Multiscale processes in the genesis of a near-equatorial tropical cyclone during the Dynamics of the MJO Experiment: Results from partial lateral forcing experiments

Tropical cyclone (TC) 05A (TC05A, November 2011) over the Indian Ocean originated near the equator and brought huge damages to Sri Lanka and India. The genesis and propagation of TC05A involves interactions with the Madden-Julian Oscillation (MJO), interannual (IA) variations, and other synoptic (SY) systems. To investigate the impacts of the SY, MJO, and IA forcings across the lateral boundaries on TC05A, regional simulation with the Weather Research and Forecast model through the partial lateral forcing approach is conducted. The control experiment reproduces realistic genesis and movement of TC05A and associated heavy rainfall, as well as the mean states and MJO. Both the SY and IA forcings make a moderate contribution to the initiation of cyclonic vorticity and rainfall enhancement in the central Indian Ocean and moderate modification of the TC05A track. However, it is the MJO forcing that fundamentally reproduces a coherent moist, low-level convergence, and a cyclonic environment that is favorable for the development of TC05A. The greatest impact of northern boundary intraseasonal (IS) forcing on the genesis location and path of TC05A is further identified, suggesting the importance of the interaction between the MJO and northeast winter monsoon. In contrast, the southern boundary IS forcing is unimportant and the western and eastern boundary IS forcings play a moderate role in slowing down TC05A and modifying its track, intensity, and rainfall. This study demonstrates that the partial lateral forcing method is an effective tool to identify the key factors controlling the extreme events through regional climate modeling.