Extended Abstract (1.1M)JP1.1
Reduction of boundary layer dropsonde winds to estimate surface winds and their comparison with SFMR data in landfalling Hurricane Katrina
Richard G. Henning, Consulting Meteorologist, Niceville, FL
As a private consulting meteorologist, this author has written over 200 reports on behalf of property owners along the Gulf Coast impacted by Hurricanes Katrina and Ivan. An essential aspect of reconstructing the wind field for specific neighborhoods along the coastline during the landfall event of a hurricane, is proper interpretation of aircraft reconnaissance dropsonde data. Since 1997, GPS dropwinsondes have been released into the core of intense tropical cyclones, providing details as to the structure of winds that had previously never been available for study.
In the vast majority of cases, the strongest winds found in vertical profiles measured by these sondes is located well below the flight level of the aircraft. Often, distinct wind maxima are noted in the boundary layer 300 to 500 meters above the surface. On most occasions, the sondes do not measure winds all the way down to the surface, as typically they lose contact with a sufficient sampling of the GPS satellite constellation at some point below 100 meters above the sea surface (or terrain if the sonde is blown onshore). If quality controlled wind data is collected to within a few meters of the surface, there is no guesswork needed to estimate the surface winds as they have effectively been measured (at least an instantaneous value has been measured, the question of whether this instantaneous value is most akin to a three second gust, a lull, or a one minute sustained value must then be addressed).
Another question that has arisen over the past several years since these GPS sondes have become operational comes when the wind measurements terminate at some point at higher altitudes above the surface. How much of the ubiquitous wind maxima seen in the boundary layer at several hundred meters should be reduced to come up with a reasonable estimate of the 10 meter standard surface winds?
Research by scientists at the Hurricane Research Division and other agencies suggests that assigning a simple percentage (ie: 90, 80, 70 percent) reduction factor, while empirically sound in a statistical sense, may not be appropriate for evaluating a single given sonde wind profile in a specific storm setting.
This author will examine a methodology of extrapolating surface winds from dropsonde wind profiles by looking at the characteristics of the boundary layer into which the sonde was released. The thermodynamic stability of the boundary layer, along with whether it was dropped into convective versus stratiform precipitation, is critical with regard to making a sound case by case estimate of the 10 meter winds.
Joint Poster Session 1, Tropical Cyclones and Probability/Statistics Posters
Monday, 21 January 2008, 2:30 PM-4:00 PM, Exhibit Hall B
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