Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
Contents lists available at SciVerse ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics
journal homepage: www.elsevier.com/locate/jastp
Cloud microphysical properties over Indian monsoon regions during
CAIPEEX-2009
Savita B. Morwal n, R.S. Maheskumar, B. Padma Kumari, J.R. Kulkarni, B.N. Goswami
Indian Institute of Tropical Meteorology, Pashan, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2011
Received in revised form
4 April 2012
Accepted 24 April 2012
Available online 6 May 2012
Cloud microphysical data collected from an instrumented aircraft in the Cloud Aerosol Interaction and
Precipitation Enhancement Experiment (CAIPEEX-2009) during May–September 2009 have been used to
examine the nature of cloud drop size distributions (DSD), cloud drop effective radius (RE) and their height
variations at different locations during tropical Indian monsoons. Single mode drop size distributions were
observed over Pathankot, Hyderabad and Bengaluru regions and bimodal DSD were recorded often over
Guwahati and Bareilly regions. DSD spectral width showed height variation, being narrow at lower heights
and broadening with increasing height. DSD spectra were narrow even at higher levels over Pathankot
during pre-monsoon season and were very broad at Bareilly and Guwahati during the active phase of
monsoon. The total concentrations of cloud droplets and percentage contribution of cloud droplet of
radiir10 mm (small) and 410 mm (large) showed interesting height variations and were different over
different regions. The RE showed nearly linear increases with height over all the regions. However, the
droplet growth rate is observed to be different over different regions, being less over north (Bareilly and
Pathankot: 1.3–1.46 mm/km), intermediate over central (Hyderabad: 1.74 mm/km) and highest over
northeast (Guwahati: 1.92 mm/km) and south (Bengaluru: 1.99 mm/km) India.
For the first time an attempt has been made to collect and explore cloud microphysical characteristics
using in-situ aircraft observations during Indian monsoon conditions.
& 2012 Elsevier Ltd. All rights reserved.
Keywords:
Atmospheric processes
Microphysics of clouds
Cloud droplet distributions
CAIPEEX
1. Introduction
Clouds play a vital role in the dynamics and thermodynamics of
the atmosphere. Cloud-scale processes are considered sub grid scale
as far as cloud dynamics and cloud microphysics is considered.
Hence, representation of these processes in large-scale weather and
climate models is a challenge. Cloud microphysics includes the
small scale properties of clouds such as material state (i.e., solid or
liquid) of the cloud particles, their size and concentration.
Clouds form in the atmosphere when the air becomes supersaturated so that water vapor condenses on particles to form
droplets. During cloud formation the concentration of cloud
droplets depends on the concentration of certain particles present
in the air mass. Therefore, preexisting particles determine cloud
properties such as droplet concentration and size. Increases in
particle (i.e., CCN) concentrations due to anthropogenic sources
leads to higher concentrations of smaller cloud droplets. Cloud
drop size distributions also depend on the development stages
n
Corresponding author. Tel.: þ91 020 25904262; fax: þ 91 020 25893825.
E-mail addresses: morwal@tropmet.res.in (S.B. Morwal),
mahesh@tropmet.res.in (R.S. Maheskumar),
padma@tropmet.res.in (B. Padma Kumari), jrksup@tropmet.res.in (J.R. Kulkarni),
goswami@tropmet.res.in (B.N. Goswami).
1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jastp.2012.04.010
i.e., an early developing stage with precipitation associated with
numerous small cloud drops to mature stage and dissipating
stage and precipitation with larger drops.
The cloud drop size spectrum has been studied by many
workers (Zaitsev, 1950; Weickmann and Aufm Kampe, 1953;
Squires, 1958a, 1958b; Warner, 1969a, 1969b, 1973; Twomey
and Warner, 1967; Hudson and Yum, 1997; Yum and Hudson,
2005; Hudson et al., 2009; Gerber, 1996). Cloud droplet number
concentrations vary with height and cloud type (Martinsson et al.,
2000). Further, it has been observed that normally the concentrations have been found to be less than a few hundreds cm 3 and
rarely above 1000 cm 3 in stratiform clouds (Hudson et al., 2010;
Anderson et al., 1994; Garrett and Hobbs, 1995; Gillani et al., 1995;
Leaitch et al., 1986, 1996; Martin et al., 1994; Twohy et al., 1995)
although a very high concentration was observed by Martinsson
et al. (2000). Fair weather continental cumulus clouds with no
precipitation have relatively narrow drop size spectra while
continental cumulus clouds which have reached more mature
stages of cumulus congetus show much broader cloud drop
spectra (Hobbs et al., 1980). Further, they showed that cumulus
clouds embedded in a stratus layer have even broader spectra.
Maritime clouds have broader drop size spectra compared to
continental clouds (Hudson and Yum, 1997; Pinsky and Khain,
2003; Wang et al., 2009; Battan and Reitan, 1957).
S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
Increases in aerosol concentrations result in decreased drop sizes
(Breon et al., 2002; Rosenfeld et al., 2008a) which may suppress
precipitation (Albrecht, 1989; Rosenfeld, 2000; Hudson and Yum,
2001; Hudson et al., 2009;). An appropriate measure of the mean
drop size is the effective radius (RE), which is the ratio between the
third and the second moment of the cloud droplet size distribution
(Stephens, 1978). This is indicative of the threshold size of cloud
drops below which precipitation seldom forms. Cloud drops at the
threshold size rapidly grow to raindrops (Houze, 1993). The RE has
been studied by many workers (Rosenfeld et al., 2008b; Martins
et al., 2007), which is related to the spectral width of the cloud
droplet spectrum (Pontikis and Hicks, 1992; and Martin et al., 1994).
In order to understand the interaction between clouds and
aerosols and also to understand the microphysical properties of
clouds over the tropical Indian region, a national experiment
called ‘Cloud Aerosol Interaction and Precipitation Enhancement
Experiment (CAIPEEX)’ was launched during 2009 and completed
successfully by making observations of cloud microphysical
parameters and aerosols from 17 May to 30 September 2009
over various geographical regions of India by the Indian Institute
of Tropical Meteorology, Pune. The observations of aerosols and
cloud microphysical and dynamical parameters were collected
onboard aircraft over various Indian regions including some close to
the coast. The following Section gives some details of the observations and meteorological conditions that prevailed over the different base stations during the observation periods. We attempt to
examine the characteristic features of cloud droplet spectra over
various regions with different geography and meteorology.
2. Data and instruments
The national scientific program CAIPEEX launched on 17th
May 2009 from Pune base station was envisaged to have three
phases. The main objective of Phase I was collection of good
spatio-temporally resolved data of aerosols and cloud microphysical parameters from May to September 2009 over various
regions of India. The specific objectives of Phase I were: (i) to
measure background concentrations of aerosols and CCN during
the monsoon season, (ii) observations of hydrometeors in clouds,
(iii) observations of space-time variability (during the monsoon
season) of trace gases over India and (iv) selection of sites for the
phase II experiments. An instrumented Piper Cheyenne aircraft
was used to collect in-situ observations of aerosols and cloud
microphysical parameters viz. Liquid Water Content (LWC), total
water content, concentrations of Cloud Condensation Nuclei (CCN)
and number concentration of cloud droplets of different sizes,
temperature, humidity, etc. The different aircraft instruments include
AIMMS Probe, Cloud Droplet Probe (CDP), Cloud Imaging Probe (CIP),
CCN counter, Gas Analyzer, LWC probe, etc. The program was
conducted in missions and IOP (Intensive Observation Periods) mode.
The mission consist of observations during the transit of the aircraft
from one base to the other. The IOP consists of aircraft observations
and other routine ground-based observations over the selected base
station regions. The IOP base stations included Pune (18.521N,
73.851E), Pathankot (32.261N, 75.651E), Bareilly (28.351N, 79.411E),
Bengaluru (12.971N, 77.591E), Hyderabad (17.381N, 78.481E) and
Guwahati (26.181N, 91.751E) (Fig. 1). Pathankot and Bareilly are
located in northern India, Hyderabad is in the Central region, Pune
and Bengaluru are in southern India and Guwahati is in the northeast. Based on the analysis of synoptic and thermodynamic conditions a work area was chosen with high potential for development of
cumulus clouds. On a particular day the observations were collected
preferably in the isolated growing cumulus clouds by profiling the
clouds so that we would have a succession of growing towers until
we reached the maximum cloud top. Sometimes clouds at different
77
heights were also chosen. All data are 1 s averages. If observed LWC
is40.0 and total cloud droplet concentration is420 cm 3 of a
minimum 3 s, it is considered as cloud. All the observations satisfying
this condition in the updraft regions have been chosen for analysis.
A Cloud Droplet Probe (CDP) from Droplet Measurement
Technologies (DMT), USA was mounted externally below the right
wing of the aircraft. CDP sized and counted cloud droplets in the
size range from 2 to 50 mm in 30 size bins at the sampling
frequencyof 1 Hz. By using the sample area at a known velocity,
particle concentrations were calculated. Other parameters that can
be computed include the average drop diameter, mass weighted
diameter, mode distributed diameter, standard deviation and liquid
water content (LWC). Cloud drop size distributions, cloud drop
effective radii and their height variations over different regions
are presented and discussed. For convenience data of one day for
each of the IOP regions is presented: 28 May 2009 (Pathankot), 15
June 2009 (Hyderabad), 1 July 2009 (Bengaluru), 24 August 2009
(Bareilly) and 4 September 2009 (Guwahati).
Liquid Water Content (LWC, g/m3) was measured onboard the
instrumented aircraft by a Johnson Williams (JW) Hotwire probe,
which was mounted externally in the nose. LWC was also derived
from measurements of CDP cloud droplet size distributions. The
CDP was size calibrated from time to time in the field with known
bead sizes. Baumgardner (1983) made an analysis to estimate measurement accuracies of the water droplet measuring probes and JW
Hotwire probe. In order to validate the LWC data computed from
CDP observations, correlation between the LWC values directly
measured from JW Hotwire probe and CDP derived LWC is examined
for all the days considered in this study. There was a high correlation
(40.85) between the two probes.
For the measurement of precloud aerosol a Passive Cavity
Aerosol Spectrometer Probe (PCASP-100X) was mounted below
the left wing of the aircraft. The PCASP counts and sizes the fine
and accumulation mode size particles (such as soot, organic carbon
and smaller mineral dust) of diameters in the range 0.1–3 mm in 30
size bins (Padma Kumari et al., 2011; Liu et al., 2009). The PCASP is
factory calibrated using known size latex spheres (Johnson et al.,
2008) and is also calibrated from time to time in the field. Data was
collected at an interval of 1 s (or 100 m) by all the above mentioned
instruments.
3. Results and discussion
The height variation of cloud droplet concentrations of different
sizes over different IOP base stations during May–September 2009
has been studied by using the number concentration spectra of
cloud droplets with radius between 1 mm and 25 mm. These drop
size spectra are available at various altitudes between 0.6 km and
8 km. Here, the observations associated only with updrafts are considered. Average cloud droplet spectra at 1 km intervals have been
computed for each km up to 8 km for all the days. Cloud Droplet
size Spectra (DSD) in different layers, percentage concentration
(normalized to total number of cloud droplets in the size range
1–25 mm) of droplets in the size ranger10 mm (small) and
410 mm (large) and effective radius in each layer for the five base
stations as mentioned above, is shown in Fig. 2. SMOKE-LBA
campaign observations suggest that when RE is close to 12 mm,
the efficiency of coalescence increases (Andreae et al., 2004). The
study carried out by Rangno and Hobbs (2005) to determine the
microphysical structures and precipitation-producing mechanisms
in cumulus and small cumulonimbus clouds over the warm pool
of the tropical Pacific Ocean showed that cloud effective radius of
12–14 mm is required for the onset of an effective collisioncoalescence process. Hence, in order to examine the increase in
coalescence efficiency/initiation of the warm rain process, the cloud
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S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
Pathankot
Bareilly
Guwahati
Patna
Pune
Hyderabad
Bengaluru
Fig. 1. Map of India showing the location and name of the different IOP (Intensive Observation Periods) base stations during CAIPEEX 2009.
droplets with radius 12 mm and greater are separated. The dark
vertical line in the DSD in Fig. 2a indicates the value of the cloud
drop radius for the initiation of the coalescence/warm rain processes (i.e., 12 mm).
3.1. Height variation of cloud droplet spectra
Cloud droplet spectra have been studied extensively (e.g., Yum
and Hudson, 2001, 2002; Derksen et al., 2009; Miles et al., 2000;
Warner, 1969a; Paul, 2000; Cooper, 1989; Pawlowska et al., 2006).
In this sub section in-situ aircraft measurements of cloud droplet
spectra collected using the CDP during CAIPEEX 2009 over various
regions at various levels are explored and shown in Fig. 2a.
May 28, 2009 was a normal pre-monsoon dry day over north
India, with surface temperatures of 40 1C and no clouds over
Pathankot except over the Himalayan mountain to the north of
Pathankot. To the north of Pathankot the observed cloud base of
4545 m was the top of a haze layer at 3 to 4 km. From the
radiosonde ascent at the base station, the lifting condensation
level was approximately 3.7 km and the freezing level was 4.5 km.
From DSD spectra in different vertical layers for Pathankot, it is
evident that the droplet radius ranges from 1.25 mm to 13.75 mm. In
all the layers the droplet spectra are very narrow and associated
with a single mode. However, the spectra showed broadening with
increasing altitude (Paul, 2000; Warner, 1969a) with simultaneous
increase of modal radius except in the 5–6 km layer. The concentrations are more or less the same for sizes below the modal value (but
modal values are different in different layers) in all the layers except
in the 4–5 km layer, which is associated with very high concentrations of small droplets. Above the modal value the concentrations
are less in the lowermost layer (4–5 km) and showed continuous
increase with height except in the 5–6 km layer where it was lower.
It is clearly evident from DSD spectra that the modal radius, in all
the vertical layers is well below 12 mm which indicates that there is
little possibility of initiation of the warm rain (Rangno and Hobbs,
2005). Suppressed convection was observed over the Pathankot IOP
region where convection was usually observed only during afternoon hours. Also, a polluted haze layer extended up to 4 km (black
line in Fig. 3. The aerosol concentration was very high up to 4 km
and decreased above this altitude.). The observed DSD spectra,
narrow and single mode with high number concentrations, represents typical super-continental types of clouds.
On 15 June 2009 over the Hyderabad region weather conditions
were partly cloudy with the possibility of development of deep
convection within radial distances of 100 nautical miles NE and SW
of Hyderabad. However, due to military restrictions, observations on
this day were made only in the region east of Hyderabad. In the
observational area, clouds with base heights above 2.7 km were
encountered. Fig. 2a shows the cloud droplet size (DSD) spectra in the
layers 2–3 km to 6–7 km. The droplet size spectra are narrow in
the lower layers (2–3 and 3–4 km) and broaden with height and the
maximum size range reaches up to 18.5 mm. According to Pawlowska
et al. (2006) the broadening of the spectra/ spectral width affects the
radiative properties of clouds (Liu and Daum, 2000) and development
of drizzle and rain (Seifert and Beheng, 2001). All the spectra showed
a single mode in droplet size distributions. The maximum concentration of different cloud droplets was 50 cm 3/mm which is very less as
compared to that over Pathankot. The modal radius showed a gradual
increase with height. However, the modal radius (o 9 mm) was
still below the value of 12 mm at all the altitudes indicating the
S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
79
Fig. 2. Variation, in different layers over Pathankot, Hyderabad, Bengaluru, Bareilly and Guwahati during CAIPEEX 2009, of (a) Number concentration spectra of cloud
droplets, (b) total concentration (black solid line with open triangle) and percentage contribution of clouds drops of radius r 10 mm (solid line with solid circles in black)
and410 mm (solid line with solid circles in red) and (c) effective radius (RE). Note that the different distances in (b) are above cloud base (mentioned below the title) and
distances are above mean sea level in (a) and (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
6000
Pathankot
Hyderabad
Bengaluru
Bareilly
Guwahati
Altitude (m)
5000
4000
3000
2000
1000
0
0
500
1000
1500
2000
2500
Aerosol Concentration (cm-3)
3000
Fig. 3. Vertical variation of cloud free aerosol concentration from PCASP-100X
at different stations viz. Pathankot (Black), Hyderabad (Red), Bengaluru (Green),
Bareilly (Blue) and Guwahati (Magenta). Very high aerosol concentration is observed
over Pathankot. The pollution is high in the lower layers over Bareilly (below 2 km)
and Guwahati (below 1 km). (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
inefficiency of the collision-coalescence processes (Pawlowska et al.,
2006). The observed DSD spectra with a single mode represent
continental type of clouds which are influenced by the prevailing
weather conditions, i.e., subdued convective activity.
Number concentration spectra on 1 July 2009 at IOP base
Bengaluru, in different layers from 1–2 km to 6–7 km are shown
in Fig. 2a. The cloud base is around 1.8 km. All the spectra show a
single mode and are associated with gradual increase in modal
radius with altitude. The maximum droplet number concentration
at the modal radius in different layers vary from 22–63 cm 3/mm.
The droplet size range is higher ( 420 mm) than that observed
over Pathankot and Hyderabad. The droplet spectra in the 1–2
and 2–3 km layers are narrow (radius rangeo10 mm) and
showed broadening in the layers above this (range 415 mm).
The broadening of the spectra may be due to the growth of the
size of the cloud droplets with increasing altitude (Pawlowska
et al., 2006 and Seifert and Beheng, 2001). Also, it can be
attributed to the incursion of moisture in the higher altitudes
under the large scale influence of the monsoon circulation
(confirmed from vertical profiles of temperature and mixing ratio
using radiosonde data over the region, not shown). As seen from
Fig. 2a, the modal radius at all the altitudes is much less than the
vertical line of 12 mm ( 10 mm). Thus, over this region, the DSD
spectra were broad and associated with a single mode.
The north–south shift of the monsoon trough has an important
bearing on the rainfall distribution over India (Rao, 1976). Under
the normal monsoon conditions, the axis of the monsoon trough
extends from Ganganagar (in Rajasthan, westward end) to Kolkata
with its eastward end over the Head Bay. When the axis of the
monsoon trough moves north and lies close to the Himalaya, a
break in rainfall is observed over most parts of India (Rajkumar and
Narasimha, 1997; Parasnis et al., 1991; Rao, 1976). The normal
position of the monsoon trough is usually associated with enhanced
monsoon activity over India (Rao, 1976; Sikka and Narasimha,
1995). Bareilly is at the meeting point between the surface SW
winds and the easterlies at the north of a west moving tropical
depression to the south. Over the IOP region Bareilly, on 24th
August 2009 the monsoon was very active due to the presence of
the monsoon trough over this region. The observed cloud base was
at 0.7 km. The cloud base was extremely warm (25.3 1C) and the
atmosphere was extremely polluted (high concentration of aerosols
below 2 km as shown with the blue line in Fig. 3) and moist. The
depth of the cloud was more than 5 km. Fig. 2a shows the number
concentration spectra of cloud droplets in different layers starting
from 0–1 km to 7–8 km at Bareilly. Over this base the spectra are
very broad starting from the lowermost layer (radius range
413.5 mm) and broadening continued with increase in height and
reached up to radius of 25 mm. Some of the droplets are bigger than
12 mm even in the lowest layer (0–1 km), suggesting the effect of
giant CCN. The spectra have single mode in the lower layers up to
2–3 km layer with modal radius of 6–7 mm. Above this layer, all the
spectra are bimodal with increase in modal radius and the second
mode reaching up to 12 mm. Even though the modal radius was
above 12 mm in all the layers above 3 km warm rain was not
observed, this may be due to the high pollution over this region (as
seen from Fig. 3. The concentration of aerosols was high in the
layers below 2 km indicating polluted air masses in the boundary
layer). Thus, this particular day indicated that pollution was able to
prevent warm rain from forming in growing convective towers up
to 5.5 km, which was associated with negative temperature.. Thus,
the observations collected on 24 August 2009 at Bareilly demonstrate that heavy air pollution can suppress warm rain from clouds
below the freezing level even in very moist tropical conditions, with
the thickest cloud depth between base and freezing level
(Rosenfeld, 2000). Aerosols serve as a source of both cloud condensation nuclei (CCN) and ice nuclei (IN) and affect microphysical
properties of clouds. Clouds forming in a more polluted atmosphere, as observed here, contain a larger number of smaller drops
and assumed to retard the cloud droplet coalescence thereby
decreasing precipitation. Also, according to Rosenfeld et al. (2002)
the polluted clouds over land need to grow beyond 6 km in height
to start precipitating.
On 4th September 2009 a weak southerly flow from the head
Bay to the north-east Indian region (Assam) was observed. The
observations were conducted over the Meghalaya region, which is
south west of IOP region Guwahati and west of Bangladesh. Over
this region, isolated thunderstorms were predicted to occur in the
afternoon. The cloud base varied from less than 1 km to 1.5 km at
different locations in Guwahati. Fig. 2a shows the cloud droplet
concentration (with base at 1.8 km) in the different vertical layers
from 1–2 km to 6–7 km. The droplet spectrum is narrow in the
1–2 km layer and shows continuous broadening with successive
increase in altitudes. The range of the spectra reaches 25 mm. The
droplet spectrum is single mode in the lowest layer with gradual
increase of the modal diameter with increasing altitude. The
spectra are bimodal in the layers above lowest layer as observed
by Warner (1969a). The second modal diameter crosses the value
of 12 mm in the layers at and above 3–4 km. Thus, the initiation of
the warm rain processes may take place above this height as
mentioned by Pawlowska et al. (2006). As per flight scientists’
day-to-day reports rainfall was noticed at altitudes above 4.5 km.
All the clouds are well grown clouds and associated with rain as
reported in the flight report. All the spectra are very broad with
modal radii above 12 mm. The characteristic of all the spectra are:
(i) droplet spectra at the top of a good growing cloud in the
3–4 km layer (at an altitude of 3.8 km, not shown in Fig. 2a) was
of single mode at radius 12.5 mm. (ii) droplet spectra in the
4–5 km layer at the top of freshly formed cloud (at an altitude
of 4.54 km) was single modal with mode radius of 13.5 mm and
(iv) the spectra in the layer 5–6 km at the top of a strongly
growing tower (at altitudes of 5.44 km, 5.8 km) and in the layer
6–7 km (at altitude of 6.36 km) showed bimodal distribution
where secondary mode was above 15 mm radius. Thus all the
growing clouds are associated with broad spectra and greater
number of larger cloud droplets as compared to small droplets.
According to the basic microphysical process in warm tropical
clouds, the particle growth by condensation begins below cloud
base and continues in clouds under supersaturated conditions,
giving rise to droplet formation (Khain et al., 2000). The process of
condensation growth of particles includes nucleation as well as
S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
diffusional growth of droplets. From cloud model simulations
Pinsky and Khain (2002) found that droplet spectral formation is
affected by three stages of in-cloud droplet nucleation which lead
to formation of primary and secondary modes, i.e., mono-modal
and bi-modal drop size distributions. They have also shown that
the secondary mode in the droplet spectrum contributes significantly to raindrop formation. Two main mechanisms have been
suggested in the literature for the formation of secondary small
droplet mode in droplet spectra. (i) Entrainment mechanism:
mixing of clouds with environmental air accompanied by partial
evaporation of cloud droplets in subsaturated zones, as well as by
fresh droplet nucleation of CCN penetrating the clouds through
the lateral cloud boundaries (Warner, 1973; Brenguier and
Grabowski, 1993; Lasher-Trapp et al., 2005; Su et al., 1998) (ii)
In-cloud nucleation mechanism: this enables in-cloud nucleation
of new droplets on CCN penetrating the cloud base and ascending
together with cloud droplets formed at low levels (Khain et al.,
2000; Pinsky and Khain, 2002; Segal et al., 2003). Such in-cloud
nucleation takes place when supersaturation within an ascending
cloud parcel exceeds its local maximum near the cloud base
(Ludlam, 1980; Pinsky and Khain, 2002; Segal et al., 2003).
In the present study the bimodal drop size distribution has
been observed over Bareilly and Guwahati. Over Bareilly, the
atmosphere was moist, extremely polluted and hazy (shown in
Fig. 3). The cloud base was very warm and at very low levels
( o1 km). The bimodal droplet spectra might have been produced
by entrainment of polluted environmental air through the lateral
cloud boundaries.
3.2. Total concentration and effective radius (RE) of cloud droplets
It is expected that cloud droplet concentrations of different
sizes change with height depending on the microphysical
processes in the cloud. To examine this, height variation of the
percentage (normalized by respective layer total concentration) of
cloud droplets of radius r10 mm (black line, small droplets)
and410 mm (red line, large droplets), for all the days over each
IOP base stations in each layer is shown in Fig. 2b. In this figure,
solid line with open triangles indicates the total concentration in
each layer. The bar diagrams for effective radius in all the vertical
layers are shown in Fig. 2c.
From Fig. 2b, it is seen that the total concentration over
Pathankot is high (390–650 cm 3) in all layers except the 5–6 km
layer (154 cm 3). The percentage contribution of small droplets is
very high (498%) throughout. This indicates the existence of super
continental clouds. This could be due to low LWC (as shown in Fig. 5
first row) or entrainment of polluted air masses (as shown in Fig. 3).
The percentage of large drops is negligible in all the layers. The
effective radius increases from the lowermost layer to the highest
layer as observed by others (Pawlowska et al., 2006; Martin et al.,
1994; Miles et al., 2000; Wood, 2000). However, the magnitude of
RE is (below 6.4 mm) very much less than 10–12 mm at which the
warm rain process initiates according to Rangno and Hobbs (2005).
Thus, during the pre-monsoon season, the possibility of precipitation initiation from the observed clouds is inhibited due to the
presence of high concentrations of small droplets.
Over the Hyderabad region the total concentration of cloud
droplets (Fig. 2b) is more or less the same in each layer and is in the
range of 210–280 cm 3. The total concentration of cloud drops is
less at all levels compared to Pathankot. The percentage of small
drops is very high up to the 4 km layer (499%) and thereafter
decreases with height. The percentage contribution of large drops
increases monotonically above 4 km to the 6–7 km layer and it is
24% in the highest layer. The RE showed continuous increase from
lower layers to higher layers (3.8 to 9.5 mm). However, the RE is still
below the value required for the initiation of warm rain at all levels.
81
Thus, the cloud microphysical features as seen on 15 June 2009 over
the Hyderabad region clearly indicates the inhibition of natural
initiation of the warm rain processes under prevailing weather
conditions.
At Bengaluru, the total concentration of cloud drops showed
less variation with height (171–308 cm 3) compared to that over
Pathankot and attains a maximum in the 3–4 km layer
(530 cm 3). This region is under the influence of widespread
monsoon activity and there was an incursion of moisture in the
mid levels (3–5 km) as seen from the radiosonde profiles (not
shown here). The percentage of small droplets is very high
( 499%) in the lower layers up to 3 km and thereafter it decreases
continuously with height. Also, the percentage of large drops
increases with height and to 33% in the highest layer. This may
be due to the (i)conversion of small cloud drops to bigger drops in
the higher layers and (ii) intrusion of moisture at higher levels
due to the prevailing large scale monsoon conditions as observed
from vertical profiles of temperature and moisture obtained from
radiosonde data. The effective radius increases continuously with
height and it is slightly more than 10 mm in the 6–7 km layer.
The cloud base at Bareilly was observed at very low level and
the environment was polluted with the existence of a haze layer
(Fig. 3 shows the high concentration of aerosols up to 2 km
indicating polluted air masses). The monsoon was very active
over this region. Total concentration varied between 70–395 cm 3
in the various vertical layers. The percentage concentration of the
smaller drops is very high (496%) below 2 km and it decreases with
height. The percentage of large cloud drops increases with height
and it exceeds the small cloud drops above 4 km. The effective
radii showed an increasing trend with height up to more than
12 mm in the layers above 4–5 km. This supports the rain observed
above 5 km as stated in the sub Section 3.1 and the criteria for the
initiation of warm rain.
Over Guwahati, where the monsoon was active on 4th September 2009, the total concentration of cloud droplets decreased with
height. The percentage contribution of small drops showed a very
sharp decrease with height up to 5 km and thereafter a slight
increase. The percentage contribution of large drops increases with
height and it becomes more than the smaller drop concentration
just below 3 km. There are many drops with radius412 mm above
this height. The effective radius showed a gradual increase in the
layers from 1–2 km to 4–5 km. RE is more than 12 mm above the
3–4 km layer and it is more or less the same in these layers. Thus,
the existence of bigger cloud drops which in turn initiate the warm
rain process supports the observed rain over this region.
Fig. 3 shows the concentration of particles of 0.1–3 mm
diameters as measured by PCASP averaged over 200 m layers in
the vertical in the cloud free regions over all the base stations. The
aerosol concentration in the lower layers (below 4 km) is highest
over Pathankot (4 1500 cm 3) as observations were taken during
the pre-monsoon conditions and least over Bengaluru (o500 cm 3)
as this observation period is associated with moist active monsoon
conditions. Even though over Bareilly the observations were conducted during the very active moist monsoon conditions under the
influence of a monsoon trough, particle concentrations were high
below 2 km (4500 cm 3) which are responsible for the formation
of haze. Same features were observed over Guwahati below 1 km.
The concentration of particles showed a decreasing trend in the
higher levels over all the stations.
Cloud droplet spectral broadening is an important problem
in warm rain processes as it is responsible for the initiation
of coalescence. Theoretically condensational growth predicts
narrowing of droplet spectra under adiabatic conditions (Rogers
and Yau, 1989). Contrary to this theory, in real clouds the standard
deviation of the droplet diameters often leads to broadening of
droplet spectra (Yum and Hudson, 2005; Yum and Hudson, 2001;
82
S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
Politovich, 1993; Martin et al., 1994; Hudson and Yum, 1997). Many
attempts have been made to explain the discrepancy between
theory and observations (Derksen et al., 2009; Feingold and
Chung, 2002; Chaumat and Brenguier, 2001; Brenguier and
Chaumat, 2001; Vaillancourt et al., 1998; Shaw et al., 1998; Khain
and Pinsky, 1997; Pinsky and Khain, 1997; Beard and Ochs, 1993).
Standard deviation (s) and mean radius of the cloud droplet spectra
considered in Section 3.1 for different vertical layers over various
base stations have been computed. Fig. 4 shows the standard
deviation against mean radius, altitude above cloud base and cloud
droplet concentration (bottom row, column 1, 2 and 3). It is evident
from the bottom row plots that increase in mean radius and cloud
depth (i.e., altitude above cloud base) and decrease in cloud droplet
concentration is associated with increase in s over all the regions
(Yum and Hudson, 2005, 2001; Hudson and Yum, 1997). The cloud
droplet spectra broadened with increase in mean radius and cloud
depth and became narrow with increasing cloud droplet concentrations. This relationship between s and altitude above cloud base
was also observed and reported by Hudson and Svensson (1995),
Martin et al. (1994), Politovich (1993), and Nicholls and Leighton
(1986). The droplet spectra are very broad over Guwahati in all
layers and narrow over Pathankot. Thus, over Guwahati, which was
associated with minimum pollution (Fig. 3) and active moist
monsoon conditions, cloud droplets were bigger with lower concentrations compared to other stations. Also, over Bareilly, under the
influence of the monsoon trough, the cloud droplets were big and
growing with altitude but due to the high pollution in the lower
layers (blue line in Fig. 3) the concentration of cloud droplets was
greater compared to Guwahati at all levels. Over Pathankot due to
the presence of very high air pollution (as seen from Fig. 3, the
concentration of particles below the cloud base was very high) there
are numerous small cloud droplets with narrow cloud droplet
spectra at all altitudes (Yum and Hudson, 2001, 2005).
The concentrations of small cloud droplets (mean radius 2–
4 mm) was very high just above cloud base over Pathankot (Fig. 4
middle row) compared to that over other stations. In general, the
droplet concentration showed decreasing trend with increasing
mean radius and cloud depth (middle row column 1 and 2 of
Fig. 4). Also the mean radius showed increasing trend with
increase in cloud depth over all the base stations (top row in
Fig. 4) and is supported by the findings of Yum and Hudson (2001,
2005) and Hudson and Yum (1997). In the lower portion of the
cloud mean radius showed maximum increasing trend over
Bengaluru, next is Guwahati and minimum is over Pathankot.
The vertical variation of cloud droplet effective radius is an
important cloud property that manifests both condensation and
coalescence growth (Chen et al., 2008). Height variation of RE of
cloud droplets over all the five regions is shown in Fig. 5. The solid
lines show the regressions. The vertical dotted line represents the
RE threshold associated with the initiation of the warm rain
process. The height at which the linear fit line intersects the
dotted line represents the height of initiation of warm rain
Altitude above Cloud base (m)
7000
Pathankot (4545)
Hyderabad (2815)
Bengaluru (1755)
Bareilly (735)
Guwahati (1980)
6000
5000
4000
3000
2000
1000
0
Cloud Droplet Concentration (cm -3)
0
2
4
6
8
10
12
14
700
700
600
600
500
500
400
400
300
300
200
200
100
100
0
0
Standard Deviation (µm)
0
2
4
6
8
10
12
0
14
1000 2000 3000 4000 5000 6000 7000
6
6
6
5
5
5
4
4
4
3
3
3
2
2
2
1
1
1
0
0
0
2
4
6
8
10
Mean Radius ( µm)
12
14
0
0
1000 2000 3000 4000 5000 6000 7000
Altitude above cloud base (m)
0
100
200
300
400
500
600
700
-3
Cloud Droplet Concentration (cm )
Fig. 4. Top Row: variation of altitude above cloud base with mean radius (column 1), Middle Row: variation of cloud droplet concentration with mean radius (column 1)
and altitude above cloud base (column 2) and Bottom Row: variation of standard deviation (of cloud droplet radius) with mean radius (column 1), altitude above cloud
base (column 2) and cloud droplet concentration (column 3) at Pathankot (Black), Hyderabad (Red), Bengaluru (Green), Bareilly (blue) and Guwahati (magenta) during
CAIPEEX-2009. The numbers within the bracket after name of the base station indicate cloud base height. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
processes in these clouds (rain height). Measured RE varied from
1.8 mm to 14.5 mm above all the stations. As the cloud droplets
ascend inside the cloud, they grow by condensation but they can
also coalesce into larger droplets, both of which will increase RE.
The aircraft data presented here shows that RE increases with
height inside clouds over all the regions. However, the rate of this
increase in droplet size is not the same at all the locations, which
is evident from the slope of the linear fit lines. These values are
shown in Table 1. From this table it is clearly evident that the
growth rate (mm/km) of the RE of cloud droplets with height is
less at Bareilly (1.30 mm/km) and Pathankot (1.46 mm/km), intermediate at Hyderabad (1.74 mm/km) and high over Bengaluru
(1.99 mm/km) and Guwahati (1.92 mm/km). Even though the
growth rate is highest over the Bengaluru region, the effective
radius at the cloud base is very much less (1.78 mm). Hence, the
8000
7000
Altitude (m)
6000
5000
4000
Pathankot
Hyderabad
Bengaluru
Bareilly
Guwahati
3000
2000
1000
0
0
2
4
8 10 12 14
6
Effective Radius (µm)
16
18
20
83
linear fit line intersects the 12 mm line above 6 km altitude which
is above the freezing level. The rain height is 10.4 km, 7.4 km and
6.45 km for Pathankot, Hyderabad and Bengaluru, respectively.
For Bareilly and Guwahati it is at 5.5 km and 4.2 km, respectively.
This implies that over these stations, above these heights, there is
a possibility of initiation of efficient warm rain processes. This
level is at very low altitude over Guwahati and at very high
altitude for Hyderabad and Pathankot. Therefore, under the then
prevailing synoptic/meteorological conditions there seems to be
no possibility of initiation of rain over these regions by warm rain
processes.
The LWC is the mass of the water in a cloud in a specified
amount of dry air and is measured per volume of air (g/m3) or
mass of air (g/kg) (Bohren and Albrecht, 1998). This variable is
important in classification of clouds and is strongly linked to
cloud microphysical variables viz. cloud droplet effective radius
(RE), cloud droplet concentration (N), and cloud droplet size
distribution (Wallace and Hobbs, 2006). Gerber (1996) and Liu
and Hallett (1997) showed that for an assumed cloud droplet size
distribution, effective radius is proportional to (LWC/N)1/3. Thus,
effective radius is directly proportional to cube root of LWC and
inversely proportional to cube root of cloud droplet concentration
(N). Fig. 5 (bottom) shows the LWC in different layers over various
base stations. Over Pathankot LWC is less as RE is less and droplet
concentration is more. There is a one to one relationship between
RE and LWC over all the stations. For Bengaluru due to the
incursion of moisture in the middle levels the RE and LWC both
showed increases in the corresponding levels.
Thus the aircraft observations described above give insight
into the microphysical processes, especially the cloud droplet size
distributions over different environments in the Indian tropical
region during the southwest monsoon season of 2009.
Altitude (m)
8000
7000
4. Summary and conclusions
6000
Aircraft observations of cloud droplet size distributions (DSD),
total and percentage contribution of small (radiusr10 mm) and
large (radius410 mm) cloud droplets and effective radius (RE) and
their height variations over different regions in the tropical Indian
monsoon region during May–September 2009 showed the following:
Single mode drop size distributions were observed over Pathankot, Hyderabad and Bengaluru regions. DSD spectra showed bimodal
distributions over Bareilly and Guwahati regions. DSD spectral
width showed height variation, being narrow at lower altitudes
(radiuso15 mm) and broadening with increasing height. However,
spectra were narrow even at higher levels over Pathankot during the
pre monsoon conditions. DSD spectra were very broad at Guwahati
and Bareilly under the influence of active monsoon conditions. The
increase in width of the spectra can be associated with the existing
weather conditions, prevailing pollution and the origin of the air
mass (continental/marine). During the pre monsoon conditions
(Pathankot) the temperatures were very warm, the cloud base was
above 4 km, air was very polluted and the origin of the air mass was
5000
4000
Pathankot (4545 m)
Hyderabad (2815 m)
Bengaluru (1755 m)
Bareilly (735 m)
Guwahati (1980 m)
3000
2000
1000
0
0.0
0.5
1.0
1.5
2.0
HW-LWC (g m-3)
Fig. 5. Vertical variation of effective radius (RE) (top) and Liquid water content
(bottom) over different regions shown with different colors Viz. Pathankot: black,
Hyderabad: red, Bengaluru: green, Bareilly: blue and Guwahati: magenta. In the
top figure the continuous line with corresponding color represents the linear fit for
each region. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
Table 1
The number of clouds samples in the updraft regions, cloud base, the effective radius at cloud base, Growth rate of cloud droplets and the height where the linear fit line
crosses the 12 mm radius for different stations is given in the table.
Base station
Date
No of samples
Cloud
base (m)
RE at Cloud
base (lm)
Growth Rate of Cloud drops
with height (lm/km)
Height where
RE 412 mm(km)
Pathankot
Hyderabad
Bengaluru
Bareilly
Guwahati
28 May
15 June
1 July
24 August
4 September
43
30
45
28
8
4545
2815
1755
735
1980
3.77
4.01
1.78
3.55
7.14
1.46
1.74
1.99
1.30
1.92
10.42
7.42
6.45
5.51
4.26
84
S.B. Morwal et al. / Journal of Atmospheric and Solar-Terrestrial Physics 81–82 (2012) 76–85
super continental (cloud drop radiuso15 mm). Hyderabad associated with subdued monsoon activity also showed narrow and
single mode DSD spectra. Over, Bengaluru, though there was incursion of moisture above 700 hPa levels, the DSD spectra showed little
widening (cloud drop radius reached up to 20 mm). The clouds were
continental in nature at both the locations. The marine originated
clouds moved over Bareilly along the monsoon trough which were
responsible for widening of the DSD spectra. However, due to heavy
air pollution, these clouds did not precipitate below 5.5 km. The DSD
spectra were very broad and bimodal over the Guwahati region and
this may be due to the prevailing active monsoon conditions and the
influence of the cyclonic circulation in the head of the Bay of Bengal
(clouding as seen from satellite pictures).
The total concentrations of cloud droplets were highest at
Pathankot, the possible causes include: (i) this observation period
was during the pre-monsoon season, (ii) high pollution and lack of
cloud scavenging during the pre-monsoon season. The observed total
concentrations were lowest over the Guwahati region. The percentage of small cloud drops is very high up to 7–8 km over Pathankot
and minimum over Guwahati. Thus, over Pathankot the nature of the
cloud drops showed the influence of continental air masses (negligible contribution of the bigger size drops) and that over Guwahati
the influence of marine originated air masses was clearly evident
(major contribution of bigger size drops). Over Hyderabad, Bengaluru
and Bareilly the percentage contribution of the larger drops increased
with height and at some level it became more than the contribution
of the small drops, except for Hyderabad. The droplet growth rate as
seen from the height-RE graph is observed to be less over north
(Bareilly: 1.3 mm/km and Pathankot: 1.46 mm/km), intermediate
over central (Hyderabad: 1.74 mm/km) and highest over south
(Bengaluru: 1.99 mm/km) and northeast (Guwahati: 1.92 mm/km).
Thus, the change in the slope of the linear fit line depends on the
prevailing weather conditions, observed air pollution, size distribution of the cloud drops and their variation in height.
Acknowledgments
Authors are thankful to Ministry of Earth Sciences, Government
of India for financial support and to all CAIPEEX team members.
Thanks are also due to Prof. Daniel Rosenfeld, The Hebrew
University of Jerusalem, Israel. The authors are also thankful to
the anonymous referee for very fruitful suggestions and comments
which helped in improvement of the manuscript.
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