Date of Award

Fall 10-2022

Access Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Engineering Physics

Department

College of Arts & Sciences

Committee Chair

Alan Z. Liu

First Committee Member

Wenjun Dong

Second Committee Member

Chester Gardner

Third Committee Member

Christopher Heale

Fourth Committee Member

Michael Hickey

College Dean

Peter Hoffmann

Abstract

GWs significantly impact the Mesosphere and Lower Thermosphere (MLT), and as a consequence of GW breaking, atmospheric turbulence plays an essential role in the mixing and transport of momentum, mass, and chemical materials in MLT. The main focus of this research for the fulfillment of the proposed Ph.D. dissertation is to improve the understanding of the dynamic process of wave breaking and their effects in the mesopause region above Andes Lidar Observatory (30.2◦ S, 70.7◦ W). A few scientific topics related to GW break- ing are addressed. What are the probabilities of the atmosphere becoming convectively or dynamically unstable in the mesopause region and how is it affected by GWs? How turbulence develops as a result of instabilities, and what are the roles of GWs and background in this process? What is the effect of turbulence on heat transport and how is it affected by instabilities?

Data from both observations and models will be used in this investigation. Observational data includes 231-night, 1300 h of lidar measurements. Model data is a simulation from the Complex Geometry Compressible Atmosphere Model (CGCAM). The observation data will be used to study the characteristics of instabilities and turbulence and their relationship with gravity waves. The model data will be used to understand the evolution of the wave braking process and interpret the observations.

We report a detailed analysis of atmospheric stabilities in the mesopause region (85-100 km) based on over 2000 hours of high-resolution temperature and horizontal wind measurements made with a Na lidar at the Andes Lidar Observatory, located in Cerro Pach ́on, Chile (30.25◦S, 70.74◦W). The square of Brunt–V ̈ais ̈al ̈a frequency and the Richardson number are calculated, and occurrence probabilities of convective and dynamic instabilities are derived. An approach to assess the biases due to measurement uncertainties is used to obtain more accurate occurrence probabilities. The overall occurrence probabilities of convective and dynamic instabilities are 2.7% and 6.7%, respectively. High-, medium-, and low-frequency gravity wave (GW) contributions to these probabilities are isolated, which shows that the high-frequency GWs contribute the most but the simultaneous presence of high- and medium-frequency GWs is much more effective in increasing the probabilities. Convective and dynamic instabilities are mainly generated because of the joint effect of different-scale GWs. Isolated parts of GWs have much less contribution to the generation of both convective and dynamic instabilities. The dynamic instability is mainly contributed by less stable stratification and large wind shear together. Either factor can lead to about 15% of dynamic instability. The observational results also show probabilities of instabilities are not only caused by GW activities. Not very stable BG can lead to large probabilities of instabilities as well.

Next, the energy conversion and transfer during GW breaking are investigated with CGCAM simulation. The total kinetic energy change and the amount of energy converted to internal energy and potential energy in a chosen region are derived. Before GW starts to break. Part of potential energy is converted into kinetic energy. Most of the kinetic energy

converted from potential energy is transported out of the chosen region. After GW breaks and turbulence develops, part of the potential energy is converted into kinetic energy. Most of the kinetic energy from potential energy is converted into internal energy. The kinetic energy transfer in GW, turbulence, and BG in the same chosen region and the contributions from different mechanisms are calculated, such as pressure gradient doing work and GW- turbulence interactions. The probabilities of instabilities are calculated at different stages of the GW-breaking process. Instabilities occur 10 minutes before gravity wave starts to break. Probabilities of instabilities reach the peak when turbulence starts to decay. The simulation suggests that GWs propagation leads to instabilities. Instabilities are responsible for GW breaking. Turbulence is generated where GW breaks. During the GW breaking process, GWs lose energy to the background. At the beginning of GW breaking, turbulence gets energy through GW-turbulence interactions and BG-turbulence interactions. After turbulence develops to contain enough energy, it absorbs energy from the background and loses energy to GWs.

With the high-resolution photon counts in Lidar measurements, the turbulence effect on heat transport is derived. The heat fluxes are calculated for convectively stable or unstable, dynamically stable or unstable flow. We found turbulence in unstable flow transports heat downward above 92km with a heat flux of around -0.8K m/s. The turbulence heat flux is about 0.4 K m/s below 92 km. The turbulence heat flux in stable flow is negligible compared with the heat flux in unstable flow, and most heat is transported downward by turbulence in the stable flow because instabilities have short durations.

Through observational and modeling studies, this dissertation investigates the dynamic processes of GW breaking in the MLT region. Turbulence effects on background flow and GWs are derived, which can provide references for parameterizing the effect of turbulence in general circulation models. The results are expected to provide more insights into atmospheric GWs, instabilities, and turbulence.

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