Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

David C. Noone

Second Advisor

Graham Feingold

Third Advisor

Balaji Rajagopalan

Fourth Advisor

Karen Rosenlof

Fifth Advisor

Darin W. Toohey

Abstract

Water vapor and clouds both respond to and influence global temperature. Consequently, understanding the processes that control humidity patterns and cloudiness is key for predicting future climate accurately. To elucidate the physical processes controlling water cycle feedbacks in the subtropics, this thesis applies new observational techniques to evaluate moisture and pollutant transport between the Hawaiian boundary layer--near ocean surface--and the dry free troposphere above. Beginning with a case study, paired measurements of humidity and the ratio of heavy-to-light oxygen isotopes in vapor are used to characterize vertical moisture transport as a simple mixing process. Strong mixing events are found to shape moisture transport for multiple days at a time, highlighting the need to understand which factors control the strength of convective mixing.

Large-scale dynamical controls on convective mixing near Hawaii are subsequently evaluated, and the relative importance of large-scale transport and microphysical processes in modifying humidity and pollutant distributions is investigated. Based on isotopic theory, strong convective events are classified as having either high or low precipitation efficiency. While the latter are associated with a branching of the subtropical jet, which facilitates advection of relatively high concentrations of methane and ozone from Eurasia, the former are associated with tropical plumes that transport relatively clean and moist air. Particle number concentrations, in comparison, exhibit clear sensitivity to the precipitation processes associated with convection, with higher precipitation efficiency events resulting in scavenging of pre-existing aerosol that, in turn, appears to facilitate new particle formation at higher altitudes.

To determine whether the resultant variations in vertical distributions of moisture and pollutants influence cloud fields, isotopic retrievals from satellite are used to define precipitation efficiency over broader regions. Precipitation efficiency is shown to be a strong indicator of low-level cloud fraction but only in areas of active convection. This finding is consistent with the idea that as cloud evaporation is favored over precipitation, the resultant drying of the boundary layer inhibits low-cloud formation. The results thus provide critical observational constraints for future studies wishing to evaluate cloud feedbacks and, ultimately, climate model sensitivity.

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