UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
13 results
Search Results
Item DATA ASSIMILATION OF THE GLOBAL OCEAN USING THE 4D LOCAL ENSEMBLE TRANSFORM KALMAN FILTER (4D-LETKF) AND THE MODULAR OCEAN MODEL (MOM2)(2011) Penny, Stephen G.; Kalnay, Eugenia; Carton, Jim; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The 4D Local Ensemble Transform Kalman Filter (4D-LETKF), originally designed for atmospheric applications, has been adapted and applied to the Geophysical Fluid Dynamics Laboratory's (GFDL) Modular Ocean Model (MOM2). This new ocean assimilation system provides an estimation of the evolving errors in the global oceanic domain for all state variables. Multiple configurations of LETKF have been designed to manage observation coverage that is sparse relative to the model resolution. An Optimal Interpolation (OI) method, implemented through the Simple Ocean Data Assimilation (SODA) system, has also been applied to MOM2 for use as a benchmark. Retrospective 7-year analyses using the two systems are compared for validation. The oceanic 4D-LETKF assimilation system is demonstrated to be an effective method for data assimilation of the global ocean as determined by comparisons of global and regional `observation minus forecast' RMS, as well as comparisons with temperature/salinity relationships and independent observations of altimetry and velocity.Item MADDEN-JULIAN OSCILLATION AND SEA SURFACE TEMPERATURE INTERACTIONS IN A MULTI-SCALE FRAMEWORK(2009) Zhou, Lei; Murtugudde, Raghu; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ocean-atmosphere coupling can play a role in initiating and sustaining the Madden-Julian Oscillations (MJOs), which are the major intraseasonal oscillations in the atmosphere. In this thesis, the oceanic influence on MJOs is studied with reanalysis products, numerical models, and idealized theoretical models. The energy sources for MJOs are calculated with NCEP reanalysis. The perturbed potential energy is found to be the most important energy source for most MJO events. In some MJO events, the sea surface is warmed due to the reduced latent heat flux during the suppressed phase of MJOs. As a result, warm sea surface temperature anomalies (SSTAs) occur, which appear to prolong the life time of these MJO events. In a minority of the MJO events, warm SSTAs can drive the atmosphere actively and trigger MJO events. In these events, the warm SSTAs are attributable to the internal oceanic processes influenced by the warm Indonesian Throughflow (ITF), which spreads from the southeastern Indian Ocean to the western Indian Ocean and modifies the subtle balance between stratification and mixing in the western Indian Ocean. In addition, during the transit period between monsoon seasons, a few MJO events are sustained by the energy obtained from the mean kinetic energy. Since the MJO events have different energy sources, their mechanisms should be considered in the context of these energy sources. While the spatial scale of the SSTAs in the Indian Ocean is only of order 100 km, the scale of MJOs is of order 1000 km, raising the potential for interactions between the oceanic and the atmospheric oscillations with different scales and this is demonstrated to be possible with analytical solutions to idealized linear governing equations. With a reasonable choice of parameters, the meso-scale oceanic and the large-scale atmospheric oscillations can interact with each other and lead to unstable waves in the intraseasonal band in this linear coupled model. The coupling and frequency shifts between oscillations with different scales and the atmospheric/oceanic responses to small variations in the external forcing are also tested with numerical models. Incorporating the oceanic influence on MJOs and the multi-scale interaction appropriately in a numerical model is supposed to help improve the simulation and forecast of MJOs. The hypothesis of multi-scale interaction is also expected to have wide applications in other studies, in addition to the MJO-SST interaction. The theoretical and numerical approach adopted here should also serve as a prototype for enhancing the process understanding of intraseasonal variability and lead to improved predictive understanding.Item Ensemble Data Assimilation and Breeding in the Ocean, Chesapeake Bay, and Mars(2009) Hoffman, Matthew Joseph; Kalnay, Eugenia; Carton, James A; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)My dissertation focuses on studying instabilities of different time scales using breeding and data assimilation in the oceans, as well as the Martian atmosphere. The breeding method of Toth and Kalnay finds the perturbations that grow naturally in a dynamical system like the atmosphere or the ocean. Here breeding is applied to a global ocean model forced by reanalysis winds in order to identify instabilities on weekly and monthly timescales. The method is extended to show how the energy equations for the bred vectors can be derived with only very minimal approximations and used to assess the physical mechanisms that give rise to the instabilities. Tropical Instability Waves in the tropical Pacific are diagnosed, confirming the existence of bands of both baroclinic and barotropic energy conversions indicated by earlier studies. For regional prediction of smaller timescale phenomena, an advanced data assimilation system has been developed for the Chesapeake Bay Forecast System, a regional Earth System Prediction model. To accomplish this, the Regional Ocean Modeling System (ROMS) implementation on the Chesapeake Bay has been interfaced with the Local Ensemble Transform Kalman Filter (LETKF). The LETKF is among the most advanced data assimilation methods and is very effective for large, non-linear dynamical systems in both sparse and dense data coverage situations. In perfect model experiments using ChesROMS, the filter converges quickly and reduces the analysis and subsequent forecast errors in the temperature, salinity, and velocity fields. This error reduction has proved fairly robust to sensitivity studies such as reduced data coverage and realistic data coverage experiments. The LETKF also provides a method for error estimation and facilitates the investigation of the spatial distribution of the error. This information has been used to determine areas where more monitoring is needed. The LETKF framework is also applied here to a global model of the Martian atmosphere. Sensitivity experiments are performed to determine the dependence of the assimilation on observational data. Observations of temperature are simulated at realistic vertical and horizontal levels and LETKF performance is evaluated. Martian instabilities that impact the assimilation are also addressed.Item Global Oceanic Mixed Layer Properties(2008) Liu, Hailong; Carton, James; Grodsky, Semyon; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, the global oceanic mixed layer properties are explored in three aspects: variability of the oceanic mixed layer, subseasonal variability of the barrier layer/ compensated layer and comparison of bulk sea surface and mixed layer temperatures. (1) The analysis of variability of the oceanic mixed layer from 1960-2007 reveals substantial variability in the winter-spring depth of the mixed layer in the subtropics and midlatitudes. In the North Pacific an Empirical Orthogonal Eigenfunction analysis shows a pattern of mixed layer depth variability peaking in the central subtropics. This pattern occurs coincident with intensification of local surface winds and may be responsible for the SST changes associated with the Pacific Decadal Oscillation. In the North Atlantic a pattern of winter-spring mixed layer depth variability occurs that is not so obviously connected to local changes in winds or SST, suggesting that other processes such as advection are more important. Over the 48-year period the winter-spring mixed layers of both basins show deepening trends by 10-100m. (2) The strongest variability of barrier layer/ compensated layer from monthly climatology is found over 100m in the subpolar latitudes of the North Atlantic in winter. Compensated layers in the eastern North Atlantic vary interannually associated with a North Atlantic Oscillation-like pattern of anomalous sea level pressure. In the winter a barrier layer exists in the subpolar North Pacific, while further south along the Kuroshio extension a compensated layer exists, both of which have variability of up to 60m and a significant long-term trend (shrinkage of the barrier layer in the subpolar gyre and growth of the compensated layer to the south) . These changes are also associated with meteorological shifts. (3) Mixed layer temperature (MLTT) and sea surface temperature (SST) are frequently used interchangeably or assumed to be proportional in climate studies. Historical analyses of bulk SST and MLTT from contemporaneous ocean profile observations show that globally and time averaged MLTT is lower than SST by approximately 0.1 oC. In the upwelling zone of the Equatorial East Pacific this negative MLTT-SST difference varies out of phase with seasonal SST, but on interannual timescales MLTT-SST varies in phase with SST with small differences during El Niños as a result of low solar heating and enhanced rainfall. On shorter diurnal timescales, during El Niños, MLTT-SST differences associated with temperature inversions occur in response to nocturnal cooling in presence of nearsurface freshening. Near surface freshening produces persistent shallow (a few meters depth) warm layers in the northwestern Pacific during boreal summer when solar heating is strong. In contrast, shallow cool layers occur in the Gulf Stream area of the Northwest Atlantic in boreal winter when fresh surface layers developed due to lateral interactions are cooled down by abundant turbulent heat loss.Item Density- and wind-driven lateral circulation and the associated transport of sediments in idealized partially mixed estuaries(2008-06-06) Chen, Shih-Nan; Sanford, Lawrence P; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lateral circulation and the associated transport of sediments in idealized partially mixed estuaries are investigated using a three-dimensional, hydrostatic, primitive equation numerical model (ROMS). The model simulates a straight estuarine channel with a triangular cross-section. Attention is focused on lateral density (salinity) gradients, the major driving force for lateral circulation. Lateral salinity gradients can result from boundary mixing on a slope and differential advection of axial salinity gradients. Without wind forcing, the numerical experiments suggest that boundary mixing on a slope can drive significant lateral circulation when the water column is stratified. Boundary mixing is at least as important as differential advection for the modeled scenarios, when the two mechanisms are evaluated using the salt balance equation. Sediments are eroded in the channel and preferentially deposited on the right slope (looking seaward), mainly due to tidal pumping. Both stratification and axial salt transport show strong responses to axial wind forcing. While stratification is always reduced by up-estuary winds, stratification shows an increase-to-decrease transition as down-estuary wind stress increases, due to the competition between wind-induced straining of the axial salinity gradient and direct wind mixing. A horizontal Richardson number modified to include wind straining/mixing is shown to reasonably represent the transition. A regime classification diagram is proposed. Axial winds also exert important controls on lateral circulation. When the water column mixes vertically, surface Ekman transport is not a significant contributor to lateral circulation. Instead, wind-induced differential advection of the axial salinity gradient establishes lateral salinity gradients that in turn drive lateral circulation. A Hansen-Rattray-like scaling shows good predictive skill for variations in lateral flow. Event-integrated sediment transport is from channel to shoals during down-estuary winds but reversed for up-estuary winds. Accounting for wind-waves results in an order-of-magnitude increase in lateral sediment fluxes. The effects of wind-waves and seagrass beds on nearshore (< 2m) sediment dynamics are explored separately using a nearshore model (NearCoM). Without seagrass beds, wind-waves greatly enhance sediment resuspension, providing a large sediment source for lateral sediment transport. Seagrass beds attenuate wind-wave energy and trap sediments, thus reducing net sediment loss from the shallow shoal.Item Interannual variability of sea surface temperature in the eastern tropical Pacific Ocean and Central American rainfall(2007-11-26) Karnauskas, Kristopher Benson; Busalacchi, Antonio J; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Sea surface temperature (SST) in the east Pacific warm pool (EPWP) plays a potentially important role in Central American rainfall, tropical cyclogenesis, ocean biology, large-scale tropical heating, and the El Niño-Southern Oscillation (ENSO). The first part of this dissertation is aimed at understanding what processes govern the interannual variability of SST in the EPWP. Interannual wind stress, shortwave radiation, and precipitation were used as forcing to an ocean general circulation model. Shortwave heating was identified as the primary driver of the interannual SST tendency in the EPWP. The high correlation between the EPWP and the equatorial Pacific Ocean is explained by the fact that equatorial SST anomalies modify the distribution of atmospheric vertical motions and therefore cloud cover and shortwave heating. In a parallel set of experiments, the low-frequency variability of the Tehuantepec gap winds was also shown to have a considerable effect on that of SST in the EPWP. Motivated by the results of the first part of this dissertation, the second part offers significant improvements to the mean state of the equatorial Pacific Ocean in a climatology ocean model experiment by including the Galápagos Islands in the model topography. In this context, the equatorial cold bias is reduced. Furthermore, when the ocean model is coupled to the atmosphere through zonal wind stress, the problem of an excessively regular and biennial ENSO is also reduced. The change in ENSO timescale is a result of the same dynamics operating on a different mean state. The third part of this dissertation is aimed at understanding the role of the interannual variability of SST in the EPWP in that of Central American rainfall. An anomalously warm EPWP can trigger a rapid enhancement of the east Pacific intertropical convergence zone (ITCZ) in rainy seasons following peak ENSO events, which leads to a rainfall anomaly over Central America. Moreover, the timing and amplitude of the SST-enhanced ITCZ depends on the persistence of the ENSO event. The longer the equatorial SST anomaly persists, the longer the EPWP is subject to anomalous shortwave heating and thus the greater the subsequent SST enhancement of the ITCZ.Item Circulation and Transport Processes for the Pocomoke River: A Tibutary to Partially Mixed Estuary(2005-12-12) Davis, Edgar Vernon; Boicourt, William C.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One and two-dimensional box models were used to estimate steady state single and two-layer gravitational circulation, transport, and residence times for the Pocomoke River, a tributary estuary on the eastern shore of Chesapeake Bay. Vertical salinity distribution in the narrow deep river varied from well mixed to stratified, both spatially and temporally. Comparison of estimated freshwater inputs to ADCP transport calculations indicates that the surrounding wetlands have the capacity to store and release a substantial amount of water to the river. The models are particularly useful in defining steady-state concentration distributions of dissolved conservative substances entering the river at a given flux. Spring and late summer residence times varied with river flow, as expected, from 52 to 102 days. The exception was 395 days during the summer drought of 1999.Item Using Ecosystem Network Analysis to Quantify Fluid Flow(2005-08-10) Zickel, Michael James; Ulanowicz, Robert E; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A type of network analysis successfully demonstrated to quantify growth and development in ecosystems is applied to the purely physical phenomenon of fluid flow. Simple two-dimensional models of fluid flow are created and represented as networks of nodes and transfers or flows between nodes. Modeling flow data as a network enables the calculation of indicators or indices that quantify the activity and organization of the represented flow field. The method of cellular automata is used to create three flow field examples, two of which introduce obstacles in the flow field to disrupt the otherwise uniform flow. Four well understood examples from fluid dynamics are described analytically and then analyzed as networks. These conceptual examples of fluid flow demonstrate the utility of network analysis as a method of quantitatively characterizing complex patterns of fluid flow.Item MULTIVARIATE ERROR COVARIANCE ESTIMATES BY MONTE-CARLO SIMULATION FOR OCEANOGRAPHIC ASSIMILATION STUDIES(2005-08-04) Borovikov, Anna Y; Carton, James A; Rienecker, Michele M; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the most difficult aspects of ocean state estimation is the prescription of the model forecast error covariances. Simple covariances are usually prescribed, rarely are cross-covariances between different model variables used. A multivariate model of the forecast error covariance is developed for an Optimal Interpolation (OI) assimilation scheme (MvOI) and compared to simpler Gaussian univariate model (UOI). For the MvOI an estimate of the forecast error statistics is made by Monte Carlo techniques from an ensemble of model forecasts. An important advantage of using an ensemble of ocean states is that it provides a natural way to estimate cross-covariances between the fields of different physical variables constituting the model state vector, at the same time incorporating the model's dynamical and thermodynamical constraints. The robustness of the error covariance estimates as well as the analyses has been established by comparing multiple populations of the ensemble. Temperature observations from the Tropical Atmosphere-Ocean (TAO) array have been assimilated in this study. Data assimilation experiments are validated with a large independent set of subsurface observations of salinity, zonal velocity and temperature. The performance of the UOI and MvOI is similar in temperature. The salinity and velocity fields are greatly improved in the MvOI, as evident from the analyses of the rms differences between these fields and independent observations. The MvOI assimilation is found to improve upon the control (no assimilation) run in generating water masses with properties close to those observed, while the UOI fails to maintain the temperature-salinity relationship. The feasibility of representing a reduced error subspace through empirical orthogonal functions (EOFs) is discussed and a method proposed to substitute the local noise-like variability by a simple model. While computationally efficient, this method produces results only slightly inferior to the MvOI with the full set of EOFs. An assimilation scheme with a multivariate forecast error model has the capability to simultaneously process observations of different types. This was tested using temperature data and synthetic salinity observations. The resulting subsurface structures both in temperature and salinity are the closest to the observed, while the currents structure is maintained in dynamically consistent manner.Item BRED VECTORS IN THE NASA NSIPP GLOBAL COUPLED MODEL AND THEIR APPLICATION TO COUPLED ENSEMBLE PREDICTIONS AND DATA ASSIMILATION(2005-04-27) Yang, Shu-Chih; Yang, Shu-Chih; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The theme of my thesis research is to perform breeding experiments with NASA/NSIPP coupled general circulation model (CGCM) in order to obtain ENSO-related growing modes for ensemble perturbations. We show for the first time that the breeding method is an effective diagnostic tool for studying the coupled ENSO-related instabilities in a coupled ocean-atmosphere general circulation model that includes physical and dynamical processes of many different time scales. We also show for the first time that it is feasible to utilize the coupled bred vectors (BV) as a way to construct perturbations for ensemble forecasts for ENSO prediction using an operational coupled climate prediction model. The results of the thesis research show that coupled breeding can detect a precursor signal associated with ENSO events. Bred vectors are characterized by air-sea coupled features and they are very sensitive to ENSO phases and background season. This indicates that bred vectors can effectively project on the seasonal-to-interannual instabilities by growing upon the slowly varying coupled instability. These results are robust: bred vectors obtained from both the NASA and NCEP coupled systems exhibit similarities in many fields, even in atmospheric teleconnected regions. We show that bred vectors have a structure similar to the one-month forecast error (analysis increment). The BV growth rate and the one-month forecast error show similar low frequency variations. Both of their subsurface temperatures have large-scale variability near the depth of thermocline. Evidence shows that bred vectors capture the eastern movement of the analysis increment (one-month forecast error) along the equatorial Pacific during 1997-1998 El Niño evolution. The results suggest that one-month forecast error in NSIPP CGCM is dominated by dynamical errors whose shape can be captured by bred vectors, especially when the BV growth rate is large. These results suggest that bred vectors should be effective coupled perturbations for ensemble ENSO predictions, compensating for the lack of coupled ENSO-related perturbations in current operational ensembles. The similarity between the bred vectors and the one month forecast errors suggests that bred vectors can capture "errors of the month" and could also be applied to improve oceanic data assimilation by providing information on the month-to-month background variability.