Publication Date

2014-03-27

Availability

Embargoed

Embargo Period

2015-04-24

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Applied Marine Physics (Marine)

Date of Defense

2014-03-11

First Committee Member

William M. Drennan

Second Committee Member

Hans C. Graber

Third Committee Member

Peter J. Minnett

Fourth Committee Member

Neil J. Williams

Fifth Committee Member

Erik Sahlée

Abstract

The marine boundary layer (MBL) is a dynamic region where the ocean is strongly coupled with the atmosphere. This coupling facilitates the exchange of momentum, mass, and heat at the air-sea interface through turbulent processes. In part, these exchanges impact the climate though driving currents, creating waves, triggering ocean mixing, influencing whitecap coverage, impacting aerosol production, and altering atmospheric and oceanic stability. The exchange of momentum and enthalpy at high wind speeds are also responsible for the genesis and dissipation of tropical cyclones and their cold wakes. However, there remains a lack of understanding about the MBL and the interaction between the air and the ocean, particularly at high wind speeds. This dissertation helps fill this knowledge gap by examining energy transfer and MBL turbulence using in situ data collected on floating instrument platforms in harsh conditions and at high wind speeds. Results are presented from two recent field campaigns: the Southern Ocean Gas Exchange Experiment (SOGasex) and the Impact of Typhoons on the Ocean in the Pacific (ITOP) experiment. ITOP data represent the first direct deep ocean measurements taken from a floating platform in typhoons. Mean wind speeds (1 min. sustained) reached 30m/s and significant wave heights above 11m were recorded. Data were analyzed using the eddy-covariance technique and parameterized in terms of the turbulent exchange coefficients for latent heat CE and momentum CD. Results show that CE is generally independent of wind speed. CD is found to have a linear dependence on wind speed up to ~22m/s at which point it features a roll-off. It was found that this was due to a reduction in the downwind component of the momentum flux at the frequency of the peak waves. This occurred during strongly forced conditions when the wind and peak wave directions were closely aligned and significant wave height was very large. Environmental conditions surrounding the drag coefficient roll-off are inductive of some form of flow separation at the wave frequency. The possibility that the roll-off occurred because measurements were made in the wave boundary layer is also discussed. At moderate wind speeds CD is also found to be suppressed in the presence of swell. The evolution of the upper ocean determined from an array of subsurface temperature sensors is also presented. Results reveal a link between surface forcing to changes in sea surface temperature, mixed layer depth, and depth averaged temperature. It was also found that, in some circumstances, cool water upwelled by typhoons was capped by a surface warm layer which masked the still present colder water below. It is noted that this capping represents a challenge when trying to describe the thermal structure of the ocean using surface temperature observations alone. Finally, ocean temperature was modeled using a 1-D mixed layer model. The model was able to determine the approximate timing of mixed layer temperature decrease associated with typhoon cold wake, but did not adequately model the magnitude of the temperature reductions or mixed layer depth changes. In periods of low to moderate wind speeds the model done a good job at capturing the magnitude of diurnal heating. However, under such conditions, the model was found to suffer from run-away foundation temperature due to insufficient nocturnal heat loss.

Keywords

air-sea interaction; marine boundary layer; turbulence; drag coefficient; typhoon; tropical cyclone

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