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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L06811, doi:10.1029/2009GL037296, 2009

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ArticleGenesis of tropical cyclone Nargis revealed by multiple satellite
observations
Kazuyoshi Kikuchi,1 Bin Wang,1,2 and Hironori Fudeyasu1
Received 16 January 2009; accepted 26 February 2009; published 27 March 2009.

[1] Tropical cyclone (TC) Nargis recently battered
Myanmar on May 2 2008 is one of the most deadly
tropical storms in history. Nargis was initiated by anabnormally strong intraseasonal westerly event associatedwith Madden-Julian oscillation (MJO) in the eastern Indian
Ocean. An incipient cyclonic disturbance emerged as an
emanation of Rossby wave-induced vortex when theintraseasonal convective anomaly reached the Maritime
Continent. The northeastward movement of MJO
convection facilitated further development of the
disturbance. The incipient disturbance became a tropicaldisturbance (TD) with a central warm-core structure on
April 26. The further development from the TD to TC
formation on April 28 is characterized by two distinctive
stages: a radial contraction followed by a rapid intensification.
The processes responsible for contraction and rapid
intensification are discussed by diagnosis of multiple satellite
data. This proposed new scenario is instrumental forunderstanding how a major TC develops in the northern
Indian Ocean. Citation: Kikuchi, K., B. Wang, and H. Fudeyasu(2009), Genesis of tropical cyclone Nargis revealed by multiple
satellite observations, Geophys. Res. Lett., 36, L06811, doi:10.1029/
2009GL037296.

major player in the Atlantic and eastern North Pacific TC
genesis [e.g., Landsea et al., 1998], and monsoon confluence zone and Rossby wave energy dispersion also play animportant role over the western North Pacific [e.g., Ritchieand Holland, 1999]. The aforementioned incipient disturbances, however, are virtually absent in the NIO. Thus,
initiation process of TD in the NIO is expected to bedifferent from the aforementioned basins. Previous studies
have documented how equatorial waves including MJO
events provide favorable large scale conditions for TC
generation [e.g., Bessafi and Wheeler, 2006; Frank andRoundy, 2006; Zhou and Wang, 2007]. The MJO andassociated equatorial waves are most active over the equatorial Indian Ocean (IO). It is conceivable that someequatorial waves which have rotational component may
contribute to TC genesis by changing environment anddirectly initiating TD.
[4] In this study, we examine how Nargis was initiatedfrom an MJO event and evolved from a weak to a strong
storm. Our strategy is to use as many available satellite dataas possible to reveal evolution of dynamic and thermodynamic structures of Nargis.

2. Data, Methodology, and Terminology
1. Introduction[2] TC Nargis, recently emerged in the northern Indian
Ocean (NIO) and battered Myanmar on May 2, 2008 [e.g.,
Lin et al., 2009], is one of the most deadly tropical storms.
Over 130,000 fatalities have been reported. Given the dense
population and inadequate infrastructures, devastating TCs
are a major threat to the livelihood in Southeast Asia.
Despite of this, the formation, development, and movementof TCs in the NIO have received little attention compared tothose in the Pacific and Atlantic. Lack of observations over
the NIO is one of the major reasons that have hinderedadvancement of our knowledge.
[3] While the general favorable environmental conditions
are well known [e.g., Briegel and Frank, 1997], the TC
genesis remains the least understood matter among TC
issues due to insufficient in situ high resolution observationand the complex nature of multi-scale interactions [Holland,1995]. It has been well recognized that TCs never arise
spontaneously [Emanuel, 2003]. Incipient disturbances areoften required in order to initiate a TD. Easterly waves are a
1
International Pacific Research Center, University of Hawaii, Honolulu,
Hawaii, USA.
2
College of Physical and Environmental Oceanography, Ocean
University of China, Qingdao, China.

Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL037296$05.00[5] The following satellite data were used in our study:(1) daily outgoing longwave radiation (OLR) data for
depicting organized convection; (2) pixel-resolution infrared (IR) brightness temperature data, globally-merged fromall available geostationary satellites for depicting development of clouds; (3) Sea surface winds data obtained byblending multiple satellites including QuikSCAT, Special
Sensor Microwave/Imagers, Tropical Rainfall Measuring
Mission (TRMM) Microwave Imager (TMI), and Advanced
Microwave Scanning Radiometer observations on a 0.25°
grid and 6-hourly time resolution for describing near surface
wind field; (4) Atmospheric Infrared Sounder (AIRS) version 5 swath data for describing three dimensional temperature structure; and (5) TRMM TMI SST data gridded with
0.25°  0.25°.
[6] Nargis was detected as a deep depression on April 26
by the Indian Meteorological Department and as a tropicalstorm (TS) at 12Z 27 by Joint Typhoon Warning Center(JTWC). To depict the early history of the TC, we objectively determined the centers of TD disturbance by using thetechnique developed by Kurihara et al. [1995]. First,
surface wind field was decomposed into a basic flow anda disturbance. The TD center was determined by optimization of the central location so that the maximum of theazimuthally mean tangential wind within 6° from the center
is maximized. After reaching the TS stage when the JTWC
best track data became available, the results derived from

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Table 1. Tropical Cyclone Classification
10-Minute SustainedWinds (Knots)

Atlantic and
ENP/WNP

NIO

28
2833
3447
4863
64119120

Tropical depression
Tropical depression
Tropical storm
Tropical storm
Hurricane/Typhoon
Hurricane/Typhoon

Depression
Deep depression
Cyclonic storm
Severe cyclonic storm
Very severe cyclonic storm
Super cyclonic storm

the technique explained above are similar to those from the best
track. Thus, the satellite-observed surface winds allow us
detect initiation process a few days ahead of TS formation.
[7] For classification of TC intensity, we use the terminology that is commonly used for Hurricanes and Typhoons
in the Atlantic and Pacific Oceans to facilitate comparison(Table 1). We categorize TC development as the following
stages: (i) pre-TD, (ii) TD, (iii) TS, and (iv) hurricane.

3. Initiation of Nargis[8] The TC Nargis formed in April 28, 2008, which isin the pre-TC season. The TC formation in the NIO
exhibits double peaks in October – November and May –June (Figure 1a), which correspond to the transitional
season when equatorial westerly prevails. Historically, thelikelihood for a TC occurring in April is smaller than 14%.
Why does such a strong TC Nargis forms in April? What we
found is that although climatologically the equatorial IO has
not entered a westerly regime, a notable lasting abnormallystrong westerly event had occurred in the mid-late April, lasting
for about 10 days from April 16 to April 26 (Figure 1b). It
was this westerly event that played a critical role in initiationof the Nargis. The westerly event was accompanied byorganized convections. Both moved eastward slowly at a
speed about 6 m s 1. Those characteristics are consistent
with well-known MJO.
[9] On April 21 when the intraseasonal westerly event
reaches its peak. Two off-equatorial vortices associated withthe equatorial westerly anomalies were first formed in thesoutheastern IO (Figure 2a), while NIO has no vortex
formed and only a cloud cluster was seen. The earlyL06811development of off-equatorial vortices in the southernhemisphere is attributed to the effect of the strong low-levelbackground cyclonic shear due to the southeast trades andthe equatorial westerly. In the ensuing two days, theorganized convections shifted eastward a little bit and the
major body reached the Maritime Continent (MC), when aweak cyclonic circulation emerged as an emanated Rossby
wave vortex [Kemball-Cook and Wang, 2001] near the northwest tip of Sumatera Island on 12Z April 23 (Figure 2b).
From April 23 to 25, the northeastward movement of theconvections facilitated the development of the embedded
incipient disturbance near Sumatera developed into a welldefined TD on 12Z April 25 (Figure 2c). At the same time, theconvections associated with southern hemisphere vortices
continuously weakened. From April 25 to 27, the northern
vortex moved northwestward and its cyclonic circulation
dominated entire Bay of Bengal (BOB) (Figure 2d), which
eventually developed into the Hurricane-strength Nargis on
06Z April 28.
[10] The northeastward shift of the MJO convection in
the equatorial IO, which is critical for initiation of Nargis, isa common feature of boreal summer MJO [e.g., KemballCook and Wang, 2001]. The northeastward movement of
MJO has been attributed to atmosphere-ocean interaction[Fu et al., 2003]. The northward migration of cyclonicvortex in the BOB is probably due to the effect of theenvironmental easterly vertical shear [Jiang et al., 2004]
and the northward SST gradient as shown in Figure 3a. The
easterly vertical shear is also a favorable condition for
cyclonic develop [Wang and Xie, 1996].
[11] The northward movement of the pre-TD disturbancemay contribute to establishment of a TD in the early stage.
From April 23 to 25, the planetary vorticity became 1.4 times
larger due to the northward movement from 5°N to 7°N(Figure 3a). As relative vorticity is generated by absolute
vorticity multiplied by divergence, the generation of relative
vorticity by convergent flow becomes more effective, evenfor the same amount of the Ekman pumping.

4. Formation Processes of Nargis[12] Here we document how the TD initiated by the MJO
event developed and turned into Hurricane-strength Nargis.

Figure 1. (a) Accumulated number of occurrence of TCs (dark), and TCs and TSs (light) during 1979 – 2007 as a functionof month. (b) Westerly wind burst index defined by averaged zonal wind speed (5°S – 5°N, 80°E – 100°E). The thick line
shows 2008 case and other thin lines indicate cases of 1998 – 2007. Five day running mean was taken and monthly mean
was subtracted.
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L06811KIKUCHI ET AL.: SATELLITE OBSERVATIONS OF TC NARGIS

Figure 2. Temporal evolution of incipient disturbance andtropical depressions. Blended sea surface winds are indicatedby vectors and infrared (IR) brightness temperature by shades(K). Closed circles represent votex centers deduced from thesurface winds.

Figure 3b shows time evolutions of satellite-derived maximum surface wind speed (MW), number of deep convection pixels (indication of precipitation), and the radius ofmaximum winds (RMW) from April 23 (pre-TD) to April
30 (Hurricane). Note that the MW derived from satellite
observation and the best track estimation show consistent
temporal evolution but with different amplitude due todifferent derivation methods and criteria. As discussed in
section 2, the TC development was categorized into fourstages.

L06811[13] Contraction is a salient process throughout the TD
and TS stages, which fills the structural gap between the
incipient TD and mature TC. A key responsible for contraction may be an interaction between convection andsynoptic dynamics of TD. On its earlier development stage,organized convections were found to the north – west and-east of the TD center by about 200– 300 km away
(Figure 4a) which corresponds to RMW value at around thistime (Figure 3b). Indeed, the relationship between strong
surface winds and active convection was robust even in theearly TD stage (Figure 5a). AIRS observation is luckily
available which gives an important evidence that warming
occurs in the places just around active convective areas,
especially around the TD center (Figure 4a). Note that thetemperature in deep convective areas cannot be retrieved withfidelity and thus missing here, but under partial cloud cover it
can be retrieved with RMS errors on the order of, or less than,
1°K in 1-km-thick layer from the surface to 300 mb [Susskind
et al., 2003]. The warming results in a warm core structure(3 K), which can be clearly seen in the horizontal-verticalprofile of temperature anomaly (Figure 4b). The formation of
the warm core at this early stage is important in spinning-up
the TD by lowering surface pressure at the TD center. During
this stage, northward migration (Figure 3a) is also favorable
for enhancing convection by enhancing boundary layer
convergence due to increase of the Coriolis parameter.
[14] As RMW reduced to 200 km, the MW started tointensify dramatically. In the 24 hours starting from 12Z
April 27 the MW increased from 17 m s 1 to 33 m s 1,
becoming a Hurricane (Figure 3b). This qualified as a rapid
intensification [Kaplan and DeMaria, 2003].
[15] An interesting feature to note is that the interaction
between convection and circulation shows odd correspondence in the rapid intensification. While MW increased rather
monotonously, deep convection suddenly dropped around
06Z April 28 after rapid development. The key process
responsible for this may be related to the structural changeof TD. Axisymmetry seems to become more evident in
the later rapid intensification period (Figures 5b– 5d). To
check this hypothesis, we define ‘‘symmetricity’’ by KsE/KtE.

Figure 3. (a) Background SST (shades, °C) and zonal wind vertical shear between 200 and 850 hPa (contours) from
NCEP/NCAR reanalysis averaged for the period April 23– May 1 2008. Positive vertical shear (denoted by solid contours)means westerly vertical shear and the contour interval is 5 m s 1. The thick solid segments indicate the track of the pre-TD,
TD, TS, and Hurricane. The numbers nearby indicate dates. (b) Time evolution of deep convection (IR  200 K)
population (bar) within 400 km, maximum surface wind speed (green line), the radius of maximum wind (blue line), andthe symmetricity of surface winds averaged within 300 km (red line) with reference to the pre-TD, TD, TS, or Hurricane
centers. Closed circles are 1-minute mean sustained 10-m wind speed provided by JTWC. Three point average was taken
for MW and RMW (thick lines).
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L06811Figure 4. AIRS observation of (a) temperature at 500 hPa (circles) around 0730UTC, April 26 2008 together with IR at
0730UTC, April 26 (shades). The weather symbol represents the TD center. The black rectangle indicates 10°  10° box
centered on the TD center. The blue box indicates the area that were used to make the composite Figure 4b. (b) Horizontalvertical distribution of potential temperature anomaly deviated from the black rectangle area in Figure 4a. Average wastaken up to four footprints along each scan shown in Figure 4a. X axis indicates average latitude over the four footprints
and the center location of the TD is shown by the thick line.

Figure 5. Horizontal distribution of IR (shades in units of K) and surface winds of disturbance component (vectors)
around the pre-TD, TD, TS, or Hurricane center. The center location (0°, 0°) provided by JTWC was used after it became
available on 12Z April 27. The center locations estimated by Kurihara et al.’s [1995] method are indicated by  forcomparison. Surface wind speed less than or equal to 6 m s 1 are represented by gray arrows and greater than 6 m s 1 byblack arrows. The blue circles represent radius of maximum wind, and the green circles 300 km radius from the center.
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Where KsE(r, q) and KtE(r) are axisymmetric component ofand total kinetic energy of surface winds, respectively,
expressed in terms of the TC-centered cylindrical coordinate
system (r, q). After all, the time evolution of the symmetricity appears to support our hypothesis (Figure 3b). The
feature of axisymmetrization characterized by outward
propagating asymmetric component of both IR and surface
winds (Figure 5) is consistent with the theoretical work by
Montgomery and Enagonio [1998] that showed the warm
core reinforcement occurs as a result of axisymmetrization.
Note that the aforementioned conclusion is the same no
matter how the center location is determined (JTWC or
Kurihara et al.’s [1995], Figure 5).

5. Discussion and Summary[16] By examining multiple satellite observations, a new
scenario of how Nargis was initiated and formed in the preTC season in the NIO was proposed. First, an abnormallystrong intraseasonal westerly occurred in mid-late April
which is associated with MJO and accompanied by organized convections. When the main body of the convections
reached the MC, the pre-TD disturbance was initiated as an
emanated Rossby wave vortex near the northern tip of
Sumatera on April 23, 2008. The northeastward movementof organized convection associated with MJO facilitatedfurther development of the pre-TD. Once the pre-TD
developed to some extent, the strong connection between
well-built surface winds and active convection made it
possible to create a warm core structure in its center,
forming the TD on April 26. The middle troposphericwarming near the TD center lowers surface pressure,
resulting in contraction of the band of maximum winds
and convection. The continuous northward migration may
further strengthen the pre-TD disturbance and the TD byincreasing boundary layer convergence due to the increaseof the Coriolis parameter. As contraction reaches a certain
threshold, the cooperation between convection and synoptic
dynamics of TD was accelerated, resulting in rapid intensification and turning into a hurricane in the end. Two stages
were involved in the rapid intensification, i) deep convection development with strong axial asymmetry and ii)
axisymmetrization process.
[17] The present study has not only presented a timely
and concise analysis of one of the most disastrous weather
event worldwide in 2008, but also provided a new perspective on the TC genesis process in the NIO. While theproposed scenario seems to be reasonable, other factors
have not been well examined due to limitation of satellite
data availability. To achieve a more comprehensible understanding, a number of issues such as the effects of theL06811steering flow and beta drift on the movement and vertical
shear on the development need to be further clarified.
[18] Acknowledgments. This research was supported by ONR grant
N00014-02-0532. Additional support was provided by the Japan Agency
for Marine-Earth Science and Technology (JAMSTEC), by NASA throughgrant NNX07AG53G, and by NOAA through grant NA17RJ1230 throughtheir sponsorship of research activities at the International Pacific ResearchCenter. The blended surface wind data are acquired from NOAA’s NCDC,
via their website http://www.ncdc.noaa.gov/oa/rsad/blendedseawinds.html.
We thank two anonymous reviewers for their insightful comments. This
manuscript is SOEST contribution 7633 and IPRC contribution 585.

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H. Fudeyasu, K. Kikuchi, and B. Wang, International Pacific ResearchCenter, University of Hawaii, POST Building 401, 1680 East West Road,
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