| NSF Org: |
AGS Division of Atmospheric and Geospace Sciences |
| Recipient: |
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| Initial Amendment Date: | September 7, 2011 |
| Latest Amendment Date: | September 7, 2011 |
| Award Number: | 1132576 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Chungu Lu
AGS Division of Atmospheric and Geospace Sciences GEO Directorate for Geosciences |
| Start Date: | September 15, 2011 |
| End Date: | August 31, 2015 (Estimated) |
| Total Intended Award Amount: | $443,956.00 |
| Total Awarded Amount to Date: | $443,956.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1400 WASHINGTON AVE ALBANY NY US 12222-0100 (518)437-4974 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
1400 Washington Avenue Albany NY US 12222-0001 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Physical & Dynamic Meteorology |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Tropical cyclone intensity change remains a difficult process to understand. Even beyond the context of real-time forecasting and with the benefit of a more comprehensive analysis, it is still sometimes impossible to explain why, in apparently similar situations, one tropical storm or hurricane will weaken while another will become stronger. Our inability to understand (let alone forecast) these events remains as a potential danger to coastal communities which are vulnerable to the possibility of a rapidly intensifying hurricane making landfall.
Intellectual merit: In general, increasing values of environmental wind shear are increasingly less favorable for tropical cyclone intensification. However, cases with unexpected (and unforecasted) intensification are often associated with moderate values of wind shear. The goal of this research is to better understand the relationships between wind shear, tropical cyclone structure, and tropical cyclone intensification, with an emphasis on cases of moderate wind shear. This will be achieved through a synthesis of observational data sets obtained in past and recent field programs as well as analyses of high-resolution numerical model simulations. In particular, a recently developed modeling technique allows for the production of idealized simulations with highly controlled environments that can be specified to be nearly identical to the environments around observed tropical cyclones. These simulations will be further validated against observations from within the cyclones. Once satisfactory agreement between the observed and simulated storms is achieved, the simulations can be used to understand the physical processes that caused intensity and structure change for those cases. Furthermore, the idealized modeling technique allows all aspects of the surrounding environment - the wind, temperature, and humidity profiles, to be varied independently. Thus it will be possible, on a case-by-case basis, to isolate which of these factors have the most control over intensification (or decay) of the storm. This will lead to a new understanding of how the environment surrounding a tropical cyclone modulates its structure and intensity, and to what extent intensity changes are either internally or externally driven.
Broader impacts: This project will lead to improvement of our understanding of environmental controls of tropical cyclone intensity and structure change. Greater understanding will lead to improved forecasting, not only through improved model development, but also through the identification of new or improved forecasting parameters based on the environmental soundings. The project will put to use existing datasets as well as lead to the generation and dissemination of new data sets derived from the raw observations obtained from recent field projects. The project will support the education and training of two graduate students.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Our research addressed two factors that influence tropical cyclone intensity change. The first is vertical wind shear, the change of wind with height, in the tropical cyclone environment. Vertical shear acts to tilt the storm away from upright, thus removing some of the warm air from the storm core in the upper troposphere. The resulting increase in surface pressure weakens the storm. Our research interests were in storms that “broke the rules” and intensified despite the presence of strong vertical wind shear.
All sheared storms contain stronger precipitation and latent heat release on the side opposite the shear, i.e., east of the center if the vertical shear is from the west. We found that new disturbances sometimes form within this downshear precipitation. The resulting low pressure area, which is only about 10 km wide, initially rotates around the primary vortex, but intensifies rapidly as it moves underneath the tilted part of the primary vortex above it. In response, the two vortices grow together and intensify while exhibiting little tilt. This “downshear reformation” process provides a way for tropical cyclones to resist the deleterious effects of vertical wind shear. Such events are difficult to observe on satellite because the downshear vortex is small and beneath the cloud tops. In future work we will design ways to observe this phenomenon in real time in order to predict when such events will occur.
The second topic of research related to the influence of the tropical cyclone “cirrus canopy” on storm intensity change. This canopy represents the extended region of high cloud that surrounds tropical cyclones in the upper troposphere. It is the most dramatic visual feature of the storm when viewed on satellite pictures. We found strong cooling of the top of this canopy due to longwave radiation. This is similar in concept to radiative cooling on a clear calm night, but occurring 15 km above the surface. In addition, longwave warming occurs deep within the cloud, and warming from the sun occurs within the cloud during the day. The combination of these processes produced significant turbulence in the upper troposphere. They also create temperature gradients between the cirrus canopy and the clear environment resulting in a condition labeled “symmetric instability”. In response to this instability, outflow in alternately enhanced and weakened as the air sloshes back and forth. This creates significant fluctuations in the storm intensity. The importance of this process was not known previously, and in future work we will study it in more detail.
The cirrus canopy has been “terra incognita” for tropical cyclones, because satellites cannot see down into the cloud layer, and reconnaissance aircraft fly beneath it. However, new data is now coming on line from stratospheric aircraft. These aircraft release instrumented “dropsondes” that sample the entire cirrus canopy. These new data sources will allow us to continue to increase our understanding of cirrus canopy physics and its role in tropical cyclone intensity change.
The broader outcomes of the work related primarily to education of four graduate students. Two MS degrees and one PhD were completed, and one PhD student is continuing in his work. These students learned to write, to program, and to do technical research in our field. In addition, we disseminated our results in numerous presentations and published papers, and used the knowledge we gained in our tropical meteorology course lectures.
Last Modified: 11/30/2015
Modified by: John E Molinari
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