Scale Continuum of Vertical Exchanges between Lower Stratosphere and Surface Layers

Exchanges of momentum, heat, and moisture between layers of atmosphere and upper ocean govern the variability of global climate down to local environments. Vertical exchange mechanisms include the largescale mean flow, planetary to small-scale waves, and ubiquitous turbulent eddies, the latter being particularly prominent in the atmospheric boundary layer (ABL). In this thesis, observational datasets covering a wide range of spatiotemporal scales are analyzed and interpreted to unravel multi-scale vertical exchange processes underpinning the atmospheric variability.

The major component of this work was a study of intraseasonal disturbances in the Indian Ocean (IO) during late boreal winter of 2015, dubbed ASIRI-RAWI. These disturbances drive tropical weather and travel as (theoretically predictable) planetary waves. Upper-air soundings from multiple IO sites as well as model reanalysis were utilized to educe wave activity. Equatorial baroclinic Kelvin waves (KWs) within the stratified lower stratosphere and upper troposphere were identified as dominant patterns repeating biweekly. The eastward- and downward-propagating KWs initiated shear instabilities in the tropical tropopause layer (~17 km). Later when phase propagation brought westerly winds and high barometric pressure to ~12-14 km altitude, KWs coupled with lower-tropospheric disturbances and initiated strong vertical motions within ‘chimney’-like columns of ‘convection’ ~300-500 km wide. High-resolution measurements from remote-sensing instruments and a flux tower at the Seychelles site captured the impacts of these events on the surface layer of ABL as westerly wind bursts (WWBs). The quasi-periodic WWBs were also studied using one-dimensional ocean mixed layer model to estimate response of upper ocean. These finding suggest new mechanisms for upper troposphere interacting with surface layers and should have implications in equatorial air-sea exchange parameterizations of coupled atmosphere-ocean global circulation models.

A case study on turbulence mixing parameters is also conducted based on fine-scale measurements from a specialized hot-film probe deployed in stably stratified shear flow of ABL. Direct measurements of mixing coefficient Γ – used widely in modeling atmospheric and oceanic flows – demonstrated its dependence on multiple parameters, thus underscoring the challenge of parameterizing turbulent mixing in environmental flows. However, during certain intervals the measurements behaved consistently with past laboratory/numerical experiments, which could be explained using physical arguments.