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Tropical Cyclone Research and Review
Volume 3, No. 2
TCRAIN – A DATABASE OF TROPICAL CYCLONE RAINFALL
PRODUCTS FOR NORTH INDIAN OCEAN
S. Balachandran, B. Geetha, K. Ramesh, and N. Selvam
Cyclone Warning Research Centre, Regional Meteorological Centre, Chennai, India
ABSTRACT
Analysis of tropical cyclone (TC) rainfall characteristics helps in understanding and improving quantitative
precipitation forecasts of TCs. Based on the Tropical Rainfall Measuring Mission (TRMM) rainfall data for
the period of 2000-2010, a TC rainfall database –Tropical Cyclone Rainfall Analysis for North Indian Ocean
(TCRAIN) is developed to study the precipitation characteristics during various stages of life cycle of TCs of
the North Indian Ocean (NIO). Three rainfall products, viz., frequency distribution of rain rate, azimuthally
averaged radial profile of rain rate and quadrant mean rain rate with respect to the TC centre and the direction
of motion of the TC are generated in a Lagrangian frame of coordinate system for 5 intensity stages of life cycle
of each cyclone over NIO. Using this TCRAIN database, composites of frequency distribution of rain rates as
well as quadrant mean rain rate for TCs of NIO are generated to bring out probable rain rates and asymmetric
structure in rainfall distribution. Rainfall profiles for individual cases are likely to deviate from the climatological profiles under different environmental and oceanic conditions. TCRAIN database would serve as a useful
tool for carrying out intensity based analytical studies on structure of rainfall associated with cyclones of NIO
through ready depiction of TC rainfall profiles.
Keywords: tropical cyclone, rainfall, radial profile, quadrant mean, TCRAIN
1.Introduction
India, having an extensive coastline is vulnerable to
the destruction caused by gale winds, storm surges and
heavy rains associated with landfalling tropical cyclones
(TCs) that form over the North Indian Ocean (NIO) basin
comprising of the Bay of Bengal (BOB) and the Arabian
Sea (AS). Understanding the structure and dynamics of
TC rainfall would help in generating reliable TC rainfall
forecasts. It has been shown that the rainfall distribution
around a TC is highly complex and is determined by
environmental factors such as wind shear, sea surface
temperature, moisture distribution as well as TC specific
factors such as intensity, location, translational speed and
direction of motion and local effects such as topography
and orientation of the coast (Chen et al. 2006; Corbosiero
and Molinari 2003; Rogers 2003). Consequently, TC
precipitation characteristics can vary greatly from one TC
to another and even from time to time for a particular TC.
However, owing to sparse data over oceanic regions, early
Corresponding author address: Dr. S. BALACHANDRAN, Cyclone
Warning Research Centre, Office of the Director General of Meteorology,
Regional Meteorological Centre, 6 College Road, Chennai - 600 006, Tamil
Nadu, India. E-mail: [email protected]
DOI: 10.6057/2014TCRR02.05
attempts on Quantitative Precipitation Forecasts for TCs
did not show good skill as they assumed a constant rain
rate, while in reality, precipitation distribution displayed
asymmetric nature. With the advent of satellite based
observations, several datasets covering oceanic regions
are being generated. Whereas the rain gauge observations
provide data over the land areas, satellite based
observations, supplement for the rainfall data associated
with a TC when it is over the ocean. The Tropical Rainfall
Measuring Mission (TRMM, available at http://trmm.
gsfc.nasa.gov, Kummerow et al. 1998), a joint U.S.Japan satellite mission to monitor tropical and subtropical
precipitation is an important initiative for generating
satellite based estimates of rain rates over the entire tropical
region of the globe. Several studies have been undertaken
based on satellite/radar estimates of rain rates around TCs
to understand the complexities in the asymmetry in rainfall
distribution.
Generally, (i) the radial profile of the azimuthally averaged rain rate (ii) frequency distribution of rain rate in different rainfall rate classes and (iii) the quadrant mean and
standard deviation of rain rate are used to study the asymmetry/symmetry structure of the precipitation distribution
around tropical cyclones. Whereas the composite approach
May 2014
BALACHANDRAN et al.
is used to bring out typical features smoothing out variabilities in individual cases, case studies would bring out salient features that draw attention for further studies. In case
of TCs both observational and numerical studies showed
significant variation of rainfall asymmetry from storm to
storm (e.g., Lonfat et al 2004). Hence development of a database of above mentioned rainfall products for individual
storms would help in validating results from numerical
simulation of TC rainfall features as well as to elucidate
TC rainfall features under different scenario like recurving
versus non-recurving TCs, under El Niño–La Niña conditions, during strengthening and weakening phases etc. In
the present study, the development of an analytical tool
Tropical Cyclone Rainfall Analysis for North Indian Ocean
(TCRAIN) which provides the following three products for
43 TCs that occurred over the NIO during the period 20002010 is presented:
a.Frequency distribution of rain rates within 500 km
from the TC centre
b.Azimuthally averaged radial profiles of mean rain rates
within 500 km from the TC centre
c.Quadrant-wise mean rain rates within 200 km from
the TC centre and with respect to the direction of
movement of the TC
2. Database and products development
As mentioned above, the present work is aimed at
generating profiles of (i) percentage frequency distribution
of rain rates, (ii) azimuthally averaged radial mean rain rate
and (iii) quadrant-wise mean rain rate for different intensity
stages of each TC of the NIO during the period 20002010. These products are generated using 3-hrly TRMM
3B42 V6 dataset. This dataset provides precipitation rate at
horizontal resolution of 0.25° X 0.25° at 3 hourly temporal
resolution over 50°N to 50°S. This dataset is prepared from
combination of TRMM precipitation RADAR, Microwave
Imager, Visible and Infrared Scanner. The 3B42 processing
has been designed to maximise data quality and has been
recommended for research work and further details are
available at http://disc.sci.gsfc.nasa.gov/. Durai et al.
(2010) and Srivastava (2011) showed that TRMM 3B42
V6 product is in good agreement with gauge observed data
over Indian region.
During the period 2000-2010, 43 TCs formed in the
NIO basin. According to India Meteorological Department
(IMD), low pressure systems are categorized, based on
maximum sustained surface wind speeds (MWS), as Low
(MWS <17 knots), Depression (D, 17-27 knots), Deep Depression (DD, 28-33 knots), Cyclonic Storms (CS, 34-47
knots), Severe Cyclonic Storms (SCS, 48-63 knots), Very
Severe Cyclonic Storm (VSCS, 64-119 knots), and Super
Cyclonic Storm ( SuCS, >120kts).
Using the IMD’s best track data of TCs (www.imd.gov.
in) which contain information on instantaneous position,
intensity and direction of motion of the TC, the life cycle
123
of a TC is stratified based on intensity during the growth
and decay of the TC and grouped into 5 stages as given
Fig. 1 (i.e.), the growing phase of the life cycle of the TC is
classified into 3 intensification stages, viz.,
1 the intensity categories of D, DD are categorised as
Stage-1 (Intensification stage 1 and indicated as iD);
2 the category CS is categorised as Stage-2 (Intensification
stage 2 and indicated as iCS);
3 the categories of SCS, VSCS and SuCS are grouped
under Stage-3 (Intensification stage 3 and indicated as
iSCS).
The decaying phase (i.e., when the intensity category of
the TC changes from the peak category to lower categories)
of the TC is classified into two stages of weakening, viz.,
1 the intensity category of CS during the decaying phase
is classified under Stage-4 (Weakening CS and indicated as
wCS)
2 the categories of DD and D during the decaying phase
as Stage-5 (Weakening D and indicated as wD).
Firstly, the direction of motion of TCs is determined
from the IMD’s best track data using position difference
method. Next, a moving coordinate system with the TC
centre as the origin and direction of motion of the TC as
the reference direction was considered. For each best track
position, the centre of the coordinate system is first shifted
to the centre of the TC and then the coordinate system is
rotated such that the direction of motion of the TC at that
specific instant of time coincides with the 0° azimuth (which
is taken as the positive Y-direction for the purpose of plot-
Fig. 1. Schematic representation of intensity stratification of a TC
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Tropical Cyclone Research and Review
ting). For this purpose, every grid point in the world coordinate system (longitude, latitude) are represented in polar co-ordinates (r, θ), in terms of radial distance from the
TC centre (r) and oriented at an angle (θ) with reference to
direction of motion of the TC . All 0.25°x0.25° rainfall data
are then represented in terms of radial distance from the TC
centre and with reference to the direction of motion of the
TC. This procedure is repeated for all 3hourly TRMM rainfall data.
Next, all three hourly rainfall data belonging to specific
category of intensification and weakening are grouped
together and the composite mean rainfall for each of these
category were generated. Using this composite mean
rainfall for each stage, the precipitation characteristics
are determined by computing (i) percentage frequency
distribution of rain rates, (ii) azimuthally averaged radial
mean rain rate and (iii) quadrant-wise mean rain rate as
detailed in the following sections.
3. Rainfall analysis products
The percentage frequency distribution of rain rates is
determined by considering all rain rates within 5° radius
(≈500 km from the TC centre) in respect of all the 3-hourly
instances of observation grouped under the specific
intensity category. For this purpose, the rain rates are
classified into nine classes (including no rain category)
as 0.0, 0.0-0.1, 0.1-0.2, 0.2-0.5, 0.5-1.0, 1.0-2.5, 2.5-5.0,
5.0-10.0 and >10.0 mm/hr and the frequency distribution
in each class is determined and expressed in percentage
by binning method for each intensity stage of the TC. A
sample product is shown in Fig. 2. The percentage of nonraining areas are computed and mentioned separately below
the graph.
Radial profile of rain rate provides information about
the radial rainfall variation with respect to the TC
centre. Radial profile of mean rain rates is obtained by
Fig. 2. Sample product of Frequency distribution of rain rates
within 5° radial distance
Volume 3, No. 2
first azimuthally averaging rain rates within an annulus
of very small thickness [0.1° (≈10 km)] for a particular
radial distance r, and then determining the same for all
radial distances from the TC centre up to 5° radius (≈500
km from the TC centre) for each instant of observation
and then determining the mean of all the mean rain
rates corresponding to each radial distance (annulus at
that radius) in respect of all observations in the specific
intensity category. For this purpose, for each observation,
the origin is shifted to the TC centre. Then an area within
5° radius (≈500 km) from the TC centre (origin) is taken
and divided in to 50 annular rings of 0.1° width (≈10 km).
Each data point within 5° radius from the TC centre is
then expressed in terms of radial distance from the centre
(r) and all rain rates within each annulus of radius r are
considered for determining the azimuthally averaged rain
rate at the radial distance r of the annulus from the TC
centre. Similarly, instantaneous profiles are obtained for all
observations grouped under a specific intensity stage from
which the mean profile of azimuthally averaged mean rain
rate for that intensity stage is determined. Similar profiles
are obtained for all intensity stages during the life cycle of
the TC. Fig. 3a depicts the steps schematically and Fig. 3b
provides a sample Radial Profile product.
The Quadrant-wise mean rain rates are obtained by
averaging all rain rates within 2 degree radius from the
TC centre with the direction of motion of the TC as the
reference direction or each instant of observation and then
determining the mean rain rate in each quadrant in the
specific intensity category of the TC. For this purpose, the
co-ordinate system is rotated through an angle θ1 which is
the direction of TC movement measured clockwise from
the reference direction (North, which is taken as 0° azimuth
as per meteorological convention) such that the direction
of movement of the TC is now oriented along 0° azimuth,
the reference direction. Then, in the rotated configuration,
the mean of all rain rates in the quadrant 0°-90° (measured
Fig. 3a. Schematic representation of steps for determination of azimuthally averaged radial profile of mean rain rate
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BALACHANDRAN et al.
125
of rain rates within 5° radial distance for each intensity
stage based on the rainfall associated with all the 43 TCs
of NIO during 2000-2010 are presented in Fig. 5 and the
number of data points in each rain rate class for each stage
of intensity are given in Table 1. The percentage frequency
of no rain is greater than 50% of the area and it generally
increases with each stage of intensity. During the stages
of intensification, non-raining areas occupy 59%, 67%
and 64% of the area and during the weakening stages, 7577% of the area do not receive any rainfall. Increasing
non-raining areas during the intensification stages indicate
organization of spiral rain bands leaving more non-raining
areas between the bands. Increasing non-raining areas
during the weakening stages indicate decreasing convective
activity.
Fig. 3b. A sample product of Radial Profile of rain rates within 5°
radial distance
clockwise from the direction of movement of the TC) from
the TC centre up to 2 degree radius is determined which
gives the mean rain rate in the Right Forward quadrant.
Similarly, mean of all rain rates in the quadrant 90°-180°
within 2 degree radius gives the mean rain rate in the Right
Rear quadrant, that in quadrants 180°-270° and 270°360° correspond to Left Rear and Left Forward quadrants
respectively. Fig. 4a depicts the steps schematically and
Fig. 4b provides a sample product.
4.Characteristics of TC rainfall distribution over
NIO
a. Frequency distribution of rain rates
To understand the variation in the amount of rainfall
around TCs of NIO, composites of frequency distribution
Fig. 4b. A sample product of quadrant mean rain rates within 2°
radial distance
Fig. 4a. Schematic representation of steps for determination of quadrant-wise mean rain rates
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Tropical Cyclone Research and Review
Volume 3, No. 2
Table 1. Stage-wise distribution of rain rates for various rain rate classes around Tropical Cyclones of North Indian Ocean within 5°
radial distance from the TC centre
Rain rate (mm/hr)
Stage
0
0-0.1 0.1-0.2 0.2-0.50.5-1.01.0-2.52.5-5.05.0-10.0>10.0
Stage-1
29240797265542 14809 31177 69111 4542519821 8041
Stage-2
298702
8335
4946
12922
22028
46035
29901
15875
7131
Stage-3
262917
9144
5536
14408
24080
46662
27068
14687
8538
Stage-4
79957
2125
1371
3436
4707
7966
3381
2006
1178
Stage-5
159504
3800
2235
5586
8291
13817
7863
4253
2128
(based on 3-hrly TRMM 3B42V6 data during the period 2000-2010)
Total
frequency
496059
444710
413040
106127
207477
Fig. 6a. Quadrant mean rain rates during various stages of intensity
of TCs of NIO. FL: front left, FR: front right, BL: back left and BR:
back right quadrants
Fig. 5. Frequency distribution of rain rates around TCs of NIO
upto 5° radial distance and for various intensity stages. (black shade:
intensification stages; gray shade: weakening stages)
It may be noted that 1-2.5 mm/hr is the most frequently
occurring rain rate during all the five intensity stages of
TCs of NIO. It covers an area of about 10-14% during the
intensification stages, but only about 6% of the area during
the weakening stages. The frequency of rain rates greater
than 5 mm/hr is about 10-15% during intensification stages
but during weakening stages, such higher rain rates are less
frequent at about 5% only.
b. Rainfall asymmetry
Next, a composite of quadrant-wise [front left (FL), front
right (FR), back left (BL) and back right (BR) with respect
to direction of motion of the TC] rainfall distribution within 200 km from the centre of 43 TCs that formed over the
NIO during 2000-2010 was prepared and same is shown in
Fig. 6. From this figure it is noted that during all the five
intensity stages, rain rates in the front quadrants are greater
than the rain rates in the rear quadrants. Highest rain rates
of 3.5-4.5 mm/hr are observed in the FL quadrant during
the intensification stages of Stage-1, Stage-2 and Stage-3
followed by the FR quadrant where the mean rain rates
are 3-3.5 mm/hr during the first three stages of intensity.
Amongst the three stages of intensification, Stage-3 has the
highest rain rates in the FL, FR and BR quadrants. In the
BL quadrant, mean rain rate during Stage-3 is marginally
less than the rain rate during Stage-2. During the same intensity stage of CS, there is a vast variation in the rain rate
distribution during intensification and weakening periods.
Whereas the mean rain rates in the four quadrants are in the
ranges 1.7-3.9 mm/hr during the cyclonic storm stage of
intensification phase, the quadrant mean rain rate is in the
range 0.5-1.3 mm/hr during cyclonic storm stage of weakening phase. The standard deviation of rain rates in each
quadrant are in the range 1-2.45 mm/hr during the stages of
intensification and about 0.7-1.58 mm/hr during the stages
of weakening. Highest standard deviation of 2.45 mm/hr is
in the FL quadrant during the Stage-2.
c. Utilisation for case studies and accessibility of products
Apart from composite approach to obtain above mentioned typical TC rainfall features, availability of such
rainfall analysis products for individual storms would
help in analysis of TC rainfall characteristics under different scenario like recurving versus non-recurving TCs, El
Niño – La Niña years, during strengthening and weakening
phases etc. For example, the tracks and the quadrant mean
rainfall plots of two storms that formed during November
2002 and 2005 over BOB are shown in Fig. 7a and b. Af-
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BALACHANDRAN et al.
Fig. 6b. Quadrant-wise distribution of rain rates during the intensity stage of cyclonic storm during the growth
(Stage-2) and decay (Stage-4) of TCs of NIO (FL, FR, BL and BR: Same as in Fig.6a)
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Volume 3, No. 2
Fig. 7. Tracks (a) and quadrant mean rain rates (b) for northward and westward moving storms
ter intensification as CS around same region of BOB, one
storm moved westward (CS-Baaz, 2005) while the other recurved northward (CS-23Nov, 2002). Highest rain rates are
observed in FR quadrant in the case of northward moving
storm while it is in the FL quadrant for westward moving
storm. For such studies on TC rainfall characteristics under
different scenario, above mentioned rainfall products for
individual storms can be used.
These products are ported in the web as TCRAIN for free
access at www. imdchennai.gov.in/ for the benefit of research community and disaster managers. Homepage of the
TCRAIN database is shown in Fig. 8. The user has to select
the Year, Cyclone, Product and Stage and the corresponding product would be displayed. Presently the rainfall analysis products are available for cyclones formed during the
period 2000-2010 and the same is being updated for further
Fig. 8. Home page of TCRAIN tool
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BALACHANDRAN et al.
129
years. Utilisation of these TC rainfall products would help
in understanding the complex TC rainfall asymmetries
which, when studied in relation to physical and dynamical
features associated with the TCs would bring out dynamics
of TC rainfall variations.
intensity based analytical studies on structure and dynamics
of rainfall associated with cyclones of NIO, TCRAIN products would provide ready-made profiles of rainfall around
TCs of NIO and hence is likely to form a useful database
for TC rainfall researchers.
5.Summary
Acknowledgements
Development of a TRMM based rainfall analytical database for depicting rainfall characteristics of TCs of NIO
for the period 2000-2010 is presented. For each TC, three
products, viz., frequency distribution of rain rates, azimuthally averaged radial profile of rain rates and quadrant mean
rain rates with respect to the storm motion direction determined in a Lagrangian frame of coordinate system with TC
centre as origin for every 3-hourly observation and then
composited based on intensity stratification (three stages of
intensification and two stages of weakening) are presented.
Climatological rainfall characteristics of TCs of NIO are
presented through composites of frequency distribution of
rain rates within 5° radial distance and quadrant mean rain
rate for various stages of intensity based on rainfall data of
43 TCs that formed over the NIO during the period 20002010. It is observed that the most frequently occurring rain
rate during all the five intensity stages of TCs of NIO is in
the range 1-2.5 mm/hr which occur over an area of about
10-14% (6%) within 5° radial distance during the intensification stages (weakening stages). The frequency of rain
rates greater than 5 mm/hr is about 10-15% during intensification stages but during weakening stages, such higher
rain rates are less frequent at about 5% only. Regarding the
asymmetric rainfall structure, among the four quadrants
around TCs of NIO, rain rates in the front quadrants are
greater than the rain rates in the rear quadrants. Highest
rain rates of 3.5-4.5 mm/hr are observed in the FL quadrant
during the intensification stages. In case of a westward
(recurving northward) moving TC, highest rain rate was
observed in the FL (FR) quadrant. Thus, for carrying out
Our acknowledgements are due to TRMM, NASA,
USA for sharing the TRMM rainfall database. We thank
Dr. L. S. Rathore, Director General of Meteorology, IMD
for his support and for providing facilities to undertake
this project. We are grateful to Shri K. Mathan Mohanram
and Shri Srimantha Haldar for technical help in the
development of the TCRAIN tool.
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