Browsing by Author "Maloney, Eric, committee member"
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Item Open Access Analysis of precipitation and convection in the west Pacific during the PISTON field campaign(Colorado State University. Libraries, 2022) Chudler, Kyle, author; Rutledge, Steven, advisor; Bell, Michael, committee member; Maloney, Eric, committee member; Reising, Steven, committee memberTropical convection is a meteorological phenomenon with important impacts on the atmosphere, both locally and globally. Consequently, it has been an intensely studied topic for many years. Importantly, several ship-based field campaigns have taken place over tropical oceans. Such field campaigns are vital to the advancement of knowledge in this field, as meteorological observations over these open oceans are otherwise scant or non-existent. The latest project to examine tropical convection is the Propagation of Intraseasonal Oscillations (PISTON) field campaign, which took place in the western North Pacific in the late-summer and early-fall of 2018 and 2019. On board the PISTON ships was the SEA-POL weather radar, the first polarimetric weather radar designed specifically for deployment at sea. In addition to taking traditional radar measurements of precipitation intensity and velocity, SEA-POL's polarimetric measurements also provide insights into the size, shape, and composition of hydrometeors within precipitating systems. By combining SEA-POL's unique measurements with other meteorological datasets, this work presented in this dissertation provides new insights in tropical convection in the Pacific warm pool. Chapter 2 of this dissertation provides an overview of the variability in convection observed during the PISTON cruises, and relates this variability to large-scale atmospheric conditions. Using an objective classification algorithm, precipitation features are identified and labeled by their size (isolated, sub-MCS, MCS) and degree of convective organization (nonlinear, linear). It is shown that although large mesoscale convective systems (MCSs) occurred infrequently (present in 13% of radar scans), they contributed a disproportionately large portion (56%) of the total rain volume. Conversely, small isolated features were present in 91% of scans, yet these features contributed just 11% of the total rain volume, with the bulk of the rainfall owing to warm rain production. Convective rain rates and 30-dBZ echo-top heights increased with feature size and degree of organization. MCSs occurred more frequently in periods of low-level southwesterly winds, and when low-level wind shear was enhanced. By compositing radar and sounding data by phases of easterly waves (of which there were several in 2018), troughs are shown to be associated with increased precipitation and a higher relative frequency of MCS feature occurrence, while ridges are shown to be associated with decreased precipitation and a higher relative frequency of isolated convective features. During PISTON, SEA-POL routinely measured extreme values of differential reflectivity in the cores of small, isolated convection, owing to the presence of large drops. Chapter 3 examines the structure and frequency of cells containing large drops. Cells with high differential reflectivity (> 3.5 dB) were present in 24% of all radar scans. The cells were typically small (8 km2 mean area), short lived (usually < 10 minutes), and shallow (3.7 km mean height). High differential reflectivity was more often found on the upwind side of these cells, suggesting a size sorting mechanism which establishes a low concentration of large drops on the upwind side. Differential reflectivity also tended to increase at lower altitudes, which is hypothesized to be due to continued drop growth, increasing temperature (dielectric effect), and evaporation of smaller drops. Rapid vertical cross section radar scans, as well as transects made by a Learjet aircraft with on-board particle probes, are also used to analyze these cells, and support the conclusions drawn from statistical analysis. In Chapter 4, the observations of precipitation from spaceborne Ku-Band precipitation radar (KuPR) from the Global Precipitation Mission Dual-Frequency Precipitation Radar is compared surface observations from SEA-POL. Over the 18 instances where KuPR and SEA-POL made concurrent measurements of precipitation, the average rain rate in KuPR was 50% lower than in SEA-POL, but the raining area was 113% higher. The net effect of these two differences of opposite sign was for KUPR to have 23% more rain volume than SEA-POL. The limited resolution of KuPR (5x5 km) causes it to underestimate rain rate in small convective cores, but also over-broaden raining features beyond their true extent. It is also shown that KuPR tends to slightly overestimate rain rate below the melting layer in stratiform rain, likely due to overcorrection of attenuation below radar bright bands. Using a statistical model to simulate KuPR rain volume, it was found that KuPR would theoretically overestimate rain volume during trough phases of the easterly waves observed during PISTON (when there was more precipitating area), and underestimate rainfall during ridge phases (when there was less precipitating area).Item Open Access Aspects of gulf surges and tropical upper tropospheric troughs in the North American monsoon(Colorado State University. Libraries, 2011) Newman, Andrew James, author; Johnson, Richard, advisor; van den Heever, Sue, committee member; Maloney, Eric, committee member; Bienkiewicz, Bogusz, committee memberGulf surges are transient events that propagate along the Gulf of California (GoC) from south to north, transporting cool moist air toward the deserts of northwest Mexico and the southwest United States during the North American monsoon (NAM). The general features and progression of surge events are well studied but the dynamical characteristics and evolution are still unclear. Tropical upper-tropospheric troughs (TUTTs) are another critical transient event occurring during the NAM that enhance precipitation on their western flank. The mechanism of precipitation enhancement associated with TUTT passage needs further refinement as well. To address these unknowns, a number of convection-permitting simulations are performed over the entire core monsoon region for the 12-14 July 2004 gulf surge and TUTT event that occurred during the North American Monsoon Experiment. This allows for extensive comparison with many observational platforms. A control simulation is able to reproduce the surge event reasonably well, capturing all the important observed features on 12 and 13 July. The dynamical evolution of the surge event notes two distinct features, a precursor event on 12 July and the actual surge on 13 July. Using shallow water theory, the feature on 12 July is likely a coastally trapped, slightly non-linear Kelvin wave. This feature is important because it introduces cooler, moister air into the southern and central GoC. The surge signature develops early on 13 July in the southern GoC and is likely a coastally trapped non-linear Kelvin wave throughout its lifetime. Sensitivity simulations show that the convective outflow is critical to the intensity of the simulated surge, in agreement with past studies. The removal of mountain gap flows into the GoC from the Pacific Ocean along the Baja Peninsula shows they are not critical in surge initiation and evolution; the surge and its general character remain. A unique approach to examine the TUTT precipitation enhancement mechanism is used where the vorticity anomaly associated with the TUTT is removed in the initial conditions. It is shown that the TUTT likely enhances convection along the Sierra Madre Occidental (SMO) through slightly increased shear and slightly more convective available potential energy (CAPE) near the SMO. These slight differences lead to enhanced precipitation and microphysical evolution. The control simulation generates 23% more precipitation during the primary period of TUTT interaction with the SMO and has enhanced graupel, cloud and precipitation ice and supercooled liquid water contents, which is related to changes in lightning production. Finally, two dimensional dry idealized simulations examine some attributes of the observed surge. The GoC LLJ, multiple convective outflows, and slope of the isentropes along the GoC all influence the character of the idealized surge. The slope of the isentropes, which is a consequence of the heat low over the Southwest US, is most important, followed by the convective outflows, and GoC LLJ. The sloped isentropes create a unique thermodynamic environment which significantly impacts gravity wave phenomena like Kelvin waves and bores. Convective outflows modulate surge intensity and its complexity while the GoC LLJ only enhances the surge intensity.Item Open Access Characteristics of current and future flood-producing storms in the continental United States(Colorado State University. Libraries, 2020) Dougherty, Erin M., author; Rasmussen, Kristen, advisor; Schumacher, Russ, committee member; Maloney, Eric, committee member; Morrison, Ryan, committee memberUnderstanding the changes to extremes in the hydrologic cycle in a future, warmer climate is important for better managing water resources and preventing detrimental impacts to society. The goal of this dissertation is to contribute to this understanding by examining the precipitation characteristics of flood-producing storms in the current climate over the continental United States (CONUS) and how these will change in a future, warmer climate. Numerous storm types are responsible for floods over the CONUS, so quantifying how their characteristics will change among a large number of flood-producing storms in the future provides a spectrum of possible changes and impacts to flood-prone regions across the country. To understand flood-producing storms in the current climate over the CONUS, a climatology of these storms from 2002–2013 is created by merging storm reports, streamflow-indicated floods, and Stage-IV precipitation data (Chapter 2). From this climatology, it is observed that flash flood-producing storms preferentially occur in the warm-season in the Mississippi River Basin, with intense rain rates and short durations. Slow-rise floods occur mostly during the cool-season, concentrated in the Ohio River Valley and Pacific Northwest, and are long-duration, low-intensity rainfall events. Hybrid floods, having characteristics of both flash and slow-rise flood-producing storms, tend to occur in the spring and summer notably in the central CONUS and Northeast, with moderate durations and rain rates. Examining these floods on a sub-basin scale in the Wabash and Willamette basins, precipitation and instantaneous streamflow correlations are spatially variable, with strong positive correlations in areas of complex terrain and urbanization (Chapter 3). These studies show that in the current climate, flood-producing storm precipitation characteristics and their hydrologic response is nuanced, which is critical to document in order to understand their behavior in a future climate. A subset of nearly 600 flash flood-producing storms from the Chapter 2 climatology are examined using high-resolution convection-permitting simulations over the CONUS to understand how these historical storms might change in a future, warmer climate (Chapter 4). Both precipitation and runoff show widespread increases in the future over the CONUS, increasing by 21% and 50%, respectively, with maximum hourly rain rates becoming more intense by 7.5% K−1. In California, 45 flood-producing storms associated with atmospheric rivers also display a future increase (decrease) in precipitation (snow water equivalent) leading to increased runoff, particularly over the Sierra Nevada Mountains, implying a shift in future water resources in California (Chapter 5). In the Mississippi River Basin–a flash flood hotspot in the CONUS––nearly 500 flash flood-producing storms exhibit a 17% average increase in precipitation and 32% average increase in runoff primarily associated with warm-season convection, and to a lesser extent, tropical cyclones (Chapter 6). When stratified by vertical velocity, the storms with the strongest vertical velocity in the current climate exhibit the greatest (least) increase (decrease) in future rainfall (vertical velocity), suggesting a potential role of storm dynamics in modulating future rainfall changes.Item Open Access Characterizing the influence of anthropogenic emissions and transport variability on sulfate aerosol concentrations at Mauna Loa Observatory(Colorado State University. Libraries, 2013) Potter, Lauren E., author; Kreidenweis, Sonia, advisor; Maloney, Eric, committee member; Farmer, Delphine, committee member; Cooley, Daniel, committee memberSulfate aerosol in the atmosphere has substantial impacts on human health and environmental quality. Most notably, atmospheric sulfate has the potential to modify the earth's climate system through both direct and indirect radiative forcing mechanisms (Meehl et al., 2007). Emissions of sulfur dioxide, the primary precursor of sulfate aerosol, are now globally dominated by anthropogenic sources as a result of widespread fossil fuel combustion. Economic development in Asian countries since 1990 has contributed considerably to atmospheric sulfur loading, particularly China, which currently emits approximately 1/3 of global anthropogenic SO2 (Klimont et al., 2013). Observational and modeling studies have confirmed that anthropogenic pollutants from Asian sources can be transported long distances with important implications for future air quality and global climate change. Located in the remote Pacific Ocean (19.54°N, 155.58°W) at an elevation of 3.4 kilometers above sea level, Mauna Loa Observatory (MLO) is an ideal measurement site for ground-based, free tropospheric observations and is well situated to experience influence from springtime Asian outflow. This study makes use of a 14-year data set of aerosol ionic composition, obtained at MLO by the University of Hawaii at Manoa. Daily filter samples of total aerosol concentrations were made during nighttime downslope (free-tropospheric) transport conditions, from 1995 to 2008, and were analyzed for aerosol-phase concentrations of the following species: nitrate (NO3-), sulfate (SO42-), methanesulfonate (MSA), chloride (Cl-), oxalate, sodium (Na+), ammonium (NH4+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+). An understanding of the factors controlling seasonal and interannual variations in aerosol speciation and concentrations at this site is complicated by the relatively short lifetimes of aerosols, compared with greenhouse gases which have also been sampled over long time periods at MLO. Aerosol filter data were supplemented with observations of gaseous radon (Rn222) and carbon monoxide (CO), used as tracers of long distance continental influence. Our study applied trajectory analysis and multiple linear regression to interpret the relative roles of aerosol precursor emissions and large-scale transport characteristics on observed MLO sulfate aerosol variability. We conclude that observed sulfate aerosol at MLO likely originated from a combination of anthropogenic, volcanic, and biogenic sources that varied seasonally and from year to year. Analysis of chemical continental tracer concentrations and HYSPLIT back trajectories suggests that non-negligible long distance influence from either the Asian or North American continents can be detected at MLO during all seasons although large interannual variability was observed. Possible influence of circulation changes in the Pacific Basin related to the El Niño-Southern Oscillation were found to be both species and seasonally dependent. We further found an increasing trend in monthly mean sulfate aerosol concentrations at MLO of 4.8% (7.3 ng m-3) per year during 1995-2008, significant at the 95% confidence level. Multiple linear regression results suggest that the observed trend in sulfate concentrations at MLO cannot reasonably be explained by variations in meteorology and transport efficiency alone. An increasing sulfate trend of 5.8 ng m-3 per year, statistically significant at the 90% confidence level, was found to be associated with the variable representing East Asian SO2 emissions. The results of this study provide evidence that MLO sulfate aerosol observations during 1995-2008 reflect, in part, recent trends in anthropogenic SO2 emissions which are superimposed onto the natural meteorological variability affecting transport efficiency.Item Open Access Comparison of convective clouds observed by spaceborne W-band radar and simulated by cloud-resolving atmospheric models(Colorado State University. Libraries, 2014) Dodson, Jason B., author; Randall, David, advisor; Birner, Thomas, committee member; Maloney, Eric, committee member; Chandrasekar, V., committee memberDeep convective clouds (DCCs) play an important role in regulating global climate through vertical mass flux, vertical water transport, and radiation. For general circulation models (GCMs) to simulate the global climate realistically, they must simulate DCCs realistically. GCMs have traditionally used cumulus parameterizations (CPs). Much recent research has shown that multiple persistent unrealistic behaviors in GCMs are related to limitations of CPs. Two alternatives to CPs exist: the global cloud-resolving model (GCRM), and the multiscale modeling framework (MMF). Both can directly simulate the coarser features of DCCs because of their multi-kilometer horizontal resolutions, and can simulate large-scale meteorological processes more realistically than GCMs. However, the question of realistic behavior of simulated DCCs remains. How closely do simulated DCCs resemble observed DCCs? In this study I examine the behavior of DCCs in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and Superparameterized Community Atmospheric Model (SP-CAM), the latter with both single-moment and double-moment microphysics. I place particular emphasis on the relationship between cloud vertical structure and convective environment. I also emphasize the transition between shallow clouds and mature DCCs. The spatial domains used are the tropical oceans and the contiguous United States (CONUS), the latter of which produces frequent vigorous convection during the summer. CloudSat is used to observe DCCs, and A-Train and reanalysis data are used to represent the large-scale environment in which the clouds form. The CloudSat cloud mask and radar reflectivity profiles for CONUS cumuliform clouds (defined as clouds with a base within the planetary boundary layer) during boreal summer are first averaged and compared. Both NICAM and SP-CAM greatly underestimate the vertical growth of cumuliform clouds. Then they are sorted by three large-scale environmental variables: total preciptable water (TPW), surface air temperature (SAT), and 500hPa vertical velocity (W500), representing the dynamical and thermodynamical environment in which the clouds form. The sorted CloudSat profiles are then compared with NICAM and SP-CAM profiles simulated with the Quickbeam CloudSat simulator. Both models have considerable difficulty representing the relationship of SAT and clouds over CONUS. For TPW and W500, shallow clouds transition to DCCs at higher values than observed. This may be an indication of the models' inability to represent the formation of DCCs in marginal convective environments. NICAM develops tall DCCs in highly favorable environments, but SP-CAM appears to be incapable of developing tall DCCs in almost any environment. The use of double moment microphysics in SP-CAM improves the frequency of deep clouds and their relationship with TPW, but not SAT. Both models underpredict radar reflectivity in the upper cloud of mature DCCs. SP-CAM with single moment microphysics has a particularly unrealistic DCC reflectivity profile, but with double moment microphysics it improves substantially. SP-CAM with double-moment microphysics unexpectedly appears to weaken DCC updraft strength as TPW increases, but otherwise both NICAM and SP-CAM represent the environment-versus-DCC relationships fairly realistically.Item Open Access Downdraft impacts on tropical convection(Colorado State University. Libraries, 2013) Thayer-Calder, Katherine, author; Randall, David, advisor; Johnson, Richard, committee member; Maloney, Eric, committee member; Strout, Michelle, committee memberDowndrafts are an integral part of the convective cycle, and have been observed and documented for more than a hundred years. But many questions still surround convective downdrafts and their most difficult to observe properties. These questions have made the parameterization of convective downdrafts in global climate models (GCMs) very difficult. Designers of parameterizations have resorted to a wide range of assumptions and unverified hypotheses in their models of convective downdrafts. In the last ten years, computing resources have advanced to a point where large domain, high resolution cloud resolving models (CRMs) can easily be run for long simulations. This study uses several simulations with 1 km horizontal resolution from the System for Atmospheric Modeling (SAM) v6.8.2 to examine convective downdrafts. We look at Radiative-Convective Equilibrium (RCE), a 21 day case from TOGA-COARE, Weak Temperature Gradient (WTG) simulations with varied shear profiles, and Lagrangian Parcel data to consider many difficult to observe properties of downdrafts. We consider a variety of assumptions and questions that arise in the development of convective parameterizations. Our results show that downdrafts are an important mass flux in all simulations, and that cold pools organize convective systems and enhance updraft Convective Available Potential Energy (CAPE). We examine the ability for downdrafts to help couple deep convection to high relative-humidity regions in the tropics, and find that entrainment is likely a more important process in this relationship. We discuss the impact of downdrafts in maintaining boundary layer quasi-equilibrium, and find that, in our simulations, environmental entrainment has a larger impact on low-level most static energy. Finally, we show results from Lagrangian parcel data that illuminate our downdrafts as existing in an unsaturated state, with increasing buoyancy as they descend. We show that many of our downdrafts have positive buoyancy perturbations, suggesting the presence of warm downdrafts and under-shooting bottoms in heavily precipitating tropical systems.Item Open Access Formation of rain layers in the Indian Ocean and their feedbacks to atmospheric convection(Colorado State University. Libraries, 2023) Shackelford, Kyle T., author; van Leeuwen, Peter Jan, advisor; DeMott, Charlotte, advisor; Maloney, Eric, committee member; Venayagamoorthy, Karan, committee memberRainfall over the tropical warm pool spanning the Indian and West Pacific Oceans is relatively colder, fresher, and less dense than the near-surface ocean. Thus, under low-to-moderate winds, rainfall can act to stably stratify the upper ocean, forming a rain layer (RL). RLs cool and freshen the ocean surface and shoal ocean mixed layer depth, confining air-sea interaction to a thin, near-surface ocean layer. The shallow, transient nature of RLs has limited their observation, and RL impact on air-sea interaction is not well understood. This two-part thesis aims to address knowledge gaps surrounding 1) RL formation and characteristic traits, and 2) RL feedbacks to the atmosphere. In the first part of this thesis, we examine Indian Ocean RLs and their potential feedbacks to the atmosphere using a 1D ocean model. Initial experiments focus on model validation, and demonstrate that the model is able to effectively replicate upper ocean response to precipitation as revealed by in situ measurements. Following model validation, Indian Ocean RL characteristics are studied by forcing a 2D array of 1D model columns with atmospheric output from an existing convection-permitting simulation. Results from this experiment demonstrate that SST reduction within RLs persists on time scales longer than those of the parent rain event. To evaluate RL feedbacks to the atmosphere, a second 2D array experiment is conducted over the same domain with identical atmospheric forcing except rainfall is set to zero at every time step. Comparison between simulations with and without rain forcing demonstrate that RLs reduce SST through cold rain input to the ocean surface, and maintain and enhance SST reductions through a stable salinity stratification. Through prolonged SST reduction, RLs also enhance spatial SST gradients that have previously been shown to excite atmospheric convection. In the second part of this thesis, RL feedbacks to the Madden-Julian Oscillation (MJO) are studied by conducting regional ocean-atmosphere coupled simulations. Output from two convection-permitting coupled simulations of the November 2011 MJO event, one with rain coupling to the ocean surface and a second without rain coupling, is used to evaluate two potential RL feedback mechanisms. The first feedback is the ''SST gradient effect,'' which refers to RL-enhanced SST gradients imposing low-level patterns of convergence/divergence in the atmospheric boundary layer. The second is the ''SST effect,'' which refers to RL-induced SST perturbations altering turbulent heat fluxes. During the MJO transition from suppressed to enhanced convection, the SST gradient effect and SST effect have opposing feedbacks to convection, as RL-enhanced SST gradients favor convective initiation, while RL-induced SST reduction hinders convection. Comparison of coupled simulations with and without rain coupling to the ocean demonstrates that RL-induced SST reduction has a more substantial impact than enhanced SST gradients during this transitory phase. A delayed pathway in which RLs feedback to the MJO through the SST effect arises from frequent RL presence during the disturbed phase, which isolates subsurface ocean heat from the atmosphere. At the onset of the MJO active phase, westerly wind bursts erode near-surface RLs and release previously trapped subsurface ocean heat to the atmosphere, amplifying the intensity of MJO convection. Between the direct and delayed SST effect, RLs are shown to modify intraseasonal tropical variability.Item Open Access Kinematic structures, diabatic profiles, and precipitation systems in West Africa during summer 2006(Colorado State University. Libraries, 2013) Davis, Adam James, author; Johnson, Richard, advisor; Maloney, Eric, committee member; Kirby, Michael, committee memberWest Africa is a region characterized by great spatial contrasts in temperature, precipitation, and topography, which combine to create many complex and interesting weather phenomena. In particular, the area is home to a seasonal monsoon, propagating easterly waves, and some of the most intense thunderstorm systems on Earth. These types of events have both local and global effects - precipitation variability has a major bearing on regional water resource issues, while West Africa is also the source of many of the disturbances that develop into tropical cyclones in the North Atlantic Ocean. Unfortunately, atmospheric data has historically been very sparse in West Africa, leading to an incomplete understanding of many of these meteorological features and a corresponding difficulty in modeling them accurately. An exceptional opportunity for improvement on these fronts exists thanks to the African Monsoon Multidisciplinary Analysis (AMMA) field campaign, which collected an unprecedented quantity of observations throughout the region, with the most concentrated effort during the summer of 2006. This work uses a gridded analysis of radiosonde measurements obtained during AMMA and places those observations in the context of AMMA radar data and satellite rainfall estimates to examine the patterns of kinematic and diabatic quantities in West Africa relative to the summer monsoon phase, easterly wave disturbances, precipitation systems, and the diurnal cycle. Many unique aspects of West African weather compared to conditions elsewhere in the tropics are revealed by this study. The meridional transitions related to the West African monsoon comprise the predominant control on the location and intensity of precipitation at seasonal time scales, with variations in convective activity related to the Madden-Julian Oscillation contributing at 25 to 60 day periods. On shorter time scales of two to six days, easterly wave disturbances look to be the principal factor governing the timing of rainfall events, though especially persistent cold pools and residual cloudiness generated by thunderstorm systems also act as constraints on convective evolution on the days following a precipitation episode. One of the most distinctive traits of the study region in West Africa compared to other tropical areas is the particular prevalence of convective downdrafts, chiefly those associated with mesoscale zones of stratiform precipitation in thunderstorm complexes. These features, along with the gravity waves forced by their characteristic heating pattern, have an especially large influence on the time-mean atmospheric structure relative to the majority of the tropics. A comparison of the diabatic profiles from the AMMA dataset with those from other field projects indicates that the signals of both convective downdrafts and diurnal variations of the planetary boundary layer are much stronger in West Africa than in the previously studied regions. Beyond the mentioned differences, though, the AMMA profiles show resemblance to those from both western Pacific and eastern Atlantic field campaigns. The vertical patterns of atmospheric variables tend to be complex and multi-layered in West Africa, suggesting that the area is home to an especially diverse cloud population, with contributions from numerous height regimes prominent enough to influence the mean state. Meridional differences within the domain of the analysis are evident, including indications of more intense convective updrafts toward the north, stronger effects of boundary layer mixing in the north, and a greater net influence of mesoscale convective system downdrafts toward the south. The diurnal cycle of precipitation appears most prominently shaped by convective initiation near areas of high topography and the subsequent development and long-distance propagation of extensive, well-organized thunderstorm systems, though there seem to be effects related to diurnal flow patterns near the Gulf of Guinea coast too. Inland, moisture transport achieved by the nocturnal low-level jet is a key influence on rainfall, with mixing by the daytime boundary layer playing an important function as well. Changes in the relative contribution and intensity of deep convective and stratiform heating and moistening patterns arise among different times of day and night, as the leading precipitation regime transitions from developing deep convection at midday to organizing thunderstorm systems by evening and propagating thunderstorm complexes with extensive stratiform rainfall overnight. The analyses in the present work demonstrate a few different issues and caveats that need to be considered when utilizing observational or remote sensing datasets. Namely, the timing of radiosonde launches and the spacing of the sounding site array combined to create a delay between when convective systems passed the Niamey, Niger measurement site and when their effects were detected in the gridded AMMA sounding data. Similarly, infrared satellite rainfall estimates from the Tropical Rainfall Measuring Mission (TRMM) are shown to have a time lag of about three hours between when precipitation actually occurs and when it appears in the estimate product, complicating the intended use of the data in evaluating the diurnal cycle of rainfall.Item Open Access Latent heating and mixing due to entrainment in tropical deep convection(Colorado State University. Libraries, 2013) McGee, Clayton J., author; van den Heever, Susan, advisor; Maloney, Eric, committee member; Eykholt, Richard, committee memberRecent studies have noted the role of latent heating above the freezing level in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with evidence of diluted tropical deep convective cores. This study evaluates recent modifications to the HTH through Lagrangian trajectory analysis of deep convective cores in an idealized, high-resolution cloud-resolving model (CRM) simulation. A line of tropical convective cells develops within a high-resolution nested grid whose boundary conditions are obtained from a large-domain CRM simulation approaching radiative-convective equilibrium (RCE). Microphysical impacts on latent heating and equivalent potential temperature are analyzed along trajectories ascending within convective regions of the high-resolution nested grid. Changes in equivalent potential temperature along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term. This residual term is composed of radiation and mixing. Due to the small magnitude of radiative heating rates in the convective inflow regions and updrafts examined here, the residual term is treated as an approximate representation of mixing within these regions. The simulations demonstrate that mixing with dry air decreases equivalent potential temperature along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase equivalent potential temperature above the freezing level. The latent heating contributions along trajectories from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation are also quantified. Finally, the source regions of trajectories reaching the upper troposphere are identified; it is found that two-thirds of backward trajectories with starting points within strong updrafts or downdrafts above 10 km have their origin at levels higher than 2 km AGL. The importance of both boundary layer and mid-level inflow in moist environments is underscored in this study.Item Open Access Links between climate feedbacks and the large-scale circulation across idealized and complex climate models(Colorado State University. Libraries, 2023) Davis, Luke L. B., author; Thompson, David W. J., advisor; Maloney, Eric, committee member; Randall, David, committee member; Pinaud, Olivier, committee member; Gerber, Edwin, committee memberThe circulation response to anthropogenic forcing is typically considered in one of two distinct frameworks: One that uses radiative forcings and feedbacks to investigate the thermodynamics of the response, and another that uses circulation feedbacks and thermodynamic constraints to investigate the dynamics of the response. In this thesis, I aim to help bridge the gap between these two frameworks by exploring direct links between climate feedbacks and the atmospheric circulation across ensembles of experiments from idealized and complex general circulation models (GCMs). I first demonstrate that an existing, widely-used type of idealized GCM — the dynamical core model — has climate feedbacks that are explicitly prescribed and determined by a single parameter: The thermal relaxation timescale. The dynamical core model may thus help to fill gaps in the model hierarchies commonly used to study climate forcings and climate feedbacks. I then perform two experiments: One that explores the influence of prescribed feedbacks on the unperturbed, climatological circulation; and a second that explores their influence on the circulation response to a horizontally uniform, global warming-like forcing perturbation. The results indicate that more stabilizing climate feedbacks are associated with 1) a more vigorous climatological circulation with increased thermal diffusivity, and 2) a weaker poleward displacement of the circulation in response to the global warming-like forcing. Importantly, since the most commonly-used relaxation timescale field resembles the real-world clear-sky feedback field, the uniform forcing perturbations produce realistic warming patterns, with amplified warming in the tropical upper troposphere and polar lower troposphere. The warming pattern and circulation response disappear when the relaxation timescale field is instead spatially uniform, demonstrating the critical role of spatially-varying feedback processes on shaping the response to anthropogenic forcing. I next explore circulation-feedback relationships in more complex GCMs using results from the most recent Coupled Model Intercomparison Projects (CMIP5 and CMIP6). Here, I estimate climate feedbacks by regressing top-of-atmosphere radiation against surface temperature for both 1) an unperturbed pre-industrial control experiment and 2) a perturbed global warming experiment forced by an abrupt quadrupling of CO2 concentrations. I find that across both ensembles, the cloud component of the perturbed climate feedback is closely related to the cloud component of the unperturbed climate feedback. Critically, the relationship is much stronger in CMIP6 than CMIP5, contrasting with many previously proposed constraints on the perturbation response. The relationship also explains the slow part of the CO2 response better than the fast, transient response. In general, the strength of the relationship depends on the degree to which the spatial pattern of the response resembles ENSO-dominated internal variability, with "El Niño-like" East Pacific warming and related tropical cloud changes. This is consistent with fluctuation-dissipation theory: Regions with stronger deep ocean heat exchange and weaker net feedbacks must always dominate both 1) internal fluctuations in the global energy budget, and 2) the slow part of the response to forcing perturbations. The stronger CMIP6 inter-model relationships are due to both an amplification of this mechanism and higher inter-model correlations between tropical cloud changes and extratropical cloud changes. Finally, I present emergent constraints on the slow response using a recent observational estimate of the unperturbed cloud feedback. I conclude by discussing some implications of these results. I consider how the relaxation feedback framework might be further developed and reconciled with traditional climate feedbacks to provide future research opportunities with climate model hierarchies.Item Open Access Midlatitude prediction skill following QBO-MJO activity on subseasonal to seasonal timescales(Colorado State University. Libraries, 2019) Mayer, Kirsten J., author; Barnes, Elizabeth A., advisor; Maloney, Eric, committee member; Anderson, Chuck, committee memberThe Madden-Julian Oscillation (MJO) is known to force extratropical weather days-to-weeks following an MJO event through excitation of Rossby waves, also known as tropical-extratropical teleconnections. Prior research has demonstrated that this tropically forced midlatitude response can lead to increased prediction skill on subseasonal to seasonal (S2S) timescales. Furthermore, the Quasi-Biennial Oscillation (QBO) has been shown to possibly alter these teleconnections through modulation of the MJO itself and the atmospheric basic state upon which the Rossby waves propagate. This implies that the MJO-QBO relationship may affect midlatitude circulation prediction skill on S2S timescales. In this study, we quantify midlatitude circulation sensitivity and prediction skill following active MJOs and QBOs across the Northern Hemisphere on S2S timescales through an examination of the 500 hPa geopotential height field. First, a comparison of the spatial distribution of Northern Hemisphere sensitivity to the MJO during different QBO phases is performed for ERA-Interim reanalysis as well as ECMWF and NCEP hindcasts. Secondly, differences in prediction skill in ECMWF and NCEP hindcasts are quantified following MJO-QBO activity. We find that regions across the Pacific, North America and the Atlantic exhibit increased prediction skill following MJO-QBO activity, but these regions are not always collocated with the locations most sensitive to the MJO under a particular QBO state. Both hindcast systems demonstrate enhanced prediction skill 7-14 days following active MJO events during strong QBO periods compared to MJO events during neutral QBO periods.Item Open Access Moist synoptic transport of CO2 along midlatitude storm tracks, transport uncertainty, and implications for flux estimation(Colorado State University. Libraries, 2011) Parazoo, Nicholas C., author; Denning, A. Scott, advisor; Randall, David, committee member; Maloney, Eric, committee member; Kawa, Randy, committee member; Paustian, Keith, committee memberMass transport along moist isentropic surfaces on baroclinic waves represents an important component of the atmospheric heat engine that operates between the equator and poles. This is also an important vehicle for tracer transport, and is correlated with ecosystem metabolism because large-scale baroclinicity and photosynthesis are both driven seasonally by variations in solar radiation. In this research, I pursue a dynamical framework for explaining atmospheric transport of CO2 by synoptic weather systems at middle and high latitudes. A global model of atmospheric tracer transport, driven by meteorological analysis in combination with a detailed description of surface fluxes, is used to create time varying CO2 distributions in the atmosphere. Simulated mass fluxes of CO2 are then decomposed into a zonal monthly mean component and deviations from the monthly mean in space and time. Mass fluxes of CO2 are described on moist isentropic surfaces to represent frontal transport along storm tracks. Forward simulations suggest that synoptic weather systems transport large amounts of CO2 north and south in northern mid-latitudes, up to 1 PgC/month during winter when baroclinic wave activity peaks. During boreal winter when northern plants respire, warm moist air, high in CO2, is swept upward and poleward along the east side of baroclinic waves and injected into the polar vortex, while cold dry air, low in CO2, that had been transported into the polar vortex earlier in the year is advected equatorward. These synoptic eddies act to strongly reduce seasonality of CO2 in the biologically active mid-latitudes by 50% of that implied by local net ecosystem exchange while correspondingly amplifying seasonality in the Arctic. Transport along stormtracks is correlated with rising, moist, cloudy air, which systematically hides this CO2 transport from satellite observing systems. Meridional fluxes of CO2 are of comparable magnitude as surface exchange of CO2 in mid-latitudes, and thus require careful consideration in (inverse) modeling of the carbon cycle. Because synoptic transport of CO2 by frontal systems and moist processes is generally unobserved and poorly represented in global models, it may be a source of error for inverse flux estimates. Uncertainty in CO2 transport by synoptic eddies is investigated using a global model driven by four reanalysis products from the Goddard EOS Data Assimilation System for 2005. Eddy transport is found to be highly variable between model analysis, with significant seasonal differences of up to 0.2 PgC, which represents up to 50% of fossil fuel emissions. The variations are caused primarily by differences in grid spacing and vertical mixing by moist convection and PBL turbulence. To test for aliasing of transport bias into inverse flux estimates, synthetic satellite data is generated using a model at 50 km global resolution and inverted using a global model run with coarse grid transport. An ensemble filtering method called the Maximum Likelihood Ensemble Filter (MLEF) is used to optimize fluxes. Flux estimates are found to be highly sensitive to transport biases at pixel and continental scale, with errors of up to 0.5 PgC/year in Europe and North America.Item Open Access On the observed and simulated responses of the extratropical atmosphere to surface thermal forcing(Colorado State University. Libraries, 2019) Wills, Samantha M., author; Thompson, David W. J., advisor; Alexander, Michael, committee member; Barnes, Elizabeth, committee member; Maloney, Eric, committee member; Venayagamoorthy, Subhas Karan, committee memberThe ocean is an integral part of the climate system, and its closely-coupled interactions with the atmosphere system have wide-ranging impacts on the large-scale and local patterns of climate and weather variability from one region of the globe to another. Improvements in the resolution of satellite observations and numerical models over the past decade have led to a series of advances in understanding the role of the ocean in extratropical air-sea interaction. While the influence of the extratropical ocean can be relatively subtle and difficult to detect, recent studies have provided a growing body of evidence suggesting that the extratropical ocean has a potentially important influence on the atmospheric circulation on a wide variety of timescales. The aim of this thesis is to improve the current understanding on the role of the extratropical ocean in climate by 1) presenting new observational analyses on the relationships between midlatitude SST anomalies and the atmospheric circulation on subseasonal timescales and 2) providing a new, simplistic framework for interpreting the atmospheric response to surface thermal forcing across the globe in an idealized global climate model. In the first theme of this thesis, observational analyses of daily-mean data are exploited to re-examine the evidence for midlatitude air-sea interaction over the Kuroshio-Oyashio Extension region, and important comparisons are drawn to a previous companion study over the Gulf Stream Extension region. The results indicate that during the boreal winter season, SST anomalies in both the Gulf Stream and Kuroshio-Oyashio Extension regions are associated with distinct spatial and temporal patterns of atmospheric variability that precede and follow peak amplitude in the SST field on daily-mean timescales. In particular, a very similar pattern of low pressure anomalies that develops over the warm SST anomalies is viewed as the most robust common aspect of the atmospheric "response" over both ocean basins. The least common aspect of the "response" is characterized by robust high pressure anomalies that develop over the North Atlantic and have a seemingly unique relationship to positive lower-tropospheric temperature anomalies generated over the Gulf Stream Extension region. These results suggest that extratropical SST anomalies on subseasonal timescales are capable of forcing significant changes in the large-scale atmospheric circulation through the transfer of heat from the ocean to the atmosphere. Partially motivated by the results from the observational analyses, the second theme of this thesis presents a simplified model framework to critically assess the one-way influence of the ocean on the atmosphere at different locations across the globe. A series of steady-state and transient numerical experiments are designed to explore the atmospheric response to surface thermal forcing in an idealized "aquaplanet" configuration of the NCAR Community Atmosphere Model, Version 5.3. The results indicate that in each of the extratropical SST perturbation experiments, there is a consistent and robust steady-state atmospheric response (of similar sign and amplitude) to surface thermal forcing. The response is characterized by a hemispheric-scale, equivalent-barotropic pattern of atmospheric circulation anomalies reminiscent of the model's leading mode of internal variability and is seemingly independent of the latitudinal placement of the heat source. This result is explored further, and a possible explanation of the consistent steady-state atmospheric circulation response is discussed.Item Open Access Quantifying and understanding current and future links between tropical convection and the large-scale circulation(Colorado State University. Libraries, 2020) Jenney, Andrea M., author; Randall, David A., advisor; Barnes, Elizabeth A., advisor; Maloney, Eric, committee member; Rasmussen, Kristen, committee member; Anderson, Georgiana Brooke, committee memberTropical deep convection plays an important role in the variability of the global circulation. The Madden Julian Oscillation (MJO) is a large tropical organized convective system that propagates eastward along the equator. It is a key contributor to weather predictability at extended time scales (10-40 days). For example, variability in the MJO is linked with variability in meteorological phenomena such as landfalling atmospheric rivers, tornado and hail activity over parts of North America, and extreme temperature and rainfall patterns across the Northern Hemisphere. Links between the MJO and atmospheric variability in remote locations are heavily studied. This is in part because the current skill of weather forecasts at extended time scales is mediocre, and because of evidence suggesting that the potential predictability offered by the MJO may not be fully captured in numerical prediction models. In the first part of this dissertation, I develop a tool for these types of studies. The "Sensitivity to the Remote Influence of Periodic Events" (STRIPES) index is a novel index that condenses the information obtained through composite analysis of variables after a periodic event (such as the MJO) into a single number, which includes information about the life cycle of the event, and for a range of lags with respect to each stage of the event. I apply the STRIPES index to surface observations and show that the MJO signal is detectable and significant at the level of individual weather stations over many parts of North America, and that the maximum strength of this signal exhibits regionality and seasonality. Tropical convection affects the extratropics primarily through the excitation of Rossby waves at the places where the upper-tropospheric divergent outflow associated with deep convection interacts with the background wind. In a future warmer climate, the strength of the mean circulation and convective mass flux is expected to weaken. A potential consequence is a weakening of Rossby wave excitation by tropical convective systems such as the MJO. In the second part of this study, I analyze a set of idealized simulations with specified surface warming and superparameterized convection and develop a framework to better understand why the mean circulation weakens with warming. I show that the decrease in the strength of the mean circulation can be explained by the slow rate at which atmospheric radiative cooling intensifies relative to the comparatively fast rate that the tropical dry static stability increases. I also show that despite a decrease in the mean convective mass flux, the warming tendency of the convective mass flux over the most deeply- convecting regions is not constrained to follow that of the global mean. In the final part of this dissertation, I consider how changes in the MJO and of the mean atmospheric state due to warming from increases in greenhouse gas concentrations may lead to changes in the MJO's impact over the North Pacific and North America. Specifically, I show that changes to the atmosphere's mean state dry static energy and winds have a larger impact on the MJO teleconnection than changes to MJO intensity and propagation characteristics.Item Open Access The changing nature of convection over Earth's tropical oceans from a water budget perspective(Colorado State University. Libraries, 2023) Leitmann-Niimi, Nicolas, author; Kummerow, Christian, advisor; Maloney, Eric, committee member; Arabi, Mazdak, committee memberConsistent spatiotemporal hydrologic measurements over Earth's oceans are only feasible with satellite remote sensing. The water budget components of an atmospheric column are precipitation (P), evaporation (E) and horizontal water vapor divergence (divQ). Physically, the sum of these components leaves a residual term: the amount of water vapor stored inside the atmospheric column. When time series of the water budget components are made using independent data products, this residual term is unphysical, which must be a result of measurement error in one or more of the water variables. This study finds that variations in lack of closure are not random, and seeks to reveal underlying sources for long-term, high amplitude trends so that errors in observations may be better understood as the climate system evolves and assumptions built into the algorithms today may bias results into the future. Trends in the residual are particularly significant over the Tropical West Pacific (TWP), Southern Tropical East Indian (STEI) and Tropical Central Pacific (TCP), where there are multi-year residual trends that maintain a consistent magnitude of 1 mm/day. While there are still residual discrepancies over the Tropical Western Indian (TWI), Tropical Eastern Pacific (TEP) and Tropical Atlantic Ocean (TAO), closure is overall better, as residual trends are more annual in variation and less unphysical in magnitude. This study hypothesizes that the first-order explanation for potential long-term biases lies in shifting convective organization. Convective organization changes are quantified using the amount of rain explained by three different regimes of convection (shallow, deep isolated and deep organized), which are dubbed convective rain states (CRS). A second-order explanation lies in relative ice amount. Relative ice amount is represented by ice-rain ratio (IRR), the amount of ice per amount of rain present in the atmospheric volume as determined from spaceborne radars. Changes in CRS can cause biases because rainfall spatial correlations related to well-known errors (e.g. beam-filling, convective/stratiform microphysics) are likely responsible for over-and underestimation of precipitation, while changes in the relative ice amount in individual convective rain states can cause the precipitation to be under or over-estimated due to scattering effects. Over the TCP these changes are purely dictated by the El-Nino Southern Oscillation (ENSO), with organization becoming a clear function of SST. Over the STEI there is a circulation that stems from the Indian Ocean Dipole (IOD), leading to a CRS and IRR dependence on vertical wind shear. Finally, over the TWP, CRS is neither a simple function of SST nor shear, but rather seems to arise from a deeper ocean change of state: a coupling/decoupling of west Pacific SST with central and east Pacific SSTs that coincides with the global warming hiatus period. Two different mechanisms may be at play when it comes to shifts in convective organization during this period. Outside of 2001-2007, the regular mechanism sees low-level shear directing convective organization. During 2001-2007, TWP SSTs are much warmer than surrounding SSTs, leading to an anomalous mechanism that sees water vapor convergence and atmospheric instability favoring isolated convection. The purpose of this study is not to locate specific algorithm retrieval or model deficiencies, but rather to connect long-term measurement errors with fundamental changes in dynamic characteristics of ocean environments. While these proposed mechanisms would explain much of the observed biases, this study cannot address quantitative biases, as these depend on algorithm details and vary from one algorithm to another – although the qualitative trends are consistent among them.Item Open Access Tropical deep convective cloud morphology(Colorado State University. Libraries, 2014) Igel, Matthew R., author; van den Heever, Susan, advisor; Eykholt, Richard, committee member; Maloney, Eric, committee member; Stephens, Graeme, committee memberA cloud-object partitioning algorithm is developed. It takes contiguous CloudSat cloudy regions and identifies various length scales of deep convective clouds from a tropical, oceanic subset of data. The methodology identifies a level above which anvil characteristics become important by analyzing the cloud object shape. Below this level in what is termed the pedestal region, convective cores are identified based on reflectivity maxima. Identifying these regions allows for the assessment of length scales of the anvil and pedestal of the deep convective clouds. Cloud objects are also appended with certain environmental quantities from the ECMWF reanalysis. Simple geospatial and temporal assessments show that the cloud object technique agrees with standard observations of local frequency of deep-convective cloudiness. Additionally, the nature of cloud volume scale populations is investigated. Deep convection is seen to exhibit power-law scaling. It is suggested that this scaling has implications for the continuous, scale invariant, and random nature of the physics controlling tropical deep convection and therefore on the potentially unphysical nature of contemporary convective parameterizations. Deep-convective clouds over tropical oceans play important roles in Earth's climate system. The response of tropical, deep convective clouds to sea surface temperatures (SSTs) is investigated using this new data set. Several previously proposed feedbacks are examined: the FAT hypothesis, the Iris hypothesis, and the Thermostat hypothesis. When the data are analyzed per cloud object, each hypothesis is broadly found to correctly predict cloud behavior in nature, although it appears that the FAT hypothesis needs a slight modification to allow for cooling cloud top temperatures with increasing SSTs. A new response that shows that the base temperature of deep convective anvils remains approximately constant with increasing SSTs is introduced. These cloud-climate feedbacks are integrated to form a more comprehensive theory for deep convective anvil responses to SST. An investigation into the physical shape and size of mature, oceanic, tropical, deep convective clouds is conducted. Mean cloud objects are discussed. For single-core clouds, the mean cloud has an anvil width of 95 km, a pedestal width of 11 km, and an anvil thickness of 6.4 km. The number of identified convective cores within pedestal correlates well with certain length scales and morphological attributes of cloud objects. As the number of cores increases, so does the size of the mean cloud object. Pedestal width is shown to regress linearly to anvil width when a 2/3rd power scaling is applied to pedestal width. This result implies continuous but retarded growth of anvils with growing pedestals and equivalence in the mass flux convecting through the pedestal and into the anvil. Trends in cloud scales with cloud base and top heights are investigated to shed light on related convective parameterization assumptions and on convective transport, respectively. Many of the results obtained using the CloudSat methodology are also examined with a large-domain radiative-convective equilibrium numerical simulation and are found to exhibit similar trends when modeled. Finally, various CloudSat sampling issues are discussed in several appendices. Utilizing the CloudSat cloud object database, an examination of the sensitivity of oceanic, mature, deep convective cloud morphology to environmental characteristics is conducted. Convective available potential energy (CAPE), aerosol optical depth, mid-level vertical velocity, and troposphere deep shear are all included as meteorological measures. The sensitivity of various aspects of convective morphology to each one of these environmental characteristics is assessed individually. The results demonstrate that clouds tend to be invigorated by higher CAPE, aerosol amount, and upward mid-level vertical velocity. Stronger shear tends to make clouds wider but also shallower. The relative importance of each of these, and some additional, environmental measures to trends in cloud morphology are compared. It is found that aerosol, mid-level vertical velocity, and sea surface temperature tend to be the most influential environmental characteristics to convective morphology. The results are shown to be insensitive to the manner in which the environment is measured. The potentially surprising insensitivity of cloud morphology to CAPE is discussed in detail.