measurements of cirrus cloud backscatter color ratio with a two-wavelength lidar

Measurements of cirrus cloud backscatter color ratiowith a two-wavelength lidar

Zongming Tao,1 M. Patrick McCormick,1 Dong Wu,1,2 Zhaoyan Liu,3,*and Mark A. Vaughan4

1Center for Atmospheric Sciences, Department of Atmospheric and Planetary Sciences,Hampton University, Hampton, Virginia 23668, USA

2Key Laboratory of Ocean Remote Sensing, Ministry of Education of China, Ocean University of China, Qingdao 266003, China3National Institute of Aerospace, Hampton, Virginia 23666-6147, USA

4Science Systems & Applications Inc. (SSAI), NASA Langley Research Center,Mail Stop 475, Hampton, Virginia 23681-2199, USA

*Corresponding author: [email protected]

Received 16 November 2007; revised 4 February 2008; accepted 4 February 2008;posted 11 February 2008 (Doc. ID 89882); published 27 March 2008

We present observations of cirrus clouds from June 2006 to July 2007 performed by using a two-wavelength lidar located at HamptonUniversity. For this time period, cirrus clouds were observedmostlyin 7–13:5km altitudes. Data analyses have been performed focusing on a color-ratio retrieval. In total,86,369 samples from 1,689 profiles (1 min average and 15m range resolution) containing cirrus cloudswith attenuated backscatter ratio (ratio of attenuated total backscatter to the molecular backscatter)larger than 10 have been selected. The cirrus color ratio distribution shows a peak value at about0.88 and a full width at half-maximum of 0.12. © 2008 Optical Society of America

OCIS codes: 280.3640, 010.1615.

1. Introduction

Satellite data showed that the occurrence proba-bility for high clouds over the Earth’s surface canbe as high as 30% [1,2]. Clouds can absorb long-wavelength outgoing radiation emanating from theEarth’s surface while reflecting short-wavelengthincoming solar radiation [3]. Therefore, clouds havea large influence on weather and climate. Lidarmeasurements can be used in deriving the range-resolved optical properties of clouds, a knowledgeessential in understanding cloud-radiation effects.To provide new insight into the role that cloudsand atmospheric aerosols play in regulating Earth'sweather, climate, and air quality, the Cloud–AerosolLidar and Infrared Pathfinder Satellite Observation(CALIPSO) satellite was launched on 28 April 2006.

The key active instrument onboard the CALIPSOpayload is a two-wavelength (532nm and 1064nm),polarization-sensitive lidar system to provide cloudand aerosol altitude-resolved optical data on a globalbasis [4].

Selected high clouds (predominantly cirrus clouds)are being used to calibrate the CALIPSO 1064nmchannel [5]. Backscatter color ratios (ratios of totalbackscatter coefficients at 1064 and 532nm) of cirrusclouds are assumed to be known. Thus, the CALIPSO532nm calibration can be transferred to the 1064nmchannel by comparing lidar signals at the two wave-lengths in the selected high clouds. Currently a con-stant color ratio of unity for selected cirrus clouds isused in the CALIPSO 1064nm signal calibration [5].The CALIPSO 532nm data are calibrated by compar-ing lidar signals at high altitudes (30–34km) to themodel reference profile where molecular scattering isdominant at 532nm. The same calibration approachfor the 532nm channel, however, cannot be used for

0003-6935/08/101478-08$15.00/0© 2008 Optical Society of America

1478 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

the 1064nm channel because the molecular signal athigh altitudes is too weak at 1064nm (about 16 timessmaller) to be used for the data calibration at thiswavelength. Therefore, the high cloud backscattercolor ratio is used in the transfer of the 532nm cali-bration to 1064nm. Backscatter color ratios are alsoused to discriminate aerosols from cloud particles [6].To date, however, a very limited number of studieshave been reported on cirrus cloud backscatter colorratios [7,8]. Clearly, investigations are necessary toobtain sufficient knowledge of high cloud color ratio.An initial analysis of cirrus cloud measurements con-ducted by the airborne Cloud Physics Lidar (CPL)during six years (2002–2007) has been performed, fo-cusing on the retrieval and characterization of thecirrus cloud backscatter color ratios to improve thecurrent CALIPSO lidar 1064nm signal calibra-tion [9].In this paper, we investigate the cirrus cloud color

ratios with the ground-based lidar measurementsperformed at Hampton University (HU), Hampton,Virginia (37:02 °N, 76:34 °W), over the period fromJune 2006 through July 2007. The lidar system is de-scribed in Section 2. The methods used to calibratethe lidar measurement data and produce cirrus cloudcolor ratios are described in Section 3. Section 4 pre-sents results for the cirrus color ratio measurement.

2. Hampton University Lidar System

The HU lidar system consists of transmitter, receiv-er, and data acquisition subsystems. The light sourcedeployed in the transmitter is a Nd:YAG laser (Con-tinuum Powerlite 8020) emitting laser pulses at20Hz at the fundamental wavelength of 1064nmand the frequency-doubled wavelength of 532nm.The receiver is built around a Cassegrainian-configured telescope whose primary mirror is 48 in:(122 cm) in diameter. A photomultiplier tube andan avalanche photodiode are used for the returnsignal detection at 532 and 1064nm, respectively.Interference filters are used to suppress the skybackground radiation to increase the detectionsignal-to-noise ratio. The lidar system uses a bi-static configuration. The geometric overlap of thereceiver and transmitter fields of view is adjust-able from a few hundred meters to a few kilometers.The data acquisition system is a lidar transient re-corder (Licel TR-20-160), which can be set to operatein either analog or photon counting modes. However,for the measurements reported herein, only the ana-log mode is used. For analog detection, lidar signalsare sampled by a 12 bit digitizer. The specifications ofour lidar system are summarized in Table 1.

3. Calibration and Data Analysis Methods

Lidar signal calibration is critical in lidar dataanalysis to derive cirrus cloud color ratios. In thissection we describe the methods that we use in cali-brating the lidar data and computing the cirrus cloudcolor ratio.

The lidar equation can be written as

XðzÞ ¼ PðzÞz2 ¼ C½β1ðzÞ

þ β2ðzÞ� exp�−2

Zz

0½α1ðz0Þ þ α2ðz0Þ�dz0

�; ð1Þ

where PðzÞ is the backscatter signal from a scatteringvolume at altitude z, C is the lidar system constant,β1ðzÞ and β2ðzÞ are the volume backscatter coefficientat altitude z for particles and molecules, and α1ðzÞand α2ðzÞ ¼ the volume extinction coefficient at alti-tude z for particles and molecules, respectively. Sub-scripts “1” and “2” represent particulate andmolecular scattering, respectively. The particulateterms α1ðzÞ and β1ðzÞ include aerosol and/or cloud.That is, β1ðzÞ ¼ βaerosolðzÞ þ βcloudðzÞ and a1ðzÞ ¼aaerosolðzÞ þ acloudðzÞ. The effective region of αcloudðzÞand βcloudðzÞ is from zb (cloud base) to zt (cloudtop). In other altitude regions, their values are as-sumed to be zero.

Lidar signal calibration is to determine the lidarsystem constant C. The atmosphere offers a naturalcalibration tool. Russell et al. [10,11] demonstratedthat, under nonvolcanic conditions and in the ab-sence of desert dust or cloud, there are generallyfew aerosols present in the free troposphere abovethe planetary boundary layer. We determine C byusing lidar returns in clean-air regions where theaerosol load is minimum (i.e., the backscatter ratioR, which is the ratio of total backscatter to molecularbackscatter, ðβ1 þ β2Þ=β2, is minimum) and the mole-cular scattering is dominant. Using a standardmolecular model as a reference, we search for acloud-free clean-air altitude zc over 6–15km alti-tudes where the ratio of the lidar signal to the modelmolecular signal is minimum [12]. The cleanest re-gion over Hampton is frequently observed between8 and 12km in the absence of cirrus clouds. Assum-ing that the aerosol backscatter ratio RðzcÞ is known,we get

Table 1. Specifications of the HU Bistatic Lidar

Nd:YAG laser (Continuum Powerlite 8020)Wavelength 1064 and 532nmPulse energy 750mJ at 1064nm, 450mJ at 532nmRepetition rate 20HzDivergence 0:45mrad

Telescope (Cassegrainian configured)Diameter 48 in: (122 cm)Focal length 480 in: (1219 cm)

Interference FiltersBandwidth 1:0nm at 532 and 1064nm

Transient Recorder (Licel TR-20-160)Sampling rate 20MHzPhoton Counting rate 250MHz

DetectorsPhotomultiplier R7400U-02Avalanche photodiode Perkin & Elmer C30956E-TC

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1479

XðzcÞ ¼ CRðzcÞβ2ðzcÞ exp�−2

Zzc


�:

ð2Þ

We define C0 as

C0 ¼ C exp�−2

Zzc


�: ð3Þ

In practice, instead of C, we determine C0 by using

C0 ¼ XðzcÞ=½RðzcÞβ2ðzcÞ�: ð4Þ

In our analysis, we assume RðzcÞ to be 1.01 for532nm [13] and 1.06 for 1064nm, between the valuesused by Russell et al. [10] and Zhou et al. [14], corre-sponding to an aerosol backscatter color ratio of0.375. For each subdataset observed in a day, we de-termine C0 by comparing a cirrus-free profile to thereference model profile. This cirrus-free profile isthen normalized by using C0. Once this is done, theprofiles with cirrus clouds are calibrated relativelyby comparing range-scaled signals below the cirruslayer to the attenuated backscatter in the same alti-tude region in the cirrus-free profile, with an as-sumption that the atmosphere in cirrus-free regionsshould be stable over the observation period (nor-mally 1–2h). Both 532 and 1064nm lidar data arecalibrated by using this method.The attenuated backscatter, β0ðzÞ, can then be com-

puted by using

β0ðzÞ ¼ ½β1ðzÞ þ β2ðzÞ� exp�−2

Zz

zc

½α1ðz0Þ þ α2ðz0Þ�dz0�

¼ XðzÞ=C0: ð5Þ

β0ðzÞ is a fundamental quantity of the calibrated lidarsignal and will be analyzed to derive the cirrus cloudcolor ratio. The particulate backscatter color ratio χis defined as

χ ¼ β1;1064=β1;532: ð6Þ

We will focus our study on the color ratios of moreoptically thick cirrus clouds with attenuated back-scatter ratio R0

532 ¼ β0532=β2;532 > 10. Strong cirrusbackscatter signals (currently R0

532 > 50) are usedin the CALIPSO 1064nm calibration. In cases of op-tically thin clouds, their optical property includingbackscatter can be retrieved by using the transmit-tance constrainedmethod [15]. However, for opticallythick clouds, because the signals above the cirrus aretoo noisy owing to the large attenuation of the cirrus,reliable retrievals cannot be achieved. Therefore, analternative method [i.e., Eq. (7)] described below isused to compute the backscatter color ratio of opti-cally thick cirrus signals.For these cirrus clouds, where α1;532 ≈ α1;1064, χ may

be approximated by

χðzÞ≂½β01064ðzÞ − β2;1064ðzÞ�=× ½β0532ðzÞ=T2;532ðzÞ − β2;532ðzÞ�;

ð7Þ

where T2;532ðzÞ ¼ exp½−2 R zzcα2;532ðzÞdz� is the molecu-

lar transmittance at 532nm from zc to z. Moleculartransmittance at 1064nm from zc to z is neglectedsince it is very close to unity. Equation (7) is usedto calculate the cirrus cloud backscatter color ratioin this study.

Equation (7) will generally overestimate the cirruscolor ratio. However, the difference is small for theselected strongly scattering cirrus signals. To dem-onstrate the validity of this approximation, we cal-culate the cirrus backscatter color ratio by usingEqs. (6) and (7) for comparison. To compute the cirrusbackscatter color ratio by using Eq. (6), lidar signalinversion is conducted to retrieve cirrus backscattercoefficients at the two wavelengths by using thetransmittance method [15–17]. First we calculatethe cirrus cloud optical depth from the transmittancethat can be determined from the molecular signalsabove and below the cirrus layer at 532nm or, inthe case when a cirrus-free profile is available, bycomparing the signals above the cirrus layer to thecorresponding part of the cirrus-free profile as de-scribed in detail by Young [15] and Liu et al. [17].The optical depth at 1064nm is assumed to be thesame as 532nm because the extinction is wavelengthindependent for large particles [18]. We then use thedetermined optical depths as an extra constraint toretrieve the lidar ratio as well as backscatter profilesfor cirrus clouds, using the method by Fernald [19].An example obtained on 1 March 2007 is shownin Fig. 1.

Figures 1(a)–1(c), respectively, present the attenu-ated backscatter profiles at 532 and 1064nm, thecolor ratio profiles computed by using Eqs. (6) (black)and (7) (red), and the difference in the color ratioscomputed by using the two equations. The differencebetween the color ratios computed by using Eqs. (6)and (7) [Fig. 1(c)] is small (<3%). Thus, Eq. (7) ap-pears to be an appropriate approximation ofEq. (6) for computing cirrus cloud color ratios. Inthe CALIPSO 1064nm calibration, the attenuatedcolor ratio

χ0 ¼ β01064=β0532 ð8Þ

is used to transfer the 532nm calibration to 1064nm.When R0 is large, χ0 is very close to χ. For this reason,strong cirrus signals with R0

532 > 50 are currentlyselected and used in the CALIPSO 1064nm calibra-tion. In this case, the impact of molecular scatter-ing is smaller than 2%. For comparison, we havealso computed the attenuated color ratio profile byusing Eq. (8) and the difference in the color ratioscomputed by using Eqs. (6) and (8) for the cirrus sig-nals with R0

532 > 10. These results are presented inFigs. 1(b) and 1(c) (green curves).


4. Results

A. Cirrus Cloud Measurements

Measurements of cirrus clouds were made duringJune 2006 through July 2007 by using theHU lidar. In total, 1,689 profiles averaged over1 min (1200 shots) with a 15m vertical resolu-tion have been observed to have visible cirrusclouds during 41 days. All the data have beencalibrated and analyzed by using the methods de-scribed in Section 3 to derive cirrus cloud color ratios.Figure 2 presents three typical cases acquired on9 March 2007 [2(a) and 2(b)], 1 May 2007 [2(c) and2(d)], and on 28 July 2007 [2(e) and 2(f)]. The pro-files of 532nm attenuated backscatter (left-handpanels) were derived from the lidar return signalsby using Eqs. (4) and (5). Cirrus color ratios (right-hand panels) were computed by using Eq. (7) for datapoints where R0

532 > 10.Three clouds, distributed in 9–11km altitudes,

were observed on 9 March 2007 [Figs. 2(a) and 2(b)].These clouds are physically thin (<0:6km), and theirhorizontal scales appear to be small, with the back-scatter color ratio changing from 0.8 to 0.95.The cloud observed on 1 May 2007 appears to be

well developed and has a medium physical thickness(∼1km). For the 1h observation, the cirrus altitudeis stable at 12–13km. The cirrus attenuated back-scatter at 532nm decreases slightly with time. Butthe backscatter color ratio remains relatively invari-ant, about 0.92, showing a better homogeneity.On 28 July 2007, the cloud appears to be convective

and shows a complex structure. For this type cloud,the physical thickness is relatively large and thecloud is multiple layered. The cirrus height is within9–12km. The 532nm attenuated backscatter variesgreatly with time, and the backscatter color ratiosdecrease as altitude increases.Figure 3 presents example profiles of 532nm at-

tenuated backscatter and color ratio profiles fromthe cases shown in Fig. 2 as indicated there by thedotted vertical lines. For the well-developed cirrusdeck [in Figs. 3(c) and 3(d)], the color ratio is rela-tively invariant with altitude, whereas for the other

two types, the variation in the color ratio with alti-tude is larger.

B. Statistics of Cirrus Cloud Color Ratios

All the cirrus data within 6–14km altitudes whereR0

532 > 10 have been used in this study to derivethe statistics of cirrus color ratios. In total, 86,36915 m data samples have been selected from 1,689profiles measured during June 2006 through July2007. Figure 4 presents the distribution of colorratios (red curve) computed by using Eq. (7) for all86,369 selected samples. The increment in the colorratio is 0.02. A peak value is seen at about 0.88. Thefull width at half-maximum (FWHM) is about 0.12(from 0.8 to 0.92).

The uncertainties contained in the color ratio com-putation are examined. Error sources include detec-tion noise, errors in the calibration, and the use of theapproximation in Eq. (7). The uncertainty rising fromthe detection noise is negligibly small for the aver-aged data. To demonstrate this, an example of uncer-tainties estimated by using the noise scale factor [20]for the measurement shown in Fig. 1 is presented inFig. 5. The noise scale factor is a factor representingthe proportionality between the standard deviationand the square-root mean of random samples thatfollow a Poisson stochastic process. Lidar returnsignals detected by using a photomultiplier tube oran avalanch photodiode have been demonstrated tofollow a Poisson distribition [20]. It is seen that therelative errors are <0:3% for the 532nm signals and<0:02% for the 1064nm signals in cloud whereR0

532 > 10.To assess the uncertainty in the color ratio due to

the use of approximation (7), the distribution of theattenuated color ratios computed by using Eq. (8) isalso shown in Fig. 4 (blue curve) for comparison.A peak is seen at about 0.85. The two distributionsappear to be very close with a shift in color ratioof about 0.03. As mentioned earlier, the color ratiocomputed by using approximation (7) is an over-estimate of the color ratio from Eq. (6), while, onthe other hand, the attenuated color ratio fromEq. (8) is an underestimate. Therefore, the distribu-tion of color ratios from Eq. (6) should be between the

Fig. 1. Example of cirrus cloud observed on 1 March 2007: (a) 532 and 1064nm attenuated backscatter, (b) backscatter color ratios com-puted by using Eqs. (6) (black), (7) (red), and (8) (green), and (c) differences of color ratios computed by using Eqs. (7) and (6) (red), andEqs. (8) and (6) (green). Color ratios using Eqs. (7) and (8) were computed only for data points where R0

532 > 10.


two distributions from Eqs. (7) and (8). Hence, theuncertainty in the color ratio distribution due tothe use of Eq. (7) is smaller than 0.03.It is hard to estimate uncertainties in the lidar ca-

librations accurately, particularly for the 1064nmdata, from lidar measurements. An obstacle is thelack of accurate knowledge of the aerosol loading(i.e., the aerosol backscatter ratio) in our selectedclean-air regions. The aerosol contribution to the sig-nals in our selected clean-air regions is small at532nm (few percent by backscatter) and has littleeffect on the 532nm calibration. The aerosol con-tribution is relatively large at 1064nm because themolecular backscatter at 1064nm is 16 times smallerthan that at 532nm; however, it seems no larger than

15% in this study. A sensitivity study shows that thecirrus color ratio can be underestimated by ∼7% ifthe backscatter ratio of the aerosol in the calibrationregions is R1064 ¼1:15 (15% aerosol loading by back-scatter) and overestimated by ∼5% if R1064 ¼1:0 (noaerosol loading), when R532 ¼1:01 and R1064 ¼ 1:06are used in the calibration [Eq. (4)]. Therefore, avalue of 7% is used as an estimate for the calibrationuncertainty. Thus, an overall uncertainty in the cir-rus color ratio is estimated to be ∼7:6% based on theerror propagation theory. The presence of aerosol inthe calibration region is the primary driving factorin our uncertainty estimation.

We note that the color ratios of strong cirrus cloudsignals have a wide distribution (∼14% of the peak

Fig. 2. Time–altitude displays of 532nm logarithmic attenuated backscatter (left) and backscatter color ratio (right). The profiles in (a)and (b) were acquired on 9March 2007 starting at 9:28 a.m., (c) and (d) on 1May 2007 starting at 11:01 a.m., and (e) and (f) on 28 July 2007starting at 2:04 p.m., respectively. Altitude and time resolutions are 0:015km and 1 min, respectively. The units of 532nm attenuatedbackscatter is km−1 sr−1. Profiles for the dotted vertical lines are discussed in Fig. 3.


value). The measured distribution in this paper issimilar to that measured by the CPL at the samewavelength pair (1064 and 532nm) [9]. An evenwider spread of cirrus color ratios (at wavelengthsof 532 and 355nm) has been reported previously[8]. In those observations, larger deviations in colorratios are due primarily to weaker cirrus signalswith R ≤ 10. In this study, the data points withR ≤ 10 have been rejected. The peak value in the cir-rus color ratio distribution is at about 0.88. This peakvalue is similar to those measured by the CPL [9].The peak value reported previously [8] is a littlesmaller than unity (5.32 for their definition of thecolor ratio C). Some discrepancy appears in thesemeasurements for the peak color ratio values. The

discrepancy in the peak values may be due to the dif-ference in the wavelength pairs used. These studiessuggest a wide spread of the cirrus color ratios with anonunity peak value. This does not appear to be in-consistent with theoretical work. Mie theory does notsuggest a constant unity value of the backscattercolor ratio for all large particles, though it does sug-gest no wavelength dependence for large particles.

Figure 6 presents the occurrence number of the cir-rus signals with R0

532 > 10 [6(a)] and the correspond-ing color ratio distribution [6(b)] as a function ofaltitude. The cirrus clouds appear to occur over alti-tudes of 7–13:5km. Figure 7 shows the distributionsof cirrus clouds as a function of 532nm attenuatedbackscatter [7(a)] and as a function of attenuated

Fig. 3. Cirrus cloud backscatter color ratio profiles and their corresponding 532nm attenuated backscatter profiles from the measure-ments shown in Fig. 2 as indicated by the dotted vertical lines. In the left-hand panels, the blue curves are calculated by using Eq. (6), andthe red curves are calculated by using Eq. (7) in regions where R0

532 > 10.


backscatter and color ratio [7(b)]. The 532nm attenu-ated backscatter is in km−1 sr−1. The increment forlogarithmic 532nm attenuated backscatter is 0.05,and for color ratio 0.02. We note that the distri-butions with respect to logarithmic attenuatedbackscatter are truncated for smaller values ofattenuated backscatter because only data pointswith R0

532 > 10 have been included. Also, theseselected cirrus clouds occur most frequently at∼0:01km−1 sr−1 (−2 logarithmic) of the 532nm atte-nuated backscatter.

5. Conclusions

Observations of cirrus clouds over one year were per-formed by using the HU two-wavelength backscatterlidar over Hampton, Virginia (37:02 °N, 76:34 °W),from June 2006 through July 2007. During this timeperiod, cirrus clouds were observed to occur mostly ataltitudes from 7 to 13:5km over Hampton.An approach to compute the color ratio of strong

cirrus signals from the attenuated backscatter hasbeen developed in this paper. Data processing hasthen been performed. The distribution of color ratios

at a wavelength pair of 1064 and 532nm for strongcirrus signals with R0

532 > 10was derived. The distri-bution has a large FWHM of 0.12 with a non-unitypeak at about 0.88 similar to those derived fromthe CPL measurements for the same wavelengthpair (1064 and 532nm) [9]. A wide distribution of cir-rus cloud color ratios at a different wavelength pairof 532 and 355nm has also been reported previously[8], which is consistent with the results of this paper.However, a smaller deviation of the peak value fromunity was observed in [8]. This may be due to the dif-ference of the laser wavelengths used in the twostudies.

Cirrus clouds are currently used in the CALIPSO1064nm channel calibration with an assumption ofknown and constant value of the cirrus color ratios[5]. Color ratios are also used in the CALIPSO lidardata processing to discriminate aerosols from clouds[6]. Investigations of cirrus color ratios are thereforenecessary to help select reasonable values of the cir-rus color ratio for the CALIPSO 1064nm calibrationand aerosol–cloud separation studies.

Approximation (7) for computing the cirrus colorratio has been demonstrated to be less sensitive to

Fig. 4. Distributions of color ratios for strong cloud signals(R0

532 > 10) within 614km altitudes measured by using the HU li-dar over Hampton during June 2006 through July 2007. The redcurve is calculated by using Eq. (7), and the blue line by usingEq. (8).

Fig. 5. Example of relative errors estimated by using the noisescale factor for the 532 and 1064nm attenuated backscattersshown in Fig. 1. The solid curves indicate the location of the cloudwhere R0

532 > 10.

Fig. 6. (a) Occurrence number and (b) color ratio distribution of cirrus cloud as a function of altitude used in this study. Altitude and colorratio resolutions are 0:15km and 0.02, respectively.


the presence of molecular scattering. Therefore, thisapproximation could be used in the CALIPSO1064nm calibration so that the criteria of R0

532 >50 for selecting strong cirrus signals could be relaxedto R0

532 > 10, and hence more cirrus signals wouldbe available for the 1064nm calibration over broaderareas of the globe.

This work is supported under NASA’s CALIPSOProgram NAS1-99108, and the National Oceanicand Atmospheric Administration’s CREST Program49866-00-05B. Dong Wu was a visiting Scholar atHampton University and is being supported by theChina Scholarship Council.

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Fig. 7. (a) Cirrus cloud occurrence number as a function of logarithmic 532nm attenuated backscatter, and (b) as a function of color ratioand logarithmic 532nm attenuated backscatter. 532nm attenuated backscatter is in km−1 sr−1. The increment is 0.05 for logarithmicattenuated backscatter and 0.02 for color ratio.


measurements of cirrus cloud backscatter color ratio with a two-wavelength lidar

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