Theses and Dissertations from UMD
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Item UNDERSTANDING CLIMATIC FACTORS DRIVING WILDFIRES IN THE WESTERN U.S.(2024) ZHANG, LEI; Li, zhanqing; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wildfires have profound and catastrophic impacts on landscapes and human society and act as important agents in the transformation of ecosystems. Over the past decades, the western United States (WUS) has experienced a significant increase of large wildfires, with substantial rise of the economic and ecological costs. Considerable research efforts towards understanding climate change as a primary driver of larger and more severe wildfires, which exacerbates summer drought, reduces spring snowpack, etc. However, the physical relationships among wildfires in North America (NA) and regional feedback processes to changes in the large-scale circulation, global dryness, and linkages to global warming are still poorly understood. Our observational analyses of wildfire-climate relationships in North America were conducted using diverse independent observations and reanalysis data sets for the period 1984–2014. Results show that the WUS has experienced the most robust increase in burned area. In addition to warming, the WUS has been under the influence of multi-decadal trends in tropospheric relative humidity deficit, reduced cloudiness, increased surface net insolation, and enhanced adiabatic warming and drying from increased tropospheric subsidence, as well as drying from enhanced offshore low-level flow. These trends are found to be associated with a widening of the descending branch of the Hadley circulation, consistent with climate model projections under greenhouse gases warming. This work sheds new light on the underlying regional climate processes affecting wildfire trends in NA and linkages with climate change under global warming. My second work focuses on analyzing the causes of the exceptional 2020 fire season in the WUS. Our comprehensive examination shows this extraordinary year for fires in the WUS is the results of “perfect storm”, a combination of multiple climate and weather extremes events. Extreme fuel aridity in September serves as a compelling example of the critical significance of tropospheric subsidence to the surface and atmospheric RH deficit. The third study evaluates performance of the Canada and US fire indices over the various ecoregions of the WUS. My study also finds Haines index combined with current index further improves the performance of conditional frequency distribution and predictive skill of large fires, suggesting the importance and merit of input from atmosphere dryness and stability into current fire indices.Item A BAROMETER OF SCIENTIFIC CULTURE: THE DEBATED ROLE OF AMERICAN SCIENCE AT THE 1850’S SMITHSONIAN INSTITUTION(2023) Buser, Allison; Woods, Colleen; History/Library & Information Systems; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)During the initial decade of the Smithsonian Institution’s existence, its first secretary, Joseph Henry, sought to establish an institution for the advancement of science that defied popular understandings of scientific work in the United States. From the end of the eighteenth century and into the nineteenth century, American science was infused with republican ideology and was widely expected to prioritize practical results that would directly benefit society at large. At the Smithsonian, Henry sought to establish a boundary between professional, theoretical science, conducted and distributed more selectively among experts, and wider public influence and demand for utilitarian scientific work. Examination of discourse in popular publications reveals that Henry’s plan created an ongoing public debate in the 1850s regarding the Smithsonian’s legitimate scientific mission. This included criticism of the Smithsonian publications program’s inaccessibility and lack of utility to the public as well as many alternative proposals for how the institution might be of better scientific use to Americans. Such expectations that Smithsonian research and resources would serve the general American population were also expressed throughout the correspondence of the Smithsonian Meteorology Project—the Institution’s first major scientific research initiative. Although Henry sought to create a boundary between theInstitution’s work and the public, the utilitarian demands of many of the project’s volunteer observers ensured that the practical goals of the public remained intertwined with Henry’s own goals to promote theoretical science in the development of the Smithsonian. The influential work of this extended scientific community was often made possible through the contributions of additional members of households. Close reading of the meteorological project correspondence reveals an extensive, although often officially unacknowledged, contribution from women and other individuals whose labor was often more fixed to the household. While the public volunteers of the project shaped the trajectory of the Smithsonian, the devalued labor of peripheral contributors to the Institution’s large-scale data work set important precedent for professional scientific frameworks at the end of the century. Overall, the relationship between the early Smithsonian and the public in the 1850s demonstrates that the process of establishing borders defining a professional/amateur dichotomy in American science was uneven. The Institution contended with republican expectations of the scientific public and its projects continued to rely upon contributors without formal or elite credentials who in turn demanded accessible and practical research and shaped scientific institutions. Despite Joseph Henry's contribution to the professionalization and specialization of science, the boundaries of science and who could participate in scientific research remained fluid through the mid-nineteenth century.Item An evaluation of convection-allowing ensemble forecast sensitivity to initial conditions(2021) Schwartz, Craig; Poterjoy, Jonathan; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation aims to advance understanding of initial conditions (ICs) for convection-allowing ensembles (CAEs). To do so, experiments with 80-member limited-area ensemble Kalman filters (EnKFs) were performed over the entire conterminous United States for a 4-week period. The EnKF data assimilation systems differed in terms of their cycling strategies (continuous or partial cycling) and horizontal grid spacings (15- or 3-km horizontal grid spacing). EnKF analyses initialized 36-h, 3-km, 10-member CAE forecasts that were evaluated with a focus on precipitation, providing insights about CAE forecast sensitivity to ICs. Additionally, EnKF analyses were leveraged to isolate CAE forecast sensitivity to resolution of both IC perturbations and central initial states about which IC perturbations were centered. A “blending” approach was also used to produce new sets of CAE ICs by combining small scales from continuously cycling EnKF analyses with large scales from Global Ensemble Forecast System (GEFS) ICs using a low-pass filter. Key results are as follows:• CAE forecasts initialized from continuously cycling 3-km EnKF analyses were more skillful and reliable than those initialized from downscaled GEFS and continuously cycling 15-km EnKF ICs through 12–18 and 6–12 h, respectively. Conversely, after 18 h, GEFS-initialized forecasts were better than forecasts initialized from continuously cycling EnKFs. Blended 3-km ICs led to ~18–36-h forecasts possessing comparable quality as GEFS-initialized forecasts while preserving short-term forecast benefits of unblended continuously cycling 3-km EnKF analyses. • Continuously cycling EnKF analyses initialized ~1–18-h forecasts that were comparable to or somewhat better than those with partial cycling EnKF ICs. Conversely, ~18–36-h forecasts with partial cycling EnKF ICs were comparable to or better than those with unblended continuously cycling EnKF ICs. However, blended ICs yielded ~18–36-h forecasts that were statistically indistinguishable from those with partial cycling ICs. • It is more important for central initial states than for IC perturbations to possess convection-allowing horizontal grid spacing for short-term CAE forecasting applications. These collective findings have important implications for model developers working on next-generation CAEs and suggest paths toward potentially saving computing resources, streamlining processes for improving CAE ICs, and unifying short-term and next-day CAE forecasting systems.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 WINTER STORM TRACKS, RELATED WEATHER, AND SUBSEASONAL-TO-SEASONAL (S2S) PREDICTION IN THE NCEP CLIMATE FORECAST SYSTEM FOR NORTH AMERICA(2019) Lukens, Katherine Elizabeth; Berbery, Ernesto H; Ide, Kayo; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The subseasonal-to-seasonal (S2S) forecast period (2 weeks—2 months) represents a major gap in operational forecasting. Advancing S2S prediction is an international priority, particularly for disaster mitigation and resource management decisions. If storm tracks contain S2S signals, their characterization in long term forecasts could advance S2S prediction by providing important information at longer lead times that may not be acquired from standard wind and precipitation forecasts. Potential damaging effects of Northern Hemisphere winter storm tracks on North American weather are investigated using the NCEP Climate Forecast System (CFS) reanalysis (CFSR). Storm tracks are described by objectively tracking 320-K isentropic potential vorticity anomalies (PV320). Large increases in deep convective heating, near-surface winds, and precipitation are found where strong storms (those with higher PV320) are most intense. The eastern US and North American coasts are most vulnerable to strong-storm related losses, which depend on the dynamics and local population density. Despite representing a small fraction (16%) of all storms, strong-storm tracks have a significant imprint on winter weather potentially leading to structural/economic loss. Storm tracks in weeks 3-4 CFS reforecasts (CFSRR) are examined to assess their potential use in S2S prediction. Removal of statistically significant positive biases in PV320 storm intensity improves general storm track features. CFSRR reproduces observed storm-related weather and the characteristic intensity/frequency of hazardous strong-storm winds. Bias-corrected reforecasts better depict the observed variability in storm-related weather. CFSRR contains useful storm track-related information supporting our hypothesis that storm track statistics contribute to the advancement of S2S prediction of hazardous weather in North America. The weeks 3-4 CFS version 2 (CFSv2) operational forecast performance is evaluated from a storm-focused perspective. CFSv2 retains the ability to predict general storm track behavior. Significant negative biases in storm intensity are apparently driven by mean static stability, with relative vorticity being a secondary driver. CFSv2 partially encapsulates the variability in storm winds and generally reproduces more extreme precipitation observations. Bias corrections improve storm wind forecasts. This work demonstrates that the use of climatological PV storm track statistics coupled with an appropriate storm track bias correction is a powerful instrument for the advancement of S2S prediction.Item IMPROVED SATELLITE MICROWAVE RETRIEVALS AND THEIR INCORPORATION INTO A SIMPLIFIED 4D-VAR VORTEX INITIALIZATION USING ADJOINT TECHNIQUES(2017) Tian, Xiaoxu; Zou, Xiaolei; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microwave instruments provide unique radiance measurements for observing surface properties and vertical atmosphere profiles in almost all weather conditions except for heavy precipitation. The Advanced Microwave Scanning Radiometer 2 (AMSR2) observes radiation emitted by Earth at window channels, which helps to retrieve surface and column integrated geophysical variables. However, observations at some X- and K-band channels are susceptible to interference by television signals transmitted from geostationary satellites when AMSR2 is scanning regions including the U.S. and Europe, which is referred to as Television Frequency Interference (TFI). It is found that high reflectivity over the ocean surface is favorable for the television signals to be reflected back to space. When the angle between the Earth scene vector and the reflected signal vector is small enough, the reflected TV signals will enter AMSR2’s antenna. As a consequence, TFI will introduce erroneous information to retrieved geophysical products if not detected. This study proposes a TFI correction algorithm for observations over ocean. Microwave imagers are mostly for observing surface or column-integrated properties. In order to have vertical temperature profiles of the atmosphere, a study focusing on the Advanced Technology Microwave Sounder (ATMS) is included. A traditional AMSU-A temperature retrieval algorithm is modified to remove the scan biases in the temperature retrieval and to include only those ATMS sounding channels that are correlated with the atmospheric temperatures on the pressure level of the retrieval. The warm core structures derived for Hurricane Sandy when it moved from the tropics to the mid-latitudes are examined. Significant improvements have been obtained for the forecasts of hurricane track, but not intensity, especially during the first 6-12 hours. In this study, a simplified four-dimensional variational (4D-Var) vortex initialization model is developed to assimilate the geophysical products retrieved from the observations of both microwave imagers and microwave temperature sounders. The goal is to generate more realistic initial vortices than the bogus vortices currently incorporated in the Hurricane Weather Research and Forecasting (HWRF) model in order to improve hurricane intensity forecasts. The case included in this study is Hurricane Gaston (2016). The numerical results show that the satellite geophysical products have a desirable impact on the structure of the initialized vortex.Item THE IMPACT OF UPPER-LEVEL PROCESSES ON THE INTENSITY AND STRUCTURAL CHANGES OF HURRICANE SANDY (2012)(2016) Shin, Jung Hoon; Zhang, Da-Lin; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The first part of this study examines the relative roles of frontogenesis and tropopause undulation in determining the intensity and structural changes of Hurricane Sandy (2012) using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy’s four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea-level pressure (SLP) falls, and (iv) re-intensification. Results show typical correlations between intensity changes, sea-surface temperature and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy’s moving northward into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak pressure gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, there is a continuous frontogenesis in the outer region during the last three stages. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical-storm-force wind and structural changes, while deep convection in the eyewall and merging of the final two survived frontal rainbands generate a spiraling jet in Sandy’s northwestern quadrant, leading to its re-intensification prior to landfall. The physical, kinematic and dynamic aspects of an upper-level outflow layer and its possible impact on the re-intensification of Sandy are examined in the second part of this study. Above the outflow layer isentropes are tilted downward with radius as a result of the development of deep convection and an approaching upper-level trough, causing weak subsidence. Its maximum outward radial velocity is located above the cloud top, so the outflow channel experiences cloud-induced long-wave cooling. Because Sandy has two distinct convective regions (an eyewall and a frontal rainband), it has multiple outflow layers, with the eyewall’s outflow layer located above that of the frontal rainband. During the re-intensification stage, the eyewall’s outflow layer interacts with a jet stream ahead of the upper-level trough axis. Because of the presence of inertial instability on the anticyclonic side of the jet stream and symmetric instability in the inner region of the outflow layer, Sandy’s secondary circulation intensifies. Its re-intensification ceases when these instabilities disappear. The relationship between the intensity of the secondary circulation and dynamic instabilities of the outflow layer suggests that the re-intensification occurs in response to these instabilities. Additionally, it is verified that the long-wave cooling in the outflow layer helps induce symmetric instability by reducing static stability.Item Observed Teleconnections in Northern Winter: Subseasonal Evolution and Tropical Linkages(2016) Baxter, Stephen Robert; Nigam, Sumant; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Teleconnections refer to the climate variability links between non-contiguous geographic regions, and tend to be associated with variability in both space and time of the climate’s semi-permanent circulation features. Teleconnections are well-developed in Northern winter, when they influence subseasonal-to-seasonal climate variability, notably, in surface temperature and precipitation. This work is comprised of four independent studies that improve understanding of tropical-extratropical teleconnections and their surface climate responses, subseasonal teleconnection evolution, and the utility of teleconnections in attribution of extreme climate events. After an introduction to teleconnection analysis as well as the major teleconnection patterns and associated climatic footprints manifest during Northern winter, the lagged impact of the Madden-Julian Oscillation (MJO) on subseasonal climate variability is presented. It is found that monitoring of MJO-related velocity potential anomalies is sufficient to predict MJO impacts. These impacts include, for example, the development of significant positive temperature anomalies over the eastern United States one to three weeks following an anomalous convective dipole with enhanced (suppressed) convection centered over the Indian Ocean (western Pacific). Subseasonal teleconnection evolution is assessed with respect to the Pacific-North America (PNA) pattern and the North Atlantic Oscillation (NAO). This evolution is analyzed both in the presence and absence of MJO-related circulation anomalies. It is found that removal of the MJO results only in small shifts in the centers of action of the NAO and PNA, and that in either case there is a small but significant lag in which the NAO leads a PNA pattern of opposite phase. Barotropic vorticity analysis suggests that this relationship may result in part from excitation of Rossby waves by the NAO in the Asian waveguide. An attempt is made to elegantly differentiate between the MJO extratropical response and patterns of variability more internal to the extratropics. Analysis of upper-level streamfunction anomalies is successful in this regard, and it is suggested that this is the preferred method for the real time monitoring of tropical-extratropical teleconnections. The extreme 2013-2014 North American winter is reconstructed using teleconnection analysis, and it is found that the North Pacific Oscillation-West Pacific (NPO/WP) pattern was the leading contributor to climate anomalies over much of North America. Such attribution is cautionary given the propensity to implicate the tropics for all midlatitude climate anomalies based on the El Niño-Southern Oscillation (ENSO) paradigm. A recent hypothesis of such tropical influence is presented and challenged.Item Micrometeorological investigation in relation to forage crop production in Maryland(1953) Decker, A. Morris; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Item The role of negative buoyancy and urbanization in warm season precipitation processes over the US.(2013) Ganeshan, Manisha; Murtugudde, Raghu; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates some important processes for better understanding and modeling warm season rainfall characteristics over the US. In the first part, the causes for commonly observed biases in the simulation of the diurnal cycle of warm season rainfall are explored. Model sensitivity analyses are carried out to identify potential deficiencies in two popular cumulus parameterization schemes, viz. Betts-Miller-Janjic (BMJ) and Kain-Fritsch (KF) schemes, considered suitable for use in mesoscale simulations. A novel approach using remote sensing observations to better understand the relevant trigger processes for convection is demonstrated. The convective trigger in both schemes is found to include weak, implicit constraints above the lifting condensation level (LCL), which may contribute to premature, light rain. In order to adjust for this behavior, a simple modification is made to the KF scheme to allow moist convection to begin only from the level of free convection (LFC). Even with the seemingly strict constraint, the scheme performs adequately in a mesoscale seasonal simulation producing an improvement in the nocturnal phase propagation of rainfall in the Central Plains region. The resolvable processes in the mesoscale model are able to overcome the negative buoyancy below the LFC, thereby reducing biases caused by sensitivity of the scheme's trigger to the grid-scale forcing at the LCL. In the future, such a modified scheme will be tested in regional and global simulations, to evaluate its robustness in varying convective regimes. In the second part of this thesis, a multi-city analysis using high-resolution surface observations over the US, investigates the impact of the Urban Heat Island (UHI) on warm season precipitation. Statistical methods are employed to study the rainfall anomalies associated with propagating and non-propagating storms. A strong variability is observed in the UHI-influence on rainfall based on geographical setting and diurnal forcing mechanisms. Coastal cities may experience a more pronounced positive rainfall anomaly during daytime due to the UHI-sea breeze (or lake breeze) interaction. Apart from the late-afternoon rainfall enhancement, a nocturnal rainfall increase occurs over and downwind of inland cities. The nocturnal urban instability, and its interaction with propagating thunderstorms, is explored in detail using model sensitivity analyses. It appears that urban areas act as "hot spots" during the nighttime favoring convergence of propagating storm cells due to the UHI and enhanced frictional drag. In the future, a better understanding of the contribution of thermal and dynamical effects is needed while planning strategies to reduce the urban land cover impact on climate. The results of this study also suggest that the varying influence of the UHI for coastal and inland cities must be further investigated for improving forecasts, urban water resources management, flood disaster planning, etc.