Hurricane Florence (2018) was unique due to its slow storm motion during landfall, causing convective rainbands to produce high amounts of precipitation along the coast of North Carolina. This study focuses on the relationship between precipitation asymmetries and atmospheric stability surrounding the tropical cyclone (TC) during the landfall period of a nearly-stationary TC. Previous research with idealized hurricane simulations suggests that atmospheric stability may vary surrounding a TC during landfall, with the atmosphere destabilizing offshore and stabilizing onshore. However, this finding has not been studied using a realistic approach. Due to Hurricane Florence's slow motion, the storm was situated at the land-ocean boundary for multiple days, providing an ideal opportunity to examine the role of atmospheric stability in modifying hurricane precipitation during landfall. This study uses the Advanced Research Weather Research and Forecasting (WRF-ARW) version 3.6.1 to produce high-resolution simulations to examine the variations in precipitation and atmospheric stability surrounding Hurricane Florence. Precipitation accumulation at different temporal scales was used to determine that asymmetries existed during the landfall period. Observed and model-simulated Convective Available Potential Energy (CAPE) were used to measure stability surrounding the TC. Simulated CAPE indicates that there was a significant difference between stability right- and left-of-track. In addition to a control simulation, two experimental simulations were conducted by modifying the land surface to vary the heat and moisture exchange coefficient (HS) and hold the surface roughness (Z0) constant. By isolating the HS to be more moist or dry, the altered low-level moisture was hypothesized to cause the precipitation and convection distributions to become more symmetrical or asymmetrical, respectively. The results from the experimental simulations showed that the altered land surface affects the relative humidity from the surface to 950 mb, which has an immediate impact on stability off-shore left-of-track. Overall, the precipitation and stability asymmetries were not significantly impacted by the altered near-surface moisture, indicating other physical factors contribute to the asymmetries. The results of this study provide insight into the role of atmospheric instability in generating asymmetrical precipitation distributions in landfalling TCs, which may be particularly important in slow-moving TCs like Hurricane Florence. / Master of Science / Landfalling tropical weather systems such as hurricanes can significantly impact coastal communities due to severe flooding and damaging winds. Hurricane Florence (2018) affected coastal and inland communities in North Carolina and South Carolina when the storm produced a significant amount of precipitation over the coastal region. During landfall, the center of Hurricane Florence moved slowly parallel to the coastline, which creates a suitable time frame to isolate and study the influence of landfall on precipitation asymmetries. Precipitation asymmetry occurs when more rainfall falls on one side of the hurricane; for example, heavier precipitation tends to occur on the right side of a hurricane during the landfall period. Hurricane rainbands that are responsible for producing heavy precipitation form in areas where there is higher moisture near the surface while lighter precipitation forms in areas where there is drier air near the surface. This study focuses on the relationship between land surface moisture and spatial variations of precipitation during the hurricane landfall period by studying observations and model simulations of Hurricane Florence. The model simulation of Hurricane Florence found that more precipitation fell on the right side of the storm, indicating that there was precipitation asymmetry. In order to understand how the precipitation asymmetries form, the model simulation of Hurricane Florence was modified to create two experiments. In the first experiment, the land surface was altered to have a moister land surface, which should cause the hurricane precipitation to be more symmetrical. In the second experiment, the land surface was altered to have a drier land surface, which should cause stronger precipitation asymmetry. However, the results did not match this expectation. Instead, both experiments simulated asymmetrical precipitation with more precipitation falling on the right side of each storm during the landfall period. These results suggest that the modified land surface moisture did not have a significant impact on the formation of precipitation asymmetries. Other factors are therefore suggested to have a more dominant influence on the development of precipitation. Overall, this work can support future studies by ruling out the impact of land surface moisture on a hurricane's precipitation formation during the landfall period.

The climatology of precipitation asymmetries in Tropical Cyclones (TCs) and their relationship to TC intensity changes using 16 years of data from the Tropical Rainfall Measuring Mission (TRMM) satellite. TC Inner core precipitation asymmetries were quantified using the Fourier wavenumber decomposition method upon the pixel level data of 3,542 TRMM TMI overpasses. Composites of wavenumber–1 and wavenumber 1–6 total precipitation asymmetries were constructed to show the distribution pattern under different storm motion speed, vertical wind shear and the combined effects of varying vertical wind shear and storm motion. Results indicate that motion–relative total precipitation asymmetry is located down–motion. The phase of motion–relative maximum asymmetry shifts cyclonically by adding the wavenumber–2–6 asymmetry to wavenumber–1. Shear is more dominant than motion on the distribution of precipitation asymmetry. The analysis of combined effects of motion and shear shows when shear is weak, and shear is to the left of motion, the precipitation asymmetry is explained more by storm motion. The main contributor to the general asymmetry pattern is from the moderate and heavy precipitation. The wavenumber 2–6 energy localizes the maximum heavy precipitation asymmetry. The quantified wavenumber 1–6 asymmetries is also applied to differentiate between different intensity change categories and the asymmetry evolution of a rapidly intensifying storm. The precipitation asymmetry properties of rapid intensification (RI) and non–RI storms are examined. The dataset of 2,186 global tropical storms through category 2 hurricanes is divided by future 24–h intensity change and includes storms with at least moderately favorable environmental conditions. The normalized wavenumber 1–6 asymmetries, indicates quantitatively that the lower asymmetry of precipitation is most strongly correlated with future intensity change. The precipitation field of non–RI storms are more asymmetric than RI storms. The 595 sampled overpasses are classified into 14 categories in the timeline of an RI event from 48 hours before RI until RI ends. The decrease of normalized wavenumber 1–6 asymmetries in the inner core region of all four types of precipitation several hours before RI onset was quantitatively demonstrated to be critical for TC RI.

Thesis advisor: Amy Frappier / Tropical cyclones (TCs), known as hurricanes in the Atlantic and Typhoons in the Pacific, are among the most destructive and deadly natural disasters that occur on Earth. Attempts to understand how TCs relate to the global climate system, and future risk assessments are dependent upon having records of TC activity that pre-date the modern meteorological records, which are commonly not older than 130 years (Nott, 2003). Paleotempestology is a sub-discipline of paleoclimatology that attempts to extend the TC record beyond the meteorological record through the use of proxies. Presented here is the establishment of a paleotempestology proxy based on clastic mud being suspended in the water column by floods caused by TCs and deposited in stalagmite CH-1, collected in June of 2007 from Cenote Chaltun-Ha, a low-lying cave from the Yucatán Peninsula. CH-1 was dated by a combination of <super>210</super>Pb, U/Th, and layer counting techniques, creating an age model for its entire length. The years with mud layers were compared to the historical TC record from 1852-2006. Nineteen mud layers were identified for this time period. All of the mud layers deposited in years with at least one TC passing within 330 km of Cenote Chaltun-Ha. A total of 265 mud layers were identified in CH-1 dating from 198 BC to 2006 AD. Relatively high TC frequency, ~16 mud layers/century, was recorded in CH-1 from 198 BC to ~1233 AD. This period was followed by eight centuries of relatively low TC frequency, ~7 mud layers/century. The low frequency period hit a low point in the 1600s with 4 mud layers. Since the 1600s the TC frequency recorded in CH-1 has been on the rise, indicating possible periods of higher than present TC frequency in the future for the Yucatán region. This trend of high TC frequency followed by a sudden drop and subsequent increase is also reported in published sand overwash deposit research. &#948;;<super>18</super>O and &#948;;<super>13</super> values collected from CH-1 provide insight into the timing of regional droughts. A low &#948;;<super>18</super>O value from 1815 provides evidence that "The Year Without a Summer" caused by the 1815 eruption of the Tambora volcano in Indonesia had a climatic effect on the Yucatán. A land-use signal related to the agricultural production boom of Agave Fourcroydes, a succulent plant known as Henequen, in the early 20th century was also detected in &#948;;<super>13</super>C values. / Thesis (MS) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

<p>This MSc thesis contributes with new information on how vulnerability to tropical cyclone (TC) induced flooding has evolved at two coastal lowland study sites in the town of Corinto in Nicaragua over a period of 50 years. The research was done through rain station data analyses, semi-structured interviews, and focus group discussions. Analyses of changing poverty, human mobility, housing conditions, and occurrence of flood-related diseases were used as additional indicators supporting the overall vulnerability assessment.</p><p>The recent increase in the North Atlantic TC activity can not be seen in the data from Corinto. The both studied neighborhoods built on dump-sites and mangrove marsh have clearly become less exposed, less sensitive, and more resilient to external stress brought by TCs. These two sites have been developing into different directions since one has reached a more prosper status whereas the other is still rather marginal. The former has been supported by the local government while the latter has been growing in a less regulated way.</p><p>This thesis suggests that there are coastal communities in developing countries, which are able to cope with and adapt to extreme climate events even though this kind of vulnerability has been predicted to increase due to global warming.</p>

This MSc thesis contributes with new information on how vulnerability to tropical cyclone (TC) induced flooding has evolved at two coastal lowland study sites in the town of Corinto in Nicaragua over a period of 50 years. The research was done through rain station data analyses, semi-structured interviews, and focus group discussions. Analyses of changing poverty, human mobility, housing conditions, and occurrence of flood-related diseases were used as additional indicators supporting the overall vulnerability assessment. The recent increase in the North Atlantic TC activity can not be seen in the data from Corinto. The both studied neighborhoods built on dump-sites and mangrove marsh have clearly become less exposed, less sensitive, and more resilient to external stress brought by TCs. These two sites have been developing into different directions since one has reached a more prosper status whereas the other is still rather marginal. The former has been supported by the local government while the latter has been growing in a less regulated way. This thesis suggests that there are coastal communities in developing countries, which are able to cope with and adapt to extreme climate events even though this kind of vulnerability has been predicted to increase due to global warming.

Tropical cyclones (TCs) can make significant size changes during their lifetime. Being able to accurately forecast TC size change is important for predicting the onset of storm surge as well as the spatial extent of damaging winds. TC size changes can occur from internal storm dynamics, such as eyewall replacement cycle or from changes in the synoptic environment. In this study, the impacts of changing the atmospheric temperature and air-sea temperature difference on TC size and structure are investigated. The study is conducted in two parts: the first part uses the WRF-ARW model to test the sensitivity of TC size changes to simple changes in the environment; the second part to validates the results from the first part by characterizing the environments associated with real cases of TC size change in the North Atlantic basin. It is found that when the simulated atmosphere is cooled, the initial specific humidity and convective available potential energy (CAPE) decrease but the surface energy fluxes from the ocean increase. The higher surface fluxes produce a wider area of radially-inflowing air in the boundary layer, which supports a larger precipitation field and the formation of outer-core spiral rainbands. The larger precipitation field translates to a larger wind field, which is likely related to the diabatic production of potential vorticity. In contrast, when the atmosphere is warmed the surface energy fluxes reduce, which ultimately inhibits the growth of the TC wind field. The higher initial CAPE and moisture content, however, allow the TC to spin up more rapidly with a compact core of intense precipitation. Thus, it is not the temperature of the atmosphere that is causing the size changes, but instead it is the higher surface energy fluxes that arise from the increased air-sea temperature difference. Diagnostics show that fluxes of angular momentum from the environment are not responsible for the simulated TC size increases, even when the gradient in Earth vorticity is included. Rather, it is the production of energy due to the fluxes from the ocean that is responsible for the TC size increases in these simulations. Finally, a larger TC will increase in size more than a smaller TC in the same environment. In the second part of the study, the environments associated with real cases of TC size change in the North Atlantic Basin were characterized. Size changes were evaluated using the Tropical Cyclone Extended Best Track Dataset, and the environments associated with these size changes were examined using the 6-hourly, ERA-Interim global reanalysis dataset. Environmental composites show that the TCs that made size changes in the deep tropics were typically associated with more environmental, mid-level humidity and higher air-sea temperature difference. The TCs that made large size changes in the extratropics were associated with highly-baroclinic environments and high mid-level moisture south of the TC-circulation center. In general, the environments that were associated with TC size increases in the North Atlantic showed similar characteristics to the size change environments simulated in the first part of this study. In addition, the presence of high, mid-level moisture in both the deep tropics and extratropics was consistent with the results of other modeling studies that have explored the impact of environmental moisture on TC size changes.

Accurate forecasts of tropical cyclone surface wind fields are essential for decisions involving evacuation preparation and damage potential. Towards addressing these actions, a comparison of the CFAN tropical cyclone surface wind field model with the H*Wind wind field reanalyzes is done to assess the accuracy of the CFAN algorithm and to determine potential limitations of its use. 16 tropical cyclones were assessed through correlation coefficient, mean bias, and root mean square error. The resolution of initial conditions to be ingested into the model was also analyzed, along with storm type and whether or not wind shear was a limiting factor. Results suggest that the CFAN wind model accurately predicts the H*Wind analyses in most regions of the TC. The center of circulation has the highest error due to the CFAN wind model treating the center of circulation as a point rather than having finite lateral extent. Results from the sensitivity analysis based on input resolution show that the minimum input resolution for the CFAN wind model to produce fine spatial resolutions with high fidelity is 0.25°. It is shown that the reproductions of weaker tropical cyclones have lower accuracy due to wind field asymmetries within these systems, while stronger TCs are better reproduced, as these systems are usually better organized. Finally, through the wind shear analysis, it is shown that the accuracy of reconstruction is not dependent on the magnitude of vertical wind shear.

Eyewall replacement cycle (ERC) is frequently observed during the evolution of intensifying Tropical Cyclones (TCs). Although intensely studied in recent years, the underlying mechanisms of ERC are still poorly understood, and the forecast of ERC remains a great challenge. To advance our understanding of ERC and provide insights in improvement of numerical forecast of ERC, a series of numerical simulations is performed to investigate ERCs in TC-like vortices on a f-plane. The simulated ERCs possess key features similar to those observed in real TCs including the formation of a secondary tangential wind maximum associated with the outer eyewall. The Sawyer-Eliassen equation and tangential momentum budget analyses are performed to diagnose the mechanisms underlying the secondary eyewall formation (SEF) and ERC. Our diagnoses reveal crucial roles of outer rainband heating in governing the formation and development of the secondary tangential wind maximum and demonstrate that the outer rainband convection must reach a critical strength relative to the eyewall before SEF and the subsequent ERC can occur. A positive feedback among low-level convection, acceleration of tangential winds in the boundary layer, and surface evaporation that leads to the development of ERC and a mechanism for the demise of inner eyewall that involves interaction between the transverse circulations induced by eyewall and outer rainband convection are proposed. The tangential momentum budget indicates that the net tendency of tangential wind is a small residual resultant from a large cancellation between tendencies induced by the resolved and sub-grid scale (SGS) processes. The large SGS contribution to the tangential wind budget explains different characteristics of ERC shown in previous numerical studies and poses a great challenge for a timely correct forecast of ERC. The sensitivity experiments show that ERCs are strongly subjected to model physics, vortex radial structure and background wind. The impact of model physics on ERC can be well understood with the interaction among eyewall/outer rainband heating, radilal inflow in the boundary layer, surface layer turbulent processes, and shallow convection in the moat. However, further investigations are needed to fully understand the exhibited sensitivities of ERC to vortex radial structure and background wind.

Climate and weather play a significant role in patterns of air pollution occurrence and severity. An analysis of the effect of weather on pollution parameters in Hong Kong was performed. Hong Kong is one of the world's most densely populated regions and air pollution can be problematic, which is a serious public health concern. Hong Kong is impacted by Tropical Cyclones which strongly affect weather patterns. In this research, a twelve-year record (2007-2018) of tropical cyclone (TC) and pollutant concentrations (carbon monoxide, ground-level ozone, nitrogen dioxide, sulfur dioxide, and particulate matter) were analyzed to investigate the effects of TC on air quality. It is found that the occurrences of TC are strongly related to days with elevated particulate matter, sulfur dioxide and carbon monoxide concentrations (above 90th percentile), and low concentrations (below the 10th percentile) for nitrogen dioxide. In particular, the spatial location of TC with respect to Hong Kong is found to be clearly associated with high or low pollutant concentrations. When the TC is located to the North/Northeast of Hong Kong, the air quality tends to be poor because polluted air from mainland China is advected over the city. Conversely, TC located to the West resulted in good air quality by ventilating the city with relatively clean air from the ocean. / Graduate

Severe weather events frequently affect important marine fish stocks and fisheries along the United States Atlantic and Gulf of Mexico coasts. However, the effects of these events on fish and fisheries are not well understood. The availability of self-reported data from two fisheries in a region frequently affected by tropical cyclones provided a unique opportunity to investigate short-term responses to past events. This study involved selecting severe weather events, calculating changes in effort and catch-per-unit- effort (CPUE), and analyzing those changes across various temporal, spatial, and species-specific scenarios. Responses in each variable were analyzed within and across scenario factors and explored for correlations and linear multivariate relationships with hypothesized explanatory variables. A negative overall directional change was identified for logbook fishing effort. Based on both correlations and linear models, changes in logbook fishing effort were inversely related to changes in average maximum wind speed. Severe weather events are more likely to affect fishing effort than catch rates of reef-fish species. However, lack of responses in CPUE may also relate to the ability of this study to detect changes. The temporal and spatial scales analyzed in this study may not have been adequate for identifying changes in effort for the headboat fishery, or in CPUE for either fishery. Although there was no region-wide response in CPUE associated with severe weather events, further research on this topic is necessary to determine if storm-induced changes in fishery data are likely strong, long-lasting, or widespread enough to influence the outcome of stock-wide assessments. / Master of Science