Interactions between the Madden-Julian oscillation and mesoscale to global scale phenomena

2019 Summer.Includes bibliographical references.The Madden-Julian Oscillation (MJO) influences and interacts with atmospheric phenomena across the globe, from the tropics to the poles. In this two-part study, the interactions of the MJO with other phenomena across a broad range of scales are considered, including mesoscale convective structures within the tropics and global teleconnection patterns. While the two studies are distinct in the scales of the interactions they discuss, each highlights an aspect of the importance of interactions between the MJO and variability across a broad range of scales within the climate system. The study of such cross-scale interactions is important for understanding our climate system, as these interactions can transfer energy between phenomena of starkly different spatial and temporal scales. Part one of the study uses a cloud-resolving model, the Regional Atmospheric Modeling System, to consider the relationship between mesoscale convective structures within the Indo-Pacific region and the regional, intraseasonal anomalies associated with the MJO. The simulation captures the entirety of a canonical boreal summertime MJO event, spanning 45 days in July and August of 2016, during which the convective anomaly associated with the MJO propagated over the Maritime Continent. The convective cloud structures, or cells, within the simulation were tracked and logged according to their location relative to the regional convective anomaly of the MJO. Using both spectral analysis and phase compositing, it was found that a progressive relationship exists between the boreal summertime MJO and mesoscale deep convective structures within the Indo-Pacific region, specifically within the convectively enhanced region of the MJO, as follows: increased cell longevity in the initial phases of the MJO, followed by increased cell number in the intermediate phases, progressing into increased cell expanse in the terminal phases. This progressive relationship is connected back to the low-frequency atmospheric response to the MJO. It is suggested that the bulk thermodynamic and kinematic anomalies of the MJO are closely related to the convective cell expanse and longevity, although the number of convective cells appears to be tied to another source of variability not identified within this study. These findings emphasize that while the MJO is commonly defined as an intraseasonal-scale convective anomaly, it is also intrinsically tied to the mesoscale variability of the convective systems that constitute its existence. The second part of the study quantifies the prevalence of the MJO within the overall climate system, along with the dependence of its teleconnections on variability in another tropical phenomena on a larger scale than itself. It is well known that the MJO exhibits pronounced seasonality in its tropical and global signature, and recent research has suggested that its tropical structure also depends on the state of the Quasi-Biennial Oscillation (QBO). We therefore first quantify the relationship between 300-mb geopotential anomalies and the MJO across the globe, then test the dependence of the relationship on both the meteorological season and the QBO phase using a derivative of cross-spectral analysis, magnitude-squared coherence Coh2. It is found that the global upper-tropospheric signature of the MJO exhibits pronounced seasonality, but also that the QBO significantly modulates the upper-tropospheric tropical and extratropical anomalies associated with the MJO. Globally, variability in upper tropospheric geopotential linked to the MJO is maximized during the boreal summertime and wintertime of easterly QBO phases, which is consistent with previous research that has shown easterly QBO phases to enhance the persistence of tropical convection associated with the MJO. Additional features are identified, such as the global maximum in upper-tropospheric variability associated with the MJO occurring during boreal summertime, rather than boreal wintertime. Overall, the MJO explains seven to thirteen percent of intraseasonal atmospheric variability in 300-mb geopotential, depending on season and QBO phase. These results highlight the importance of considering the phase of the QBO in analyses related to either global or local impacts of the MJO, along with the importance of cross-scale relationships, such as those between the MJO and QBO, in governing the coupling between the MJO and teleconnections across the globe. This thesis considers the relationship between the MJO and processes that operate on both longer and shorter timescales than itself, including tropical convection and the Quasi-Biennial Oscillation. In doing so, this work highlights the importance of considering relationships between the MJO and atmospheric phenomena on different spatial and temporal scales and with origins distinct from the MJO itself. While theories exist describing the MJO as its own distinct entity, this research corroborates the idea that it is at its core fundamentally linked to the rest of the climate system, both modulating and being modulated by a broad range of atmospheric processes

Equations discovery of organized cloud fields: Stochastic generator and dynamical insights

The emergence of organized multiscale patterns resulting from convection is ubiquitous, observed throughout different cloud types. The reproduction of such patterns by general circulation models remains a challenge due to the complex nature of clouds, characterized by processes interacting over a wide range of spatio-temporal scales. The new advances in data-driven modeling techniques have raised a lot of promises to discover dynamical equations from partial observations of complex systems. This study presents such a discovery from high-resolution satellite datasets of continental cloud fields. The model is made of stochastic differential equations able to simulate with high fidelity the spatio-temporal coherence and variability of the cloud patterns such as the characteristic lifetime of individual clouds or global organizational features governed by convective inertia gravity waves. This feat is achieved through the model's lagged effects associated with convection recirculation times, and hidden variables parameterizing the unobserved processes and variables.Comment: 11 pages, 9 figure

Examining the impacts of convective environments on storms using observations and numerical models

2022 Summer.Includes bibliographical references.Convective clouds are significant contributors to both weather and climate. While the basic environments supporting convective clouds are broadly known, there is currently no unifying theory on how joint variations in different environmental properties impact convective cloud properties. The overaching goal of this research is to assess the response of convective clouds to changes in the dynamic, thermodynamic and aerosol properties of the local environment. To achieve our goal, two tools for examining convective cloud properties and their environments are first described, developed and enhanced. This is followed by an examination of the response of convective clouds to changes in the dynamic, thermodynamic and aerosol properties using these enhanced tools. In the first study comprising this dissertation, we assess the performance of small temperature, pressure, and humidity sensors onboard drones used to sample convective environments and convective cloud outflows by comparing them to measurements made from a tethersonde platform suspended at the same height. Using 82 total drone flights, including nine at night, the following determinations about sensor accuracy are made. First, when examining temperature, the nighttime flight temperature errors are found to have a smaller range than the daytime temperature errors, indicating that much of the daytime error arises from exposure to solar radiation. The pressure errors demonstrate a strong dependence on horizontal wind speed with all of the error distributions being multimodal in high wind conditions. Finally, dewpoint temperature errors are found to be larger than temperature errors. We conclude that measurements in field campaigns are more accurate when sensors are placed away from the drone's main body and associated propeller wash and are sufficiently aspirated and shielded from incoming solar radiation. The Tracking and Object-Based Analysis of Clouds (tobac) tracking package is a commonly used tracking package in atmospheric science that allows for tracking of atmospheric phenomena on any variable and on any grid. We have enhanced the tobac tracking package to enable it to be used on more atmospheric phenomena, with a wider variety of atmospheric data and across more diverse platforms than before. New scientific improvements (three spatial dimensions and an internal spectral filtering tool) and procedural improvements (enhanced computational efficiency, internal re-gridding of data, and treatments for periodic boundary conditions) comprising this new version of tobac (v1.5) are described in the second study of this dissertation. These improvements have made tobac one of the most robust, powerful, and flexible identification and tracking tools in our field and expanded its potential use in other fields. In the third study of this dissertation, we examine the relationship between the thermodynamic and dynamic environmental properties and deep convective clouds forming in the tropical atmosphere. To elucidate this relationship, we employ a high-resolution, long-duration, large-area numerical model simulation alongside tobac to build a database of convective clouds and their environments. With this database, we examine differences in the initial environment associated with individual storm strength, organization, and morphology. We find that storm strength, defined here as maximum midlevel updraft velocity, is controlled primarily by Convective Available Potential Energy (CAPE) and Precipitable Water (PW); high CAPE (>2500 J kg-1) and high PW (approximately 63 mm) are both required for midlevel CCC updraft velocities to reach at least 10 m s-1. Of the CCCs with the most vigorous updrafts, 80.9% are in the upper tercile of precipitation rates, with the strongest precipitation rates requiring even higher PW. Furthermore, vertical wind shear is the primary differentiator between organized and isolated convective storms. Within the set of organized storms, we also find that linearly-oriented CCC systems have significantly weaker vertical wind shear than nonlinear CCCs in low- (0-1 km, 0-3 km) and mid-levels (0-5 km, 2-7 km). Overall, these results provide new insights into the joint environmental conditions determining the CCC properties in the tropical atmosphere. Finally, in the fourth study of this dissertation, we build upon the third study by examining the relationship between the aerosol environment and convective precipitation using the same simulations and tracking approaches as in the third study. As the environmental aerosol concentrations are increased, the total domain-wide precipitation decreases (-3.4%). Despite the overall decrease in precipitation, the number of tracked terminal congestus clouds increases (+8%), while the number of tracked cumulonimbus clouds is decreased (-1.26%). This increase in the number of congestus clouds is accompanied by an overall weakening in their rainfall as aerosol concentration increases, with a decrease in overall rain rates and an increase in the number of clouds that do not precipitate (+10.7%). As aerosol particles increase, overall cloud droplet size gets smaller, suppressing the initial generation of rain and leading to clouds evaporating due to entrainment before they are able to precipitate

CIRA annual report FY 2017/2018

Reporting period April 1, 2017-March 31, 2018

Tropical waves and rainfall over Africa: Variability, mechanisms and potential for forecasting

Excessive rains or prolonged drought can have severe impacts on the economy, agriculture, water resources, spread of diseases and ecosystems in many African countries. As current global numerical weather prediction systems fail to deliver accurate rainfall forecasts over tropical Africa, novel forecasting strategies are needed. Tropical waves are known to modulate precipitation over this region on timescales of a few days to several weeks. The aim of this dissertation is to quantify the influence of all major waves on rainfall variability over Africa, to investigate the involved mechanisms and, to test their potential for forecasting rainfall, with a focus on northern tropical Africa during the extended monsoon season. Despite the importance of rainfall variability for vulnerable societies in tropical Africa, the relative influence of tropical waves for this region is largely unknown. This thesis closes this gap and presents the first systematic comparison of the impact of six wave types on precipitation over northern tropical Africa during the transition and full monsoon seasons, using two satellite products and a dense rain gauge network. Composites of rainfall anomalies based on different datasets show comparable modulation intensities in the West Sahel and at the Guinea Coast, varying from less than 2 to above 7 mm per day depending on the wave type. Tropical disturbances (TDs, including African Easterly Waves, AEWs) and Kelvin waves dominate the 3-hourly to daily timescale and explain 10-30% of precipitation variability locally. On longer timescales (7–20 days), only the Madden-Julian Oscillation (MJO) and Equatorial Rossby (ER) waves remain as modulating factors and explain up to one third of rainfall variability. Eastward inertio-gravity (EIG) waves and mixed Rossby-gravity (MRG) waves are comparatively unimportant. An analysis of wave superposition shows that low-frequency waves (MJO, ER) in their wet phase amplify the activity of high-frequency waves (TD, MRG) and suppress them in the dry phase. Furthermore, this dissertation gives the first systematic comparison of the dynamics and thermodynamics associated with tropical waves affecting rainfall variability over northern tropical Africa: Reanalysis and radiosonde data were analyzed for the period 1981–2013 based on space-time filtering of outgoing longwave radiation. The identified circulation patterns are largely consistent with equatorial shallow water theory. The slow modes, MJO and ER, mainly impact precipitable water, whereas the faster TDs, Kelvin waves, and MRG waves primarily modulate moisture convergence. Monsoonal inflow intensifies during wet phases of the MJO, ER, and MRG waves, associated with a northward shift of the intertropical discontinuity for MJO and ER waves. This study reveals that MRG waves over Africa have a distinct dynamical structure that differs significantly from AEWs. During passages of vertically tilted imbalanced wave modes, such as MJO, TDs, Kelvin, and partly MRG waves, increased vertical wind shear and improved conditions for up- and downdrafts facilitate the organization of mesoscale convective systems. The balanced ER waves are not tilted and rainfall is triggered by large-scale moistening and stratiform lifting. The MJO and ER waves interact with intraseasonal variations of the Indian monsoon and extratropical Rossby wave trains. The latter causes a trough over the Atlas Mountains associated with a tropical plume and rainfall over the Sahara. The presented results unveil which dynamical processes need to be modeled realistically to represent the coupling between tropical waves and rainfall in northern tropical Africa. The potential of tropical waves as predictors for African rainfall was tested. The spatio-temporal correlation patterns of tropical waves highlight their potential for synoptic rainfall forecasting. The observed spatio-temporal properties agree with values predicted by shallow-water theory, with the exception of MRG and EIG waves, which have a strong phase dispersion at low wavenumbers. Unfiltered precipitation fields show correlations patterns that are physically explainable by tropical waves and other atmospheric phenomena such as the position of the tropical rainbelt. These correlations serve as predictors in a logistic regression model. It was shown that this model successfully predicts rainfall occurrence over Africa with a lead time of one day. The statistical model is calibrated and outperforms the climatological forecast and current numerical weather prediction models by about 20%. The fact that tropical waves explain large portions of synoptic to intraseasonal rainfall variability in almost the entire tropics emphasize the potential of the proposed statistical model. This PhD thesis has laid the foundation to exploit this potential and to significantly improve short-term weather forecasts in Africa and throughout the tropics

Impacts of convective treatment on tropical rainfall variability in realistic and idealized simulations

The prediction of precipitation in the tropics is a challenge for numerical weather prediction (NWP), meaning very low practical predictability there. However, previous studies indicated that intrinsic predictability in the tropics is up to a few weeks and thus longer than in the extratropics. Equatorial waves (EWs) from the linear shallow-water theory are considered the source of this long predictability. Most weather and climate models still struggle to accurately capture EWs, which is often attributed to parameterized convection. With advanced computing power, model development is moving toward high-resolution models with explicit convection. To evaluate the value of these high-resolution models, this thesis aims to provide important insights into the behavior of tropical precipitation due to the treatment of deep and shallow convection using the ICOsahedral Nonhydrostatic (ICON) model. First, we examine the sensitivity of EWs to model configuration using realistic ICON simulations with varying horizontal grid spacings (80-2.5 km) and with different convectivetreatments between parameterized versus explicit deep and shallow convection. To robustly identify wave signals, we use two objective methods, one filtering rainfall using a fast Fourier transform and the other projecting two-dimensional wind and geopotential onto theoretical wave patterns. The results demonstrate that large-scale EWs are surprisingly consistent in terms of phase speed and wave amplitude with little sensitivity to model resolution, convective treatment and wave identification method. Rainfall signals of westward inertio-gravity waves (WIGs), however, show a large difference between parameterized and explicit convection with the latter showing marked rainfall signals but with no corresponding wind patterns. A composite analysis to link rainfall and wind fields of waves reveals that the identified signals in rainfall appear to be associated with mesoscale convective systems, the spatiotemporal scales of which overlap with those of WIGs, and thus are isolated as waves through space-time filtering. Secondly, we analyze idealized ICON simulations in a tropical aquachannel configuration with zonally symmetric sea surface temperatures and with rigid walls at 30°N/S. The aquachannel simulations vary in the representation of deep and shallow convection but with the same horizontal grid spacing of 13 km. All aquachannel simulations have maximum rainfall at the equator, showing an intertropical convergence zone (ITCZ) there, but the rainfall amount increases by 35% with explicit deep convection. To physically understand this difference, we adapt a diagnostic based on a conceptual model by Emanuel (2019), assuming boundary-layer quasi-equilibrium (BLQE), the weak temperature gradient approximation, and mass and energy conservation. BLQE implies that moist entropy is in balance between surface enthalpy fluxes, which import high moist entropy to the BL, and convective downdrafts, which transport low moist entropy from the free troposphere into the BL. The results reveal that the rainfall differences are primarily associated with surface enthalpy fluxes through BLQE, while precipitation efficiency is surprisingly constant in the ITCZ. Further detailed analysis demonstrates that mean surface wind speed, which is closely related to the large-scale circulation, contributes most to the differences in surface enthalpy fluxes. Thus, the treatment of deep convection alters mean rainfall through tight links between surface winds, associated surface fluxes and convective mass flux. Lastly, variability associated with EWs is examined in the aquachannel simulations by using the same wave identification methods used for the realistic simulations. All simulations show prominent signals of Kelvin waves (KWs) with large variations among them. Parameterized deep convection produces various eastward propagation with speeds of 5–27 m/s, while explicit deep convection exhibits a dominance of KWs with a zonal wavenumber of one and with a propagation speed of 24 m/s. Furthermore, explicit deep convection causes more pronounced structures of zonal wind and temperature in the lower stratosphere and a stronger link of wind-induced surface enthalpy flux exchange to the development of convection. Meanwhile, the treatment of shallow convection plays a role for temperature variation below 2.5 km. However, BL warming is in phase with maximum rainfall associated with KWs, which is opposite to observations. Parameterized deep convection generates a feature sharing similarities with the Madden Julian Oscillation, which is not found in the other aquachannel simulations. The novelty of this thesis lies in understanding the behavior of tropical rainfall in both realistic and idealized simulations by using diagnostics adapted for systematic comparisons between different simulations, mainly due to different convective treatments. This allows us to obtain valuable insights into the sensitivity of tropical rainfall and its variability to model configuration, ultimately paving the way for developing more accurate weather and climate predictions in the tropics

The Role of Oceanic Processes In The Initiation of Atmospheric Intraseasonal Oscillations Over The Indian Ocean

Observational analyses and post-processing of a hierarchy of ocean general circulation model (OGCM) experiments were performed to understand the influence of oceanic processes on the warm sea surface temperature anomalies (SSTAs) prior to the convection initiation of intraseasonal oscillations (ISOs) in the tropical Indian Ocean. We show results for boreal summer, when the Indian Summer Monsoon Intraseasonal Oscillation (MISO) is active, as well as for boreal winter, when the canonical Madden-Julian Oscillation (MJO) is active. We found 41 (39) strong ISO events that initiated in the tropical Indian Ocean during boreal summer (winter) of the 2001-2012 period. Furthermore, 17/39 boreal winter events initiated in the Seychelles-Chagos Thermocline Ridge (SCTR) before propagating eastward; the remaining winter events initiated in the Warm Pool (16) or southern Arabian Sea (6) regions. Using MJO indices and a set of event selection criteria, we determined that 71% of the SCTR ISO events were global-scale MJOs, while only 25% of WP ISO events were, a result which emphasizes the SCTR&rsquo;s importance in the MJO initiation. About 20% (24%) of the boreal summer ISO (winter SCTR ISO) events were associated with SSTAs that were strongly influenced by wind stress-driven oceanic processes during the initiation period. Composite analyses revealed that about one third of the pre-convection surface warming resulted from oceanic processes for the strong events. We show case studies of global-scale MISO and MJO events prior to which oceanic Rossby waves passed through the initiation region and were associated with strong ocean dynamically influenced SSTAs. Based on an ocean mixed layer heat budget analysis, the oceanic processes that affected SSTAs varied seasonally; horizontal advection (reduced upwelling and entrainment cooling) dominated during boreal summer (winter). We also show the results of a linear atmospheric boundary layer convergence model which revealed that the +SSTAs during boreal winter contributed strongly to the boundary layer convergence during convection initiation, suggesting that the ocean may act as a &ldquo;trigger&rdquo; for some MJO events. These results underscore the need for climate prediction models to accurately represent the three-dimensional ocean structure so that oceanic predictors of intraseasonal SSTAs and associated convection can be fully utilized.</p

Proceedings of the 2011 New York Workshop on Computer, Earth and Space Science

The purpose of the New York Workshop on Computer, Earth and Space Sciences is to bring together the New York area's finest Astronomers, Statisticians, Computer Scientists, Space and Earth Scientists to explore potential synergies between their respective fields. The 2011 edition (CESS2011) was a great success, and we would like to thank all of the presenters and participants for attending. This year was also special as it included authors from the upcoming book titled "Advances in Machine Learning and Data Mining for Astronomy". Over two days, the latest advanced techniques used to analyze the vast amounts of information now available for the understanding of our universe and our planet were presented. These proceedings attempt to provide a small window into what the current state of research is in this vast interdisciplinary field and we'd like to thank the speakers who spent the time to contribute to this volume.Comment: Author lists modified. 82 pages. Workshop Proceedings from CESS 2011 in New York City, Goddard Institute for Space Studie