Pure appl. geophys. 154 (1999) 163–1820033–4553/99/010163–20 $ 1.50+0.20/0

A Diagnostic Study of the Cyclonic Storm over the Arabian Seaduring June 1994

L. GEORGE,1 P. S. SALVEKAR1 and PREM SINGH1

Abstract—Based on the NCMRWF analysis over the Arabian Sea, a complete energy cycle of thesevere cyclonic storm that formed in the beginning of June 1994 in the east central Arabian Sea is carriedout, using the in-house developed energy package. Both barotropic and baroclinic energy conversionsare responsible for the maintenance of the system, however dominance of one over the other is noticedat different stages of the system at different heights. Dynamical characteristics of synoptic scale monsoonflow surrounding the cyclonic storm are also investigated. By examining the generation and dissipationterms, it is observed that both zonal and eddy components of the synoptic scale flow act as source ofenergy for the cyclonic storm, both in the predeveloped and developed stages.

Key words: Monsoon onset, cyclonic storm, Arabian Sea vortex, energetics.

Introduction

Various aspects of the onset and advance of the monsoon across the west coastof India have been studied by many people. During the onset phase of thesouthwest monsoon over India, spectacular changes take place in the atmospherecirculation pattern over the monsoon region. The circulation and moisture changesover India associated with the onset of the southwest monsoon over Kerala areexamined by SOMAN and KRISHNAKUMAR (1993). From the surface synopticclimatology, it is found that cyclonic systems are forming at the leading edge of theadvancing monsoon current during the onset of the southwest monsoon (ANAN-

THAKRISHNAN et al., 1968; MUKHERJEE and PAUL, 1980; KRISHNAMURTI et al.,1981). The association of a major cyclonic disturbance over the Arabian Sea isevidently not unusual during the onset phase (PHILIP et al., 1973). Once themonsoon sets, its further progress takes place due to the rain-bearing systems likemonsoon trough, lows, depressions, mid-tropospheric cyclones, etc. These synopticscale systems are considered as perturbations embedded in the basic monsoon

1 Indian Institute of Tropical Meteorology, Pune-411008, India.

L. George et al.164 Pure appl. geophys.,

current. The most common form of this disturbance is a trough of low pressure ora low pressure area (onset-vortex). Generally, the formation of a low pressure areaand its further northward movement takes place over the Arabian Sea during theonset phase of the monsoon. An analysis of past data for the period 1901–1993 hasrevealed that onset of the monsoon was associated with an onset vortex on 50occasions. Out of these, the vortex developed in the Arabian Sea in 44 years and inthe Bay of Bengal in only 6 years (1941, 1951, 1971, 1974, 1987 and 1991).Numerous studies exist relating to the synoptic features during the onset phase ofthe southwest monsoon over India. All these features have broad similarities as wellas interannual variations.

To understand the dynamical and physical processes involved in the initialformation and subsequent intensification of the onset vortex, many dynamicinstability and energetics studies are carried out using summer MONEX-1979 datasets (KRISHNAMURTI et al., 1981; KRISHNAMURTI and RAMANATHAN, 1982;PEARCE and MOHANTY, 1984). A few studies on barotropic (KRISHNAMURTI et al.,1981; MISHRA et al., 1985; PARK and SIKDAR, 1985) and combined barotropic-baroclinic (MAK and KAO, 1982) instability of the observed zonal flow have beencarried out to understand the initial formation of the onset vortex of 1979. Thesestudies concluded that barotropic energy conversion arising from strong horizontalshear of zonal wind might have been the dominant process in the initiation of thedisturbance. RAMANATHAN (1981) computed the energetics of the onset vortex inorder to diagnose the results of the numerical prediction experiment, using amulti-level primitive equation limited area model, and he found that even thoughbarotropic conversions were dominant in the lower levels, the disturbance’s kineticenergy was maintained by baroclinic processes in the upper levels and theseprocesses were responsible for the further growth of the disturbance. GEORGE andMISHRA (1993) studied the various energy characteristics associated with the onsetvortex in order to establish the relative roles of barotropic and baroclinic processesin the growth and maintenance of the vortex formed in the Arabian Sea duringJune 1979. Further, they investigated the relative role of various momentum andheat transports in barotropic and baroclinic conversions, respectively. They foundthat the barotropic energy conversion from KZ to KE dominates over the baroclinicconversion from AE to KE to maintain the onset vortex and the energy transferfrom KZ to KE, due to the down-gradient eddy transport of meridional momentumis more than that due to the down-gradient eddy transport of zonal momentum.Further, it is seen that the barotropic and baroclinic conversions due to the verticaltransport of heat and momentum are two to three orders smaller and are insignifi-cant.

During 1994, the onset of the southwest monsoon was not associated with anysynoptic scale disturbance. However, after the onset date i.e., 28 May, 1994, acyclonic storm developed in the east central Arabian Sea during 5–8 June.Therefore, in the present study, an attempt is made to examine the various energy

Diagnostic Study of the Cyclonic Storm 165Vol. 154, 1999

characteristics associated with the predeveloped (2–4 June) and the developed (5–8June) stages of the cyclonic storm in order to understand the formation and growthmechanisms. Time variation of the vertically integrated energetics is computed forthe period 2–8 June, 1994. In addition to the above, vertical distribution of the timemean energetics as well as the energy cycle for the predeveloped and developedstages are also computed separately.

Energy Equations

The study of the energetics of the monsoon systems is an important approachto understand the source of kinetic energy for the growth and maintenance ofmonsoon disturbances. The intensification (weakening) of weather system indicatesthe gain (loss) of kinetic energy.

The origin of the study of atmospheric energy cycle began with LORENZ (1955).MUENCH (1965) derived the energy equations with which to identify the energysource for regional synoptic scale disturbances. NORQUIST et al. (1977) computedthe energetics of the African wave disturbances observed during phase III ofGATE. However they have not reported the values of boundary fluxes as well asgeneration and dissipation terms. For the first time, the boundary flux terms wereincluded by BRENNAN and VINCENT (1980) which were derived in (l, 8, p, t)system to examine the energy budget of hurricane Carmen. The expressions forbasic and eddy energies as well as energy conversions used in the present study arethe same as those given in GEORGE and MISHRA (1993), who studied the energeticsof the onset vortex formed in the Arabian Sea during June 1979. Further, theboundary flux terms are derived for a limited area in (x, y, p, t) system and the finalexpressions are given below.

BKZ=−1

2g& P0

PT

![a1u ]E− [a1u ]WXE−XW

+(a16)N− (a16)S

YN−YS

"dp

BKE=−1

2g& P0

PT

![a2u ]E− [a2u ]WXE−XW

+(a26)N− (a26)S

YN−YS

"dp

BAZ=−1

2s

& P0

PT

![a3u ]E− [a3u ]WXE−XW

+(a36)N− (a36)S

YN−YS

"dp

BAE=−1

2s

& P0

PT

![T %2u ]E− [T %2u ]WXE−XW

+(T %26)N− (T %26)S

YN−YS

"dp

where

a1= (u2+62−u %2−6%2), a2= (u %2+6%2), a3=2T %T*+T*2.

L. George et al.166 Pure appl. geophys.,

( ( ) and [( )] denote the zonal and meridional average, respectively. Prime andasterisk represent a departure from the zonal average and a departure of the zonalaverage from the area average, respectively. The subscripts N, S, E and W representthe values at the north, south, east and west boundary, respectively. YN−YS andXE−XW represent the distance between north and south, and east and west,respectively. Other standard symbols have their standard meanings.

Data and Computation Procedure

The daily initial analyzed data consist of contour height (z) and zonal (u) andmeridional (6) wind components during 2–8 June 1994 on a horizontal resolutionof 1.5°×1.5° latitude-longitude grid over the region. Eq. −25.5°N and 51°E–81°Eare used in the present study and are obtained from NCMRWF2 at standard levels1000, 850, 700, 500, 400, 300, 250 and 200 hPa. By using the cubic spline technique,z, u, and 6 are interpolated vertically up to 200 hPa at the interval of 50 hPa.

For all the computations, vertical derivatives are obtained by a cubic splinetechnique. Horizontal derivatives are obtained by using a centered differencescheme except for the boundaries, and at the boundaries the forward/backwardschemes are used. The temperature values are obtained from height data using thehydrostatic approximation. A kinematic method with O’Brien’s correction is usedto determine the vertical velocity (v), and the static stability parameter (s) isobtained using the expression

s=−gR�1

cp

( [f( ]( log p

−1R(2[f( ]((log p)2

�.

Synoptic Features and Basic State

Synoptic Features

A well marked low pressure area was formed off the Maharashtra coast and theadjoining Arabian Sea on the evening of 5th June. Initially it moved in anorthwesterly direction and then intensified into a cyclonic storm on 6th June overthe east central Arabian Sea and moved in a west northwesterly direction until themorning of the 7th. It further intensified into a severe cyclonic storm on the eveningof the 7th and moved in a westerly direction. Moving further westward, itweakened into a depression on the morning of the 9th and became unimportant off

2 National Centre for Medium Range Weather Forecast, New Delhi, India.

Diagnostic Study of the Cyclonic Storm 167Vol. 154, 1999

the Oman coast by the same evening. The track of the cyclonic storm is shown inFigure 1. Due to the influence of this system, the southwest monsoon advanced intoKonkan and strengthened over the west coast of India. Since the cyclonic stormdeveloped and intensified during 5–8 June, we have chosen 2–4 June as predevel-oped and 5–8 June as developed stages of the system.

Basic State

Meridional plane distribution of time-averaged zonal flow (u) ) for predeveloped(2–4 June) and developed (5–8 June) stages is displayed in Figures 2a and 2b,respectively. During the predeveloped stage (Fig. 2a), westerlies are found from thesurface to 400 hPa up to 15°N with a maximum (20 m s−1) near 850 hPa at 9°N.Beyond 15°N, easterlies are seen throughout in the vertical, with a maximum valueof 8 m s−1 near 600 hPa at 17°N. Once the predeveloped stage is crossed, strongmeridional shear is seen between 10°N and 20°N in the mid-troposphere and thestrength of easterlies beyond 15°N are found to be reduced and the core ofeasterlies move upwards (Fig. 2b). During the developed stage, the zonal flow hastwo westerly maxima, the primary one of value 18.8 m s−1 is seen at 10°N near 850hPa and the secondary one of value 16.5 m s−1 is found near the equator around600 hPa. The region of westerlies remains similar. From the figure, it is seen thatthe westerly flow is slopping downward with latitude. Beyond 20°N easterlies exist

Figure 1Observed track of the cyclonic storms.

L. George et al.168 Pure appl. geophys.,

Figure 2Meridional plane distribution of time-averaged zonal flow u) for (a) predeveloped and (b) developed

stages. Units: m s−1.

throughout in the vertical from 900 hPa. Further, it can be seen that upper leveleasterlies are increasing with height beyond 350 hPa in the latitudinal belt Eq.-17°N.

Figures 3a and b are the meridional plane distribution of meridional flow (6) ) forthe predeveloped and developed stages, respectively. From Figure 3a, it is seen thatupto 10°N, southwesterlies are prominent up to 300 hPa, except for a small layernear 600 hPa. A southerly maximum 7.5 m s−1 is at 8°N near 900 hPa and anortherly maximum 3.8 m s−1 is at 20°N near 650 hPa. As time progresses (Fig. 3b)low-level southerlies remain similar but the tongue of the northerlies from thenorthern boundary is extended to the equatorial belt in the mid-troposphere, witha maximum value of 3.5 m s−1 at 650 hPa near 15°N, and the low latitude uppertropospheric southerlies are enhanced.

Diagnostic Study of the Cyclonic Storm 169Vol. 154, 1999

Time-averaged zonal temperature (T( ) in the meridional plane for both thestages (Figs. 4a and b) are found similar. The maximum value is seen in the layer1000–875 hPa between 12°N and 25.5°N. Meridional shear of zonal mean temper-ature (T( y ) in the meridional plane is shown in Figure 5. T( y is positive except in thelayer 600–350 hPa beyond 15°N. Maximum value 0.8°K/100 km is found at 800hPa near 17°N.

Vertical shear of zonal flow (u) p ) in the meridional plane for the developed stageis shown in Figure 6. A maximum value of 8.5 m s−1/100 hPa is found below thewesterly jet core in the layer 1000–950 hPa at 10°N and a secondary maximum of2 m s−1/100 hPa is seen in the layer 600–400 hPa beyond 15°N. The easterly windshear of maximum value 12.3 m s−1/100 hPa is found near 250 hPa level at 10°Nand the secondary maximum value of 6.8 m s−1/100 hPa is found near 750 hPa at15°N. The thermal wind relationship implies the presence of easterly wind shear in

Figure 3Meridional plane distribution of time-averaged meridional flow 6) for (a) predeveloped and (b) developed

stages. Units: m s−1.

L. George et al.170 Pure appl. geophys.,

Figure 4Meridional plane distribution of time-averaged temperature T( for (a) predeveloped and (b) developed

stages. Units: °K.

the region of positive T( y. In the present case the easterly wind shear in the region(Fig. 6) is found in the region of T( y\0 and the region of maximum easterly windshear almost coincides with that of T( y maximum. The observed strong wind shearis mainly confined to the boundary layer and as expected it is not supported by thethermal wind relationship.

Distribution of vertical shear of meridional flow (6) p ) in the meridional plane isshown in Figure 7. Weak southerly shear is found in the 1000–950 hPa layer in thelatitudinal belt Eq.-8°N. Northerly shear increases with height above 900 hPa andreaches a maximum value of 5.6 m s−1/100 hPa at 800 hPa. Another northerlyshear maximum is found near 250 hPa around 4°N. Beyond 10°N northerly shearis seen throughout in the lower troposphere up to 650 hPa with a maximum value(3.2 m s−1/100 hPa) around 18°N at 800 hPa. The southerly shear maximum (2.2m s−1/100 hPa) is also seen at the same latitude around 600 hPa.

Diagnostic Study of the Cyclonic Storm 171Vol. 154, 1999

Figure 5Meridional plane distribution of meridional shear of time-averaged temperature T( y for developed stage.

Units: 10−1 °K/100 km.

From Figures 2 and 3, it is seen that meridional winds are nearly 1/3 of thezonal wind, while from Figures 6 and 7, it is found that maximum values for u) p and6) p in the layer 800–700 hPa are comparable, where the cyclonic storm attains themaximum intensity. Therefore, it can be concluded that vertical shear of meridionalwind is also of equal importance to that of zonal wind and must be considered forany study of the dynamics of monsoon systems. Earlier studies (MAK and KAO

1982; MISHRA and SALVEKAR 1980; MOORTHY and ARAKAWA, 1985, etc.) haveestablished the positive role of vertical shear of zonal flow in the formation and thedevelopment of monsoon systems.

Meridional shear of zonal wind (u) y ) for the developed stage in the meridion-al plane is presented in Figure 8. The maximum cyclonic shear of the value

Figure 6Meridional plane distribution of vertical shear of time-averaged zonal flow, u) p for developed stage.

Units: 10−1 m s−1/100 hPa.

L. George et al.172 Pure appl. geophys.,

Figure 7Meridional plane distribution of vertical shear of time-averaged meridional flow, 6) p for developed stage.

Units: 10−1 m s−1/100 hPa.

4 m s−1/100 km is seen at 15°N in the 850–650 hPa layer. Anticyclonic shearmaximum is found near the equator and 19°N around 700 hPa. Figure 9 exhibitsthe meridional shear of meridional wind (6) y ). 6) yB0 implies convergence whichcontributes to the upward motion.

A comparison of u) y (=j) and 6) y (=D) distributions indicates that in theboundary layer, cyclonic vorticity (−u) y\0) beyond 10°N is associated withconvergence (6) yB0) while the anticyclonic vorticity (−u) yB0) in the equatoriallatitude is associated with weak divergence (6) y\0). On the basis of Ekman’s theoryof boundary layer it can be said that the observed zonal average convergence anddivergence in the boundary layer are induced by the frictional effect. In the free

Figure 8Meridional plane distribution of meridional shear of time-averaged zonal flow u) y for developed stage.

Units: 10−1 m s−1/100 km.

Diagnostic Study of the Cyclonic Storm 173Vol. 154, 1999

Figure 9Meridional plane distribution of meridional shear of time-averaged meridional flow, 6) y for developed

stage. Units: 10−1 m s−1/100 km.

atmosphere, cyclonic shear and convergence are highly correlated, however anticy-clonic shear and divergence are weakly correlated. This can be seen clearly fromFigures 8 and 9.

Vertical velocity (v), which is a derived parameter, plays an important role inenergy conversion equations. Vertical velocity distribution in the y-p plane forpredeveloped stage (Fig. 10a) indicates strong upward motion in the latitudinal belt8°N to 16°N throughout the troposphere. The main ascending branch for themonsoon circulation is located around the latitude 13°N near 750 hPa. Anotherbranch of significant upward motion is seen north of 20°N in the lowertroposphere.

As time progresses (Fig. 10b), the narrow belt 16°N to 20°N of downwardmotion vanishes and upward motion is seen from 6°N to 24°N throughout thevertical. Comparing Figures 8 and 10b, it is found that the maximum upwardmotion takes place in the cyclonic shear zone of the basic flow u) , while thedownward motion is mainly confined to the anticyclonic shear zone of u) .

Results

Zonal and eddy components of kinetic and available potential energy, theirconversions and boundary fluxes are computed each day during the predevelopedand developed stages of the cyclonic storm in the vertical layer 1000–200 hPa. Thevertically integrated energy values are presented as time series and time-averagedvalues are chosen to illustrate the vertical resolution.

L. George et al.174 Pure appl. geophys.,

Time Mean Energetics

Area averaged vertical profile of basic and eddy energies for predeveloped (2–4June) and developed (5–8 June) stages are presented in Figures 11a and b,respectively. Figure 11a shows that basic kinetic energy KZ has a large magnitudein the lower layers from the surface to 600 hPa with a peak near 850 hPa.Thereafter KZ decreases monotonically with height and then increases rapidly above400 hPa. Basic available potential energy AZ also has made a significant contribu-tion in the lower layers. The bell shaped profile of AZ peaks near 775 hPa anddecreases on either side. Above 500 hPa, AZ starts redeveloping. Eddy kineticenergy KE is relatively small in the lower layers and its magnitude is enhanced in thelayer 800 hPa to 200 hPa with a maximum near 550 hPa. Generation of eddyavailable potential energy AE in the boundary layer can be attributed to the sensible

Figure 10Meridional plane distribution of time-averaged vertical velocity v) for (a) predeveloped and (b)

developed stages. Units: 10−5 hPa s−1.

Diagnostic Study of the Cyclonic Storm 175Vol. 154, 1999

Figure 11Vertical profile of time- and area-averaged basic and eddy energies for (a) predeveloped and (b)

developed stages. Units: 103 J m−2.

heat transport due to air-sea interaction. Above 850 hPa, AE decreases but a slightincrease near 550 hPa may be due to the diabatic process. AZ is considerably higherthan AE in the lower layers up to 650 hPa, and thereafter AE is slightly higher thanAZ up to 425 hPa. Thereafter AZ increases and again it is higher than AE. The largemagnitudes of AZ and KZ seen in the lower levels indicate that the energy source isin the lower levels. Figure 11b demonstrates that the behaviour of basic and eddyenergies in the developed stage is qualitatively the same as that of the predevelopedstage, except the enhancement in the quantities of eddy energies and thereafter a

L. George et al.176 Pure appl. geophys.,

reduction in the quantities of basic energies, particularly in the lower and middlelevels.

Vertical profiles of barotropic energy conversion C(KZ, KE ), baroclinic energyconversion C(AE, KE ), as well as conversions C(AZ, KZ) and C(AZ, AE ) forpredeveloped and developed stages are shown in Figures 12a and b. Figure 12aillustrates that the barotropic conversion C(KZ, KE ) is positive in the layer 950–325hPa with its maximum near 725 hPa. The same conversion for the developed stage(Fig. 12b) has two maximum at 775 hPa and 600 hPa and also a secondary

Figure 12Vertical profile of time- and area-averaged different energy conversions for (a) predeveloped and (b)

developed stages. Units: W m−2.

Diagnostic Study of the Cyclonic Storm 177Vol. 154, 1999

maximum in the upper troposphere near 250 hPa. C(AZ, AE ) is positive in thelower layer 950–750 hPa and thereafter negative up to 600 hPa during thepredeveloped stage (Fig. 12a), however as the system is in the developed stage, themagnitude of C(AZ, AE ) is enhanced both in the lower and upper layers.C(AZ, AE ) profiles for predeveloped and developed stages indicate that the contri-bution is positive below 825 (775) hPa and above 625 (675) hPa with three maximaaround 925, 550 and 250 hPa (900, 550 and 300 hPa). The wave-like structure ofC(AE, KE ) profile in Figures 12a and b from surface to 200 hPa with three zonesof positive contribution indicates that at least a six-layer model is needed tounderstand the dynamics of the present cyclonic storm. The conversion C(AZ, AE )is more than the conversion C(AE, KE ) in the layer 850–700 hPa. Thus AZ isadequate to maintain the observed increase of AE, despite its loss due to transfer toKE. Further, in the layer 1000–850 hPa, C(AZ, AE ) is less than C(AE, KE ). Thismay be quite likely that the transfer of sensible heat due to air-sea interaction maybe responsible for the generation of AE in the boundary layer (SEETARAMAYYA andMASTER, 1984). The generation of AE due to the diabatic process may be takingplace in the mid-troposphere. Thus the contribution of baroclinic conversionC(AE, KE ) towards the growth of cyclonic storm is confined in both the upper andlower levels. The positive values of C(AZ, KZ ) with two maxima near 850 hPa and400 hPa in the predeveloped stage indicate that the direct monsoon circulation inthe y-p plane over the Arabian Sea is very vigorous and the same is also confirmedfrom the meridional plane distribution of the strong upward motion throughout thetroposphere (Fig. 10a). In the developed stage, C(AZ, KZ ) is found to reduceslightly above 700 hPa.

Time Variation of Energy and Con6ersion

Time variation of energy from 2–8 June for a unit horizontal area from surfaceto 200 hPa is shown in Figure 13. It is observed that initially KZ decreases slowlyreaching June 5th and then increases almost linearly throughout the period. AZ hasshown small oscillations, i.e., initially small decreases up to June 3rd, then increasesslowly up to June 5th and then again decreases. AE is similar throughout the periodwith a slight increasing trend from June 3rd to June 6th, whereas KE is unalteredup to June 4th and manifests an increasing trend from the 4th to the 5th of Juneand also from the 6th to the 7th June. A sudden increase in KE on both the 5th and7th of June is found to be consistent with the formation and intensification of thesystem. On the 5th of June a low was formed and on June 7th it developed into acyclonic storm.

The time series of energy conversions is seen in Figure 14. Barotropic conver-sion C(KZ, KE ) has a wave-like pattern with a period of two days. C(AZ, AE ) ispositive and increases during the predeveloped stage and starts decreasing fromJune 5th, even though C(AE, KE ) is positive and increasing. It is important to note

L. George et al.178

Figure 13Time variation of vertically integrated basic and eddy energies. Units: 105 J m−2.

that the contribution of C(AE, KE ) is the highest among all the conversion terms.It can be inferred that the generation of AE is stronger than the conversion fromAZ. Therefore we may conclude that during the mature stage of the cyclone (i.e., onJune 7th) both barotropic conversion and CISK process contribute significantly.

Figure 14Time variation of vertically integrated different energy conversions. Units: W m−2.

Diagnostic Study of the Cyclonic Storm 179Vol. 154, 1999

Figure 15Vertically integrated time-averaged total energy cycles for (a) predeveloped and (b) developed stages.Units: Energy components 105 J m−2; conversions, boundary fluxes, generation and dissipation, W m−2.

Energy Cycle

Time-averaged total energy cycles for the predeveloped and developed stages ofthe cyclonic storm are shown in Figures 15a and b, respectively. The generationterms of AZ and AE (GAZ and GAE) and the dissipation of KZ and KE (DKZ andDKE) are obtained by residue method.

The predeveloped stage (Fig. 15a) shows that zonal kinetic energy KZ is themost abundant form of energy. The main terms which maintain the balance of

L. George et al.180 Pure appl. geophys.,

zonal available potential energy AZ are the conversion C(AZ, KZ ) which depletesAZ by thermally-driven mean-meridional circulation and the generation GAZ bydiabatic heating processes. The contribution of C(AZ, AE ) which describes thetransport of sensible heat along the mean temperature gradient and the boundaryflux BAZ across the region are small.

For eddy available potential energy AE, the generation GAE by diabatic heatingacts to increase AE and the conversions C(AE, KE ) and C(KZ, KE ) act as sources ofKE where BKE and DKE act as the sink of KE. The large values of KZ content areprimarily due to the conversion C(AZ, KZ ) process. The boundary flux BKZ, thedissipation term DKZ which shows the friction on KZ and the conversionC(KZ, KE ) which is accomplished by momentum transport along the mean windgradient, act to decrease KZ.

During the developed stage (Fig. 15b) a slight decresae in AZ is noted. Thegeneration GAZ and boundary flux BAZ act as sources of AZ, however the valueof GAZ is significantly small compared to its predeveloped stage. This may be thereason for the reduction of AZ in the developed stage. The magnitude of AE

increases (about 30%) from its predeveloped stage. The generation GAE andboundary flux BAE become weak sources of AE. The reduction in the magnitudesof GAZ and GAE in the developed stage indicates the decrease of latent heat.Further, the conversion C(AZ, AE ) is significantly enhanced in the developed stagewhich in turn increases AE, indicating sensible heat transport is very strong in theArabian Sea.

A slight decrease in KZ occurs during the developed stage. Though the dissipa-tion DKZ and the conversion C(AZ, KZ ) act as sources of KZ, the conversionC(KZ, KE ) and the boundary flux DKZ destroy KZ. It is interesting to note that themagnitudes of C(AZ, KZ ) and C(KZ, KE ) are approximately equal. This impliesthat the portion of KZ created by thermally-driven mean-meridional circulation isquickly destroyed by down gradient momentum transport. Though the conversionC(KZ, KE ) through the down gradient momentum transport is found to be of thesame magnitude in both stages, the enhancement of KE in the developed stage isdue to the baroclinic conversion C(AE, KE ) which acts as a major source of KE.The boundary flux BKE is also found to be a weak source.

Conclusion

The present study investigates various dynamical parameters as well as the zonaland eddy energy processes of the synoptic scale circulation in which the severecyclonic storm was developed. Results of the complete three-dimensional energybudget indicate the following main conclusions.

1. The reduction in the generation of both zonal and eddy available potentialenergy (GAZ and GAE) from the predeveloped stage to the fully developedstage suggests a decrease in the latent heating.

Diagnostic Study of the Cyclonic Storm 181Vol. 154, 1999

2. Zonal and eddy energy contents remain relatively comparable throughout theperiod, with a 35 to 40% increase in the eddy circulation processes.

3. Both barotropic and baroclinic conversions are important for the develop-ment of the system. Barotropic conversions remain unchanged throughoutthe period, whereas baroclinic conversions increased significantly due to thefact that the system reached its mature stage, and the process of sensible heattransport is found to be responsible for the baroclinic process.

4. It is important to note that in the fully developed stage an increase ofsensible heat transport is responsible for the increase in C(AZ, AE ) whereasweak latent heating is responsible for reducing GAZ and GAE in the maturestage.

Acknowledgement

The authors are thankful to Director, Indian Institute of Tropical Meteorology,Pune for providing necessary facilities for this research work. They are also gratefulto Director, NCMRWF, New Delhi for providing the data sets for this study.

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(Received February 10, 1998, accepted August 10, 1998)

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