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The Role of Sensible Heat Flux on Convective Initiation in Oklahoma - A Case Study
Abstract
Oklahoma Mesonet and OASIS instrumentation aided in the calculation of sensible
heat fluxes that played a role in convective initiation in Oklahoma on 26 May
2000. Past research has indicated that landscape characteristics can cause differential
sensible heat fluxes, which can induce mesoscale circulations and enhance surface
convergence. Crawford (1998) used a model to calculate sensible heat fluxes
and observed that gradients of sensible heat fluxes caused the development of
the inland sea breeze, which then enhanced surface convergence. The results
of this project extended Crawford's work and illustrated and discussed the effects
of the inland sea breeze on the surface wind field and convergence in southwestern
and northeastern Oklahoma on 26 May 2000.
Radar Algorithm Output Associated With Damage and No Damage: A Comparison
Abstract
Thunderstorms producing wind, hail, and tornado damage are compared with thunderstorms
that produce no damage in the Pittsburgh, PA, WSR-88D radar coverage area by
investigating various parameters in the output of the National Severe Storms
Laboratory Mesocyclone and Tornado Detection Algorithms. Results show there
is some correlation between these parameters and tornado-causing storms. There
is also evidence that any differences between parameter values of the two different
sets of thunderstorms are either too small to be detected by the algorithms
or the differences are so small that it is very difficult to distinguish between
the two sets. Results also indicate it is more likely a tornado will be reported
given that it occurred and caused damage than wind or hail reported given damage
was observed.
Derecho Longevity: Predicting Storm Persistence or Decay Using Simulated Airflow Over a Cold Pool
Abstract
A case list of 17 derechos and 26 proximity soundings are compiled and characteristics
of the storm system are calculated and analyzed. Using a model to simulate airflow
over a cold pool, further development or decay of the system is inferred. Additional
soundings are added to the cases to further test model output. Results indicate
that the simple cold pool model is able to correctly determine derecho persistence
or decay for 31 of the 37 proximity soundings. Suggestions for further research
and applications to operational forecasting are made.
Variability of Significant Tornado Climatology in the United States
Abstract
In studying the variability of significant tornado climatology in the United
States, the goal is to model the variability of significant tornado occurrence
in the country. This study looked at the regional and the national tornado variability
in the United States.
In order to model variability, "raw" climatology data was first analyzed. Five different percentiles, the 10th, 25th, median, 75th and 90th, can be analyzed and from this, the number days for each gridpoint where the 90th percentile is greater than the maximum median for the year were plotted on a graph. The greater number of days this occurs on during the year, the higher the variability of significant tornadoes. Since the climatology data might not be entirely right, the beta distribution was used to model the tornado data by analyzing the distribution's two parameters, particularly parameter p, in the distribution. The number of days when p is greater than one, representing low variability, in a year was then plotted on a national map. The Komolgorov-Smirnoff test was run to test our beta distribution's results against the climatology data. A more sophisticated model using the beta distribution was used in order to study the variability of these tornadoes and the average number of tornado days per century was plotted.
The results from this study indicate that the Monte Carlo model using the beta distribution for the input is a good model of the significant tornado climatology in the country. The climatology data with the Monte Carlo model produced a small variance. The Komolgorov-Smirnoff test indicated that the beta distribution had good and bad fits and that there was no pattern to which places did well and which places did poorly. One of the most important results is that there is low variability over the Plains states and areas to the east have high variability. Therefore, the Plains states have a definite tornado season and significant tornadoes are likely almost all year for places east of the Plains.
Climatological and Synoptic Aspects of November Tornado Outbreaks
Abstract
Tornado outbreaks occurring during the month of November during 1955-99 have
been analyzed to determine their climatological and synoptic aspects. Significant
climatological results include a southeast regional frequency with a progressive
type outbreak scenario as well as a temporal frequency of outbreaks in the final
half of November. Similar synoptic patterns include tornado formation primarily
east of a deep mid-level trough along a quickly moving cold front after rapid
return flow from the Gulf of Mexico.
Upper-Tropospheric Inertial Instability: Climatology and Possible Relationship to Severe Weather Occurrence
Abstract
The spatial and temporal occurrence of inertial instability in the upper-troposphere
is explored, as is the hypothesis that regions of inertial instability may be
related to the occurrence of severe convective storms. The first part of the
study presents a climatology of 250-hPa inertial instability in the Northern
Hemisphere. Inertially unstable conditions occur most frequently in the low
latitudes and where anticyclonic shear and curvature in the subtropical jet
is strongest in the midlatitudes. Inertial instability is most common in the
winter months when the subtropical jet is at its strongest and farthest south.
A study of 74 cases where high-risk outlooks for severe weather were issued
reveals no definite relationship between upper-tropospheric inertial stability
and severe weather occurrence. The upper-level absolute vorticity advection
in these events is also examined, and cyclonic vorticity advection (CVA) is
present in most of the cases. However, the data do not show a definite relationship
between the strength of the CVA and the occurrence of severe weather. Four cases
are examined in further detail, emphasizing the wide range of conditions that
are present during severe weather outbreaks. While inertial instability is shown
to exist above some severe weather events, the exact function that it serves
is unclear.
Low-Level Winds Over Southeastern Mexico Using PACS-SONET Data
Abstract
The Pan American Climate Studies Sounding Network has provided data that allowed
the behavior of a low-level jet over the Yucatan Peninsula to be analyzed and
compared with winds across southeastern Mexico. Composite studies were performed
for days when a low-level jet was present at Frontera and Ciudad del Carmen,
pilot balloon stations in southeast Mexico. These indicated the presence of
a low-level jet on the western coast of the Yucatan Peninsula at 1200 UTC; a
jet was not present along the east coast. It is likely that a low-level jet
forms over the western part of the peninsula as the trade wind boundary layer
flow stabilizes overnight. The morning low-level jet gives way to afternoon
sea breezes by 0000 UTC. Observations from theodolite stations on the Pacific
coast of southeastern Mexico, separated from the Yucatan Peninsula by the ~2-3
km high Sierra Madre de Chiapas, showed that the wind fluctuations there were
not correlated with the winds over the Yucatan as expected. Rather than enhancing
the winds at the Pacific coast site of Salina Cruz, stronger easterly trade
winds at Frontera and Ciudad del Carmen corresponded to weaker meridional winds
at Salina Cruz. Winds at Puerto Madero could not be clearly correlated to wind
fluctuations at either Salina Cruz or Frontera and Ciudad del Carmen, even though
all three stations are in the same region. NCEP reanalyses captured the general
wind flow over this region, but do not show the details of the gap flow and
the diurnal variations.
Tornado Frequency in the Southern Plains as Related to SSTs in the Pacific Ocean
Abstract
Forty years of tornado data are analyzed for their relation to Pacific sea surface
temperature anomalies (SSTA) to detect linkages between the Pacific and Southern
Plains tornado days. The Southern Plains is divided into eight regions that
group counties with similar tornado day behavior. These regions serve as a springboard
for further analyses. Visual inspection of the tropical and north Pacific Ocean
suggest that above normal tornado days are associated with unique regions. These
regions are identified as rectangular boxes in the western Pacific, central
Pacific and eastern Pacific in mid-latitudes. Additionally, two areas in the
ENSO region of the tropical Pacific are tested since they are hypothesized to
be related to tornado behavior. Two tornado day regions, one in west Texas,
the Oklahoma panhandle and western Kansas and the second in west central Texas
extending into central Oklahoma are subject to an intensive analysis. This included
identification of March through May years with above normal tornado days that
are subsequently formed into a time series for compositing.
Results of compositing above normal years minus below normal years for each spring month on SSTA, 500 hPa geopotential heights and .210 sigma total vorticity are consistent in identification of ridges overlaying warm SSTAs in the mid-latitudes and troughs overlaying cold SSTAs. The height patterns frequently form Rossby wave trains over the US. The positive anomalous vorticity is consistently oriented along the base and eastern side of the aforementioned troughs and always terminate over or immediately adjacent to the tornado regions. This suggests that storm tracks are displaced from their normal positions by the anomalous SSTAs through a feedback mechanism with the overlaying atmosphere.
Three Case Studies of Conditions Associated With Tornadoes Occurring Near Mid-Level Vorticity Maxima
Abstract
Tornado episodes associated with two types of mid-level vorticity maxima-synoptic
scale and mesoscale--were examined to try to find correlations between the two.
Two mesoscale vorticity center (MVC) cases were chosen as the focus for the
study, and one synoptic scale case was chosen for comparison. It was found that
the synoptic scale case and the MVC cases were distinctly different. However,
key similarities between the two MVC cases are noted, including the idea that
convective feedback may play an important role in tornado development. Ideas
for further research on the topic of MVC related tornado episodes are discussed.
Wind Power in Oklahoma: Using Windmap Software to Estimate the Wind Resource Potential of Southwestern Oklahoma
Abstract
Wind power in Oklahoma is an untapped resource. The purpose of this research
was to use a software package called WindMap and perform a threefold assessment
of its effectiveness in determining Oklahoma's wind resource potential. First,
WindMap's ability to estimate wind speeds was evaluated. An R2 correlation of
0.4168 was found between the data points that were used as the basis for the
wind speed estimations, so given the input parameters used, WindMap's wind speed
estimations were mediocre at best. The results show that the accuracy of the
surface roughness values in the study area had a large affect on the software's
output. The proximity of a station's nearest neighboring stations and localized
terrain effects also affected the software's accuracy, but the connection was
not as clear with these variables as with surface roughness. It was concluded
that WindMap may have the ability to estimate wind speeds and wind power density
well enough to indicate good regions for installing wind turbines, but more
testing needs to be done using a broader range of input parameters.