Mountain Scholar :: Browsing by Author "Maloney, Eric D., committee member"

Browsing by Author "Maloney, Eric D., committee member"

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Open Access
A simplified approach to understanding boundary layer structure impacts on tropical cyclone intensity
(Colorado State University. Libraries, 2018) Delap, Eleanor G., author; Bell, Michael M., advisor; Maloney, Eric D., committee member; Venayagamoorthy, Subhas Karan, committee member
The relationship between tropical cyclone boundary layer (TCBL) structure and tropical cyclone (TC) intensity change is difficult to understand due to limited observations of the complex, non-linear interactions at both the top and bottom boundaries of the TCBL. Consequently, there are debates on how the TCBL interacts with surface friction and how these interactions affect TC intensity change. To begin to address these questions, a conceptual framework of how axisymmetric dynamics within the TCBL can impact TC intensity change is developed from first principles in the form of a new, simple logistic growth equation (LGE). Although this LGE bears some similarities to the operational LGE Model (LGEM; DeMaria 2009), the difference is that our growth-limiting term incorporates TCBL structure and surface drag. The carrying capacity of the LGE—termed the instantaneous logistic potential intensity (ILPI) in this study—is used to explore the relationship between TCBL structure and TC intensity. The LGE is also further solved for the drag coefficient (CD) to explore the relationships between it and both TCBL structure and TC intensity. The validity of this new LGE framework is then explored in idealized numerical modeling using the axisymmetric version of Cloud Model 1 (CM1; Bryan and Fritsch 2002). Results show that CM1 exhibits changes to TCBL structure and TC intensity that are consistent with the LGE framework. Sensitivity of these results to the turbulent mixing lengths, Lh and Lv, are also explored, and general LGE relation- ships still hold as CD is increased. Finally, the LGE framework is applied to observations, and initial CD retrievals indicate that while this new method is low compared to Bell et al. (2012), they are still plausible estimates.
Open Access
A simplified model for understanding natural convection driven biomass cooking stoves
(Colorado State University. Libraries, 2010) Agenbroad, Joshua Nicholas, author; DeFoort, Morgan W., advisor; Willson, Bryan D., advisor; Kirkpatrick, Allan Thomson, committee member; Maloney, Eric D., committee member
It is estimated that half the world's population cooks over an open biomass fire; improved biomass cooking stove programs have the potential to impact indoor air quality, deforestation, climate change, and quality of life on a global scale. The majority of these cooking stoves operate in a natural convection mode (being driven by chimney effect buoyant fluid forces). Simplified theories for understanding the behavior of this unexpectedly complex combustion system, along with practical engineering tools to inform its design are markedly lacking. A simplified model of the fundamental stove flow physics is developed for predicting bulk flow rate, temperature, and excess air ratio based on stove geometry (chimney height, chimney area, viscous and heat release losses) and the firepower (as established by the stove operator). These parameters are intended to be fundamental inputs for future work understanding and improving biomass cook stove emissions and heat transfer. Experimental validation is performed and the simplified model is shown to be both accurate and applicable to typical stove operation. Carbon monoxide and particulate matter emissions data has been recorded in conjunction with the validation data. The initial results are presented and indicate that the excess air ratio may be a promising tool for reducing carbon monoxide emissions. A dimensionless form of the simplified stove flow model is then developed. This form offers several advantages, including scale similarity and a reduction of independent experimental parameters. Plotting with dimensionless parameters, various stove configurations can be plotted concurrently, and general stove flow behavior common to all natural convection stoves is observed. With a dimensionless firepower axis, emissions trends for both carbon monoxide and particulate matter become apparent, and a region of improved combustion efficiency and lowered emissions is identified.
Open Access
An examination of the large-scale drivers of North Atlantic vertical wind shear and seasonal tropical cyclone variability
(Colorado State University. Libraries, 2021) Jones, Jhordanne J., author; Bell, Michael M., advisor; Klotzbach, Philip J., advisor; Barnes, Elizabeth A., committee member; Maloney, Eric D., committee member; Florant, Gregory L., committee member
This dissertation characterizes and examines the large-scale sources of variability driving tropical North Atlantic deep-layer vertical wind shear (VWS). VWS is a key variable for the seasonal prediction of tropical cyclone (TC) activity and can be used to assess sources of predictability within a given season. Part 1 of the dissertation examines tropical versus subtropical impacts on TC activity by considering large-scale influences on boreal summer tropical zonal VWS variability, a key predictor of seasonal TC activity. Through an empirical orthogonal function analysis, I show that subtropical anticyclonic wave breaking (AWB) activity drives the second mode of variability in tropical zonal VWS, while El Niño-Southern Oscillation (ENSO) primarily drives the leading mode of tropical zonal VWS variability. Linear regressions of the four leading principal components against tropical North Atlantic zonal VWS and accumulated cyclone energy show that, while the leading mode holds much of the regression strength, some improvement can be achieved with the addition of the second and third modes. Furthermore, an index of AWB-associated VWS anomalies, a proxy for AWB impacts on the large-scale environment, may be a better indicator of summertime VWS anomalies. The utilization of this index may be used to better understand AWB's contribution to seasonal TC activity. Part 2 shows that predictors representing the environmental impacts of subtropical AWB on seasonal TC activity improve the skill of extended-range seasonal forecasts of TC activity. There is a significant correlation between boreal winter and boreal summer AWB activity via AWB-forced phases of the quasi-stationary North Atlantic Oscillation (NAO). Years with above-normal boreal summer AWB activity over the North Atlantic region also show above-normal AWB activity in the preceding boreal winter that forces a positive phase of the NAO that persists through the spring. These conditions are sustained by continued AWB throughout the year, particularly when ENSO plays less of a role at forcing the large-scale circulation. While individual AWB events are synoptic and nonlinear with little predictability beyond 8-10 days, the strong dynamical connection between winter and summer wave breaking lends enough persistence to AWB activity to allow for predictability of its potential impacts on TC activity. We find that the winter-summer relationship improves the skill of extended-range seasonal forecasts from as early as an April lead time, particularly for years when wave breaking has played a crucial role in suppressing TC development. Part 3 characterizes VWS variability within the Community Earth System Model version 1 Large Ensemble (CESM1-LE). The 35 historical runs of the CESM1-LE provide substantially larger samples of the environment and various large-scale drivers than the ERA5 reanalysis that spans 1979 to present. Firstly, ENSO is shown to be the leading mode of tropical Atlantic variability and explains most, if not all, of the structured variance. Secondly, while the CESM1-LE shows robust physical representations of known climate phenomena, their relationships with tropical Atlantic VWS remain marginal except for ENSO. Eigenanalysis applied to the CESM1-LE shows that the principal components are ill-defined and gives no distinct pattern for non-ENSO associated large-scale drivers. Thirdly, composite analyses show that despite the narrow range of VWS variability associated with non-ENSO large-scale drivers, their individual contribution to VWS is noticeably stronger during ENSO-neutral conditions as represented by the large ensemble.
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
Application of neural networks to subseasonal to seasonal predictability in present and future climates
(Colorado State University. Libraries, 2022) Mayer, Kirsten J., author; Barnes, Elizabeth A., advisor; Hurrell, James W., committee member; Maloney, Eric D., committee member; Anderson, Charles, committee member
The Earth system is known for its lack of predictability on subseasonal to seasonal timescales (S2S; 2 weeks to a season). Yet accurate predictions on these timescales provide crucial, actionable lead times for agriculture, energy, and water management sectors. Fortunately, specific Earth system states – deemed forecasts of opportunity – can be leveraged to improve prediction skill. Our current understanding of these opportunities are rooted in our knowledge of the historical climate. Depending on societal actions, the future climate could vary drastically, and these possible futures could lead to varying changes to S2S predictability. In recent years, neural networks have been successfully applied to weather and climate prediction. With the rapid development of neural network explainability techniques, the application of neural networks now provides an opportunity to further understand our climate system as well. The research presented here demonstrates the utility of explainable neural networks for S2S prediction and predictability changes under future climates. The first study presents a novel approach for identifying forecasts of opportunity in observations using neural network confidence. It further demonstrates that neural networks can be used to gain physical insight into predictability, through neural network explainability techniques. We then employ this methodology to explore S2S predictability differences in two future scenarios: under anthropogenic climate change and stratospheric aerosol injection (SAI). In particular, we explore subseasonal predictability and forecasts of opportunity changes under anthropogenic warming compared to a historical climate in the CESM2-LE. We then investigate how future seasonal predictability may differ under SAI compared to a future without SAI deployment in the ARISE-SAI simulations. We find differences in predictability between the historical and future climates and the two future scenarios, respectively, where the largest differences in skill generally occur during forecasts of opportunity. This demonstrates that the forecast of opportunity approach, presented in the first study, is useful for identifying differences in future S2S predictability that may not have been identified if examining predictability across all predictions. Overall, these results demonstrate that neural networks are useful tools for exploring subseasonal to seasonal predictability, its sources, and future changes.
Open Access
Balanced and transient aspects of the intertropical convergence zone
(Colorado State University. Libraries, 2015) Gonzalez, Alex O., author; Schubert, Wayne H., advisor; Maloney, Eric D., committee member; Birner, Thomas, committee member; Estep, Donald J., committee member
The Intertropical Convergence Zone (ITCZ) is one of the primary drivers of tropical circulations and because of its interactions with the extratropics, contributes significantly to Earth's general circulation. This dissertation investigates dynamical aspects of the ITCZ using a variety of analytical and numerical models. In the first chapter, we learn that deep and shallow balanced Hadley circulations are forced by deep diabatic heating and Ekman pumping at the top of the boundary layer, respectively. Also, when the ITCZ is located off of the equator there is an inherent asymmetry between the winter and summer Hadley cells due to the anisotropic nature of the inertial stability. The second study examines shallow and deep vertical motions over the eastern Pacific Ocean (80°W--150°W) using the Year of Tropical Convection reanalysis (YOTC). Vertical motions in the eastern Pacific tend to be bimodal, with both shallow and deep vertical motions occurring throughout the year. Shallow vertical motions are typically narrow and restricted to low latitudes (ITCZ-like) while deep vertical motions tend to be broad and are located poleward of shallow regimes, except during El Niño conditions. The study of balanced Hadley circulations is also extended to investigate the role of transient aspects of the Hadley circulation. The solutions illustrate that inertia-gravity wave packets emanate from the ITCZ and bounce off a spectrum of turning latitudes when the ITCZ is switched on at various rates. These equatorially trapped wave packets cause the Hadley cells to pulsate with periods of 1--3 days. In the last part of this dissertation, we focus on boundary layer aspects of the formation of the ITCZ. Since the ITCZ boundary layer is a region of significant meridional convergence, meridional advection should not be neglected. Using a zonally symmetric slab boundary layer model, shock-like structures appear in the form of near discontinuities in the horizontal winds and near singularities in the vorticity and Ekman pumping after 1--2 days. The numerical model also agrees well with dynamical fields in YOTC while adding important details about the boundary layer pumping and vorticity. In closing, we believe that the ITCZ is a highly transient region vital to the general circulation of the atmosphere, and many of its features can be explained by dry dynamics.
Open Access
Climate model error in the evolution of sea surface temperature patterns affects radiation and precipitation projections
(Colorado State University. Libraries, 2024) Alessi, Marc J., author; Rugenstein, Maria A.A., advisor; Barnes, Elizabeth A., committee member; Maloney, Eric D., committee member; Willis, Megan D., committee member
Atmosphere-ocean general circulation models (AOGCMs) are the primary tool climate scientists use in predicting the effects of climate change. While they have skill in reproducing global-mean temperature over the historical period, they struggle to replicate recently observed sea surface temperature (SST) trend patterns. In this dissertation, we quantify the impact of potential future model error in SST pattern trends on projections of global-mean temperature and Southwest U.S. (SWUS) precipitation. We primarily use a Green's function (GF) approach to identify which SST regions are most relevant for changes in these variables. Our findings demonstrate significant sensitivity of both global-mean temperature and SWUS precipitation to the pattern of sea surface warming, meaning that a continuation of AOGCM error in SST trend patterns adds uncertainty to climate projections which are currently not accounted for. In Chapter 1, we quantify the relevance of future model error in SST to global-mean temperature projections through convolving a GF with physically plausible SST pattern scenarios that differ from the ones AOGCMs produce by themselves. We find that future model error in the pattern of SST has a significant impact on projections, such as increasing total model uncertainty by 40% in a high-emissions scenario by 2085. A reversal of the current cooling trend in the East Pacific over the next few decades could lead to a period of global-mean warming with a 60% higher rate than currently projected. These SST pattern scenarios work through a destabilization of the shortwave cloud feedback to affect temperature projections. In Chapter 2, we focus on near-term projections of precipitation in the SWUS. The observed decrease in SWUS precipitation since the 1980s and heightened drought conditions since the 2000s have been linked to a cooling sea surface temperature (SST) trend in the Equatorial Pacific. Notably, climate models fail to reproduce this observed SST trend, and they may continue doing so in the future. In this chapter, we assess the sensitivity of SWUS precipitation projections to future SST trends using a GF approach. Our findings reveal that a slight redistribution of SST leads to a wetting or drying of the SWUS. A reversal of the observed cooling trend in the Central and East Pacific over the next few decades would lead to a period of wetting in the SWUS. In Chapter 3, we analyze SWUS precipitation sensitivity to SST patterns on long timescales (7+ years) according to a GF approach and a convolutional neural network (CNN) approach. The GF and CNN identify different SST regions as having greater influence on SWUS precipitation: the GF highlights the Central Pacific known from theory to be relevant, while the CNN highlights the South-Central Pacific. To determine if the South-Central Pacific has a physically meaningful and so far overlooked influence on SWUS precipitation, rather than just a statistical relationship, we force an atmosphere-only climate model with an SST anomaly inspired by an Explainable Artificial Intelligence (XAI) method. We find that SSTs in the South-Central Pacific influence SWUS precipitation through an atmospheric bridge dynamical pathway, justifying the CNN's sensitivity physically. The fact that we cannot fully trust the evolution of SST patterns in AOGCMs has many implications for the field of climate science and for how the world's governments and organizations respond to global warming. It is critical for climate change adaptation and mitigation assessments to consider this previously unaccounted for uncertainty in climate projections. Climate scientists can do this by developing SST pattern storylines based on theory, observations, and our understanding of the ocean-atmosphere system. If we fail to communicate known uncertainties for both global-mean and regional projections, the world could lose faith in the climate science community, resulting in less of a global response to climate change.
Open Access
CloudSat tropical cyclone database
(Colorado State University. Libraries, 2010) Tourville, Natalie Dawn, author; Stephens, Graeme L., 1952-, advisor; DeMaria, Mark, committee member; Eykholt, Richard Eric, 1956-, committee member; Maloney, Eric D., committee member
CloudSat (CS), the first 94 GHz space borne cloud profiling radar (CPR), launched in 2006 to study the vertical distribution of clouds. Not only are CS observations revealing inner vertical cloud details of water and ice globally but CS overpasses of tropical cyclones (TC's) are providing a new and exciting opportunity to study the vertical structure of these storm systems. CS TC observations are providing first time vertical views of TC's and demonstrate a unique way to observe TC structure remotely from space. Since December 2009, CS has intersected every globally named TC (within 1000 km of storm center) for a total of 5,278 unique overpasses of tropical systems (disturbance, tropical depression, tropical storm and hurricane/typhoon/cyclone (HTC)). In conjunction with the Naval Research Laboratory (NRL), each CS TC overpass is processed into a data file containing observational data from the afternoon constellation of satellites (A-TRAIN), Navy's Operational Global Atmospheric Prediction System Model (NOGAPS), European Center for Medium range Weather Forecasting (ECMWF) model and best track storm data. This study will describe the components and statistics of the CS TC database, present case studies of CS TC overpasses with complementary A-TRAIN observations and compare average reflectivity stratifications of TC's across different atmospheric regimes (wind shear, SST, latitude, maximum wind speed and basin). Average reflectivity stratifications reveal that characteristics in each basin vary from year to year and are dependent upon eye overpasses of HTC strength storms and ENSO phase. West Pacific (WPAC) basin storms are generally larger in size (horizontally and vertically) and have greater values of reflectivity at a predefined height than all other basins. Storm structure at higher latitudes expands horizontally. Higher vertical wind shear (≥ 9.5 m/s) reduces cloud top height (CTH) and the intensity of precipitation cores, especially in HTC strength storms. Average zero and ten dBZ height thresholds confirm WPAC storms loft precipitation sized particles higher into the atmosphere than in other basins. Two CS eye overpasses (32 hours apart) of a weakening Typhoon Nida in 2009 reveal the collapse of precipitation cores, warm core anomaly and upper tropospheric ice water content (IWC) under steady moderate shear conditions.
Open Access
Computational modeling of wind turbine wake interactions
(Colorado State University. Libraries, 2012) Davis, Cole J., author; Venayagamoorthy, S. Karan, advisor; Heyliger, Paul R., advisor; Maloney, Eric D., committee member
The rapid expansion of the wind energy market necessitates the need for advanced computational modeling and understanding of wind turbine aerodynamics and wake interactions. The following thesis work looks to study turbulence closure methods widely used in computational fluid dynamics (CFD) and their applicability for modeling wind turbine aerodynamics. The first investigation is a parametric study of turbulence models and their performance on geometries of stationary in-line turbines and disks spaced at different intervals. A variety of Reynolds-averaged Navier-Stokes (RANS) closure schemes (Spalart-Allmaras, Standard k-ε, k-ε Realizable, k-ε RNG, Standard k-ω, k-ω SST) were studied as well as a large eddy simulation (LES) with a dynamic Smagorinsky-Lilly sub-grid scale (SGS) model. The simulations showed the grid refinement to be inadequate for LES studies. The RANS closure schemes did not indicate a dominant model. However, relevant literature on separating flows has shown the k-ω SST model to be preeminent. The second investigation uses only the k-ω SST RANS closure scheme to model wake development and resolution for both a single fully resolved rotating turbine as well as two in-line fully resolved rotating turbines. These simulations were successful in predicting wake development and resolution, as well as predicting velocity deficits experienced by the downstream turbine. Vorticity results also showed an accurate wake structure and helical tendencies. In the third investigation, a grid independence study was performed to gain an accurate pressure distribution on the blade surfaces for a separate, collaborative, non-linear, structural study of wind turbine blades. This study showed a strong asymptotic relationship of the maximum pressure on the blades to the predicted Bernoulli pressure on the blade. The results of this research show clear wake development, structure and resolution. The velocity deficits found translate directly in to power deficits for downstream turbines and the vorticity translates directly into increased fatigue experienced by the blades. In contrast to the vast super-computer simulations found in literature, all simulations in this thesis work were calculated using four parallel processors. The accuracy was achieved through assumptions, which were designed to maintain the desired physics while simplifying the complexity of the problem to the capabilities of desktop computing. This research demonstrates the significance of model design and capabilities and accuracy achievable using desktop computing power. This has vast implications of accessibility into academia and the further development of the wind power industry.
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
Deep and shallow overturning circulations in the tropical atmosphere
(Colorado State University. Libraries, 2013) Rojas, Gabriela Mora, author; Schubert, Wayne H., advisor; Birner, Thomas, committee member; DeMaria, Mark, committee member; Maloney, Eric D., committee member; Trumbo, Craig W., committee member
This dissertation examines the dynamics of zonally symmetric, deep and shallow overturning circulations in the tropical atmosphere. The dynamics are discussed in the context of idealized analytical solutions of the equatorial β-plane version of the Eliassen meridional circulation equation that arises in balanced models of the Hadley circulation. This elliptic equation for the meridional circulation has been solved analytically by first performing a vertical normal mode transform that converts the partial differential equation into a system of ordinary differential equations for the meridional structures of all the vertical modes. These meridional structure equations can be solved via the Green's function, which can be expressed in terms of parabolic cylinder functions of half-integer order. The analytical solutions take simple forms in two special cases: (1) Forcing by deep diabatic heating that projects only onto the first internal mode in the absence of Ekman pumping; (2) Forcing by Ekman pumping in the absence of any diabatic heating. Case (1) leads to deep overturning circulations, while case (2) leads to shallow overturning circulations. Both circulations show a marked asymmetry between the winter hemisphere and summer hemisphere overturning cells. This asymmetry is due to the basic anisotropy introduced by the spatially varying inertial stability coefficient in the Eliassen meridional circulation equation. A simple physical interpretation is that fluid parcels forced near the equator to overturn by diabatic and frictional processes tend to move much more easily in the horizontal direction because the resistance to horizontal motion (i.e. inertial stability) is so much less than the resistance to vertical motion (i.e., static stability).
Open Access
Heat transfer efficiency of biomass cookstoves
(Colorado State University. Libraries, 2010) Zube, Daniel Joseph, author; DeFoort, Morgan W., advisor; Willson, Bryan D., committee member; Maloney, Eric D., committee member
Nearly half of the world's human population burns biomass fuel to meet home energy needs for heating and cooking. Biomass combustion often releases harmful chemical compounds, greenhouse gases, and particulate matter into the air which all have a detrimental effect on both human health and global climate and ecology. In order mitigate the harmful effects of biomass combustion, thermal efficiency of the combustion process must be improved. Thermal efficiency is influenced equally by combustion efficiency and heat transfer efficiency, but the emphasis of this research is on heat transfer efficiency since it offers the most room for improvement. A theoretical approach is taken to understand the fundamental physics underpinning the three modes of heat transfer: conduction, convection, and radiation. A strong theoretical understanding of each mode as it applies to cookstoves is established and used as a tool to evaluate potential design enhancements. Based on these evaluations, certain design modifications are suggested as a practical means to boost heat transfer efficiency. Future research topics are suggested which will further increase the accuracy of theoretical predictions surrounding stove performance.
Open Access
Interactions between the Madden-Julian oscillation and mesoscale to global scale phenomena
(Colorado State University. Libraries, 2019) Toms, Benjamin A., author; van den Heever, Susan C., advisor; Barnes, Elizabeth A., committee member; Maloney, Eric D., committee member; Cooley, Daniel, committee member
The Madden-Julian Oscillation (MJO) influences and interacts with atmospheric phenomena across the globe, from the tropics to the poles. In this two-part study, the interactions of the MJO with other phenomena across a broad range of scales are considered, including mesoscale convective structures within the tropics and global teleconnection patterns. While the two studies are distinct in the scales of the interactions they discuss, each highlights an aspect of the importance of interactions between the MJO and variability across a broad range of scales within the climate system. The study of such cross-scale interactions is important for understanding our climate system, as these interactions can transfer energy between phenomena of starkly different spatial and temporal scales. Part one of the study uses a cloud-resolving model, the Regional Atmospheric Modeling System, to consider the relationship between mesoscale convective structures within the Indo-Pacific region and the regional, intraseasonal anomalies associated with the MJO. The simulation captures the entirety of a canonical boreal summertime MJO event, spanning 45 days in July and August of 2016, during which the convective anomaly associated with the MJO propagated over the Maritime Continent. The convective cloud structures, or cells, within the simulation were tracked and logged according to their location relative to the regional convective anomaly of the MJO. Using both spectral analysis and phase compositing, it was found that a progressive relationship exists between the boreal summertime MJO and mesoscale deep convective structures within the Indo-Pacific region, specifically within the convectively enhanced region of the MJO, as follows: increased cell longevity in the initial phases of the MJO, followed by increased cell number in the intermediate phases, progressing into increased cell expanse in the terminal phases. This progressive relationship is connected back to the low-frequency atmospheric response to the MJO. It is suggested that the bulk thermodynamic and kinematic anomalies of the MJO are closely related to the convective cell expanse and longevity, although the number of convective cells appears to be tied to another source of variability not identified within this study. These findings emphasize that while the MJO is commonly defined as an intraseasonal-scale convective anomaly, it is also intrinsically tied to the mesoscale variability of the convective systems that constitute its existence. The second part of the study quantifies the prevalence of the MJO within the overall climate system, along with the dependence of its teleconnections on variability in another tropical phenomena on a larger scale than itself. It is well known that the MJO exhibits pronounced seasonality in its tropical and global signature, and recent research has suggested that its tropical structure also depends on the state of the Quasi-Biennial Oscillation (QBO). We therefore first quantify the relationship between 300-mb geopotential anomalies and the MJO across the globe, then test the dependence of the relationship on both the meteorological season and the QBO phase using a derivative of cross-spectral analysis, magnitude-squared coherence Coh2. It is found that the global upper-tropospheric signature of the MJO exhibits pronounced seasonality, but also that the QBO significantly modulates the upper-tropospheric tropical and extratropical anomalies associated with the MJO. Globally, variability in upper tropospheric geopotential linked to the MJO is maximized during the boreal summertime and wintertime of easterly QBO phases, which is consistent with previous research that has shown easterly QBO phases to enhance the persistence of tropical convection associated with the MJO. Additional features are identified, such as the global maximum in upper-tropospheric variability associated with the MJO occurring during boreal summertime, rather than boreal wintertime. Overall, the MJO explains seven to thirteen percent of intraseasonal atmospheric variability in 300-mb geopotential, depending on season and QBO phase. These results highlight the importance of considering the phase of the QBO in analyses related to either global or local impacts of the MJO, along with the importance of cross-scale relationships, such as those between the MJO and QBO, in governing the coupling between the MJO and teleconnections across the globe. This thesis considers the relationship between the MJO and processes that operate on both longer and shorter timescales than itself, including tropical convection and the Quasi-Biennial Oscillation. In doing so, this work highlights the importance of considering relationships between the MJO and atmospheric phenomena on different spatial and temporal scales and with origins distinct from the MJO itself. While theories exist describing the MJO as its own distinct entity, this research corroborates the idea that it is at its core fundamentally linked to the rest of the climate system, both modulating and being modulated by a broad range of atmospheric processes.
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
Investigation of relationships between tropical cyclone structure and intensity change
(Colorado State University. Libraries, 2022) Casas, Eleanor G., author; Bell, Michael M., advisor; Randall, David A., committee member; Maloney, Eric D., committee member; Venayagamoorthy, Subhas Karan, committee member
Rapid intensification (RI) of a tropical cyclone (TC) remains one of the largest sources of intensity forecast error, due in part to internal dynamics that are complex and less well understood. Part of the difficulty in improving understanding of RI is due to complex interactions across a wide range of TC intensities, shapes, and sizes. In this doctoral study, I investigate these interactions by first simplifying the complexity and reducing the dimensionality of the intensity and structure parameter space to distill the key aspects of variability from observations, and then re-introducing physical complexity back into the experimental design through idealized modeling. In Chapter 2, an Empirical Orthogonal Function (EOF) analysis is used to develop the intensity-size framework that lays the foundation for the rest of this doctoral study. In addition to commonly-used TC metrics, a new structural parameter is introduced that describes the decay of tangential wind outside the radius of maximum wind (RMW). The utility of this framework is demonstrated for describing key TC evolutionary features with observations of Hurricanes Rita (2005) and Charley (2004) and numerical simulations of Rita. In Chapter 3, simplified TC analytic profiles are used to construct physically realistic wind fields that can explore the intensity-size phase space. Results suggest that while there are systematic differences between the details of the reconstructed wind fields using different methods, they all are representative of observed variability in TC structure despite being derived from a relatively small set of parameters derived from the EOFs. In Chapter 4, these simplified TC wind profiles are used to investigate the tropical cyclone boundary layer (TCBL) response across our intensity-size phase space using both height-averaged (slab) and height-resolved TCBL numerical models. The results suggest that while there are some different dynamical ramifications of the specific analytic profiles used, the response depends more on the location in the intensity and size phase space than on the differences between analytic wind formulations. The results indicate that (1) strong, big TC profiles produce the strongest supergradient wind within the TCBL; (2) weak, big TCs have the largest RMW contraction as the TCBL adjusts; and (3) weak TCs regardless of size have TCBL responses that are less conducive for intensification. Finally, in Chapter 5, full-physics, axisymmetric models are used to test whether the one-way TCBL responses found in Chapter \ref{c4_results} are consistent with two-way TCBL interactions with influences from convection, and explore the dependencies of intensification rates on TC internal structure. The results suggest that small, strong TCs can achieve the highest rapid intensification rates. The findings suggest that while intensification rates do not systematically vary with contraction rates of the RMW, both intensification and contraction rates do have some dependence on different aspects of TC intensity and size across the phase space. When visualized in the phase space, there is a relatively smooth transition between a "initially large mode" and "initially small mode" of RI. The findings of this doctoral study provide new insights into the role of TC intensity and size in the RI process.
Open Access
Morphology, lifecycles, and environmental sensitivities of tropical trimodal convection
(Colorado State University. Libraries, 2022) Sokolowsky, George Alexander, author; van den Heever, Susan C., advisor; Maloney, Eric D., committee member; Kreidenweis, Sonia M., committee member; Jathar, Shantanu, committee member
Convective clouds are ubiquitous in the tropics and typically follow a trimodal distribution of cumulus, congestus, and cumulonimbus clouds. Due to the crucial role each convective mode plays in tropical and global transport of heat and moisture, there has been both historical and recent interest in the characteristics, sensitivities, and lifecycles of these clouds. However, designing novel studies to further our knowledge has been challenging due to several limitations: the extensive computing resources needed to conduct modeling studies at sufficient resolution and scale to capture the trimodal distribution in detail; the lack of analysis tools which can objectively detect and track these clouds throughout their lifetime; and a need for more observational and modeling data of the tropical convective environments that produce these clouds. In this dissertation, three distinct but related studies that address these problems to advance the knowledge of our field on the morphology, lifecycles, and environmental sensitivities of tropical trimodal convection are presented. The first study examines the sensitivities of the tropical trimodal distribution and the convective environment to initial aerosol loading and low-level static stability. The Regional Atmospheric Modeling System (RAMS) configured as a Large Eddy Simulation (LES) is utilized to resolve all three modes in detail through two full diurnal cycles. Three initial static stabilities and three aerosol profiles are independently and simultaneously varied for a suite of nine simulations. This research found that (1) large aerosol loading and strong low-level static stability suppress the bulk environment and the intensity and coverage of convective clouds; (2) cloud and environmental responses to aerosol loading tend to be stronger than those from static stability; (3) the effects of aerosol and stability perturbations modulate each other substantially; (4) the deepest convection and highest dynamical intensity occur at moderate aerosol loading, rather than at low or high loading; and (5) most of the strongest feedbacks due to aerosol and stability perturbations are seen in the boundary layer (the latter being applied within the boundary layer themselves), though some are stronger above the freezing level. The second study presented seeks to further enhance an artificial intelligence analysis tool, the Tracking and Object-Based Analysis of Clouds (tobac) Python package, from both a scientific and procedural standpoint to enable a wider variety of research uses, including process-level studies of tropical trimodal convection. Scientific improvements to tobac v1.5 include an expansion of the tool from 2D to 3D analyses and the addition of a new spectral filtering tool. Procedural enhancements added include greater computational efficiency, data regridding capabilities, and treatments for processing data with singly or doubly periodic boundary conditions (PBCs). My distinct contributions to this work focused on the 2D to 3D expansion and the PBC treatment. These new capabilities are presented through figures, schematics, and discussion of the new science that tobac v1.5 facilitates, such as the analysis of large basin-scale datasets and detailed simulations of layered clouds, that would have been impossible before. Finally, the last study in this dissertation is a process-focused modeling study on the sensitivities of upscale growth of tropical trimodal convection to environmental aerosol loading. This project was enabled by the scientific and procedural improvements to tobac discussed in the second study, in particular the new abilities of tobac to detect and track objects in 3D and with model PBCs. Here, we used a subset of RAMS simulations from the first study, where only aerosol loading was changed and the upscale growth from shallow cumulus through congestus and cumulonimbus during the nighttime hours was investigated. This study revealed that moderately increasing aerosol loading enhances collision-coalescence processes in the middle of the cloud, which delays initial glaciation but promotes it later in the growth period. Greatly increasing aerosol, however, produces a cloud structure with a more extreme aspect ratio and greater entrainment aloft that rapidly loses buoyancy and vertical velocity with height, as well as exhibiting a greater amount of condensate loading towards the top of the cloud. We also found the relative timing of these processes to be especially important, with more rapid initial growth and lofting of condensate often inhibiting deeper convective growth.
Open Access
Multi-scale interactions leading to tropical cyclogenesis in sheared environments
(Colorado State University. Libraries, 2021) Nam, Chaehyeon Chelsea, author; Bell, Michael M., advisor; Rutledge, Steven A., committee member; Maloney, Eric D., committee member; Reising, Steven C., committee member
To be, or not to be, that is the question of tropical cyclogenesis. Only a small fraction of tropical disturbances eventually develop into tropical cyclones (TCs). Accurate forecasts of tropical cyclogenesis are difficult because TC development involves a wide range of scales from the stochastic convective scale to a quasi-balanced large-scale flow. This dissertation examines the factors that increase uncertainty around the multi-scale tropical cyclogenesis problem, namely, vertical wind shear (VWS), environmental humidity, and convective organization. These factors were explored using multiple data sources including observations such as dual-Doppler radar, dropsonde soundings, and satellite data for mesoscale case studies, reanalyses data for synoptic and climatological analysis, and extensive ensemble mesoscale modeling for controlled experiments. First, this dissertation presents a detailed observational analysis for multi-scale processes around an incipient wave pouch of Hagupit (2008) that survived through strong VWS and underwent TC genesis. The strong deep-layer VWS (> 20 m s-1) had a negative impact on the development through misalignment of the low and mid-level circulations and dry air intrusion. However, the low-level circulation persisted and the system ultimately formed into a tropical cyclone after it had left the high-shear zone. Here we propose that a key process that enabled the pre-depression to survive through the upper-tropospheric trough interaction was persistent vorticity amplification on the meso-γ scale that was aggregated on the meso-α scale within the wave pouch. In the second part, twelve sets of Weather Research and Forecasting ensemble simulations were created to examine the combined impacts of VWS, environmental moisture, and the structure of the precursor vortex on the uncertainty of TC genesis. Here we hypothesized that the combination of moderate shear and dry air makes an unstable condition for a vortex to intensify or decay, which implies that TC genesis in such environments may be intrinsically unpredictable in a deterministic sense. Based on the close examination of selected ensemble members and statistical analysis of geometric probability distribution and time-lagged correlations for all ensemble sets, we propose a theoretical pyramid diagram of the five processes leading to TC genesis in sheared and dry environments. First, inside their low-level circulations, deep convection emerged over a wider area. Second, a new smaller scale mid-level vortex formed inside the deep convection where the pre-existing mid-level vortex was carried away by VWS. Third, the mid-level vortex and low-level vortex went through a vertical alignment process. Fourth, with sustained vortex alignment, convection organized near the low-level center. Fifth, central pressure fell and wind speed increased; and the system reached tropical cyclone intensity. The results suggest that all successfully developing TCs share a common set of precursor events that lead to TC genesis, while a deficiency in any of the precursor events leads to a failure of genesis. In the third part, we investigated the likelihood of subsequent TC genesis from the "monsoon tail" rainband for TCs in the monsoonal area of the western North Pacific (WNP). The monsoon tail rainband—an elongated rainband in the southwestern quadrant of the TC—is shown to be a common feature for TCs in the WNP due to the climatological northeasterly VWS. Variations in the convective activity are shown to be related to the strength of the low-level and upper-level monsoonal flow on synoptic and seasonal timescales, with VWS having the highest correlation to cold cloud tops in the southwest quadrant. Some monsoon tail rainbands sustain convective organization even after they separated from the pre-existing TCs, but despite the enhanced convective activity, the persistent VWS that produced the rainbands was an overriding negative factor that inhibits genesis. This dissertation provides a detailed look at the complex interactions between VWS and the incipient TC depending on spatial scales, the vertical depth of shear, environmental moisture, and the structure of the TC vortex. The findings herein improve our process-based understanding of why moderate VWS, especially in combination with environmental dry air, produces unstable and uncertain conditions for TC genesis.
Open Access
Simple analytical solutions for potential vorticity intrusions
(Colorado State University. Libraries, 2012) Masarik, Matthew Thomas, author; Schubert, Wayne H., advisor; Eykholt, Richard E., committee member; Maloney, Eric D., committee member; Randall, David A., committee member
Using potential temperature (θ) as the vertical coordinate, we derive analytical solutions of the potential vorticity (PV) invertibility principle for the case in which the flow is y-independent and an isolated PV anomaly is confined within an ellipse in the (x, θ)-plane. The solutions aid in understanding the dynamics of low latitude PV intrusions whose associated cloud patterns are often referred to as moisture bursts, or tropical plumes and whose flow patterns are often referred to as tropical upper tropospheric troughs (TUTTs). The solutions illustrate the phenomenon of isentropic upglide below an upper tropospheric positive anomaly in PV. They also quantify how the partitioning of PV between vorticity and static stability depends on the shape and strength of the PV anomaly. The solutions also apply to the problem of determining the balanced flow induced by a surface temperature anomaly, which is equivalent to a very thin layer of infinite PV at the surface. Reanalysis data is consulted as a check on the solutions. Finally, a numerical model is constructed where approximations made in the analytical theory can be examined.
Open Access
Steady-state circulations forced by diabatic heating and wind stress in the intertropical convergence zone
(Colorado State University. Libraries, 2011) Gonzalez, Alex Omar, author; Schubert, Wayne H., advisor; Maloney, Eric D., committee member; Estep, Don J., committee member
A number of studies have shown the importance of using idealized models to gain insight into large-scale atmospheric circulations in the tropics, especially when investigating phenomena that are not well understood. The recent discovery of the Shallow Meridional Circulation (SMC) in the tropical East Pacific and West Africa is a perfect example of a phenomenon that is not well understood (Zhang et al., 2004). The vertical structure of the SMC is similar to the Hadley circulation, but its return flow is located at the top of the boundary layer. The current theory of the SMC is entirely different dynamically than the Hadley circulation because it has been thought of as a large-scale "sea-breeze" circulation rather a geostrophic balance in the meridional momentum equation. The SMC is a vital aspect of the general circulation since it can transport more moisture than the traditional deep Hadley circulation. Climate models often misrepresent the SMC, making many model simulations incomplete (Zhang et al. 2004; Nolan et al. 2007). We aim to better understand the dynamics near the Intertropical Convergence Zone (ITCZ) that involve both deep and shallow circulations using a steady-state linearized model on the equatorial β-plane that is solved analytically. The model is forced by prescribed diabatic heating and boundary layer wind stress curl. The circulations that arise from deep diabatic heating profiles suggest that both the Hadley and Walker circulations are always present, with the Hadley circulation being more prevalent as the deep heating is elongated in the zonal direction, similar to the ITCZ in the East Pacific. The Hadley circulation strengthens because the horizontal surface convergence increases in the meridional direction. Also, the zonal and meridional surface wind anomalies enhance as the deep heating is displaced farther from the equator. The surface wind field associated with this deep heating also forces a significant wind stress curl north of the equator. The atmosphere responds to the wind stress curl by opposing the initial dynamical fields, and generating Ekman pumping in the boundary layer. For example, the surface consists of anomalous negative vorticity in a region that previously contained positively vorticity. This is often referred to as spin down. The Ekman pumping in the boundary layer forces shallow circulations when the frictional forcing is zonally-elongated and sufficiently displaced off of the equator. This shallow circulation makes sense in the East Pacific, where the ITCZ is always north of the equator and is often zonally-elongated. There are two SMCs that develop, one north of the Ekman pumping, and the other to its south. The cross-equatorial SMC is shallower and is stretched in the meridional direction compared to the SMC north of the Ekman pumping since the Rossby length is very large near the equator. It turns out that the frictional forcing does not provide enough vertical or meridional motion to be seen when deep diabatic heating is also present using our simple model. Since the ITCZ is a transient phenomenon and the frictional forcing is more steady, there are days where this Ekman pumping can be seen when deep convection is suppressed. Future research should concentrate on better understanding the effect of the wind stress and surface temperatures on the buildup of subsequent convection using idealized models.
Open Access
Structure of the Madden-Julain oscillation in coupled and uncoupled versions of the superparameterized community atmosphere model
(Colorado State University. Libraries, 2010) Benedict, James J., author; Randall, David A. (David Allan), 1948-, advisor; Maloney, Eric D., committee member; Ramirez, Jorge A., committee member; Schubert, Wayne H., committee member; Thompson, David W. J., committee member
The Madden-Julian Oscillation (MJO), an eastward-propagating atmospheric disturbance resembling a transient Walker cell, dominates intraseasonal (20-100 days) variability in the tropical Indian and West Pacific Ocean regions. The phenomenon is most active during the Northern Hemisphere winter and is characterized by cyclic periods of suppressed (dry phase) and active (wet phase) cloudiness and precipitation. Numerous complexities—multi-scale interactions of moist convection and large-scale wave dynamics, air-sea fluxes and feedbacks, topographical impacts, and tropical-extratropical interactions— challenge our ability to fully understand the MJO and result in its poor representation in most current general circulation models (GCMs). This study examines the representation of the MJO in a modified version of the NCAR Community Atmosphere Model (CAM). The modifications involve substituting conventional boundary layer, turbulence, and cloud parameterizations with a configuration of cloud-resolving models (CRMs) embedded into each GCM grid cell in a technique termed "superparameterization" (SP). Unlike many GCMs including the standard CAM, the SP-CAM displays robust intraseasonal convective variability. Two SP-CAM simulations are utilized in this study: one forced by observed sea-surface temperatures (SSTs; "uncoupled") and a second identical to the first except for a new treatment of tropical SSTs in which a simplified mixed-layer ocean model is used to predict SST anomalies that are coupled to the atmosphere ("coupled"). Key physical features of the MJO are captured in the uncoupled SP-CAM. Ahead (east) of the disturbance there is meridional boundary layer moisture convergence and a vertical progression of warmth, moisture, and convective heating from the lower to upper troposphere. The space-time dynamical response to convective heating is also reproduced, especially the vertical structure of anomalous westerly wind and its migration into the region of heavy rainfall as the disturbance propagates eastward. Advective drying processes in the MJO wake are also represented well. The coupled SP-CAM shows more realistic MJO eastward propagation, signal coherence and spatial structure relative to the uncoupled SP-CAM. The improvement varies with longitude but generally stems from better space-time relationships among MJO convective heating, its dynamical response, SSTs, surface fluxes, boundary layer properties, and vertical moisture structure. Coupled MJO events in the Indian Ocean display more realistic intensity; in the West Pacific, the coupled SP-CAM overestimates convective strength but shows an improved vertical structure relative to the uncoupled SP-CAM. Biases related to MJO convection are also examined. Overestimated convective intensity in the West Pacific appears to be linked to basic state biases, Maritime Continent topographical impacts, unrealistic convection-wind-evaporation feedbacks, and the neglect of convective momentum transport in the model. Phase errors between observed and simulated boundary layer moisture appear to stem from an unrealistic representation of shallow cumuli.