Date of Degree

2-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

James F. Booth

Committee Members

Yi Ming

Gillian Bayne

Johnny Luo

Tobias Schaefer

Subject Categories

Atmospheric Sciences | Climate | Educational Methods | Fluid Dynamics | Meteorology | Other Education | Science and Mathematics Education

Keywords

Atmospheric Blocking, Geophysical Fluid Dynamics, Climate Change, Weather, STEM Education, Mentoring

Abstract

The field of geophysical fluid dynamics (GFD) includes the study of both the motion and thermodynamic aspects of the atmosphere. These properties are of particular importance because they directly influence both local and large-scale weather and climate and are associated with various phenomena. One phenomena that is particularly influential is atmospheric blocking. Atmospheric blocks are persistent, quasi-stationary anticyclones (a.k.a. high-pressure systems) that occur in the atmosphere and disrupt the flow. Blocks are known to induce heat extremes and cold spells, as well as steer storms and cause numerous types of hazards. Yet despite the hazards associated with blocks, our current physical understanding and simulations of blocking are lacking. For example, no one has fully explained why blocking occurs more in some regions than others or how blocking will change in future climates. Another open question regarding blocking relates to the numerical models used to create climate projections: the models have biases in their representation of blocking, but the cause of the biases has not been determined. As such, this thesis investigates the climatology, dynamics, and impacts of atmospheric blocking within various models and climate forcing conditions. This is divided into three original research components.

The first research component utilizes an idealized moist general circulation model (GCM) to: (1) investigate the dynamics of blocking in an aquaplanet, and (2) determine how the orographically-induced mean circulation features affect the climatology of blocking. For this, a model integration using an aquaplanet configuration is compared to reanalysis and separate idealized model integrations that include mountains. Blocks in the aquaplanet are found to exhibit a similar evolution of eddy momentum flux convergence and geopotential height compared to reanalysis and orographic integrations. In the orographic integrations, we find that blocking is anchored upstream of the high-pressure stationary wave anomaly induced by orography. On the other hand, blocking minimizes near the low-pressure stationary wave anomaly where zonal flow maximizes. As the height of orography is increased, we find a correlation between stationary wave amplitude and hemispherically-averaged blocking frequency. Overall, these results help explain the regional variations in blocking location and emphasize the importance of mean circulation features in setting the frequency and location of blocking.

The second research component compares blocking in two comprehensive GCMs with contrasting ocean forcing. The first model, AM4, is an atmosphere only model forced by observed sea-surface-temperatures. The second model, CM4, is a coupled atmosphere-ocean model. Because it has a free-running ocean, CM4 has more atmospheric mean state biases than AM4. In particular, it exhibits equatorward-shifted mean zonal winds and contracted Hadley circulation compared to reanalysis and AM4. Despite the difference in zonal circulation between AM4 and CM4, however, both models produce similar blocking climatology biases: too much blocking in the Pacific and too little in the Atlantic. This result is consistent with both models’ biases in the stationary wave, which is amplified near the Pacific blocking maxima, and weakened in the Atlantic. This result is not simply self-consistent. We know this because we find that the stationary wave bias exists even if we only consider the time periods when no blocking is detected in the reanalysis. Our results suggest that biases in the stationary wave are more relevant than biases in the climatological jet in generating biases in blocking for these models. Block-centered compositing analysis confirms that the models reproduce realistic onset of Atlantic and Pacific blocking in terms of geopotential height and transient eddy momentum flux convergence.

The third research component focuses on blocking and persistent extreme heat events in summer. This is done by analyzing reanalysis as well as historical and climate change integrations of CM4. First, the regional variation of blocking and heat extreme co-location is investigated. We find that blocks and heat extremes are co-located most often over northeastern and northwestern North America and over Scandinavia and northern Eurasia. For heat events in northeastern North America, we compare those that are associated with blocking to those that are not. The duration and evolution of 500 hPa geopotential height, 2 m temperature, sea-level-pressure, and the 1000 hPa temperature budget equation are analyzed. We find that, on average, blocked heat events are warmer and longer in duration for this region. We then shift focus to CM4, confirming the model’s fidelity in simulating blocking and the association of blocking with heat extremes in the historical integration. In the climate change projection integration, less blocking occurs as compared to the historical integration and the association of blocks with heat extremes also decreases. Consistent with less blocking, composited heat events in the climate change projection exhibit anomalous temperatures that are 1-2 K cooler than in the historical integration. However, this is outweighed by a 6-7 K warming of the mean state.

Chapter 5 provides a summary and conclusions for the three research components. This chapter puts all of the novel physics research completed in for this dissertation in context. It also includes a discussion on possible future work.

Chapter 6 is a standalone chapter focused on original research in STEM education and diversity, in which the effects of mentorship on adolescents of color from underserved communities are investigated. This research grew out of a mentoring program that I co-founded. For this, college student mentors from similar sociocultural backgrounds guide youth throughout two 12-week semesters. Interpretive phenomenological analysis (IPA) is implemented to extract superordinate and subordinate themes from adolescent accounts within the program. The superordinate (subordinate) themes identified are as follows: social-emotional support (support with mental health, and family-like bonds and inclusivity), building paths to academic and professional success (demystifying success, and skills for success), and supporting connections to STEM (nurturing interests in STEM, and seeing oneself in STEM careers).

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