The Role of the Subtropical Jet Stream in a North American Monsoonal Surge
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The desert regions of the southwestern United States and northern Mexico are noted for rainy summer seasons observed during the North American Monsoon (NAM). The motivation for this thesis is to investigate the mechanisms that trigger multiple precipitation events over the NAM region during the period from 13 - 17 July 2004. This study fully documents a moisture surge event that originates from the Gulf of Mexico , as a mechanism that triggers precipitation over the NAM and the Great Basin regions. The investigation is divided within three chapters. Chapter 1 evaluates the dynamical and thermodynamical origin of the organizational mechanism for a widespread convective system, located over the Gulf of Mexico, analyzed from 28 June - 9 July. Chapter 2 investigates the mid-upper tropospheric convective outflow as a primary mechanism that transports a plume of moisture, which developed subsequently from the widespread convection over Gulf of Mexico, and its propagation into the NAM region, during 10 - 13 July. Chapter 3 analyzes subsequent precipitation events that are related to the Gulf of Mexico moisture surge from 13 - 17 July. The first chapter investigates the extratropical forcing that creates the origin of the moisture surge source. A plume of moisture is enhanced within strong convective system that is organized over the Gulf of Mexico from 28 June - 6 July. Two primary components contributed to a convectively active environment allowing widespread convection to trigger over Gulf of Mexico: component 1) represents an extratropical Rossby wave breaking (RWB) event that induced a relatively cool middle - upper tropospheric layer; and component 2) represents the subtropical jet stream enhanced vertical motion and integrated pressure falls enhancing the convergence that created convective forcing just above the marine planetary boundary layer. The interactions of those two components triggered widespread moist convection, allowing a middle tropospheric moisture plume to develop. This middle tropospheric moistened layer becomes a source of a moisture surge that propagates downstream. The long distance transport of this moisture is investigated in Chapter 2. Chapter 2 investigates the transport process of this middle tropospheric moistened layer which led to the development of various moisture surge features. The subsequent development of the convective moisture plume is tracked from 10 - 13 July by 1) trajectory analysis from observational data and 2) simulations from the Operational Multiscale Environmental Model with Grid Adaptivity (OMEGA) to substantiate convective forcing as a strong signal to trigger a moisture surge over the Gulf of Mexico. Moisture surge features are identified and related precipitation cases are analyzed in depth in Chapter 3. Chapter 3 tracks the elevated moisture plume further in time to fully evaluate the extent of this moisture surge event. Multiple precipitation events are analyzed over the Great Basin region from 13 - 17 July with significant rainfall amounts detected over multiple states including, Nevada, Utah, Colorado and Omaha.