Browsing by Author "Maloney, Eric D., advisor"

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Open Access
A statistical prediction model for east Pacific and Atlantic tropical cyclone genesis
(Colorado State University. Libraries, 2012) Slade, Stephanie A., author; Maloney, Eric D., advisor; Thompson, David, committee member; Chong, Edwin, committee member
A statistical model is developed via multiple logistic regression for the prediction of weekly tropical cyclone activity over the East Pacific and Atlantic Ocean regions using data from 1975 to 2009. The predictors used in the model include a climatology of tropical cyclone genesis for each ocean basin, an El Niño-Southern Oscillation (ENSO) index derived from the first principal component of sea surface temperature over the Equatorial East Pacific region, and two indices representing the propagating Madden-Julian Oscillation (MJO). These predictors are suggested as useful for the prediction of East Pacific and Atlantic cyclogenesis based on previous work in the literature and are further confirmed in this study using basic statistics. Univariate logistic regression models are generated for each predictor in each region to ensure the choice of prediction scheme. Using all predictors, cross-validated hindcasts are developed out to a seven week forecast lead. A formal stepwise predictor selection procedure is implemented to select the predictors used in each region at each forecast lead. Brier skill scores and reliability diagrams are used to assess the skill and dependability of the models. Results show a significant increase in model skill at predicting tropical cyclogenesis by the inclusion of the MJO out to a three week forecast lead for the East Pacific and a two week forecast lead for the Atlantic. The importance of ENSO for Atlantic genesis prediction is highlighted, and the uncertain effects of ENSO on East Pacific tropical cyclogenesis are re-visited using the prediction scheme. Future work to extend the prediction model with other predictors is discussed.
Open Access
African easterly wave energetics on intraseasonal timescales
(Colorado State University. Libraries, 2014) Alaka, Ghassan J., Jr., author; Maloney, Eric D., advisor; Schubert, Wayne H., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee member
African easterly waves (AEWs) are synoptic-scale eddies that dominate North African weather in boreal summer. AEWs propagate westward with a maximum amplitude near 700 hPa and a period of 2.5-6-days. AEWs and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer, which directly impacts local agriculture and tropical cyclogenesis. This study performs a comprehensive analysis of the 30-90-day variability of AEWs and associated energetics using both reanalysis data and model output. Specifically, the PKE and perturbation available potential energy (PAPE) budgets are used to understand the factors that contribute to PKE maxima in West Africa and the extent to which these surges of AEW activity are modulated by the Madden-Julian oscillation (MJO). The role of the MJO in the intraseasonal variability of AEWs is assessed by comparing PKE sources as a function of an MJO index and a local 30-90-day West African PKE index. Since East Africa is an initiation zone for AEW activity and is modulated by the MJO, the relationship between this region and West Africa is a primary focus in this study. The intraseasonal variability of AEW energetics is first investigated in reanalysis products. While reanalysis data depicts a similar evolution of 30-90-day PKE anomalies in both the MJO and a local PKE index, the MJO index describes only a small (yet still significant) fraction of the local 30-90-day variance. In boreal summers with more significant MJO days, the correlation between the two indices is higher. Baroclinic energy conversions are important for the initiation of 30-90-day West African PKE events east of Lake Chad. In West Africa, both barotropic and baroclinic energy conversions maintain positive PKE anomalies before they propagate into the Atlantic. The primary role of diabatic heating is to destroy PAPE in a negative feedback to baroclinic energy conversions in West Africa. More frequent East Atlantic tropical cyclone generation is associated with positive PKE events than with negative PKE events. Easterly wave activity is then examined in a regional model. The Advanced Research Weather Research and Forecasting (WRF-ARW) simulates West African monsoon climatology more accurately than the WRF Nonhydrostatic Mesoscale Model (WRF-NMM). Although the WRF-NMM produces more realistic boreal summer rainfall than the WRF-ARW, it fails to accurately simulate the AEJ and other key West African monsoon features. Parameterizations within the WRF-ARW are scrutinized as well, with the WRF single-moment 6-class microphysics and the Noah land surface model outperforming Thompson microphysics and the RUC land surface model. Three ten-year WRF-ARW experiments are performed to investigate the role of external forcing on intraseasonal variability in West Africa. In addition to a control simulation, two sensitivity experiments remove 30-90-day variability from the boundary conditions (for all zonal wavenumbers and just for eastward zonal wavenumbers 0-10). Overall, intraseasonal variability of AEWs shows only modest differences after the removal of all 30-90-day input into the model boundary conditions. PKE and PAPE budgets reveal that simulated positive PKE events in West Africa are preceded by extensions of the AEJ into East Africa, which enhance barotropic and baroclinic energy conversions in this region. This jet extension is associated with warm lower-tropospheric temperature anomalies in the eastern Sahara. In West Africa, the amplitude of PKE and PAPE budget terms exhibit a similar evolution (even in the sensitivity experiments) as in the reanalysis products.
Open Access
An investigation of an east Pacific easterly wave genesis pathway and the impact of the Papagayo and Tehuantepec wind jets on the east Pacific mean state and easterly waves
(Colorado State University. Libraries, 2021) Whitaker, Justin W., author; Maloney, Eric D., advisor; Bell, Michael M., committee member; Rasmussen, Kristen L., committee member; Niemann, Jeffrey D., committee member
Part one of this dissertation investigates the transition of a Panama Bight mesoscale convective system (MCS) into the easterly wave (EW) that became Hurricane Carlotta (2012). Reanalysis, observations, and a convective-permitting Weather Research and Forecasting (WRF) model simulation are used to analyze the processes contributing to EW genesis. A vorticity budget analysis shows that convective coupling and vortex stretching are very important to the transition in this case, while horizontal advection is mostly responsible for the propagation of the system. In the model, the disturbance is dominated by stratiform vertical motion profiles and a mid-level vortex, while the system is less top-heavy and is characterized by more prominent low-level vorticity later in the transition in reanalysis. The developing disturbance starts its evolution as a mesoscale convective system in the Bight of Panama. Leading up to MCS formation the Chocó jet intensifies, and during the MCS to EW transition the Papagayo jet strengthens. Differences in the vertical structure of the system between reanalysis and the model suggest that the relatively more bottom-heavy disturbance in reanalysis may have stronger interactions with the Papagayo jet. Field observations like those collected during the Organization of Tropical East Pacific Convection (OTREC) campaign are needed to further our understanding of this east Pacific EW genesis pathway and the factors that influence it, including the important role for the vertical structure of the developing disturbances in the context of the vorticity budget. In parts two and three of this dissertation, the Weather Research and Forecasting (WRF) model is used to quantify the impact that the Papagayo and Tehuantepec wind jets have on the east Pacific mean state and east Pacific easterly waves. Specifically, a control run simulation is compared with a gaps filled simulation, where mountain gaps in the Central American mountains are "filled in" to block the Papagayo and Tehuantepec wind jets. In the absence of these wind jets, the northern half of the east Pacific mean state becomes drier, supporting a reduction in convective activity and precipitation there. Further, a 700 hPa positive vorticity feature that is linked to the Papagayo jet is reduced. An easterly wave tracking algorithm is developed and shows that easterly wave track density and genesis density are generally reduced in the eastern half of the basin for the gaps filled run. An eddy kinetic energy (EKE) budget is also calculated and highlights that EKE, barotropic conversion, and eddy available potential energy (EAPE) to EKE conversion all decrease for easterly waves when the wind jets are blocked. A composite analysis reveals that there are slight horizontal structural changes between waves in the simulations, while the waves have surprisingly similar strengths. Overall, the Papagayo and Tehuantepec wind jets are shown to be supportive influences on east Pacific easterly waves.
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
Errors of opportunity: using neural networks to predict errors in the unified forecast system (UFS) on S2S timescales
(Colorado State University. Libraries, 2023) Cahill, Jack, author; Barnes, Elizabeth A., advisor; Maloney, Eric D., advisor; Ross, Matthew, committee member
Making predictions of impactful weather on timescales of weeks to months (subseasonal to seasonal; S2S) in advance is incredibly challenging. Dynamical models often struggle to simulate tropical systems that evolve over multiple weeks such as the Madden Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO), and these errors can impact geopotential heights, precipitation, and other variables in the continental United States through teleconnections. While many data-driven S2S studies attempt to predict future midlatitude variables using current conditions, here we instead focus on post-processing of the National Oceanic and Atmospheric Association's (NOAA) Unified Forecast System (UFS) to predict UFS errors. Specifically, by looking at when/where there are errors in the UFS, neural networks can be used to understand what atmospheric conditions helped produce these errors via explainability methods. Our 'Errors of Opportunity' approach identifies phase 4 of the MJO and phases 1 and 2 of the BSISO as significant factors in aiding UFS error prediction across different regions and seasons. Specifically, we see high accuracy for underestimates of geopotential heights in the Pacific Northwest during Spring and as well as high accuracy for overestimates of geopotential heights in Northwest Mexico during Summer. Furthermore, we demonstrate enhanced error prediction skill for overestimates of Summer precipitation in the Midwest following BSISO phases 1 and 2. Most notably, our findings highlight that the identified errors stem from the UFS's failure to accurately forecast teleconnection patterns.
Open Access
Intraseasonal variability in the diurnal cycle of precipitation in the Philippines
(Colorado State University. Libraries, 2019) Natoli, Michael B., author; Maloney, Eric D., advisor; Bell, Michael M., committee member; Niemann, Jeffrey D., committee member
Precipitation in the region surrounding the South China Sea (SCS) over land and coastal waters exhibits a strong diurnal cycle associated with a land-sea temperature contrast that drives a sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of the daily mean precipitation rate and the amplitude of the diurnal cycle. Using 19 years of the CMORPH precipitation product for the Philippines, it is shown that in aggregate the diurnal cycle amplitude is maximized before the arrival of the broader oceanic convective envelope associated with the BSISO. Over Luzon Island in the northern Philippines, the diurnal cycle amplitude is not in phase with daily mean precipitation, which peaks with the large-scale BSISO convection. An increase in nocturnal and morning precipitation more than compensates for the reduced precipitation rates during the afternoon peak amidst the BSISO active period. This pattern is not seen over Mindanao Island in the southern Philippines, where diurnal cycle amplitude tends to determine daily mean precipitation. A strong diurnal cycle in coastal waters west of the Philippines is evident in the transition from the inactive to active phase, due to offshore propagation of convection generated over land. This behavior is dramatically different on small spatial scales within the Philippine archipelago, depending strongly on topography. For example, the BSISO influence on the diurnal cycle on the eastern side of the high mountains of Luzon is nearly opposite to the western side. It is proposed, using wind, moisture, and radiation budget products from the ERA-Interim reanalysis, that the enhanced diurnal cycle over land and coastal waters west of the mountains during BSISO suppressed phases is a consequence of increased insolation and weaker prevailing onshore winds. Offshore propagation, and thus the diurnal cycle over the coastal waters of the SCS, is suppressed until ambient mid-level moisture increases during the transition to the active BSISO phase. In the BSISO enhanced phases, strong low level winds out of the southwest combine with increased cloudiness to suppress the sea-breeze circulation and thus the diurnal cycle of precipitation in the SCS region. Strong frictional moisture convergence leading the BSISO is not found to be concurrent with the peak in the diurnal cycle. Results are consistent when examined in other precipitation products or BSISO indices, and support conclusions derived from studies focusing on intraseasonal modulation of precipitation in other regions of the Maritime Continent, with some important local distinctions owed to geography.
Open Access
Intraseasonal variability of the west African monsoon and African easterly waves during boreal summer
(Colorado State University. Libraries, 2010) Alaka, Ghassan J., Jr., author; Maloney, Eric D., advisor; Schubert, Wayne H., committee member; Venayagamoorthy, Subhas, committee member
Substantial subseasonal variability in African easterly wave (AEW) activity and cyclogenesis frequency occurs in the main hurricane development region of the Atlantic during boreal summer. A complete understanding of intraseasonal variability in the Atlantic and west Africa during boreal summer requires analysis of how the Madden-Julian Oscillation (MJO) modulates the west African monsoon and consequently AEWs. Because the MJO is predictable a few weeks in advance, understanding how and why the MJO impacts the west African monsoon may have a profound influence on Atlantic tropical cyclone prediction. This study documents the MJO influence on the west African monsoon system during boreal summer using a variety of reanalysis and satellite datasets. This study aims to identify and explain the MJO teleconnection to the west African monsoon, and the processes that induce precipitation and AEW variability in this region. Intraseasonal west African and Atlantic convective anomalies on 30-90 day timescales are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and west Pacific by the MJO. Previous studies have hypothesized that an area including the Darfur mountains and the Ethiopian highlands is an initiation region for AEWs. It is shown here that the initial MJO influence on precipitation and AEW activity in the African monsoon appears to occur in these regions, where eddy kinetic energy (EKE) anomalies first appear in advance of MJO-induced periods of enhanced and suppressed AEW activity. In the initiation region, upper tropospheric temperature anomalies are reduced, the atmosphere moistens by horizontal advection, and an eastward extension of the African easterly jet occurs in advance of the MJO wet phase of the African monsoon, when AEW activity is also enhanced. These factors all support strong precursor disturbances in the initiation region that seed the African easterly jet and contribute to downstream development of AEWs. Opposite behavior occurs in advance of the MJO dry phase. Moisture and eddy kinetic energy (EKE) budgets are examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the initiation region by anomalous flow acting across the mean moisture gradient. Additionally, positive (negative) upstream EKE tendency anomalies in advance of the MJO convective maximum (minimum) over tropical north Africa suggest wave growth (decay) near the entrance of the AEJ, while enhanced (suppressed) conversion of eddy available potential energy (EAPE) to EKE and barotropic conversion maintains downstream AEW growth (decay).
Open Access
Madden-Julian oscillation teleconnections and their influence on Northern Hemisphere winter blocking
(Colorado State University. Libraries, 2017) Henderson, Stephanie A., author; Maloney, Eric D., advisor; Barnes, Elizabeth A., committee member; Thompson, David W. J., committee member; Chong, Edwin K. P., committee member
Winter blocking events are characterized by persistent and quasi-stationary patterns that re-direct precipitation and air masses, leading to long-lasting extreme winter weather. Studies have shown that the teleconnection patterns forced by the primary mode of tropical intraseasonal variability, the Madden-Julian Oscillation (MJO), influence extratropical factors associated with blocking, such as the North Atlantic Oscillation. However, the influence of the MJO on winter blocking is not well understood. Understanding this relationship may improve the mid-range forecasting of winter blocking and the associated weather extremes. The impact of the MJO on Northern Hemisphere winter blocking is examined using a two-dimensional blocking index. Results suggest that all MJO phases demonstrate significant changes in west and central Pacific high-latitude blocking. East Pacific and Atlantic blocking are significantly suppressed following phase 3 of the MJO, characterized by anomalous convection in the tropical East Indian Ocean and suppressed convection in the west Pacific. A significant increase in east Pacific and Atlantic blocking follows the opposite-signed MJO heating during MJO phase 7. Over Europe, blocking is suppressed following MJO phase 4 and significantly increased after MJO phase 6. Results suggest that the European blocking increase may be due to two precursors: 1) a pre-existing anomalous Atlantic anticyclone, and 2) a negative Pacific North American (PNA) pattern triggered by the MJO. The influence of the MJO on winter blocking may be different if a change occurs to the basic state and/or MJO heating, such as during El Niño – Southern Oscillation (ENSO) events. MJO teleconnections during ENSO events are examined using composite analysis and a nonlinear baroclinic model and their influence of winter high-latitude blocking is discussed. Results demonstrate that the ENSO-altered MJO teleconnection patterns significantly influence Pacific and Atlantic blocking and the impacts depend on ENSO phase. During El Niño, Pacific and Atlantic blocking is significantly increased following MJO phase 7, with maximum Atlantic blocking frequency anomalies reaching triple the climatological winter mean blocking frequency. Results suggest that the MJO forces the initial anomalous Atlantic dipole associated with the blocking increase, and transient eddy activity aids in its persistence. During La Niña, significant changes to high-latitude blocking are mostly observed during the first half of an MJO event, with significant suppression of Pacific and Atlantic blocking following MJO phase 3. MJO teleconnection patterns may also be altered by basic state and MJO heating biases in General Circulation Models (GCMs), important for mid-range forecasting and future climate studies of weather and climate patterns significantly altered by the MJO, such as winter blocking. Data from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is used to investigate MJO teleconnection biases due to basic state and MJO biases, and a linear baroclinic model is used to interpret the results. Results indicate that poor basic state GCMs (but with a good MJO) can have equally poor skill in simulating the MJO teleconnection patterns as GCMs with a poor MJO. Large biases in MJO teleconnection patterns occur in GCMs with a zonally extended Pacific subtropical jet relative to reanalysis. In good MJO GCMs, bias in the location and horizontal structure of Indo-Pacific MJO heating is found to have modest impacts on MJO teleconnection patterns. However, East Pacific heating during MJO events can influence MJO teleconnection amplitude and the pathways over North America. Results suggest that both the MJO and the basic state must be well represented in order to properly capture the MJO teleconnection patterns.
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
The effect of projected sea surface temperature change on MJO activity in a warmer climate
(Colorado State University. Libraries, 2023) Bowden, Amanda Francine Marie, author; Maloney, Eric D., advisor; Hurrell, Jim, committee member; Ross, Matthew, committee member
The Madden Julian Oscillation (MJO) consists of a convective region that propagates eastward in the tropics on repeat every 30-90 days with peak amplitude during the Boreal Winter (November - March). Since the MJO modulates extreme weather such as tropical cyclones, atmospheric rivers, and monsoon variability, future MJO changes in a warmer climate have implications for prediction of extreme events. Understanding precipitation pattern changes in a changing climate is critical for fresh-water resources and societal planning for oceanic regions. Decadal variability in the climate system causes patterns of sea surface temperature (SST) change in the tropical Pacific and associated precipitation, humidity, and wind pattern changes to vary from one decade to the next. MJO changes are strongly dependent on the pattern of SST change, and so understanding uncertainty in MJO change in future decades in the context of this decadal variability is the primary motivation for this investigation. Since climate models contain climate variability on decadal timescales, different initial conditions across ensemble members can result in diverse projection outcomes in any given decade. This investigation examines the impact of projected SST and moisture pattern changes over the 21st Century on MJO precipitation and zonal wind (850 mb) amplitude changes using 80 members with the SSP370 radiative forcing scenario from the Community Earth System Model 2 (CESM2) Large Ensemble. The projected SST and moisture pattern changes can be weighted more toward the central or eastern equatorial Pacific in earlier parts of the 21st Century across ensemble members, although becomes strongly El Niño-like later in the century. Ensemble members with stronger MJO precipitation amplitude in a given period are characterized by stronger El Niño-like east Pacific warming, associated with a strengthened meridional moisture gradient. As interpreted through moisture mode theory, greater east Pacific warming supports a stronger MJO by enhancing propagation through a stronger meridional moisture gradient, and enhancing MJO amplitude through a stronger vertical moisture gradient. The investigation supports the hypothesis that projected SST and moisture pattern changes influence MJO activity, and also highlights the importance of understanding decadal climate variability for interpreting changes in water resources of oceanic regions.
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 moisture-convection feedbacks in simulating the intraseasonal oscillation
(Colorado State University. Libraries, 2009) Hannah, Walter, author; Maloney, Eric D., advisor; Randall, David A. (David Allan), 1948-, committee member; Venayagamoorthy, Subhas Karan, committee member
The sensitivity of the intraseasonal oscillation (ISO) in the National Center for Atmospheric Research's (NCAR) Community Atmosphere Model (CAM) version 3.1 with Relaxed Arakawa-Schubert (RAS) convection modified with the moisture trigger of Tokioka et al. (1988) is analyzed with respect to changes to the specified minimum entrainment rate. Implementation of the Tokioka moisture trigger results in a drier and cooler troposphere due to the suppression of deep convection. A higher minimum entrainment threshold leads to more suppressed deep convection and improves the sensitivity of convection to free tropospheric humidity. This is accompanied by enhanced intraseasonal variability in the tropics. The simulated ISO which results from a non-zero minimum entrainment rate resembles a moisture mode. Variance of the column integrated saturation fraction is increased when minimum entrainment rate is increased, and precipitation becomes an increasingly non-linear function of saturation fraction which indicates that moisture-convection feedbacks are enhanced in the model. A reduction in the mean column moist static energy export by divergent motions indicates that the simulations with non-zero minimum entrainment thresholds are able to achieve negative gross moist stability which has been suggested as a necessary condition to be able to produce a moisture mode. This decrease in gross moist stability with increased minimum entrainment rate is accompanied by a lowering of the mean diabatic heating profile maximum. Additional simulations are analyzed to investigate the impact of a rain re-evaporation fraction parameter on the simulated ISO. A higher rain re-evaporation fraction leads a stronger ISO signal in the model. However, In contrast to the effect of increased minimum entrainment rate, increased rain re-evaporation fraction yields a mean state which is warmer and moister. This discrepancy in mean state humidity change indicates that intraseasonal variability has no unique dependence on basic state humidity, in contradiction to that suggested in previous studies.