A technique based solely on QuikSCAT data is developed for determining suspect differences between QSCAT and ECMWF pressure gradients. Pressure fields are computed from scatterometer winds using a variational method that applies a gradient wind conversion. Kinematic analysis of the satellite wind field is performed in order to determine which parameters are physically related to the suspect pressure gradients. It is discovered that the likelihood of these suspect occurrences has the greatest dependence on relative vorticity, total deformation, and the curvature Rossby number. A broad range of these values is tested and a single assessment criterion is derived based upon the value of several skill scores. Overall, the assessment criterion is able to correctly identify the majority of suspect pressure gradients; yet considerable over-flagging does occur in many instances. However, the over-flagging is not random: the false alarms are tightly clustered around the suspect areas, resulting in flagged regions that are too large. Identification of the location of suspect areas in pressure products should be useful to forecasters. / A Thesis Submitted to the Department of Meteorology in Partial Fulfillment of the Requirements for the Degree of Masters of Science. / Fall Semester, 2006. / August 15, 2006. / Flagging Techniques / Includes bibliographical references. / Mark Bourassa, Professor Directing Thesis; Phillip Cunningham, Committee Member; Paul Ruscher, Committee Member.

The role of the Tropical Easterly Jet (TEJ) in West African rainfall climatology has received little attention in the research community to date. Therefore, this dissertation will examine the instabilities and wave activity associated with the TEJ and their implications regarding interannual rainfall variability over western Africa. First, the instability of the TEJ is examined using potential vorticity (PV) concepts to contrast wet and dry years in West Africa. Analyses of the meridional PV gradient indicate an abrupt shift in both location and magnitude of the instability associated with the TEJ during the transition from wet to dry years in the Sahel. Additionally, the signs of the climatological anomalies of PV at the TEJ level strongly reflect the four primary modes (wet, dry, wet dipole, and dry dipole) of interannual rainfall variability in West Africa. Several examples of PV perturbation analyses at the TEJ level confirm that the upper-level development of African Easterly Waves (AEWs) differs considerably between the two periods. These results support recent observations and modeling studies that suggest that the interaction between the TEJ and the African Easterly Jet (AEJ) plays an important role in the development and structure of AEWs. In addition to the observational study, a multi-layer primitive equation model is utilized to examine easterly wave activity and vertical motion patterns based on the juxtaposition of the three primary jets located over western Africa. Idealized simulations based on the basic states of the low-level westerly jet (LLWJ), African Easterly Jet (AEJ), and the Tropical Easterly Jet (TEJ) for several anomalously wet and dry years in the Sahel are studied. Results are compared to several linearized GCM simulations that are initialized with NCEP observational data. Results show that the location, intensity, and scale of wave perturbations are sensitive to the position and intensity of the jets. Vertical motion patterns also indicate that maximum upward motion is generally located between the cores of the AEJ and TEJ and was more intense in the wet years. These results generally agree with observational and other modeling studies, further emphasizing that jet position and intensity are important factors in determining interannual rainfall variability across western Africa / A Dissertation Submitted to the Department of Meteorology in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy. / Spring Semester, 2008. / November 30, 2007. / Sahel, Rainfall variability, African easterly waves, Potential vorticity, Tropical Easterly Jet, Idealized modeling / Includes bibliographical references. / Philip Cunningham, Professor Directing Dissertation; William Dewar, Outside Committee Member; Robert Hart, Committee Member; Sharon E. Nicholson, Committee Member; T. N. Krishnamurti, Committee Member.

Five years of cloud-to-ground (CG) lightning data (2002 – 2006) from the National Lighting Detection Network (NLDN) are examined to describe the nature of CG lightning in Florida. Our focus is on positive CG lightning (+CG; i.e., that which lowers positive charge from cloud to ground). Flash densities are computed on a 2×2 km grid for +CG and total CG during the warm season (May – September), the cool (cold) season (remaining months), and the entire year. The maximum annual total CG flash density of 28.1 flashes km-2 yr-1 is located just north of Tampa. Relative minima of cool season CG flash density extend from Tallahassee northeastward and south of the Tampa – Cape Canaveral corridor. +CG flash density shows a tendency towards greater values in the northwestern portion of the domain during both seasons. Two relative maxima of +CG flash densities are located near the Apalachicola National Forest in the panhandle (0.5 – 1.05 positive flashes km-2 yr-1), and near Naples in Southwest Florida (0.35 – 0.5 positive flashes km-2 yr-1). The median peak current and number of return strokes (multiplicity) of +CG and –CG flashes are quite different during the cool season, but are more similar during the warm season. The greatest peak current (~ 30 kA) and smallest multiplicity (~ 1.4) of +CG flashes occur during the cool season. The warm season is characterized by the smaller peak current (~ 20 kA) and larger multiplicity (> 1.5) of +CG flashes. Since +CG lightning is generally thought to consist of a single return stroke, our warm season multiplicities of ~1.7 are unexpected. This value may represent cloud pulses that are misclassified by the NLDN as weak peak current +CG flashes, or may actually describe characteristics of the CG lightning. The threshold for classifying these weak positive events recently was increased from +10 kA to +15 kA. An important finding is that greater than 40 % of all positive events (> 10 kA) in Florida during June, July, August, and October ranged between 10 kA – 15 kA. The unusual warm season characteristics of +CG lightning suggest that numerous ambiguous events are retained in our dataset, even when using the new threshold of +15 kA. Daily CG lightning patterns in Jacksonville and Miami were analyzed during March and July 2003. On a given day, there is more +CG lightning during March than July in both Jacksonville and Miami. Also, when lightning does occur, the percentage of positive flashes generally is greater in Jacksonville than Miami. 1.5) of +CG flashes. Since +CG lightning is generally thought to consist of a single return stroke, our warm season multiplicities of ~1.7 are unexpected. This value may represent cloud pulses that are misclassified by the NLDN as weak peak current +CG flashes, or may actually describe characteristics of the CG lightning. The threshold for classifying these weak positive events recently was increased from +10 kA to +15 kA. An important finding is that greater than 40 % of all positive events (> 10 kA) in Florida during June, July, August, and October ranged between 10 kA – 15 kA. The unusual warm season characteristics of +CG lightning suggest that numerous ambiguous events are retained in our dataset, even when using the new threshold of +15 kA. Daily CG lightning patterns in Jacksonville and Miami were analyzed during March and July 2003. On a given day, there is more +CG lightning during March than July in both Jacksonville and Miami. Also, when lightning does occur, the percentage of positive flashes generally is greater in Jacksonville than Miami. 10 kA) in Florida during June, July, August, and October ranged between 10 kA – 15 kA. The unusual warm season characteristics of +CG lightning suggest that numerous ambiguous events are retained in our dataset, even when using the new threshold of +15 kA. Daily CG lightning patterns in Jacksonville and Miami were analyzed during March and July 2003. On a given day, there is more +CG lightning during March than July in both Jacksonville and Miami. Also, when lightning does occur, the percentage of positive flashes generally is greater in Jacksonville than Miami. / A Thesis Submitted to the Department of Meteorology in Partial Fulfillment of the Requirements for the Degree of Master of Science. / Summer Semester, 2007. / June 28, 2007. / Positive Lightning, Multiplicity, Peak Current, Cloud-To-Ground Lightning / Includes bibliographical references. / Henry E. Fuelberg, Professor Directing Thesis; Mark Bourassa, Committee Member; Phillip Cunningham, Committee Member; Andrew I. Watson, Committee Member.

Rainfall patterns over the Southeast U.S. have been found to be connected to the El Niño-Southern Oscillation (ENSO). Warm ENSO events cause positive precipitation anomalies and cold ENSO events cause negative precipitation anomalies. With this level of connection, models can be used to test the predictability of ENSO events. Using the Florida State University Global Spectral Model (FSUGSM), model data over a 50-year period will be evaluated for its similarity to observations. The FSUGSM is a global spectral model with a T63 horizontal resolution (approximately 1.875°) and 17 unevenly spaced vertical levels. Details of this model can be found in Cocke and LaRow (2000). The experiment utilizes two runs using the Naval Research Laboratory (NRL) RAS convection scheme and two runs using the National Centers for Environmental Prediction (NCEP) SAS convection scheme to comprise the ensemble. The simulation was done for 50 years, from 1950 to 1999. Reynolds and Smith monthly mean sea surface temperatures (SSTs) from 1950-1999 provide the lower boundary condition. Atmospheric and land conditions from January 1, 1987 and January 1, 1995 were used as the initial starting conditions. The observational precipitation data being used as the basis for comparison is a gridded global dataset from Willmott and Matsuura (2005). Phase precipitation differences show higher precipitation amounts for El Niño than La Niña in all model runs. Temporal correlations between model runs and the observations show southern and eastern areas with the highest correlation values during an ENSO event. Skill scores validate the findings of the model/observation correlations, with southern and eastern areas showing scores close to zero. Temporal correlations between tropical Pacific SSTs and Southeast precipitation further confirm the model's ability to predict ENSO precipitation patterns over the Southeast U.S. The inconsistency in the SST/precipitation correlations between the models can be attributed to differences in the 200-mb jet stream and 500-mb height anomalies. Slight differences in position and strength for both variables affect the teleconnection between tropical Pacific SSTs and Southeast. / A Thesis Submitted to the Department of Meteorology in Partial Fulfillment of the Requirements for the Degree of Master of Science. / Summer Semester, 2006. / April 21, 2006. / Correlation, Model, Precipitation, ENSO, Skill Score / Includes bibliographical references. / James J. O'Brien, Professor Directing Thesis; Carol Anne Clayson, Committee Member; Fei-Fei Jin, Committee Member.

This study focuses on the effects of two different cumulus parameterizations and two different radiation schemes on tropical cyclone frequency and structure in a coupled global climate model, the Florida State University Coupled Global Spectral Model. The model, run at resolution T63, simulates the global climate for 90 days initialized at 12Z on July 29 for each of the years analyzed. The FSUCGSM allows for a choice of two physical parameterizations: cumulus parameterizations and radiation schemes. The two cumulus parameterizations are a modified Kuo scheme and a modified Arakawa-Schubert scheme. The two radiation schemes are the absorptivity-emissivity model and the band model. Using objectively defined model- and basin-dependent thresholds will allow for the detection and analysis of tropical cyclones in each configuration of the model. For each configuration of the model, two tropical cyclones are selected from all the tropical cyclones available from all the years available and are analyzed. The first tropical cyclones analyzed in each configuration of the model all possess similar statistics with respect to each configuration's joint distribution of vertically integrated temperature anomaly and low-level vorticity. The second tropical cyclones analyzed in each configuration of the model are all the strongest tropical cyclones in each configuration in terms of low-level vorticity value. It is found that for a given radiation scheme, tropical cyclones in configurations using the Kuo cumulus parameterization have the ability to become more intense than those forming in the configurations using the Arakawa-Schubert cumulus parameterization. It is also found that tropical cyclones in the configurations using the band model display a more realistic heating structure in the vertical. In terms of the frequency of storms predicted per year, the configuration of the FSUCGSM that pairs the Kuo cumulus scheme with the band model produces the most accurate results when analyzing the entire forecast period and shows skill against climatology-persistence forecasts. When the first 15 days of the model run are excluded, to account for model spin-up, the average of all four configurations becomes the most accurate and shows skill against climatology-persistence forecasts. It is also shown that the configurations using the band model simulate a more realistic subtropical high in the northern Atlantic, as well as a more realistic temperature profile in the vertical in the tropical Atlantic. / A Thesis Submitted to the Department of Meteorology in Partial Fulfillment of the Requirements for the Degree of Master of Science. / Fall Semester, 2007. / October 23, 2007. / Cumulus Parameterization, Tropical Cyclones, Climate Modeling, Radiation Scheme / Includes bibliographical references. / T. N. Krishnamurti, Professor Directing Thesis; Robert E. Hart, Committee Member; Mark A. Bourassa, Committee Member.

The warm seclusion or mature stage of the extratropical cyclone lifecycle often has structural characteristics reminiscent of major tropical cyclones including eye-like moats of calm air at the barotropic warm-core center surrounded by hurricane force winds along the bent-back warm front. Many extratropical cyclones experience periods of explosive intensification or deepening (bomb) as a result of nonlinear dynamical feedbacks associated with latent heat release. Considerable dynamical structure changes occur during short time periods of several hours in which lower stratospheric and upper-tropospheric origin potential vorticity combines with ephemeral lower-tropospheric, diabatically generated potential vorticity to form a coherent, upright tower circulation. At the center, anomalously warm and moist air relative to the surrounding environment is secluded and may exist for days into the future. Even with the considerable body of research conducted during the last century, many questions remain concerning the warm seclusion process. The focus of this work is on the diagnosis, climatology, and synoptic-dynamic development of the warm seclusion and surrounding flank of intense winds. To develop a climatology of warm seclusion and explosive extratropical cyclones, current long-period reanalysis datasets are utilized along with storm tracking procedures and cyclone phase space diagnostics. Limitations of the reanalysis products are discussed with special focus on tropical cyclone diagnosis and the recent dramatic decrease in global accumulated tropical cyclone energy. A large selection of case studies is simulated with the Weather Research and Forecasting (WRF) mesoscale model using full-physics and "fake dry" adiabatic runs in order to capture the very fast warm seclusion development. Results are presented concerning the critical role of latent heat release and the combination of advective and diabatically generated potential vorticity in the generation of the coherent tower circulation characteristic of the warm seclusion. To motivate future research, issues related to predictability are discussed with focus on medium-range forecasts of varying extratropical cyclone lifecycles. Additional work is presented relating tropical cyclones and large-scale climate variability with special emphasis on the abrupt and dramatic decline in recent global tropical cyclone accumulated cyclone energy. / A Dissertation submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester, 2010. / May 28, 2010. / Tropical Cyclone, Extratropical Cyclone, Climatology, Warm Seclusion / Includes bibliographical references. / Mark A. Bourassa, Professor Directing Dissertation; Kevin G. Speer, University Representative; Xiaolei Zou, Committee Member; Vasubandhu Misra, Committee Member; Robert E. Hart, Committee Member.

Many tropical cyclones move poleward, encounter vertical shear associated with the midlatitude circulation, and undergo a process called extratropical transition (ET). One of the many factors affecting the post-transition extratropical storm in terms of reintensification, frontal structure, and overall evolution is the upper-level flow pattern. Schultz et al. (1998) categorized extratropical cyclones according to two of the many possible cyclone paradigms in terms of the upper-level trough configuration: The Norwegian cyclone model (Bjerknes and Solberg 1922) associated with high-amplitude diffluent trough flow and the Shapiro-Keyser cyclone lifecycle (1990) with low-amplitude confluent troughs. Broadly speaking, the former category is associated with a strong, meridionally oriented cold front with a weak warm front while the latter lifecycle usually entails a prominent, zonally oriented warm front. However, as will be shown, simple antipode lifecycle definitions fail to capture hybrid or cross-lifecycle evolution of transitioned tropical cyclones. To exemplify the importance upper-level features such as jet streaks and troughs, a potential vorticity framework is coupled with vector frontogenesis functions to diagnose the interaction between the poleward transitioning cyclone and the midlatitude circulation. Particular focus is concentrated upon the evolution and strength of frontal fracture from both a PV and frontogenesis viewpoint. The final outcome of extratropical transition is highly variable depending on characteristics of the tropical cyclone, SSTs, and environmental factors such as strength of vertical shear. Here, three storms (Irene 1999, Fabian 2003, and Kate 2003) typify the inherent variability of one such ET outcome, warm seclusion. Very strong winds are often observed in excess of 50 ms-1 along the southwestern flank of the storm down the bent-back warm front. The low-level wind field kinematics are examined using vector frontogenesis functions and QuikSCAT winds. A complex empirical orthogonal function (CEOF) technique is adapted to temporally interpolate ECMWF model fields (T, MSLP) to overpass times of the scatterometer, an improvement over simple linear interpolation. Overall, the above diagnosis is used to support a hypothesis concerning the prevalence of hurricane-force winds surrounding secluded systems. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester, 2004. / November 12, 2004. / Extratropical Transition, Frontogenesis, Fronts, Quikscat, Cyclone Lifecycles, Warm Seclusion, Frontal Fracture, Potential Vorticity, Hurricane Kate, Hurricane Irene, Hurricane Fabian, Tropical Cyclones / Includes bibliographical references. / James J. O’Brien, Professor Directing Thesis; Mark Bourassa, Committee Member; Robert Hart, Committee Member.

This study examines the changes in boundary layer of landfalling tropical cyclones. Several storms which made landfall near surface observation platforms capable of high resolution data storage were examined. These records were subjected to spectral methods to explore the characteristics of the changing boundary layer turbulence. These results were compared to recent observations of boundary layer roll features noted in some landfalling storms. Spectra were also used for determining turbulence dissipation rates in the storms. It was determined that only the highest resolution datasets available with a sampling rate of 5 Hz were adequate to explore the small scale features of the flow and to accurately describe the turbulence dissipation rates. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester, 2004. / July 19, 2004. / turbulence dissipation rate, turbulence, boundary layer, tropical cyclones, hurricanes / Includes bibliographical references. / Paul Ruscher, Professor Directing Thesis; Robert Hart, Committee Member; T. N. Krishnamurti, Committee Member.

This study examined inner-core sources and sinks of angular momentum in Atlantic tropical cyclones. Each advective and torque term in the Eulerian absolute angular momentum tendency equation was calculated in a storm-relative reference frame using modeled and observational data. 18 storms between 2004 and 2006 were simulated using the hurricane weather research and forecast (HWRF) model. In addition, 30 composite observational data sets from the Atlantic oceanographic and meteorological laboratory's (AOML) H*Wind archive were gathered. These included aircraft, satellite, offshore buoy, coastal, and ship observations. Through methods of statistical correlation, categorical composition and linear regression, it was found that mid-level horizontal advection of relative angular momentum was most relevant to 12 hour strength change in the modeled tropical cyclones, while observed storms favored mid-level horizontal advection of Earth's angular momentum. / A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science. / Spring Semester, 2009. / March 6, 2009. / Angular Momentum, Hurricanes, Tropical Cyclones, Eulerian / Includes bibliographical references. / T. N. Krishnamurti, Professor Directing Thesis; Robert Hart, Committee Member; Paul Ruscher, Committee Member.

NCEP multi-sensor hourly rainfall data and ground-baed GPS zenith total delay (ZTD) were used for data assimilation and evaluation of quantitative precipitation forecasts (QPFs) through three case studies. Improvements in QPFs were obtained through direct assimilation of these rainfall observations and ZTD data using 4-dimensional variational assimilation (4D-Var). Inclusion of the observed no-rain information was shown to be beneficial to QPFs. Although the assimilation of ZTD observations does not produce a rainfall distributionas close to the observations as does the assimilation of rainfall within the assimilation window, the improvement in the QPFs beyond the window from the ZTD experiment is comparable to that from the rainfall experiment. Assimilation of ZTD and rainfall observations modifies the thermodynamic structures of the atmosphere, favoring development of precipitation in the observed rainy areas. The horizontal and vertical wind velocities are also adjusted consistent with the precipitation process. Sensitivity studies indicated that the adjustments in the moisture and temperature fields resulting from precipitation assimilation played a more important role than those of other state variables for improving QPFs. Spectral analysis indicates that rainfall assimilation adjusts the model variables on smaller scales (25 to 50 km) while the ZTD assimilation adjusts the model variables mainly on larger scales (>50 km).50 km). A modified digital filter for intensifying mesoscale gravity wave signatures is developed and applied to a real case study of rainfall assimilation. The results show that the rainfall assimilation experiment with the modified digital filter produced further improvements in quantitative precipitation forecasts compared with the rainfall assimilation experiment with a regular digital filter. Spectral analysis confirms that the mesoscale gravity waves are intensified not only within the rainfall assimilation window during which the modified digital filter is applied, but also beyond the assimilation window. The gravity-wave-induced vertical motions along the direction of wave propagation are also intensified, resulting in a more realistic time evolution of the pecipitation pattern. It is also found that the assimilation of 6-h accumulated rainfall outperforms the assimilation of hourly rainfall within the same 6-h window. / A Dissertation Submitted to the Department of Meteorology in Partial FulfiLlment of the Requirements for the Degree of Doctor of Philosophy. / Summer Semester, 2004. / July 8, 2004. / Mesoscale Gravity Wave, Data Assimilation, Digital Filter, 4D-Var, QPF / Includes bibliographical references. / Xiaolei Zou, Professor Directing Dissertation; I. M. Navon, Outside Committee Member; James J. O’Brien, Committee Member; Peter S. Ray, Committee Member; Albert I. Barcilon, Committee Member.