Intellectual Merit: Tropical cyclogenesis remains one of the outstanding mysteries of the tropical atmosphere, yet a proper understanding of how hurricanes and typhoons are born is the first of several steps in relating the effects of climate change on these storms and the impact of severe weather on coastal regions of eastern North America. Fundamentally a mesoscale process, tropical cyclogenesis integrates the large-scale circulation, synoptic waves, and mesoscale processes (ranging from critical layer formation in tropical waves to vortical hot towers on the cloud scale) into a hurricane monolith at intermediate scales (20-200 km). Although many of the relevant dynamical and thermodynamical processes at this scale have been investigated, virtually nothing is known about their relative importance, chronological sequence and mutual interaction in the various stages of tropical cyclogenesis. A field campaign (PRE-depression Investigation of Cloud systems in the Tropics; PREDICT) has been designed to obtain in situ measurements of mesoscale processes in developing tropical depressions in the western Atlantic. Data from the PREDICT field campaign will be analyzed in conjunction with operational and retrospective meteorological analyses, short-term forecasts and satellite imagery in order to address mesoscale issues and the interaction of physical processes across multiple scales. A new "marsupial paradigm" developed by the Principal Investigator and colleagues at the Naval Postgraduate School will be applied in real time to the PREDICT data acquisition and interpretation efforts. This paradigm focuses on the role of the tropical wave critical layer (outer mesoscale, 200-2000 km) in genesis, and on key environmental conditions that are necessary for development within the "gyre-pouch" of a parent wave. This study is concerned mainly with mesoscale processes operating in the gyre-pouch, and how these processes are affected by environmental conditions nearby and within. A null hypothesis of PREDICT is that mesoscale processes do not vary across a wide range of synoptic patterns in which tropical cyclogenesis occurs. According to this idea, it is the precise way that mesoscale processes interact with, and within, the gyre's moist vortical environment that determines whether storm development is successful. The marsupial paradigm provides the necessary Lagrangian framework for interpretation of high-resolution atmospheric data and for improved understanding of processes. Post-analysis of field data will be integrated with large-scale analyses to refine and extend the marsupial paradigm.
Broader Impacts: Public education on severe weather is important to help society utilize weather information and to help them understand their environment. A longer-term concern is to understand how our changing climate has, and will, affect these storms and their impacts. An improved scientific understanding of how to interpret the data input stream will be passed on to the public in terms of a more reliable classification of storm status and better prediction of the likelihood of storm formation and intensification when these systems, in many cases, are still far from land.
A new model for tropical cyclogenesis was formulated by the PI and collaborators and became the scientific basis for a field campaign known as PREDICT, utilizing the NSF/NCAR G-V, based in St. Croix in August-September 2010. The value of in situ airborne sampling of tropical cyclone formation is that dropsonde data and onboard instrumentation provide essential information on the dynamics, thermodynamics, kinematics and composition that cannot be obtained by routine observations in data-sparse regions of genesis over the remote tropical oceans. The formation of tropical cyclones (TCs) has been regarded, until recently, as a fundamental mystery of the tropical atmosphere. While the satellite perspective of mature TCs available since the early 1960s has been valuable operationally and largely eliminated the element of surprise that existed prior to this era, the genesis of these storms continues to pose a challenge. Tropical forecasters at the National Hurricane Center (NHC) who issue bulletins 4x daily rely primarily on the high spatio-temporal resolution of geostationary (GEO) imagery to supplement sparse rawinsonde data. Over the last few decades forecasters have developed empirical methods to interpret the imagery sequences and to issue predictive guidance for the next 48 hours. Uncertainty remains in NHC real-time genesis probability beyond 48 hours, and in retrospective analysis of data that attempts to identify necessary and sufficient conditions for genesis. Although the mystery of genesis has not been resolved by the tireless efforts of forecasters, their written record provides important clues for development of a scientific understanding of the problem. Central to the marsupial paradigm is a wave-vortex duality that anticipates proto-vortex development at the center of the critical-layer cat’s eye of the parent wave. This idea gives scientific credence to the forecasters’ notion of a "surface low along the wave" as a necessary condition for tropical depression development from within tropical waves in the Atlantic and East Pacific basins. The paradigm identifies necessary conditions for genesis (quasi-closed recirculation above the boundary layer, diabatic activation by moist convection). It identifies, in forecast data, a locus of points (approximately east to west) along which genesis may occur, without specifying the time of development. The paradigm also identifies in analyzed fields certain flow properties that are sufficient for declaring that a tropical depression (TD) or tropical storm (TS) has formed. For example, there is the aforementioned quasi-closed recirculation (TD) in the co-moving frame, and establishment of a vortex core in approximate solid-body rotation (TS). Still lacking is a way to predict reliably the time, and hence, longitude along the forecast track, of storm formation. Forecasters have determined empirically that moist convection must be organized by a low-level cyclonic circulation. Convective organization that accompanies recirculation and formation of a vortex core may be regarded as fully sufficient for storm formation; it is primarily our inability to anticipate convective organization by the vortical flow that impairs genesis predictability. Knowledge is required of processes by which vorticity organizes convection, and how such organized convection alters the vorticity distribution to further its own organization. To address this need, the PI's research and PREDICT post-analysis supported by NSF aims to integrate quantitative information on wave pouch and proto-vortex structure and temporal development, as seen by high-resolution meteorological analyses (25-75 km grid resolution), with GEO satellite imagery which depict the spatio-temporal evolution of clouds and water vapor in extraordinary detail (1-4 km at sub-satellite point). The relevant dynamical structures are visible in the imagery for their swirling deformation of low stratocumulus cloud and water vapor in the outer wave pouch, and organization of deep convective cloud in the inner pouch region. Our preliminary research has focused on three storms (Lisa 2010, Nate 2011, Nadine 2012) in the Atlantic sector, along with storms sampled during PREDICT, for which additional data are available from G-V dropsondes. Feature tracking algorithms have been developed to extract quantitative information on horizontal motions within the wave pouch, including development of a cyclonic low swirl that is responsible for organization of deep convection leading to proto-vortex formation via aggregation of cyclonic vorticity at pouch center. In this way, our scientific research provides a quantitative interpretation of empirical guidelines developed by NHC forecasters. For instance, we can determine objectively when a quasi-closed circulation (sufficient for tropical depression declaration) has formed in the lower troposphere in the co-moving frame of the parent wave. A long-term goal of the research is to develop objective metrics for operational use that will improve the forecasters' ability to anticipate tropical cyclogenesis in the Atlantic sector and elsewhere, complementing their subjective guidance based on decades of experience with satellite imagery.
