An Analysis of Environmental Dynamical Control of Tropical Cyclone Intensity over the Bay of Bengal during 1981-2010 - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 80

wind shear has negative effect on TC intensification (Gray, 1968; McBride
and Zeh, 1981). Kotal et al. (2009) found that vertical shear of magnitude less
than 11.0 ms -1 was favourable for developing TC over the Bay of Bengal. Zehr
(1992) determined a threshold value for vertical wind shear of 12.5 ms -1 for
intensification of TCs over the western North Pacific. By considering the effect
of various environmental forcing in his coupled axisymmetric TC model,
Emanuel et al. (2004) found the greatest role of environmental vertical wind
shear on TC intensity among other environmental forcing.
2. Data
For the present study, we consider a sample of 88 tropical cyclones (TCs)
formed over the Bay of Bengal during the period from 1981 to 2010. TC data
used to analysis the effects of translation speed (TS) and vertical wind shear
(VWS) on intensification of TCs consist of intensity (maximum sustained
surface wind) and position obtained from the Joint Typhoon Warning Centre
(JTWC) best track data. Vertical wind shear (ms -1 ) is derived from European
Centre for Medium Range Weather Forecasting (ECMWF) ERA-40 Re-
Analysis daily fields available at 2.5° latitude-longitude grid. As ECMWF
(ERA-40) reanalysis data is available freely on the Internet up to August 2002,
for this exercise NCEP (National Centre for Environmental Prediction)
reanalysis data has been used after August 2002 available at 2.5° latitude-
longitude grid. National Oceanic and Atmospheric Administration (NOAA)
optimum interpolation (OI) sea surface temperature (SST) analysis data
available at 1° latitude-longitude grid intervals (Reynolds et al., 2002) is used
to derive climatological SST. The climatological vertical wind shear (ms -1 ) is
derived from NCEP reanalysis data (horizontal resolution 2.5° × 2.5°) for pre-
monsoon season (March, April, May), post-monsoon season (October,
November, December) and monsoon season (June, July, August, September)
(available at http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/
printpage.pl).
3. Effect of Translational Speed on Intensification
3.1 Translational Speed (TS) Group and Intensification
Figures 1(a-f) present a function fit between maximum intensification and
corresponding TS for 12 h, 24 h, 36 h, 48 h, 60 h and 72 h intervals respectively
(Dotted line indicates maximum intensity change line and solid line indicates
best-fit line). The figures illustrate that the maximum intensification (MI) of
tropical cyclone is non-linear function of translational speed (TS). To find the
nature of the polynomial, the range of TS from 0 to 10 ms -1 is stratified into
twenty TS groups each of 0.5 ms -1 interval for 12 h, 24 h, 36 h, 48 h, 60 h and
72 h intervals. Each observation was assigned to the nearest midpoint of each
translational speed group. The maximum intensity changes were plotted against