Nadir reflectance of cirrus cloud measured by POLDERon board the Falcon (leg F3) as a function of infrared emissivityobtained from PRT-5 on board ARAT (leg A2). Radiative transfersimulation using Henyey–Greenstein scattering phase function withasymmetry factor ranging between 0.5 and 0.9 is reported on thisfigure (see text).

manner in which the different data have been processed.We also check the self-consistency of the dataset, whichis used in Part II, to derive the optical and microphysicalproperties of the cirrus cloud documented on 17 April1994.

Acknowledgments. This work was supported by theEuropean Community under Contract EV5V-CT 92-0130 (EUCREX-2), the Centre National d’Etudes Spa-tiales, and the European Space Agency. The authors arespecially grateful to Y. Fouquart who has managed EU-CREX-2, and to the members of the Centre d’AviationMeteorologique, INSU, and DLR who operated theMerlin, the ARAT and the Falcon, respectively. Thanksare also due to the French weather service and to manyscientists, engineers, technicians, and students, partic-ularly H. Fimpel (DLR), R. Valentin (LMD), J. Y. Bal-ois, and C. Verwaerde (LOA) who participated in theexperiment. The authors are grateful to two anonymousreviewers for their fruitful comments.

APPENDIX

Analysis of Lidar Signal

The lidar signal S(x, z) after digitization can be writ-ten as

zKS(x, z) 5 b(x, z) exp 22h a(x, z9) dz9 ,E2 [ ](z 2 z )0 z0

(A1)

where z9 is a dummy variable, x is the horizontal locationof measurements, z is the altitude, and z0 is the aircraftflight altitude. In addition, b(x, z) and a(x, z) are the

total backscattering coefficient (in km21 sr21) and ex-tinction coefficient (km21) for particles and molecules,respectively; h is a multiple scattering coefficient (Platt1979). The instrumental constant K depends on the sys-tem characteristics, that is, transmitted laser energy, re-ceiver collecting area, optical efficiency, photodetectorquantum yield, and electronic gain. The noise is com-puted at long ranges after the lidar signal is negligible(SNR k 1).

The cloud boundaries—top, base, transitions betweenclean air (low aerosol content and no cloud), and cloudlayers—are assigned by lidar using a threshold algo-rithm (Young 1995). The uncertainty is estimated to be15–30 m for SNR $ 3.

In order to solve Eq. (A1) for b (or a) an additionalequation linking these two parameters is required:

b(x, z) 5 ka(x, z); (A2)

k is the backscatter-to-extinction lidar ratio also writtenas k 5 P(p)/4p, where P(p) is the backscattering phasefunction for cloud particles whose integral value is equalto unity. Inside the cloud k is constant, for example,only the particle number density changes to allow b(x, z)and a(x, z) to vary with range. Equation (A2) holds alsofor apparent parameters accounting for multiple scat-tering ka(x, z) 5 kaaa(x, z) with ka 5 k/h and aa 5 ha.

An apparent backscatter-to-extinction ratio ka, at 532nm, accounting for multiple scattering, is determinedusing two different expressions for the integrated back-scattering coefficient over the entire cirrus cloud (Spin-hirne et al. 1996): g 5 b(x, y) dy and g 5 ka(1 2zt∫zb

T 2)/2, where zb and zt are the cloud base and top, re-spectively, and T 2 is the cirrus two-way transmission.It results in a linear dependence of T 2 as a function ofg. The experimental lidar data are fitted by a straightline using a least squares method. An extrapolation atT 2 5 0 gives ka 5 2g.

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