Anantha Aiyyer

The Southcentral and Southeast US, comprising six water resource regions, has been experiencing significant growth in population over the last three decades. Among different hazards the region faces, floods occur in all the four seasons accounting more than quarter of the economic losses. The region has faced several major hurricanes ������������������ Matthew (2016), Irma (2017), Harvey (2016), Florence (2018) ������������������ over the last three seasons resulting in catastrophic flooding and loss of life. The objective of this proposal is to improve the predictability of the hazards of hydrologic extremes of floods and droughts through better understanding of (a) quantifying the changes in climatology, (b) describing their organizational patterns and (c) attributing the sources/drivers ������������������ land surface, atmosphere and ocean ������������������ that modulate their spatio-temporal variability over the region. Given the continually increasing population over study region, a synthesis on the role of various drivers and their interactions in influencing the predictability of floods will provide related agencies additional insights on developing strategies to improve the community resilience.

Easterly waves - the generic name given to the synoptic-scale disturbances that originate over west Africa and the eastern Atlantic during the summer monsoon - are the primary precursors to Atlantic tropical cyclones. While most easterly waves weaken and dissipate after leaving west Africa, many are able to survive for over 2 weeks. These long-lived easterly waves lead to tropical cyclone formation in the Caribbean, Gulf of Mexico, and even eastern Pacific. In preparation for this proposal, we examined the individual storm reports prepared by the National Hurricane Center and found that during 1995--2019, nearly 65% of eastern Pacific tropical storms were attributed to easterly waves originating over Africa. These waves can be tracked in satellite images which reveal cycles of convective organization and rapid weakening. These vacillations in the wave structure and intensity pose a major challenge to the operational forecasting of tropical cyclone formation. There are several open problems related to the maintenance of these long-lived waves and their transformation into tropical cyclones. We examine the following science questions in this proposal: 1) How do cloud populations, precipitation and vertical heating profiles vary during the life-cycle of long-lived AEWs?} 2) How does the transition from land to ocean (west Africa-eastern Atlantic) and ocean to land (western Atlantic-south/central America) impact the AEWs? 3) How do AEWs respond to the vacillation in convection once they have left their source region (i.e., the region of dynamic instability)? What theoretical constructs can we apply to this issue?} 4) Can merger of decaying AEWs and vorticity generated by antecedent convection lead to its amplification, leading to tropical cyclogenesis?} Method A series of testable hypothesis and extensive analysis using GPM/TRMM data is proposed. Our work will combine observations and theory to address the questions posed above. The maintenance of these long-lived easterly waves can be viewed as a competition between factors that damp (e.g., Rossby wave dispersion, radiative damping, and viscous spin down) and those that amplify (e.g., balanced response to diabatic heating). By focusing on periods where the convection decays and periods where it reestablishes - as is seen dramatically in satellite images - within the dynamical envelope of the wave, we will attempt to unravel some of the underlying scale interactions and the path to tropical cyclogenesis. Cloud and precipitation processes will be analyzed using products from GPM and TRMM retrievals of vertical structure of heating, precipitation and distribution of surface rain rates. Additionally, a variety of TRMM/GPM derived data sets for convective and cold-pool structures will be employed. These data, along with global reanalysis (MERRA-2, ERA5) will be used to characterize convective storm structures, attendant cold pools, and their interaction with synoptic-scale easterly waves. We will conduct idealized and cloud resolving real-case numerical simulations to arrive at a conceptual model for the maintenance of long-lived easterly waves. Relevance to Program: This project directly relates to opportunity 2.2 (Science and Process studies) of the request for proposals (RFP) and to NASA SMD science goals. It covers more than one broad topic identified in the RFP, including analysis of TRMM, GPM, and other satellite-based precipitation information for observational and modeling tropical convection and science studies to improve the indicators of tropical cyclone evolution.

Kelvin waves and easterly waves are among the most prominent modes of synoptic-scale convective variability in the tropics. Recent studies suggest that interactions between these waves can lead to tropical cyclogenesis. However, many questions remain regarding how these waves affect one another and how cyclogenesis ensues. The most significant ways that Kelvin waves might affect easterly waves relate to their modulation of low-level winds, which may alter the background shear and gradient of vorticity and enhance wave-mean flow interaction. The Kelvin wave westerlies could also enhance surface enthalpy fluxes within the easterly wave, which would lead to intensification through diabatic heating. While the kinematic view of the interaction appears simple, the inherent dynamics are expected to be complex and nonlinear. The imminent launch of the CYGNSS constellation of satellites will provide an unprecedented opportunity to observe and model these interactions. The high spatial and temporal resolution of CYGNSS is ideally suited for studying Kelvin waves and easterly waves. They have a phase speed of ~20 m s�����������������1 relative to one another and each have wavelengths of just 2000������������������4000 km. The proposed research aligns clearly with NASA objectives by leveraging CYGNSS data to investigate this important problem. Case studies from the first year of CYGNSS data will be compared with climatological composites from MERRA-2 and TRMM/GPM. This investigation will identify the evolution of surface winds and enthalpy fluxes in Kelvin and easterly waves separately as well as during interactions between them. The role of mesoscale convective systems (MCS) in mediating the interaction will also be investigated. The final phase of this project will use idealized simulations of the interactions between Kelvin waves and easterly waves. These simulations will shed light into the causal relationships between changes in the surface winds and fluxes derived from CYGNSS observations and changes in the amplitude of easterly waves. Particular focus will be placed effect of moisture and vorticity advections as well as surface energy fluxes.

The Madden������������������Julian Oscillation (MJO) is the largest source of tropical intraseasonal variability with impacts spanning the globe. Unfortunately, numerical models fail to adequately simulate its convection, limiting their opportunity to harness its long-range predictability. Nowhere is this shortcoming more apparent than over the Maritime Continent (MC). Many MJO events terminate before crossing the MC, a tendency that is exaggerated in most numerical models. The MC poses complex topography that may reduce the MJO������������������s moisture source from surface fluxes and impede the MJO������������������s low-level circulation. The exceptional diurnal cycles in the vicinity of the large islands in the MC can also drain the MJO������������������s energy. Most models fail to capture this diurnal cycle properly and result in large biases in rainfall over the MC. Many studies have examined the interactions between the MJO and convection over the MC. Far fewer have looked at the role of convectively coupled equatorial waves, even though models that faithfully represent these waves also tend to produce more representative MJO signals. The proposed study will identify avenues for model improvement by investigating the interactions between the MJO, equatorial waves, and the diurnal cycle over the MC. It will be conducted in two intertwined branches that will each complement a major international field campaign, the Year of Maritime Convection, proposed for 2017-2018. The first branch will identify the MJO and equatorial waves using Fourier filtering of GPM IMERG data. These data will be used to identify the modulation of the diurnal cycle in GPM IMERG by equatorial waves. They will also be compared with TRMM and geostationary satellite datasets that identify convective cloud populations. These results will show differences in the interactions between equatorial waves and convection over the MC during MJO events that cross the MC barrier with those that do not. The second branch will compare these observational results with reforecast data from the Climate Forecast System Reanalysis and Reforecast (CFSRR). It will test a novel approach for combining observed GPM IMERG data with CFSRR forecasts to identify the MJO and equatorial waves. With this method, it is hypothesized that the model can predict the convective envelope of the MJO and equatorial waves better than it can forecast the individual convective elements. The results will also be used to identify differences in the equatorial wave state over the MC that might lead to more or less skillful forecasts of the MJO.

The objective of this study is to advance our understanding of the dynamics of African easterly waves (AEWs). Specifically, we will: (1) Investigate further, a mechanism for AEW origin that emphasizes energy dispersion; (2) Investigate the the structure of the AEW storm track and intermittency; and (3) Investigate the feedback between AEWs and moist convection. Intellectual Merit: This study seeks to address some of the limitations of existing theories for AEWs that include normal mode instability and convective triggering. We propose to investigate the local instability of the African easterly jet (AEJ) and upstream and downstream energy dispersion of AEW wave packets to explain their origin and evolution. We also propose to undertake formal analysis of AEW storm track dynamics, an area that has not been previously investigated in any substantial detail. We examine issues related to AEW storm track structure, eddy scales for moist and dry waves, and interaction with the AEJ. Finally, we also investigate the feedbacks with moist convection and examine whether the diabatic Rossby vortex is an appropriate synoptic model for some AEWs. Broader Impacts: This study, in part, draws an analogy with midlatitude baroclinic waves while adapting the relevant conceptual framework to the complex background over North Africa. Thus, in addition to implications for AEWs, this study has potential to contribute to the problem of moist dynamics of baroclinic eddies. Given the importance of AEWs for local weather over North Africa as well as Atlantic tropical cyclones, the results will be of relevance to a wide community of stakeholders. This study will support the scientific training and career development of three graduate students.

This proposal for career development is motivated by two overarching goals: [A] Investigation of African easterly wave (AEW) origin and storm track dynamics; and [B] Improving the current paradigm for mathematical instruction in undergraduate atmospheric science curriculum. AEWs are important synoptic-scale systems that impact West Africa and are precursors of the majority of Atlantic Tropical cyclones (TCs). Despite significant advancement in our understanding of these disturbances, several conceptual gaps remain. The research objectives of this study encompass open problems within two areas: (i) AEW origin from non-modal and stochastic forcing of the African easterly jet (AEJ); (ii) AEW evolution within a streamwise varying waveguide.

Summary: The objectives of this collaborative project are to improve understanding and prediction of hazardous convective and landfalling tropical weather systems in the Southeastern United States. The foci for the proposed effort are based upon input provided by regional NWS forecast offices as well as National Centers, and is aligned with NWS Eastern Region priorities; this approach embodies a operations-to-research-to-operations (O2R2O) paradigm. The specific weather hazards forming the basis of the proposed research program include (i) inland and coastal wind accompanying tropical cyclones, (ii) heavy precipitation and localized flooding associated with tropical cyclones, and (iii) severe convective storms forming under conditions of marginal instability and strong vertical wind shear. The research program will utilize two unique elements, including state-of-the-art numerical weather prediction systems that will serve as testbeds for algorithm development and numerical model configuration. For tropical cyclone prediction, we will utilize a realtime system that was developed by two of the PIs and which has been run for the past two tropical seasons at the Renaissance Computing Institute (RENCI). While output from this system is available to NWS forecasters, our primary objective will be to utilize this system to develop and test wind and precipitation prediction techniques that can be applied at regional NWS offices. Additionally, an ensemble computing system, developed by PI Etherton at RENCI, will be developed and utilized in the prediction of heavy precipitation. The second unique element in our approach will be to establish a collaborative structure that includes site visits to remote offices and national centers, regular meetings with the students, PIs, and key NWS personnel, and the production of training materials to facilitate the transfer of research results to operations.

This is a proposal for a US-India collaborative workshop on air quality. The overarching goal is to bring together experts from U.S. and India in order to identify and promote collaborative research with specific emphasis on improving the science of the current regional/global air quality modeling system being developed in the US. It is anticipated that this workshop will result in major research initiatives in the US which will enhance the state of science in air quality measurement and modeling. The specific objectives of this proposal are to: (1) Organize a workshop in India with participants from academic, industrial and government agencies; (2) Identify the scientific challenges stemming from variability in both meteorological and emission conditions in multi-scale atmospheric photochemical models; (3) Develop a plan for future collaborative research programs involving US and Indian scientists; and (4) Develop research proposals to NSF and other agencies in the US and India. The proposed workshop will support five experts (air-quality/atmospheric science) and two graduate students from the US. The proposed activities are of interest to both US-EPA and NOAA who have offered in-kind support. Furthermore, the Ministry of Earth Sciences, Government of India, has committed to provide financial support for participants from India. Intellectual merit: This initiative will bring together leading experts in air-quality measurement, modeling and analysis from the US and India. The US scientists have published widely on these issues in Nature, Journal of Geophysical Research, Atmospheric Environment and other prominent international journals. It will provide a unique opportunity to further air quality science by focusing on environmental issues (physical and chemical meteorological) in the tropics that have global ramifications. The workshop will provide improved understanding of emission sources and meteorological conditions that contribute to regional-to-urban-scale air quality issues of relevance to public health and the environment. Broader Impacts: The team involves a combination of senior and young investigators, and graduate students. The activities of the workshop will advance scientific discovery and understanding while promoting teaching and learning. Furthermore, the proposed activities will enhance the infrastructure for research and education by providing online resources for air quality modeling, data mining and analysis, and facilitate active collaborations among the workshop participants from the U.S., India and elsewhere. Finally, the activities undertaken in collaboration with Indian scientists can serve as a catalyst for engaging participation with stakeholder groups from other parts of South Asia.

Applied research is proposed with the following objectives: (i) to determine the most likely level of tropical cyclone intensity and frequency in future climate regimes, (ii) to provide a quantitative measure of uncertainty in these predictions, and (iii) to improve understanding of the linkage between tropical cyclones and the planetary-scale circulation. Current mesoscale weather forecasting models, such as the Weather Research and Forecasting (WRF) model, are capable of simulating the full intensity of tropical cyclones (TC) with realistic structures. However, in order to accurately represent both the primary and secondary circulations in these systems, model simulations must be configured with sufficient resolution to explicitly represent convection (omitting the convective parameterization scheme). Most previous numerical studies of TC activity at seasonal and longer time scales have not utilized such explicit convection (EC) model runs. Here, we propose to employ the moving nest capability of WRF to optimally represent TC activity on a seasonal scale using a downscaling approach. The statistical results of a suite of these high-resolution TC simulations will yield a realistic representation of TC intensity on a seasonal basis, while at the same time allowing analysis of the feedback that TCs exert on the larger-scale climate system. Experiments will be driven with analyzed lateral boundary conditions for several recent Atlantic seasons, spanning a range of activity levels and TC track patterns. Results of the ensemble of WRF simulations will then be compared to analyzed TC data in order to determine the extent to which this modeling setup can reproduce recent levels of TC activity. Next, the boundary conditions (sea-surface temperature, tropopause height, and thermal/moisture profiles) from the recent seasons will be altered in a manner consistent with various future GCM/RCM scenarios, but that preserves the large-scale shear and incipient disturbance activity. This will allow (i) a direct comparison of future TC activity that could be expected for an active or inactive season in an altered climate regime, and (ii) a measure of the level of uncertainty and variability in TC activity resulting from different carbon emission scenarios.

The Madden-Julian Oscillation (MJO)--a large-scale intraseasonal circulation feature in the tropics---is known to be climatologically favorable to formation of tropical cyclones (TC). While previous studies have have delineated the generic conditions associated with MJO that are conducive for TC genesis, and have documented the statistics of TC formation, a clear synoptic model of this process is yet to be established. In particular, the details of the evolution of TC precursors within the convective envelope of the MJO and the process of decoupling of the TC and MJO are not fully understood. Furthermore, it is not clear whether there are any systematic differences in the structure, intensity and tracks of TCs that form in association with the MJO as compared to those that form independently. In order to address these issues, the current study proposes a combined approach of climatology and phenomenological case studies. The results are anticipated to help construct a synoptic model of TC genesis within the MJO and to motivate a larger, in-depth study that incorporates numerical modeling in order to address the predictability of such events.