Role of VHTs and Helical flow organization in the formation of Tropical Cyclogenesis over North Indian Ocean

Abstract:

Tropical Cyclogenesis remains one of the most complex enigmas of dynamic meteorology. Many environmental factors contribute to the genesis of tropical cyclogenesis. Earlier studies confirm tropical cyclogenesis originates from the easterly waves and ITCZ. Over the Atlantic Ocean the genesis takes place from the easterly wave and over different basin in presence of tropical equatorial waves. The main objective of this study is to understand ‘why’ and ‘how’ Mesoscale convective vortices (MCVs) cluster and the role of Vortical Hot towers (VHTs) in the genesis of tropical cyclogenesis. Also the determination of commencement of genesis from the helicity density along with the role of horizontal and vertical components of helicity in developing and non-developing cyclones.

Case study of few tropical cyclones over the North Indian Ocean is carried out. Observation from reanalysis and satellite data shows that Mesoscale convective system (MCS) interaction in presence of large Scale provides the favorable environment of cyclogenesis. Over the North Indian Ocean multiple waves interacts and modulates different requisite parameters compared to the presence of individual wave. Presence of MCS and merging prior to the depression provides the probability of genesis over the region. Aggregation of MCVs till the depression provides the genesis of tropical cyclone. Dynamical parameter Okubu-Weiss is also examined to see the building up of vorticity aggregation prior to the genesis. In order to understand the interaction of MCVs and VHTs in the Pregenesis period, numerical simulation experiment is carried out using global model called ‘Model for Prediction across Scales (MPAS)’ at higher resolution (60-3km) focused over the Indian region(domain:0-150◦E & lat 15◦S-15◦N) to understand the dynamic evolution of VHTs in presence of Large Scale interaction. Initial condition is obtained from ERA-5(Era-Interim) having spatial 0.25◦.Multiple combination of Microphysics along with boundary layer is incorporated to obtain the best simulation of VHTs in the lead time. Different evolution of dynamical and thermodynamical terms is examined during the Pregenesis of tropical cyclones. In order to understand the dynamics that governs the linkage of the primary and secondary circulation in the developing and non-developing Tropical Cyclones, the total kinetic terms <Eˢ> & <Eᵖ> are examined along with the scalar quantity, total helicity <h>. Role of different component of horizontal and vertical helicity are also examined to understand the contribution of each components in the Pregenesis of Tropical cyclones over the Indian ocean in presence of different mesoscales.The importance of the study is to investigate the validity of helicity as an indication of large scale vortex instability.