Browsing by Author "Randall, David A., committee member"

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Open Access
A theory of topographically bound balanced motions and application to atmospheric low-level jets
(Colorado State University. Libraries, 2011) Silvers, Levi Glenn, author; Schubert, Wayne H., advisor; Randall, David A., committee member; Thompson, David W. J., committee member; Eykholt, Richard E., committee member
The response of a stratified fluid to forcing from the lower boundary is studied both analytically and numerically. The lower boundary forces a flow field through orographic obstacles and potential vorticity anomalies. It is argued that these mechanisms contribute to the maintenance of low-level jets that are observed regularly in the vicinity of the Rocky Mountains and the Andes. Low-level jets function as one of the primary mechanisms through which topography and surface heating influence regional and global climates. On the ƒ-plane a horizontal transform of the governing equation for potential vorticity leads to a vertical structure equation that is solved using Green's functions. On the sphere a vertical transform of this system leads to a horizontal structure equation that is solved using spheroidal harmonics. These analytic solutions lead to a conceptually simple picture of the fluid response to forcing. However, these derivations only lead to closed analytic solutions for the case of an isentropic lower boundary. When the lower boundary is not isentropic a massless layer must be included in the domain and the solution is then found iteratively. For the cases including a massless layer the system is approximated using finite differences and solved with an over-relaxation procedure. Solutions are presented for the geostrophically balanced, steady response of the fluid to three idealized lower boundaries. An isentropic ridge is studied to determine the role non-heated orography plays on the wind field. Then a flat heated lower boundary and a non-isentropic ridge are studied. The cases with a heated lower surface result in a cyclonic wind field that is anchored over the topography. Observations show a prominent cyclonic wind field centered on both the Rocky Mountains and the Andes. The idealized cases studied in this work allow for the examination of fluid systems analogous to the Great Plains low-level jet and the South American LLJ. Both the mean behavior of these jets and their variability have important climatological and economic impacts on the plains regions of North and South America. One of the purposes of this work is to interpret particular low-level jet systems as part of the orographically bound, balanced motion associated with the potential vorticity anomalies produced by solar heating. This research proposes the jets on opposite sides of the mountains to be a single response to potential vorticity forcing that is the result of radiative heating on the Rockies and the Andes. The orographically bound circulations can also impact monsoon circulations. Although the importance of heated orography to LLJs has tended to be downplayed in the literature, it is shown here to be a significant component in the maintenance of LLJs.
Open Access
Applying two binned methods to the simple biosphere model (SiB) for improving the representation of spatially varying precipitation and soil wetness
(Colorado State University. Libraries, 2011) Medina, Isaac D., author; Denning, A. Scott, advisor; Randall, David A., committee member; Ramirez, Jorge A., committee member
Representing subgrid-scale variability is a continuing challenge for modelers, but is crucial for accurately calculating the exchanges of energy, moisture, and momentum between the land surface and atmospheric boundary layer. Soil wetness is highly spatially variable and difficult to resolve at grid length scales (~100 km) used in General Circulation Models (GCMs). Currently, GCMs use an area average precipitation rate that results in a single soil wetness value for the entire grid area, and due to the nonlinear relationship between soil wetness and evapotranspiration, significant inaccuracies arise in the calculation of the grid area latent heat flux. Using a finer GCM resolution will not solve this problem completely and other methods of modeling need to be considered. For this study, the binned and alternative binned method of Sellers et al. (2007) are applied to the Simple Biosphere Model (SiB) for improving the representation of spatially varying precipitation, soil wetness and surface-atmosphere fluxes. The methods are tested in a dry, semi-arid, and wet biome for two off-line precipitation distribution experiments, and results are compared to an explicit method, which is ideal for resolving subgrid-scale variability, and the bulk method (area averaged), which is currently in use with GCMs. Results indicate that the alternative binned method better captures the spatial variability in soil wetness and grid area flux calculations produced by the explicit method, and deals realistically with spatially varying precipitation at little additional computational cost to the bulk method.
Open Access
Approaching Arctic-midlatitude dynamics from a two-way feedback perspective
(Colorado State University. Libraries, 2019) McGraw, Marie C., author; Barnes, Elizabeth A., advisor; Randall, David A., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Karan, committee member
Arctic variability and the variability of the midlatitude circulation are closely intertwined. Although these connections are interrelated and bi-directional, and occur on a variety of timescales, they are not often studied together. Modeling studies generally focus on a single direction of influence--usually, how the midlatitude circulation responds to the Arctic. Studying these relationships in a two-way feedback perspective can offer new insights into these connections, providing information on how they feed back upon each other. This work approaches Arctic-midlatitude dynamics from a two-way feedback perspective, mostly on sub-monthly timescales. Particular emphasis is placed on the influence of midlatitude circulation variability upon the Arctic, as this direction of influence is less-studied than the converse pathway. Reinforcing feedback loops are identified between the North Pacific and North Atlantic jet streams and the Arctic. Variability in both the North Atlantic and North Pacific jet streams drives Arctic variability, which then drives further variability in the jet streams. The circulation variability in many regions, including North America, the east Pacific and Alaska, and Siberia, drives Arctic variability far more than it is driven by Arctic variability. These relationships exhibit substantial regional variability, stressing the important role of an analytical approach that incorporates this spatial heterogeneity. The two-way nature of Arctic-midlatitude connections is also explored in the context of Arctic moisture fluxes. The circulation response to sea ice loss also drives changes in Arctic moisture fluxes, with moisture flux out of the Arctic increasing more than moisture flux into the Arctic. The two-way feedback perspective explored in this research is built around the ideas of causal discovery, particularly Granger causality. Most of these two-way Arctic-midlatitude relationships are considered in the context of added variance explained, or added predictive power. That is, these relationships are characterized by comparing how much an additional predictor improves predictability beyond autocorrelation. Limiting the ability of autocorrelation to color these results emphasizes added variance explained--how much additional variance in the circulation can be explained by Arctic temperature variability, and vice versa? As an example, many recent studies have concluded that warm Arctic temperatures or low sea ice conditions drive a strengthening of high pressures and an increase in cold temperatures over Siberia. However, when memory and autocorrelation are accounted for, it emerges that the circulation variability over Siberia drives a response in the Arctic more than the other way around--results that are in concordance with modeling studies that have also disputed the veracity of the claim of the Arctic driving a strong response in Siberia. Ultimately, this research seeks to offer a different perspective on analyzing climate dynamics, with an emphasis on two-way feedbacks. While targeted climate modeling studies offer great physical insights, and provide substantial opportunities to explore and test physical mechanisms, they are often limited to exploring only one pathway of influence. In reality, these relationships do go in both directions, and a comprehensive understanding of such large-scale interactions between different parts of the atmosphere must ultimately consider the two-way relationships. The causal discovery methods used in much of this research can be used in conjunction with modeling studies to better understand these two-way relationships, and improve interpretation of results. While this research has focused on the relationships between the Arctic and the midlatitude circulation on sub-seasonal timescales, the broad framework and ideas presented within can be more widely applied to many other questions in climate variability studies. Thus, this work has also put a special emphasis on describing and implementing these causality-based methods in a manner that is accessible and interpretable for atmospheric and climate scientists.
Open Access
Climatology and variability of atmospheric rivers over the north Pacific
(Colorado State University. Libraries, 2017) Mundhenk, Bryan D., author; Barnes, Elizabeth A., advisor; Maloney, Eric D., advisor; Randall, David A., committee member; Ham, Jay M., committee member
Atmospheric rivers (ARs) are plumes of intense water vapor transport that dominate the flux of water vapor into and within the extratropics. Upon landfall, ARs are a major source of precipitation and often trigger weather and/or hydrologic extremes. Over time, landfalling AR activity, or a lack thereof, can influence periods of regional water abundance or drought. An objective detection algorithm is developed to identify and characterize these features using gridded fields of anomalous vertically integrated water vapor transport. Output from this algorithm enables the investigation into the relationships between tropical variability and ARs over the North Pacific undertaken in this dissertation. In the first segment of this study, an all-season analysis of AR incidence within the North Pacific basin is performed for the period spanning 1979 to 2014. The variability of AR activity due to the seasonal cycle, the El Nino-Southern Oscillation (ENSO) cycle, and the Madden-Julian oscillation (MJO) is presented. The results highlight that ARs exist throughout the year over the North Pacific. In general, the seasonal cycle manifests itself as northward and westward displacement of AR activity during boreal summer, rather than a seasonal change in the total number of ARs within the domain. It is also shown that changes to the North Pacific mean-state due to the ENSO cycle and the MJO may enhance or completely offset the seasonal cycle of AR activity, but that such influences vary greatly based on location within the basin. The second segment of this study investigates ARs at high northern latitudes. Comparatively little is known about the dynamics supporting these ARs in contrast to their mid-latitude counterparts. ARs are found to occur near the Gulf of Alaska and the U.S. West Coast with similar frequency, but with different seasonality. Composited atmospheric conditions reveal that a broad height anomaly over the northeast Pacific is influential to AR activity near both of these regions. When a positive height anomaly exists over the northeast Pacific, AR activity is often deflected poleward toward Alaska, while the U.S. West Coast experiences a decrease in AR activity, and vice versa. This tradeoff in AR activity between these two regions applies across a range of time scales, not just with respect to individual transient waves. Both ARs and height anomalies are found to be associated with Rossby wave breaking, thereby dynamically linking the modulation of AR activity with broader North Pacific dynamics. The third segment of this study explores the predictability of anomalous landfalling AR activity within the subseasonal time scale (approximately 2-5 weeks). An empirical prediction scheme based solely on the initial state of the MJO and the stratospheric quasi-biennial oscillation (QBO) is constructed and evaluated over 36 boreal winter seasons. This scheme is based on the premise that the MJO modulates landfalling AR activity along the west coast of North America within the subseasonal time scale by exciting large-scale circulation anomalies over the North Pacific. The QBO is found to further modulate the MJO--AR relationship. The prediction scheme reveals skillful subseasonal "forecasts of opportunity" when knowledge of the MJO and the QBO can be leveraged to predict periods of increased or decreased AR activity. Moreover, certain MJO and QBO phase combinations provide predictive skill competitive with, or even exceeding, a state-of-the-art numerical weather prediction model.
Open Access
Cold pool processes in different environments
(Colorado State University. Libraries, 2018) Grant, Leah Danielle, author; van den Heever, Susan C., advisor; Randall, David A., committee member; Rutledge, Steven A., committee member; Niemann, Jeffrey D., committee member
Cold pools are localized regions of dense air near Earth's surface. They form in association with precipitating clouds in many environments ranging from moist tropical to semi-arid continental conditions, and they play important roles in weather in climate. The overarching goal of this dissertation research is to improve our process-level understanding of cold pool interactions with different components of the Earth system, focusing on two key knowledge gaps: (1) interactions with Earth's surface in continental environments; and (2) interactions with organized convective systems in tropical oceanic environments. The primary goal of the first study conducted in this dissertation is to evaluate how surface sensible heat fluxes impact cold pool dissipation in dry continental environments via two pathways: (a) by directly heating the cold pool, and (b) by changing mixing rates between cold pool air and environmental air through altering turbulence intensity. Idealized 2D simulations of isolated cold pools are conducted with varying sensible heat flux formulations to determine the relative importance of these two mechanisms. The results demonstrate that the impact of sensible heat fluxes on mixing, i.e. mechanism (b), contributes most significantly to cold pool dissipation. Cold pool – land surface interactions in semi-arid continental conditions are investigated in the second study. Two questions are addressed: (1) how does the land surface respond to the cold pool; and (2) to what extent do land surface feedbacks modulate the cold pool evolution? Idealized 3D simulations of a cold pool evolving in a turbulent boundary layer are conducted to answer these questions. The land surface cools in response to the cold pool, resulting in suppressed sensible heat fluxes in the center of the cold pool. However, sensible heat fluxes are enhanced near the edge of the cold pool in association with higher wind speeds, leading to cold pool dissipation from the edge inwards. The land surface interactions are shown to strongly affect the cold pool, reducing its lifetime, size, and intensity by up to 50%. Preliminary analysis of a cold pool that was observed in northeastern Colorado on 17 May 2017 ("The Bees Day") during the C3LOUD-Ex field campaign is presented in the third study. The observed case exhibits similar environmental and cold pool characteristics to the first two numerical studies, thereby providing observational context for their hypotheses and conclusions. The objective of the fourth study presented in this dissertation is to determine the role of cold pools in organized tropical oceanic convective systems. To address this goal, two convective systems embedded in a weakly sheared cloud population approaching radiative-convective equilibrium are simulated at high resolution. The cold pools are weakened in the sensitivity tests by suppressing evaporation rates below cloud base. Both of the convective systems respond in a consistent manner as follows: (a) when cold pools are weakened, the convective intensity increases; and (b) the mesoscale structure, propagation speeds, and system lifetimes are insensitive to the changes in the cold pools, in contrast to the prevailing (RKW) theory that cold pools are critical to the mesoscale organization of convective systems. In summary, these high-resolution modeling and observational studies demonstrate new insights into cold pool – surface – convection interactions. The results suggest that cold pool interactions with different components of the Earth system are not all created equally; rather, these interactions depend on the environment in which the cold pools find themselves.
Open Access
Connections between climate sensitivity and large-scale extratropical dynamics
(Colorado State University. Libraries, 2019) Davis, Luke L. B., author; Thompson, David W. J., advisor; Birner, Thomas, advisor; Randall, David A., committee member; Pinaud, Olivier, committee member
The response of the extratropical storm tracks to anthropogenic forcing is one of the most important but poorly understood aspects of climate change. The direct, thermodynamic effects of climate change are relatively well understood, but their two-way interactions with large-scale extratropical dynamics are extremely difficult to predict. There is thus continued need for a robust understanding of how this coupling evolves in space and time. The dry dynamical core represents one of the simplest possible numerical models for studying the response of the extratropical storm tracks to climate change. In the model, the extratropical circulation is forced by relaxing to a radiative equilibrium profile using linear damping. The linear damping coefficient plays an essential role in governing the structure of the circulation. But despite decades of research with the dry dynamical core, the role of the damping coefficient in governing the circulation has received relatively little scrutiny. In this thesis, we systematically vary the damping rate and the equilibrium temperature field in a dry dynamical core in order to understand how the amplitude of the damping influences extratropical dynamics. Critically, we prove that the damping rate is a measure of the climate sensitivity of the dry atmosphere. The key finding is that the structure of the extratropical circulation is a function of the climate sensitivity. Larger damping timescales – which are equivalent to higher climate sensitivities – lead to a less dynamically active extratropical circulation, equatorward shifts in the jet, and a background state that is almost neutral to baroclinic instability. They also lead to increases in the serial correlation and relative strength of the annular modes of climate variability. It is argued that the climate sensitivity of the dry atmosphere may be identifiable from its dynamical signatures, and that understanding the response of the circulation to climate change is critically dependent on understanding its climate sensitvity.
Open Access
Impacts of Arctic warming and sea ice loss on the Northern Hemisphere mid-latitude large-scale circulation
(Colorado State University. Libraries, 2020) Ronalds, Bryn, author; Barnes, Elizabeth A., advisor; Thompson, David, committee member; Randall, David A., committee member; Eykholt, Richard, committee member
The consequences of the rapid warming of the Arctic and associated sea ice loss on the Northern Hemisphere atmospheric circulation is still largely debated. The uncertainty in the circulation response stems from a poor understanding of the underlying physical mechanisms of the remote response, regional and seasonal differences, differences between models and experimental set-ups, the large internal variability of the system, and the short observational record. This research seeks to address some of this uncertainty, specifically the uncertainty related to the physical mechanisms, regionality, and modeling differences. The wintertime Northern Hemisphere eddy-driven jet streams over the North Pacific and North Atlantic basins exhibit differing responses to Arctic warming and sea ice loss in a fully coupled climate model. In the North Atlantic the jet weakens, narrows along the poleward flank, and shifts slightly equatorward. This response is similar to previous studies examining the Northern Hemisphere zonal mean jet response. In contrast, the North Pacific jet strengthens and extends eastward in response to Arctic sea ice loss, with no change in latitude, and narrows slightly along the poleward flank. In both cases, there are high latitude anomalous easterlies in the region of sea ice loss, where the local surface temperature gradients are weakening. This can lead to changes in locations and frequency of wave-breaking, thus leading to changes in the mean zonal winds further south, in the vicinity of the jet. This work relates the differing changes in the North Pacific and North Atlantic to these changes in wave-breaking in a simplified atmospheric model, and posits that the location of the jet relative to the region of Arctic sea ice loss is a dominant factor in determining the mean jet response to the sea ice loss and local warming. Changes in the mean wintertime Northern Hemisphere midlatitude zonal winds are found to be indicative of changes to the sub-seasonal variability of the wintertime zonal winds. The sub-seasonal circulation patterns over the ocean basins are closely linked with continental weather regimes, including changes in temperature and precipitation. While establishing a causal link between Arctic sea ice loss and changes to remote weather regimes in the observational record remains difficult, the Polar Amplification Model Intercomparison Project (PAMIP) provides insight into possible relationships and consequences. The design of the project eliminates differences in experimental set-ups across models and aids in addressing the uncertainty in regional responses. Across four climate models, Arctic sea ice loss leads to a strengthened and extended North Pacific jet in the January-February mean. This mean change is also associated with changes to the sub-seasonal, wintertime North Pacific zonal wind variability. All four models show an increase in strengthened and extended North Pacific eddy-driven jet stream events and a decrease in weakened, retracted and equatorward-shifted North Pacific jet events in January-February. Previous work has also established the relationships between North Pacific jet stream variability and downstream, North American weather regimes, and changes to the former are expected to impact the latter. Again, there is model agreement in an increase of a warm west/cold east temperature dipole over North America, associated with the strengthened and extended jet events. There is also a decrease in cold air temperature anomalies over North America, associated with weakened and equatorward-shifted jet events.
Open Access
Importance of boundary layer entrainment for surface fluxes over land
(Colorado State University. Libraries, 2011) McGrath-Spangler, Erica L., author; Denning, A. Scott, advisor; Randall, David A., committee member; Heald, Colette, committee member; Zupanski, Dusanka, committee member; Hoeting, Jennifer A., committee member
An idealized experiment examined the impacts of entrainment in a coupled ecosystem-atmosphere model by implementing an enhanced entrainment parameterization based on the assumption that the heat flux at the top of the PBL is negatively proportional to the heat flux at the surface. This experiment found that entrainment produced a warmer, drier, and deeper PBL and that the surface fluxes of heat and moisture were modified by the vegetative response to the altered atmospheric conditions. A realistic simulation for the summer of 1999 found that enhanced entrainment produced stronger early morning growth of the PBL and a deeper midday depth. This better captured the monthly mean diurnal cycle of PBL depth from observations by a radar sounding system in northern Wisconsin. Additionally, the complex land-atmosphere interactions produced a time-mean spatial CO2 gradient of 7 ppm over 1000 km. A sensitivity analysis performed for June 2007 to the strength of the PBL-top entrainment flux found subtle spatial variations in the time mean. The addition of entrainment from overshooting thermals weakened the Bermuda high circulation and weakened the spatial gradients between the warm, dry semiarid southwestern United States and cooler, moister locations in eastern North America. These subtle variations produced a 3.5 ppm CO2 change in the time mean across 280 km. One possible explanation for these more subtle results is that additional changes to the coupled model resulted in persistent cloud cover that produced relatively cold and dark conditions. In order to evaluate and improve model simulations, PBL depth has been estimated using the backscatter from the LIDAR onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. Using an automated method, millions of estimates have been derived to which model results can be compared. This method evaluates the maximum vertical variance of the backscatter in order to identify backscatter features associated with the top of the PBL and helps to identify the vertical extent of turbulent mixing. This analysis sheds some light on the spatial heterogeneity of boundary layer processes. The derived depths are shallower over water than over land and show a local minimum along the Mississippi River valley. Deeper features are found over the desert Southwest and deeper than expected values are retrieved over the Boreal forests.
Open Access
Intraseasonal variability in the tropical diurnal cycle
(Colorado State University. Libraries, 2022) Natoli, Michael B., author; Maloney, Eric D, advisor; Bell, Michael M., committee member; Randall, David A., committee member; Niemann, Jeffrey D., committee member
The relationship between large-scale intraseasonal variability in tropical convection and the local diurnal cycle on tropical islands is explored with observations and an idealized model. In part one, the impact of quasi-biweekly variability in the monsoon southwesterly winds on the precipitation diurnal cycle in the Philippines is examined using CMORPH precipitation, ERA5 reanalysis, and outgoing longwave radiation (OLR) fields. Both a case study during the 2018 Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign and a 23-year composite analysis are used to understand the effect of the QBWO on the diurnal cycle. QBWO events in the west Pacific, identified with an extended EOF index, bring increases in moisture, cloudiness, and westerly winds to the Philippines. Such events are associated with significant variability in daily mean precipitation and the diurnal cycle. It is shown that the modulation of the diurnal cycle by the QBWO is remarkably similar to that by the boreal summer intraseasonal oscillation (BSISO). The diurnal cycle reaches a maximum amplitude on the western side of the Philippines on days with average to above average moisture, sufficient insolation, and weakly offshore prevailing wind. This occurs during the transition period from suppressed to active large-scale convection for both the QBWO and BSISO. Westerly monsoon surges associated with QBWO variability generally exhibit active precipitation over the South China Sea (SCS), but a depressed diurnal cycle. These results highlight that modes of large-scale convective variability in the tropics can have a similar impact on the diurnal cycle if they influence the local scale environmental background state similarly. In part two, a specific large-scale mode is neglected, and the impact of variability in the background wind at any timescale on the local diurnal cycle is isolated. Luzon Island in the northern Philippines is used as an observational test case. Composite diurnal cycles of CMORPH precipitation are constructed based on an index derived from the first empirical orthogonal function (EOF) of ERA5 zonal wind profiles. A strong precipitation diurnal cycle and pronounced offshore propagation in the leeward direction tends to occur on days with a weak, offshore prevailing wind. Strong background winds, particularly in the onshore direction, are associated with a suppressed diurnal cycle. Idealized high resolution 2-D Cloud Model 1 (CM1) simulations test the dependence of the diurnal cycle on environmental wind speed and direction by nudging the model base-state toward to composite profiles derived from the reanalysis zonal wind index. These simulations can qualitatively replicate the observed development, strength, and offshore propagation of diurnally generated convection under varying wind regimes. Under strong background winds, the land-sea contrast is reduced, which leads to a substantial reduction in the strength of the sea-breeze circulation and precipitation diurnal cycle. Weak offshore prevailing winds favor a strong diurnal cycle and offshore leeward propagation, with the direction of propagation highly sensitive to the background wind in the lower free troposphere. Offshore propagation speed appears consistent with density current theory rather than a direct coupling to a single gravity wave mode, though several gravity wave modes apparent in the model likely contribute to a destabilization of the offshore environment. In part three, the hypotheses developed in parts one and two regarding the mechanisms regulating the diurnal cycle response are rigorously tested. A novel probabilistic framework is applied to the Luzon test case to improve the understanding of diurnal cycle variability. High amplitude diurnal cycle days tend to occur with weak to moderate offshore low-level wind and near to above average column moisture in the local environment. The transition from the BSISO suppressed phase to the active phase is most likely to produce the wind and moisture conditions supportive of a substantial diurnal cycle over western Luzon and the South China Sea (SCS). Thus, the impact of the BSISO on the local diurnal cycle can be understood in terms of the change in the probability of favorable environmental conditions. Idealized high-resolution 3-D Cloud Model 1 (CM1) simulations driven only by a base-state derived from BSISO composite profiles are able to reproduce several important features of the observed diurnal cycle variability with BSISO phase, including the strong, land-based diurnal cycle and offshore propagation in the transition phases. Background wind appears to be the primary variable controlling the diurnal cycle response, but ambient moisture distinctly reduces precipitation strength in the suppressed BSISO phase, and enhances it in the active phase. A land-breeze, lingering deep convection over land after sunset, and strong mechanical convergence appear to all be required in order to produce offshore propagation in CM1. Simulations in which the diurnal cycle of insolation is removed suggest the potential for a natural timescale for convective regeneration related to the island size.
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
On the environments and dynamics of nocturnal mesoscale convective systems
(Colorado State University. Libraries, 2018) Hitchcock, Stacey, author; Schumacher, Russ S., advisor; Randall, David A., committee member; van den Heever, Susan C., committee member; Eykholt, Richard, committee member
The 2015 Plains Elevated Convection at Night (PECAN) field campaign was motivated by unanswered questions about key processes in elevated mesoscale convective systems (MCSs) and the difficulty in accurately forecasting them. During the campaign, 15 MCS environments were sampled by an array of instruments including radiosondes launched by fixed and mobile sounding teams. Cluster analysis of observed vertical profiles established three primary pre-convective categories. Only one of these groups fits well with the common conceptual model of nocturnal MCS environments where equivalent potential temperature increases in an elevated layer with the onset of the low-level jet (LLJ). Post-convective soundings demonstrate substantial variability, but cold pools were observed in nearly every PECAN MCS case. However, stronger, deeper stable layers appear to lead to structures where the largest cooling is observed above the surface. On 24-25 June 2015, a 'bow-and-arrow' MCS structure was observed in an environment with strong low-level stability. Previous work on the mechanisms that support the structure in the arrow region (also sometimes referred to as rearward off-boundary development or ROD) has relied on a combination of a surface cold pool and large scale ascent provided by the interaction of a LLJ with a baroclinic zone. A horizontally homogeneous simulation initialized with a near-storm pre-convective PECAN sounding from the 24-25 June 2015 produces nearly the same MCS structure in the absence of a surface cold pool. Instead, outflow takes on several different forms in different regions of the MCS. Ultimately, the ROD (or arrow) is most likely supported by gravity wave amplified by vertical wind shear over the same layer, and maintained by persistent downdrafts. The success of both MCS initiation and development of ROD despite the strong stable layer and idealized horizontally homogeneous initial conditions suggests that the interactions between convective outflow and a stable layer in a sheared environment are important in both of these processes. Very few studies to date have explored these interactions, and even less in 3D. A series of 2D and 3D experiments were designed to explore 1) What happens when a downdraft impacts a stable layer with and without shear? 2) What low-level shear profiles support MCS development in an idealized simulation with strong stability and why? 3) What shear characteristics are favorable for ROD development? Results indicate that strong low level shear is critical for sustaining convection, that low-level shear may be as important as stability in determining the effective inflow layer, and that upper level winds play a critical role in the development of ROD.
Open Access
On the structure of climate variability near the tropopause and its relationship to equatorial planetary waves
(Colorado State University. Libraries, 2011) Grise, Kevin M., author; Thompson, David W. J., advisor; Birner, Thomas, committee member; Eykholt, Richard E., committee member; Randall, David A., committee member; Randel, William J., committee member
The tropopause is an important interface in the climate system, separating the unique dynamical, chemical, and radiative regimes of the troposphere and stratosphere. Previous studies have demonstrated that the long-term mean structure and variability of the tropopause results from a complex interaction of stratospheric and tropospheric processes. This project provides new insight into the processes involved in the global tropopause region through two perspectives: 1) a high vertical resolution climatology of static stability and 2) an observational analysis of equatorial planetary waves. High vertical resolution global positioning system radio occultation profiles are used to document fine-scale features of the global static stability field near the tropopause. Consistent with previous studies, a region of enhanced static stability, known as the tropopause inversion layer (TIL), exists in a narrow layer above the extratropical tropopause and is strongest over polar regions during summer. However, in the tropics, the TIL possesses a unique horizontally and vertically varying structure with maxima located at ~17 and ~19 km. The upper feature peaks during boreal winter and has its largest magnitude between 10° and 15° latitude in both hemispheres; the lower feature exhibits a weaker seasonal cycle and is centered at the Equator. The spatial structure of both features resembles the equatorial planetary wave response to the climatological distribution of deep convection. Equatorial planetary waves not only dominate the climatological-mean general circulation near the tropical tropopause but also play an important role in its intraseasonal and interannual variability. The structure of the equatorial planetary waves emerges as the leading pattern of variability of the zonally asymmetric tropical atmospheric circulation. Regressions on an index of the equatorial planetary waves reveal that they are associated with a distinct pattern of equatorially symmetric climate variability characterized by variations in: 1) the distribution of convection in the deep tropics; 2) the eddy momentum flux convergence and the zonal-mean zonal wind in the tropical upper troposphere; 3) the mean meridional circulation of the tropical and subtropical troposphere; 4) temperatures in the tropical upper troposphere, the tropical lower stratosphere, and the subtropical troposphere of both hemispheres; and 5) the amplitude of the upper tropospheric anticyclones that straddle the Equator over the western tropical Pacific Ocean. The pulsation of the equatorial planetary waves in time provides a framework for interpreting a broad range of climate phenomena. Variability in the equatorial planetary waves is associated with variability in the tropical TIL and is linked to both the El Niño-Southern Oscillation and the Madden-Julian Oscillation (MJO). Evidence is presented that suggests that the MJO can be viewed as the linear superposition of: 1) the pulsation of the equatorial planetary waves at a fixed location and 2) a propagating component. Variability in the equatorial planetary waves may also contribute to variability in troposphere/stratosphere exchange and the width of the tropical belt.
Open Access
Radiative feedbacks in tropical organized convection and the Madden-Julian oscillation
(Colorado State University. Libraries, 2024) Hsiao, Wei-Ting, author; Maloney, Eric D., advisor; Rugenstein, Maria A. A., committee member; Kummerow, Christian D., committee member; Randall, David A., committee member; Mueller, Nathaniel D., committee member
The organization of tropical deep convection is supported by radiative feedbacks, in which high clouds and moisture anomalies associated with convection imposes anomalous longwave (LW) radiative heating in the atmosphere, further supporting convection. Despite an abundance of studies using numerical simulations, the interactions between tropical convective organization, radiative feedbacks, and the large-scale atmospheric environment have not been comprehensively examined in real-world observations. The present dissertation examines such interactions among tropical mesoscale organized convection, radiative feedbacks, and the Madden-Julian oscillation (MJO) using a set of observation-derived data products, including retrievals using spaceborne satellites and ground-based precipitation radar, along with combined products and reanalyses. The main findings in each chapter are summarized as follows: (1) higher sea surface temperature and stronger low-level wind shear strength enhance tropical mesoscale convective activity, increasing cirrus cloud cover and LW heating generated per unit precipitation. (2) the estimation of LW cloud-radiative feedback (LW CRF), defined as the LW cloud-radiative heating produced per unit precipitation, is sensitive to the precipitation data set used. (3) radiatively driven circulation and the associated moistening effects in the MJO can be derived in a weak-temperature-gradient framework and a linear baroclinic model. The result suggests that LW heating moistens the MJO more efficiently than the total apparent heat source, while shortwave (SW) radiative effects dry the MJO. (4) The LW CRF of the MJO is spatially inhomogeneous, with stronger feedbacks over the tropical Indian ocean and to the northwest of Australia, but weaker feedbacks over the tropical western and central Pacific. The spatial pattern may be determined by the spatial distribution of preferred convective types and precipitation efficiency.
Open Access
Seasonal greening in grasslands
(Colorado State University. Libraries, 2013) Orescanin, Biljana, author; Denning, A. Scott, advisor; Randall, David A., committee member; Paustian, Keith H., committee member
Grasslands cover about one quarter of the Earth's land and are currently considered to act as carbon sinks, taking up an estimated 0.5 Gt C per year. Thus, robust understanding of the grassland biome (e.g. representation of seasonal cycle of plant growth and the amount of green mass, often referred to as phenology, in global carbon models) plays a key role in understanding and predicting the global carbon cycle. The focus of this research is on improvement of a grassland biome representation in a biosphere model, which sometimes fails to correctly represent the phenology of vegetation. For this purpose, as a part of Simple Biosphere model (SiB3), a phenology model is tested and improved to provide more realistic representation of plant growth dependence on available moisture, which along with temperature and light controls plant growth. The new methodology employs integrated soil moisture in plant growth simulation. This new representation addresses the nature of the plants to use their root system to access the water supply. At same time it represents the plant's moisture recourses more accurately than the currently used vapor pressure method, which in grasslands is often non-correlated with soil conditions. The new technique has been developed and tested on data from the Skukuza flux tower site in South Africa and evaluated at 6 different flux tower sites around the world covering a variety of climate conditions. The technique is relatively easy and inexpensive to implement into the existing model providing excellent results capturing both the onset of green season and greening cycle at all locations. Although the method is developed for grasslands biome its representation of natural plant processes provides a good potential for its global use.
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
Simulations of Arctic mixed-phase clouds using a new aerosol-linked ice nuclei parameterization in a prognostic ice prediction scheme
(Colorado State University. Libraries, 2013) Carpenter, James Michael, author; Kreidenweis, Sonia M., advisor; DeMott, Paul J., advisor; Randall, David A., committee member; Eykholt, Richard, committee member
Despite the nearly universally-accepted notion that the Arctic is one of the most important areas to fully understand in the face of a changing global climate, observations from the region remain sparse, particularly of clouds and aerosol concentrations and sources. Low-level, mixed-phase clouds in the Arctic are capable of remarkable persistence, lasting for several days when our knowledge of the Wegener-Bergeron-Findeisen (WBF) process suggests that complete conversion to ice, or glaciation, should occur much faster, within a couple of hours. Multiple attempts at simulating these long-lived, mixed-phase clouds have been unable to accurately reproduce all cloud properties observed, with a major consequence being poor representation of radiative transfer, with important consequences for long-term climate simulations. Recent observational campaigns have sought to characterize ice-nucleating particles (IN) not just in the Arctic, but around the planet. A product of these campaigns, the DeMott IN parameterization (DeMott et al., 2010) seeks to provide a means for accurately implementing IN concentration calculations in a global model using minimal, readily-available proxy measurements or estimates of number concentrations of particles having diameters larger than 0.5 microns. In this study, the performance of this parameterization is tested in a cloud-resolving model capable of high resolution simulations of Arctic mixed-phase boundary layer stratus clouds. Three mixed-phase cloud case studies observed during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) and Mixed-Phase Arctic Cloud Experiment (M-PACE) are simulated with varying complexity in their cloud microphysical packages. The goal is to test the new aerosol-linked parameterization as well as the sensitivity of the observed clouds to ice nuclei concentrations. In an effort to increase the realism of the aerosol-cloud interactions represented in the cloud-resolving model, a new, simple prognostic scheme for the activation of ice nuclei is incorporated. The new scheme imposes a finite budget on potential ice nuclei, which are depleted through ice activation and growth, and can potentially be replenished by sublimating ice crystals. Results are contrasted with simulations in which no depletion of IN is assumed. In this study, we found that while the DeMott IN parameterization successfully predicted available IN concentrations within observational error, the model was unable to predict sufficiently high pristine ice concentrations for one of the case studies. There were likely issues with the model or initialization in this case. For two of the case studies, the model performed exceptionally well, predicting accurate ice number concentrations as well as cloud droplet concentrations, leading to reasonable predictions of downwelling longwave radiation at the surface. In all cases, the model failed to predict reasonable cloud ice water contents. In the future, tests of ice crystal habits and growth rates may improve microphysical representation and predicted ice water contents. Replenishment of scavenged ice nuclei via surface fluxes and long-range transport can be included in the simulations to increase realism, but more observations are needed to accurately quantify these effects.
Open Access
The dynamics of Hadley circulation variability and change
(Colorado State University. Libraries, 2017) Davis, Nicholas Alexander, author; Birner, Thomas, advisor; Randall, David A., committee member; Barnes, Elizabeth A., committee member; Venayagamoorthy, Subhas K., committee member; Randel, William J., committee member
The Hadley circulation exerts a dominant control on the surface climate of earth's tropical belt. Its converging surface winds fuel the tropical rains, while subsidence in the subtropics dries and stabilizes the atmosphere, creating deserts on land and stratocumulus decks over the oceans. Because of the strong meridional gradients in temperature and precipitation in the subtropics, any shift in the Hadley circulation edge could project as major changes in surface climate. While climate model simulations predict an expansion of the Hadley cells in response to greenhouse gas forcings, the mechanisms remain elusive. An analysis of the climatology, variability, and response of the Hadley circulation to radiative forcings in climate models and reanalyses illuminates the broader landscape in which Hadley cell expansion is realized. The expansion is a fundamental response of the atmosphere to increasing greenhouse gas concentrations as it scales with other key climate system changes, including polar amplification, increasing static stability, stratospheric cooling, and increasing global-mean surface temperatures. Multiple measures of the Hadley circulation edge latitudes co-vary with the latitudes of the eddy-driven jets on all timescales, and both exhibit a robust poleward shift in response to forcings. Further, across models there is a robust coupling between the eddy-driving on the Hadley cells and their width. On the other hand, the subtropical jet and tropopause break latitudes, two common observational proxies for the tropical belt edges, lack a strong statistical relationship with the Hadley cell edges and have no coherent response to forcings. This undermines theories for the Hadley cell width predicated on angular momentum conservation and calls for a new framework for understanding Hadley cell expansion. A numerical framework is developed within an idealized general circulation model to isolate the mean flow and eddy responses of the global atmosphere to radiative forcings. It is found that it is primarily the eddy response to greenhouse-gas-like forcings that causes Hadley cell expansion. However, the mean flow changes in the Hadley circulation itself crucially mediate this eddy response such that the full response comes about due to eddy-mean flow interactions. A theoretical scaling for the Hadley cell width based on moist static energy is developed to provide an improved framework to understand climate change responses of the general circulation. The scaling predicts that expansion is driven by increases in the surface latent heat flux and the width of the rising branch of the circulation and opposed by increases in tropospheric radiative cooling. A reduction in subtropical moist static energy flux divergence by the eddies is key, as it tilts the energetic balance in favor of expansion.
Open Access
The Madden Julian oscillation and tropical-extratropical teleconnections
(Colorado State University. Libraries, 2019) Tseng, Kai-Chih, author; Barnes, Elizabeth A., advisor; Maloney, Eric D., advisor; Randall, David A., committee member; Ebert-Uphoff, Imme, committee member
The Madden Julian Oscillation (MJO) excites strong variations in extratropical circulations that have important implications for subseasonal-to-seasonal (S2S) prediction. In particular, certain MJO phases are characterized by a consistent modulation of geopotential height patterns in the North Pacific and North America. Although the MJO's influence in the downstream weather has been widely explored in previous studies, the relationship between robust MJO teleconnection patterns and model prediction skills has received little attention. In this study, the reanalysis data and ensemble hindcasts from numerical weather forecast system are used to quantify the influence of robust MJO teleconnection on model prediction skills. By calculating the pattern consistency of MJO teleconnection, the ability of MJO convection to modulate extratropical weather is quantified over different time lags and phases. The diagnostic result demonstrates that the robust MJO teleconnection in specific MJO phases/lags are also characterized by excellent agreement in the prediction of geopotential height anomalies across model ensemble members at forecast lead of up to 3 weeks. The mechanisms that lead some MJO phases to have more consistent teleconnections than others are examined by using a linear baroclinic model (LBM). The simulation results show that MJO phases 2, 3, 6 and 7 consistently generate Pacific-North America like (PNA-like) pattern on S2S timescales while other phases do not. By employing a Rossby wave source analysis, the result shows that a dipole-like Rossby wave source patterns on each side of the jet in MJO phase 2, 3, 6 and 7 can increase the pattern consistency of teleconnection due to the constructive interference of similar teleconnection signals. On the other hand, the symmetric patterns of Rossby wave source in other phases can dramatically reduce the pattern consistency due to destructive interference. The consistency of MJO teleconnections is also characterized by an interannual variability. During the El Niño years, the pattern consistency is dramatically decreased compared to the La Niña years. Employing the numerical experiments in LBM and applying a Rossby wave ray tracing algorithm, we demonstrate two factors largely determine the interannual variability of MJO teleconnection consistency. During El Niño years, the eastward extension of subtropical jet and a less-dipole like Rossby wave source pattern on each side of the jet dramatically decrease poleward propagating wave signals. By contrast, the competing effect between these two factors results in modest changes in pattern consistency during La Niña years. Thus, the observed consistency of MJO teleconnections is much smaller during El Niño years than La Niña years. The dynamics associated with the pattern consistency of MJO teleconnection are addressed in the first half of this work. What is still unclear, however, is the importance of the accumulated influence of past MJO activity on these results. To examine the importance of past MJO phases in determining future states of extratropical circulations, a LBM and one of the simplest machine learning algorithm: logistic regression are used. By increasing the complexity of logistic regressions with additional informational about past MJO phases, we show that 15 additional lags before lag 0 play a dominant role in determining the future state of MJO teleconnections. This result is supported by the numerical LBM simulations. We further demonstrate that this 15-day span is characterized by a phase/lead time dependent feature, which is relevant to the dynamics of MJO teleconnections and explained in this work. Ultimately, a particular emphasis is placed on the role of model MJO in influencing the winter climatol- ogy of extratropical circulations. The MJO is known for consistently modulating the extratropical weather. In addition, simulating the MJO continues to be a challenge for many state-of-art climate models, and it is unclear the extent to which these biases in the MJO may cause biases in midlatitude variability. By analyzing 22 climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and the reanalysis data, we demonstrate that one of leading variability of daily geopotential height is associated with MJO activity, and can be identified without prior knowledge of MJO in both observations and CMIP5 data. This shows the dominant role of MJO in modulating extratropical circulations. However, due to this strong relationship between MJO and extratropical circulations, the model biases in the MJO convection is also reflected in the wintertime climatology of extratropical circulations.
Open Access
The role of inner-core and boundary layer dynamics on tropical cyclone structure and intensification
(Colorado State University. Libraries, 2018) Slocum, Christopher J., author; Schubert, Wayne H., advisor; DeMaria, Mark, advisor; Schumacher, Russ S., committee member; Randall, David A., committee member; Kirby, Michael, committee member; Fiorino, Michael, committee member
Inner-core and boundary layer dynamics play a vital role in the tropical cyclone life cycle. This study makes use of analytical solutions and numerical models to gain insight into the role of dynamical processes involved with the incipient, rapidly intensifying, and eyewall replacement stages. A simplified, axisymmetric, one-layer, analytical model of tropical cyclone intensification is developed. Rather than formulating the model with the gradient balance approximation, the model uses the wave-vortex approximation, an assumption to the kinetic energy of the system, which limits its use to flows with small Froude numbers. Through filtering the inertia-gravity waves and adding a mass sink so that potential vorticity is not conserved in the system, the model is solved and provides analytical, time-evolving solutions that provide insight into long incubation periods prior to rapid intensification, potential vorticity tower development without frictional effects, and storm evolution in time through the maximum tangential velocity, total energy phase space. To understand the applicability of the forced, balance model for tropical cyclone intensification, the model is compared to a model using gradient balance. The comparison shows that the model based on the wave-vortex approximation is appropriate for fluids with flow speeds indicative of the external vertical normal mode in which case the deviation to the fluid depth is small. To understand another aspect of the inner-core dynamics that influence the radial location of the mass sink associated with the eyewall convection in the tropical cyclone, boundary-layer dynamics are considered. Motivated by abrupt jumps in the horizontal wind fields observed in flight-level aircraft reconnaissance data collected in Hurricanes Allen (1980) and Hugo (1989), an axisymmetric, f-plane slab boundary layer numerical model with a prescribed pressure forcing is developed. From this model, two simple analytic models are formulated in addition to two local, steady-state models. These models allow for the role of shock dynamics in the tropical cyclone boundary layer to be assessed. Two local models are also developed to evaluate the role of the nonlinear terms in the full numerical slab model. The local models adequately describe the boundary layer winds outside of the eyewall region. If a storm is weak or broad, the local models can explain a portion of the structure that develops in the eyewall region. This result shows that, to capture the hyperbolic nature of the eyewall region, the nonlinear terms are needed. The nonlinear response allows for the boundary-layer Ekman pumping to shift radially inward into the region of high inertial stability. The results from the local models and full numerical model also show that as the vortex wind field broadens, the convergence associated with the primary eyewall decays and that a secondary maximum displaced radially outward forms, a feature indicative of the formation of a secondary eyewall.
Open Access
Tropical rainfall regimes and their evolution on hourly to daily timescales
(Colorado State University. Libraries, 2011) Elsaesser, Gregory Scott, author; Kummerow, Christian D., advisor; Maloney, Eric D., committee member; Moncrieff, Mitchell W., committee member; Randall, David A., committee member; Reising, Steven C., committee member
Data from multiple satellite and in situ sources are used to investigate the dominant raining cloud populations in the tropics, with the purpose of documenting how diverse the raining cloud populations are at any given time over a scale similar in size to the grid-box (~100 - 200 km) of a present-day global climate model (GCM). For all locations in the tropics, three similar rainfall clusters (defined according to their ensemble of clouds) are found. Differences in mean-state rainfall (e.g. East versus West Pacific Ocean) are largely the result of similar rainfall clusters occurring at ocean basin-dependent relative frequencies of occurrence. Area-average rainfall rates are substantially different for each cluster. While each rainfall cluster is observed in all tropical basins, differing relative frequencies of occurrence imply that rainfall lifecycles (i.e. the time duration for transition from light to deep rainfall) vary as a function of basin. Among the processes influencing this transition, both mesoscale cold pools (inferred from QuikSCAT surface wind field retrievals) and convective inhibition (CIN, derived from radiosonde-observations) emerge as important parameters driving the transition from light rainfall to deep convection at the spatial scale of 100 - 200 km. Associated with significant increases in rainfall are substantial decreases (40%) in convective available potential energy (CAPE). The temporal evolution of rainfall clusters is derived for different lifecycle stages of a composite Madden-Julian Oscillation (MJO) event. It is found that the rainfall cluster consisting of shallow (<3 km) and congestus raining clouds exhibits little temporal variation for all stages of the composite event, while non-raining scenes and deeper clouds are modulated as a function of time for all stages. Instead of a "transition" from shallow to deep convection, the results suggest an "addition" of deep convection at the expense of non-raining scenes. Unique to the initiation stage, deep organized convective systems are rare until 1 - 5 days before the development of a convective anomaly that finally begins propagating eastward. The lack of deep convection during the initiation stage relative to other stages is associated with both decreased values of columnar water vapor (TPW) and increased stability in the lower-troposphere. Both are hypothesized to preclude the development of deeper convection, thus allowing for the slow (10-30 day) increase in TPW by evaporation to continue, in contrast to later stages of the MJO when moisture convergence serves as the largest contributor to moistening. The analyses described above are applied to output from a novel multiscale-modeling framework (MMF) coupled with a slab ocean model. The extent to which the MMF yields results similar to the observational depictions outlined above is discussed in great detail.