Mountain Scholar :: Browsing by Author "Chandrasekar, V., committee member"

Browsing by Author "Chandrasekar, V., committee member"

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Open Access
A CMOS compatible optical biosensing system based on local evanescent field shift mechanism
(Colorado State University. Libraries, 2011) Yan, Rongjin, author; Lear, Kevin L., advisor; Dandy, David S., committee member; Chandrasekar, V., committee member; Notaros, Branislav, committee member
The need for label-free integrated optical biosensors has dramatically increased in recent years. Integrated optical biosensors have many advantages, including low-cost, and portability. They can be applied to many fields, including clinical diagnostics, food safety, environmental monitoring, and biosecurity applications. One of the most important applications is point-of-care diagnosis, which means the disease could be tested at or near the site of patient care rather than in a laboratory. We are exploring the issues of design, modeling and measurement of a novel chip-scale local evanescent array coupled (LEAC) biosensor, which is an ideal platform for point-of-care diagnosis. Until now, three generations of LEAC samples have been designed, fabricated and tested. The 1st generation of LEAC sensor without a buried detector array was characterized using a commercial near field scanning optical microscope (NSOM). The sample was polished and was end-fire light coupled using single mode fiber. The field shift mechanism in this proof-to-concept configuration without buried detector arrays has been validated with inorganic adlayers [1], photoresist [2] and different concentrations of CRP proteins [3]. Mode beating phenomena was predicted by the beam propagation method (BPM) and was observed in the NSOM measurement. A 2nd generation LEAC sensor with a buried detector array was fabricated using 0.35μm CMOS process at the Avogo Technologies Inc., Fort Collins, Colorado. Characterizations with both single layer patternings, including photoresist as well as BSA [4] and immunoassay complexes [5] were done with cooperative efforts from various research groups. The BPM method was used to study the LEAC sensor, and the simulation results demonstrated the sensitivity of the LEAC sensor is 16%/nm, which was proved to match well with the experimental data [6]. Different antigen/antibodies, including mouse IgG and Hspx (a tuberculosis reactive antigen), have been used to test the immunoassay ability of LEAC sensor [7]. Many useful data have been collected by using the 2nd generation LEAC chip. However, during the characterization of the Avago chips, some design problems were revealed, including incompatibility with microfluidic integration, restricted detection region, strong sidewall scattering and uncoupled light interference from the single mode fiber. To address these problems, the 3rd generation LEAC sensor chip with buried detector arrays was designed to allow real-time monitoring and compatibility with microfluidic channel integration. 3rd generation samples have been fabricated in the CSU cleanroom and the mesa detector structure has been replaced with the thin insulator detector structure to solve the problems encountered during the characterizations. PDMS microfluidic channels and a multichannel measurement system consisting of a probe card, a multiplexing/amplification circuit and a LabVIEW program have been implemented into the LEAC system. In recent years, outbreaks of fast spreading viral diseases, such as bird flu and H1N1, have drawn a lot of concern of the point-of-care virus detection techniques. To test the virus detection ability of LEAC sensor, 40nm and 200nm polystyrene nanoparticles were immobilized onto the waveguide, and the increased scattered light was collected. Sensitivities of 1%/particle and 0.04%/particle were observed for 200nm and 40nm particles respectively.
Open Access
A new look at the Earth's radiation balance from an A-train observational perspective
(Colorado State University. Libraries, 2010) Henderson, David Scott, author; Stephens, Graeme L., 1952-, advisor; Heald, Colette L., committee member; Chandrasekar, V., committee member
The weather and climate of the Earth are driven by interactions of the longwave and shortwave radiation between the Earth's atmosphere and surface. Past studies have tried to derive the Earth radiative budget through the use of models and passive satellite sensors. These past efforts did not have information about the vertical distribution of cloud or aerosols within the atmosphere that significantly influence radiative transfer within the atmosphere. This problem was improved upon with the launch of CloudSat and CALIPSO in 2006. These satellites provide the information on the vertical distribution of clouds. From CloudSat, a fluxes and heating rates product was produced to study the radiative budget, but this was limited to some degree because of undetected clouds and aerosol that have non-negligible effects on the radiative balance. This study addresses these issues by combining CALIPSO and MODIS data with CloudSat to detect and obtain the properties of cloud and aerosol undetected by the CloudSat CPR. The combined data were used to create a cloud and aerosol mask that identified distributions of undetected cloud and aerosol globally and quantified their radiative effects both seasonally and annually. Low clouds were found to have the highest impacts of nearly -6 Wm-2. High clouds globally have little effect, trapping 1 Wm-2, with the majority of the impact in the tropics. Four case studies are presented to show how heating rates change in the vertical due low cloud, cirrus, precipitation, and aerosol. The cloud and aerosol mask was used to create seasonal global distributions of cloud radiative effect using all clouds detected by CloudSat and CALIPSO, and the direct effect of aerosols estimated at the TOA. Using fluxes at the top and bottom of the atmosphere global distributions of outgoing and incoming radiation are shown, and an annual radiation budget of the Earth is derived. Clouds globally are found to have a radiative forcing of -20 Wm-2 at the TOA. The radiative budget of the Earth is calculated in two ways; using normalized shortwave fluxes by the average solar daily insolation, and by changing the solar zenith angle to simulate the diurnal cycle. Finally, the product is validated by comparing the outgoing and surface fluxes with CERES and ISCPP flux products.
Open Access
Analysis of the diurnal cycle in Taiwan during the terrain-influenced monsoon rainfall experiment
(Colorado State University. Libraries, 2012) Ruppert, James Howard, author; Johnson, Richard H., advisor; Chandrasekar, V., committee member; Fletcher, Steven J., committee member; Maloney, Eric D., committee member
The diurnal cycle is investigated in Taiwan during the summer monsoon ("Mei- yu" or plum rain) season using enhanced observations from the 2008 Terrain-influenced Monsoon Rainfall Experiment (TiMREX). The diurnal cycle of an undisturbed period is compared with that of a disturbed period in an aim to 1) better understand the variability of the diurnal cycle as a function of large-scale forcing, 2) describe the complex relationships between rainfall and orographically modified flow, and 3) determine the governing environmental characteristics that distinguish disturbed and undisturbed periods. The study is performed using a regional reanalysis generated by employing three-dimensional variational data assimilation techniques, 0.5° 6-h forecasts from the NCEP GFS (National Centers for Environmental Prediction Global Forecast System), and multiple observation platforms (from TiMREX datasets and others). The undisturbed period (UNDIST) was characterized by southwesterly monsoon flow at low levels, zonal flow in the upper troposphere, suppressed daily-mean rainfall, and unimpeded insolation. Accordingly, pronounced diurnal land-sea breeze (LSB) and mountain-valley (MV) circulations strongly controlled rainfall patterns, which exhibited patterns consistent with low-Froude-number (Fr) flow diverting around the mountainous island of Taiwan. Maximum daytime onshore/upslope flows were associated with enhanced rainfall along the coastal plains and foothills of Taiwan (as opposed to the high peaks), until the nighttime transition brought offshore/downslope flows and development of offshore rainfall where nocturnal density currents converged with the impinging southwesterly monsoon flow. During the disturbed period (DIST), the positioning of a prominent upper- tropospheric trough put Taiwan in a favorable area for large-scale ascent and convective organization, while a shallow, northerly cold intrusion (the Mei-yu front) provided a low- level triggering mechanism for vigorous deep convection. Although the amplitude of diurnal LSB/MV circulations was suppressed during this period (in association with reduced insolation), rainfall diurnal variability was noteworthy, suggesting heightened sensitivity of rainfall to diurnal flows. Consistent with moist conditions and higher-Fr flow, rainfall during this period was maximized over the high mountain peaks. Analysis of vertical profiles of vertical motion and apparent heat sources and moisture sinks for UNDIST demonstrates a predominance of shallow vertical circulations and bottom-heavy convection. In contrast, vigorous deep convection was the dominant rainfall mode during DIST. That the environment was more conducive for vigorous deep convection during DIST explains the increased sensitivity of rainfall to diurnal flows. Common to both periods was an afternoon transition from shallow to deep convection to stratiform rainfall (heating above the freezing level and cooling below; consistent with previous studies). The evolution of rainfall prior to, during, and following DIST exhibited a similar transition. This reflects the "self-similar" nature of tropical convective rainfall systems across spatial and temporal scales.
Open Access
Characteristics and organization of precipitating features during NAME 2004 and their relationship to environmental conditions
(Colorado State University. Libraries, 2008) Pereira, Luis Gustavo P., author; Rutledge, Steven A., advisor; Johnson, Richard H., committee member; Kummerow, Christian D., committee member; Cifelli, Robert C., committee member; Chandrasekar, V., committee member
The focus of this study is to examine the characteristics of convective precipitating features (PFs) during the 2004 North American Monsoon Experiment (NAME) and their precursor environmental conditions. The goal is to gain a better insight into the predictability and variability of warm season convective processes in the southern portion of the North American Monsoon core region. The organization and characteristics of PFs are evaluated using composite radar reflectivity images over the southern portion of the Gulf of California. The environmental conditions are assessed using satellite images and a plethora of atmospheric observational analysis maps, such as winds at multiple levels, upper-level divergence, vorticity, vertical air motion, moisture and vertical cross-sections. Our study reveals that most PFs occurred during the afternoon and evening over land, especially near the foothills of the Sierra Madre Occidental. The vast majority of the precipitating features (~95%) were small, isolated, unorganized, short-lived convective cells. Mesoscale convective systems (MCSs) made up only 5% of the PF population. Nonetheless, these large, long-lived, precipitating features were responsible for 72% of the total precipitation within the radar composite region. An analysis of the number and rainfall produced by these MCSs revealed that they were not constant from day to day, but rather, varied significantly throughout NAME. We found that MCSs were more frequent when the atmosphere is thermodynamically unstable and the wind shear or large-scale dynamics favors the development of organized convection. Lastly, we examined the synoptic conditions associated with episodes of above average MCS rainfall in the southern portion of the NAME core region. Tropical waves were found to be an essential source of moisture and instability in the region. We also found that transient upper-level inverted troughs interact with the upper-level anticyclone to produce a "North American Monsoon Jet Streak" that created favorable dynamical uplift and wind shear conditions for MCS development.
Open Access
Characteristics of hailstorms and enso-induced extreme storm variability in subtropical South America
(Colorado State University. Libraries, 2019) Bruick, Zachary S., author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., committee member; Chandrasekar, V., committee member
Convection in subtropical South America is known to be among the strongest anywhere in the world. Severe weather produced from these storms, including hail, strong winds, tornadoes, and flash flooding, causes significant damages to property and agriculture within the region. These insights are only due to the novel observations produced by the Tropical Rainfall Measuring Mission (TRMM) satellite since there are the limited ground-based observations within this region. Convection is unique in subtropical South America because of the synoptic and orographic processes that support the initiation and maintenance of convection here. Warm and moist air is brought into the region by the South American low-level jet from the Amazon. When the low-level jet intersects the Andean foothills and Sierras de Córdoba, this unstable air is lifted along the orography. At the same time, westerly flow subsides in the lee of the Andes, which provides a capping inversion over the region. When the orographic lift is able to erode the subsidence inversion, convective initiation occurs and strong thunderstorms develop. As a result, convection is most frequent near high terrain. Additionally, convection in this region often remains stationary for many hours by back-building over the high terrain, as the low-level jet continues to orographically lift unstable air over the mountains. This thesis expands the TRMM-based findings on convection in this region in two separate studies: (1) Examination of the El Nino-Southern Oscillation (ENSO)-induced convective variability and (2) Characteristics and environmental conditions supporting hailstorms. The first study uses 16 years of TRMM and reanalysis data to identify how El Nino and La Nina affect storm occurrence and characteristics in this region. While the frequency of storms does not vary greatly between ENSO phases, El Nino conditions tend to promote deeper storms with stronger convection, with more robust synoptic environments supporting convective initiation and maintenance. The second study focuses on the characteristics of the powerful hailstorms that frequent subtropical South America. Using TRMM precipitation radar and microwave imager data, hailstorms are investigated based on their probability of containing hail. Results from this study show that hailstorms have an extended diurnal cycle, often occurring in the overnight hours relative to other locations around the world. High-probability hailstorms tend to be taller and larger than storms that contain low probabilities of hail. They also tend to be supported by strong synoptic forcing, including enhanced lower- and upper-level jet streams, an anomalously warm and moist surface, and increased instability. These conditions can be forecast days in advance, which will help promote readiness and preparation for these damaging storms. Overall, these two studies further the knowledge of convection in subtropical South America, providing new information for short- and long-term forecasts of convection and context to the results of the recent RELAMPAGO field campaign.
Open Access
Comparison of convective clouds observed by spaceborne W-band radar and simulated by cloud-resolving atmospheric models
(Colorado State University. Libraries, 2014) Dodson, Jason B., author; Randall, David, advisor; Birner, Thomas, committee member; Maloney, Eric, committee member; Chandrasekar, V., committee member
Deep convective clouds (DCCs) play an important role in regulating global climate through vertical mass flux, vertical water transport, and radiation. For general circulation models (GCMs) to simulate the global climate realistically, they must simulate DCCs realistically. GCMs have traditionally used cumulus parameterizations (CPs). Much recent research has shown that multiple persistent unrealistic behaviors in GCMs are related to limitations of CPs. Two alternatives to CPs exist: the global cloud-resolving model (GCRM), and the multiscale modeling framework (MMF). Both can directly simulate the coarser features of DCCs because of their multi-kilometer horizontal resolutions, and can simulate large-scale meteorological processes more realistically than GCMs. However, the question of realistic behavior of simulated DCCs remains. How closely do simulated DCCs resemble observed DCCs? In this study I examine the behavior of DCCs in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and Superparameterized Community Atmospheric Model (SP-CAM), the latter with both single-moment and double-moment microphysics. I place particular emphasis on the relationship between cloud vertical structure and convective environment. I also emphasize the transition between shallow clouds and mature DCCs. The spatial domains used are the tropical oceans and the contiguous United States (CONUS), the latter of which produces frequent vigorous convection during the summer. CloudSat is used to observe DCCs, and A-Train and reanalysis data are used to represent the large-scale environment in which the clouds form. The CloudSat cloud mask and radar reflectivity profiles for CONUS cumuliform clouds (defined as clouds with a base within the planetary boundary layer) during boreal summer are first averaged and compared. Both NICAM and SP-CAM greatly underestimate the vertical growth of cumuliform clouds. Then they are sorted by three large-scale environmental variables: total preciptable water (TPW), surface air temperature (SAT), and 500hPa vertical velocity (W500), representing the dynamical and thermodynamical environment in which the clouds form. The sorted CloudSat profiles are then compared with NICAM and SP-CAM profiles simulated with the Quickbeam CloudSat simulator. Both models have considerable difficulty representing the relationship of SAT and clouds over CONUS. For TPW and W500, shallow clouds transition to DCCs at higher values than observed. This may be an indication of the models' inability to represent the formation of DCCs in marginal convective environments. NICAM develops tall DCCs in highly favorable environments, but SP-CAM appears to be incapable of developing tall DCCs in almost any environment. The use of double moment microphysics in SP-CAM improves the frequency of deep clouds and their relationship with TPW, but not SAT. Both models underpredict radar reflectivity in the upper cloud of mature DCCs. SP-CAM with single moment microphysics has a particularly unrealistic DCC reflectivity profile, but with double moment microphysics it improves substantially. SP-CAM with double-moment microphysics unexpectedly appears to weaken DCC updraft strength as TPW increases, but otherwise both NICAM and SP-CAM represent the environment-versus-DCC relationships fairly realistically.
Open Access
Cumulus moistening, the diurnal cycle, and large-scale tropical dynamics
(Colorado State University. Libraries, 2015) Ruppert, James H., author; Johnson, Richard H., advisor; Maloney, Eric D., committee member; van den Heever, Sue, committee member; Chandrasekar, V., committee member
Observations and modeling techniques are employed to diagnose the importance of the diurnal cycle in large-scale tropical climate. In the first part of the study, soundings, radar, and surface flux measurements collected in the Indian Ocean DYNAMO experiment (Dynamics of the Madden–Julian Oscillation, or MJO) are employed to study MJO convective onset. According to these observations, MJO onset takes place as follows: moistening of the low–midtroposphere is accomplished by cumuliform clouds that deepen as the drying by large-scale subsidence and horizontal advection simultaneously wane. This relaxing of subsidence is tied to decreasing column radiative cooling, which links back to the evolving cloud population. A new finding from these observations is the high degree to which the diurnal cycle linked to air-sea and radiative fluxes invigorates clouds and drives column moistening each day. This diurnally modulated cloud field exhibits pronounced mesoscale organization in the form of open cells and horizontal convective rolls. Based on these findings, it is hypothesized that the diurnal cycle and mesoscale cloud organization represent two manners in which local convective processes promote more vigorous day-to-day tropospheric moistening than would otherwise occur. A suite of model tests are carried out in the second part of the study to 1) test the hypothesis that the diurnal cycle drives moistening on longer timescales, and 2) better understand the relative roles of diurnally varying sea surface temperature (SST) and direct atmospheric radiative heating in the diurnal cycle of convection. Moist convection is explicitly represented in the model, the diurnal cycle of SST is prescribed, and cloud-interactive radiation is simulated with a diurnal cycle in shortwave heating. The large-scale dynamics are parameterized using the spectral weak temperature gradient (WTG) technique recently introduced by Herman and Raymond. In this scheme, external (i.e., large-scale) vertical motion wwtg is diagnosed based on the internal diabatic heating in the model. wwtg is then used to advect model temperature and humidity. wwtg opposes domain-averaged temperature anomalies via adiabatic warming and cooling, thereby yielding a feedback between the model diabatic heating and the large-scale column moisture source associated with large-scale vertical motion. With a control simulation that successfully replicates a regime of shallow convection similar to nature, it is found through sensitivity tests that the diurnal cycle in tropospheric radiative heating is the dominant driver of both diurnal column moisture variations and nocturnal rainfall in this regime, the latter of which agrees with previous findings by Randall et al. The diurnal cycle in SST and surface fluxes, in turn, drives the daytime convective regime, which is distinct from the nocturnal regime by its rooting in the boundary layer. A simulation in which the diurnal cycle is stretched to 48 h amplifies an important nonlinear feedback at work in the diurnal cycle, which owes to the high-amplitude diurnal cycle in column relative humidity RH. This diurnal cycle in RH limits the amount of evaporation, and hence evaporative cooling, that takes place in the cloud layer. By throttling down the diabatic cooling, the diurnal cycle throttles down the daily-mean moisture sink driven by large-scale subsidence, such that the environment drifts toward a more moist state, all else being equal. When the diurnal cycle is not present, this nonlinear moisture source is weaker, and the environment drier. This feedback rectifies diurnal moistening onto longer timescales, thereby linking the diurnal cycle to longer timescales. These findings suggest that 1) the diurnal cycle of moist convection, as observed in DYNAMO, cannot be ruled out as an column moisture source important to MJO initiation, and 2) that proper representation of the diurnal cycle is prerequisite to accurate representation of large-scale climate, at least within the regime studied herein.
Open Access
Development of a polarimetric radar based hydrometeor classification algorithm for winter precipitation
(Colorado State University. Libraries, 2012) Thompson, Elizabeth Jennifer, author; Rutledge, Steven A., advisor; Dolan, Brenda, committee member; Chandrasekar, V., committee member; van den Heever, Susan, committee member
The nation-wide WSR-88D radar network is currently being upgraded for dual-polarized technology. While many convective, warm-season fuzzy-logic hydrometeor classification algorithms based on this new suite of radar variables and temperature have been refined, less progress has been made thus far in developing hydrometeor classification algorithms for winter precipitation. Unlike previous studies, the focus of this work is to exploit the discriminatory power of polarimetric variables to distinguish the most common precipitation types found in winter storms without the use of temperature as an additional variable. For the first time, detailed electromagnetic scattering of plates, dendrites, dry aggregated snowflakes, rain, freezing rain, and sleet are conducted at X-, C-, and S-band wavelengths. These physics-based results are used to determine the characteristic radar variable ranges associated with each precipitation type. A variable weighting system was also implemented in the algorithm's decision process to capitalize on the strengths of specific dual-polarimetric variables to discriminate between certain classes of hydrometeors, such as wet snow to indicate the melting layer. This algorithm was tested on observations during three different winter storms in Colorado and Oklahoma with the dual-wavelength X- and S-band CSU-CHILL, C-band OU-PRIME, and X-band CASA IP1 polarimetric radars. The algorithm showed success at all three frequencies, but was slightly more reliable at X-band because of the algorithm's strong dependence on specific differential phase. While plates were rarely distinguished from dendrites, the latter were satisfactorily differentiated from dry aggregated snowflakes and wet snow. Sleet and freezing rain could not be distinguished from rain or light rain based on polarimetric variables alone. However, high-resolution radar observations illustrated the refreezing process of raindrops into ice pellets, which has been documented before but not yet explained. Persistent, robust patterns of decreased correlation coefficient, enhanced differential reflectivity, and an inflection point around enhanced reflectivity occurred over the exact depth of the surface cold layer indicated by atmospheric soundings during times when sleet was reported at the surface. It is hypothesized that this refreezing signature is produced by a modulation of the drop size distribution such that smaller drops preferentially freeze into ice pellets first. The melting layer detection algorithm and fall speed spectra from vertically pointing radar also captured meaningful trends in the melting layer depth, height, and mean correlation coefficient during this transition from freezing rain to sleet at the surface. These findings demonstrate that this new radar-based winter hydrometeor classification algorithm is applicable for both research and operational sectors.
Open Access
Development, fabrication and testing of the scanning and calibration subsystems for the Tropospheric Water and Cloud ICE instrument for 6U CubeSats
(Colorado State University. Libraries, 2019) Kilmer, Braxton, author; Reising, Steven C., advisor; Chandrasekar, V., committee member; Chiu, Christine, committee member
Global observations of ice cloud particle size and ice water content are needed to improve weather forecasting and climate prediction. The interaction between ice particles and upwelling radiation at sub-millimeter-wavelengths strongly depends on ice particle size and observation frequency. Sub-millimeter-wavelength radiometry provides the capability to fill an observational gap by allowing the detection and sizing of ice particles with diameters between 50 μm and 1 mm. Atmospheric temperature and water vapor profiles can also be yielded at sub-millimeter-wavelengths. The Tropospheric Water and Cloud ICE (TWICE) millimeter- and sub-millimeter-wave radiometer instrument is currently under development for 6U CubeSats in a joint effort among Colorado State University (lead), NASA/Caltech Jet Propulsion Laboratory, and Northrop Grumman Corporation. The TWICE radiometer instrument is designed to provide global measurements of cloud ice, as well as temperature and water vapor profiles in the upper troposphere/lower stratosphere. The TWICE radiometer instrument has 16 frequency channels near 118 GHz for temperature profiling, near 183 and 380 GHz for water vapor profiling, and centered on 240, 310, 670, and 850 GHz quasi-window channels for ice particle sizing. The TWICE radiometer instrument uses a conical scanning strategy to observe the Earth's atmosphere and surface. The complete TWICE scan is designed to sweep out a 200° arc once per second, and the scan direction reverses every second interval. The TWICE scanning system is designed to fit inside a 6U CubeSat in terms of volume and mass, while meeting the torque and acceleration requirements of the scanning radiometer instrument. A stepper motor and gearbox mechanism were selected for the TWICE scanning system. Precisely placed position sensors, in combination with stepper motor step calculation, provide sufficient angular position data, in place of a traditional encoder. The TWICE scanning system has been tested, and angular position analysis has been performed. The TWICE instrument performs end-to-end, two-point radiometric calibration by observing an ambient temperature calibration target and cosmic microwave background reflector during each conical scan. The ambient calibration target is designed to enable simultaneous blackbody measurements at all TWICE millimeter- and sub-millimeter-wave channels. Calibration target design parameters, including size, geometry, thermal and electromagnetic properties, have been chosen to meet the performance requirements of the ambient target and to minimize temperature gradients. Reflection coefficient measurements have been performed in the millimeter to sub-millimeter wavelength range of the TWICE channels. Thermal analysis of the ambient calibration target has been performed using ANSYS software. The resulting ambient calibration target design meets functional requirements as well as size and weight constraints to fit into a 6U CubeSat. The TWICE radiometer instrument employs several subsystems that need to communicate during nominal operation. An interface board was designed to meet the communication needs of and provide power regulation for the various interfacing subsystems of the instrument. The interface board is responsible for controlling the scanning subsystem of the radiometer instrument, performing temperature data acquisition for the radiometer instrument front end and the ambient calibration target, routing signals to and from the control and data handling subsystem of the radiometer instrument, and regulating power to the on-board computer. The interface board has been manufactured and its performance has been tested.
Open Access
Electromagnetic model subdivision and iterative solvers for surface and volume double higher order numerical methods and applications
(Colorado State University. Libraries, 2019) Manić, Sanja B., author; Notaroš, Branislav, advisor; Reising, Steven, committee member; Chandrasekar, V., committee member; Oprea, Iuliana, committee member; Ilić, Milan, committee member
Higher order methods have been established in the numerical analysis of electromagnetic structures decreasing the number of unknowns compared to the low order discretization. In order to decrease memory requirements even further, model subdivision in the computational analysis of electrically large structures has been used. The technique is based on clustering elements and solving/approximating subsystems separately, and it is often implemented in conjunction with iterative solvers. This thesis addresses unique theoretical and implementation details specific to model subdivision of the structures discretized by the Double Higher Order (DHO) elements analyzed by i) Finite Element Method - Mode Matching (FEM-MM) technique for closed-region (waveguide) structures and ii) Surface Integral Equation Method of Moments (SIE-MoM) in combination with (Multi-Level) Fast Multipole Method for open-region bodies. Besides standard application in decreasing the model size, DHO FEM-MM is applied to modeling communication system in tunnels by means of Standard Impedance Boundary Condition (SIBC), and excellent agreement is achieved with measurements performed in Massif Central tunnel. To increase accuracy of the SIE-MoM computation, novel method for numerical evaluation of the 2-D surface integrals in MoM matrix entries has been improved to achieve better accuracy than traditional method. To demonstrate its efficiency and practicality, SIE-MoM technique is applied to analysis of the rain event containing significant percentage of the oscillating drops recorded by 2D video disdrometer. An excellent agreement with previously-obtained radar measurements has been established providing the benefits of accurately modeling precipitation particles.
Open Access
Enhancing collaborative peer-to-peer systems using resource aggregation and caching: a multi-attribute resource and query aware approach
(Colorado State University. Libraries, 2012) Bandara, H. M. N. Dilum, author; Jayasumana, Anura P., advisor; Chandrasekar, V., committee member; Massey, Daniel F., committee member; Ray, Indrajit, committee member
To view the abstract, please see the full text of the document.
Open Access
Estimation and correction of wet ice attenuation for x-band radar
(Colorado State University. Libraries, 2010) Leon Colon, Leyda V., author; Bringi, V. N., 1949-, advisor; Chandrasekar, V., committee member; Reising, Steven C., committee member; Rutledge, Steven A., committee member
In the past, single polarized X-band radars were primarily used (along with S-band radars) for hail detection, first by the Russians and then later for the National Hail Research Experiment (NHRE). But X-band radars were not used alone because of the large attenuation at frequencies around 10 GHz and higher, until dual-polarized radars were developed. This fact has brought attention to development and evaluation of correction techniques for rain attenuation in order to exploit the advantages of dual-polarized data. Past developed methods make use of the close relation between the differential propagation phase ΦDP and path attenuation PIA. Their use is known to be successful in rain events, but in the presence of wet ice, these methods are no longer useful because the differential propagation phase is not affected by the isotropic wet ice. This factor was the basis to develop herein two different techniques for estimating the attenuation due to rain and wet ice separately and correct for the wet ice induced attenuation. In this dissertation, two methods are investigated and evaluated. The first method uses the Surface Reference Technique (SRT) α-adjustment method to correct for the attenuation. This method was first developed for the Tropical Rainfall Measuring Mission (TRMM) precipitation radar. We assume that S-band data is un-attenuated and is used as a reference. The difference in reflectivities at the end of the beam (defined as the average of the last ten gates with 'good' data) is attributed to the total attenuation (sum of rain and any wet ice) along the propagation path. The attenuation due to the rain component, if any, is corrected for using the differential propagation phase. Then the α value in the Ah(X)wet ice-Zh(X) power law relationship (with fixed exponent β) is adjusted such that the reflectivities at S-band and the rain-corrected reflectivity at X-band at the end of the beam are forced to match. This adjusted α is used to apportion the reflectivity backwards, which assumes the α parameter is constant along the beam. Using the adjusted value, the attenuation due to wet ice is estimated separately from that of rain. This method is termed here as the SRT-modified correction method. This method has been applied to different datasets. It was evaluated in both simulated and measured radar data. Using the Regional Atmospheric Modeling System (RAMS) model a supercell was simulated by Professor's Cotton's group at Colorado State University (CSU). A radar emulator was used to simulate radar measurements from this supercell at both X-band and S-bands. Results showed good agreement of both corrected reflectivity profiles and wet ice specific attenuation estimation. A dataset from the International H2O Project (IHOP) that had rain mixed with wet ice particles (mixed phase region) was analyzed too. It showed good agreement also, when comparing profiles; moreover wet ice attenuation contours showed agreement with high values of reflectivity as expected in wet ice regions. Data collected by the Center for Adaptive Sensing of the Atmosphere (CASA) radar network was analyzed along with both Next Generation Radars (NEXRAD) KTLX and KOUN data. For the light rain event (CASA/KTLX), the dual wavelength ratio at the maximum range was close to unity as expected for Rayleigh scattering. When corrected for wet ice, the specific attenuation showed agreement with high values in reflectivity at both bands. Finally, this method was applied to two different Cloud Physics Radar (CP2) radar data sets. In the CP2 data analysis, Mie hail signals were eliminated for the purpose of this research. Results from both datasets showed that resulting corrected reflectivity was comparable to the un-attenuated S-band data. Given that an un-attenuated reference signal, like the one described before, might not always be available, a second method was developed without this assumption. This second method estimates the wet ice specific attenuation using a Ah(X)wet ice-Zh(X) power law relation with fixed coefficients. These fixed coefficients were retrieved using the same CP2 datasets and compared with previous findings. Then, assuming that the reflectivity is already corrected for rain attenuation, the Hitschfeld-Bordan forward correction method is used. To determine the areas where the correction method will be applied the Hydrometeor Identification (HID) algorithm was used. The HID data is used here to locate the first 'good' range gate of the mixed phase region containing the wet ice. This method is termed as the Piece-Wise Forward correction method (PWF). Similar to the first method, this second method was applied to different datasets. First it was applied to one of the two CP2 datasets available, where the Mie 'hail' signal was eliminated. The resulting corrected reflectivity showed good agreement compared with the S-band un-attenuated reflectivity. Also this method was applied to the same convective dataset (from CASA) as the one previously analyzed with the SRT-modified method. It presented higher reflectivity values in wet ice identified areas, but lower values than those presented by the SRT-modified method. The results were also compared with the Networked Based (NB) method.
Open Access
Features based assessments of warm season convective precipitation forecasts from the high resolution rapid refresh model
(Colorado State University. Libraries, 2017) Bytheway, Janice L., author; Kummerow, Christian, advisor; Schumacher, Russ, committee member; Randall, David, committee member; Chandrasekar, V., committee member; Alexander, Curtis, committee member
Forecast models have seen vast improvements in recent years, via increased spatial and temporal resolution, rapid updating, assimilation of more observational data, and continued development and improvement of the representation of the atmosphere. One such model is the High Resolution Rapid Refresh (HRRR) model, a 3 km, hourly-updated, convection-allowing model that has been in development since 2010 and running operationally over the contiguous US since 2014. In 2013, the HRRR became the only US model to assimilate radar reflectivity via diabatic assimilation, a process in which the observed reflectivity is used to induce a latent heating perturbation in the model initial state in order to produce precipitation in those areas where it is indicated by the radar. In order to support the continued development and improvement of the HRRR model with regard to forecasts of convective precipitation, the concept of an assessment is introduced. The assessment process aims to connect model output with observations by first validating model performance then attempting to connect that performance to model assumptions, parameterizations and processes to identify areas for improvement. Observations from remote sensing platforms such as radar and satellite can provide valuable information about three-dimensional storm structure and microphysical properties for use in the assessment, including estimates of surface rainfall, hydrometeor types and size distributions, and column moisture content. A features-based methodology is used to identify warm season convective precipitating objects in the 2013, 2014, and 2015 versions of HRRR precipitation forecasts, Stage IV multisensor precipitation products, and Global Precipitation Measurement (GPM) core satellite observations. Quantitative precipitation forecasts (QPFs) are evaluated for biases in hourly rainfall intensity, total rainfall, and areal coverage in both the US Central Plains (29-49N, 85-105W) and US Mountain West (29-49N, 105-125W). Features identified in the model and Stage IV were tracked through time in order to evaluate forecasts through several hours of the forecast period. The 2013 version of the model was found to produce significantly stronger convective storms than observed, with a slight southerly displacement from the observed storms during the peak hours of convective activity (17-00 UTC). This version of the model also displayed a strong relationship between atmospheric water vapor content and cloud thickness over the central plains. In the 2014 and 2015 versions of the model, storms in the western US were found to be smaller and weaker than the observed, and satellite products (brightness temperatures and reflectivities) simulated using model output indicated that many of the forecast storms contained too much ice above the freezing level. Model upgrades intended to decrease the biases seen in early versions include changes to the reflectivity assimilation, the addition of sub-grid scale cloud parameterizations, changes to the representation of surface processes and the addition of aerosol processes to the microphysics. The effects of these changes are evident in each successive version of the model, with reduced biases in intensity, elimination of the southerly bias, and improved representation of the onset of convection.
Open Access
Higher order volume/surface integral equation modeling of antennas and scatterers using diakoptics and method of moments
(Colorado State University. Libraries, 2015) Chobanyan, Elene, author; Notaros, Branislav M., advisor; Reising, Steven, committee member; Oprea, Iuliana, committee member; Chandrasekar, V., committee member; Pezeshki, Ali, committee member
The principal objective of this dissertation is to develop, test, and optimize accurate, efficient, and robust computational methodology and tools for modeling of general antennas and scatterers based on solutions of electromagnetic integral equation formulations using the method of moments (MoM) and diakoptics. The approaches and implementations include the volume integral equation (VIE) method and its hybridization with the surface integral equation (SIE) method, in two ways. The first way combines the VIE method for dielectric parts and the SIE method for metallic parts of the structure. The second way performs subdivision of the entire structure into SIE domains of different constant permittivities, while modeling the inhomogeneity within each domain by the VIE method and employing different Green's functions, with describing the inhomogeneity within each domain in terms of a perturbation with respect to the background permittivity. The first approach is very suitable for analysis of composite wire-plate-dielectric radiation/scattering structures. The second approach provides a particularly efficient solution to problems involving inhomogineities embedded within high-contrast homogeneous dielectric scatterers. The efficiency of computation is enhanced by applying the diakoptic domain decomposition. In the VIE-SIE diakoptic method, the interior diakoptic subsystems containing inhomogeneous dielectric materials are analyzed completely independently applying the VIE-SIE MoM solver, and the solution to the original problem is obtained from linear relations between electric and magnetic surface-current diakoptic coefficients on diakoptic surfaces, written in the form of matrices. The techniques implement Lagrange-type generalized curved parametric hexahedral MoM-VIE volume elements and quadrilateral MoM-SIE and diakoptic patches of arbitrary geometrical-mapping orders, and divergence-conforming hierarchical polynomial vector basis functions of arbitrary current expansion orders. The hexahedra can be filled with inhomogeneous dielectric materials with continuous spatial variations of the permittivity described by Lagrange interpolation polynomials of arbitrary material-representation orders. Numerical computation is further accelerated by MPI parallelization to enable analysis of large electromagnetic problems.
Open Access
Influence of terrain on the characteristics and life cycle of convection observed in subtropical South America
(Colorado State University. Libraries, 2023) Rocque, Marquette N., author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., committee member; Miller, Steven D., committee member; Chandrasekar, V., committee member
Subtropical South America (SSA) is a hotspot for deep, intense convection that often grows upscale into large mesoscale convective systems (MCSs) overnight. The local terrain, including the Andes and a secondary feature known as the Sierras de Córdoba (SDC) are hypothesized to play a major role in the initiation, development, and evolution of convection in the region. Some satellite studies have investigated this role, but storm-scale and life cycle characteristics of these MCSs have not been studied in depth due to the lack of high-resolution, ground-based instruments in the region. However, in 2018-2019, several research-quality platforms were deployed to Córdoba, Argentina as part of the Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) and the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaigns. The data collected during these campaigns is used in the studies presented in this dissertation to investigate how the Andes and SDC contribute to convection initiation and rapid upscale growth under varying synoptic conditions. Determining why convection is so unique in SSA may provide insight into characteristics of other storms around the world. The first two studies in the dissertation evaluate how the Andes and SDC modulate the large-scale environment and storm-scale characteristics under strong vs. weak synoptic forcing. High resolution, convection-permitting simulations in which the terrain is modified are designed to investigate synoptic (Chapter 2) and mesoscale (Chapter 3) processes related to the development of two severe mesoscale convective systems (MCSs) observed during RELAMPAGO-CACTI. Results from the simulations are also compared with radar observations to determine how well the model performs. Under strong synoptic forcing, when the Andes are reduced by 50%, the lee cyclone that develops is weaker, the South American Low-level Jet (SALLJ) is weaker and shallower, and the MCS that develops is weaker and moves quickly off the terrain. When the SDC are removed, there are no substantial changes to the large-scale environment. However, there is no back-building signature of deep convection, likely because cold pools are no longer blocked by the SDC. Under weak synoptic forcing, there are no significant changes to the large-scale environment, even when the Andes are halved. Similar to the strongly forced case though, when the SDC are removed, there are fewer deep convective cores toward the west. In both cases, the model tends to overestimate convection compared to observations. These studies show that the terrain plays varying roles in the evolution of convection in SSA. The third and fourth studies use ground-based lightning observations from RELAMPAGO-CACTI to better understand the electrical and microphysical characteristics of these intense storms. Three-dimensional storm structures are identified in the radar data and lightning flashes are matched with these storm modes to evaluate how lightning varies throughout the convective life cycle (Chapter 4). Results show that lightning flashes associated with deep convective cores are most common along the higher terrain of the SDC and occur in the afternoon hours. They also tend to be the smallest in size. Flashes associated with wide convective cores occur more frequently along the eastern edge of the SDC and are observed around midnight local time. Stratiform flashes are found most frequently in the early morning hours about 50-100 km east of the SDC, and they tend to be the largest in area and occur lower within the cloud. These distributions highlight the life cycle of systems, which initiate along the SDC and grow upscale as they move towards the plains overnight. Flash rates are then related to microphysical properties such as graupel mass and ice water path (Chapter 5). The first lightning flash rate parameterizations are developed for storms in SSA. We find these storms have considerably more graupel associated with them compared to storms in the U.S. These new parameterizations are tested on the simulated strongly forced MCS, and results agree well with observed flash rates. If parameterizations based on U.S. storms had been used instead, the flash rates would have been overestimated by up to a factor of 8. This work, in conjunction with other studies in this dissertation, highlights just how different storms in SSA are compared to the U.S.
Open Access
Intraseasonal and diurnal variations of precipitation features observed during DYNAMO
(Colorado State University. Libraries, 2020) Rocque, Marquette N., author; Rutledge, Steven A., advisor; Maloney, Eric D., committee member; Chandrasekar, V., committee member
The diurnal cycle (DC) of rainfall over the tropical oceans and within the Madden–Julian oscillation (MJO) has been investigated in numerous studies, but there has been limited research on how the DC of precipitation and convective organization evolve throughout phases of the MJO over the open ocean. Cloud and precipitation parameterizations in models have been the source of low MJO predictability, so understanding the fundamental convective processes occurring within the MJO, both on the intraseasonal and diurnal timescales, will be beneficial in improving these model simulations. This study employs measurements collected during the Dynamics of the MJO (DYNAMO) field campaign (1 Oct. – 4 Dec. 2011) to investigate how the distribution of precipitation features (PFs) varies across MJO phase groups, throughout the day, and on-/off-equator. PFs identified from radar volume scans at the R/V Roger Revelle (80.5°E, 0°N) and R/V Mirai (80.5°E, 8°S) were classified into five morphologies based on shape and size. Additionally, several environmental parameters including sea surface temperature (SST), convective available potential energy (CAPE), and latent and sensible heat fluxes were analyzed to understand local interactions between the ocean, atmosphere, and convection. The largest rain events occurred during MJO phases 2&3 at the Revelle. Mesoscale events were found in all phase groups at the Mirai. However, convection was generally weaker at the Mirai, most likely due to extremely dry air (RH < 20%) in the mid-troposphere, and little variation in SST. Two westerly wind bursts (WWBs) were observed in phases 2&3 of the second MJO event (21–30 Nov.) at the Revelle which enhanced surface winds and air–sea fluxes and allowed stratiform precipitation to persist. Additionally, these WWBs enhanced the near-surface equatorial current known as the Yoshida–Wyrtki jet, which caused a large amount of upper ocean mixing and significantly cooled SSTs into December. The DC of rainfall was greatest during phases 8&1 and 2&3 at the Revelle with peaks in rain rate occurring in the afternoon and early morning hours. The afternoon peak was attributed to isolated and sub-MCS nonlinear PFs, apparently forced by SST heating and significant air–sea fluxes. These features then grew upscale through the evening into MCS nonlinear events, peaking in intensity just after midnight. MCS nonlinear features contributed the most to the rain volume during phases 2&3 at the Revelle at roughly 70%. Isolated and sub-MCS nonlinear features were the dominant mode of convection during the suppressed phases at the Revelle (4&5 and 6&7). Mesoscale systems were not observed in these two phase groups. MCS nonlinear systems were found in at least 15% of all radar scans for each phase group at the Mirai, and there was significantly less variability in environmental parameters between phase groups. Additionally, the DC of SST at the Mirai was much weaker than at the Revelle, which was attributed to enhanced surface winds that mixed out any diurnal warm layers. Thus, it was found the MJO had little modulation on the local environment off-equator.
Open Access
Millimeter and sub-millimeter wave radiometers for atmospheric remote sensing from CubeSat platforms
(Colorado State University. Libraries, 2018) Ogut, Mehmet, author; Reising, Steven C., advisor; Chandrasekar, V., committee member; Kummerow, Christian, committee member; Vivekanandan, Jothiram, committee member
To view the abstract, please see the full text of the document.
Open Access
Near real-time processing of voluminous, high-velocity data streams for continuous sensing environments
(Colorado State University. Libraries, 2020) Hewa Raga Munige, Thilina, author; Pallickara, Shrideep, advisor; Chandrasekar, V., committee member; Ghosh, Sudipto, committee member; Pallickara, Sangmi, committee member
Recent advancements in miniaturization, falling costs, networking enhancements, and battery technologies have contributed to a proliferation of networked sensing devices. Arrays of coordinated sensing devices are deployed in continuous sensing environments (CSEs) where the phenomena of interest are monitored. Observations sensed by devices in a CSE setting are encapsulated as multidimensional data streams that must subsequently be processed. The vast number of sensing devices, the high rates at which data are generated, and the high-resolutions at which these measurements are performed contribute to the voluminous, high-velocity data streams that are now increasingly pervasive. These data streams must be processed in near real-time to power user-facing applications such as visualization dashboards and monitoring systems, as well as various stages of data ingestion pipelines such as ETL pipelines. This dissertation focuses on facilitating efficient ingestion and near real-time processing of voluminous, high-velocity data streams originating in CSEs. Challenges in ingesting and processing such streams include energy and bandwidth constraints at the data sources, data transfer and processing costs, underutilized resources, and preserving the performance of stream processing applications in the presence of variable workloads and system conditions. Toward this end, we explore design principles to build a high-performant and adaptive stream processing engine to address processing challenges that are unique to CSE data streams. Further, we demonstrate how our holistic methodology based on space-efficient representations of data streams through a controlled trade-off of accuracy, can substantially alleviate stream ingestion challenges while improving the stream processing performance. We evaluate the efficacy of our methodology using real-world streaming datasets in a large-scale setup and contrast against the state-of-the-art developments in the field.
Open Access
Optimal higher order modeling methodology based on method of moments and finite element method for electromagnetics
(Colorado State University. Libraries, 2011) Klopf, Eve Marian, author; Notaroš, Branislav M., advisor; Chandrasekar, V., committee member; Reising, Steven C., committee member; Oprea, Iuliana, committee member
General guidelines and quantitative recipes for adoptions of optimal higher order parameters for computational electromagnetics (CEM) modeling using the method of moments and the finite element method are established and validated, based on an exhaustive series of numerical experiments and comprehensive case studies on higher order hierarchical CEM models of metallic and dielectric scatterers. The modeling parameters considered are: electrical dimensions of elements (subdivisions) in the model (h-refinement), polynomial orders of basis and testing functions (p-refinement), orders of Gauss-Legendre integration formulas (numbers of integration points - integration accuracy), and geometrical orders of elements (orders of Lagrange-type curvature) in the model. The goal of the study, which is the first such study of higher order parameters in CEM, is to reduce the dilemmas and uncertainties associated with the great modeling flexibility of higher order elements, basis and testing functions, and integration procedures (this flexibility is the principal advantage but also the greatest shortcoming of the higher order CEM), and to ease and facilitate the decisions to be made on how to actually use them, by both CEM developers and practitioners. The ultimate goal is to close the large gap between the rising academic interest in higher order CEM, which evidently shows great numerical potential, and its actual usefulness and application to electromagnetics research and engineering applications.
Open Access
Single cancer cell detection with optofluidic intracavity spectroscopy
(Colorado State University. Libraries, 2012) Wang, Weina, author; Lear, Kevin, advisor; Chandrasekar, V., committee member; Krapf, Diego, committee member; Reardon, Kenneth, committee member
The detection of cancer cells is the basis for cancer diagnostics, cancer screening and cancer treatment monitoring. Non-destructive and non-chemical optical methods may help reduce the complexity and cost of related test, making them more available to the public. The label-free technique of optofluidic intracavity spectroscopy (OFIS) uses light transmitted through a cellular body in a microfluidic optical resonator to distinguish different types of cells by their spectral signatures. The OFIS chips are fabricated in the CSU semiconductor clean room and the fabrication process was reported by a previous Ph.D student, Hua Shao. She also did some initial exploration on combining dielectrophoresis (DEP) with the OFIS technique. Since then, some revisions to the fabrication technique have been made to improve the alignment, bonding and sealing of this microfluidic chip. In addition, new DEP electrode designs have been designed and fabricated to further improve the trapping performance of the traps and facilitate automated cell trapping and analysis. Viability tests were carried out to investigate the effect of heating (induced by DEP electrodes) on cells in chips built with borosilicate and sapphire substrates. These experiments used specially designed DEP electrodes that help more accurately control the DEP exposure time and strength. The survival rate of cells out of DEP enabled OFIS system is greatly affected by the substrate type and DEP exposure dose. The OFIS technique has differentiated red and white human blood cells, as well as canine lymphoma and lymphocytes based on their distinctive transmission spectra. Using OFIS chips fabricated with the modified process, OFIS spectra of settled cells from canine hemangiosarcoma (HSA) cell lines and monocytes in peripheral blood mononuclear cells (PBMCs) were collected and analyzed. To quantify the strength of transverse modes in their spectra, a single characteristic parameter was determined for each cell by forming a linear combination of the mean and standard deviation of the transmission spectra over one free spectral range excluding the residual longitudinal peaks of the bare Fabry-Pérot (F-P) cavities filled with cell suspending medium only. The difference in the characteristic parameters of HSA and monocyte samples was highly statistically significant with a p-value as low as 10-6. A receiver operating characteristic (ROC) curve constructed from t-distributions fit to the HSA and monocytes spectra indicates that the cell classification based on their characteristic parameters can achieve 95% sensitivity and 98% specificity simultaneously. Furthermore, some features observed in the spectra of HSA cells motivated a new optical model of the cell loaded F-P cavity. The OFIS spectra of individual cells from canine HSA and canine lymphoma cancer cell lines exhibit relatively uniformly spaced multiple transverse modes repeated in each free spectral range of a microfluidic F-P cavity while similar spectra of healthy canine monocytes and lymphocytes only have up to 2 or no transverse mode peaks. Modeling of the cells as thin lenses allows paraxial Gaussian beam resonator analysis that produces spectral features that quantitatively match the frequencies of transverse modes and qualitatively agree with the trends in maximum transmission of the modes when aperture losses are included. The extracted experimental focal lengths are significantly larger for cancerous cells than for noncancerous cells and can be used as a potential cell malignancy indicator. Furthermore, a thick lens model was developed, allowing manipulation of more parameters related to cell morphology and its location in the cavity. This model was used to interpret experimental results acquired from settled and suspended cells.