This effort will address the genesis of tropical cyclones (and antecedent regions of low pressure and near-surface cyclonic wind patterns termed tropical depressions) through further study of the "marsupial paradigm." This conceptual model refers to a theory of hurricane formation in which the translation speed of one type of parent atmospheric disturbance (termed an "easterly wave") is matched the environmental flow so as to create a protective zone (or so-called "pouch") in which developing small-scale cyclonic circulation(s) are protected from hostile environmental conditions until further strengthening occurs. This study will be supported by data to be collected during the 2010 NSF-supported PREDICT (PRE-Depression Investigation of Cloud-systems in the Tropics) field campaign involving the NSF/NCAR GV research aircraft as well as complementary resources being provided by NASA and NOAA.
This research will focus on conditions within the meso-alpha scale wave pouch region (having 200-2000 km horizontal extent), its interaction with the larger-scale environment, and impacts of this interaction on ultimate tropical cyclone formation. It will incorporate flight-level, dropsonde and radar data from the PREDICT field campaign, with reanalysis data, various satellite datasets, and the WRF-ARW numerical model to address three main questions: 1) Is a well-defined wave pouch a necessary condition for tropical cyclone development, and if so, how does it form? 2) Why do some waves exhibiting a deep pouch fail to develop into a tropical storm? 3) How does the Saharan Air Layer (SAL) affect tropical cyclogenesis, and why do some wave pouches provide protection from dry air intrusion while others do not?
The intellectual merit of this effort centers on exploitation of the focused PREDICT dataset in conjunction with numerical model simulations conducted in the marsupial framework to lead to better understanding of tropical cyclogenesis. Broader impacts of this research include the education and training of two graduate students and the integration of tools and products into classroom learning activities, as well as support for an investigator drawn from an underrepresented group. Benefits to society will come through the increased understanding of tropical cyclogenesis and associated increases in warning lead time for hurricane formation.
Intellectual Merit: The objective of the proposal was to examine tropical cyclone (TC) formation within the so-called tropical easterly waves in the framework of the "marsupial paradigm". Tropical easterly waves have the typical wave length of 2,500 km to 4,000 km and propagate westward over the tropical/subtropical Atlantic. It is an intriguing question how a TC-scale proto-vortex forms and intensifies within such synoptic-scale waves. The marsupial paradigm hypothesized that a meso-alpha scale closed circulation in a synoptic-scale wave provides a protective environment for the development of the tropical cyclone embryo (Fig. 1). The tropical cyclone formation sequence was likened to the development of a baby kangaroo that is carried along and protected in its mother’s pouch. The framework was thus coined the "marsupial paradigm", and the protective region of closed circulation in the synoptic-scale wave was dubbed the "wave pouch". We examined the structure and evolution of the wave pouch prior to tropical cyclogenesis through the analysis of the PREDICT field data, satellite and reanalysis data as well as numerical model simulations. It was found that 1) the inner pouch region, due to its unique dynamic and thermodynamic conditions, is the preferred location for tropical cyclogenesis (Fig. 2), 2) cumulus congestus (clouds terminating around 6-8 km above the ground) plays a dominant role in preconditioning the atmosphere for transition to deep convection and genesis (Fig. 3), and 3) the wave pouch is generally well-protected in the middle troposphere but the dry air in the upper troposphere in combination of synoptic-scale forcing may hinder genesis (Fig. 4). Also, 4) the vertical structure of the wave pouches was examined for developers and non-developers, and it was found that a wave pouch extending from 600-700 hPa down to the boundary layer is a necessary condition for genesis; 5) the statistics of convective processes were documented from the tropical wave to tropical cyclone stage using high-resolution numerical model simulations, and 6) a pouch-tracking algorithm was developed for real-time prediction of tropical cyclogenesis location (Fig. 5), and an algorithm was also developed for northwestward-propagating waves that, on occasion, play a role in tropical cyclogenesis over the western oceans. Broader Impacts: The research supported by the above grants led to a better understanding of the key factors/processes for tropical cyclogenesis, which ultimately contributes to the improvement of tropical cyclone forecasting and climate projection. In particular, the operational "marsupial" products developed by Wang (now maintained by the Montgomery research group) provided useful information to forecasters and researchers for flight planning in several recent field experiments (Fig. 5). The products have also been used in the daily briefings at the Hurricane Research Division. In addition, the diagnostic tools and products were integrated into a graduate course on tropical meteorology at the UIUC, and the grant supported the graduation of two M.S. students and a Ph.D. student under Wang’s supervision at UIUC.
