Publication Date

2019-07-20

Availability

Embargoed

Embargo Period

2021-07-19

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2019-06-27

First Committee Member

Sharanya J. Majumdar

Second Committee Member

David S. Nolan

Third Committee Member

Brian E. Mapes

Fourth Committee Member

Mohamed Iskandarani

Fifth Committee Member

Robert F. Rogers

Abstract

This study investigates the outflow of a tropical cyclone (TC) as an agent to relate the large-scale environmental flow to TC inner-core structure and intensity evolution. As the upper part of the TC secondary circulation, the outflow often interacts with the upper-level environmental flow, leading to the formation of an outflow channel. In an idealized simulation of a TC that interacts with an upper-level westerly jet, we find that the outflow-jet interaction induces a rainband outside the eyewall, creating conditions necessary for a secondary eyewall to form. Asymmetric convective cells outside the eyewall are advected radially inward and cyclonically by the low-level inflow and TC primary circulation. The secondary eyewall then forms after the deep convection has surrounded the TC. In contrast, simulations without the jet do not show secondary eyewall formation. Motivated by the limitations of bulk measures of vertical wind shear in representing the complete environmental flow, we find that the outflow channel and asymmetric rainband are directly connected under the presence of the environmental flow. Not only is the rainband an important source of the outflow, but they are also inherently connected through a descending inflow below the outflow, and both serve as the TC response to the environmental flow. Some of the main characteristics of the rainband-outflow relationship is also supported by a real-case simulation of Hurricane Bill (2009). This finding suggests that, under the presence of the environmental flow, the rainband might be an important part of the TC secondary circulation. The last part is related to the TC weakening and recovery under strong vertical wind shear (VWS). We use a new numerical framework to keep the environmental VWS nearly constant with time. Using that method, we are able to separate the TC-induced flow from the environmental flow. We find that the TC outflow is induced to withstand the strong environmental flow in the upper level. Moreover, the TC-induced shear vector, which is a representation of TC outflow, together with the total VWS vector, builds up a region within which most of the asymmetric convection is located. This finding might enable us to more accurately evaluate the asymmetric precipitation and wind distribution around the TC. The TC-induced shear difference might also indicate TC intensification. Although strong environmental shear reduces the TC maximum wind or minimum sea-level pressure, the TC integrated energy is increased as a result of the wind field expansion outside the eyewall by the shear effect. This dissertation is an effort to illustrate the asymmetric forcing created by the environmental flow around TCs. We have not only found that the TC outflow is important in the interaction between the environmental flow and the TC, but also established a simple metric to represent that asymmetric forcing. Hopefully, the findings here can help the improvement of the TC intensity and structure prediction.

Keywords

tropical cyclones; environmental flow; vertical wind shear

Available for download on Monday, July 19, 2021

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