QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
Q. J. R. Meteorol. Soc. 135: 1630–1637 (2009)
Published online 25 June 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/qj.432
Notes and Correspondence
Differences in raindrop size distribution from southwest
monsoon to northeast monsoon at Gadanki
T. Narayana Rao,a * B. Radhakrishna,a K. Nakamurab and N. Prabhakara Raoc
a National
b Hydrospheric
Atmospheric Research Laboratory, Gadanki, India
Atmospheric Research Laboratory, Nagoya University, Nagoya, Japan
c
Sri Venkateswara University, Tirupati, India
ABSTRACT: Characteristics of raindrop size distribution (DSD) are studied during the southwest (SW) and northeast
(NE) monsoon seasons using 4 1/2 years of DSD measurements made at Gadanki (13.5◦ N, 79.2◦ E) by an impact-type
disdrometer. The observed DSD is found to be distinctly different in the NE monsoon from that of the SW monsoon. The
stratified DSD (based on rain rate) shows more small drops and fewer bigger drops in the NE monsoon than in the SW
monsoon, particularly in the low rain rate regime. This feature is not an anomalous one, rather observed consistently in
all the years. The diurnal variation of DSD (in terms of mass-weighted mean diameter, Dm ) plots in both monsoons show
large values of Dm in the evening hours. Several possible reasons, from instrumental problems to geophysical mechanisms,
for the observed DSD differences are examined. The wind field and the range of Dm values indicate that the DSDs
in the SW and NE monsoons are continental and oceanic in nature, respectively. Further, high temperature and intense
convective activity (vigorous updraughts) in the SW monsoon modify the DSD through evaporation, drop sorting, and
c 2009 Royal Meteorological Society
collision-coalescence processes. Copyright KEY WORDS
Indian monsoon; continental and oceanic rain; mass-weighted mean diameter
Received 16 September 2008; Revised 6 February 2009; Accepted 20 April 2009
1.
Introduction
Monsoons are the major systems for rainfall over the
Indian subcontinent and can affect most of the people living under their influence. Though the principal rainy season for the Indian subcontinent as a whole is the summer
monsoon (southwest (SW) monsoon, June–September),
some parts of India, particularly the east coast of the
southern peninsula, get about 30–50% of the annual
rainfall in the post-monsoon season (northeast (NE) monsoon, October–December) (Figures 1(a), (b)). The gross
features, such as the rainfall distribution, circulation patterns, onset date, and the variation of the monsoon on
different time-scales (intraseasonal, interannual and interdecadal) associated with these monsoons, are well documented (Webster et al., 1998; Kripalani and Kumar,
2004; Goswami, 2005; and references therein). What is
relatively less known are the microphysical properties of
rainfall systems (for example, the raindrop size distribution (DSD)) in these monsoon seasons, mainly due to the
paucity of reliable long-term DSD measurements over
Indian region. For the reasons mentioned above, southeast India becomes an ideal test bed to study the variation
of rainfall, including DSD, from SW to NE monsoon.
∗
Correspondence to: T. Narayana Rao, National Atmospheric Research
Laboratory, Gadanki–517 112, Chittoor District, Andhra Pradesh, India
E-mail: tnrao@narl.gov.in
c 2009 Royal Meteorological Society
Copyright A few attempts have been made earlier to see the differences in rain DSD using the data collected at Gadanki
(13.5◦ N, 79.2◦ E), located in the southeast peninsular
region. Rao et al. (2001), first noticed the differences in
rain DSD in these monsoon seasons. They obtained different reflectivity–rainfall rate (Z –R) relations in SW
and NE monsoon seasons. Subsequent studies, using
stratified DSD data into three groups based on R also
observed some differences in DSD from SW to NE
monsoon (Reddy and Kozu, 2003; Kozu et al., 2006).
However, as also reported by them, the rainfall classification was not considered seriously in their studies and they assumed that R in the ranges 1–3 and
10–30 mm hr−1 represented stratiform and convective
rain, respectively (Kozu et al., 2006). Therefore a better
classification/stratification of DSD is required to understand the differences in DSD in these monsoon seasons.
Moreover, the data utilized in the above studies are limited to few monsoon seasons. In the present study, the
DSD data collected over 4 1/2 years are utilized to
(1) examine the differences in DSD from SW to NE monsoon season, and (2) study the diurnal and interannual
variations of DSD. The above studies have been made
after stratifying the data properly into various classes,
based on R (section 2). The DSD variations are reported
in section 3. The possible reasons for the observed differences in DSD from SW to NE monsoon are discussed in
section 4.
MONSOON RAINDROP SIZE DISTRIBUTIONS
1631
difference between the accumulated rain is less than 15%.
The loss of data due to this quality control is insignificant
(∼10%). A total of 286 events from ∼40 000 1-minute
samples are retained for further analysis.
As mentioned above, earlier studies stratified the DSD
measurements into only three rain rate classes. In the
present study, the quality-controlled data are stratified
into different classes (11) based on R. Five groups with
different class intervals (or different intervals of R) are
considered (Figure 3(a)). The class intervals are selected
randomly. For selecting the ideal group, the following
criteria are used: (1) the number of data points should
be large in each class, so that the results will be robust,
and (2) the mean R for each class interval should be
nearly equal in both the seasons. As can be seen from
Figures 3(b) and (c), group 1 satisfied all the above
conditions and is therefore considered here for stratifying
the DSD data.
3.
Figure 1. Spatial distribution of seasonal rainfall, retrieved from highresolution (1◦ × 1◦ ) rainfall maps generated by the India Meteorological Department (Rajeevan et al., 2006), and vector winds at
850 hPa from NCEP for the (a) SW monsoon and (b) NE monsoon, respectively. This figure is available in colour online at
www.interscience.wiley.com/journal/qj
2.
Data and methodology
The Joss and Waldvogel (1969) disdrometer (RD-69)
(hereafter JWD) is used to measure high-resolution (1minute) DSD at Gadanki. The range of raindrops that a
JWD measures is 0.3–5.1 mm and it arranges them in
20 channels. Standard algorithms are utilized to convert
the DSD into integral rainfall parameters. The pros and
cons of the instrument are well known and are discussed
in several earlier papers (Tokay et al., 2001; Atlas and
Ulbrich, 2006). The JWD measurements are available for
four SW (1999, 2000, 2006 and 2007) and five NE (1997,
1999, 2000, 2006 and 2007) monsoon seasons. Since the
data are taken from different years and also given the
instrument’s sensitivity to acoustic noise, strong winds
and electrical interference, it is important to ascertain the
quality of the data before using it for further analysis. A
collocated optical rain gauge (ORG-815), which measures
the rainfall rate at a resolution of 1 min, and a tippingbucket rain-gauge are used to check the quality of the
data. The tipping-bucket rain-gauge measurements are
available only from 2006. The accumulated rainfall in
each event measured by these instruments is compared
in Figure 2. An event is defined as the period in which
rainfall is continuous, although relatively small gaps
(<1 hour) are ignored. However, if the time gap in the
rainfall exceeds 1 hour, then the events are considered as
separate. Figure 2 shows that the accumulated rainfalls
measured by different instruments compare very well
in all the cases, except for very small rain amounts
(<1 mm). The data are retained for further analysis if the
c 2009 Royal Meteorological Society
Copyright Observations
Figure 4 shows the variation of number of raindrops per
unit volume per diameter range, N (D), with raindrop size,
D, for each class interval in SW and NE monsoon seasons. In general, the distributions look similar in both
monsoons, for all rain rate classes, nearly linear at low
rain rates and show curvature at large rain rates (with
increasing curvature with rain rate). It is clearly apparent from the figure that the number of smaller drops
is larger in the NE monsoon than in the SW monsoon
for the same rain rate. As expected, a reverse feature is
seen at the bigger drop end, i.e. relatively more bigger
drops in the SW monsoon for the same R. The diameter at which the number of drops is nearly equal in both
the monsoon seasons changes with R, being 0.77 mm
for R < 1 mm hr−1 , 1.12 mm for 1 < R < 8 mm hr−1 ,
and 1.5 mm for R > 8 mm hr−1 . The significant difference in N (D) from SW to NE monsoon is pronounced
in low rain rate classes (R < 4 mm hr−1 ). This difference seems to be weakening continuously with increase
of R and almost negligible in the last two rain rate classes
(R > 32 mm hr−1 ). It is partly due to the problems associated with JWD in heavy rain (Tokay et al., 2001) and
partly geophysical (Sauvageot and Lacaux, 1995) and will
be discussed in detail in the next section. The contribution of number of drops in the NE monsoon season to
the total number of drops (NE+SW) is ∼60–90% in the
smaller drop regime during weak to moderate rain. As
expected, an opposite feature is seen at the bigger drop
end with a higher fraction of bigger drops in the SW
monsoon season.
To examine whether the observed differences in DSD
between the SW and NE monsoon seasons is a regular feature or an anomalous one, the above analysis is
repeated on data for individual years. The mass-weighted
mean diameter, Dm , is considered to compare the DSDs
in SW and NE monsoon seasons and also to study the
interannual variability. Dm is the ratio of fourth to third
moment of DSD. Figures 5(a), (b) show the difference in
Q. J. R. Meteorol. Soc. 135: 1630–1637 (2009)
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T. NARAYANA RAO ET AL.
Figure 2. Comparison of accumulated rainfall estimated from ORG, disdrometer and tipping-bucket rain-gauge measurements for
each event. Tipping-bucket rain-gauge measurements are available only from 2006. This figure is available in colour online at
www.interscience.wiley.com/journal/qj
Figure 3. (a) Stratification of DSD data based on R into different groups with varying class intervals. Different symbols are shown for different
groups and the locations of the symbols on each line indicate the rain rate interval for each class. (b) The mean difference in R between SW and
NE monsoons in each class interval for all groups. (c) The number of 1-minute DSD spectra in each class interval from SW and NE monsoon
season for all groups. This figure is available in colour online at www.interscience.wiley.com/journal/qj
R between the SW and NE monsoons in each class and
the variation of mean Dm as a function of class for each
year. The mean difference in R between the SW and NE
monsoons for individual years is very small (<1 mm hr−1
for first eight class intervals and <7 mm hr−1 in last
three class intervals), indicating that the differences in
DSD, if any, are not due to differential R. Although
there exists some interannual variability in both SW and
NE monsoon seasons, the composite Dm profiles (solid
and dashed lines in Figure 5(b)) are distinctly different
in SW and NE monsoon seasons. It is clearly apparent
from the figure that Dm is larger in the SW monsoon
than in the NE monsoon in all classes, except for the last
class (or last two classes in some years). A difference
of 0.2–0.4 mm is observed in mean Dm between the
SW and NE monsoon in all classes, except in heavy rain
(R > 32 mm hr−1 ). This clearly indicates that the difference in DSD between SW and NE monsoons is a common
feature in all years. The observed large and small values
c 2009 Royal Meteorological Society
Copyright of Dm in SW and NE monsoons, respectively, are consistent with Figure 4. A large number of small drops reduces
the Dm value, as seen in the NE monsoon season, while
the presence of fewer small drops and relatively more big
drops in the SW monsoon increase the Dm in that season.
Recently, Rao et al. (2004) and Kozu et al. (2006) have
seen significant differences in the diurnal pattern of rain
in the SW and NE monsoon seasons. They observed large
amplitude in the diurnal variation of rainfall in the SW
monsoon with an evening to midnight peak, while only
a weak diurnal variation in the NE monsoon season. To
examine whether these different diurnal rainfall patterns
have any implications for DSD, the data in each monsoon
season are stratified into four categories with 6-hour
intervals centred on 0000, 0600, 1200, and 1800 local
time (LT). The differences in R between NE and SW
monsoon seasons for these categories and corresponding
mean Dm values are plotted as a function of class interval
in Figures 6(a) and (b), respectively. The variation of
Q. J. R. Meteorol. Soc. 135: 1630–1637 (2009)
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MONSOON RAINDROP SIZE DISTRIBUTIONS
1633
Figure 4. The averaged DSD spectra with standard error for each of 11 class intervals for SW (solid) and NE (dashed) monsoons. This figure is
available in colour online at www.interscience.wiley.com/journal/qj
Figure 5. (a) The mean difference in R between SW and NE monsoons and (b) the variation of seasonal mean Dm with each class interval in
different years. This figure is available in colour online at www.interscience.wiley.com/journal/qj
mean difference in R between SW and NE monsoons
with class interval is similar to that of seen in Figure 5(a).
Figure 6(b) also shows a clear separation of Dm profiles
in SW and NE monsoon seasons, except for the last
class. The mean Dm values in the SW monsoon are
c 2009 Royal Meteorological Society
Copyright larger than their counterparts in the NE monsoon by
0.15–0.5 mm, however 72% of the values are within
0.2–0.4 mm. In general, Dm values are large during
the evening (1500–2100 LT) and small during early
morning (0300–0900 LT) in most of the classes in both
Q. J. R. Meteorol. Soc. 135: 1630–1637 (2009)
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T. NARAYANA RAO ET AL.
Figure 6. As Figure 5, but for the diurnal variation of Dm . This figure is available in colour online at www.interscience.wiley.com/journal/qj
monsoon seasons. The diurnal variation is clearly visible
in the moderate to heavy rain (R > 8 mm hr−1 ) in both
monsoon seasons. From Figures 5(b) and 6(b), one can
clearly see large values of Dm in the last two classes,
but the variability (both interannual and diurnal) is not
very clear. This is mainly due to the limitation of JWD
measurements in heavy rain.
4.
Discussion
The variability of DSD within the storm, from storm to
storm and from one region to the next, has been elucidated in many earlier studies (Rosenfeld and Ulbrich,
2003, and references therein). It is important to understand whether the observed DSD variability is random
or associated with any complex microphysical process.
Several earlier studies have shown that the DSDs are not
random, but can be reproduced under homogeneous environmental conditions (Jameson and Kostinski, 2001), and
have investigated the variability of DSD and statistics
of DSD parameters as a function of R (Sauvageot and
Lacaux, 1995; Tokay et al., 2001; Nzeukou et al., 2004;
Kozu et al., 2006). Therefore, the DSDs stratified based
on R should look similar, unless they are fundamentally
different. We now examine several possible reasons, starting from instrumental limitations to geophysical, for the
observed differences in DSD during SW and NE monsoon
seasons.
It is well known that the JWD underestimates smaller
drops in moderate to heavy rain, due primarily to the
increase in acoustic noise. The underestimation is seen
clearly in comparisons between the DSDs measured by
JWD and by 2D video disdrometer (Williams et al., 2000;
Tokay et al., 2001). On the other hand, Sauvageot and
Lacaux (1995) and Atlas and Ulbrich (2000) argued that
the observed fewer small drops during heavy rain in the
Tropics is a real atmospheric phenomenon and attributed
it to drop sorting and evaporation. Notwithstanding these
c 2009 Royal Meteorological Society
Copyright arguments, the acoustic noise was measured in both the
monsoon seasons, when all the known acoustic noise
sources were turned on (e.g. generator, a radio acoustic
sounding system (RASS) separated by a distance of
100 m), except for rain. The acoustic noise was found
to be in the range of 50–55 dB in both seasons. During
rain, the acoustic noise may increase due to the impact
of raindrops on the rooftop where the disdrometer is
placed. Therefore, it is possible that the instrument may
underestimate smaller drops and thereby increase Dm . But
this problem, if at all present, will exist in both monsoon
seasons.
Ulbrich (1983) observed a wide distribution of raindrops with large numbers of smaller drops (small median
volume diameter, D0 ) in orographic rain. The terrain surrounding Gadanki (shown in Figure 7(a)) is complex
with many small hills (altitudes <1 km). Therefore, the
effect of complex topography on rainfall, in particular on
DSD, is difficult to assess. In general, orographic convection generates small D0 or Dm values (<1 mm) even
in the presence of moderate R (Rosenfeld and Ulbrich,
2003). The R versus Dm plot for SW and NE monsoons (Figure 7(b)) depicts Dm always greater than 1 mm
for R > 8 mm hr−1 . We therefore assume the orographical influence is either negligible or the same in both
seasons.
The differences in DSD in different monsoon seasons
can arise due to several geophysical processes, either at
the formation stage of the cloud or in the evolution phase.
As shown in Figures 1(a) and (b), at Gadanki the wind
flow in the SW monsoon is over the continent, while in
the NE monsoon it is over the ocean (Bay of Bengal).
Therefore, the rainfall and DSD are mostly continental
and oceanic in nature during SW and NE monsoon
seasons, respectively, consistent with the observations
of Kozu et al. (2006). The ranges of Dm observed
(Figures 5(b) and 6(b)) during the SW and NE monsoons
are also in good agreement with Dm and D0 values
reported in continental and oceanic locations elsewhere
Q. J. R. Meteorol. Soc. 135: 1630–1637 (2009)
DOI: 10.1002/qj
MONSOON RAINDROP SIZE DISTRIBUTIONS
(a)
(b)
Figure 7. (a) Topography around Gadanki. The location of Gadanki
is marked by a star. (b) The scatter plot between Dm and R for SW
and NE monsoon seasons. This figure is available in colour online at
www.interscience.wiley.com/journal/qj
(Tokay et al., 2002; Rosenfeld and Ulbrich, 2003; Bringi
et al., 2003; Ulbrich and Atlas, 2007). For instance,
during moderate to heavy rain (R > 10 mm hr−1 ), 80% of
the Dm population at Gadanki is in the range 1.5–2.6 and
1.3–2.2 mm in the SW and NE monsoons, respectively.
Further, all the above studies observed relatively fewer
small drops and more big drops, and therefore large
D0 and Dm values, in continental precipitation than in
maritime precipitation, consistent with our observations
(Figures 5(b) and 6(b)).
The differences in microphysical processes from SW
to NE monsoon during the descent of rainfall also play
a crucial role in modifying the DSD. The DSD evolves
into an equilibrium distribution under the influence of
processes such as collision, coalescence and break-up,
if the 0◦ C isotherm level is sufficiently high (Hu and
Srivastava, 1995; Atlas and Ulbrich, 2006). Therefore, if
the height of melting level is different in these seasons,
it may cause some differences in rain DSD at the
surface. However, Rao et al. (2008) found no significant
difference in the height of the melting level from SW to
NE monsoon at Chennai (120 km away from Gadanki).
But the surface temperature and convective activity are
different in these seasons. For instance, the mean surface
c 2009 Royal Meteorological Society
Copyright 1635
temperatures in the SW and NE monsoons are 28◦ C
and 23.7◦ C, respectively. Similarly, the number and the
percentage of R greater than any threshold value for
convection (10, 20, and 30 mm hr−1 ) are larger in the
SW monsoon than in the NE monsoon (Figure 8). High
temperatures in the SW monsoon cause evaporation of
raindrops. Earlier studies at Gadanki using VHF and UHF
profilers have noticed significant evaporation of raindrops
in many precipitating systems during the SW monsoon
(Rao et al., 2006; Kirankumar et al., 2008). Evaporation
reduces the number of smaller drops and increases
Dm , as seen in the present study. Further, the intense
convective activity (and vigorous updraughts) in the SW
monsoon affects rain DSD in two ways: drop sorting
and enhancing the collision–coalescence process. Strong
updraughts carry small drops aloft into divergent regions
above updraughts, but allow bigger drops to precipitate
locally, thereby increasing Dm values. The updraughts
can also hold drops aloft, thereby increasing the chance of
collision and also the size of the drop. As seen above, both
these processes increase Dm , the former by not allowing
the smaller drops to fall and the later by consuming the
smaller drops for the growth of medium-sized drops. The
efficacy of these microphysical processes (evaporation
and due to updraughts) is more in afternoon – evening
hours. Therefore, one can expect large Dm values during
this period. Indeed, Dm values are found to be relatively
large in the evening hours (1500–2100 LT) in both
monsoons.
Strong updraughts and downdraughts are generally
observed in convection. Therefore, one can expect large
differences in Dm values between the seasons in the last
two classes (R > 32 mm hr−1 ). However, as discussed
above, the disdrometer severely underestimates smaller
drops in heavy rain. The effect of evaporation and
drop sorting is more on smaller drops. Because of
the underestimation of these drops, the differences in
DSD are not clearly apparent in the last two classes.
But, inspection of data reveals that the N (D) values
are, indeed, larger in the NE (SW) monsoon than in
the SW (NE) monsoon at the smaller (bigger) drop
end. Nevertheless, because of instrumental problems, the
differences are not as pronounced as in the case of
weak – moderate rain (R < 16 mm hr−1 ).
This result has important implications on rainfall estimation with scanning weather radars and space-borne
radars (such as the precipitation radar in the Tropical
Rainfall Measuring Mission – TRMM). Different DSDs
in different seasons will lead to different Z –R relationships (Figure 9). The prefactor and exponent of the relations are completely different from each other and also
from that used at other geographical locations (Rosenfeld
and Ulbrich, 2003; Ulbrich and Atlas, 2007; and references therein). Thus, the usage of a single Z –R relation
for converting radar reflectivity into R will underestimate
in one season and overestimate in the other season. Also
the seasonal differences in DSD will have implications
on cloud modelling studies and parametrization schemes
that are imperative for passive remote sensing of rainfall
(Ferrier et al., 1995).
Q. J. R. Meteorol. Soc. 135: 1630–1637 (2009)
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T. NARAYANA RAO ET AL.
Figure 8. The distribution of R (in terms of percentage occurrence) in SW and NE monsoon seasons. This figure is available in colour online at
www.interscience.wiley.com/journal/qj
and more big drops than their counterparts in the NE
monsoon. This feature is not anomalous, but seen in all
years, indicating that the observed distributions are characteristic features of the respective monsoons. Several
possible reasons for the observed differences in DSD
are examined, including instrumental problems to geophysical mechanisms. From the observed wind field and
similarities in the distribution (and the range) of Dm during the SW (NE) monsoon and continental (oceanic) DSD
elsewhere (Tokay et al., 2002; Bringi et al., 2003; Rosenfeld and Ulbrich, 2003; Ulbrich and Atlas, 2007), it is
considered that the rain DSD at Gadanki in the SW and
NE monsoons are, respectively, continental and oceanic
in nature. Except for the first study mentioned above,
all other studies used ensembles of rain DSDs collected
from several geographical locations with an assumption
that all oceanic (continental) DSD are influenced by similar microphysical processes. The study by Tokay et al.
(2002), similar to the present study, has shown that the
DSD at a single location can be influenced by both
oceanic and continental precipitating systems. Nevertheless, the range of Dm or D0 values in all these studies
is nearly equal. Even though the primary control on the
observed DSD differences comes from the nature of the
rain (oceanic or continental), the evolution of DSD is also
important. As discussed above, the predominance of processes like evaporation, drop sorting and enhancement of
collision–coalescence processes during the SW monsoon
reduces the number of smaller drops and increases Dm .
The diurnal variation of Dm is also consistent with this
view.
Figure 9. The reflectivity (Z) – rain rate (R) relations for SW
and NE monsoon seasons, showing the seasonal differences in the
prefactor and exponent. This figure is available in colour online at
www.interscience.wiley.com/journal/qj
5.
Acknowledgements
The authors thank IMD and NCAR-NCEP for providing
rain and wind information, respectively.
Conclusions
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