Ocean-Atmosphere Interactions in Tropical Cyclones over North Indian Ocean: an Investigation through Observation and Modeling Approaches

Abstract

Tropical cyclones (TCs) are regarded as one of the most catastrophic meteorological and oceanic phenomena, in which the Northern Indian Ocean (NIO; including the Bay of Bengal (BoB) and the Arabian Sea (AS)) is one of the active basins worldwide. They usually result in severe property damage and significant loss of life through destructive winds, higher storm surges, torrential rains, and severe floods. The TC evolution mainly depends on multiscale airsea processes starting from largescale to synoptic to vortex scales. The NIO has unique airsea interaction due to peculiar water properties, wind systems, and mesoscale ocean and convective vortices. TCs generally form over the warm oceans where the dense network of observations is limited. These insitu observations address the temporal variability of parameters and can also be useful to validate model outputs, remote sensing data, etc. However, the scarcity of these observations limits the spatial variability assessment of airsea parameters and the representation of the ocean mesoscale vortices. The warm ocean plays a critical role in determining the genesis, intensification, maintenance, and weakening of TCs. Identifying and integrating the ocean atmospheric parameters is extremely beneficial to improve the skill of numerical models. The oceanic parameters such as tropical cyclone heat potential (TCHP), barrier layer thickness (BLT), sea surface temperature (SST), and rainfall are undergone profound spatial and temporal changes in the presence of TCs. Understanding the TC evolution response to these parameters helps in better representation of TC processes in the numerical model. However, the NIO basin lacks the baseline climatology of these parameters and hinders the numerical models’ skill. Therefore, better establishing the relationship of BL, TCHP, SST, and rainfall parameters with TC evolution by developing the composite structures helps in understanding of the TC intensification/weakening processes over the basin. The theoretical and modeling studies provided the SST effect on the TC evolution. Concurrently, the knowledge about the TC size changes and destructive potential under ocean warming conditions is limited and needs to be modelled to minimize the potential threat. The errors in initializing/defining the TCrelated circulation (vortex characteristics) from the largescale environment (global analysis) is identified as one of the reasons for model deficiency. It can overcome by providing the quality of the threedimensional information during the initial TC vortex for the improved TC predictions. The present thesis takes the opportunity to study the airsea interaction processes over the NIO basin based on observations and modeling efforts. Correspondingly, the thesis is framed starting viii from introduction to conclusion chapters along with four working chapters. The working chapters, Chapter3 and Chapter4 elaborates on the response of the TC evolution and rainfall to the ocean parametersTCHP/BL/SST. While, Chapter5 discusses the feedback response of ocean parameters to TC rainfall and wind intensity. However, it may be noted that the feedback of modified ocean parameters (SST change by the TC rainfall) is part of the results of Chapter5, but they are not exclusively studied. The SST in the model is essentially modified by the forecasted wind and rainfall, which can be seen in the temporal evolution of SST in Chapter5. The last working Chapter6 demonstrates the impact of TC vortex initialization and relocation in improving TC track, intensity, and rainfall prediction. It also reveals the importance of TC size on defining the TC initial vortex and subsequent forecast. The climatological SSTwind relation holds well over the NIO basin, except in the northern BoB, where the salinity stratification dominates and hindered it. The presence of strong ACE and its associated increase in the SST by ∼ 3−4 °C triggers an increase of (i) TCHP by ∼ 250 − 280% and (ii) enthalpy fluxes by ∼ 370% over the BoB region showing the positive feedback to the TC. The composite analysis shows that most of the TC intensification occurred when the TCHP ranging between ∼ 50 − 80 kJ cm−2 and BLT of ∼ 10 − 30 m over the BoB. The relationship of SST and TCHPA through the composite analysis revealed that VSCS and above intensity stages produce an SST cooling of >0.8 °C, TCHPA of∼ 25−30 kJ cm−2, and rainfall of >9 mm h−1 respectively. Similarly, considering the translation speed, slowmoving TCs induce a maximum SST cooling of 0.5–1.2 °C, TCHPA of 1520 k J cm−2, and rainfall of ∼ 2 − 4 mm h−1 in the TC inner core (0–100 km) region. The individual basin analysis confirms that BoB TCs produces extremely heavy rainfall (∼910 mm h−1) as compared to the AS TCs (very heavy rainfall: ∼____________7–8 mm h−1) in the inner core region. The sensitive experiments on the response of ocean warming (from 1 to 3 °C) to the TC rainfall, size, and intensity show a linear increase and whereas an exponential growth is seen in the case of destructive potential parameter to SST increase. The vortex initialization method in the ARW model improves the track, intensity, rainfall prediction of landfalling TCs Giri (2010) and Jal (2010) over the BoB. The present dissertation provides (i) a baseline for evaluating numerical models, (ii) understanding/identifying the processes that might not be modeled appropriately, (iii) information to improve vortex initialization in the coupled model environment. It is intended that these efforts helps to achieve better improvements in the TC predictions over the North Indian Ocean.