UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Improving our Understanding of Tropical Cyclone Unusual Motion and Rapid Intensification(2019) Miller, William James Schouler; Zhang, Da-Lin; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite steady improvement in their tropical cyclone (TC) track and intensity forecasts over recent decades, operational numerical weather prediction (NWP) models still struggle at times in predicting two TC phenomena: climatologically unusual motion and rapid intensification (RI). Atlantic TCs typically move clockwise along curved tracks skirting the southern, western, and northwestern periphery of the Western Atlantic Ridge. Hurricane Joaquin (2015) followed a particularly unusual hairpin loop-shaped track that was poorly predicted by most operational NWP models, including the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). Over recent years, considerable interest has also developed in understanding the cause-and-effect relationship between RI, defined here as a maximum surface wind (VMAX) intensification rate exceeding 15 m s-1 (24 h-1), and outbreaks of inner core deep convection, known as convective bursts (CBs), that have been observed to precede or coincide with RI in some TCs. A deeper physical understanding of the atmospheric processes governing TC unusual motion and RI, together with retrospective case study analyses of model forecast errors, will help us to identify NWP model components – data assimilation and physical parameterizations, for example – that may need further improvement. This research project seeks to (i) identify the atmospheric features that steered Hurricane Joaquin (2015) along the southwestward leg of its looping track and (ii) investigate the thermodynamic and three-dimensional characteristics of CBs as a first step toward developing a more comprehensive understanding of how CBs may facilitate RI. To accomplish (i), we generate a high-resolution Weather Research and Forecasting (WRF) model Control (CTL) simulation of Hurricane Joaquin (2015) that reproduces its looping track and intensification trends. Comparing CTL forecast fields against sensitivity WRF simulations initialized from perturbed analyses and against two representative GFS forecasts, we find that a sufficiently strong mid-to-upper level ridge northwest of Joaquin and a vortex sufficiently deep to interact with northeasterly geostrophic flows surrounding the ridge are both necessary for steering Joaquin southwestward. These results suggest that more accurate track forecasts for TCs developing in vertically sheared environments may be at least partly contingent on improved vortex initialization; for these cases, assimilation of more inner-core observations such as cloudy radiances and airborne radar-derived winds could be particularly beneficial. We address (ii) by comparing parcel traces, thermodynamic variables, and vertical accelerations along trajectories run through CB updraft cores with trajectories representative of the background eyewall ascent in a Hurricane Wilma (2005) WRF simulation. We compute three-dimensional trajectories from WRF-output winds using a model developed for this study that implements an experimental advection correction algorithm designed to reduce time interpolation errors, with the latter confirmed by tests on analytical and numerically-simulated flows. Results show that Wilma’s CBs are characterized by significant thermal buoyancy, particularly in the upper troposphere; this is consistent with their lower environmental air entrainment rates and reduced midlevel hydrometeor loading relative to the background ascent, and with their updrafts being rooted in portions of the boundary layer where ocean surface heat and moisture fluxes are locally higher.Item Institutions, Poverty, and Tropical Cyclone Mortality(2019) Tennant, Elizabeth; Patwardhan, Anand; Public Policy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tropical cyclones can result in thousands of deaths when the exposed population is unprepared or ill-equipped to cope with the hazard. Evaluating the importance of institutions and socioeconomic conditions for these deaths is challenging due to the extreme variability in hazard exposure. Studies of socioeconomic risk factors that do not account for exposure will be imprecise and possibly biased, as a storm’s path and intensity are important determinants of mortality and may be correlated with socioeconomic conditions. I therefore model and then control for hazard exposure by spatially interacting meteorological and socioeconomic data, allowing me to develop novel evidence of socioeconomic risk factors. In essay 1, I construct a global dataset of over one thousand tropical cyclone events occurring between 1979 and 2016. Controlling for population exposure to strong winds and rainfall, I find that higher levels of national government effectiveness are associated with lower tropical cyclone mortality. Further, deaths are higher when exposure is concentrated over a subset of the population that is already less well off. In essay 2, I investigate whether local government capacity and poverty alleviation can reduce tropical cyclone deaths, using panel data from 78 provinces and 1,426 municipalities in the Philippines. Tropical cyclone exposure is concentrated in wealthier regions of the Philippines, but once wind exposure and rainfall are controlled for I find robust evidence of a link between local poverty rates and cyclone deaths. In essay 3, I investigate the potential for leveraging policy experiments for causal inference about the effects of development interventions on disaster mortality using an existing randomized control trial in the Philippines. This empirical example illustrates how randomization overcomes issues of multicollinearity and omitted variable bias; however, the presence of outliers in exposure and vulnerability to natural hazards interact to make average treatment effect estimates highly imprecise. Strong evidence of an association between government effectiveness and cyclone deaths suggests that capacity constraints need to be addressed in tandem with risk-specific strategies and financial transfers. Further, evidence that local poverty rates and socioeconomic conditions matter highlights the need for equitable and inclusive approaches to mitigating the risk from tropical cyclones.Item Theoretical and Numerical Studies of Tropical Cyclone Development(2008-08-07) Kieu, Chanh Q; Zhang, Da-Lin; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Part I of this dissertation is devoted to a theoretical study of tropical cyclones (TCs), in which a class of exact solutions is obtained. These solutions capture well many important dynamical aspects of the TC development. Major results include: A strong dependence of the TC growth rate on the vertical structure, i.e., the lower the level of the maximal tangential wind, the faster TCs will grow; A much faster TC growth rate inside the radius of the maximal wind than that outside; and The key dynamical roles of the secondary circulation in controlling the evolution and structures of TCs. In particular, the bottom-upward development of the cyclonic flow is demonstrated to be a consequence of the secondary circulation. The new analytical model provides a systematic way to construct the three-dimensional storm structures needed for initialization of TC models. An application of the new theory in deriving the pressure-wind relationship is also presented. In Part II, the genesis of Tropical Storm Eugene (2005) is studied, using a cloud-resolving, multiple-grid simulation with the Weather Research and Forecast (WRF) model. It is shown that the genesis of Eugene is a result of the merger of two mesovortices associated with the ITCZ breakdowns. The simulation captures well the vortex merger as well as Eugene's life-cycle developments. Some key findings include: The merger of mesoscale vortices is critical for the genesis of Eugene; The total potential vorticity associated with the merging vortices increases substantially during the merging phase as a result of the net internal dynamical forcing between the PV condensing and diabatic production and partly from the continuous PV fluxes from the ITCZ; and Cyclonic vorticity grows from the bottom upward during the merger due to deep convection caused by the low-level frictional convergence and latent heating. Without deep convection, little vorticity growth could result from the vortex merger.