Sensitivity of initial conditions for the tropical cyclone simulation (Formerly Paper number 2.3)

On October 26, 1999 a tropical cyclone was formed off the Burma coast, which subsequently moved towards the north-west direction and turned into a super-cyclone during its passage over the Bay of Bengal. Finally, the super cyclone hit the coastal city of Bhubaneswar, India and its surrounding areas with a maximum surface wind speed of around 70 m/s and a central pressure of about 910 mb (Category 5 in Saffir-Simpson Scale).

We used the parallel version of the Florida State University global spectral model at T126 resolution. The model was integrated for 120 hours starting 1200 UTC on Oct. 26, 1999. This version is parallelised using message passing interface (MPI) and run on an IBM SP machine. NCEP-AVN global analyses at 1x1 degree horizontal resolution and at 15 vertical levels are used to initialise the model. Several numerical experiments have been conducted to simulate the cyclone track and intensity; i) by shifting the cyclone center and its environment by 0.5 degree in the four directions, (ii) by choosing various SSTs, e.g., climatological mean, monthly mean, weekly mean, satellite derived values once in two days and interactive SSTs, (iii) also by increasing/decreasing the SST fields. The SST data is available in about two day time-intervals, and for our interactive SST scheme, we have interpolated it to 24 hour intervals.

The control model run, performed with weekly mean SST, shows that for first 48 hours the simulated and observed tracks follow quiet closely. Then the forecast starts deviating until the landfall that occurs about 250 km away from the observed location of the landfall. The simulation performed in the case (i) does not improve the track forecast significantly. In this case, the ensemble average of maximum wind speeds simulated at 850 mb is only about 60 m/s, and the minimum central pressure was about 976 mb. Similar results were obtained in the second set of experiments (case ii) where we used various kind of SST products. On the other hand, (case iii) shows that if SSTs are uniformly increased by 2 degree, the maximum wind speed at 850 mb reached about 70 m/s and minimum central pressure was as low as 965 mb. In addition various diagnostic parameters such as total precipitable water, eddy and zonal averages of kinetic and available potential energy have also been estimated to examine the various aspects of this tropical cyclones.