Factors Driving Intensification of Pre-Monsoon Tropical Cyclones Over the Bay of Bengal: A Comparative Study of Cyclones Fani and Yaas

Abstract

This research paper investigates the factors responsible for tropical cyclones’ intensification process, focusing on pre-monsoon cyclones Fani and Yaas. Using the WRF model, the study examines the dynamics and thermodynamics of the two cyclones in three phases: pre-intensification, intensification, and post-intensification. The results are compared with available satellite data sets and it was found that Fani had higher wind speeds and a more structured system with high specific humidity concentrations closer to the storm center than Yaas. Before the RI of Fani, there was a sudden increase in moisture convergence rate, hydrometeors, and diabatic heating, facilitating the formation of cloud ice at the upper level of the troposphere and the release of higher latent heat energy. In contrast, the lack of moisture convergence in Yaas resulted in a weaker updraft and reduced latent heat release. Further analysis suggests that the heating in Fani was more intense and widespread in the inner eyewall region than the core, compared to Yaas. In general, it is also noted that an enhanced bulk kinetic energy (BKE) is a clear indicator for intensification and during the intensification phase, sustained BKE production and moderate dissipation are evident. The latent heating budget parameters indicate a continuous accumulation of moisture with nearly constant moisture consumption during the intensification process. These features are more distinct for Fani compared to the Yaas. The study concludes that during RI, dynamical processes play a dominant role compared to thermodynamical processes.