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 82

occurs in a narrow range of translational speeds between 3 ms -1 and 5 ms -1 and
decreases with both the increase and the decrease of the translational speed.
This feature can be explained based on previous theoretical and numerical
studies. If TCs move too slow, oceanic cooling induced by turbulent mixing
generated by surface wind stress curl under the TC will disrupt the intensification
(Schade and Emanuel, 1999; Schade, 2000), while if they move too fast the
resulting asymmetric structure will also inhibit intensification as shown by
Peng et al. (1999).
The scattered diagram in Fig. 1(a-f) suggests that the nature of maximum
intensification is a 3 rd order polynomial function of TS of the form:

dv max(t) =a 3 TS 3 + a 2 TS 2 + a 1 TS + a 0

(1)

for t = 12, 24, 36, 48, 60 and 72 h where dv max = maximum intensification (MI)
in kt and TS = translational speed (ms -1 ).
The nature of relationship specified in (1) shows too slow and too fast
translation which prohibits TC intensification and maximum intensification
can only develop under a narrow range of translational speeds.

3.2 Translational Speed and Relative Intensification

Comparing the different percentile and maximum intensification curve, it is
found that most of the cyclones do not reach their MI. MIs are calculated
during the time of their peak intensification using equation (1) and then relative
intensification (RI) is derived for each cyclone using the following definition
(Zeng et al., 2007):

RI (%) = dv max × 100/MI
The inter-seasonal variability (Table 2) shows cyclones in the pre-monsoon
season tend to reach a larger fraction of their MI than in the post-monsoon and
lowest in the monsoon season. The climatological vertical wind shear over the
Bay of Bengal during the monsoon season is stronger than pre-monsoon and
post-monsoon seasons as shown in Fig. 3(a-c) and the climatological SST is
lower during the post-monsoon season than pre-monsoon season (Fig. 4(a-c)).

Table 2: Seasonal variations of mean Relative Intensification (%)

Season Relative intensification (RI)

12 h 24 h 36 h 48 h 60 h 72 h

Pre-monsoon 57 41 47 44 55 56

Monsoon 21 26 25 20 23 22

Post-Monsoon 31 29 34 37 33 39