Browsing by Author "Maloney, Eric, advisor"
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Item Open Access Data-driven models for subseasonal cyclogenesis forecasts in the east Pacific and north Atlantic(Colorado State University. Libraries, 2024) Carlo Frontera, Zaibeth, author; Barnes, Elizabeth A., advisor; Maloney, Eric, advisor; Anderson, G. Brooke, committee memberTropical cyclones (TCs) are hazardous and financially burdensome meteorological events. Previous studies have revealed that longer timescale phenomena, including the El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), and African Easterly Waves, influence TC development by modifying large-scale environmental conditions such as vertical wind shear, mid-level moisture, and sea surface temperatures. Statistical models have been developed to forecast TCs in the Atlantic and Pacific basins by incorporating information about ENSO and the MJO. Expanding on this work, we employ logistic regression (LR) and neural network (NN) models with an extended set of variables to predict cyclogenesis on subseasonal timescales for the east Pacific and Atlantic regions. These models utilize ENSO and MJO indices, along with other local environmental information, and demonstrate enhanced forecasting skill relative to models that only use TC climatology. Overall, the NN model shows superior performance compared to the LR model, retaining skill out to three weeks leadtime for the east Pacific, and out to four weeks for the Atlantic basin. The predictive capabilities of the model are demonstrated for the years 1983 and 2021. To gain insights into the decision-making process of the NN models, an AI explainability technique is employed to understand which features are considered important in making the predictions. For both basins, the addition of ENSO and MJO information prove to be essential for the superior forecast skill of the NN model.Item Open Access Influence of the Madden-Julian Oscillation and Caribbean Low-Level Jet on east Pacific easterly waves(Colorado State University. Libraries, 2018) Whitaker, Justin W., author; Maloney, Eric, advisor; Bell, Michael, committee member; Niemann, Jeffrey, committee memberThe east Pacific warm pool exhibits basic state variability associated with the Madden-Julian Oscillation (MJO) and Caribbean Low-Level Jet (CLLJ), which affects the development of easterly waves (EWs). This study compares and contrasts composite changes in the background environment, eddy kinetic energy (EKE) budgets, moisture budgets, and EW tracks during MJO and CLLJ events. While previous studies have shown that the MJO influences jet activity in the east Pacific, the influence of the MJO and CLLJ on EWs is not synonymous. The CLLJ is a stronger modulator of the ITCZ than the MJO, while the MJO has a more expansive influence on the northeastern portion of the basin. Anomalous low-level westerly MJO and CLLJ periods are associated with favorable conditions for EW development paralleling the Central American coast, contrary to previous findings about the relationship of the CLLJ to EWs. Easterly MJO and CLLJ periods support enhanced ITCZ EW development, although the CLLJ is a greater modulator of EW tracks in this region, which is likely associated with stronger moisture and convection variations and their subsequent influence on the EKE budget. ITCZ EW growth during easterly MJO periods is more reliant on barotropic conversion than in strong CLLJ periods, when EAPE to EKE conversion associated with ITCZ convection is more important. Enhanced background state moisture anomalies during strong CLLJ periods lead to stronger diluted CAPE anomalies in the mean state and EWs that support convection. Thus, the influence of these phenomena on east Pacific EWs should be considered distinct.1 1 This abstract is adapted from the abstract of: Whitaker, J.W., and E. D. Maloney, 2018: Influence of the Madden-Julian Oscillation and Caribbean Low-Level Jet on East Pacific Easterly Wave Dynamics. J. Atmos. Sci., in press. ©American Meteorological Society. Used with permission.Item Open Access Initation and intensification of east Pacific easterly waves(Colorado State University. Libraries, 2015) Rydbeck, Adam V., author; Maloney, Eric, advisor; Johnson, Richard, committee member; Birner, Thomas, committee member; Niemann, Jeffrey, committee memberThe background atmospheric state of the east Pacific (EPAC) warm pool in which easterly waves (EWs) develop varies dramatically on intraseasonal time scales. EPAC intraseasonal variability is well known to modulate local convective and circulation patterns. Westerly intraseasonal phases are associated with westerly lowlevel wind and positive convective anomalies and easterly intraseasonal phases are associated with easterly low-level wind and negative convective anomalies. This study first investigates the perturbation available potential energy (PAPE) and perturbation kinetic energy (PKE) budgets of easterly waves composited during westerly, easterly, and neutral intraseasonal phases, respectively. During neutral and westerly intraseasonal phases, the generation of PAPE associated with perturbation diabatic heating that is subsequently converted to PKE is enhanced and is the dominant energy source for EWs. EWs draw energy from low-level barotropic conversion, regardless of phase. A novel and previously unrecognized result is the detection of strong barotropic generation of PKE at midlevels during westerly intraseasonal phases. This previously unidentified source of PKE at midlevels is in part due to strong intraseasonal modulation of the background midlevel winds. Processes associated with the local amplification of EWs in the EPAC warm pool are then explored. Developing EWs favor convection in the southwest and northeast quadrants of the disturbance. In nascent EWs, convection favors the southwest quadrant. In these quadrants, lower tropospheric vorticity is generated locally through vertical stretching that supports a horizontal tilt of the wave from the southwest to the northeast. EWs with such tilts are then able to draw energy via barotropic conversion from the background cyclonic zonal wind shear present in the east Pacific. EWs during westerly and neutral intraseasonal periods are associated with robust convection anomalies. Easterly intraseasonal periods are, at times, associated with very weak EW convection anomalies due to weaker moisture and diluted CAPE variations. The in-situ generation of EWs in the EPAC is then investigated using the Weather Research and Forecasting Model (WRF). Sensitivity tests are performed to examine the atmospheric response to the removal of external and internal EW forcing in the EPAC warm pool. External forcing of EPAC EWs is removed by filtering EWs in wavenumber frequency space from the model’s boundary forcing. Internal forcing of EWs is removed by reducing the terrain height in portions of Central and South America to suppress the strong source of diurnal convective variability in the Panama Bight. These regions of high terrain are associated with mesoscale convective systems that routinely initiate in the early morning and propagate westward into the EPAC warm pool. In both sensitivity tests, EW variance is significantly reduced in the EPAC, suggesting that both EWs propagating into the EPAC from the east and EWs generated locally in association with higher frequency convective disturbances are critical to EPAC EW variability. A new mechanism is proposed to explain the in-situ generation of EPAC EWs. Serial mid-level diurnal vorticity and divergence anomalies generated in association with deep convection originating in the Panama Bight underpin the local generation, intensification, and spatial scale selection of EW vorticity by vertical vorticity stretching. Diurnal vorticity anomalies in the Panama Bight are able to initiate disturbances capable of growing into robust EWs through a tendency to organize vorticity upscale.Item Open Access Moist static energy and the Madden-Julian oscillation: understanding initiation, maintenance and propagation through the application of novel diagnostics(Colorado State University. Libraries, 2014) Wolding, Brandon, author; Maloney, Eric, advisor; Johnson, Richard, committee member; Cooley, Dan, committee memberAs the dominant mode of tropical intraseasonal variability, the Madden-Julian Oscillation (MJO) has enormous societal impacts. Despite four decades of research motivated by these impacts, the processes that drive the initiation, maintenance and propagation of the MJO are still poorly understood. The development of large scale moisture anomalies plays an important role in many recent theories of the MJO, including moisture mode theory. This study identifies processes that support the development, maintenance and propagation of moisture anomalies associated with the MJO. A new set of objective MJO diagnostics, obtained as an extension of CEOF analysis, are introduced. These diagnostics provide useful measures of previously overlooked information yielded by CEOF analysis, including an objective measure that allows geographically disparate locations to be compared and contrasted throughout a reference MJO lifecycle. Compositing techniques based on this measure are applied to the MJO in an attempt to determine key physical processes affecting the MSE budget, identify prominent geographical variability of these processes, and highlight changes in the mean state winds and moisture field that explain this variability. The MSE budget reveals that variations in MSE associated with the MJO are largely the result of variations in column integrated moisture content (~90%), the majority of which occur between 850-500 hPa (~75%). Easterly(westerly) low level wind anomalies to the east(west) of the MJO result in a reduction(enhancement) of drying due to horizontal advection, which is only partially offset by a reduction(enhancement) of surface latent heat flux. In the deep tropics (5°N-5°S) of the eastern hemisphere, anomalous horizontal advection is primarily the result of the anomalous winds acting on the mean state moisture gradient. Over the broader tropics (15°N-15°S), the anomalous horizontal advection appears to result primarily from the modulation of synoptic scale eddy activity. The incomplete cancelation that occurs between anomalous horizontal advection and anomalous surface latent heat flux allows for the enhancement(reduction) of MSE to the east(west) of the MJO, enhancing(reducing) convection and helping drive propagation of the MJO. Anomalous vertical moisture advection is the primary process maintaining moisture and MSE anomalies against dissipation by anomalous precipitation throughout the MJO lifecycle. Anomalously positive(negative) vertical moisture advection appears to slightly exceed anomalous precipitation during periods of enhanced(suppressed) convection, suggesting a potential positive feedback that could act to destabilize the MJO. Geographical changes in the MSE budget of the MJO are primarily associated with changes in the mean state winds and the mean state moisture gradient. These results suggest that MJO convective anomalies are maintained by anomalous vertical moisture advection, and that propagation of these convective anomalies results from the large scale asymmetrical dynamical response to equatorial heating occurring in a specific arrangement of mean state winds and mean moisture gradient. The findings of this study support the hypothesis that the MJO is a moisture mode.Item Open Access Remote versus local controls of east Pacific intraseasonal variability(Colorado State University. Libraries, 2012) Rydbeck, Adam, author; Maloney, Eric, advisor; Birner, Thomas, committee member; Niemann, Jeffrey, committee memberThe Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability and propagates eastward at 5 m/s with primary signals in wind and precipitation. During boreal summer, interactions between intraseasonal variability in the eastern Hemisphere and the east Pacific warm pool are often described as a local amplification of the propagating MJO. However, the precise mechanism by and degree to which intraseasonal variability in the eastern Hemisphere affects the east Pacific warm pool are not well understood. One school of thought holds that the MJO initiates a dry intraseasonal Kelvin wave response in the west Pacific that rapidly propagates into the Western Hemisphere and initiates intraseasonal convective variability there. To quantify the relationship between the source (Eastern Hemisphere) and amplification region (east Pacific warm pool), sensitivity tests in two separate models are used to determine the importance of local versus remote controls of east Pacific warm pool intraseasonal variability. The two models include the National Center for Atmospheric Research Community Atmosphere Model 3 (CAM3) and the International Pacific Research Center Regional Atmosphere Model (IRAM). The two models use different schemes to isolate the east Pacific from eastward-propagating intraseasonal variability that impinges from the west. Removing the influence of the MJO on the east Pacific warm pool in these two models reveals different insights into local versus remote control of intraseasonal variability in the east Pacific. The CAM3 produces comparable intraseasonal variability in winds and precipitation in the east Pacific when Kelvin wave signals from the west are removed, suggesting that the Eastern Hemisphere MJO helps to pace east Pacific intraseasonal variability, although east Pacific variability can exist in isolation from the MJO. Thus, the CAM3 supports independent intraseasonal variability in the east Pacific warm pool that may be phase locked to intraseasonal variability in the Eastern Hemisphere in observations. However, the IRAM has very small east Pacific intraseasonal variability when isolated from global MJO signals. The weak intraseasonal variability in IRAM may be a result of mean low-level wind biases that cause 30 to 90 day surface flux anomalies to be out of phase with 30 to 90 day precipitation and low level wind anomalies. As a result, the IRAM model does not support an independent local mode of intraseasonal variability in the east Pacific.Item Open Access Surface heat fluxes and MJO propagation through the Maritime Continent(Colorado State University. Libraries, 2022) Hudson, Justin, author; Maloney, Eric, advisor; Rasmussen, Kristen, committee member; Rugenstein, Jeremy, committee memberThe 'barrier effect' of the Maritime Continent (MC) is a known hurdle in understanding the propagation of the Madden-Julian Oscillation (MJO). To understand the differing dynamics of MJO events that propagate versus stall over the MC, a new MJO tracking algorithm utilizing 30-96 day filtered NOAA Interpolated OLR anomalies is presented. Using this algorithm, MJO events can be identified, tracked, and described in terms of their propagation characteristics. Latent heat flux from CYGNSS and OAFLUX as well as CYGNSS surface winds are used to compare large-scale patterns for MJO events that do and do not propagate through the MC. Local area-averaged surface fluxes and OLR anomalies are 7-14% and 18-22% of the value of precipitation anomalies, respectively. While differences in these contributions do not change substantially for propagating versus terminating events, precipitation events that successfully propagate through the MC demonstrate surface flux anomalies that are stronger and more spatially-coherent. The spatial scale of precipitation events that propagate through the MC region is also larger than terminating events. It is also shown that large-scale enhancement of latent heat fluxes near and to the east of the Dateline accompanies MJO events that successfully propagate through the MC. This large-scale enhancement of latent heat fluxes to the east of the Dateline is equally driven by dynamic and thermodynamic effects. These findings are placed in the context of recent theoretical models of the MJO in which latent heat fluxes are important for propagation and destabilization. The tracking algorithm is also used to show for historical and greenhouse gas warming scenarios in CESM2 that MJO propagation speed increases and precipitation anomalies propagate further east with warming. However, the CESM2 inadequately represents the 'barrier effect' of the MC region on propagating MJO events.Item Open Access The impact of tropical intraseasonal variability on subseasonal-to-seasonal predictability(Colorado State University. Libraries, 2021) Hsiao, Wei-Ting, author; Maloney, Eric, advisor; Barnes, Elizabeth, advisor; Mueller, Nathan, committee memberSubseasonal-to-seasonal (S2S) timescales have been identified as a gap in weather forecast skill at 2 weeks to 2 months lead times. This timescale is set by midlatitude synoptic predictability limits, and sits between the typical weather timescale and the longer annual to interannual periods that may have skill due to knowledge of low-frequency phenomena such as El Niño-Southern Oscillation (ENSO). Previous studies have shown that tropical intraseasonal variability serves as an important source of S2S predictability in the midlatitudes based on a linear Rossby wave theory. The theory suggests that consistent weather patterns are excited by tropical divergence and associated teleconnections to the extratropics on S2S timescales that influence predictability. However, those physical processes that provide sources of S2S forecast skill have yet to be fully characterized. This thesis examines aspects of tropical intraseasonal variability that are important for S2S prediction, including how tropical intraseasonal variability has changed with warming over the last century and how the misrepresentation of such variability in a weather forecast model leads to errors in midlatitude precipitation S2S forecasts. In the first part of this thesis, three reanalyses datasets (ERA5, MERRA-2, and ERA 20-C) are examined to quantify the amplitude changes in a dominant mode of intraseasonal tropical variability, the Madden-Julian oscillation (MJO), over the last century. MJO-associated precipitation and vertical velocity amplitude are found to exhibit a complex evolution over the observational record, where the precipitation has larger increases than the vertical velocity. A decrease in the ratio of MJO circulation to precipitation anomaly amplitude is detected over the observational period. Tropical weak temperature gradient theory is used to show that this decrease is consistent with the change in tropical dry static stability that has occurred under climate warming. The weakening MJO circulation per unit precipitation over the past century may have modified associated teleconnections and has implications for S2S prediction in the tropics and midlatitudes. In the second part of the thesis, emphasis is placed on understanding S2S precipitation forecast errors for the western United States (U.S.) in an operational weather model. A set of hindcasts during boreal winter, where the tropics are nudged toward reanalysis, is compared to hindcasts without nudging. The western U.S. precipitation forecasts are found to improve with nudging at 3-4 week lead times. Using a multivariate k-means clustering method, hindcasts are grouped by their initial states and one cluster that exhibits an initially strong Aleutian Low is found to provide better forecast improvement. The improvement originates from the poor representation in the non-nudged hindcasts of the destructive interference between (1) the anomalous Aleutian Low and (2) the teleconnection pattern generated by certain phases of the MJO during non-cold ENSO conditions. These results suggest that improving the simulation of tropical intraseasonal precipitation during the early MJO phases under non-cold ENSO may lead to better 3-4 week precipitation forecasts in the western U.S.Item Open Access Tropical deep convection, entrainment, and dilution during the DYNAMO field campaign(Colorado State University. Libraries, 2014) Hannah, Walter, author; Maloney, Eric, advisor; Randall, David, committee member; Johnson, Richard, committee member; Venayagamoorthy, Karan, committee memberThis dissertation presents a study of outstanding questions in tropical meteorology relating to tropical deep convection, entrainment, and dilution. Much of the discussion in this study will focus on an important convectively-coupled phenomenon in the tropical atmosphere known as the Madden-Julian Oscillation (MJO), which is an eastward propagating atmospheric disturbance over the Indian and West Pacific Oceans that dominates the tropical variability on intraseasonal timescales (30-90 days). The MJO is most active during the Northern Hemisphere winter season and is characterized by alternating periods of enhanced and suppressed convective activity. A field campaign known as the "Dynamics of the MJO" (DYNAMO) was conducted in the boreal winter months from October 2011 through February 2012 to study the initialization of the MJO with in-situ observations. The first part of this study examines hindcast simulations of the first two MJO events during DYNAMO in a general circulation model (GCM). The model used for this is the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM5) version 5, which uses parameterized convection. In these simulations, an entrainment rate parameter is varied to test its effects on the representation of the MJO, following previous studies. Hindcast simulations with CAM5 reveal that the entrainment parameter can improve the representation of the MJO. However, analysis of the column integrated moist static energy (MSE) budget reveals that this improvement is the right answer for the wrong reason. CAM5 incorrectly enhances vertical MSE advection, which compensates for cloud radiative feedbacks that are too weak. A promising theory for the MJOs fundamental dynamics is that of a moisture mode. The gross moist stability (GMS) describes the ratio of advective MSE import to a measure of convective activity. Negative GMS, and specifically the vertical component of GMS (VGMS), is thought to be a necessary condition for the destabilization of a moisture mode. In CAM5, VGMS becomes negative when the entrainment parameter is increased, indicating that the model can more easily destabilize moisture modes. However, this is inconsistent with re-analysis data, which exhibits positive VGMS. The second part of the study examines hindcasts using the super-parameterized version of CAM5 (SP-CAM) that uses embedded cloud-resolving models (CRM) to explicitly simulate convection on the sub-grid scale. SP-CAM was used for these hindcast simulations because previous studies have shown this type of model can reproduce the MJO much better than conventional GCMs. SP-CAM hindcasts yield a more robust MJO representation than CAM5, as expected. SP-CAM has an interesting systematic drift away from the initial conditions that projects well on the Real-time Multivariate MJO index (RMM), which negatively impacts the RMM skill scores. Analysis of the column MSE budget shows that SP-CAM has more realistic cloud-radiative feedbacks when compared to CAM5. SP-CAM also has a bias towards stronger import by vertical MSE advection that is similar CAM5 and inconsistent with re-analysis data. VGMS in SP-CAM is also found to be negative, which is similar to CAM5 and inconsistent with re-analysis data. The results from the first part of this study highlight a paradox surrounding entrainment. Although, previous studies have shown that entrainment rates should be larger than typical values used in parameterizations, increasing the entrainment rate does not make global model simulations more realistic. This prompted a detailed investigation into entrainment processes in high-resolution CRM simulations. A series of simulations are conducted where deep convection is initiated with a warm humid bubble. The bubble simulations are compared to a more realistic field of deep convection driven by forcing derived from the DYNAMO northern sounding array data. Entrainment and detrainment are found to be associated with toroidal circulations, consistent with recent studies. Analysis of the directly measured fractional entrainment rates does support an inverse relationship between entrainment and cloud radius, as is often assumed in simple models of deep convection. A method for quantifying the dilution by entrainment is developed and tested. Dilution and entrainment are generally not synonymous, but dilution is found to have a weak inverse relationship to cloud core radius. Sensitivity experiments show that entrainment and total water dilution are enhanced with environmental humidity is increased, contrary to the assumptions of some parameterizations. More vigorous convection in a more humid environment is better explained by a reduction of buoyancy dilution. An additional sensitivity experiment shows that entrainment and dilution are enhanced when convection is organized by the presence of vertical wind shear. The enhanced dilution is associated with entrainment of drier air on average.Item Open Access Vertically resolved weak temperature gradient analysis of the Madden-Julian Oscillation(Colorado State University. Libraries, 2017) Wolding, Brandon, author; Maloney, Eric, advisor; Randall, David, committee member; van den Heever, Susan, committee member; Kiladis, George, committee member; Ham, Jay, committee memberInteractions between moisture, convection, and large-scale circulations are thought to play an important role in destabilizing the Madden-Julian Oscillation (MJO). A simplified framework for understanding such interactions is developed, building upon the work of Chikira (2014). Tropical weak temperature gradient (WTG) balance is used to diagnose intraseasonal variations in large-scale vertical velocity from variations in apparent heating, allowing intraseasonal variations in large-scale vertical moisture advection to be decomposed into contributions from various apparent heating processes (e.g. radiative heating, microphysical processes). The WTG diagnosis captures the vertical structure and magnitude of large-scale vertical velocity and vertical moisture advection with exceptional accuracy throughout the free troposphere. Moisture and moisture variance budgets are used to investigate the MJO in ERA-interim (ERAi) reanalysis and the Superparameterized Community Earth System Model (SP-CESM). Moisture budgets indicate that, during the enhanced phase of the MJO, anomalous moistening by large-scale vertical moisture advection exceeds anomalous drying by microphysical processes and sub-grid scale (SGS) eddy fluxes, such that the net effect of these large and opposing processes (hereafter the column process) is to further moisten regions that are anomalously moist. Moisture variance budgets indicate that the column process helps grow moisture variance, acting to destabilize the MJO. Horizontal advective damping of moisture variance, associated with the modulation of higher frequency convective variability on intraseasonal timescales, acts to stabilize the MJO. The vertically resolved WTG balance framework is used to assess the contribution various apparent heating processes make to the column process, and its ability to destabilize the MJO. Intraseasonal variations in longwave radiative heating enhance variations in large-scale vertical moisture advection at low and mid levels, strongly supporting destabilization of the MJO in both ERAi and SP-CESM. The effect of convection alone (i.e. without radiative and surface flux feedbacks) is to weakly grow moisture variance in SP-CESM, and weakly damp moisture variance in ERAi, suggesting that the MJO is unrealistically unstable in the former. Surface flux feedbacks appear to play a more important role in destabilizing the real world MJO. Moisture variance budget analysis of periods of weak, moderate, and strong MJO activity suggests that changes in the vertical structure of apparent heating do not play a dominant role in limiting the amplitude of the MJO in SP-CESM in the current climate. WTG balance provides a useful framework for investigating how the MJO, and its impacts, may change as the climate system warms. Two simulations of SP-CESM, one at pre-industrial levels of CO2 (280 ppm, hereafter PI) and one where CO2 levels have been quadrupled (1120 ppm, hereafter 4xCO2), were analyzed. MJO convective variability increases considerably in the 4xCO2 simulation, a consequence of more favorable mean state moist thermodynamic conditions. A steepened mean state vertical moisture gradient allows MJO convective heating to drive stronger variations in large-scale vertical moisture advection, helping to support enhanced MJO convective variability in the 4xCO2 simulation. The dynamical response to MJO convective heating weakens in the warmer climate, a result of increased tropical static stability. One consequence of this weakened dynamical response is that the MJO's ability to influence the extratopics, which is closely tied to the strength of its associated divergence, is reduced considerably in the 4xCO2 simulation.