11.4 Upper Ocean Temperature Response to Hurricane Ignacio (2015) as Observed by ALAMO Profiling Floats

Wednesday, 25 January 2017: 4:45 PM
607 (Washington State Convention Center )
Elizabeth R. Sanabia, U.S. Naval Academy, Annapolis, MD; and S. R. Jayne and W. Swick

The dynamic nature of the ocean beneath Hurricane Ignacio in 2015 was observed by several Air Launched Autonomous Micro Observer (ALAMO) profiling floats. The development of a small profiling float that can be launched from Hurricane Hunter aircraft has enabled the ability to monitor the upper-ocean thermal structure over a time span of many months.  These Argo-type profiling floats can be deployed in advance of, or during, a tropical cyclone from any aircraft equipped with an A-sized (AXBT) launch tube.  The floats have same dimensions as an AXBT and weigh about 8.5 kg.  Upon deployment, the floats parachute to the surface, detach and automatically begin their programmed mission.  The recorded temperature data is subsampled to 1-meter bins that are reported back via the Iridium satellite phone network, which is then automatically processed and posted to the GTS.  The floats are also reprogrammable via the 2-way communication afforded by Iridium.

The high temporal sampling rate of these observations enabled collection of pre-storm conditions, followed by a large depression of the thermocline, and then subsequent oscillations of the upper 200m of the water column for more than one week following the passage of Hurricane Ignacio.  Analysis of the impact of this Category IV hurricane includes storm and buoy geometry, atmospheric conditions, and the upper ocean thermal response to the observed forcing. 

First deployed in tropical cyclones in 2014, the ALAMO float offers substantial benefit to both ocean and coupled model forecasts, improving accuracy of the initial temperature conditions within the upper kilometer of the ocean.  These sensors are also capable of rapidly sampling within shallower depths, and in the case of Hurricane Ignacio (2015), to 300m every 2 hours.  This capability enabled collection of a unique data set, with more than a dozen observations from 2 locations within 200 km of the center of Hurricane Ignacio, including 2 observations beneath the eyewall.  Forcing mechanisms are evaluated using wind observations from the 53rd Weather Reconnaissance Squadron Stepped Frequency Microwave Radiometer data from flights through the storm and over the buoy locations.  The observed ocean response is compared to 1-D and 3-D versions of the Price-Weller-Pinkel model.

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