Thesis (M. S.)--Texas A&M University, 2006. / "Major Subject: Atmospheric Sciences" Title from author supplied metadata (automated record created on Apr. 27, 2007.) Vita. Abstract. Includes bibliographical references.

Thesis (M. S.)--Texas A&M University, 2008. / "Major Subject: Atmospheric Sciences" Title from author supplied metadata (automated record created on Oct. 13, 2008.) Vita. Abstract. Includes bibliographical references.

Thesis (Ph. D.)--Texas A&M University, 2008. / "Major Subject: Atmospheric Sciences" Title from author supplied metadata (automated record created on Oct. 13, 2008.) Vita. Abstract. Includes bibliographical references.

<p> Deep convection is an important component of atmospheric circulations that affects many aspects of weather and climate. Therefore, improved understanding and realistic simulations of deep convection are critical to both operational and climate forecasts. Large-eddy simulations (LESs) often are used with observations to enhance understanding of convective processes. This study develops and evaluates a nested-LES method using the Weather Research and Forecasting (WRF) model. Our goal is to evaluate the extent to which the WRF nested-LES approach is useful for studying deep convection during a real-world case. The method was applied on 2 September 2013, a day of continental convection having a robust set of ground and airborne data available for evaluation. A three domain mesoscale WRF simulation is run first. Then, the finest mesoscale output (1.35 km grid length) is used to separately drive nested-LES domains with grid lengths of 450 and 150 m. Results reveal that the nested-LES approach reasonably simulates a broad spectrum of observations, from reflectivity distributions to vertical velocity profiles, during the study period. However, reducing the grid spacing does not necessarily improve results for our case, with the 450 m simulation outperforming the 150 m version. We find that simulated updrafts in the 150 m simulation are too narrow to overcome the negative effects of entrainment, thereby generating convection that is weaker than observed. Increasing the sub-grid mixing length in the 150 m simulation leads to deeper, more realistic convection, but comes at the expense of delaying the onset of the convection. Overall, results show that both the 450 m and 150 m simulations are influenced considerably by the choice of sub-grid mixing length used in the LES turbulence closure. Finally, the simulations and observations are used to study the processes forcing strong midlevel cloud-edge downdrafts that were observed on 2 September. Results suggest that these downdrafts are forced by evaporative cooling due to mixing near cloud edge and by vertical perturbation pressure gradient forces acting to restore mass continuity around neighboring updrafts. We conclude that the WRF nested-LES approach provides an effective method for studying deep convection for our real-world case. The method can be used to provide insight into physical processes that are important to understanding observations. The WRF nested-LES approach could be adapted for other case studies in which high-resolution observations are available for validation.</p>

<p> The goal of this work is to assess the feasibility of using a two-CubeSat constellation to make continuous solar science measurements from low Earth orbit. There is a growing interest in using CubeSats for scientific missions since they are relatively inexpensive, can be manufactured quickly, and they have a standard form factor. CubeSats have increased access to space, and there is a growing interest in the solar science community to be able to conduct remote sensing solar science missions from a CubeSat platform. By using a constellation separated by differential drag, this mission concept enables continuous measurements of the sun, allowing scientists to have a complete record despite the spacecraft's eclipse periods. In this thesis, I have developed a two-body propagator that takes various inputs for starting altitude, density model, attitude, and spacecraft configuration to enable investigation over a large trade space. Following the model development, I ran a series of simulations to explore the feasibility of this concept, finding that there are many combinations of parameters that produce a feasible mission design. I show that the model is validated by altitude decay data from the MinXSS CubeSat, I will discuss areas of the design that require further study, and I explore the logical next steps for future development of this concept.</p><p>

<p> Heat stress is a global issue that crosses socioeconomic status. Heat stress leads to reduced worker capacity on seasonal scales, and weekly to sub-daily timescales, incapacitation, morbidity, and mortality. This dissertation focuses on 2 distinct parts: quantification methods of heat stress, and heat stress applications.</p><p> <b>Quantification methods of heat stress:</b> Chapters 1&ndash;3 focus on historical analysis of heat stress. Chapter 1 is a detailed assessment of previous work in heat stress&mdash;methods, history, and future research out- look. Chapter 2 focuses on the implementation and quantification of a battery of heat stress metrics within the global circulation model framework. The ultimate outcome is a Fortran module, the HumanIndexMod [1], that may be run independently on individual datasets, or used with the Community Earth System Model 1, Community Land Model Version 5 (released February 2018 w/HumanIndexMod). Chapter 3 is an analysis of a battery of heat stress metrics with the focus on showing their differences in global circulation models, and thermodynamic predictability and scalability.</p><p> <b>Heat stress applications:</b> Chapters 4 and 5 focus on applications for physical impact modeling and economic outcomes. Chapter 4 quantifies labor impacts from heat stress due to the covariance or temperature, humidity, and radiation. My predictions of labor productivity losses from heat stress are amenable to Integrated Assessment Modeling. Chapter 5 is a preliminary economic impacts analysis&ndash;a 1^st order sensitivity perturbation study for labor impacts&ndash;which will guide a flagship application for the Purdue University Big Idea Project, GLASS: Global to Local Analysis of Systems Sustainability. My labor productivity losses from heat stress will become a boundary condition for a series of sensitivity assessments intended to inform the policy making process to help achieve the United Nations Sustainability Development Goals.</p><p>

<p> The climate over Ecuador is complex due to several interacting factors, such as its location at the equator, the Andean topography, and several modes of internal variability, including the El Ni&ntilde;o&ndash;Southern Oscillation (ENSO), affecting the region. In addition, the rapid increase in greenhouse gas concentrations will continue to affect both the mean state and climate variability in Ecuador over the coming decades. Hence, a thorough understanding of both natural and anthropogenic forcings and how they combine to influence Ecuadorian climate is a necessity for decision-making and implementation of adequate adaptation measures. However, the lack of observational data, both in space and time, severely limits our ability to study climate changes that affect Ecuador today. Employing a high-resolution regional climate model (RCM) can help to better diagnose the mechanisms and feedbacks that lead to climate changes and how they differ in space and time, as long as the model is able to adequately reproduce what is observed in the limited observational data. </p><p> With the purpose of contributing to a better understanding of how and why Ecuador&rsquo;s climate will change in the coming decades, three topics of specific relevance for this country are addressed in this dissertation: a) how well can a RCM simulate the mean climate state and its variability over a region of complex topography such as Ecuador under different parameterization schemes? b) what feedbacks are involved in producing elevation-dependent warming (EDW) in the Ecuadorian Andes? And c) how are the characteristics of climate extreme events (CEEs) over Ecuador projected to change by the middle of the 21st century? These three questions are addressed by use of observations and simulations using the Weather Research and Forecasting Model (WRF) configured as a RCM with a high-resolution of 10 km horizontal grid spacing and 51 vertical levels. </p><p> Sensitivity test runs were performed to choose a proper combination of parameterization schemes for conducting four WRF simulations comprising the territory of Ecuador and spanning 30 years. The first simulation was driven by the Climate Forecast System Reanalysis (CFSR) for the period 1980&ndash;2010 and used to evaluate the model&rsquo;s ability to realistically portray present-day climate over the region. The other three simulations used the output from the Community Climate System Model version 4 (CCSM4) as the boundary conditions to produce a baseline simulation (1976&ndash;2005) and two future simulations (2041&ndash; 2070) following the moderate-emissions scenario RCP 4.5 and the high-emissions scenario RCP 8.5. </p><p> EDW over the Ecuadorian Andes is studied by analyzing observations and the present-day WRF-simulation, while the future simulations were used to test the contribution to this effect caused by future changes in feedback mechanisms. Surface net radiation changes due to future changes in cloudiness were identified as the most important mechanisms leading to EDW over the Ecuadorian Andes, with future reductions in cloudiness dominating at high elevations. The model results also indicate different future warming signals on both sides of the Andes, with higher warming rates at the high elevations of the western Andes, likely due to enhanced subsidence and adiabatic warming in the mid-troposphere. </p><p> CEEs are analyzed by using annual climatic indices. First the present-day relationship between CEEs and Pacific (ENSO) and Atlantic modes of variability are investigated in both models and observations. Results confirm the dominant role played by ENSO in governing the occurrence of many CCEs over Ecuador, while calling for more studies on the potential influence of Atlantic modes over Ecuador&rsquo;s CEEs. The model projections suggest significant future changes in CEEs, with large increases in warm and wet extremes over most regions, but the simulations also highlight significant spatial heterogeneity, which suggests that it is important to study changes in extreme events using high-spatial resolution data.</p><p>

<p> Unforced internal variability abounds in the climate system and often confounds the identification of climate change due to external forcings. Given that greenhouse gas concentrations are projected to increase for the foreseeable future, separating forced climate change from internal variability is a key concern with important implications. Here, we leverage a 40-member ensemble, the Community Earth System Model Large Ensemble (CESM-LE) to investigate the influence of internal variability on the detection of forced changes in two climate phenomena. First, using cyclone identification and compositing techniques within the CESM-LE, we investigate precipitation changes in extratropical cyclones under greenhouse gas forcing and the effect of internal variability on the detection of these changes. We find that the ensemble projects increased cyclone precipitation under twenty-first century business-as-usual greenhouse gas forcing and this response exceeds internal variability in both near- and far- futures. Further, we find that these changes are almost entirely driven by increases in cyclone moisture. Next, we explore the role of internal variability in projections of the annual cycle of surface temperature over Northern Hemisphere land. Internal variability strongly confounds forced changes in the annual cycle over many regions of the Northern Hemisphere. Changes over Europe, North Africa and Siberia, however, are large and easily detectable and further, are remarkably robust across model ensembles from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive. Using a simple energy balance model, we find that changes in the annual cycle over the three regions are mostly driven by changes in surface heat fluxes. </p><p> The thesis also presents a novel ensemble-based framework for diagnosing forced changes in regional climate variability. Changes in climate variability are commonly assessed in terms of changes in the variances of climate variables. The covariance response has received much less attention, despite the existence of large-scale modes of variability that induce covariations in climate variables over a wide range of spatial scales. Addressing this, the framework facilitiates a unified assessment of forced changes in the regional variances and covariances of climate variables.</p><p>

Ionospheric data relating to the height and incidence of Sporadic-E (E ) has been considered for a number of stations, the normal parameter used being the percentage incidence of greater than a certain specified frequency. Variations in this incidence have been studied on a diurnal and seasonal basis, with respect to magnetic activity, and for different levels of solar activity as represented by the IGY (1957/58) and IqsY (1964/65) periods. The behaviour of E in the auroral region has been examined in detail both in relation to occurrences of all types of E and those types which occur specifically at high latitudes. An E auroral zone has been defined and its properties determined in relation to diurnal, seasonal, solar cycle and magnetic activity changes. The behaviour of the critical frequency and equivalent height of E layers has been interpreted in terms of a two zone model containing a diffuse and a discrete zone. The properties of the two zones and of the E layers are related to the energy of the precipitated electrons. Rocket and satellite measurements of electron energy spectra have been used to calculate electron density profiles as a function of latitude and reflection heights compared with those based on ground based observations. Reasonable agreement is obtained between the two sets of measurements. The Phillips Rule relating to the cumulative distribution of E has been found to apply to all the stations considered at a frequency above the most probable frequency possible interpretations have been given for the gradient of the straight line obtained with the distribution and this gradient is found to show a dependence on latitude which is similar for quiet and disturbed magnetic conditions. This study has also been made of different types of E and their relationship to magnetic activity. Evidence is given to suggest that the changes in the incidences of low type E may be related to changes in the magnetic field. Changes in the behaviour of cusp-type E are considered to be related to the interaction of the quiet solar and disturbed polar current systems. It has also been found that the superposition of low and high type E has a magnetic dependence which is very similar to that of flat type.

<p> Upper-tropospheric thermodynamic processes can play an important role in tropical cyclone (TC) structure and evolution. Despite its importance, until recently few <i>in-situ</i> observations were available in the upper levels of TCs. Two recent field campaigns&mdash;the National Aeronautics and Space Administration (NASA) Hurricane and Severe Storm Sentinel (HS3) and the Office of Naval Research Tropical Cyclone Intensity (TCI) experiment&mdash;provided a wealth of high-altitude observations within TCs. These observations revealed that the upper-level static stability and tropopause structure of TCs can change dramatically with both space and time. </p><p> The TCI dropsonde dataset collected during the rapid intensification (RI) of Hurricane Patricia (2015) revealed dramatic changes in tropopause height and temperature within the storm's inner core. These changes in tropopause structure were accompanied by a systematic decrease in tropopause-layer static stability over the eye. Outside of the eye, however, an initial decrease in static stability just above the tropopause was followed by an increase in static stability during the latter stages of RI. </p><p> Idealized simulations were conducted to examine the processes that might have been responsible for the tropopause variability observed in Hurricane Patricia. A static stability budget analysis revealed that three processes&mdash;differential advection, vertical gradients of radiative heating, and vertical gradients of turbulent mixing&mdash;can produce the observed variability. These results support the theoretical assumption that turbulent mixing plays a fundamental role in setting the upper-level potential temperature stratification in TCs. The existence of turbulence in the upper troposphere of TCs is corroborated by the presence of low-Richardson number layers in a large number of rawinsonde observations. These layers were more common in hurricanes than in weaker TCs, as hurricanes were characterized by both smaller static stability and larger vertical wind shear in the upper troposphere. </p><p> HS3 dropsondes deployed within and around TC Nadine (2012) observed two distinct upper-level stability maxima within the storm's cirrus canopy. Outside of the cirrus canopy, however, only one stability maximum was present in the upper levels. This maximum, just above the tropopause, was stronger over the cirrus canopy than outside of the cirrus canopy. Observations from a large rawinsonde dataset also show this structure, with a stronger temperature inversion located above the tropopause within regions of cold cirrus than outside of cold cirrus. It is hypothesized that vertical gradients of radiative heating, differential advection within the upper-tropospheric outflow layer, and vertical gradients of turbulence all could contribute to producing multiple stability maxima and the stronger temperature inversions in the lower stratosphere. </p><p>