The effects of atmosphere-ocean-wave coupling during tropical cyclone

• Main Author: Garg, Nikhil
• Other Authors: Ng Yin Kwee, Eddie
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2018
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics; DRNTU::Science::Physics::Meteorology and climatology; DRNTU::Social sciences::Geography::Oceanography
• Online Access: http://hdl.handle.net/10356/73516
• Institution: Nanyang Technological University

It is well recognized that air-sea interaction affects the development and modulation of tropical cyclone (TC). However, open questions remain as to how and to what extent air-sea interaction affects the structure and intensity of a TC. This research project investigates and provides a better understanding of the coupling of atmosphere, ocean and ocean wave systems at the interface (i.e. sea surface) and its impacts on the TC structure. It is intuitive to think that, at the earth's surface, the fluid (both air and water) layer belongs to a single system, thus, justifying the construction of a single dynamically coupled model where atmosphere and ocean are interacting on different temporal and spatial scales. Within this coupled system, it can be thought that the ocean waves generated at the sea surface are due to the momentum transfer from atmosphere, which subsequently gets transferred to ocean. This transfer of momentum (from atmosphere to ocean) is not a one-way link, i.e., there are feedback effects from ocean (both in terms of enthalpy and momentum) on the atmospheric boundary layer. Earlier studies using coupled models have shown improvements in TC modelling and prediction. Studies using theoretical analyses and laboratory experiments have argued that the state of the art bulk parametrisation schemes that are currently implemented in these models are unable to account for the ocean wave related processes (e.g., sea state dependent surface stress, sea spray mediated heat and momentum flux). In summary, the atmosphere-ocean-wave coupling affects the boundary layer and the TC structure. However, the highly complex and nonlinear nature of these processes makes them difficult to parametrise, which necessitates more work in the field of air-sea interaction. Moreover, given the growing interest in renewable energy and wider acceptance of numerical modelling for the assessment of available renewable energy resources, improved modelling systems with better representation of air-sea interaction have the potential to contribute towards better forecasts of wind, wave and ocean currents. These models also have the potential to be used within the early warning systems, and to aid in human activities in areas such as fisheries, coastal constructions, marine transportation and oil exploration. This research project utilises a dynamically coupled atmosphere-ocean-wave model to quantify the effects of air-sea interaction on the TC. A new fully coupled modelling system was designed and implemented. This coupled model was designed with the capability to conduct simulations in coupled and uncoupled configurations, so that the effects of different physical processes can be isolated. In addition to the coupling interface, the modelling system also includes several modules and modifications for including effects of processes such as sea spray and ocean currents. A series of simulations using different model configurations are carried out to study the effects of coupling wave model, sea spray mediated heat and momentum fluxes, sea surface temperature and ocean currents on a TC named Hurricane Arthur, that traversed through North Atlantic in 2014. During the course of this study, special attention was paid to the often inadvertently overlooked factors that can modulate the intensity of a TC. For instance, recent studies using field observations and laboratory measurements have concluded that the momentum exchange coefficient saturates at wind speed >33m/s. However, little is known about the behaviour of coefficient of enthalpy at such wind speeds. Furthermore, a number of studies have argued that the model skill can be potentially improved by including the effects of sea spray. However, most if not all, of the sea spray parametrisation depend only on the wind speed. Due to this wind speed dependency, these sea spray parametrisations have a tendency to overestimate the spray flux contributions. It can be posited that the sea spray generation should depend on both wind speed and sea state. A sea state dependent spray parametrisation was developed, where the spray generation was estimated from the wave energy dissipated through whitecapping was developed as part of this study. Furthermore, the effects of atmosphere-ocean coupling on a TC are investigated, where the influence of sea surface temperature and ocean currents is considered. Within the atmosphere-ocean coupled model, the effects of surface waves were also considered, where the surface waves affected ocean model by means of radiation stress, while affecting the momentum flux in atmosphere model. Model results are evaluated using a comprehensive set of atmosphere and oceanic observations obtained from airborne radar, dropwindsonde, satellite altimetery and moored buoys. Model results have shown that coupling a wave model enhances the momentum flux from atmosphere to ocean, where this enhanced momentum flux acts to reduce the wind speeds in high wind region, while increasing the enthalpy flux. Additionally, the coupling of ocean model affects the intensity of modelled TC, where the reduced sea surface temperature decreases the intensity of TC. The coupling of ocean currents also influences the intensity of TC, by reducing the momentum flux from atmosphere to ocean. In conclusion, coupling of atmosphere, ocean and wave models affects the intensity and structure of TC. The coupling modifies the low-level boundary layer structure and the physical properties of near surface flow in TCs, which subsequently affects the physical mechanisms such as convective organisation. Lastly, it was shown that at the air-sea interface, there are a number of physical processes that can affect the TC. Unfortunately, these complex physical processes cannot be completely parametrised, thus, obviating the usage of fully coupled models.