Browsing by Author "Matthew D. Parker, Committee Chair"

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The Dynamics of Wildfire-Generated Dry Convection: Fundamental Processes and Complicating Factors
(2009-04-27) Kiefer, Michael Thomas; Matthew D. Parker, Committee Chair; S. Pal Arya, Committee Member; Joseph J. Charney, Committee Member; Gerald S. Janowitz, Committee Member
Wildfires are capable of inducing atmospheric circulations due predominately to the large temperature anomalies produced by the fire. The fundamental dynamics through which a forest fire and the atmosphere interact to yield different convective regimes is still not well understood. The work described in this dissertation is aimed at understanding, from the perspective of atmospheric dynamics, how different modes of convection (e.g. plumes and multicells) develop. This research is conducted through the use of a numerical model in which the fire is parametrized by a surface heat flux, and atmospheric variables (e.g. wind) and fire parameters (e.g. dimension, intensity) are varied independently. Although the focus of this work lies in the atmospheric processes, effort is also made to apply the findings to the problem of fire behavior by examining relevant atmospheric variables such as surface wind speed and temperature near the fire for different convective modes. In the first set of experiments, two-dimensional simulations are performed wherein the upstream surface wind speed and mixed-layer mean wind speed are varied independently in order to better understand the fundamental processes governing the organizational mode and updraft strength. The result of these experiments is the identification of two primary classes of dry convection: plume and multicell. Simulated plume cases exhibit weak advection by the mean wind and are subdivided into intense plume and hybrid classes based on the degree of steadiness within the convection column. Hybrid cases contain columns of largely discrete updrafts versus the more continuous updraft column associated with the intense plume mode. Multicell cases develop with strong mixed layer advection and are subdivided into strong and weak classes based on the depth of convection. Intense plume and strong multicell (hybrid and weak multicell) cases occur when the surface buoyancy is large (small). The multicell (strong and weak) and intense plume modes are forced by a combination of buoyancy and dynamic pressure gradient forcing associated with the perturbation wind field, while the hybrid mode is forced by a combination of buoyancy and dynamic pressure gradient forcing associated with the strong background shear. In the second set of experiments, the simulations of the first set are extended in several ways. First, the impact of surface wind speed and cross-fireline dimension on parcel potential temperature is examined. It is found that the buoyancy parameter fails as a control parameter when values of the parameter exceed unity. However, the parameter may be useful as a gauge of fire behavior predictability in such cases, since the first set of experiments indicated a relationship between parameter B and the potential for feedback to the atmosphere. A second way in which the first set of experiments are extended is by performing simulations in a 3D model. In general, salient results from the earlier 2D study are reproduced in the 3D model. In one case with strong vertical wind shear, new convection develops away from the main convective line as a result of local changes to parcel speed and heating. The third way in which results are extended is through 3D experiments wherein fireline shape and along-line inhomogeneity are varied. It is found that a sinusoidal-shaped fireline with along-line uniform intensity induces stronger parcel heating where the fireline bows into the wind (i.e. the back of the fire). In a separate simulation with a more realistic fire structure wherein surface heat fluxes are strongest where the fireline bows out in the direction of the wind (i.e. the head of the fire), parcel heating and convection are weaker than for a straight, uniformly heated fireline. In the third set of experiments, the impact of Kelvin-Helmholtz (i.e. shear) instability and a critical level on dry convection above a prescribed heat source is examined. An analysis of the advection parameter indicates that prior to convection penetrating the critical level, multicell convection dominates. After convection reaches the critical level, overturning develops as a result of shear instability. The relationship between the advection parameter and organizational mode deduced from the first set of experiments does not apply after convection reaches the critical level. In addition to the intense plume and multicell modes, a third mode termed the deep wave mode is simulated. This mode consists of disturbances with wavelengths of 7-10 km, and results from the multicell convection perturbing the unstable layer centered at the critical level. The presence of an unstable wind profile at the critical level is shown to be crucial to development of both the deep wave and intense plume modes; in the absence of a critical level and shear instability, multicell convection dominates. For the third set of experiments, knowledge of the mean and surface wind speeds, and thus the values of parameters A and B, respectively, is not sufficient to understand what processes will dominate the generation of convection.
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.
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.