Now showing items 609-628 of 1029

• #### A new sensitivity analysis and solution method for scintillometer measurements of area-average turbulent fluxes

Scintillometer measurements of the turbulence inner-scale length l₀ and refractive index structure function C²n allow for the retrieval of large-scale area-averaged turbulent fluxes in the atmospheric surface layer. This retrieval involves the solution of the non-linear set of equations defined by the Monin-Obukhov similarity hypothesis. A new method that uses an analytic solution to the set of equations is presented, which leads to a stable and efficient numerical method of computation that has the potential of eliminating computational error. Mathematical expressions are derived that map out the sensitivity of the turbulent flux measurements to uncertainties in source measurements such as l₀. These sensitivity functions differ from results in the previous literature; the reasons for the differences are explored.
• #### Nicotine exposure in the developing bullfrog: influences on neuroventilatory responses to CO₂

Developmental exposure to the neuroteratogen nicotine may affect ventilatory responses to hypercapnia. Developmental changes in normocapnic and hypercapnic neuroventilation of the isolated bullfrog brainstem preparation have been previously characterized. I investigated the effect of 3- and 10-wk chronic nicotine (30 [mu]g/L) exposure on lung burst frequency exhibited by early and late metamorphic bullfrog tadpoles during normocapnia (1.5 % CO₂) and hypercapnia (5.0 % CO₂). Chronic nicotine exposure impairs the hypercapnic neuroventilatory response of early metamorphic tadpoles following both 3- and 10-wk exposure. Late metamorphic tadpoles demonstrated an impaired hypercapnic neuroventilatory response only after 10-wk exposure. Chronic nicotine exposure had no effect on normocapnic neuroventilation. Brainstem preparations from early and late metamorphic tadpoles and juvenile bullfrogs were exposed acutely to 18 [mu]g/L nicotine. Acute nicotine had no effect on normocapnic or hypercapnic neuroventilation of early metamorphic tadpoles. Late metamorphic tadpoles and juvenile bullfrogs demonstrated depressed normocapnic neuroventilation in response to acute nicotine exposure, while late metamorphic tadpole brainstems responded significantly to hypercapnia during acute exposure. This suggests that bullfrogs have a differential response to acute nicotine exposure that increases with development. Collectively these data suggest that the consequences of developmental nicotine exposure differ between acute and chronic exposure and throughout bullfrog development.
• #### Nitrogen cycling at treeline: latitudinal and elevational patterns across the boreal landscape

We studied spatial and temporal patterns of soil nitrogen pools and fluxes at treeline and forested sites within three Alaskan mountain ranges along a latitudinal transect of 785 km during 2001- 2002. We measured soil temperatures, pools of soil mineral (ammonium and nitrate) and organic (amino acid and microbial biomass) nitrogen, in situ rates of net mineralization, net nitrification, and net amino acid production, conducted a decomposition experiment at all sites using common litter, and studied soil carbon turnover in a laboratory incubation experiment. Soils at treeline were colder than forested soils, particularly during fall and over winter, and had reduced rates of nitrogen cycling and litter decomposition relative to soils in forested stands. During incubation, treeline soils had lower respiration rates per unit carbon, suggesting lower soil organic matter quality relative to forested soils. 70% of annual net nitrogen mineralization occurred from August-May, suggesting that fall and winter are critical periods for soil nitrogen transformations in forested and treeline ecosystems. Among mountain ranges, nitrogen pools and fluxes were similar, despite variation in growing season length and mean annual temperatures. Soil moisture and organic matter quality may have stronger effects on variation in nitrogen cycling than temperature at our sites.
• #### Nitrogen oxide photochemistry in high northern latitudes during spring

The transport of NOy reservoir species from midlatitudes into the Arctic and the thermal and photochemical breakup of these species has been proposed to be the most important NOx source during spring, and may have an important influence on the ozone budget. This has not yet been shown to be correct. The objective of this research is to understand the sources of NOx and ozone in high latitudes during spring. To measure NOx, a high sensitivity chemiluminescence NO detector and a photolytic converter for NO$\sb2$ were constructed. The detection limits for NO and NO$\sb2$ were 1.70 and 5.67 part per trillion (pptv) in a one-hour average, respectively. Springtime NOx measurements were carried out concurrently with measurements of ozone, PAN, J(NO$\sb2$), and other species during 1994 at the Zeppelin station on Svalbard, and during 1993 and 1995 at Poker Flat, Alaska. The median mixing ratios of NOx, PAN and ozone at Svalbard were 23.7, 237.0 pptv, and 39.0 parts per billion (ppbv), respectively. During a few ozone depletion events in the Arctic marine boundary layer ozone and NOx mixing ratios were as low as 4 ppbv and 0.9 pptv, respectively. Halogen chemistry is probably responsible for both effects. The median NOx, PAN and ozone mixing ratios at Poker Flat were 79.5 pptv, 85.9 pptv, and 40.6 ppbv, respectively. During April and May diurnal cycles of PAN, ozone and temperature were observed and anticorrelated with the water mixing ratio. We interpret this to be the result of mixing with higher layers of the troposphere during the day. At both locations thermal PAN decomposition was an important NOx source. At Svalbard PAN decomposition was small, and the in-situ ozone production rates are an insignificant contribution to the ozone budget. Because of the higher temperatures, PAN decomposition rates, NOx mixing ratios, and in-situ ozone production rates are higher at Poker Flat. A contribution from this production to the overall ozone budget was visible during some periods. These results indicate that stable ozone precursors which are transported into the Arctic from anthropogenic sources can influence the ozone budget in high latitudes.
• #### Nitrogen oxide photochemistry in marine air over the eastern Pacific

Models predict that photochemistry is the primary contribution to the observed spring surface ozone maxima in the Northern Hemisphere. Nitrogen oxides (NOx) are the limiting reagents for photochemical ozone production. This fact leads to an apparent paradox, because the short lifetime of NOx should lead to a low mixing ratio of NOx far from source regions. However, chemical reservoirs of NOx such as peroxyacetyl nitrate (PAN) may provide a transport pathway for delivering NOx to 'clean' marine Pacific air masses. A coastal site in Washington, was chosen to investigate the relationship of NOx, PAN and ozone in marine air from the Pacific during two consecutive spring campaigns in 1997 and 1998. In this work, we develop a simple PAN decomposition model and comparie the modeled to the measured PAN mixing ratios. We estimate ozone formation using the deviation in the measured NO to NO₂ ratio, from the Leighton cycle.
• #### Nitrogen oxides in the arctic stratosphere: Implications for ozone abundances

In the high latitude winter stratosphere, NO$\sb2$ sequesters chlorine compounds which are extremely efficient at destroying ozone. During the nighttime, NO$\sb2$ reacts with ozone to form $\rm N\sb2 O\sb5$ which acts as a reservoir of NO$\sb2$. Under heavy aerosol loading, $\rm N\sb2O\sb5$ may react with water on aerosol surfaces to form HNO$\sb3$, a reservoir more resistant to photolysis. This heterogeneous reaction results in reduced NO$\sb2$ concentration when the sun returns at the end of the winter. A spectrograph system has been developed to measure scattered zenith skylight and thereby determine stratospheric NO$\sb2$ slant column abundance. Conversion of the measured slant column abundance to vertical column abundance requires dividing by the air mass. The air mass is the enhancement in the optical path for the scattered twilight as compared to a vertical path. Air mass values determined using a multiple scattering radiative transfer code have been compared to those derived using a Monte Carlo code and were found to agree to within 6% at a 90$\sp\circ$ solar zenith angle for a stratospheric absorber. Six months of NO$\sb2$ vertical column abundance measured over Fairbanks during the winter 1992-93 exhibited the daylight diminished and increased as the sunlight hours lengthened. The overall seasonal behavior was similar to high-latitude measurements made in the Southern Hemisphere. The ratios of morning to evening column abundance were consistent with predictions based on gas-phase chemistry. The possible heterogeneous reaction of $\rm N\sb2O\sb5$ on sulfate aerosols was investigated using Fourier Transform Infrared Spectrometer measurements of $\rm HNO\sb3$ column abundance and lidar determinations of the aerosol profile. Using an estimated $\rm N\sb2O\sb5$ column abundance and aerosol profile as input to a simple model, significant $\rm HNO\sb3$ production was expected. No increase in $\rm HNO\sb3$ column abundance was measured. From this set of data, it was not possible to determine whether significant amounts of $\rm N\sb2O\sb5$ were converted to $\rm HNO\sb3$ by this heterogeneous reaction. Better estimates of the $\rm N\sb2O\sb5$ and aerosol profile, and a more continuous set of $\rm HNO\sb3$ measurements, are needed to determine if $\rm HNO\sb3$ was actually produced.
• #### Nitrogen oxides in the Arctic troposphere

Nitrogen oxides play a critical role in tropospheric photochemistry. In order to characterize these compounds in the arctic troposphere, ground-level concentrations of total reactive nitrogen (NO$\sb{y}$) and NO were determined over an extended period at a site near Barrow, Alaska. A high-sensitivity instrument developed for this purpose was used in three measurement campaigns: summer 1988, spring 1989, and March-December 1990. During the 1990 campaign, the detection limit for NO was 3-10 pptv (depending on averaging period), and the NO$\sb{y}$ uncertainty was $\pm$26%. A screening algorithm was applied to the data to eliminate effects from local (Barrow) sources, and the remaining data were divided into "background periods" (unaffected by local or regional NO$\sb{x}$ sources), and "events" (periods when emissions from a regional NO$\sb{x}$ source--the Prudhoe Bay oil-producing region--apparently impacted Barrow). These measurements revealed a sharp seasonal cycle of background NO$\sb{y}$ concentrations, with high values in early spring (median 560-620 pptv) and $\sim$70 pptv (median) during summer. This cycle is similar to that of other compounds in arctic haze but is partially attributed to a reduction in NO$\sb{y}$ lifetime due to organic nitrate decomposition as temperatures and insolation increased. Evidence indicates that the springtime arctic NO$\sb{y}$ reservoir was primarily composed of stable removal-resistant species, including PAN and other organic nitrates. PAN decomposition as temperatures rose in late spring likely caused an observed pulse of NO to $\sim$35 pptv (maximum hourly average); hourly-average NO concentrations were otherwise generally $<$8 pptv. NO$\sb{x}$ production from PAN decomposition due to the onset of spring or southward advection may affect springtime O$\sb3$ levels both in the Arctic and in the northern mid-latitudes. NO$\sb{y}$ and O$\sb3$ concentrations were positively correlated during summer, possibly indicating long-range transport of both and/or the presence of a mid-tropospheric NO$\sb{y}$ reservoir combined with a stratospheric O$\sb3$ source. A number of events with substantially elevated NO$\sb{y}$ concentrations (to 16 ppbv) were observed in air not impacted by emissions from the town of Barrow. Substantial evidence indicates that these events were a result of NO$\sb{x}$ emissions from the Prudhoe Bay region ($\sim$300 km to the ESE), which is also expected to affect measurements of other compounds at the Barrow site.
• #### Nitrogen retention in the riparian zone of watersheds underlain by discontinuous permafrost

Riparian zones function as important ecotones for reducing nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. Two research questions were addressed in this study: 1) how does riparian zone nitrogen retention vary in watersheds underlain by discontinuous permafrost, and 2) what is the contribution of denitrification to riparian zone nitrogen retention? To estimate the contribution of the riparian zone to watershed nitrogen retention, I analyzed groundwater chemistry using an end-member mixing model. To assess the importance of denitrification as a mechanism of nitrogen retention, I conducted field denitrification assays using the acetylene block technique. Over the summer, nitrogen retention averaged 0.75 and 0.22 mmol N m⁻² d⁻¹ in low and high permafrost watersheds, respectively. Compared with the fluvial export of nitrogen, the retention rate of nitrogen in the riparian zone is 10 - 15% of the loss rate in stream flow. Denitrification accounted for a small proportion (3%) of total nitrogen retention in the riparian zone. Variation in nitrogen retention between watersheds did not account for differences in stream nitrate concentration between watersheds.
• #### Nmda Receptors In Hibernating Arctic Ground Squirrels

Hibernation is a unique physiological state characterized by suppressed metabolism and body temperature that is interrupted by multiple, brief periods of arousal throughout the hibernation season. Blood flow fluctuates during hibernation and arousal in a reperfusion-like manner without causing neurological damage. Previous studies show that hippocampal slices from hibernating animals tolerate experimental oxygen nutrient deprivation and N-methyl-D-aspartate (NMDA) toxicity better than slices from euthermic animals. However, the cellular mechanisms underlying these examples of tolerance remain unclear. Tolerance to NMDA toxicity suggests that modulation of NMDA receptors (NMDAR) contributes to intrinsic tissue tolerance in slices from hibernating Arctic ground squirrels (hAGS, Spermophilus parryii). NMDAR are one subtype of glutamate receptors. NMDAR play critical roles in excitatory synaptic transmission, synaptic plasticity, learning and memory, and excitotoxicity. NMDAR1 (NR1) is a fundamental subunit of NMDAR and required for receptor function. The main focus of the current project was to test the hypothesis that NMDAR are down-regulated in hAGS compared with interbout euthermic AGS (ibeAGS) and to explore the potential mechanisms of this down-regulation. NMDAR function can be modulated by protein phosphorylation, subunit composition, and internalization. Hence, the aim of chapter 2 was to determine the distribution of NRl in hAGS and ibeAGS using immunohistochemistry. The aim of chapter 3 was to examine NMDAR function in cultured hippocampal slices from hAGS, ibeAGS, and rats using calcium imaging, and to investigate potential modulation of NMDAR such as phosphorylation and internalization for altered function using western blot analysis. Given that synaptic remodeling and functional changes after arousal from hibernation, and NMDAR play an important role in learning and memory, the aim of chapter 4 was to address the effects of hibernation on learning and memory in AGS using an active avoidance task. Here, we report that NMDAR in hAGS are down-regulated via decreased phosphorylation of NR1. This down-regulation is not due to changes in NR1 distribution and internalization. In addition, the fraction of NR1 in the functional membrane pool in AGS is less than in rats. These findings provide evidence that modulation of NMDAR contributes to neuroprotection observed in hAGS.
• #### Nocturnal Processing Of Nitrogen Oxide Pollution At High Latitudes: Off-Axis Cavity Ring-Down Spectroscopy Method Development And Field Measurement Results

Nitrogen oxides, or NOx, play a central role in ozone and nitric acid (HNO3) pollution in the troposphere. Reactions of nitrate radical (NO3) and dinitrogen pentoxide (N2O 5) result in the removal of NOx and ozone from the nighttime atmosphere. In this thesis, we describe the configuration, operation, and performance of an off-axis cavity ring-down spectroscopy (oa-CRDS) field instrument designed for measuring NO3 and N2O5. Furthermore, we report results of an N2O5 instrument intercomparison conducted using an atmospheric simulation chamber in Julich, Germany. The results of the intercomparison demonstrate that the oa-CRDS instrument is an excellent tool for measuring NO3 and N2O 5. Also reported in this thesis are the results of two field campaigns aimed at characterizing NOx removal from the nocturnal pollution plume arising from Fairbanks, AK. The results from the field campaigns suggest ice is responsible for catalyzing N2O5 heterogeneous hydrolysis in cold, high-latitude plumes. When air masses are sub-saturated with respect to ice, the data show longer lifetimes (&sim;20 minutes) and elevated N2O5 levels while ice-saturated air masses show shorter lifetimes (&sim;6 minutes) and suppressed N2O 5 levels. Lastly, we present vertical profiles of N2O 5 measured above the seasonal snow pack. The results of the profiling studies suggest that N2O5 can be removed by heterogeneous hydrolysis on ice in the snow pack. Our findings indicate that catalysis on ice surfaces is largely responsible for nocturnal processing of N2O 5 leading to nitric acid production and loss of NOx in high latitude plumes.
• #### Non-invasive methods for obtaining occupancy probabilities and density estimates of Interior Alaska's mesocarnivore populations

Mesocarnivore species worldwide have been shown to be significant drivers of ecological communities. Changes in their abundance and distributions are known to cause cascading effects throughout ecosystems, and changes to the landscape and climate will likely lead to shifts in mesocarnivore population sizes and distributions. However, the current status of these species in some of the world's most susceptible landscapes is not known. I assessed the impacts of abiotic factors on the distributional patterns and abundance of boreal mesocarnivores and evaluated methods commonly used to estimate density and occupancy. I conducted non-invasive winter surveys of coyotes (Canis latrans), red foxes (Vulpes vulpes), lynx (Lynx canadensis), wolverines (Gulo gulo), and marten (Martes americana) in the interior of Alaska. Overall, mesocarnivore occupancy was most strongly influenced by snow depth and snow compaction as well as habitat type. Canid species used areas with shallow and compact snow while mustelid species used deeper and fluffier snow conditions most often, and lynx used areas with shallow and fluffy snow. Forested habitat types were used most commonly across all mesocarnivores. Prey abundance and the presence of human activity were less influential to mesocarnivore occupancy patterns than snow conditions and habitat, suggesting that a changing boreal climate may have a strong, direct influence on the distribution of these mesocarnivores. Estimating current population status of these species is particularly important in areas that are most susceptible to change, and I used two occupancy-modeling methods and a spatially explicit capture-recapture density estimator to assess coyote and red fox populations. Occupancy and density are two distinct parameters, however, the simplicity of occupancy (both in terms of sampling and modeling) makes its use as a proxy for density an appealing possibility. I found that occupancy and density estimates were not consistent and led to significantly different inference about coyote and red fox populations. Coyotes and red fox occupancy probabilities were similar to each other (range: 0.34-0.48), but red fox density was nearly four times greater than coyote density. While both methods produced precise parameter estimates, top-ranking occupancy and density models were different. I suggest that managers use caution when using occupancy as a proxy for density. Occupancy is best used to address questions related to spatial use, while density should be used to assess population size. Together, these findings provide valuable information about the current status of a previously unstudied mesocarnivore community and provide managers with useful insight into study design and management actions that should be taken to best protect this guild.
• #### Non-invasive nocturnal surveys of sea turtle nesting beaches at La Flor (Pacific) Nicaragua, and Pacuare Reserve and Tortuguero (Caribbean) Costa Rica, July 2013

Marine turtles on the nesting beaches of La Flor public beach (latitude11.14282, longitude 11.14282, geographic datum WGS84), Pacific Nicaragua, and Pacuare Reserve public beach (latitude 10.20123, longitude 83.25925) and Tortuguero (latitude 10.59583, longitude 83.52520) , Caribbean, Costa Rica were observed during late hours after sunset. Observations where noninvasive, geo-referenced and observers stayed three meters away from the sea turtles, according to the national requirements (no light, and some limited red light was used for field clarifications). Many surrounding attributes were taken into consideration and measured including date, time, species, location, observed cysts present in the facial region, visually estimated carapace length, other disturbances present on the beach, number of people/tourists and dogs present, plastic encountered, and if applicable, start and end time of specific activities of nesting (such as start of nest time, start of egg laying and start of Ridley dance.) These data are part of a citizen science project and from a sea turtle fieldclass with Maderas Rainforest Conservancy. This dataset consists of an MS Excel sheet and is less than 1MB in size. Some photoes were taken to present the beaches and procedures.
• #### Non-Normality In Scalar Delay Differential Equations

Analysis of stability for delay differential equations (DDEs) is a tool in a variety of fields such as nonlinear dynamics in physics, biology, and chemistry, engineering and pure mathematics. Stability analysis is based primarily on the eigenvalues of a discretized system. Situations exist in which practical and numerical results may not match expected stability inferred from such approaches. The reasons and mechanisms for this behavior can be related to the eigenvectors associated with the eigenvalues. When the operator associated to a linear (or linearized) DDE is significantly non-normal, the stability analysis must be adapted as demonstrated here. Example DDEs are shown to have solutions which exhibit transient growth not accounted for by eigenvalues alone. Pseudospectra are computed and related to transient growth.
• #### Nonlinear analysis of the ground-based magnetometer network

When the first magnetometer was created by Frederick Gauss in 1833, scientists gained a powerful tool for studying the structure, dynamics, and strength of the Earth's magnetic field: the magnetosphere. Since Gauss' time, the world's scientific community has established ground-based magnetometer stations around the globe in an effort to study local and global perturbations and patterns of the Earth's magnetic field. The main focus of this network has been monitoring the magnetic flux and impact from the Sun's constant outflow of radiation and particles known as the solar wind, as well as its more violent eruptive events. There has been little work, by comparison, into the signals and correlations within the network itself. Since the Earth's field can roughly be mapped to a dipole and disturbances often have a large scale structure, one can surmise there should be some correlation between stations based on their relative positions to one another. What that correlation is or represents is not clear. To investigate this possible correlation and its nature, a set of nonlinear analytic methods were conducted on magnetic data collected from stations scattered across North America over an 18 year period. The analysis was focused on searching for spatial and temporal correlations of nonperiodic signals in the magnetometer network. The findings from that analysis suggest there exist nonlocal correlations between stations that are dependent on position, which could be useful in the development of a space weather risk assessment.
• #### Novel fungal taxa in an Alaskan boreal forest: phylogenetic affinities, ecologies, and ribosomal RNA secondary structures

Phylogenetic analyses suggest that a novel DNA sequence (NS1) found in a boreal forest soil-clone library belongs to the fungal kingdom but does not fall unambiguously within any known class. In order to determine if NS1 codes for an authentic ribosomal RNA (rRNA) gene-copy, I modeled ribosomal RNA secondary structure for four gene regions. Such analyses have never been used on environmental ribosomal sequences before. It appears that NS1 does code for an authentic gene-copy and is not a biological or lab artifact. I also elucidated the habitat preferences, horizon preferences, and fine-scale spatial structure of NS1 using molecular methods. I determined that NS1 was associated with spruce and was found in both the organic and mineral soil horizons. It appears to have a clumped distribution on the scale of a few meters and its spatial distribution shows little inter-annual variability. Together these findings suggest that NS1 does represent an authentic gene-copy and also shed light on the ecology of this putative taxon. I hope future efforts will expand our understanding of both its identity and function.
• #### Novel methods of disease surveillance in wildlife

Both infectious and noninfectious disease agents in wildlife impact human health and accurate research, monitoring, and diagnostic methods are necessary. The objectives of the research reported here were to develop and implement novel methods for bacterial and toxicological disease agent surveillance in wildlife. This dissertation begins with a review of tularemia, an important zoonotic disease to the state of Alaska and the Northern hemisphere. In chapter two, I show the development and implementation of broad-based PCR and quantitative PCR (qPCR) surveillance methods for bacterial DNA in tissue samples; 1298 tissue samples were assayed, numerous potential bacterial pathogens were identified and qPCR detection limits were quantified for various tissue matrices. Chapter three describes an investigation into microbial infection as a source of embryo mortality in greater white-fronted geese (Anser albifrons) in Arctic Alaska. This chapter builds upon our previously developed PCR surveillance techniques by which I demonstrated that bacterial infection is responsible for some greater white-fronted goose embryo mortality in Arctic Alaska. Chapter four describes the development and validation of a cellulose filter paper method for quantifying total mercury in whole blood. I determined that filter paper technology is useful for monitoring total mercury in whole blood, with excellent recoveries (82 - 95% of expected values) and R2 values (0.95 - 0.97) when regressed against the concentration of total mercury in whole blood, the technique generally considered as the "gold standard" for mercury detection. These methods will aid in the accurate detection of disease agents in wildlife as demonstrated by our white-fronted goose work.
• #### Numerical modeling and remote sensing to determine depths of lava tubes and buried cylindrical hot sources

Estimating depths of buried lava tubes is important for determining the thermal budgets and effusion rates of certain volcanic systems. This research uses a laboratory experiment scaled to an observed lava tube system to measure the 3D temperature field surrounding a buried depth adjustable glass tube with hot honey flowing through it at varying conditions such as flow rate and temperature. Numerical techniques are used to model the laboratory experiment. The input parameters are then applied to non-laboratory situations. The surface thermal distributions from these models are analyzed to empirically derive a depth estimation function using regression techniques. This depth function is the first scaleable depth estimation technique which can be solved with remote sensing data alone. The minimum temperature, maximum temperature and width of a Lorentzian distribution, fit to a surface thermal transect, are used in the function to predict depth to the hot source. Sensitivity and error analysis of the function is carried out for depths ranging from 0.01 m to ±60 m with good results. The function gives accurate depth estimates of 0.2 m for extreme arctic environments, ±0.3 m for lava tubes and ± 55 m for subsurface coalfires.
• #### Numerical modeling of lava flow cooling applied to the 1997 Okmok eruption: comparison with AVHRR thermal imagery

Throughout February and March of 1997, Okmok volcano, in the eastern Aleutian Islands of Alaska, erupted a 6 km long lava flow of basaltic a'a within its caldera. A numerical model for lava flow cooling was developed, building upon existing lava cooling models, and applied to the flow to better understand the nature of its cooling. The model predictions were then compared to Advanced Very High Resolution Radiometer (AVHRR) data collected over the flow. Daily data of rainfall and ambient temperature, as opposed to yearly averages used in comparable models, greatly increased the accuracy of the model. Furthermore, convective cooling of the lava surface was observed to be the dominant heat loss process during extended cooling indicating the convective heat transfer coefficient is a prime determinant of the accuracy of the model. The model's flexibility allows application to flows beside that of the 1997 Okmok eruption.
• #### Numerical Modeling Of Seasonally Freezing Ground And Permafrost

This thesis represents a collection of papers on numerical modeling of permafrost and seasonally freezing ground dynamics. An important problem in numerical modeling of temperature dynamics in permafrost and seasonally freezing ground is related to parametrization of already existing models. In this thesis, a variation data assimilation technique is presented to find soil properties by minimizing the discrepancy between in-situ measured temperatures and those computed by the models. The iterative minimization starts from an initial approximation of the soil properties that are found by solving a sequence of simple subproblems. In order to compute the discrepancy, the temperature dynamics is simulated by a new implementation of the finite element method applied to the heat equation with phase change. Despite simplifications in soil physics, the presented technique was successfully applied to recover soil properties, such as thermal conductivity, soil porosity, and the unfrozen water content, at several sites in Alaska. The recovered properties are used in discussion on soil freezing/thawing and permafrost dynamics in other parts of this thesis. Another part of this thesis concerns development of a numerical thermo-mechanical model of seasonal soil freezing on the lateral scale of several meters. The presented model explains observed differential frost heave occurring in non-sorted circle ecosystems north of the Brooks Range in the Alaskan tundra. The model takes into account conservation principles for energy, linear momentum and mass of three constituents: liquid water, ice and solid particles. The conservation principles are reduced to a computationally convenient system of coupled equations for temperature, liquid water pressure, porosity, and the velocity of soil particles in a three-dimensional domain with cylindrical symmetry. Despite a simplified rheology, the model simulates the ground surface motion, temperature, and water dynamics in soil and explains dependence of the frost heave on specific environmental properties of the ecosystem. In the final part, simulation of the soil temperature dynamics on the global scale is addressed. General Circulation Models are used to understand and predict future climate change, but most of them do not simulate permafrost dynamics and its potentially critical feedback on climate. In this part, a widely used climate model is evaluated and the simulated temperatures are compared against observations. Based on this comparison, several modifications to the Global Circulation Models are identified to improve the fidelity of permafrost and soil temperature simulations. These modifications include increasing the total soil depth by adding new layers, incorporating a surface organic layer, and modifying the numerical scheme to include unfrozen water dynamics.
• #### Numerical modeling of two-dimensional temperature dynamics across ice-wedge polygons

The ice wedges on the North Slope of Alaska have been forming for many millennia, when the ground cracked and the cracks were filled with snowmelt water. The infiltrated water then became frozen and turned into ice. When the annual and summer air temperatures become higher, the depth of the active layer increases. A deeper seasonal thawing may cause melting of ice wedges from their tops. Consequently, the ground starts to settle and a trough begins to form above the ice wedge. The forming trough creates a local temperature anomaly in the surrounding ground, and the permafrost located immediately under the trough starts degrading further. Once the trough is formed, the winter snow cover becomes deeper at the trough area further degrading the permafrost. In this thesis we present a computational approach to study the seasonal temperature dynamics of the ground surrounding an ice wedge and ground subsidence associated with ice wedge degradation. A thermo-mechanical model of the ice wedge based on principles of macroscopic thermodynamics and continuum mechanics was developed and will be presented. The model includes heat conduction and quasi-static mechanical equilibrium equations, a visco-elastic rheology for ground deformation, and an empirical formula which relates unfrozen water content to temperature. The complete system is reduced to a computationally convenient set of coupled equations for temperature, ground displacement and ground porosity in a two-dimensional domain. A finite element method and an implicit scheme in time were utilized to construct a non-linear system of equations, which was solved iteratively. The model employs temperature and moisture content data collected from a field experiment at the Next-Generation Ecosystem Experiments (NGEE) sites in Barrow, Alaska. The model describes seasonal dynamics of temperature and the long-term ground motion near the ice wedges and helps to explain destabilization of the ice wedges north of Alaska's Brooks Range.