Study of cyclogenesis of developing and non-developing tropical systems of NIO using NCUM forecasting system

Abstract

A study has been carried out to understand the similarities and differences in a pair of developing and a non-developing tropical disturbances using NCMRWF Unified Model (NCUM) and the concepts of marsupial theory of tropical cyclogenesis. We focused on two systems, tropical cyclone (TC) Ockhi that attained Very Severe Cyclonic Storm (VSCS) strength, and another system that attained deep depression state, however, subsequently weakened. As per the first hypothesis of Marsupial Paradigm, genesis of both the systems is found to be associated with a westward moving parent disturbance. The pouch region associated with the parent disturbance is identified as a region of enhanced moisture. The vertical profiles of vorticity and convergence show that the pathway of genesis is bottom-up for both the systems. Further analyses have been carried out to understand why the depression failed to attain tropical cyclone strength. It has been evidenced that the vertical wind shear is too high (> 50 ms− 1) in the premises of the depression core during the post-genesis evolution. At the same time, the center of the depression happened to be in the close proximity of a ridge present over central Indian land mass. Additionally, significant evidence of dry air intrusion into the core of the depression is seen in the Total Precipitable Water (TPW) analysis and HYSPLIT back trajectory analysis. Furthermore, the depression immediately interacts with the colder sea water as it moves northwestward. Though the marsupial paradigm of tropical cyclogenesis is applicable in the North Indian Ocean as well, factors such as SST, wind shear, closeness to the land mass may play a critical role in further development of the system.