Genesis of severe cyclonic storm Mora in the presence of tropical waves over the North Indian Ocean

Abstract

Tropical cyclone (TC) genesis is still a crucial mysterious problem in tropical meteorology. Over the North Indian Ocean (NIO), TC Mora genesis commenced as a depression on May 28, 2017. Kalpana‐IR satellite images indicate a disturbance that developed as a depression within Intertropical Convergence Zone (ITCZ). Tropical Rainfall Measuring Mission (TRMM) rainfall shows the organization of convection that started with clustering of clouds on May 24; thereafter, cloud patches remained intact and became a depression on May 28. During the pre‐genesis period (May 20–28), the Madden–Julian oscillation (MJO) was in phase 2 with amplitude greater than 1 and equatorial Rossby (ER) wave was also active over the Indian Basin. These waves facilitated intense convection through the supply of mid‐level moisture and low‐level vorticity required for the genesis. The presence of ER wave resulted in the formation of circulation at 850 hPa over the equatorial region. From May 22 to 23, the existence of a quasiclosed circulation (QCC) region within ER wave indicates a continuous buildup of moisture and vorticity, separating the region of moist and dry air. Later, the QCC became self‐sustained and evolved into a depression on May 28. The streamline overturning point is the inflection point (IP), and the approximate circulation center is the stagnant point (SP). The coexistence of IP and SP along with the minimum stream function and maximum column integrated total precipitable water (TPW) confirms the potential area of genesis. This criterion is applied to track the seeding of vorticity as the preferred genesis location, 5 days before genesis. These locations match well with the best track data locations provided by the India Meteorological Department after May 28. Processes within the QCC are also studied following the QCC during the formation of the depression.