Modeling of moisture transport associated with tropical cyclones

Abstract

Tropical cyclones (TCs) play an important role in the hydrological cycle in the tropics and subtropics. This thesis aims to analyze multiple parameters through modelling to objectively identify the moisture sources for the precipitation of TCs during the genesis, lifetime maximum intensity (LMI), and dissipation stages from 1980 to 2018 in the North Atlantic (NATL), the Eastern and Central North Pacific (NEPAC), the North Indian Ocean (NIO), the South Indian Ocean (SIO), the South Pacific Ocean (SPO), and the Western North Pacific (WNP). The information on TCs in the NATL and NEPAC basins was from the HURDAT2 databases provided by the United States National Hurricane Center (NHC) and the historical records from the Joint Typhoon Warning Center (JTWC) for the remaining basins. Using a Lagrangian moisture source diagnostic method applied to the global outputs of the Lagrangian FLEXible PARTicle dispersion model (FLEXPART) v9.0, the precipitating air parcels over the area enclosed by the outer radius of the TCs were backtracked in time up to 10 days to identify the origin of the humidity that caused the precipitation associated with them. The target region was defined as the area enclosed by the outer radius of the TC. In the NATL basin, the Lagrangian analysis revealed that the Atlantic Ocean north of the mean position of the Intertropical Convergence Zone (ITCZ), including the Caribbean Sea and the Gulf de México, acted as the principal source of moisture for the precipitation of the TCs formed in this basin, contributing approximately 87% of the total moisture uptake. The Atlantic Ocean south of the ITCZ contributed ~11%, being more relevant during the genesis phase. Meanwhile, the contribution from the eastern region of the tropical North Pacific Ocean was small (~2%) but not neglected. In general, the easterly winds and the circulation associated with North Atlantic Subtropical High were the main mechanisms of moisture transport toward TCs locations. The combined contribution from the Arabian Sea, the Bay of Bengal, the Indian Peninsula and the Ganges Basin accounted for ~70% of the total amount of moisture for TCs in the NIO basin, followed by the Indochina Peninsula and the South China Sea with ~20%, and the Western Indian Ocean with ~10%. The wind flow linked to the Somali low-level jet acted as the principal moisture transport mechanism, while the Indian Summer Monsoon and the East Asian Summer Monsoon highly modulated the intensity and extent of sources. In the case of the SIO basin, the highest moisture contribution (~65%) came from the central Indian Ocean and the Wharton and Perth basins (located in western Australia), while the western Indian Ocean supplied ~22% of the moisture. Likewise, the remaining ~13% was supported from northern Australia and the Coral Sea. The circulation of the Mascarene High and westerly monsoon winds over northern Australia were the main drivers of moisture for the TCs in SIO. The moisture that caused the precipitation of the TCs in NEPAC mainly came from the eastern tropical North Pacific Ocean, including Central America (~65%), the eastern tropical South Pacific Ocean (~20%) and the Caribbean Sea (~15%); the trade winds from both hemispheres and the easterly winds that cross over the tropical North Atlantic Ocean and the Caribbean Sea were the main moisture transport mechanisms. Meanwhile, the analysis of moisture sources revealed that during the genesis and peak of maximum intensity, moisture sources for TCs formed over WNP extended eastward with the highest contribution (~60%) from the Western Tropical North Pacific Ocean (WTNPac) and the Philippine Sea, followed by (~25%) the China Seas. Meanwhile, the Bay of Bengal, South Asia, and the central Pacific Ocean near the Southwest Hawaiian Islands supplied the remaining ~15%. However, during the dissipation stage, moisture sources shifted northward, with the largest contributions (~85%) coming from the WTNPac, the East China Sea, the Japan Sea, East China, and the Korean peninsula. The moisture that precipitated in the area enclosed by the outer radius of the TCs was mostly transported by the circulation of winds associated with the western North Pacific Subtropical High and westerly winds associated with the South Asian monsoon. In the SPO basin, the moisture for the precipitation of the TCs in SPO came mainly from the Coral Sea (~40-50%), the western tropical South Pacific Ocean (~20-35%) and northern Australia (~20-30%). The central South Pacific Ocean also contributed about ~10-15% of the moisture. Meanwhile, the convergence of westerly and easterly winds that form the South Pacific Convergence Zone was identified as the main moisture driver towards each cluster. The findings also reveal that moisture uptake in all basins was higher during the hurricane category (Saffir-Simpson Wind Scale Category 1 and 2 hurricanes) than at any other stage. Additionally, the pattern of moisture sources showed that TCs gained more moisture from oceanic sources than terrestrial sources, confirming previous findings of the ocean's role as the source of energy and moisture for the genesis and development of TCs. Furthermore, the analysis of the origin of the precipitation associated with the major hurricanes (Saffir category 3+ hurricane--Simpson wind scale) formed in 2017 in the NATL basin suggests that the highest moisture uptake generally occurs within approximately 3º to 5º of their trajectories. It also shows that evaporation from local sources cannot fully explain the precipitation of TCs, highlighting the role of low-level convergence associated with secondary circulation in transporting moisture to the eyewall. The Lagrangian method of diagnosis of moisture sources applied in this thesis also allows the evaluation of the mean water vapour residence time (MWVRT) for the precipitation associated with the TCs. The highest MWVRT was found in the SIO and SPO basins, being approximately 3.08 days, followed by WNP (~2.98 days), NEPAC (~2.94 days), NIO (~2.85 days), and NATL (~2.72 days). The MWVRT exhibited the highest values towards the equator and decreased poleward. In summary, this thesis shows that the Lagrangian moisture source diagnostic method is a suitable tool to provide useful information on the geographic position of moisture sources for precipitation associated with TCs and to quantify that precipitation. The results can support the forecast of rainfall produced by TCs and, in turn, the possible negative (floods) and positive (drought period attenuation) impacts on the continental hydrological cycle and the associated socioeconomic effects. Identifying the regions where the moisture-producing precipitation accompanies TCs and where they originated can help improve the seasonal prediction of TC activity and related precipitation. Therefore, in the context of global warming and the projected increase in the low-level moisture content at a rate of 6-7\% per degree of sea surface temperature warming, our results could be used as a reference to identify the changes in the moisture sources for the precipitation of TCs in a warmer climate.