Browsing by Author "Dr. Gary Lackmann, Committee Member"

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An Analysis of Hurricane Debby (2000) and the Impact of Vertical Shear on the GFDL Forecast Performance.
(2002-09-04) Rhome, Jamie Robert; Dr. Al Riordan, Committee Member; Dr. Sethu Raman, Committee Chair; Dr. Gary Lackmann, Committee Member
The need for improved tropical cyclone (TC) intensity guidance has never been greater given recent upward population trends in coastal areas. The difficulty in forecasting rapid intensity change remains one of the more challenging aspects of TC forecasting and was recently highlighted by the unexpected weakening of Hurricane Debby (2000) along the northern coast of Hispaniola on August 23, 2000. To address the need for improved understanding of rapid intensity change and the ability of dynamical TC models to accurately forecast intensity, a three-dimensional dynamical TC model (GFDL) is analyzed during the lifecycle of Debby. This was accomplished by first performing a comprehensive observational analysis making use of observed in-situ data as well as remotely sensed satellite data and derived products. Results from this analysis indicate that rapidly increasing vertical shear was the primary catalyst for the sudden weakening. Accordingly, vertical shear was analyzed within several operational simulations of the GFDL model near the time of weakening. This was accomplished by comparing the GFDL initial and forecast intensity with the National Hurricane Center official best track data as well as comparing the GFDL vertical shear with the AVN global analysis. Deviations in the GFDL intensity and vertical shear from the analysis data were considered to represent forecast error. These errors were then analyzed further by comparing the GFDL model forecast environmental wind field with a suite of observed data including GOES-8 satellite-derived winds, NOAA G-IV dropwindsondes, and upper-air observations supplemented by the GFDL initial analysis (F00). Results indicate that errors in vertical shear were nearly coincident with deviations in observed intensity versus forecast intensity. These deviations were primarily the result of a misrepresentation of the upper-level flow in the model due to an overdeveloped downstream upper-level ridge. Additionally, an erroneous anticyclone developed over the model storm in several cases, resulting in significant weakening of the upper level westerly flow and associated vertical shear. In this case, the downstream anticyclone was more intense and closer to the storm in nearly all simulations analyzed. These findings are similar to previous studies where a storm to environment interaction has been identified as the result of redistribution of latent heat release due to convection and the downstream advection of the associated low Potential Vorticity (PV) outflow. The misrepresentation of convection and the associated effects on the surrounding environment is identified as the primary factor for both track and intensity forecast errors by the GFDL model during Debby.
A Climatology of the Sea Breeze Front in the Coastal Carolinas and Georgia.
(2006-11-08) Crouch, Andrew David; Dr. Sethu Raman, Committee Member; Dr. Gary Lackmann, Committee Member; Dr. Allen Riordan, Committee Chair
The sea breeze circulation forms as a result of differential heating across the land-sea surface interface. The resultant pressure gradient force (PGF) induces an onshore flow at the surface and a return flow higher in the atmosphere. The sea breeze front is a reflection of this circulation, a boundary between the cool, moist, maritime air mass advancing landward and the warm, dry ambient air mass in place over inland areas. Studies have shown that the circulation forms more often in the spring and summer months when the temperature difference between land and sea surfaces is greatest. The following study is based on the analysis of satellite imagery and standard hourly measurements of air temperature, dewpoint, wind speed, and wind direction recorded at four coastal sites: Savannah, GA, Charleston and Myrtle Beach, SC, and Wilmington, NC. One of the objects of this study is to establish specific values associated with the changes induced by passage of the sea breeze front, and to examine differences in the station-to-station incarnation of the sea breeze circulation. Variability from station to station in the nature and timing of sea breeze frontal passage was found to be a function of relative proximity to the coast. For example, sea breeze frontal passage was found to occur earliest at Myrtle Beach (the closest station to the coast), around 1300 LST on average. Savannah, the farthest of the four coastal sites from the water, was affected by the sea breeze last, with an average passage time of between 1630 and 1700 LST. Previous studies indicate that the extent to which the circulation and associated front penetrate inland is usually on the order of about 20-60 km. GOES satellite imagery was accumulated from the North Carolina State Climate Office and analyzed with GIS (Geographic Information System) software for the purpose of determining the inland horizontal extent of the sea breeze circulation. Penetration distances of 20-40 km were common, but occasionally the sea breeze penetrated as far as 80 to 120 km. The second part of this study attempts to develop a scheme for the prediction of the development and evolution of the sea breeze front. The factors most significant to this prediction include the synoptic wind flow regime and values for the temperature difference between land and sea surfaces. North American Regional Reanalysis data were downloaded and analyzed to gather daily geostrophic wind vectors. Water temperature data were collected and compared with air temperature measurements to determine the temperature difference, the driving mechanism for sea breeze development. Data for the Myrtle Beach site were analyzed first to provide a framework for the other stations because of relative completeness and reliability of the data sources. At Myrtle Beach, ninety percent of sea breeze development occurred with land-sea temperature differences of 2.0 degrees Celsius or higher.
Cold Air Damming Erosion and Associated Precipitation in the Southeastern United States
(2006-01-18) Green, Thomas A., Jr.; Dr. Gary Lackmann, Committee Member; Dr. Allen Riordan, Committee Chair; Dr. Lian Xie, Committee Member
Appalachian cold air damming (CAD) is often associated with significant sensible weather effects in Virginia and the Carolinas. Impacts include below-normal temperatures, reduced visibility, and the potential for freezing or frozen precipitation. Current operational numerical weather prediction (NWP) models have difficulty in predicting the end of CAD events, often prematurely eroding the cold dome. The poor model performance increases the difficulty of correctly forecasting the end of an event. In addition, NWP models are known for having low skill scores with quantitative precipitation forecasting (QPF). While precipitation can occur during CAD events without the presence of a cyclone, the synoptic system eroding the cold dome may be responsible for different precipitation patterns over the damming region. The combination of poor model performance on CAD erosion and QPF creates difficulty for the human forecaster in developing a precipitation forecast. The objectives of this study are to (i) develop high-resolution maps of synoptic and mesoscale patterns associated with CAD; (ii) determine whether particular erosion scenarios have different precipitation signatures, and if so, identify these signatures; (iii) identify physical processes associated with specific precipitation signatures; and (iv) establish parameters to forecast whether a given CAD event will yield above or below average precipitation amounts in the Carolinas. High-resolution maps of synoptic patterns associated with cold air damming were developed using Rapid Update Cycle (RUC) model data for five erosion scenarios. The scenarios include Coastal Lows, Northwestern (NW) Lows, NW Lows with Cold Front, Residual Cold Pool, and Southwestern (SW) Lows. From the five erosion scenarios, three (Coastal, NW, and SW) were chosen for further study of the spatial and temporal distribution of precipitation. Precipitation data from the North American Regional Reanalysis (NARR) was used to label cases as 'wet' or 'dry' in an individual erosion scenario, using the mean normalized precipitation value for each storm over the damming region. Differences were noted in the synoptic distribution of precipitation according to the location of the cyclone that was eroding the cold dome. Further investigation of composites from the wettest and driest cases in each scenario revealed differences between composites of sea-level pressure and 500 hPa geopotential heights. Greater differences were seen between erosion scenarios when examining precipitation coverage both spatially and temporally. NW Lows produced the smallest precipitation totals and had the smallest spatial coverage through the center of the damming region. Coastal Lows had moderate amounts of precipitation, with spatial coverage that peaked approximately nine hours before CAD demise, with decreasing coverage after that time. SW Lows produced the greatest precipitation totals and spatial coverage, with a peak in spatial coverage approximately three hours after CAD demise. Several variables were examined at multiple levels to determine what quantities and physical processes were important for precipitation in the three erosion scenarios. For NW Lows, dynamic and moisture variables such as relative humidity and mixing ratio had the highest correlation with precipitation amounts, suggesting that moisture and synoptic-scale lifting are limiting factors for large amounts of precipitation to occur in this scenario. The low center is well removed from the damming region; therefore, maximum amounts of moisture are necessary because of the weak dynamics. Advective variables such as temperature advection, vorticity advection, and moisture transport yielded the highest correlations with precipitation for SW Lows. For that scenario, moisture is not a limiting factor, and the location of dynamical support with the cyclone in the damming region is more important in determining precipitation amounts. No category of high correlating variables existed for Coastal Lows, suggesting that the track of the cyclone may be most important in determining precipitation amounts. Between all scenarios, relative humidity at 500 hPa, moisture transport at 700 and 850 hPa, and temperature advection at 700 and 850 hPa have the highest correlations with precipitation totals. Several findings from this study can be directly used by operational forecasters. Differences in spatial and temporal coverage of precipitation were found between erosion scenarios that can be used for forecasting or nowcasting. Some differences were further explained by the correlations between particular variables and precipitation totals. Focus can also be placed on whether adequate moisture and lift is present in NW Lows, whether dynamical support is strong enough to produce large amounts of precipitation in SW Lows, and if the cyclone track in Coastal Lows is close enough to the coastline for heavy precipitation to move inland.
The Dynamics of Orographic Rainfall and Track Deflection Associated with the Passage of a Tropical Cyclone over a Mesoscale Mountain
(2004-03-31) Witcraft, Nicholas Charles; Dr. Fred Semazzi, Committee Member; Dr. Gary Lackmann, Committee Member; Dr. Yuh-Lang Lin, Committee Chair
This thesis is composed of two papers concerning tropical cyclones affecting Taiwan. The first paper investigates the dynamics of heavy orographic rainfall associated with the passage of a typhoon over the Central Mountain Range (CMR) of Taiwan. Included in this paper are sensitivity tests of various precipitation parameterizations. The second paper examines the dynamics of track deflection associated with the passage of typhoons over the CMR. In the first paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate Supertyphoon Bilis (2000) in order to investigate the dynamics of orographic rainfall associated with the passage of typhoons over the Central Mountain Range (CMR) of Taiwan. In the first part, we identified many factors present in this case to support heavy rainfall, based on Lin et al. (2001). The most important factors appear to be the presence of potential and convective instability, a very moist air impinging on the CMR, and a low level wind maximum associated with the outer circulation of the typhoon. A moisture flux model was also used to estimate rainfall and the relevant dynamics. The remaining portion of the first paper concerns the sensitivity of track, intensity, and rainfall to various subgrid-scale cumulus parameterization and resolvable scale microphysics schemes in simulations of Bilis at 21 and 7 km. The amount of rainfall over the CMR was sensitive to the cumulus scheme, even though most of the rainfall over the CMR was generated by the microphysics scheme. The more active schemes modified and stabilized the environment around Bilis, resulting in less rainfall over the CMR. The track and intensity were also highly sensitive to the cumulus scheme. Varying the microphysical parameterization had relatively minor effects on the simulation. In the second paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate Supertyphoon Bilis (2000) and Typhoon Toraji (2001) in order to investigate the dynamics of track deflection associated with the passage of typhoons over the Central Mountain Range (CMR) of Taiwan. Bilis was an intense fast moving storm with a continuous track over the CMR. The upper and lower level potential vorticity (PV) centers remained coupled as the center traversed the CMR. The forward speed of Bilis also helped prevent any significant lee-side cyclone reformation. Toraji was weaker and slower moving, and had a discontinuous track over the CMR. Partially due to the slower forward speed, Chinook winds (foehn), a combination of the release of latent heat over the CMR, followed by adiabatic downsloping and warming, had a longer time to generate lower heights in the lee of the CMR. The original low-level center made landfall and dissipated, while the upper level center continued to move northwestward. Without lower level support, the original upper level center also weakened and dissipated. Over time, PV banners/filaments wrapped into the secondary center, which ultimately became dominant. As the secondary center pulled away from Taiwan, it extended into the upper levels. Control parameters for track continuity from idealized studies are calculated for Bilis and Toraji. A conceptual model proposed by Lin et al. 2004 is applied to explain the behavior of the track for each storm.
The Effects of Altered Vegetation on Local Climate Change with Respect to the Glaciers atop Mount Kilimanjaro
(2010-04-28) Heuser, Sean; Dr. Frederick Semazzi, Committee Chair; Dr. Gary Lackmann, Committee Member; Dr. Adel Hanna, Committee Member
HEUSER, SEAN PATRICK. The Effects of Altered Vegetation on Local Climate Change with Respect to Glaciers atop Mount Kilimanjaro. (Under the direction of Dr. Fredrick H.M. Semazzi) The objective of this study is to determine how changes in vegetation around the region of Mount Kilimanjaro effect the glaciers atop the mountain. Using the Weather Research and Forecasting Model (WRF) Advanced Research WRF (ARW) model, we are able to alter albedo, roughness length, and vegetation for a given area around the mountain during the June, July, August season of 2000. These simulations are also done for the March, April, May season of 2000 to determine lag effects of vegetation in comparison to soil moisture for the region. We use the Normalized Difference Vegetation Index (NDVI) to determine the severity of vegetation change for the region. For this study, we are comparing temperature, precipitation, as well as radiation balances to infer whether glaciers will thrive or decline in a changed environment. Using this information, we make inferences on what should be happening atop Mount Kilimanjaro. Our study concludes that the albedo plays a larger role in changing temperatures with roughness length playing a larger role in terms of affecting precipitation. Changing vegetation from grasslands to savanna show more change to the glacier than when being altered to cropland. Furthermore, the changes made in this study conclude that the glacier itself may actually decline in an environment with more cropland than with more savanna. This is due to the decrease in orographic effects which dominate the precipitation patterns in the region.
An Examination of Tropical Cyclone Dynamics Utilizing the 3-Way Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) Model
(2009-12-01) Zambon, Joseph Brendan; Dr. Ruoying He, Committee Chair; Dr. John Warner, Committee Member; Dr. Gary Lackmann, Committee Member
Tropical Cyclones are fundamentally connected to the environment in which they exist. Currently most numerical models do not represent the interactions between the atmosphere, ocean, and wave environments. These environments are drastically modified by the existence of the tropical cyclone and therefore drastically modify the tropical cyclone as a continuous feedback mechanism. As a result, improvement of solutions provided by the individual numerical models representing the atmosphere, ocean, and waves is sought through coupling these models together. In the first chapter, the dynamic feedback mechanisms are explored in depth through literature review of previous studies into the reaction of the ocean to tropical cyclones. Several analytical and numerical studies are researched in order to provide sufficient background into the problem, provide motivation into developing a coupled numerical model, and provide a base from which hypotheses for experimentation may be drawn. In the second chapter, experiments will be based off of an atmospheric model tied to a simple 1-dimensional ocean model. Three different experiments are carried out, with the sea surface condition as the only variable between them. By including this simple configuration allowing ocean feedback, hypotheses regarding track, intensity, and sea surface temperature changes will tested. In the third chapter, the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) model is introduced. An idealized tropical cyclone is placed into the model domain and the COAWST model is tested. Three experiments of increasing complexity are used in testing the coupling scheme and examining the dynamical differences in the modeled solution. The idealized tropical cyclone is used to test several hypotheses based on modeled track, intensity, size, sea surface temperature change, and significant wave height. The COAWST model performs as expected and the initial proof of concept is successful. In the fourth chapter, the COAWST model is tested with a realistic case, a hindcast simulation of Hurricane Ivan. The model is initialized within a spatial and temporal domain that was found to provide the best solution for a Hurricane Ivan hindcast using only the atmospheric model. Five experiments are carried out, with increasing complexity in resolving the ocean condition. Hypotheses for the realistic case are tested based on modeled track, intensity, size, sea surface temperature change, heat exchange, and significant wave height. The COAWST model demonstrates reasonable skill in the Hurricane Ivan hindcast, although additional improvement in the initial condition is desired. The final chapter serves to review the discussions of the previous chapters and seeks to provide a platform for future research. The utility of coupled numerical modeling is reiterated and the success of the study highlighted. Likewise, significant improvement of the initial condition in the realistic hindcast will be sought in future research. In addition, several questions remain in improving and examining the coupled numerical solution of a tropical cyclone.
Formation and Maintenance Mechanisms of the Stable Layer over the Po Valley and Genoa Cyclone Movement during MAP IOPs
(2005-10-18) Hoggarth, Allison M; Dr. Gerald Janowitz, Committee Member; Dr. Gary Lackmann, Committee Member; Dr. Yuh-Lang Lin, Committee Chair
This thesis is composed of two papers concerning events that occurred during the Mesoscale Alpine Programme (MAP). The first paper investigates the formation and maintenance mechanisms of a stable layer over the Po Valley during MAP IOP-8. The second paper examines the dynamics of the movement associated with the MAP IOP-1 and IOP-8 Genoa cyclones as they approached the Apennines. In the first paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate the stable layer over the Po Valley during MAP IOP-8 in order to investigate the mechanisms that led to the formation and maintenance of the stable layer. Blocking and deflection of cool, easterly flow by the western flank of the Alps was found to play a significant role in the formation of the cool, stable layer by helping to build up the cool air over the Po Valley. When the flow shifted to southerly, the western flank of the Alps and the northern flank of the Alps acted in concert to help retain the cool air over the Po Valley. As a result, warm, less stable southerly flow originating from over the Mediterranean Sea was advected atop the cool, stable layer in the Po Valley. This differential advection, along with blocking by the western and northern flanks of the Alps, helped to maintain the stable layer and extend its longevity over the Po Valley. An additional test showed that one commonly assumed mechanism that acts to maintain stable layers in cases of cold air damming, evaporative cooling, did little to alter the development or life cycle of the stable layer in this case study. In the second paper, the Penn State/NCAR MM5 Mesoscale Model was used to simulate the MAP IOP-1 and IOP-8 Genoa cyclones and their tracks. Both cyclones formed near the Gulf of Genoa, propagated eastward, and approached the Apennines over the Italian Peninsula. The IOP-8 surface cyclone slowed down along the upstream (west) side of the mountain and accelerated over the mountain range, but was slightly deflected towards the south and became discontinuous as it crossed the Apennines. For IOP-1, the surface cyclone was deflected towards the south as it approached the upstream side of the Apennines, but remained on the western side of the mountain range until reaching the southern end of Italy. In this study, the tracks of those Genoa cyclones were examined to investigate what led to their differences. The theory of Lin et al. (2005 JAS) on track deflection is hypothesized to explain the differences in the movement of the IOP-8 and IOP-1 cyclones. Based on ECMWF reanalysis data and MM5 modeling results, it was found, however, that the basic-flow and vortex Froude numbers for both cyclones were nearly identical. Thus, the cyclone movement cannot be explained simply by the orographic blocking effects. Instead, it appears that the movement of the IOP-1 surface cyclone was mainly controlled by synoptic forcing.
Geostatistical Modeling of Subclimatic Tropical Precipitation in the Carolinas
(2008-08-08) Palmer, Joshua Michael; Dr. Lian Xie, Committee Chair; Dr. Montserrat Fuentes, Committee Member; Dr. Gary Lackmann, Committee Member
Numerical Simulations of Mesoscale Boundary Layer Structure over New York City
(2003-07-30) Childs, Peter Phipps; Dr. S. Pal. Arya, Committee Member; Dr. Gary Lackmann, Committee Member; Dr. Sethu Raman, Committee Chair
Roughness length variations and the urban heat island effect are the dominating influences of highly urbanized terrain on boundary layer structure and evolution. Variations in roughness length can alter the surface wind flow by slowing it down, turning it, or a combination of both. The urban heat island effect keeps surface temperatures warmer than surrounding rural areas, leading to a more turbulent nocturnal boundary layer over the urbanized terrain than the surrounding regions. With a pronounced heat flux gradient, surface wind speeds are often enhanced as they flow across the urban regions. This thesis explores the influences of New York City on the structure and evolution of the boundary layer through a combination of numerical model simulations and observational analysis following the destruction of the World Trade Center buildings on 11 September 2001. Mesoscale processes, such as sea breeze circulations, urban heat island, and terrain modified flows are addressed in this research through the use of observations and several numerical simulations. Surface based observations from the National Weather Services' ASOS network are examined. Additionally, observations from an independent 10 m micrometeorological tower and two Sound Detection and Ranging (SODARS) are used. These observations are also used for model validation. An observational analysis of 10 m tower data and SODAR data is conducted for an extended study period between 10 September 2001 and 10 December 2001. Tower measurements of wind speed and direction (10 m) and temperature (2 m) are presented. SODAR data of wind speed and direction is also examined. Several different synoptic flow regimes were analyzed during this study period. Aerodynamic roughness length calculations were also made for two independent flow direction sectors. Results from this analysis showed that roughness lengths less than 1 m, if the predominant flow was between 180 to 359 degrees. Twenty-four hour averaged surface temperatures were observed to be warmer over the city center than the surrounding rural areas. Near surface wind speeds were also observed to be lower over the highly urbanized terrain associated with New York City. Simulations using 1 km grid spacing output from the Advanced Regional Prediction System (ARPS) and PSU/NCAR Mesoscale Model 5 (MM5) are examined during a high ground level pollutant concentration episode in lower Manhattan. The ARPS simulation showed a more defined sea breeze frontal formation and propagation than the MM5 simulation did over lower Manhattan. The ARPS simulation also showed a better defined slowing and turning of the 10 m wind speed over the highly urbanized terrain of lower Manhattan and Brooklyn relative to the MM5 simulation. Since both simulations used the same landuse data and roughness length parameterization, the planetary boundary layer scheme in both models is likely contributing to the observed differences. The urban heat island effect, urban blocking effect and sea breeze front are analyzed using the ARPS mesoscale model. The sea breeze frontal development and inland propagation agrees well with previous research by Michael (1998) and Bornstein (1994), who showed similar results using WSR-88D imagery and numerical simulations, respectively. Additionally, the turning of the surface (10 m) wind flow agrees well with previous research by Bornstein and Johnson (1977) that showed nighttime conditions during stronger flow regimes (>4 m/s) to be associated with distinctive roughness induced cyclonic turning in the winds over the main core of Manhattan and Brooklyn.
Risk Assessment of North Carolina Tropical Cyclones (1925-2000)
(2003-02-19) Hilderbrand, Douglas Clarence; Dr. Lian Xie, Committee Chair; Dr. Gary Lackmann, Committee Member; Dr. Len Pietrafesa, Committee Member
Previous tropical cyclone risk assessment studies have been national in scope. This study demonstrates the need for regional risk assessments, using the tropical cyclone history of North Carolina as an example. The standard normalization procedure for historical damage data was reevaluated. A housing factor was used instead of the more conventional population factor to go along with inflation and changes in wealth. For coastal counties in North Carolina, housing figures from 1940-2000 increased 780% while population figures increased only 370%. It is believed that use of housing data in lieu of population data in the normalization procedure provides a more realistic measure of impact. Using the new normalization method, 1954-55 tropical cyclone storm totals in North Carolina added together would have caused over $18 billion in damage (expressed in 2000 dollars). By comparison, the destructive period from 1996 to 1999 in North Carolina added up to $13 billion. Storm damage totals were separated into damages caused by wind, flooding, and storm surge. For all 36 direct landfalling tropical cyclones in North Carolina from 1925-2000, flooding caused approximately 40% of all damages, while wind and storm surge caused an estimated 35% and 25%, respectively. From these results, it is clear that flooding produced relatively greater damage in North Carolina compared to the United States in general. Rainfall was correlated to meteorological parameters of tropical cyclones making landfall in North Carolina. There was a weak relationship between intensity of the tropical cyclone and maximum rainfall totals. There was a stronger relationship between rainfall and translation speed. For those tropical cyclones that did not directly interact with synoptic-scale features such as upper-level troughs, lows, or surface fronts, the relationship between rainfall and translation speed can be expressed by the equation Y=29.529X-0.6134 where Y is the average of the five highest recorded rainfall totals and X is the translation speed (expressed in knots). Rain volume calculations quantified the magnitude of the September 1999 flood event in eastern North Carolina. Hurricane Floyd yielded an estimated 4.14 cubic miles of water on North Carolina only 10 days after Tropical Storm Dennis brought North Carolina out of drought conditions with 3.67 cubic miles of water. The next-highest value from previous tropical cyclones was Hurricane Fran (1996) with 3.14 cubic miles of water. While major hurricanes accounted for 83% of the overall damage due to hurricanes nationally, this percentage changes regionally. Using the total normalized damage numbers for North Carolina, 70 % of all tropical cyclone damage was caused by major hurricanes, while category-2 hurricanes added a significant percentage (21.4%). The results of this study suggest a stronger consideration for weaker tropical cyclones, especially category-2 hurricanes, in risk management decisions.
Three-Dimensional Radar and Total Lightning Characteristics of Mesoscale Convective Systems
(2003-08-11) McCormick, Tracy Lynn; Dr. Al Riordan, Committee Member; Dr. Gary Lackmann, Committee Member; Dr. Lawrence D. Carey, Committee Chair
The radar and electrical characteristics of three linear leading convective/trailing stratiform midlatitude mesoscale convective systems (MCSs) that passed through Dallas-Fort Worth, Texas on the following dates are examined: 1) 7-8 April 2002, 2) 12-13 October 2001, and 3) 16 June 2002. Quantitative results from the April and June MCSs are presented, but data problems with the October MCS restricted partitioned analysis to qualitative results. The convective line produced ~69% and ~93% of the total cloud-to-ground (CG) lightning flashes in the April and June MCSs, respectively. The convective line CG flash rate averaged 12.3 flashes min-1 (53.6 flashes min-1) in the April (June) case study, and only 7.5% (2%) of these flashes were positive in polarity. Lightning Detection and Ranging (LDAR II) source data identified two main electrically-active regions present within the convective line in the following temperature layers: 1) 0 to -25 °C, and 2) -35 to -55 °C. The lower region (1) was most likely a combination of the main negative and the lower positive charge centers of the thunderstorm tripole, and the upper region (2) was most likely the upper positive charge center of this tripole. Convective echo volume aloft (≥ 30 dBZ, 0 to -40 °C) was strongly correlated to convective lightning activity, suggesting that the presence of strong updrafts and differential sedimentation caused convective line electrification via the non-inductive charging (NIC) mechanism. The stratiform region CG flash rate averaged 2.2 flashes min-1 (4.5 flashes min-1) in the April (June) case study, and ~45% (~27%) of these flashes were positive in polarity. LDAR II source data identified one primary electrically-active layer (at -10 to -25 °C) that was sloped from the upper portions of the convective line rearward to just above the bright band in the stratiform region. A small and spatially distinct secondary electrically-active layer (at ~ -40 °C) was located towards the rear of the stratiform region. These two layers had smaller average source concentrations than the convective line had, resulting in significantly less lightning production in the stratiform region than in the convective line. Hydrometeor trajectory analyses using storm-relative vertical and horizontal motions determined from synthetic dual-Doppler results indicate that the these two stratiform region layers become electrified by a combination of 1) positive charge advection from the upper-positive convective charge center to the stratiform region and 2) stratiform in situ charging via NIC, likely creating an inverted dipole. This inverted dipole may explain why the +CG flash percentage was significantly higher in the stratiform region than in the convective region (where a normal dipole is present). In addition, stratiform echo volume aloft (≥ 25 dBZ, -10 to -40 °C) was strongly correlated to stratiform lightning activity, suggesting that differential sedimentation as a result of the presence of larger ice aggregates (i.e. Z > 25 dBZ) at these temperatures was required for stratiform region electrification via both charge advection and in-situ charging.