Pantina, P. (2010). Characterizing the Variability of the Indian Monsoon: Changes in Evaporative Sources for Summertime Rainfall Events . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: This study focuses on the interannual and intraseasonal variability of evaporative sources for rainfall events during the Indian monsoon. The monsoon is an important part of the economy and lifestyle in India, thus, any improvements in our understanding of its mechanisms would be directly beneficial to society. We first discuss the use of evaporative sources for rainfall events as an important tool to help increase our knowledge of the variations of the monsoon. We then outline the variability of the monsoon on an interannual (wet and dry years) and intraseasonal (active and break periods) time scale. We use three reanalyses (NCEP-R2, CFSR, and MERRA) and an IMD gridded rainfall dataset to trace the location and strength of evaporative sources via a quasi-isentropic back trajectory program. The program uses reanalysis winds and evaporation, among other parameters, to estimate these sources back in time. We discuss the differences in parameters between the datasets on a seasonal, interannual, and intraseasonal time scale. We then thoroughly investigate the strength and location of evaporative sources between datasets on interannual and intraseasonal time scales, and we attempt to explain the variations by analyzing the differences in the input parameters and circulation mechanisms themselves. The study finds that the evaporative sources for given interannual or intraseasonal rainfall events do vary in strength and location. Interannually, the strongest change in evaporative source occurs over central India and the Arabian Sea, suggesting that the overall monsoon flow contributes moisture for Indian rainfall on this time scale. Intraseasonally, the strongest change in evaporative source occurs over the Bay of Bengal, suggesting that low pressure systems contribute moisture for Indian rainfall on this time scale. All three reanalyses yield similar fields of evaporative source. We conclude that accurate prediction of the Indian monsoon requires improved understanding of both interannual and intraseasonal oscillations since the sources of moisture for these events are unique.
Moroni, D. F. (2008). Global and Regional Diagnostic Comparison of Air-Sea Flux Parameterizations during Episodic Events . Ph.D. thesis, Florida State University, Tallahassee, FL.
Abstract: Twenty turbulent flux parameterizations are compared globally and regionally with a focus on the differences associated with episodic events. The regional focus is primarily upon the Gulf Stream and Drake Passage, as these two regions contain vastly different physical characteristics related to storm and frontal passages, varieties of sea-states, and atmospheric stability conditions. These turbulent flux parameterizations are comprised of six stress-related parameterizations [i.e., Large and Pond (1981), Large et al. (1994), Smith (1988), HEXOS (Smith et al. 1992, 1996), Taylor and Yelland (2001), and Bourassa (2006)] which are paired with a choice of three atmospheric stability parameterizations ['Neutral' assumption, Businger-Dyer (Businger 1966, Dyer 1967, Businger et al. 1971, and Dyer 1974) relations, and Beljaars-Holtslag (1991) with Benoit (1977)]. Two remaining turbulent flux algorithms are COARE version 3 (Fairall et al. 2003) and Kara et al. (2005), where Kara et al. is a polynomial curve fit approximation to COARE; these have their own separate stability considerations. The following data sets were used as a common input for parameterization: Coordinated Ocean Reference Experiment version 1.0, Reynolds daily SST, and NOAA WaveWatch III. The overlapping time period for these data sets is an eight year period (1997 through 2004). Four turbulent flux diagnostics (latent heat flux, sensible heat flux, stress, curl of the stress) are computed using the above parameterizations and analyzed by way of probability distribution functions (PDFs) and RMS analyses. The differences in modeled flux consistency are shown to vary by region and season. Modeled flux consistency is determined both qualitatively (using PDF diagrams) and quantitatively (using RMS differences), where the best consistencies are found during near-neutral atmospheric stratification. Drake Passage shows the least sensitivity (in terms of the change in the tails of PDFs) to seasonal change. Specific flux diagnostics show varying degrees of consistency between stability parameterizations. For example, the Gulf Stream's latent heat flux estimates are the most inconsistent (compared to any other flux diagnostic) during episodic and non-neutral conditions. In all stability conditions, stress and the curl of stress are the most consistent modeled flux diagnostics. Sea-state is also a very important source of modeled flux inconsistencies during episodic events for both regions.
Keywords: Parameterizations, Parameterization, Algorithm, Probability Density, Probability Distribution, Pdf, Drake Passage, Kuroshio, Gulf Stream Ect, Cold Tongue, Indian Ocean, Pacific Ocean, Southern Oceans, Atlantic Ocean, Tropics, Sea-State
Samuelsen, A. (2005). Modeling the Effect of Eddies and Advection on the Lower Trophic Ecosystem in the Northeast Tropical Pacific . Ph.D. thesis, Florida State University, Tallahassee, FL.
Abstract: A medium complexity, nitrogen-based ecosystem model is developed in order to simulate the ecosystem in the northeast tropical Pacific. Several physical processes have major impact on the ecosystem in this region, most importantly intense wind jets along the coast and upwelling at the Costa Rica Dome (CRD). The ecosystem model is run “offline”, using a realistic physical ocean model hindcast as input. The physical model is a subdomain of the global Navy Coastal Ocean Model, which is a hybrid sigma-z level model. The model assimilates Modular Ocean Data Assimilation System temperature and salinity profiles derived from altimetry and sea surface temperature data. The model is forced by daily heat and momentum fluxes, and therefore captures short-term wind events such as the Tehuantepec jet. Because the model has high horizontal resolution (~1/8 degree) and assimilates sea surface height data, it has a realistic representation of eddies and mesoscale variability. The ecosystem model includes two nutrients (nitrate and ammonium), two size-classes of phytoplankton, two size-classes of zooplankton, and detritus. The model is run for 4 years from 1999 to 2002, with analyses focused on 2000-2002. The model is validated using SeaWiFS data and ship-based observations from the STAR-cruises (Stenella Abundance Research Project) of 1999 and 2000. The northernmost and most intense of the wind jets along Central America is the Tehuantepec jet. The Tehuantepec jet is responsible for upwelling large amounts of nutrient rich water south of the Gulf of Tehuantepec. The jet also occasionally produce large anti-cyclonic eddies that transport organic matter away from the coast. Because organic matter that is transported into the open ocean will eventually sink to the deep ocean, this has implications for the carbon export in this region. The model results are used to calculate cross-shelf fluxes in this region in order to estimate how much organic material is transported across the shelf break. Results show that at the Gulf of Tehuantepec there is high offshore export of organic material, particularly during eddy generation events, but also in fall. The highest export is on the order of 10 Mg C per meter of coastline per day and happens during eddy events. During these events there is a comparable onshore flux to the south of the gulf. Typically there is onshore flux to the south of the gulf during the summer. The model estimated transport away from the coast at the Gulf of Tehuantepec is 167 Tg C/year, and the onshore transport to the south of the gulf is 704 Tg C/year. The second subject of interest is the CRD. In this region, upwelling at the surface is caused by Ekman upwelling during the summer, although the dome is thought to be present at depth throughout the year. The doming of the isotherms below the thermocline is a result of vortex stretching and is decoupled from the wind-driven processes at the surface. A mass-balance budget is calculated at the CRD, and the horizontal and vertical fluxes are related to the abundance of plankton at the dome. There is upwelling (7.2X10-2 Sv ) at the dome throughout the year, but around the location of the dome (90° W), the upwelling is largest in the winter. Further west, input of nutrients from below is larger in the fall and summer. The results suggest that about 80% of the nitrate that is supplied to the dome during summer is actually brought up to the west of the dome and transported eastward by the North Equatorial Counter Current.
Petraitis, D. C. (2006). Long-Term ENSO-Related Winter Rainfall Predictions over the Southeast U.S. Using the FSU Global Spectral Model . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: 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.
Taylor, J. P. (2006). Comparison of ECMWF and Quikscat-Derived Surface Pressure Gradients . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: 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.
Lombardi, K. C. (2004). Resolving the Diurnal and Synoptic Variance of Scatterometer Vector Wind Observations . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: Scatterometer observations of vector winds are used to examine the amplitudes of synoptic and diurnal cycles. Scatterometers have the advantage of providing global coverage over water; however, irregular temporal sampling complicates the analyses. A least squares technique is used in determination of the amplitudes and phases of the diurnal and synoptic cycles on spatial scales of 5°, 15°, and 30°. In open ocean areas and regions with sufficient open water, the magnitudes of the diurnal and synoptic cycles are 1.0 ms-1 and 3.5ms-1, respectively. Diurnal amplitudes are highest in the polar regions and close to land surfaces due to sea breeze effects. The fraction of variance explained by the diurnal cycle is greatest near the equator. Synoptic amplitudes are consistently larger downwind of land from storm tracks and in the southern polar region as the time analyzed is during the southern winter season.
Banks, R. (2006). Variability of Indian Ocean Surface Fluxes Using a New Objective Method . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: A new objective technique is used to analyze monthly mean gridded fields of air and sea temperature, scalar and vector wind, specific humidity, sensible and latent heat flux, and wind stress over the Indian Ocean. A variational method produces a 1°x1° gridded product of surface turbulent fluxes and the variables needed to calculate these fluxes. The surface turbulent fluxes are forced to be physically consistent with the other variables. The variational method incorporates a state of the art flux model, which should reduce regional biases in heat and moisture fluxes. The time period is January 1982 to December 2003. The wind vectors are validated through comparison to monthly scatterometer winds. Empirical orthogonal function (EOF) analyses of the annual cycle emphasize significant modes of variability in the Indian Ocean. The dominant monsoon reversal and its connection with the southeast trades are linked in eigenmodes one and two of the surface fluxes. The third eigenmode of latent and sensible heat flux reveal a structure similar to the Indian Ocean Dipole (IOD) mode. The variability in surface fluxes associated with the monsoons and IOD are discussed. September-October-November composites of the surface fluxes during the 1997 positive IOD event and the 1983 negative IOD event are examined. The composites illustrate characteristics of fluxes during different IOD phases.
Lowry, M. R. (2009). Developing a Unified Superset in Quantifying Ambiguities Among Tropical Cyclone Best Track Data for the Western North Pacific . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: In the western North Pacific basin, several agencies archive “best track” data of tropical cyclones. The Joint Typhoon Warning Center (JTWC) in Hawaii is responsible for the issuance of tropical cyclone warnings for United States Department of Defense interests and has a record of tropical cyclones extending back to 1945. The Japanese Meteorological Agency (JMA) is the World Meteorological Organization (WMO) official Regional Specialized Meteorological Center (RSMC) for the western North Pacific basin and has best track tropical cyclone data extending back to 1951. The Shanghai Typhoon Institute (STI) of the Chinese Meteorological Administration and the Hong Kong Observatory (HKO) of the Government of the Hong Kong Special Administrative Region also have 6-hourly tropical cyclone data records from 1949 and 1961, respectively. Western North Pacific (WNP) data sets are investigated in order to quantify ambiguities in position and intensity estimates among the forecast institutions through the development of a unified Superset. Ambiguities among the two primary warning centers (JMA and JTWC) are presented in the context of a changing observation network, observational tools, and analysis techniques since the beginning of tropical cyclone records. Mean differences in position estimates are found between the two centers on the order of 60 km prior to the introduction of meteorological satellites in 1961 and near 50 km following the deactivation of aircraft reconnaissance in 1987. Results show a step function change among intensity in JTWC and JMA best track data from 1989 to 1990 due to varying applications of the Dvorak intensity estimation technique. Parsing best track data into landfall subsets does not ameliorate interagency differences in position or intensity estimates. Additionally, analyses from Superset data call into question the veracity of JTWC best track data during the period from 1995-1999. The applicability of adopting an individual data set in discerning long term climate trends is examined in light of these differences. Past efforts to analyze, assemble, and maintain a complete, reliable best track tropical cyclone data set for the WNP are discussed among topical methods of incorporating the Superset within a basin-wide re-analysis.
Williams, M. (2010). Characterizing Multi-Decadal Temperature Variability in the Southeastern United States . Master's thesis, Florida State University, Tallahassee, FL.
Abstract: Prior studies of the long-term temperature record in the Southeastern United States (SE US) mostly discuss the long-term cooling trend, and the inter-annual variability produced by the region's strong ties to El Niño Southern Oscillation (ENSO). An examination of long-term temperature records in the SE US show clear multi-decadal variations in temperature, with relative warm periods in the 1920's through the mid 1950's and a cool period in the late 1950's through the late 1990's. This substantial shift in multi-decadal variability is not well understood and has not been fully investigated. It appears to account for the long-term downward trend in temperatures. An accurate characterization of this variability could lead to improved interannual and long-term forecasts, which would be useful for agricultural planning, drought mitigation, water management, and preparation for extreme temperature events. Statistical methods are employed to determine the spatial coherence of the observed variability on seasonal time scales. The goal of this study is to characterize the nature of this variability through the analysis of National Weather Service Cooperative Observer Program (COOP) station data in Florida, Georgia, Alabama, North Carolina, and South Carolina. One finding is a shift in the temperature Probability Distribution Function (PDF) between warm regimes and cool regimes.
Winterbottom, H. (2010). The Development of a High-Resolution Coupled Atmosphere-Ocean Model and Applications Toward Understanding the Limiting Factors for Tropical Cyclone Intensity Prediction . Ph.D. thesis, Florida State University, Tallahassee, FL.
Abstract: The prediction of tropical cyclone (TC) motion has improved greatly in recent decades. However, similar trends remain absent with respect to TC intensity prediction. Several hypotheses have been proposed attempting to explain why dynamical NWP models struggle to predict TC intensity. The leading candidates are as follows: (1) the lack of an evolving ocean (i.e., sea-surface temperature) boundary condition which responds as a function of the atmosphere (e.g., TC) forcing, (2) inappropriate initial conditions for the TC vortex (e.g., lack of data assimilation methods), (3) NWP model grid-length resolutions which are unable to resolve the temporal and length scale for the features believed responsible for TC vortex intensity. modulations (i.e., eye-wall dynamics, momentum transport, vortex Rossby wave interactions, etc.), and (4) physical parametrization which do not adequately represent the air-sea interactions observed during TC passage. In this study, a coupling algorithm for two independent, high-resolution, and state-of-the-art atmosphere and ocean models is developed. The atmosphere model -- the Advanced Weather Research and Forecasting (WRF-ARW) model is coupled to the HYbrid Coordinate Ocean Model (HYCOM) using a (UNIX) platform independent and innovative coupling methodology. Further, within the WRF-ARW framework, a dynamic initialization algorithm is developed to specify the TC vortex initial condition while preserving the synoptic-scale environment. Each of the tools developed in this study is implemented for a selected case-study: TC Bertha (2008) and TC Gustav (2008) for the coupled-model and TC vortex initialization, respectively. The experiment results suggest that the successful prediction (with respect to the observations) for both the ocean response and the TC intensity cannot be achieved by simply incorporating (i.e., coupling) an ocean model and/or by improving the initial structure for the TC. Rather the physical parametrization governing the air-sea interactions is suggested as the one of the weaknesses for the NWP model. This hypothesis is (indirectly) supported through a diagnostic evaluation of the synoptic-scale features (e.g., sea-level pressure and the deep-layer mean wind beyond the influence of the TC) while the assimilated TC vortex is nudged toward the observed intensity value. It is found -- in the case of TC Gustav (2008) using WRF-ARW, that as the assimilated TC vortex intensity approaches that of the observed, the balance between the mass and momentum states for WRF-ARW is compromised leading to unrealistic features for the environmental sea-level pressure and deep-layer (800- to 200-hPa) mean wind surrounding the TC. Forcing WRF-ARW to assimilate a TC vortex of the observed maximum wind-speed intensity may ultimately compromise the prediction for the TC's motion and subsequently mitigate any gains for the corresponding intensity prediction.Suggestions for additions to the coupled atmosphere-ocean model include a wave-model (WAVEWATCH3), the assimilation of troposphere thermodynamic observations, and modifications to the existing atmospheric boundary-layer parametrization. The current suite of atmosphere model parametrizations do not accurately simulate the observed azimuthal and radial variations for the exchange coefficients (e.g., drag and enthalpy) that have been indicated as potentialpredictor variables for TC intensity modulation. However, these modifications should be implemented only after the limitations for the current coupled-model and TC vortex initialization methods are fully evaluated.
