Stanford University

*** Ph.D. Thesis/ Oral Defense ***

Thermodynamic behavior of atmospheric heat engines in different environments

Laurel Regibeau-Rockett

Monday, December 2, 9:00 AM

Y2E2 – ROOM 300

Zoom link: Here

Department of Earth System Science

Advisor: Dr. Morgan O’Neill

Abstract In this thesis, I investigate the impacts of environmental variability on the behavior of atmospheric systems, viewed through the atmospheric heat engine conceptual model. I focus on the highly organized convective systems known as Tropical Cyclones (TCs) and disorganized atmospheric convection.

The first portion of this work is concerned with the energetic response of TCs to environmental changes. The TC mechanical efficiency is equal to the ratio of the kinetic energy of the full TC wind field to the total heat input to the TC. In this dissertation, I examine the impact of two environmental variables on the mechanical efficiency and related TC energetics: the sea surface temperature (SST) and solar radiative forcing.

First, I study the variability of TC energetics and mechanical efficiency in a suite of axisymmetric TC simulations with constant SSTs ranging from 295 to 307.5 K. I find that an increase in SST leads to a decrease in this efficiency. This occurs despite an increase in the wind kinetic energy because there is a stronger increase in the total heating of the TC. Additionally, at a higher SST, a much larger fraction of the net heating is consumed by moist processes, instead of as wind kinetic energy, than at lower SST due to the strong increase in atmospheric moisture content expected from the Clausius-Clapeyron relation.

Next, I study how the diurnal cycle of solar radiative forcing impacts the diurnal variability of TC mechanical efficiency and energetics. I analyze output data from both a hurricane nature run and two idealized three-dimensional simulations of TCs, where one is forced by the solar diurnal cycle and the other is a control under perpetual afternoon conditions. I find evidence of diurnal cycles in the mechanical efficiency and most related TC energetics.

The final portion of this dissertation is concerned with the impacts of changes in surface moisture on localized atmospheric convection. Projections of future climate change include an anticipated shift in the global distribution of atmospheric moisture, with some regions expected to dry while others moisten.  I apply the technique of isentropic analysis to a suite of idealized simulations of localized convection with differing values of the surface relative humidity. Isentropic analysis supports the projection of simulated atmospheric convection into different thermodynamically-relevant spaces. Key results include the following: first, we find that the temperature of convective tops in our simulations does not vary with the amount of surface moisture. This supports the generalization of the Fixed Anvil Temperature hypothesis- which proposes that cloud top temperatures are constant as the surface temperature varies- to convection under varied surface humidity. We also observe a deepening of the planetary boundary layer under surface drying, in line with the deeper boundary layers seen over drier regions on Earth. Finally, our driest simulations have a stronger convective mass flux compared to the simulations with greater surface moisture.