Browsing by Author "Sandra Yuter, Committee Member"

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Effects of Appalachian Topography on Precipitation from Landfalling Hurricanes
(2009-04-13) Harville, Steven L.; Sandra Yuter, Committee Member; Gary Lackmann, Committee Member; Anantha Aiyyer, Committee Chair
A thorough analysis of rainfall distributions associated with tropical cyclones that have impinged upon or impacted the southern and central Appalachian mountain range is conducted using the North America Regional Reanalysis (NARR) and the Weather Research and Forecasting (WRF) model. The primary objective of this study is to improve the skill and precision of future forecasts by identifying specific areas where enhancement of the precipitation associated with landfalling tropical cyclones due to the direct and indirect effects of orography most frequently occurs. Based on the relative positions between the tropical cyclone tracks and the orientation of the Appalachian Mountains, four storm tracks are classified. We identify locations with the highest potential for flooding using local maximum analysis for each representative track. For storms that run parallel along the eastern side of the Appalachians (Track-B), heavy rainfall is located along eastern slopes with the heaviest precipitation falling across western North Carolina and central Virginia. Storm tracks that run parallel on the western side of the Appalachians (Track-C) show heaviest precipitation falling on the eastern slopes of western North Carolina. For storms that track more perpendicular to the mountain range, maximum rainfall is located over the mountains of central Virginia (Track-A) and across the southern Appalachians (Track-D). A second goal of this work is to document some of the effects of these mountains on landfalling tropical cyclones, on the synoptic environment as a whole, and on the interactions of these tropical and mid-latitude cyclones. Work here is focused on expanding upon the synoptic approach of Atallah et al. (2007). This is accomplished through examination of the precipitation climatology, analysis of composites and case studies, and by numerical simulation. Hart and Evans (2006) find that the orientation of the approaching upper-level mid-latitude trough is one of the most significant factors in determining the occurrence of extratropical transition (ET) and the potential for reintensification. Results suggest that through orographic enhancement of the downstream ridge, these storms play an active role in tilting the approaching mid-latitude trough towards a more negative orientation, thus increasing the likelihood of ET. At the same time synoptic-scale frontal boundaries slow and strengthen as they approach the Appalachians from the west, similar to the findings of O'Handley and Bosart (1996). As a result, numerical simulations run with topography show greater precipitation over areas northwest of the Appalachians than experimental simulations run with flattened terrain.
Evolution and Maintenance of the 22-23 June 2003 Nocturnal Convection during BAMEX
(2007-08-01) Billings, Jerilyn Marie; Matthew D. Parker, Committee Chair; Gary Lackmann, Committee Member; Sandra Yuter, Committee Member
On 22-23 June 2003 two mesoscale convective systems (MCSs) evolved throughout the evening and night time hours and were observed by the Bow Echo and Mesoscale convective vortex Experiment (BAMEX). These two MCSs were studied by analyzing the observations, and performing both case study and idealized model simulations. The first of these MCSs originated from a group of supercells that had been initiated in a north-south line along a pre-existing outflow boundary in eastern Nebraska. These supercells anchored to the pre-existing outflow boundary leading to large rainfall totals and facilitating cell mergers. These cell mergers increased the depth and strength of the surface cold pool, which became the forcing mechanism for new convection. As this happened, the convection reoriented from a north-south line of isolated supercells into an east-west, southward propagating squall line. While the squall line was developing and reorienting, isolated supercells developed along the dryline in north-central Kansas. These supercells moved northeastward, eventually passing the southward propagating squall line and evolving into a small MCS that continued to move northeastward during the night. These two modes of convection developed and evolved in a similar nocturnal environment suggesting that each MCS was being forced differently or feeding off of a different source layer. A northeastward mean wind vector explains the motion of all of the cells, including individual cells within the squall line, however, does not account for the differing storm motions of the two resulting MCSs. This can be explained by te presence of a deep cold pool at the surface that was responsible for the maintenance of the southward propagating squall line throughout the nocturnal hours. The nocturnal boundary layer cooled and stabilized, however, convection was able to remain surface-based as long as a mechanism existed to lift air to its level of free convection (LFC). In this study, both cold pool dynamics and supercell dynamics played an important roll in lifting air to the LFC throughout the nocturnal hours.
The Initiation and Evolution of Multiple Modes of Convection Within a Meso-Alpha Scale Region
(2007-10-25) French, Adam James; Matthew D. Parker, Committee Chair; Gary Lackmann, Committee Member; Sandra Yuter, Committee Member
On 30 March 2006 a convective episode occurred featuring isolated supercells, a mesoscale convective system (MCS) with parallel stratiform (PS) precipitation, and an MCS with leading stratiform (LS) precipitation. These three distinct convective modes occurred simulataneously across the same region in eastern Kansas. Multi-modal events are especially challenging for forecasters given the wide range of severe weather threats that accompany the different modes. In order to better understand the mechanisms that govern such events, this study examined the 30 March 2006 episode through a combination of an observation-based case study and numerical simulations. From the results of this study we conclude that, for this event, localized environmental variations were largely responsible for the eventual convective mode, with the method of storm initiation having only limited effects. The resultant mode was very sensitive to both the environmental thermodynamic and shear profiles, as variations in either led to different convective modes within the numerical simulations. Finally, we conclude that while the individual modes each developed within an environment distinctly favorable for that mode, they were able to persist in close proximity to one another due to a "middle ground" environment permissive of all three. Strong vertical shear and moderate instability led to the development of supercells in western Oklahoma and similarly strong shear oriented parallel to a surface dryline coupled with dry air in the middle and upper levels led to the development of the PS linear MCS in central Kansas. Meanwhile, moderate wind shear coupled with high instability and strong linear forcing led to the development of the LS MCS in eastern Kansas. Without this linear forcing, the moderate shear environment was supportive of both linear and isolated supercell modes, resulting in the storms that moved into this region maintaining their original organization.
Investigation of Aerosol Optical Properties in the Ultraviolet Spectrum
(2006-05-15) Hamrick, David Fowler; Gary Lackmann, Committee Member; Frederick Semazzi, Committee Chair; Sandra Yuter, Committee Member; Bill Barnard, Committee Member
Atmospheric aerosols can reduce the amount of ultraviolet (UV) radiation reaching the Earth's surface by scattering this radiation towards space and mitigating the increase of UV irradiance due to stratospheric ozone depletion. The objective of this study is to determine the amount of scattering of ultraviolet radiation by aerosol particles in the earth's atmosphere. Aerosol single scattering albedo (SSA) at UV wavelengths is an important aerosol radiative parameter in determining how aerosols affect the surface UV irradiance. An ultraviolet multi-filter rotating shadowband radiometer (UVMFR-SR) situated in Raleigh, NC, is used to gather the UV irradiances and the aerosol optical depth (AOD). These data are collected at 300 nm, 305 nm, 311 nm, 318 nm, 325 nm, 332 nm, and 368 nm. A total of 15 cloudless days were studied in the Raleigh, NC area from March to September of 2004 at a solar zenith angle of 45 degrees. The values of aerosol SSA in this study ranged from 0.68 to 0.99, with an overall average value of 0.867, indicating that aerosols are scattering most of the incident UV radiation. These results are in good agreement with previous studies. Two of these clear days were also studied for trends in SSA and black carbon levels during different times of the day. It was found that less scattering occurs during the middle of the day. A tropospheric radiative transfer model (TUV) was used to determine the SSA when values of AOD, diffuse-to-direct ratios, solar zenith angle, and time are known. The asymmetry parameter and ground albedo were given assumed values in the model, 0.70 and 0.04, respectively. The SSA values were determined by comparing the output diffuse-to-direct ratios in the model to those actually observed. More values of SSA in the ultraviolet spectrum will allow for better estimation of this parameter for future UV radiative transfer modeling and also reduce the error in estimation of surface UV irradiances.
Mesoscale Convective Systems Crossing the Appalachian Mountains
(2009-07-27) Letkewicz, Casey Elizabeth; Sandra Yuter, Committee Member; Gary Lackmann, Committee Member; Matthew D. Parker, Committee Chair
Forecasting the maintenance of mesoscale convective systems (MCSs) is a unique problem in the eastern United States due to the influence of the Appalachian Mountains. At times these systems are able to traverse the terrain and produce severe weather in the lee, while at other times they instead dissipate upon encountering the mountains. Thus, there exists a need to differentiate between crossing and noncrossing MCS environments. Examination of twenty crossing and twenty noncrossing MCS cases revealed that the environment east of the mountains best separated the cases. The thermodynamic and kinematic variables which had the most discriminatory power included those associated with instability, several different shear vector magnitudes, and also the mean tropospheric wind. Crossing cases were unsurprisingly characterized by higher instability; however, these cases unexpectedly also contained weaker shear and a smaller mean wind. Idealized simulations using a thermodynamic profile favorable for convection revealed that the wind profile is indeed an important factor, but does not uniquely determine whether systems have a successful crossing. All simulated convective systems underwent a cycle orographic enhancement, suppression, and subsequent reinvigoration, the magnitude of which was sensitive to the wind profile. Increasing (decreasing) the mean wind led to greater (less) enhancement and suppression of vertical velocities on the windward and lee sides of the mountain, respectively. The strength of the mean wind also influenced the scale of terrain-induced gravity waves which played a significant role in the reintensication of the convection, along with a hydraulic jump of the cold pool at the base of the mountain in the lee. Variations in low-level shear impacted the intensity of the MCS, yet the simulated systems were always able to successively traverse the barrier due to the influence of the hydraulic jump and mountain waves. Simulations utilizing crossing and noncrossing observed wind profiles suggested that the mean wind exerts a stronger influence than the shear. Despite the differing impacts of the wind profile, the availability of instability appears to be the most important factor to consider when predicting the maintenance of convective systems crossing mountain ridges.
Momentum Transport in Mesoscale Convective Systems
(2009-08-27) Mahoney, Kelly Marie; Anantha Aiyyer, Committee Member; Sandra Yuter, Committee Member; Gary Lackmann, Committee Chair; Matthew D. Parker, Committee Member
The transport of horizontal momentum by vertical motions within a mesoscale convective system (MCS) affects storm dynamics, sensible weather, and the connection between the system and its surrounding environment. Earlier works have examined this process for a number of purposes, but understanding of its significance to both MCS motion and the generation of convectively-driven surface winds remains incomplete. This study describes the convective momentum transport (CMT) process both qualitatively and quantitatively; this is pursued through the analysis of quasi-idealized numerical simulations. Momentum budgets illustrate that the motion of a numerically-simulated MCS is significantly impacted by CMT within the MCS. Vertical advection of the perturbation wind is found to contribute largely to the momentum field at the leading edge of the cold pool, which is the region in which the resulting accelerated winds drive system motion. Results also show that the pressure gradient acceleration and, to a lesser degree the vertical advection of the background environmental wind, contribute to the acceleration of rear-to-front-directed momentum in the middle- to rearward portions of the storm, thereby generating and reinforcing transport of the perturbation flow into the cold pool and accelerating the MCS. The second part of this dissertation uses a series of experimental simulations to examine the sensitivity of CMT, MCS motion, and surface wind speed generation to environmental humidity and microphysical processes. Results reveal modest changes in MCS motion, but marked differences in the generation of convectively-driven surface winds. Drier air at mid-levels increases descent within the trailing stratiform region and enhances CMT; this slightly increases average MCS speed by ~1 ms-1, but produces a much larger number of severe surface winds. CMT is also shown to be a contributing factor to the occurrence of severe surface winds produced via the favorable superpositioning of a descending rear inflow jet and the low-level circulation associated with gust front mesovortices. The potential for a descending rear inflow jet to cause strong surface winds at locations away from the leading edge of the gust front is discussed as well. While such surface wind patterns may occur in a variety of storm environments, it is shown that the additional downward motion imparted by decreasing the relative humidity of the mid-levels leads to additional acceleration through CMT and contributes to an increase in occurrence of strong to severe surface winds. Reducing evaporation yields the most marked decrease in both MCS motion and strength of surface wind speeds, followed by the removal of melting and sublimation, respectively. The challenge of completely isolating the contribution of Ã¢â‚¬Å“cold pool dynamicsÃ¢â‚¬Â (i.e., density current propagation) from Ã¢â‚¬Å“CMT-forcedÃ¢â‚¬Â MCS motion is also discussed. Avenues for future work are outlined, with a focus on adding or improving the representation of CMT in existing cumulus parameterization schemes, incorporating CMT into conceptual models of both MCS motion and severe surface wind generation, and further exploring of the sensitivity of CMT to a greater variety of storm environments and kinematic profiles.
Nonlinear Structure and Evolution of African Easterly Waves
(2008-07-24) Hardin, Nathan R; Anantha Aiyyer, Committee Chair; Sandra Yuter, Committee Member; Gary Lackmann, Committee Member
A summertime radar climatology of convection in the coastal region of South Carolina
(2008-08-08) Booth, William Jay; Sethu Raman, Committee Member; Sandra Yuter, Committee Member; Matthew Parker, Committee Chair