[Eoas-seminar] Special Seminar: Dr. Greg Foltz AOML/PO, Monday, April 29, 3:00PM, Room 6042

[eoas-seminar at lists.fsu.edu]

Dear all,



Please join us next MONDAY, April 29, in Room 6042 for a special seminar (made possible by Meteorology alumnus Dr. Mark Powell) given by Dr. Gregory Foltz of the Physical Oceanography Division of NOAA/AOML.



He will speak about “Direct measurements of the air-sea momentum flux and drag coefficient in hurricanes"



The abstract is below.



Dr. Foltz is also available for individual meetings in EOAS on Monday and Tuesday, and will be visiting COAPS Tuesday afternoon. If you’d like to meet with him in EOAS, please contact Bob Hart (rhart at fsu.edu) to schedule a day and time from those slots available.



Graduate students: please join lunch with the speaker in Room 3067 at 11:45 AM on Monday. Please RSVP to Bob Hart by the end of the day Thursday.



Summary:


DATE: Monday, April 29

SEMINAR TIME: Refreshments at 2:45 PM, Talk 3:00 - 4:00 PM

SEMINAR LOCATION: EOA 6042 (special time and room due to final exam scheduling)



A reception will follow in the Mark Powell Observatory (Room 6067 Maproom).



Seminar Title:  Direct measurements of the air-sea momentum flux and drag coefficient in hurricanes

The turbulent exchange of momentum between the ocean and atmosphere, known as wind stress, drives ocean currents and waves and acts to reduce near-surface wind speed. Wind stress is normally parameterized in terms of the product of the mean wind speed at a height of 10 meters, U10, and a drag coefficient, Cd. It is known that Cd increases with increasing U10 until about 25-30 m/s. However, there is considerable uncertainty regarding the behavior of Cd for U10 > 25 m/s, and there are conflicting results on the impact of surface waves on Cd. The large uncertainties are due in part to extremely limited concurrent and collocated direct measurements of wind stress, U10, and wave properties for U10 > 20 m/s and a lack of direct measurements for U10 > 30 m/s. Improved understanding of the air-sea momentum flux and the dependence of Cd on wind and ocean properties has the potential to improve forecast models and hurricane intensity prediction. During the past three Atlantic hurricane seasons, Saildrone uncrewed surface vehicles were steered into hurricane eyewalls to acquire continuous and direct measurements of the upper ocean and near-surface atmosphere, including U10, wind stress, and surface waves. Data were obtained from a complete transect through the eyewall of category-4 Hurricane Sam (2021), with measured wind gusts of up to 66 m/s, and from several other hurricanes. Results show an increase in Cd to a maximum of 0.0028 at U10 = 25 m/s, then a slight decrease and leveling off as U10 increases from 25 to 45 m/s. Interestingly, for U10 between 20 and 30 m/s, Cd is highly variable and is larger, on average, for greater misalignment between wind and dominant wave directions. Potential reasons for the large scatter of Cd in this wind range, and higher Cd for wind-wave misalignment, are discussed with the help of videos of the ocean's surface recorded during Hurricane Sam. Plausible explanations include sea state transitions within the 20-30 m/s wind range and delayed or accelerated transitions due to wind-wave alignment/misalignment.
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