Browsing by Author "Bell, Michael M., advisor"
Now showing 1 - 11 of 11
Results Per Page
Sort Options
Item Open Access A potential vorticity diagnosis of tropical cyclone track forecast errors(Colorado State University. Libraries, 2023) Barbero, Tyler Warren, author; Bell, Michael M., advisor; Barnes, Elizabeth A., committee member; Chen, Jan-Huey, committee member; Klotzbach, Philip J., committee member; Zhou, Yongcheng, committee memberA tropical cyclone (TC) can cause significant impacts on coastal and near-coastal communities from storm surge, flooding, intense winds, and heavy rainfall. Accurately predicting TC track is crucial to providing affected populations with time to prepare and evacuate. Over the years, advancements in observational quality and quantity, numerical models, and data assimilation techniques have led to a reduction in average track errors. However, large forecast errors still occur, highlighting the need for ongoing research into the causes of track errors in models. We use the piecewise potential vorticity (PV) inversion diagnosis technique to investigate the sources of errors in track forecasts of four high-resolution numerical weather models during the hyperactive 2017 Atlantic hurricane season. The piecewise PV inversion technique is able to quantify the amount of steering, as well as steering errors, on TC track from individual large-scale pressure systems. Through the systematic use of the diagnostic tool, errors that occur consistently (model biases) could also be identified. TC movement generally follows the atmospheric flow generated by large-scale environmental pressure systems, such that errors in the simulated flow cause errors in the TC track forecast. To understand how the environment steers TCs, we use the Shapiro decomposition to remove the TC PV field from the total PV field, and the environmental (i.e., perturbation) PV field is isolated. The perturbation PV field was partitioned into six systems: the Bermuda High and the Continental High, which compose the negative environmental PV, and quadrants to the northwest, northeast, southeast, and southwest of the TC, which compose the positive environmental PV. Each piecewise PV perturbation system was inverted to retrieve the balanced mass and wind fields. To quantify the steering contribution in individual systems to TC movement, a metric called the deep layer mean steering flow (DLMSF) is defined, and errors in the forecast DLMSF were calculated by comparing the forecast to the analysis steering flow. Lag correlation analyses of DLMSF errors and track errors showed moderate-high correlation at -24 to 0 hrs in time, which indicates that track errors are caused in part by DLMSF errors. Three hurricanes (Harvey, Irma, and Maria) were analyzed in-depth and errors in their track forecasts are attributed to errors in the DLMSF. A basin-scale analysis was also performed on all hurricanes in the 2017 Atlantic hurricane season. The DLMSF mean absolute error (MAE) showed the Bermuda High was the highest contributor to error, the Continental High showed moderate error, while the four quadrants showed lower errors. High error cases were composited to examine potential model biases. On average, the composite showed lower balanced geopotential heights around the climatological position of the Bermuda High associated with the recurving of storms in the North Atlantic basin. The analysis techniques developed in this thesis aids in the identification of model biases which will lead to improved track forecasts in the future.Item Open Access A simplified approach to understanding boundary layer structure impacts on tropical cyclone intensity(Colorado State University. Libraries, 2018) Delap, Eleanor G., author; Bell, Michael M., advisor; Maloney, Eric D., committee member; Venayagamoorthy, Subhas Karan, committee memberThe relationship between tropical cyclone boundary layer (TCBL) structure and tropical cyclone (TC) intensity change is difficult to understand due to limited observations of the complex, non-linear interactions at both the top and bottom boundaries of the TCBL. Consequently, there are debates on how the TCBL interacts with surface friction and how these interactions affect TC intensity change. To begin to address these questions, a conceptual framework of how axisymmetric dynamics within the TCBL can impact TC intensity change is developed from first principles in the form of a new, simple logistic growth equation (LGE). Although this LGE bears some similarities to the operational LGE Model (LGEM; DeMaria 2009), the difference is that our growth-limiting term incorporates TCBL structure and surface drag. The carrying capacity of the LGE—termed the instantaneous logistic potential intensity (ILPI) in this study—is used to explore the relationship between TCBL structure and TC intensity. The LGE is also further solved for the drag coefficient (CD) to explore the relationships between it and both TCBL structure and TC intensity. The validity of this new LGE framework is then explored in idealized numerical modeling using the axisymmetric version of Cloud Model 1 (CM1; Bryan and Fritsch 2002). Results show that CM1 exhibits changes to TCBL structure and TC intensity that are consistent with the LGE framework. Sensitivity of these results to the turbulent mixing lengths, Lh and Lv, are also explored, and general LGE relation- ships still hold as CD is increased. Finally, the LGE framework is applied to observations, and initial CD retrievals indicate that while this new method is low compared to Bell et al. (2012), they are still plausible estimates.Item Open Access Airborne radar quality control and analysis of the rapid intensification of Hurricane Michael (2018)(Colorado State University. Libraries, 2020) DesRosiers, Alexander J., author; Bell, Michael M., advisor; Barnes, Elizabeth, committee member; Chen, Suren, committee memberImprovements made by the National Hurricane Center (NHC) in track forecasts have outpaced advances in intensity forecasting. Rapid intensification (RI), an increase of at least 30 knots in the maximum sustained winds of a tropical cyclone (TC) in a 24 hour period, is poorly understood and provides a considerable hurdle to intensity forecasting. RI depends on internal processes which require detailed inner core information to better understand. Close range measurements of TCs from aircraft reconnaissance with tail Doppler radar (TDR) allow for the retrieval of the kinematic state of the inner core. Fourteen consecutive passes were flown through Hurricane Michael (2018) as it underwent RI on its way to landfall at category 5 intensity. The TDR data collected offered an exceptional opportunity to diagnose mechanisms that contributed to RI. Quality Control (QC) is required to remove radar gates originating from non meteorological sources which can impair dual-Doppler wind synthesis techniques. Automation of the time-consuming manual QC process was needed to utilize all TDR data collected in Hurricane Michael in a timely manner. The machine learning (ML) random forest technique was employed to create a generalized QC method for TDR data collected in convective environments. The complex decision making ability of ML offered an advantage over past approaches. A dataset of radar scans from a tornadic supercell, bow echo, and mature and developing TCs collected by the Electra Doppler Radar (ELDORA) containing approximately 87.9 million radar gates was mined for predictors. Previous manual QC performed on the data was used to classify each data point as weather or non-weather. This varied dataset was used to train a model which classified over 99% of the radar gates in the withheld testing data succesfully. Creation of a dual-Doppler analysis from a tropical depression using ML efforts that was comparable to manual QC confirmed the utility of this new method. The framework developed was capable of performing QC on the majority of the TDR data from Hurricane Michael. Analyses of the inner core of Hurricane Michael were used to document inner core changes throughout RI. Angular momentum surfaces moved radially inward and became more vertically aligned over time. The hurricane force wind field expanded radially outward and increased in depth. Intensification of the storm became predominantly axisymmetric as RI progressed. TDR-derived winds are used to infer upper-level processes that influenced RI at the surface. Tilting of ambient horizontal vorticity, created by the decay of tangential winds aloft, by the axisymmetric updraft created a positive vorticity tendency atop the existing vorticity tower. A vorticity budget helped demonstrate how the axisymmetric vorticity tower built both upward and outward in the sloped eyewall. A retrieval of the radial gradient of density temperature provided evidence for an increasing warm core temperature perturbation in the eye. Growth of the warm core temperature perturbation in upper levels aided by subsidence helped lower the minimum sea level pressure which correlated with intensification of the near-surface wind field.Item Open Access An examination of the large-scale drivers of North Atlantic vertical wind shear and seasonal tropical cyclone variability(Colorado State University. Libraries, 2021) Jones, Jhordanne J., author; Bell, Michael M., advisor; Klotzbach, Philip J., advisor; Barnes, Elizabeth A., committee member; Maloney, Eric D., committee member; Florant, Gregory L., committee memberThis dissertation characterizes and examines the large-scale sources of variability driving tropical North Atlantic deep-layer vertical wind shear (VWS). VWS is a key variable for the seasonal prediction of tropical cyclone (TC) activity and can be used to assess sources of predictability within a given season. Part 1 of the dissertation examines tropical versus subtropical impacts on TC activity by considering large-scale influences on boreal summer tropical zonal VWS variability, a key predictor of seasonal TC activity. Through an empirical orthogonal function analysis, I show that subtropical anticyclonic wave breaking (AWB) activity drives the second mode of variability in tropical zonal VWS, while El Niño-Southern Oscillation (ENSO) primarily drives the leading mode of tropical zonal VWS variability. Linear regressions of the four leading principal components against tropical North Atlantic zonal VWS and accumulated cyclone energy show that, while the leading mode holds much of the regression strength, some improvement can be achieved with the addition of the second and third modes. Furthermore, an index of AWB-associated VWS anomalies, a proxy for AWB impacts on the large-scale environment, may be a better indicator of summertime VWS anomalies. The utilization of this index may be used to better understand AWB's contribution to seasonal TC activity. Part 2 shows that predictors representing the environmental impacts of subtropical AWB on seasonal TC activity improve the skill of extended-range seasonal forecasts of TC activity. There is a significant correlation between boreal winter and boreal summer AWB activity via AWB-forced phases of the quasi-stationary North Atlantic Oscillation (NAO). Years with above-normal boreal summer AWB activity over the North Atlantic region also show above-normal AWB activity in the preceding boreal winter that forces a positive phase of the NAO that persists through the spring. These conditions are sustained by continued AWB throughout the year, particularly when ENSO plays less of a role at forcing the large-scale circulation. While individual AWB events are synoptic and nonlinear with little predictability beyond 8-10 days, the strong dynamical connection between winter and summer wave breaking lends enough persistence to AWB activity to allow for predictability of its potential impacts on TC activity. We find that the winter-summer relationship improves the skill of extended-range seasonal forecasts from as early as an April lead time, particularly for years when wave breaking has played a crucial role in suppressing TC development. Part 3 characterizes VWS variability within the Community Earth System Model version 1 Large Ensemble (CESM1-LE). The 35 historical runs of the CESM1-LE provide substantially larger samples of the environment and various large-scale drivers than the ERA5 reanalysis that spans 1979 to present. Firstly, ENSO is shown to be the leading mode of tropical Atlantic variability and explains most, if not all, of the structured variance. Secondly, while the CESM1-LE shows robust physical representations of known climate phenomena, their relationships with tropical Atlantic VWS remain marginal except for ENSO. Eigenanalysis applied to the CESM1-LE shows that the principal components are ill-defined and gives no distinct pattern for non-ENSO associated large-scale drivers. Thirdly, composite analyses show that despite the narrow range of VWS variability associated with non-ENSO large-scale drivers, their individual contribution to VWS is noticeably stronger during ENSO-neutral conditions as represented by the large ensemble.Item Open Access Axisymmetric and asymmetric processes contributing to tropical cyclone intensification and expansion(Colorado State University. Libraries, 2020) Martinez, Jonathan, author; Bell, Michael M., advisor; Birner, Thomas, committee member; Schumacher, Russ S., committee member; Davis, Christopher A., committee member; Anderson, Brooke Georgiana, committee memberThis dissertation endeavors to advance our understanding of tropical cyclones (TCs) by investigating axisymmetric and asymmetric processes contributing to TC intensification and expansion. Chapter 2 examines the extreme rapid intensification (RI) and subsequent rapid over-water weakening of eastern North Pacific Hurricane Patricia (2015). Spline-based analyses are created from high-resolution observations collected between 22--23 October during the Office of Naval Research Tropical Cyclone Intensity (TCI) experiment and the National Oceanic and Atmospheric Administration Intensity Forecasting Experiment (IFEX). The first full-tropospheric analysis of the dry, axisymmetric Ertel's potential vorticity (PV) in a TC is presented without relying on balance assumptions. Patricia's structural evolution is characterized by the formation of a "hollow tower" PV structure during RI that persists through maximum intensity and subsequently breaks down during rapid over-water weakening. Transforming the axisymmetric PV analyses from radius-height to potential radius-potential temperature coordinates reveals that Patricia's RI occurs "in-place"; eyewall heating remains fixed to the same absolute angular momentum surfaces as they contract in physical space, contributing to rapid PV concentration. Eddy mixing processes are inferred to concentrate PV radially inward of the symmetric heating maximum during RI and hypothesized to be a primary factor underlying the rearrangement of Patricia's PV distribution during rapid over-water weakening, diluting the PV tower while approximately conserving the absolute circulation. Chapter 3 raises the question: do asymmetries facilitate or interfere with TC intensification? An idealized, high-resolution simulation of a rapidly intensifying TC is examined to assess asymmetric contributions to the intensification process. The inner-core asymmetric PV distribution remains on the same order of magnitude as the symmetric PV distribution throughout the intensification period. Scale-dependent contributions to the azimuthal-mean PV tendency are assessed by partitioning asymmetries into low-wavenumber (large-scale) and high-wavenumber (small-scale) categories. Symmetric PV advection and generation are approximately twice larger than asymmetric contributions throughout the intensification period, but the two symmetric contributions largely oppose one another in the eyewall region. Low-wavenumber advection concentrates PV near the axis of rotation during the early and middle stages of RI and low-wavenumber heating concentrates PV in the hollow tower during the middle and late stages of RI. Therefore, asymmetric processes produce non-negligible contributions to TC intensification and may indeed facilitate the intensification process. Chapter 4 investigates the contributions of incipient vortex circulation and environmental moisture to TC expansion with a set of idealized simulations. The incipient vortex circulation places the primary constraint on TC expansion and in part establishes the expansion rate. Increasing the mid-level moisture further promotes expansion but mostly expedites the intensification process. One of the more common findings related to TC expansion in the literature illustrates a proclivity for relatively small TCs to stay small and relatively large TCs to stay large. Findings reported herein suggest that an initially large vortex can expand more quickly than its relatively smaller counterpart; therefore, with all other factors contributing to expansion held constant, the contrast in size between the two vortices will increase with time. Varying the incipient vortex circulation is associated with subsequent variations in the amount and scale of outer-core convection. As the incipient vortex circulation decreases, outer-core convection is relatively scarce and characterized by small-scale, isolated convective elements. On the contrary, as the incipient vortex circulation increases, outer-core convection abounds and is characterized by relatively large rainbands and mesoscale convective systems. A combined increase in the amount and scale of outer-core convection permits an initially large vortex to converge a substantially larger amount of absolute angular momentum compared to its relatively smaller counterpart, resulting in distinct expansion rates.Item Open Access Eyewall replacement cycle of Hurricane Matthew (2016) observed by Doppler radars(Colorado State University. Libraries, 2018) Cha, Ting-Yu, author; Bell, Michael M., advisor; Rasmussen, Kristen Lani, committee member; Reising, Steven C., committee memberAn eyewall replacement cycle (ERC) can cause significant changes to the intensity and structure of a tropical cyclone, but the physical mechanisms involved in the ERC process are not fully understood due to a lack of detailed observations. Hurricane Matthew was observed by the NEXRAD KAMX, KMLB, and KJAX polarimetric radars and NOAA P-3 airborne radar when it approached the southeastern United States during an ERC event. The radar observations indicate that Matthew's primary eyewall was replaced with a weaker outer eyewall, but unlike a classic ERC, Matthew did not reintensify after the inner eyewall disappeared. The evolution of Matthew's ERC is analyzed by examining the observations from the airborne and ground-based radars near the Florida coast. Triple Doppler analysis is performed by combining the NOAA P-3 airborne fore and aft radar scanning with KAMX radar data during the period of secondary eyewall intensification and inner eyewall weakening from 19 UTC 6 October to 00 UTC 7 October. Four passes of the P-3 aircraft show the evolution of the reflectivity, tangential winds and secondary circulation as the outer eyewall became well-established. Further evolution of the ERC is analyzed through reflectivity and tangential wind derived from the single ground-based Doppler radar observations for 35 hours with high temporal resolution every 6 minutes from 19 UTC 6 October to 00 UTC 8 October using the Generalized Velocity Track Display (GVTD) technique. The single-Doppler analyses indicate that the inner eyewall decayed a few hours after the P-3 flight, while the outer eyewall contracted but did not reintensify and the asymmetries increased episodically. The analysis suggests that the resilient outer eyewall was influenced by both environmental vertical wind shear and an internal vortex Rossby wave damping mechanism during the ERC evolution.Item Open Access From surface to tropopause: on the vertical structure of the tropical cyclone vortex(Colorado State University. Libraries, 2024) DesRosiers, Alexander J., author; Bell, Michael M., advisor; Barnes, Elizabeth A., committee member; Rasmussen, Kristen L., committee member; Davenport, Frances V., committee memberThe internal vortex structure of a tropical cyclone (TC) influences intensity change. Beneficial structural characteristics that allow TCs to capitalize on favorable environmental conditions are an important determinant as to whether a TC will undergo rapid intensification (RI) or not. Accurately forecasting RI is a significant challenge and past work identified characteristics of radial and azimuthal structure of the tangential winds which favor RI, but vertical structure has received less attention. This dissertation aims to define vertical structure in a consistent manner to improve our understanding of how it influences intensity change in observed and modeled TCs, as well as discern when strong winds are more likely to reach the surface with potential for greater impacts. Part 1 investigates the height of the vortex (HOV) in observed TCs and its potential relationships with intensity and intensification rate. As a TC intensifies, the tangential wind field expands vertically and increases in magnitude. Past work supports the notion that vortex height is important throughout the TC lifecycle. The Tropical Cyclone Radar Archive of Doppler Analyses with Recentering (TC-RADAR) dataset provides kinematic analyses for calculation of HOV in observed TCs. Analyses are azimuthally-averaged with tangential wind values taken along the radius of maximum winds (RMW). A threshold-based technique is used to determine the HOV. A fixed-threshold HOV strongly correlates with current TC intensity. A dynamic HOV (DHOV) metric quantifies vertical decay of the tangential wind normalized to its maximum at lower levels with reduced intensity dependence. DHOV exhibits a statistically significant relationship with TC intensity change with taller vortices favoring intensification. A tall vortex is always present in observed cases meeting a pressure-based RI definition in the following 24-hr period, suggesting DHOV may be useful to intensity prediction. In Part 2, numerical modeling simulations are utilized to discern mechanisms responsible for the observed relationships in Part 1. Vertical wind shear (VWS) can tilt the TC vortex by misaligning the low- and mid-level circulation centers which prevents intensification until realignment occurs. Both observed and simulated TCs with small vortex tilt magnitudes possess DHOV values consistent with those observed prior to RI. In aligned TC intensification, DHOV and intensity have a mutually increasing relationship, indicating the metric provides useful information about vertical structure in both tilted and aligned TCs. Vertical vortex growth during RI is sensitive to internal processes which strengthen the TC warm core in the upper-levels of the troposphere. Comparison of a TC simulated in the presence of a concentrated upper-level jet of VWS to a control simulation in quiescent flow indicates that disruption of intensification in the upper levels limits vortex height and intensity without appreciable low- to mid-level tilt. Part 3 focuses on decay of the TC wind field as it encounters friction near the surface in the planetary boundary layer (PBL). Surface winds are important to operational TC intensity estimation, but direct observations within the PBL are rare. Forecasters use reduction factors formulated with wind ratios (WRs) from winds observed by aircraft in the free troposphere and surface winds. WRs help reduce stronger winds aloft to their expected weaker values at the surface. Asymmetries in the TC wind field such as those induced by storm motion can limit the accuracy of static existing WR values employed in operations. A large training dataset of horizontally co-located wind measurements at flight level and the surface is constructed to train a neural network (NN) to predict WRs. A custom loss function ensures the model prioritizes accurate prediction of the strongest wind observations which are uncommon. The NN can leverage relevant physical relationships from the observational data and predict a surface wind field in real-time for forecasters with greater accuracy than the current operational method, especially in high winds.Item Open Access Investigation of relationships between tropical cyclone structure and intensity change(Colorado State University. Libraries, 2022) Casas, Eleanor G., author; Bell, Michael M., advisor; Randall, David A., committee member; Maloney, Eric D., committee member; Venayagamoorthy, Subhas Karan, committee memberRapid intensification (RI) of a tropical cyclone (TC) remains one of the largest sources of intensity forecast error, due in part to internal dynamics that are complex and less well understood. Part of the difficulty in improving understanding of RI is due to complex interactions across a wide range of TC intensities, shapes, and sizes. In this doctoral study, I investigate these interactions by first simplifying the complexity and reducing the dimensionality of the intensity and structure parameter space to distill the key aspects of variability from observations, and then re-introducing physical complexity back into the experimental design through idealized modeling. In Chapter 2, an Empirical Orthogonal Function (EOF) analysis is used to develop the intensity-size framework that lays the foundation for the rest of this doctoral study. In addition to commonly-used TC metrics, a new structural parameter is introduced that describes the decay of tangential wind outside the radius of maximum wind (RMW). The utility of this framework is demonstrated for describing key TC evolutionary features with observations of Hurricanes Rita (2005) and Charley (2004) and numerical simulations of Rita. In Chapter 3, simplified TC analytic profiles are used to construct physically realistic wind fields that can explore the intensity-size phase space. Results suggest that while there are systematic differences between the details of the reconstructed wind fields using different methods, they all are representative of observed variability in TC structure despite being derived from a relatively small set of parameters derived from the EOFs. In Chapter 4, these simplified TC wind profiles are used to investigate the tropical cyclone boundary layer (TCBL) response across our intensity-size phase space using both height-averaged (slab) and height-resolved TCBL numerical models. The results suggest that while there are some different dynamical ramifications of the specific analytic profiles used, the response depends more on the location in the intensity and size phase space than on the differences between analytic wind formulations. The results indicate that (1) strong, big TC profiles produce the strongest supergradient wind within the TCBL; (2) weak, big TCs have the largest RMW contraction as the TCBL adjusts; and (3) weak TCs regardless of size have TCBL responses that are less conducive for intensification. Finally, in Chapter 5, full-physics, axisymmetric models are used to test whether the one-way TCBL responses found in Chapter \ref{c4_results} are consistent with two-way TCBL interactions with influences from convection, and explore the dependencies of intensification rates on TC internal structure. The results suggest that small, strong TCs can achieve the highest rapid intensification rates. The findings suggest that while intensification rates do not systematically vary with contraction rates of the RMW, both intensification and contraction rates do have some dependence on different aspects of TC intensity and size across the phase space. When visualized in the phase space, there is a relatively smooth transition between a "initially large mode" and "initially small mode" of RI. The findings of this doctoral study provide new insights into the role of TC intensity and size in the RI process.Item Open Access Investigation of the dynamics of tropical cyclone precipitation structure using radar observations and numerical modeling(Colorado State University. Libraries, 2023) Cha, Ting-Yu, author; Bell, Michael M., advisor; Rasmussen, Kristen L., committee member; Schumacher, Russ S., committee member; Lee, Wen-Chau, committee member; Reising, Steven C., committee memberPrecipitation from tropical cyclones (TCs) produces significant damage and causes fatalities worldwide. Forecast skill of the structure of precipitation in a TC remains challenging, due in part to limited fundamental understanding of the underlying complex dynamics and limitations in our observational capability. This dissertation seeks to improve our understanding of the underlying dynamics of TC precipitation structure by using and improving radar retrieval techniques and numerical modeling. In Part 1, the vortex dynamics of TC polygonal eyewall structure during rapid intensification (RI) of Hurricane Michael (2018) are examined from ground-based single‐Doppler radar analysis. Although the organization of polygonal precipitation asymmetries has been theorized to be related to vortex Rossby wave (VRW) dynamics, prior studies have had observational limitations that prevent a detailed description of the phenomena. Here, we present the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the axisymmetric and asymmetric winds at 5‐min intervals. Novel single Doppler radar retrievals show that both tangential wind and reflectivity asymmetries rotate at speeds that are consistent with linear VRW theory. Dual‐Doppler winds from airborne radar provide further evidence of the vortex structure that supports growth of asymmetries during RI. In Part 2, the relationship between VRWs and the polygonal precipitation structure is further explored through a simple modeling framework. A two-layer model consisting of a shallow water fluid on top of a slab boundary layer is used to understand the dynamical relationship. The model maintains an approximate gradient wind balance in the free atmospheric layer and parameterizes the diabatic heating produced by convection from the vertical motion out of the boundary layer. The two-layer model provides insight into the essential dynamics of Hurricane Michael's intensification and precipitation structure observed by radar in Part 1. The results show that the convective maxima located at the vertices of an elliptical vortex are due to boundary layer processes and not the free atmospheric convergence. The simulations further show that continuous intensification of the vortex can happen in the presence of elliptical asymmetries and even after the potential vorticity ring breakdown when the diabatic heating is continuously maintained by boundary layer processes. When TCs approach land they can produce voluminous rainfall totals, especially when interacting with complex terrain. Doppler radar can provide the capability to monitor extreme rainfall events over land, but our understanding of airflow modulated by orographic interactions remains limited. In Part 3, a new Doppler radar technique is developed to retrieve three-dimensional wind fields in precipitation over complex terrain. New boundary conditions are implemented in a variational multi-Doppler radar technique to represent the topographic forcing and surface impermeability. A series of observing simulation sensitivity experiments using a full-physics model and radar emulator simulating rainfall from Typhoon Chanthu (2021) over Taiwan are conducted to evaluate the retrieval accuracy and parameter settings. Analysis from real radar observations from Chanthu demonstrates that the improved retrieval technique can advance scientific analyses for the underlying dynamics of orographic precipitation using radar observations.Item Open Access Multi-scale interactions leading to tropical cyclogenesis in sheared environments(Colorado State University. Libraries, 2021) Nam, Chaehyeon Chelsea, author; Bell, Michael M., advisor; Rutledge, Steven A., committee member; Maloney, Eric D., committee member; Reising, Steven C., committee memberTo be, or not to be, that is the question of tropical cyclogenesis. Only a small fraction of tropical disturbances eventually develop into tropical cyclones (TCs). Accurate forecasts of tropical cyclogenesis are difficult because TC development involves a wide range of scales from the stochastic convective scale to a quasi-balanced large-scale flow. This dissertation examines the factors that increase uncertainty around the multi-scale tropical cyclogenesis problem, namely, vertical wind shear (VWS), environmental humidity, and convective organization. These factors were explored using multiple data sources including observations such as dual-Doppler radar, dropsonde soundings, and satellite data for mesoscale case studies, reanalyses data for synoptic and climatological analysis, and extensive ensemble mesoscale modeling for controlled experiments. First, this dissertation presents a detailed observational analysis for multi-scale processes around an incipient wave pouch of Hagupit (2008) that survived through strong VWS and underwent TC genesis. The strong deep-layer VWS (> 20 m s-1) had a negative impact on the development through misalignment of the low and mid-level circulations and dry air intrusion. However, the low-level circulation persisted and the system ultimately formed into a tropical cyclone after it had left the high-shear zone. Here we propose that a key process that enabled the pre-depression to survive through the upper-tropospheric trough interaction was persistent vorticity amplification on the meso-γ scale that was aggregated on the meso-α scale within the wave pouch. In the second part, twelve sets of Weather Research and Forecasting ensemble simulations were created to examine the combined impacts of VWS, environmental moisture, and the structure of the precursor vortex on the uncertainty of TC genesis. Here we hypothesized that the combination of moderate shear and dry air makes an unstable condition for a vortex to intensify or decay, which implies that TC genesis in such environments may be intrinsically unpredictable in a deterministic sense. Based on the close examination of selected ensemble members and statistical analysis of geometric probability distribution and time-lagged correlations for all ensemble sets, we propose a theoretical pyramid diagram of the five processes leading to TC genesis in sheared and dry environments. First, inside their low-level circulations, deep convection emerged over a wider area. Second, a new smaller scale mid-level vortex formed inside the deep convection where the pre-existing mid-level vortex was carried away by VWS. Third, the mid-level vortex and low-level vortex went through a vertical alignment process. Fourth, with sustained vortex alignment, convection organized near the low-level center. Fifth, central pressure fell and wind speed increased; and the system reached tropical cyclone intensity. The results suggest that all successfully developing TCs share a common set of precursor events that lead to TC genesis, while a deficiency in any of the precursor events leads to a failure of genesis. In the third part, we investigated the likelihood of subsequent TC genesis from the "monsoon tail" rainband for TCs in the monsoonal area of the western North Pacific (WNP). The monsoon tail rainband—an elongated rainband in the southwestern quadrant of the TC—is shown to be a common feature for TCs in the WNP due to the climatological northeasterly VWS. Variations in the convective activity are shown to be related to the strength of the low-level and upper-level monsoonal flow on synoptic and seasonal timescales, with VWS having the highest correlation to cold cloud tops in the southwest quadrant. Some monsoon tail rainbands sustain convective organization even after they separated from the pre-existing TCs, but despite the enhanced convective activity, the persistent VWS that produced the rainbands was an overriding negative factor that inhibits genesis. This dissertation provides a detailed look at the complex interactions between VWS and the incipient TC depending on spatial scales, the vertical depth of shear, environmental moisture, and the structure of the TC vortex. The findings herein improve our process-based understanding of why moderate VWS, especially in combination with environmental dry air, produces unstable and uncertain conditions for TC genesis.Item Open Access The unconventional eyewall replacement cycle of Hurricane Ophelia (2005)(Colorado State University. Libraries, 2018) Razin, Muhammad Naufal Bin, author; Bell, Michael M., advisor; Rasmussen, Kristen L., committee member; Bangerth, Wolfgang, committee memberOne of the mechanisms proposed for the spin-up of the tropical cyclone (TC) mean tangential circulation is the convergence of absolute angular momentum above the boundary layer. This mechanism is important for the outer primary circulation and results in the broadening of the TC wind field. We hypothesize that the mid-level inflow associated with the stratiform precipitation in TC rainbands may be instrumental in spinning up the broader circulation, and may be important in the development of secondary eyewalls. Hurricane Ophelia (2005) underwent an unconventional eyewall replacement cycle (ERC) as it was a Category 1 storm located over cold sea surface temperatures near 23°C. The ERC was observed using airborne radar observations during the Hurricane Rainband and Intensity Change Experiment (RAINEX). Data was collected from the single-parabolic X-band radar aboard the National Oceanic and Atmospheric Administration (NOAA) P-3 aircraft and from the dual-beam X-band Electra Doppler Radar (ELDORA) aboard the Naval Research Laboratory (NRL) P-3 aircraft. The two aircraft flew simultaneously along Ophelia's primary rainband during a research flight beginning around 1700 UTC on 11 September 2005, allowing for quad-Doppler wind retrievals along the rainband. Analyses were conducted using a spline-based three-dimensional variational wind synthesis technique. Results showed a broadened tangential wind field associated with the ERC was observed in the stratiform-dominant rainbands of Ophelia. The broadening of the tangential wind field was collocated with the strongest radial advection of angular momentum through the stratiform mid-level inflow and is consistent with the proposed mechanism for TC intensity change.