Tuesday, 12 January 2016
Room 344 ( New Orleans Ernest N. Morial Convention Center)
The rain and wind fields of tropical cyclones begin to affect people hours prior to landfall, particularly in the case of large hurricanes such as Katrina (2005). Preparedness actions such as securing windows and outdoor property, and evacuating are hindered when heavy rainfall commences. Defining the leading edge of the rain field is a complex task as a tropical cyclone begins to interact with the land surface and continental air masses. This study focuses on the outer rainbands associated with Hurricane Katrina's (2005) Louisiana landfall. Our objectives were to 1) determine the hour when rainfall begins at land-based locations relative to the time of the eye's landfall, and 2) track the position of the leading edge of the outermost rainband over time. We defined the onset of rainfall as the start of the six-hour segment in which rain rates derived from the Stage IV product were at least 1.21 mm, corresponding to a radar reflectivity value of 25 dBZ. We identified rainfall onset at coastal locations for 43 landfalling tropical cyclones 2002-2012 and determined that the average rainfall onset occurred 12 hours prior to landfall. Given Katrina's large size, it was not unexpected that rainfall in Louisiana began earlier, averaging 15 hours prior to landfall for locations along the coastline.
To track the evolution of the outer rainbands, we utilized a Geographic Information System to mosaic the NEXRAD composite reflectivity product from the four radars closest to Katrina's landfall location. We rendered one line at the top of each hour along the outermost rainband containing 40 dBZ reflectivity values, and computed the start, midpoint, and end of these lines as well as their length. The distance and bearing of these points were calculated in relation to the storm center. We identified five main regions that could be considered the leading edge during 28 August 1800 UTC – 29 August 1400 UTC. Each edge lasted 5-8 hours. Edge A formed near the mouth of the Mississippi river and extended east, was present 1800-0000, and traveled 250 km towards the northwest before eroding. Edge C took a similar trajectory starting 9 hours later. Edge B formed over Lake Borgne at 2200 and moved north. Edge D formed nine hours later along the Louisiana, Mississippi, and Alabama coastlines, moved the shortest distance at 110 km, but had the longest length at 470 km. Edge E developed 180 km west of the eye near the time of landfall, encircling more than 180 degrees around the eye within five hours. We hypothesize that changes in friction near the coastline and interaction between tropical and somewhat drier continental air masses are responsible for these patterns. Our future work will include simulations with the Weather Research and Forecasting model to test these hypotheses.
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