The life cycle dynamics and intensification processes of three long-duration tropical cyclones (TCs), viz., Fani (2019), Luban (2018), and Ockhi (2017) formed over the North Indian Ocean (NIO) have been investigated by developing a high-resolution (6 km ${\times}$ 6 km) mesoscale analysis using WRF and En3DVAR data assimilation system. The release of CAPE in nearly saturated middle-level relative humidity caused intense diabatic heating, leading to an increase in low-level convergence triggering rapid intensification (RI). The strengthening of the relative vorticity tendency terms was due to vertical stretching (TC Fani) and middle tropospheric advection (TCs Luban and Ockhi). The increase or decrease in uppertropospheric divergence led to RI through two different mechanisms. The increase in upper divergence strengthens the vortical convection (in TC Luban and Fani) by enhancing the moisture and heat transport, whereas its decrease caused a reduction in the upper-level ventilation flow at 200 hPa followed by moisture accumulation, enhanced diabatic heating, and strengthened the warm core (TC Ockhi). The RI caused the vortex of three cyclones to extend up to the upper troposphere. The well organised wind during RI led the unorganised, weak, discontinuous vertical vortex columns to become organised with intense vertical velocity throughout the column. Spatial distributions of Okubo–Wiess (OW) parameter showed TC core dominated by vorticity than strain, since deep depression (DD) stages.

$\bf{Highlights}$

$\bullet$ The saturated middle-level relative humidity caused intense diabatic heating, and then release of CAPE led to a rise in low-level spin-up triggering the RI.

$\bullet$ The strengthening of the relative vorticity tendency terms was due to stretching (TC Fani) and middle tropospheric advection (TCs Luban and Ockhi).

$\bullet$ The increase or decrease in upper-tropospheric divergence led to RI through two different mechanisms.

$\bullet$ The RI caused the vortex of three cyclones to extend up to the upper troposphere.

$\bullet$ RI led unorganised, weak, discontinuous vertical vortex columns to become organised with intense vertical velocity throughout the column.