A lidar system has been used at Gadanki (13.5º,

79.2ºE) to study the characteristics of aerosol layer

(cloud) occurring just below the tropical tropopause.

The preliminary results of the lidar observations indicate

that the cloud occurs ~ 2 km below the tropopause.

The top and bottom edges of the cloud have

propensity for ice crystal presence with liquid droplets/

vapours in-between. The clouds show temporal fluctuations

(in their backscattering ratio) with temporal

scales of the order of 30–90 min

Bhavanikumar, Y.

Krishna Moorthy, K.

Krishna Murthy, B. V.

Krishnaiahi, M.

Nair, Prabha R.

Parameswaran, K.

Prabhakaran Nayar, S. R.

Raghunath, K.

Rao, P. B.

Revathy, K.

Satheesan, K.

English

Publications of the IAS Fellows

INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 166
Lidar observations of aerosol layers just below 
the tropopause level during IFP-INDOEX 
K. Parameswaran, Prabha R. Nair, K. Krishna Moorthy*,  
B. V. Krishna Murthy*,§, S. R. Prabhakaran Nayar*, K. Revathy*,  
K. Satheesan*, P. B. Rao†, Y. Bhavanikumar†, K. Raghunath† and M. Krishnaiah‡ 
Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, India 
*Department of Physics, University of Kerala, Kariavattom, Thiruvananthapuram 695 581, India 
†NMRF, Tirupati 517 502, India 
‡Department of Physics, S.V. University, Tirupati 517 502, India 
A lidar system has been used at Gadanki (13.5º, 
79.2ºE) to study the characteristics of aerosol layer 
(cloud) occurring just below the tropical tropopause. 
The preliminary results of the lidar observations indi-
cate that the cloud occurs ~ 2 km below the tropo-
pause. The top and bottom edges of the cloud have 
propensity for ice crystal presence with liquid droplets/ 
vapours in-between. The clouds show temporal fluc-
tuations (in their backscattering ratio) with temporal 
scales of the order of 30–90 min. 
TROPICAL tropopause is a very cold region with tempera-
tures as low as 190 K in some months of the year1. Such 
low temperatures are favourable for nucleation, condensa-
tion and formation of clouds provided vapours, e.g. 
H2O/H2SO4 are present with sufficient pressure. In fact, 
thin persistent ice clouds (of H2O/H2SO4) have been  
detected near the tropical tropopause and reported as 
early as 1977 from ground-based lidar observations2. 
From satellite-based measurements, Prabhakara et al.3,4 
found that the seasonal average cloud cover produced by 
thin cirrus (near tropopause) is as large as 50% over the 
tropical western Pacific in all seasons. They also reported 
that the thin cirrus occurrence frequency peaks sharply 
over the western Pacific particularly during northern winter 
when the tropopause is coldest. SAGE II solar occultation 
particle extinction measurements5 also revealed optically 
thin cirrus clouds near tropopause in exc ss of 50% of the 
time in the western Pacific region and that they occur 
within top 2km of the tropopause. Lidar measurements at 
Kwajelein2 in the Pacific region showed the presence of 
these clouds persisting even for days. Air-borne lidar meas-
urements during the Central Equatorial Pacific Experi-
ment6 (CEPEX) revealed the cloud layer with a thickness 
of the order of 200 m. These measurements showed  
that the cloud layer sometimes occurred not only in the 
outflow region of deep convective systems but are also 
present in regions apparently not affected by convection. 
 Optically the thin cirrus clouds are very thin and it is 
inferred that their optical depth6 is less than 0.03. Gage  
et al.7 suggested that the subvisible cirrus cloud may be 
responsible for local radiative energy absorption and 
hence, responsible for the observed upward vertical wind 
speeds of 0.3 cm/s near tropopause with implications to 
lower stratospheric circulation. 
 Jenson et al.6 evaluated two mechanisms for the forma-
tion of the thin cirrus clouds: (i) Dissipation of optically
thick cumulonimbus outflow anvils leaving behind an  
optically thin layer of ice crystals, (ii) In situ nucleation of 
ice crystals near the tropopause due to homogeneous 
freezing of sulphuric acid particles. The supersaturation 
required for the ice nucleation in the second mechanism 
may be due to either slow synoptic scale uplift or shear-
driven turbulent mixing. Jenson et al. found that both the 
above two mechanisms are plausible for the formation of 
the thin cirrus clouds. 
 The microphysical properties of the clouds and their 
temporal variations are crucial information to understand 
and gain insight into the cloud formation mechanisms. For 
this ground-based lidar offers an excellent means though 
limited to nighttime operation. At Gadanki (13.5ºN, 
79.2ºE), a tropical station, where a powerful MST radar 
has been in operation for the last few years, a lidar capa-
ble of operation in the Mie and Rayleigh scatter modes 
has been installed recently. The lidar is capable of provid-
ing signal strength measurements of the cross polarized 
component along with the parallel component in the Mie 
scatter mode, so that depolarization ratio which is a very 
imp rtant factor in the cloud microphysics can be 
obtained. During the Indian Ocean Experiment-Intense 
Field Phase (INDOEX-IFP) the lidar experiment at 
Gadanki has been conducted during nighttime from 18 
January 1999 to 5 March 1999. We present here the pre-
liminary results of an analysis of the lidar data on aerosol 
(cloud) layers just below the tropopause level. (We are 
using the more general term ‘aerosol (cloud) layer’ than 
the usual term ‘cirrus cloud’ as cirrus clouds usually com-
prise ice crystals whereas our observations reveal the 
presence of liquid phase also as shown later in the paper.) §For correspondence. (e-mail: bvkmurthy@123india.com) 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 167
Lidar description and method of analysis 
The lidar employs a Nd : YAG laser as the transmitter and 
it is operated at its harmonic wavelength of 532 nm. The 
laser pulse energy is 0.4 J and the pulse width is 7 n . The 
laser beam is directed vertically upwards by an optical 
mirror system. The receiving telescope primary mirror is 
~ 30 cm dia parabolic for Mie mode of operation (for 
aerosol studies in the troposphere and lower stratosphere). 
The transmitter beam and the receiving telescope are 
separated by a horizontal distance of ~2 m. The system is 
operated with an altitude resolution of 300 m and a pulse 
repetition frequency of 20 Hz. The system provides back-
scattered signal strength integrated over 5000 transmitted 
pulses for both polarizations (i.e.) parallel nd perpen-
dicular (to the laser beam polarization) polarization com-
ponents. The integration over 5000 pulses corresponds to 
a time of 250 s. Three such integrated backscatter signal 
strength profiles are averaged providing a profile aver-
aged over 750 s (12.5 min) and these profiles are the basic 
data used for subsequent analysis. The lidar has been 
operated on almost all the nights during 18 January 1999 
to 5 March 1999 from ~ 2200 h to ~ 0500 h with few  
interruptions. 
 The backscatter signal strength profiles for the paral-
lelly polarized (main) component are used to obtain the 
backscatter ratio. The backscatter ratio (BR) is given by 
  BR = (bR + bA)/bR, 
where bR and bA are the backscatter coefficients for air 
molecules (Rayleigh scatter) and aerosols respectively. 
The reference height for this evaluation is taken as 40 km 
where the aerosol number density can be assumed to be 
nil and the air density profiles for estimation of Rayleigh 
backscatter coefficient are taken from the model of Sasi 
and Sengupta8. The depolarization factor (D) which is 
essentially the ratio of the backscatter signal strength of 
the perpendicular to parallel component is also evaluated. 
 The lidar data obtained during 18 January 1999 to 5 
March 1999 have been analysed to obtain the above  
parameters. 
Results 
We have examined the backscatter ratios derived from the 
lidar data for the presence of aerosol layers (clouds) 
below the tropopause level. The clouds will be characte-
rized by high backscatter ratios. We have found that aero-
sol clouds are present just below the tropopause (which is 
taken to be at ~ 16 km) more frequently in January than in 
February and also in March till 5th during the period  
under examination. This probably is due to the generally 
colder tropopause (and upper troposphere) temp atur s 
in January compared to February (and March). The aero-
sol clouds (hereafter referred as aerosol clouds than the 
more frequently used term cirrus clouds) are also stronger 
in January and last longer than in February when they 
occurred for short durations and were weaker. 
 Figure 1 shows the profile of backscatter ratio (BR) on 
the night of 20–21 January averaged over the period 
2240 h to 0440 h. The aerosol cloud is present (Figure 1) 
between 12 and 16 km with two peaks in-between. In  
order to quantify the cloud characteristics we defined 
three parameters namely cloud strength, m0; mean height 
f t  cloud, m1; and vertical thickness of the cloud, m2. 
Th se are given by 
 
  m0 = åBR(h) 
  m1 = åBR(h).h/m0 
  m2 = {å (h – m1)
2BR(h)/m0},  
where h1 and h2 are the lower and upper height limits of 
the cloud. 
 The above parameters are similar to the first three  
moments of frequency spectrum which give the total 
power, mean weighted fr quency and the spectral width. 
 We have obtained the parameters 0, m1 and m2 taking 
h1 and h2 as 12 and 18 km respectively (as the lower and 
upper height of the cloud). Based on the cloud strength 
m0, the clouds can be broadly classified into strong 
(m0 > 20) and weak (m0 < 20) though the demarcation is 
not very distinct. Quite often it is found that the cloud 
strength (backscatter ratio) fluctuates rapidly with a time 
scale of 30 min. We present in the following features of four 
different cases of clouds which occurred on four diffe ent 
Figure 1. Backscatter ratio (parallel component, P) vs height on the 
night of 20–21 January 1999. This is the average over the period from 
2240 h to 0440 h. 
 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 168
nights. These are typical of the clouds observed during the 
period under study. 
18–19 January 1999 
Figure 2 shows an example of a fluctuating cloud, which 
occurred on the night of 18–19 January. The figure shows 
m0, m1 and m2. The cloud strength (m0) shows rapid fluc-
tuations with time with scales as short as ~ 30 min. The 
mean height m1 (km) is around 14.9 km and appears to 
fluctuate similar but opposite in sense to m0. The cloud 
vertical thickness varied between 1.45 km and 1.7 km and 
the cloud appears, in general, to be thinner when optically 
strong as seen from a comparison of the top and bottom 
panels of the figure. 
 Figure 3 shows the depolarization factor D of he cloud. 
It is known8,9 that the depolarization D f moist air is less 
than that of dry air and that it is more for solid crystals 
(H2O/H2SO4) of non-spherical shape (compared to dry 
air). Examining Figure 3 we find that D in the cloud is 
more than that in the ambient (above and below the cloud) 
till about midnight. Later it is high in the top and bottom 
edges of the cloud with lower values in-between. So, it 
appears that the top and bottom edges of the cloud have 
strong ice crystal (of presumably H2O/H2SO4) presence 
while within the cloud liquid phase dominates. Further, D
appears to be more, thus indicating stronger ice crystal 
p esenc , in the top edge than in the bottom edge. D does 
not show the rapid fluctuations of m0. 
20–21 January 1999 
Figure 4 shows features of a strong cloud which occurred 
on the night of 20– 1 January. The value of m0 (top 
panel) is between 20 and 50 with superposed fluctuations 
till ~ 0400 h (20 on the x-axis scale in the figure). Then it 
raises sharply to a very high value > 100 followed by a 
sharp decrease. The total duration of this excursion is 
~ 1 h. The mean height of the cloud m1 (middle panel) is 
between 14.3 km and 15 km till 0340 h in the night. Later, 
when the cloud strength rose sharply the mean height 
decreased rapidly to 13.8km  The cloud thickness (m2)
Figure 2. Cloud parameters m0 (top panel), m1 (middle panel) and m2 
(bottom panel) on the night of 18–19 January 1999 for the main (para-
llel) component. The starting time (corresponding to 0 on the time axis) 
is 2240 h. One unit on the x-axis corresponds to 750 s (12.5 min). 
 
Figure 3. Depolarization ratio on the night of 18–19 January 1999. 
 
Figure 4. Same as Figure 2, but for the night of 20–21 January 1999 
and starting time 2352 h. 
 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 169
varied between 1 to 1.4 km. It appears that the cloud is 
thinner when strong. 
 Figure 5 shows the depolarization factor D. It is higher 
in the top and bottom edges of the cloud with a minimum 
in-between. The minimum is not deep. An interesting fea-
ture is the development of a region of high D at lower 
heights (12.5–13.0 km) in the early morning hours. There 
is again a region of low D (rather broad) between this 
cloud and the bottom edge of the main cloud. 
26–27 January 1999 
Figure 6 shows the three parameters on this night. The 
cloud on this night is not as strong as in the previous case 
but moderately strong. The cloud becomes stronger in the 
early morning hours as revealed by m0. It fluctuated 
between 20 and 40 till early morning hours. The mean 
height of the cloud remained around 14.4 km for most of 
the night. In the early morning hours when the cloud  
became stronger there is a sudden decrease in its mean 
height to 13.7 km. The cloud thickness remained around 
1 km except for some fluctuations. 
 Depolarization factor D is shown in Figure 7. D is quite 
low within the cloud compared to the ambient. This means 
the cloud is mainly in the vapour/liquid droplet state. In 
the early morning a region (broad) of high D developed 
below the bottom edge of the cloud as in the previous 
case. So, in the early morning hours there is a develop-
ment of ice crystal layer below the bottom edge of the 
main cloud. This is also the period of high m0. 
3–4 February 1999 
Figures 7 and 8 show the three parameters and D repec-
tively. The cloud on this night is quite different from the 
earlier ones. It shows two bursts of higher m0 values last-
ing for 30–90 min before midnight. Except for these 
bursts the cloud is very weak throughout the night with 
minor fluctuations. The mean height is steady around 
1.5 km except for sharp troughs corresponding to the 
peaks in m0. The cloud thickness also is high on this night 
(~ 1.75 km) except that it is thinner during the bursts. 
 The depolarization D is low within the cloud throug-
out the night compared to the ambient. This indicates that 
the cloud is in vapour/liquid droplet state. 
Discussion and conclusions 
The aerosol clouds appeared approximately 2 km below 
the tropical tropopause. The fact that they appeared more 
strongly in January than in February could be linked to the 
lower tropopause temperatures (and also upper tropo-
sphere) in January compared to in February. Low tem-
perature is one of the conditions favourable for the 
presence of these clouds. 
 The trong clouds showed high depolarization factors 
in the top and bottom edges of the cloud with low values 
within the cloud. This indicates that the cloud edges have 
prope sity for solid ice crystal formation. When the cloud 
Figure 5. Same as Figure 3, but for night of 20–21 January 1999. 
 
Figure 6. Same as Figure 2, but for night of 26–27 January 1999 and 
starting time 2205 h. 
 
Figure 7. Same as Figure 3, but for night of 26–27 January 1999. 
 
INDIAN OCEAN EXPERIMENT 
CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001 170
 
is wear/moderately strong, it is basically in the vapour/ 
liquid droplet state. 
 Jenson et al. suggested that dissipation of optically 
thick cumulonimbus outflow anvils leaving behind ice 
crystal layer and i  situ nucleation of ice crystals near the 
tropopause due to freezing of sulphuric acid particles are 
two plausible mechanisms for cirrus cloud formation near 
the tropopause. In the case of cloud formation due to the 
first process, which is a large-scal  process, the cloud 
dissipation would be due to adiabatic process and the 
clouds are likely to persist for days. (Large wind shears 
can also dissipate through diffusion.) We have found that 
the clouds often show very deep fluctuations with time 
scales of the order of 30 to 90 min. Thus, it appears that 
the source of the clouds is of variable strength (and/or the 
dissipation process is quite rapid). In the second process 
of cloud formation, turbulence plays a major role in trans-
porting the vapours from lower heights to near tropopause 
heights where in situ nucleation can occur. It is quite 
likely that the turbulence strength itself is quite variable in 
time, thus modulating the cloud strength. Internal cloud 
dynamics/microphysics (including turbulence within the 
cloud) also can cause the fluctuations in the cloud 
strength. Thin layers of turbulence interspersed with lay-
ers of stability may occur below the tropopause. The thin 
stable layers where the mixing will be weak could facili-
tate formation of ice crystal layers due to high density of 
cloud condensation nuclei/aerosols. In between these sta-
ble layers of ice crystals, there can be condensed vapour 
in liquid state (or high vapour density without condensa-
tion) as observed in the present study when the clouds are 
strong. Further work is in progress on these lines. 
 
1. The following are the main conclusions from the pre-
sent study. 
2. Thin aerosol clouds occur ~ 2 km below the tropical 
tropopause. They occur more frequently and strongly 
when the tropopause (and upper tropopause) is colder. 
3. As the cloud becomes stronger it comes down in height 
and becomes thinner, may be due to cloud mass content. 
4. The top and bottom edges of the cloud have propensity 
for presence of ice crystals with liquid phase in between. 
 
 The strength of the cloud shows very prominent 
fluctuations with time scales of the order of 30 to 60 min. 
 
 
1. Krishna Murthy, B. V., Parameswaran, K. and Rose, K. O.,  
J. Atmos. Sci. 1986, 43, 914–922. 
2. Uthe, E. E. and Russell, P. B., Proc. of the Radar. Conf. IAMAP, 
IUGG, 1977, pp. 242–244. 
3. Prabhakara, C., Frase, R. S., Dalu, G., Man-Li, C. Wu and Curan, 
R. J., J. Appl. Meteorol., 1988, 27, 379–399. 
4. Prabhakara, C., Kratz, D. P., Yoo, J-M. Dalu, G. and Vernekar, 
A. J., Quant. Spectrosc. Radiat. Transfer, 1993, 49, 467–483. 
5. Wang, P-H., McCormick, M. P., Poole, L. R., Chu, W. P., Yue, 
G. K., Kent, G. S. and Skeens, K. M., Atmos. Res., 1994, 34,  
53–83. 
6. Jenson, E. J., Toon, O. B., Selkirk, H. B., Spinhirne, J. D. and 
Schoeberl, M. R., J. Geophys. Res., 1996, 101, 21361–21375. 
7. Gage, K. S., McAfee, J. R., Carter, D. A., Ecklund, W. L., Riddle, 
A. C., Reid, G. C. and Balsley, B. B., Science, 1991, 254, 1771–
1773. 
8. Sasi, M. N. and Sengupta, K., Sci. Report, VSSC:1979:SR:19:79, 
ISRO, 1979. 
9. Hoyt, D. V., J. Appl. Meteorol., 1977, 16, 432–436. 
10. Frohlich, C. and Shaw, G. E., Appl. Opt., 1980, 19, 1773–1775. 
 
ACKNOWLEDGEMENTS. The National MST Radar Facility (NMRF) 
at Gadanki is operated as an Autonomous Facility under DOS with 
partial support from CSIR. We thank the technical staff of NMRF 
whose dedicated efforts made it possible to conduct the experiments. 
B. V. Krishna Murthy thanks CSIR for a grant under Emeritus Scientist 
Sch me. 
 
Figure 8. Same as Figure 2, but for night of 3–4 February 1999 and 
starting time 2206 h. 
 
Figure 9. Same as Figure 3, but for night of 3–4 February 1999. 
 


Lidar Observations of aerosol layers just below the tropopause level during IFP-INDOEX

http://repository.ias.ac.in/69462/1/69462.pdf

Lidar Observations of aerosol layers just below the tropopause level during IFP-INDOEX

Abstract

Similar works

Full text

Available Versions

Publications of the IAS Fellows