Role of land in the unusual track of cyclones Gulab and Shaheen

Cyclones lead to heavy precipitation in a very short period causing severe damage to life and socio-economy along its track. Globally, it is projected that there will be an increase in extreme weather events, which will lead to flooding in places like the Indian subcontinent because of irregular monsoon patterns and cyclonic storms. Extremely rare climatic events occasionally display unexpected phenomena, and cyclone Gulab and Shaheen's formation was one such extraordinary occurrence. Cyclone Gulab developed over the Bay of Bengal on 25^th September 2021. The cyclone moved westward and made landfall on the east coast of India in the state of Andhra Pradesh on 26^th September. Cyclone Shaheen formed in the North East Arabian sea from the remnants of cyclone Gulab. Although these cyclones were not particularly powerful compared to others in this region, it followed a very unusual track. As the cyclone entered the land, it started losing energy but continued to move across the Indian peninsula as a low-pressure system before emerging into the North Eastern Arabian Sea. Favorable atmospheric and oceanic factors for cyclogenesis in this region caused the system to reintensify on 1^st October 2021. The system continued to move westward steadily for two days and intensified into a severe cyclonic storm, Shaheen. On 3^rd October, cyclone Shaheen made landfall on the Northeastern coast of Oman and made history as the first severe cyclone to strike the Northern coast of Oman for one and a half-century.

After the landfall of cyclone Gulab, the low-pressure system sustained over land and eventually developed into cyclone Shaheen, suggesting that land was a significant source of moisture. Thus, in this study, we quantified the moisture contributed by land in the form of evapotranspiration to the cyclones Gulab and Shaheen. We used an Eulerian water tracking technique incorporated in the state-of-the-art Weather Research and Forecasting (WRF) model to track moisture. The model allows us to specify a source region of moisture originating as evapotranspiration, which can be tracked throughout the atmosphere. This moisture is tracked till it results in precipitation or advects out of the domain. The precipitation associated with this tracked moisture is termed recycled precipitation. ERA5, a fifth-generation ECMWF atmospheric reanalysis data, is used to set up the model's initial and boundary conditions. The microphysical, cumulus, and planetary boundary layer schemes used are WSM6, Kain-Fritsch, and YSU, respectively. Eulerian water tracking being one of the most accurate tracking techniques, will enable us to get accurate contributions of different regions and land use to the cyclonic system. In this study, we mainly focus on the contributions of moisture from the forested areas and understanding the role of antecedent soil moisture in sustaining the low-pressure system across the Indian landmass. Our results showed that Northeast India and Myanmar's dense vegetated regions contributed copious amounts of moisture to the cyclonic systems in the Bay of Bengal.