-
Indian Journal of Radio & Space Physics Vol 43, February
2014, pp 57-66
Raindrop size distribution variations in JAL and NILAM cyclones
induced precipitation observed over Kadapa (14.47oN, 78.82oE), a
tropical
semi-arid region of India
N P Amrutha Kumari, S Balaji Kumar, J Jayalakshmi & K
Krishna Reddy$,*
Semi-arid-zonal Atmospheric Research Centre (SARC), Department
of Physics, Yogi Vemana University, Kadapa 516 003, Andhra Pradesh,
India
$E-mail: [email protected]
Received 16 April 2013; revised 10 June 2013; accepted 12 June
2013
Raindrop size distributions (RSD) of JAL (7 November 2010) and
NILAM (29 October 2012) cyclones induced precipitation were
measured with PARticle SIze and VELocity (PARSIVEL) disdrometer
deployed at Kadapa (14.47°N, 78.82°E), a semi-arid continental site
in Andhra Pradesh. Small and mid drops below 2 mm diameter have
higher concentration in JAL and NILAM cyclones. RSD characteristics
stratified on the basis of rainrate showed that the concentration
of small (large) drops is higher (lesser) for JAL cyclone than
NILAM cyclone. The concentration of mid size drops of JAL cyclone
are greater than or equal to that of NILAM cyclone. The JAL cyclone
induced precipitation is associated with higher (lesser)
concentration of small drops (small and mid drops) in stratiform
(convective) region than that of NILAM cyclone precipitation. JAL
cyclone has long duration of stratiform rainfall with smaller
raindrop compared to NILAM cyclone, which had a short duration of
stratiform rainfall with more number of mid and large drops. In
both convective and stratiform regimes, the coefficient value of
Z-R relations is higher in NILAM cyclone than JAL cyclone. The
average mass weighted diameter, Dm of JAL cyclone is smaller
(larger) in stratiform (convective) than that of NILAM
precipitation.
Keywords: Raindrop size distribution (RSD), Rainrate, Mass
weighted diameter, Cyclone induced precipitation, Z-R
coefficient
PACS Nos: 92.60.jf; 92.60.Qx
1 Introduction Knowledge of raindrop size distribution (RSD)
is
essential in determining the characteristics of precipitation.
Precipitation is an integral product of RSD and is highly variable
in space and time. The variability of precipitation is directly
linked to the variability of RSD. Over Southern India, the major
period of rainfall is October to December, particularly the eastern
half of the peninsula. By October, a low pressure establishes over
the central and southeast Bay of Bengal, moving southward as the
season progresses. Under the impact of this low pressure area,
tropical cyclones originate in the Bay of Bengal between 8°N and
14°N and influence the southern peninsula of India. The process of
raindrop formation, growth, transformation and decay occur on a
microphysical scale within a cyclone. Each process, such as
condensation growth, evaporation or collision/coalescence, leaves a
signature on RSD of a rain event. The rain parameters, like
rainrate, radar reflectivity, liquid water content and rainfall
accumulation can be determined with the help of RSD
measurements1. For weather radar, a relationship between radar
measured reflectivity, Z (in dBZ), and surface rainrate, RR (in mm
h-1), has been traditionally derived by employing RSD
measurements1. The characteristic differences in RSD of
precipitations results in significant errors in radar rainfall
estimation. Moreover, detailed information about the RSD is
essential in cloud microphysical processes2, numerical weather
modeling3 and weather radar applications4,5. This shows that a
concrete effort is needed in understanding the RSD of various rain
regimes. There are a number of studies carried out over the globe
on RSD characteristics in terms of diurnal, seasonal variation6,7,
different precipitations8-10 and type of precipitation11-13.
Systematic analysis on the form of RSD, its temporal and
rainrate dependent evolution at the surface and also aloft is
essential in understanding the process of rainfall formation.
However, there are few studies on RSD characteristics of
cyclones/typhoons
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
58
over the globe14,15. The differences in RSD characteristics
prior to and during the passage of remnants of Hurricane Helene
(2000) are presented by Ulbrich & Lee16. Typhoon systems RSD
are reported by Tokay et al.17 and Chen et al.18. The RSD of
cyclonic precipitation is studied by Radhakrishna & Rao19.
Recently, RSD variations in cyclonic and northeast monsoon
thunderstorm precipitation are investigated by Kumar & Reddy20.
The earlier studies
are mainly focused on seasonal or intra-seasonal variation of
cylone activity. However, in the present paper, an attempt is made
to get the RSD variations between two cyclones (JAL and NILAM)
induced precipitations using a laser disdrometer. In addition, the
one minute resolution RSD of two cyclone events are categorized
into stratiform and convective rainfall as well as the physical
reasons for the corresponding spectral shape.
2 Data and Methodology
For the present study, data obtained from a ground-based laser
(PARSIVEL) disdrometer, installed at Yogi Vemana University
(14.47°N, 78.82°E) in Kadapa district, during the land fall of JAL
and NILAM cyclone is utilized. The observational site, Kadapa, is
indicated by square box in Fig. 1. The infrared cloud coverage
images (obtained from the Kalpana satellite) of JAL and NILAM
cyclones are shown in Fig. 2. Complete details of measurement
technique along with the assumptions made in determining the size
and velocity of hydrometeors from the PARSIVEL disdrometer are
provided by Loffler-Mang & Joss21, Tapidor et al.22, Jaffrain
et al.
23 and in brief by Kumar & Reddy20. The raindrop
concentration N(D), in mm–1m–3,
at an instant of time from the PARSIVEL disdrometer counts is
obtained from the following equation:
Fig. 1 — Track of JAL and NILAM cyclones, observational site
location (Kadapa) and Chennai IMD Doppler radar location (denoted
with square boxes)
Fig. 2 — Cloud coverage images of: (a) JAL (7 November 2010) and
(b) NILAM (31 October 2012) cyclones observed from Kalpana
satellite images
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AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES
OVER KADAPA
59
( )32
ij
i
j=1 j i
N =.∆ . .∆
nD
A t V D∑ …(1)
where, nij, is the number of drops reckoned in the size bin i
and velocity bin j; A, the sampling area in m2; ∆t, time in
seconds; Di, the drop diameter in mm for the size bin i; ∆Di, the
corresponding diameter interval; and Vj, the fall speed in ms
-1 for the velocity bin j. From the raindrop concentration N(D),
the radar reflectivity factor Z in mm6 m–3 and rainrate RR in mm
h-1 are derived by:
( )32
6
i i i i
j=1
Z = N( ) ∆D D D D∑ …(2)
32 323
j i i i4i=1 j=1
6πRR= N( ) ∆
10V D D D∑∑ …(3)
The mass-weighted mean diameter Dm in mm, shape parameter µ (-)
and slope parameter Λ in mm–1 are briefly explained by Kumar &
Reddy20.
3 Comparison of disdrometer and tipping bucket
rain gauge A large number of drop sizing instruments have
been used in the past in the measurements of the RSD. They can
be divided into different groups depending on the physical
principle used: impact disdrometers, PARSIVEL disdrometer/optical
disdrometers, and Doppler radar disdrometers. All these instruments
are able to operate continuously and unattended. It is essential to
determine the accuracy of PARSIVEL disdrometer in measuring the
observed rainrates. The rainrates calculated from the RSD measured
by disdrometer were compared with the values measured
by a tipping bucket rain gauge as integral part of Automatic
Weather Station (AWS) at the same site. The disdrometer and AWS are
spatially separated about 6 m in the Yogi Vemana University
Meteorological Observatory. For the present study, three months
(September to November 2012) precipitation data was collected from
disdrometer and AWS have been utilized to understand the accuracy
of the PARSIVEL disdrometer in measuring the observed rainrates.
The 5-minutes averaged rainrates calculated from the RSD measured
by the disdrometer were compared with the values measured by a rain
gauge at the same site. Figure 3 demonstrates the regression
relation [Fig. 3(a)] and deviation [Fig. 3(b)] between the two
sources of measurement. The least square fitting of the two
measurements is close to 0.95. The observational results are in
fairly good agreement and acceptable in view of the differences in
the measurement technique.
4 RSD analyses of JAL and NILAM cyclones
induced precipitation
4.1 Comparison of raindrop concentration for JAL and
NILAM cyclones precipitation
The time series of the RSD during the passage of JAL cyclone on
7 November 2010 revealed three different segments (S1, S2 and S3)
that were separated by rain intermittence of half an hour to an
hour with total rain accumulation of 41.0 mm occurred for a period
of 8 hours 49 minutes [Fig. 4(a)]. The raindrops below 1 mm
diameter were considered as small drops, above 3 mm as large drops
and those in the diameter range 1-3 mm as mid-size drops11,20. The
first segment (S1) occurred for
Fig. 3 — (a) Linear regression and (b) deviation comparison of
rainrate (RR, mm h-1) measured with PARSIVEL disdrometer and
tipping bucket rain gauge
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
60
42 minutes during 06:33 - 07:15 hrs IST. The highest
reflectivity of 27.6 dBZ, and maximum rainrate 1.8 mm h-1 with a
maximum drop concentration of 218.8 m-3 mm-1 and rainfall
accumulation of 0.5 mm was observed during this event. The
concentration of small drops was more compared to mid-size and
large drops during this segment. The second segment (S2) lasted
over 2 hours 41 minutes from 08:01 to 11:46 hrs IST with rain gaps
ranging from one minute to seventeen minutes with maximum rainrate
3.4 mm h-1, reflectivity 34.1 dBZ with a maximum concentration of
2249.1 m-3mm-1 and rainfall of 0.9 mm. The concentration of the
second segment is in between first and third segment. In this
segment (S2), relatively low concentrations (1000-1500 m-3 mm-1) of
small drops were observed. The third and the most intense regime
(S3) of the cyclonic precipitation had 5 hours 26 minutes of
continuous rainfall from 12:34 to 24:00 hrs IST. In this segment,
maximum rainrate of 55.6 mm h-1, radar reflectivity of 46.5 dBZ and
drop concentration of 4305.3 m-3 mm-1 with a rain accumulation of
39.6 mm was observed. At the end of the third portion from 21:00
hrs IST, large concentrations of small drops were present, while an
appreciable number of medium drops were also present.
Interestingly, higher reflectivity and rainrate values were
observed during this third segment of the cyclonic precipitation
between 21:00 and 23:00
hrs IST. Relatively high concentrations of mid-size and large
drops are responsible for the heavy rain and high reflectivity. The
rain integral parameters values of these three segments (S1, S2 and
S3) are given in Table 1.
The time series of the RSD of NILAM cyclonic precipitation,
occurred on 31 October 2012, revealed three different segments (S1,
S2 and S3) that were separated by rain intermittence of thirty nine
minutes to one hour eight minutes with total rain accumulation of
32.6 mm for a period of 8 hours 27 minutes [Fig. 4(b)]. The first
segment (S1) occurred for 48 minutes between 09:28 and 10:46 hrs
IST. The
Fig. 4 — Time series of raindrop size distributions of: (a) JAL
(7 November 2010) and (b) NILAM (31 October 2012) cyclones
precipitation days
Table 1 — Maximum values of rainrate (RR, mm h-1), radar
reflectivity (Z, dBZ) and raindrop concentration [N(D),
m-3mm-1]
of JAL (7 November 2010) and NILAM (31 October 2012) cyclonic
precipitation day in different segments (S1-S3)
Parameter, unit JAL cyclonic precipitation day
NILAM cyclonic precipitation day
S1 S2 S3 S1 S2 S3
Duration, min 42 161 326 48 188 271
RRmax, mm h-1 1.8 3.4 55.6 4.06 12.59 60.27
Zmax, dBZ 27.6 34.1 46.5 32.81 37.12 47.92
N(D)max, m-3 mm-1
218.8 2249.1 4305.3 3247.0 3396.0 3329.0
Rainfall, mm 0.5 0.9 39.6 1.3 6.2 25.1
Total rainfall, mm
41.0 32.6
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AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES
OVER KADAPA
61
maximum reflectivity of 32.81 dBZ, maximum rain intensity of
4.06 mm h-1 with a maximum drop concentration of 3247.0 m-3 mm-1
and rainfall accumulation of 1.3 mm was observed during this event.
The concentration of small drops was more compared to mid-size
drops during this segment. The second segment (S2) occurred for 3
hours 8 minutes between 11:25 and 15:16 hrs IST with a rain gap of
11 minutes. In this segment, maximum reflectivity of 37.12 dBZ,
maximum rain intensity of 12.59 mm h-1, and maximum drop
concentration of 3396.0 m-3 mm-1 with rainfall accumulation of 6.2
mm was observed. The third segment (S3) occurred for 4 hours 31
minutes between 16:24 and 21:34 hrs IST with rain gaps ranging one
minute to eight
minutes. In this segment, maximum reflectivity of 47.92 dBZ,
maximum rain intensity of 60.27 mm h-1 with a maximum drop
concentration of 3329.0 m-3 mm-1 and rainfall accumulation of 25.1
mm were observed. The rain parameter values of these three segments
(S1, S2 and S3) are given in Table 1.
4.2 Variation of RSD with rainrate
The RSD of JAL (7 November 2010) and NILAM (31 October 2012)
cyclone precipitations at different rainrate ranges are depicted in
Fig. 5. The rainrates (RR) of JAL and NILAM cyclonic precipitations
have been classified into eight categories (0.5
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
62
averaged to get the mean RSD of each rainrate range. In the
rainrate range 0.5-1.0 mm h-1, 104 minutes of JAL cyclonic
precipitation and 42 minutes of NILAM cyclonic precipitation are
observed [Fig. 5(a)]. In this rainrate range, mid-size drops up to
2 mm have same concentration in both JAL and NILAM cyclonic
precipitations. For the rainrate range 1-5 mm h-1, 289 minutes of
JAL cyclonic and 168 minutes of NILAM cyclonic precipitations are
observed [Fig. 5(b)]. In this rainrate range, concentration of
small drops is higher in JAL cyclone than NILAM cyclone and a
reverse pattern is observed for mid and large drops. In the
rainrate range 5-10 mm h-1, 60 minutes of JAL cyclonic and 62
minutes of NILAM cyclonic precipitations are observed [Fig. 5(c)].
In the rainrate range 10-15 mm h-1, 31 minutes of JAL cyclonic and
36 minutes of NILAM cyclonic precipitations are observed [Fig.
5(d)]. In these two rainrate ranges (5
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AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES
OVER KADAPA
63
convective precipitation for a period of 22 minutes and
stratiform precipitation for 783 minutes. The NILAM cyclone is
associated with 500 minutes of stratiform precipitation and 33
minutes of convective precipitation. The raindrop concentration of
convective precipitation is higher than stratiform precipitation in
both JAL and NILAM cyclones [Fig. 7(a and b)]. In the stratiform
region [Fig. 7(c)], the JAL cyclone induced precipitation is having
high drop concentration up to 1.2 mm diameter than NILAM cyclone
precipitation and a reverse pattern is observed for raindrops above
1.2 mm diameter. In convective region [Fig. 7(d)], small drops are
associated with slightly higher concentration in JAL cyclone than
NILAM cyclone; whereas mid and large drops are having less
concentration in JAL cyclone than NILAM cyclone precipitation. The
Z= A*Rb relations of stratiform and convective regions of JAL and
NILAM cyclones induced precipitations are obtained from scatter
plots of radar reflectivity (Z, dBZ) and rainrate (RR, dBR) (Fig.
8). The coefficient 'A' and exponent 'b' of stratiform and
convective regions of JAL and NILAM cyclones are given in Table 2.
The JAL cyclone is having
less coefficient values in both convective and stratiform
regimes than NILAM cyclone. From the Z-R scatter plots of
stratiform and convective regions, it is clear that the JAL cyclone
induced precipitation is having raindrops of size less than or
equal to that of NILAM cyclone.
4.4 Variation of mean drop diameter (Dm), shape (µ) and
slope
(Λ) parameters
The variation of shape (µ, -) and slope parameter (Λ, mm-1),
mean drop diameter (Dm, mm) and total drop concentration (Nt, m
-3) with rainrate of JAL and NILAM cyclones induced
precipitation are shown in Fig. 9. From the figure, it is clear
that the range of variability of µ, Λ, Dm and Nt is more for JAL
cyclone than NILAM cyclone for the rainrate less than 20 mm h-1.
This variation decreases with increase in rainrate and becomes more
uniform above 20 mm h-1. The shape (µ, -) and slope parameter (Λ,
mm-1) decreases with the increase in rainrate and this decrease is
sharper in NILAM cyclone than JAL cyclone. The Dm increases with
increase in rainrate but this increase is steeper in NILAM cyclone
than the JAL cyclone. In the JAL cyclone induced precipitation, the
average Dm value is 1 mm for the
Fig. 7 — Raindrop concentration vs drop diameter of convective
and stratiform precipitation of JAL (7 November 2010) and NILAM (31
October 2012) cyclone precipitations
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
64
rainrate below 20 mm h-1 and 1.6 mm for above 20 mm h-1; and for
NILAM cyclone these values are 1.26 mm and 1.25 mm below and above
20 mm h-1 rainrate. In both the cyclones (JAL & NILAM), Nt
increases with the increase in rainrate with a large spread in Nt
in JAL cyclone than NILAM cyclone. The shape (µ, -), slope
parameter (Λ, mm-1), mean diameter
Fig. 8 — Radar reflectivity (Z, dBZ) and rainrate
(dBR=10*log10(RR), mm h-1) relations for convective and stratiform
regions of JAL
(7 November 2010) and NILAM (31 October 2012) cyclone
Fig. 9 — Variation of: (a) shape (µ, -); (b) slope parameter (Λ,
mm-1); (c) mean drop diameter (Dm, mm); and (d) total drop
concentration (Nt, m
-3) with rainrate (RR, mm h-1) for JAL (7 November 2010) and
NILAM (31 October 2012) cyclone precipitations
Table 2 — Z=a*R^b values of stratiform and convective regions of
JAL and NILAM cyclonic precipitation
Day Stratiform Convective
A b R^2 A b R^2
7 Nov 2010 96.04 1.33 0.9521 122.65 1.46 0.8688
31 Oct 2012 193.19 1.399 0.947 246.03 1.304 0.788
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AMRUTHA KUMARI et al.: RSD VARIATIONS IN JAL AND NILAM CYCLONES
OVER KADAPA
65
(Dm, mm) and total drop concentration (Nt, m-3) values
of stratiform and convective regions of JAL cyclone induced
precipitation and NILAM cyclone induced precipitation are given in
Table 3. From the values of Dm and Nt, it is clear that in the JAL
cyclone, the convective region is mainly composed of small to
mid-size drops rather than large drops, whereas the stratiform
region is composed of only small drops.
5 Drop size and fall velocity relation
Raindrop fall velocity is useful for the measurement of rain
integral parameters like radar reflectivity, liquid water content
and rainrate. Fall velocity plays an important role in rain related
studies in numerical
simulation and remote sensing2. Figure 10 (a and b) shows the
observed drop concentration as a function of drop diameter and the
fall velocity for the JAL and NILAM cyclone precipitation. For the
JAL cyclone induced precipitation, small and mid drops up to 2 mm
diameter have high concentration with fall velocities less than 4
ms-1, whereas for NILAM cyclone induced precipitation, small and
mid drops up to 2 mm diameter have high concentration with fall
velocity ranging 4-5 ms-1. There is a larger spread in the drop
fall velocities of mid and large drops in the NILAM cyclonic
precipitation compared to JAL cyclonic precipitation. 6 Results and
Conclusions
The raindrop size distribution (RSD) of JAL (7 November 2010)
and NILAM (31 October 2012) cyclones induced precipitations are
studied using PARSIVEL disdrometer deployed at Kadapa (14.47°N;
78.82°E ), a semi-arid, temperate, plateau climate region of India.
From rainrate comparison studies between disdrometer and rain
gauge, the rainrate data corrected for instrumental error, matches
very well with a tipping bucket rain gauge measured values at
5-minute resolution placed at the same site.
In the JAL cyclone induced precipitation, the concentration of
small drops is high compared to mid-size and large drops; whereas
in NILAM cyclone induced precipitation, the concentration of all
the drops (small, mid and large) is almost same. The maximum
raindrop diameter does not exceed 4 mm even at higher rainrates in
JAL cyclone induced precipitation but for NILAM cyclone induced
precipitation, raindrop diameter exceeds 4 mm at higher rainrates.
From the RSD characteristics, it is clear that both JAL and NILAM
cyclone precipitations contain more small and mid drops up to 1.3
mm diameter at lower rainrates (< 5 mm h-1). The JAL cyclone
precipitation is associated with lower rainrates (< 5 mm h-1)
having longer duration than NILAM cyclone precipitation. At the
lower rainrate range (
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INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014
66
1.5 mm (1.2 mm) diameter in stratiform (convective) regions have
higher concentration in JAL cyclone than NILAM cyclone
precipitation. From stratiform and convective regions of the Z-R
scatter plots, it is clear that the JAL cyclone induced
precipitation has raindrops of size less than or equal to that of
NILAM cyclone precipitation raindrops. There is a large spread in
total drop concentration (Nt, mm
-3), shape parameter (µ, -), slope parameter (Λ, mm-1) and mass
weighted diameter (Dm, mm) in JAL cyclone precipitation than NILAM
cyclone. A more spread in fall velocities of mid and large drops is
observed for NILAM cyclonic precipitation than JAL cyclone induced
precipitation. JAL cyclone has long duration of stratiform rainfall
with smaller raindrop when compared to NILAM cyclone, which had a
short duration of stratiform rainfall with more number of mid and
large drops.
It may be noted that the results presented in this study are
based on one disdrometer measurement, which represents the time
evolution of the raindrop size distribution over a single site.
More observation sites are needed to measure the spatial
variability of microphysical characteristics of tropical cyclone
precipitation.
Acknowledgement
The authors gratefully acknowledge India Meteorological
Department (IMD), Government of India for providing the JAL and
NILAM cyclone track information and Kalpana satellite images. One
of the authors (SBK) acknowledges the Ministry of Earth Sciences
(MoES), Government of India for providing the fellowship to carry
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