Advances in Space Research 34 (2004) 845–850

www.elsevier.com/locate/asr

Lidar observations of high altitude cirrus clouds nearthe tropical tropopause

K. Parameswaran a,*, S.V. Sunilkumar a, B.V. Krishna Murthy b, K. Satheesan c

a Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 022, Indiab B1-Ceebros, 47/20, 3rd Main Road, Gandhi Nagar, Adayar, Chennai 600 020, India

c Oceanic Sciences Division, Meteorology and Oceanography Group, Space Application Centre, Ahmedabad 380 015, India

Received 19 October 2002; received in revised form 8 July 2003; accepted 1 August 2003

Abstract

The physical properties of high altitude cirrus occurring near the tropical tropopause are studied at the tropical station Gadanki

(13.5�N, 79.2�E) using a monostatic lidar. This study indicates that the altitude region 14–16 km is more favourable for the for-

mation of cirrus. The cloud depth varies from 0.4 to 4 km. Clouds with optical depth less than 0.03 (subvisual cirrus) is found to

occur quite frequently at this tropical region. They introduce significant depolarisation for backscatter radiation indicating the

presence of non-spherical ice crystals. Formation and persistence of these clouds is found to be closely associated with the strength

of tropospheric turbulence, estimated from vertical wind data obtained from MST radar.

� 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Lidar observations; High altitude cirrus clouds; Tropical tropopause; Tropospheric turbulence

1. Introduction

Extreme cold temperatures well below 200 K and

prevailing convective turbulence which increases the

availability of water vapour, precursor gases and the

aerosols makes the upper tropospheric region conducive

for super saturation of air with respect to ice which isfavourable for the formation of non-spherical ice crys-

tals by condensation and nucleation leading to the

manifestation of thin clouds (Jensen et al., 1996). Space

borne lidars, in situ measurements and the ground-based

experiments indicates frequent manifestation of cirrus

clouds in the upper troposphere. These studies show

that the cirrus clouds are globally distributed in all lat-

itudes irrespective of season with a global coverage ofabout 20–30%. Over the tropics the cirrus cover is more

than 70%. But the formation of cirrus, its maintenance,

lifetime, vertical and horizontal extend, dynamics, op-

tical/scattering property differs from location to loca-

tion. High altitude cirrus plays a significant role in

* Corresponding author. Tel.: +91-471-256-3927; fax: +91-471-270-

6535.

E-mail address: k_parameswaran@vssc.org (K. Parameswaran).

0273-1177/$30 � 2004 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2003.08.064

atmospheric chemistry, radiation and troposphere–

stratosphere exchanges. But their microphysical, dy-

namical and radiative properties are yet not well

understood. The monostatic Nd:YAG lidar and the

Mesosphere–Stratosphere–Troposphere (MST) radar

located at National MST Radar Facility (NMRF),

Gadanki (13.5�N, 79.2�E) provides an excellent oppor-tunity to study the characteristics of tropical cirrus and

their association with tropospheric turbulence.

2. System description and data analysis

The Nd:YAG lidar system essentially consists of a

transmitter emitting linearly polarised 7 ns pulsed laserradiation at 532 nm wavelength with 0.55 J energy with

a pulse repetition rate of 20 Hz. The receiver system

consists of 350 mm telescope with dual-polarisation

measurement capability. The co-polarised and cross-

polarised components (relative to transmitter beam po-

larisation) of the backscattered irradiance (referred to as

P and S channels, respectively) are separated using a

cubical polarised beam splitter and measured separatelythrough two identical photomultiplier (PMT) channels

ved.

846 K. Parameswaran et al. / Advances in Space Research 34 (2004) 845–850

coupled with photon counting system. The dwell time of

the counting system is 2 ls, which corresponds to an

altitude resolution of 300 m. The backscattered returns

(photon counts) summed for 250 s corresponding to

5000 laser shots are stored in the PC hard disk through

the data acquisition system. The Doppler informationfrom collocated MST radar operating at 53 MHz in the

vertical beam mode is used to derive the mean vertical

wind in the altitude region 3–22 km.

Altitude profile of particulate backscatter coefficient

(ba) in the altitude region 7–30 km is estimated from the

lidar data employing Fernald’s algorithm (Fernald,

1984) by taking 30 km as the reference altitude where bais assumed to be negligible. The full overlapping oftransmitted beam with the receiver field of view governs

the lower altitude limit. Altitude profile of molecular

backscatter coefficient (bm) required for lidar signal in-

version is estimated from the mean model profiles of

temperature and pressure for that month applicable for

this tropical region, derived from M100 rocket mea-

surement conducted at Trivandrum (8.5�N, 77�E) for aperiod of 16 years from 1970 to 1986 (Sasi, 1994). Fromthe altitude profiles of ba and bm, the backscatter ratio Ris estimated as

RðhÞ ¼ baðhÞ þ bmðhÞbmðhÞ

; ð1Þ

where h is the altitude. Altitude profiles of R estimated

separately from the two quadrate-polarised channels (Rp

and Rp from P and S channels, respectively) are used to

estimate the altitude profile of linear depolarisation ratio

(LDR), d, in cirrus clouds as

dðhÞ ¼ dmRsðhÞRpðhÞ

; ð2Þ

where dm is the molecular depolarisation which is taken

as 0.028 as suggested by Bodhaine et al. (1999). The

subscripts ‘p’ and ‘s’ refer to the value of the respective

parameters for co-polarised and cross-polarised com-ponents, respectively. From these estimated values of

Rp, Rs and d the effective backscatter ratio Re, used for

describing the unbiased backscatter property (Osborn

et al., 1990), is estimated as

ReðhÞ ¼RpðhÞ þ dmRsðhÞ

1þ dm: ð3Þ

The cloud parameters, namely the cloud strength nor-

malised to cloud vertical extent (M0), mean altitude (M1)

and half-width (M2) are estimated (Parameswaran et al.,2001) from the first three moments of ReðhÞ. The cloud

optical depth (sc) is estimated by integrating the par-

ticulate extinction coefficient within the cloud bound-

aries.

The lidar was operated regularly on 2–3 nights in a

week for �5 h under clear sky conditions, subject to

system availability, during the period from January

1999 to March 2000. In addition to this the lidar ob-

servations were taken in campaign mode on all the

nights from 18 January 1999 to 5 March 1999 and from

17 February 2000 to 2 April 2000 to study the day-

to-day variability in cirrus activity. A good data set for

121 apparently clear nights available during this periodis used to study the mean characteristics of tropical

cirrus and the short-term temporal variation in cloud

properties. The vertical wind data from MST radar

at 45 s interval for �2 h from 22:00 IST is used for

estimating the temperature profile and turbulence

characteristics in troposphere. The association of cirrus

formation with tropospheric turbulence is examined

using these two data sets.

3. Temporal variation of cloud parameters

The altitude profiles of backscatter ratio (R) ob-

tained for the two polarised components (P and S) are

used to identify the presence of cirrus clouds. The

cloud is sought in the altitude region 8–20 km andidentified as the region where the backscatter ratio

P 2 in either of the two channels. The cloud base (hcb)and cloud top (hct) are fixed, respectively, at one range

bin (300 m) below, and one range bin above this

identified region. For a thinnest cloud appearing (with

Rp or Rs P 2) only in one range bin (with value of R in

the upper and lower rangebins below 2) of the lidar

receiver, this selection criterion fixes the geometricalextent as 2 bins (600 m). This is the thinnest cloud

observable by the lidar for its available range resolu-

tion. In order to include fine structure and multi-layer

clouds it is ascertained that the value of R (Rp or Rs as

the case may be) remains below 2 continuously for at

least three range bins below the cloud base and three

range bins above the cloud top. This criterion is fol-

lowed throughout the analysis. The backscatter ratioand LDR reveal the different cloud patterns occurring

within �4 km below the tropopause. Figs. 1(a) and (b)

show the contours of Re and d for 25 January 1999 as

a typical case. Fig. 1(c) shows the temporal variation

of cloud strength M0 and cloud optical depth sc for

this night. The altitude and temporal structure of the

cloud optical properties are observable in these figures.

The cloud shows an unusual descent of �2 km withinthe time period of �6 h. A wavy structure is also seen

in these plots. The tropical tropopause on this night

identified from the temperature profile is around 17

km and is cold at �193 K. Within the cloud the

backscatter ratio and LDR show significant spatial

and temporal variations. While the backscatter ratio

varies in the range 2–12, the LDR varies from 0.03 to

0.16. High values of LDR in the cloud indicate thepresence of randomly oriented highly non-spherical ice

crystals. Low values of LDR could be due to the

Fig. 2. Temporal variation of cloud optical depth and asymmetry

factor of cirrus on 25 January 1999.

Fig. 1. Contour plots showing the temporal and altitude variation of (a) effective backscatter ratio (Re) and (b) linear depolarisation ratio (d) in the

cirrus cloud observed on 25 January 1999 along with (c) the temporal variation of cloud strength and cloud optical depth on that night.

K. Parameswaran et al. / Advances in Space Research 34 (2004) 845–850 847

presence of more regular quasi-spherical ice crystals ordue to the horizontal alignment of flat ice crystals

causing specular reflection of vertically pointed lidar

beam (Thomas et al., 1990) resulting in an enhance-

ment of the co-polarised component in the backscat-

tered signal. Continuous curve in Fig. 1(c) shows the

temporal variation of M0 and dotted curve shows the

same for sc. As expected the temporal variation of

cloud strength and cloud optical depth are well cor-related. The optical depth varies from 0.02 to 0.12. At

the start of the lidar observation sc is low. It increases

with time reaching a maximum mean value of �0.07

during midnight followed by decrease. In the early

morning period the cloud again becomes optically thin.

The cloud depth is �2 km throughout the period of

observation. A perusal of the features of cirrus clouds

on different nights during the period of study showedthat while on some nights they are generally strong

and persists throughout the period of lidar observation

on some other nights they occur only for a short du-

ration and then disappear for the rest of the night or

appear intermittently. Thick cirrus topped by thin

cirrus was also observed on some other nights. The

backscatter ratio within the cloud shows significant

variation with altitude. The value R is high near thecloud centre and decreases towards its boundaries.

Nature of variation of R within the cloud can be pa-

rameterised (Sassen and Cho, 1992) using the asym-

metry factor (n) given by

n ¼ M1 � hcbhct � hcb

: ð4Þ

The asymmetry factor gives information on the relativeposition of M1 with respect to a normalised cloud

thickness profile (n ¼ 0 and 1.0 represent optical centre

to be near the cloud base and cloud top, respectively,

and 0.5 the same in the geometric middle portion of the

cloud). If the altitude distribution of Re with in the cloud

is Gaussian in nature, n will be close to 0.5. A value of nless than 0.5 indicates that the Re distribution is skewed

with more scattering-occurring from the lower half of

the cloud and vice versa. Even though n shows largetemporal variations, the mean value for a night lies close

to 0.5. Fig. 2 shows the temporal variations of n along

with the optical depth sc for 25 January 1999. At the

early night hours (up to 00 IST) temporal variation in scand n are positively correlated. The ascending and de-

scending nature of optic centre (cloud mean altitude M1)

from the geometric centre of the cloud is seen from this

figure. After 00 IST for most of the time temporal var-iation in sc and n are negatively correlated with the mean

optical centre below the geometric centre. This indicates

that for the cloud during the post mid-night period on

this day most of the scattering occurs from its lower

portion.

Fig. 3 shows the temporal variation of cloud strength

(M0), mean altitude (M1) and half-width (M2) for 19, 20

and 26 January 1999 depicting different types of cirrusmanifestation. On 19 January 1999 the cloud is optically

thin with steady mean altitude just above 16 km with

half-width �0.2 km. But on 20 January 1999 the cloud is

optically thin during the initial observation period and

gets optically denser after 00 IST. On this night the

mean altitude of the cloud decreases steadily from 16 km

during the pre midnight period to 14 km in the early

Fig. 3. Temporal variations of optical depth, mean altitude and half-

width of cirrus on three nights in January 1999.

848 K. Parameswaran et al. / Advances in Space Research 34 (2004) 845–850

morning hours. The average half-width of the cloud is

�1 km. On 26 January 1999 the cloud is optically dense

during the start and end of the lidar observation whereas

it is optically thin during mid-night hours. The cloud

forms at an altitude �14 km and its vertical extent varies

from 0.8 to 2 km.

4. General characteristics of cloud parameters

High altitude cirrus clouds observed near the tropical

tropopause are classified based on their optical depth

(Sassen and Cho, 1992) as Subvisual Cirrus (with

Fig. 4. Bar charts showing the statistics of lidar observation and occurrence

March 2000.

sc 6 0:03), Thin Cirrus (with 0:03 < s6 0:3) and Dense

Cirrus (s > 0:3). Lidar observations taken during the

period from January 1999 to March 2000 on clear sky

nights are used for the present study. Lidar signals

summed for 250 s yield one profile. Each lidar profile is

examined for the presence of cirrus cloud. Fig. 4 showsthe occurrence of the clouds for different nights. Hollow

bars indicate total number of lidar profiles taken on that

night. Filled region of this bar indicates the number of

profiles in which the cloud could be detected. Hatched

portion of the bar in this figure indicates the number of

profiles showing subvisual cirrus (SVC), a light shade

indicates those showing thin cirrus (TC) and dark shade

indicates those showing dense cirrus (DC). As the lidardata is not evenly spaced and also have long gaps in

May and July–October period it is not possible to ex-

amine the seasonal dependence of the occurrence of

these clouds. However, from the available data it is seen

that while the occurrence of SVC is large during winter

months (when rain and thunder storm activity is less)

that of TC or DC is large during the SW monsoon pe-

riod (when thunder clouds are seen frequently). Table 1shows occurrence satistics of cirrus clouds examined

using the 7455 profiles of R obtained at 250 s intervals

on different nights during the period of study. Cirrus

clouds could be detected in the R profile on 51% of cases.

The profile statistics shows that the frequency of oc-

currence of SVC is large compared to that of TC and

DC. Frequency of occurrence of different types of cirrus

clouds have also been examined based on average

of cirrus clouds at Gadanki during the period from January 1999 to

Table 1

Occurrence frequency of high altitude cirrus observed at a tropical

station Gadanki during the period from January 1999 to March 2000

Cloud type Profile statistics Night statistics

No. Percentage No. Percentage

SVC 2010 60.2 55 63.2

TC 1042 31.2 24 27.6

DC 286 8.6 8 9.2

K. Parameswaran et al. / Advances in Space Research 34 (2004) 845–850 849

optical depth for each night. The mean cloud optical

depth for each night is obtained by averaging the scvalues estimated from individual lidar profiles on that

night. Of the 121 nights of lidar observations consid-

ered, on 87 nights cirrus clouds were present. On 34

nights no cirrus clouds could be detected during the

entire period of lidar observation. The frequency of

occurrence of different cirrus types based on this mean

statistics is also presented in Table 1. From the table it is

seen that the frequency of occurrence of different typesof cirrus clouds estimated based on profile statistics

matches fairly well with that derived from night statis-

tics. Both these statistics indicate the high occurrence of

SVC at this location.

Fig. 5 shows frequency distribution of mean (aver-

aged for each night) cloud parameters M0, M1 and M2

for SVC, TC and DC. The cloud strength is generally

low (<3) for SVCs. Most of the observed SVCs have thevalues of M0 around 1.5. For TCs the maximum prob-

able value of M0 is between 3 and 8. For DC the values

of M0 lies in the range 9–20. While SVCs are observed

anywhere in the altitude region 10–18 km, with favoured

altitude above 15 km, TCs and DCs usually occur

around 14 and 13 km, respectively. Studies on cloud

Fig. 5. Frequency distribution of M0, M1 and M2 for different type of

cirrus clouds.

morphology using satellite (Stratospheric Aerosol and

Gas Experiment, SAGE) data by Wang et al. (1996)

suggest that over the tropics, locations centered over

Southern Asia, India and Mexico are conducive for the

formation of SVCs and they occur quite frequently in

altitude region 14–16 km. The present study also indi-cates that the altitude region 14–16 km is more condu-

cive for cirrus formation. The cloud depth (2.36 M2)

varies from 0.4 to 4 km. Even though the half-width for

SVC gose up to 1.5 km, in most of the cases these clouds

will have half-width around 0.4 ± 0.2 km. In the case of

TCs also it is seen that clouds with low M2 occur more

frequently. But DCs with M2 < 0:8 km seldom occur.

The role of shear driven turbulent mixing in the for-mation of cirrus clouds (Jensen et al., 1996) through

condensation/nucleation is examined. As stated above

during the period from 18 January 1999 to 5 March

1999 lidar observations are available on almost all the

nights. From Fig. 4 it can be seen that the occurrence of

cirrus is high in January 1999 compared to that in

February 1999. A detailed perusal of data on different

nights in this period also indicated that in January 1999the clouds were stronger and persisted almost through-

out the period of lidar observations whereas in February

1999 the clouds were weak with low frequency of oc-

currence and also on most of the nights they appeared

intermittently. This prompted a study on the association

of cirrus with tropospheric turbulence. From the tem-

poral spectra of vertical wind obtained from simulta-

neous observations of MST radar with a rangeresolution of 150 m on these nights, the altitude profile

of turbulence kinetic energy (TKE) dissipation rate (e)

Fig. 6. Altitude profiles of mean TKE dissipation rate (e) and mean

vertical eddy diffusion coefficient (Km) for January 1999 and February

1999.

850 K. Parameswaran et al. / Advances in Space Research 34 (2004) 845–850

and the eddy diffusion coefficient for momentum (Km)

are estimated (Satheesan and Krishnamurthy, 2002) by

integrating the vertical wind variance at frequencies

above the Brunt Vaisala frequency. The altitude profile

of monthly average TKE dissipation rate and eddy

diffusion coefficient for January 1999 and February 1999are presented in Fig. 6. These two parameters are in-

dicative of the strength of prevailing tropospheric tur-

bulence. These profiles indicate an enhancement in e andKm (and thus turbulence) with a broad maximum in the

altitude region �10–16 km. In January (when cirrus

occurrence is high) Km profile shows a broad peak

(Fig. 6(c)) in the altitude region 11–17 km superposed on

which a sharp peak at �13 km. In contrast, in Februarywhen cirrus clouds were weak and intermittent in their

occurrences, the Km profile is much broader (Fig. 6(d))

with no sharp peak superposed. The values of Km in the

peak region also are smaller in February 1999 than those

in January 1999. An important result of the present

study is that the region below tropopause is highly tur-

bulent and the eddy diffusion has a sharp positive alti-

tude gradient between the region 10 and 12.5 km whichis followed by a region of negative altitude gradient (of

Km) within �2 km. As positive and negative altitude

gradient of Km will lead to divergence and convergence

of species in the presence of negative altitude gradient in

their concentrations, it appears that conditions are fa-

vourable for accretion of aerosols and water vapour at

altitudes close to tropopause leading to the formation of

the cirrus. In general, when the convergence/divergenceis sharp and strong the probability of cloud occurrence

is high and when it is broad and weak the cloud is rather

weak and discontinuous (Fig. 4). This observation sup-

ports the formation of thin clouds due to in situ nucle-

ation near tropopause. The super saturation required

for the in situ nucleation could be provided by the

turbulence.

Acknowledgements

The National MST Radar Facility at Gadanki is op-

erated as an autonomous facility under Department of

Space (DOS) with partial support from Council of Sci-entific and Industrial Research (CSIR). The authors are

thankful to the technical and scientific staff of NMRF for

their dedicated efforts in conducting this experimental

programme. B.V. Krishna Murthy acknowledge CSIR

for grant of Emeritus Scientist Scheme.

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