In Situ Observations at the Air-Sea Interface by Expendable Air-Deployed Drifters Under Hurricane Michael (2018)

An array of surface drifters deployed ahead of Hurricane Michael measured the surface temperature, pressure, directional wind and wave spectra, and surface currents one day before it made landfall as a Category 5 Hurricane. The drifters, 25-50 km apart, spanned two counter-rotating ocean eddies as Hurricane Michael rapidly intensified. The drifters measured the shift of wave energy between frequency bands in each quadrant of the storm, the response of upper ocean currents, and the resulting cold wake following Michael's passage. Wave energy was greatest in the front quadrants and rapidly decreased in the left-rear quadrant, where wind and wave energy were misaligned, and components of the wave field were aligned with currents. Hurricane Michael's wave field agreed with previous studies of nondirectional wave spectra across multiple tropical cyclones but had some unique characteristics. The analysis demonstrates how co-located surface wind and wave observations can complement existing airborne and satellite observations. Lagrangian drifters were air-deployed ahead of Hurricane Michael and measured the direction and strength of waves and surface wind, sea surface temperature, and sea-level pressure as the storm transited through the central Gulf of Mexico. As Hurricane Michael passed over the drifters, the drifters observed the cyclonic structure of the wind, the shift of wave energy from swell to wind-sea, and the relative mismatch in direction of wind, waves, and ocean currents. In the rear quadrants of the storm, low-frequency waves opposed the wind direction. The drifters, caught in counter-rotating eddies, were ultimately entrained in different sides of the storm. The observations illustrate the importance of a suite of in situ surface observations to complement airborne observing strategies of tropical cyclones. Ten surface drifters measured temperature, pressure, currents, directional wave spectra, and wind under Hurricane Michael Wave energy was greatest in the front quadrants with misalignment of wave energy by frequency band in rear-quadrants of the storm Waves agreed with fetch-limited wave growth and there were significant differences in wind-wave-current alignment in each quadrant