NUMERICAL SIMULATION OF A BAY OF BENGAL TROPICAL CYCLONE:
A COMPARISON OF THE RESULTS FROM EXPERIMENTS WITH JRA-25 AND
NCEP REANALYSIS FIELDS
Dodla Venkata Bhaskar Rao, Desamsetti Srinivas and Dasari Hari Prasad
Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, Indiae-mail: dvb_1949@yahoo.com
INTRODUCTION
Tropical cyclones are known for their devastation and damage all over the world. Predictionof the intensification and the movement of the tropical cyclones is important for planning anddisaster management. Numerical models based on well defined dynamical and physical processes
provide quantitative weather prediction. Numerical modeling studies of tropical cyclones are being
continuously attempted since 1960s to understand the physical and dynamical mechanisms oftropical cyclone development and movement. With the developments in atmospheric modeling andcomputer technology, mesoscale atmospheric models with non-hydrostatic dynamics are currently
being used for tropical cyclone studies. As of short range weather prediction, numerical prediction oftropical cyclones is dependent on the accuracy of the initial conditions provided for the modelintegration. Numerical models require specification of the initial state of the atmosphere (i.e.) of the3-D fields of wind, pressure/ geopotential, temperature and humidity in addition to the 2-D fields ofsurface pressure, sea surface temperature, land surface characteristics and topography. These data
are to be prescribed at the grid points of the model domain as per the chosen horizontal and verticalresolutions. Since observations are not available at the required grid points of the model domain,
they are interpolated using an objective analysis procedure. This was the method adopted till theglobal reanalysis fields have become available. The first global reanalysis were provided by
NCEP/NCAR (NCEP-RA) for the period starting from 1948 (Kalnay et al, 1996). These data areavailable at a horizontal resolution of 2.5° latitude/ longitude. ECMWF have produced global
reanalysis fields for the 15 year period from 1979 to 1993 with a resolution of 200 km (Gibson et al,1997) and then second reanalysis for the 45 years from September 1957 to August 2002 at a higher
resolution of about 120 km (Uppala et al, 2005). The JRA-25 (Japanese 25-year) Reanalysis data at aresolution of 120 km for the period from 1979-2004 have become available recently (Onogi et al2007). These data facilitated their application for numerical models in lieu of objective analysis toproduce the gridded initial fields.
In the present study numerical prediction experiments of an intense tropical cyclone over
North Indian Ocean have been made with NCAR MM5 model with the initial conditions taken from
NCEP-RA and JRA-25 global reanalysis fields. The model predicted intensification and movementof the tropical cyclone with the two different data sets have been compared to discuss the relative
merits of the utilization of NCEP-RA and JRA-25 data.
DESCRIPTION OF THE MODEL, DATA AND THE CASE STUDYNCAR MM5, a non-hydrostatic primitive equation model, developed by Pennsylvania State
University (PSU)/ National Center for Atmospheric Research (NCAR), has versatility to choose thedomain region of interest; horizontal resolution; interactive nested domains and with various options
to choose parameterization schemes for convection, planetary boundary layer (PBL), explicit

1moisture; radiation and soil processes (Grell et al, 1994). For the present study, the model isdesigned to have two interactive nested domains with horizontal resolutions at 90 and 30 km
covering the Bay of Bengal and neighborhood region as shown in Figure 1. The model is integrated
for 120 hours starting from 0000 UTC 25 October 1999.
The initial conditions for the two model
domains were taken from JRA-25 and NCEPRA global fields available at 1.25 and 2.5 degree
resolution respectively corresponding to 0000
UTC 25 October 1999. The model landuse has
13 categories as per USGS classification and themodel topography for the two domains wereinterpolated from the USGS topography data at30´ and 10´ resolutions. Time varying lateralboundary conditions were derived at every 6 hr
interval during the period 0000 UTC of 25
October to 0000 UTC of 30 October, 1999. Theintensity and the position of the Orissa superFigure 1: Model domains withcyclone are taken from the reports of the IMD
90 and 30 km resolution(India Meteorological Department, 2000) forcomparison with the model results.
Orissa super cyclone (OSC) is the most intense cyclone occurred over the North Indian
Ocean during the past century. The life cycle of this cyclone is about 7 days i.e. from 25 -31
October 1999. This system was identified as a low pressure at 99102500, intensified to attain thestrength of a very severe cyclonic storm at 99102718 and of super cyclone (with estimated central
surface pressure of 912 hPa and wind speed of 140 knots) at 99102818. The system consistently
moved westnorthwestward throughout its life cycle and had its landfall near Paradip (20.5 N, 86 E)
around 99102903.
RESULTS
Two numerical prediction experiments were performed with NCAR MM5 model to predictthe development and movement of the Orissa super cyclone taken up as a case study. For both theexperiments, the model domain and the choice of the parameterization schemes are same but differ
only in the specification of the initial and lateral boundary conditions taken from JRA-25 andNCEP-RA fields. The model was integrated for 120 hours starting from 00UTC of 25 October 1999.
The model predicted intensity in terms of CSLP (central sea level pressure) and maximum wind andthe track positions are compared with IMD reports. The results are briefly presented as follows.
The model predicted track positions from the two experiments along with the IMD reports
are shown in Figure 2. The numerical experiment with JRA-25 produces very good track prediction
up to 72 hours and then the predicted movement deviates slightly towards right of the observed
track. The vector track errors (Figure 2) are less than 70 km up to 72 hours and then slowly increase
to 150 km in 96 hr and 200 km at 120 hours. Correspondingly the experiment with NCEP-RA showsdeviation right from the beginning, first towards right up to about 24 hours and then towards left ofthe observed track with the vector errors consistently increasing (i.e.) with errors of 150 km at 36hours increasing to 200 km at 60 hours, 350 km in 96 hours and 490 km at 120 hours. This clearly2demonstrates the superiority of the JRA-25 reanalysis fields in the specification of not only theinitial vortex but also the large scale environmental fields surrounding the vortex region.
Vector track errors (km)JRA-25
NCEP-RATime (hr)Figure 2. Model predicted track positions along with IMD estimates (left) and vector track errors(right) at different times of prediction
The model predicted intensity, in terms of CSLP and maximum wind associated with thetropical cyclone, for the two experiments are compared with IMD estimates for validation (Figure3a). The numerical experiment with JRA-25 predicts gradual deepening up to about 48 hours
attaining 985 hPa, and then rapidly deepens to attain a CSLP of 960 hPa at 78 hours and then littlevariation indicating the attainment of the mature stage. Correspondingly the experiment with NCEPRA has pre- deepening up to 54 hours attaining CSLP of 989 hPa, then deepens to reach 965 hPa at
90 hours and slowly gets filled up. It is to be noted that though JRA-25 has an initial CSLP of 1000hPa and NCEP-RA has 1005 hPa, the model predicted CSLP coincides with IMD estimates at 12hours and produces the deepening period between 48 hours and 96 hours nearly agreeing with theobservations. However both the experiments could not predict the observed intensity of the cyclonei.e., the CSLP of 912 hPa.
The time variation of the model predicted maximum wind (Figure 3b) shows that theexperiment with JRA-25 has an initial strength of 20 m/s which weakens slightly during the first 12hours and then gradually increases up to 48 hours followed by rapid intensification attaining amaximum strength of 55 m/s at 96 hours. Correspondingly the experiment with NCEP-RA shows
intensification up to 24 hours, then remains constant up to 48 hours and then rapidly increases toattain a maximum of 50 m/s at 90 hours. Though neither of the two experiments produce theobserved intensity of the cyclone, experiment with JRA-25 produces slightly stronger cyclone but
the time of attainment of maximum wind in both the experiments nearly coincide with IMD reports.
It is to be noted that in the experiment with JRA-25, the cyclonic vortex strength decreases during
the first 12 hours and then undergoes intensification as indicated by slight increase of CSLP anddecrease of maximum wind. This may be due to the adjustment of vortex circulation to the large
scale environment as JRA-25 incorporate assimilation of TCR wind data.
3(b) Maximum wind (m/s)(a) Central sea level pressure (hPa)JRA-25
NCEP-RAIMD
JRA-25
NCEP-RA
IMD

Time (hr)Time (hr)Figure 3. Time variation of (a) central sea level pressure (hPa) and (b) maximum wind (m/s) fromthe experiments with JRA-25 and NCEP-RA along with IMD estimates.
In view of the above described results that the numerical experiment with the initial and timevarying boundary conditions taken from JRA-25 has shown better simulation of the intensificationand movement of the Orissa super cyclone, the differences in the initial conditions from JRA-25 andNCEP-RA are examined. The JRA-25 fields show slightly stronger tropical cyclone vortex at theinitial time (i.e.) 00 UTC of 25 October 1999. The distribution of mean sea level pressure (Figure 4a,b) show CSLP of 998 hPa in the JRA-25 as compared to 1004 hPa in NCEP-RA. This indicates thevortex as an intense low pressure system in the JRA-25 and closer to the observations as comparedto the weak low pressure in the NCEP-RA. The wind field at 925 hPa (Figure 4 c,d) show stronger
vortex with winds exceeding 16 m/s over a small region to the south of the centre with the radius of
the maximum wind at about 300 km. Correspondingly NCEP-RA shows organized vortex withmaximum wind strength of 8-16 m/s and with radius of maximum winds of about 400 km. Thisshows better representation of the magnitude of the cyclone vortex in the JRA-25 which may be dueto the special features of JRA-25 to assimilate wind profiles around tropical cyclone (TCR) as
reconstructed from the best track information. The vorticity at 925 hPa (Figure 4 e,f) shows thevortex to have stronger cyclonic vorticity in the JRA-25 with a magnitude of 12e-5 /s as compared to8e-5 with the NCEP-RA. The cyclonic vorticity is noted to extend up to 200 hPa (Figure 5a) in both
the data but with stronger intensity in the JRA-25 at all levels. A comparison of vertical profiles ofthe divergence at the respective locations of the surface maximum convergence show that theconvergence at lower levels is nearly the same in both the fields, but the divergence at upper levels
above 300 hPa divergence is stronger in the JRA-25 (Figure 5b). These differences clearly indicate abetter specification of the vortex structure at the initial time in the JRA-25 reanalysis fields. The
essential reason for the better representation of the initial vortex may be due to the impact of theassimilation of tropical cyclone retrieval wind data on the tropical cyclone representation in JRA-25
reanalysis. The impact is probably evident for the Bay of Bengal region where observations aresparse. This also demonstrates the superiority of JRA-25 to the NCEP-RA which did not have thiskind of TCR wind assimilation.

4JRA-25

NCEP-RA

(a)

(b)

(c)

(d)

(e)

(f)

MSLP

WindVort

Figure 4. Horizontal distributions of (a,b) mean sea level pressure (hPa);(c,d) wind vectors and magnitude (m/s) and (e,f) vorticity (*1e-5) )/s) at theinitial time of model integration (00 UTC of 25OCT1999) as derived fromthe JRA-25 (left panel) and NCEP-RA fields (right panel) .

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(a)NCEP-RA(b)NCEP-RAJRA-25JRA-25Figure 5: Vertical profile of (a) vorticity (*1e-5) (/s) and (b) divergence(*1e-5) (/s) at the locations of their maximum cyclonic vorticity/
convergence value at the 925 hPa level.
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