The Rapid Intensification of Hurricane Michael (2018): Storm Structure and the Relationship to Environmental and Air–Sea Interactions

• Published in: Monthly weather review 2021-01, Vol.149 (1), p.245-267
• Main Authors: Wadler, Joshua B., Zhang, Jun A., Rogers, Robert F., Jaimes, Benjamin, Shay, Lynn K.
• Format: Article
• Language: English
• Subjects: Air; Air-sea flux; Air-sea interaction; Amplification; Boundary layers; Cyclone structure; Cyclones; Distribution; Downdraft; Enthalpy; Entrainment; Entropy; Fluxes; Hurricanes; Life cycle; Life cycles; Oceanographic parameters; Outflow; Precipitation distribution; Quadrants; Recovery; Sea surface; Sea surface temperature; Shear; Storm structure; Storms; Surface temperature; Temporal variations; Tropical climate; Tropical cyclone structure; Tropical cyclones; Tropical oceanography; Updraft; Vertical shear
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/MWR-D-20-0145.1

The spatial and temporal variation in multiscale structures during the rapid intensification of Hurricane Michael (2018) are explored using a coupled atmospheric–oceanic dataset obtained from NOAA WP-3D and G-IV aircraft missions. During Michael’s early life cycle, the importance of ocean structure is studied to explore how the storm intensified despite experiencing moderate vertical shear. Michael maintained a fairly symmetric precipitation distribution and resisted lateral mixing of dry environmental air into the circulation upshear. The storm also interacted with an oceanic eddy field leading to cross-storm sea surface temperature (SST) gradients of ~2.5°C. This led to the highest enthalpy fluxes occurring left of shear, favoring the sustainment of updrafts into the upshear quadrants and a quick recovery from low-entropy downdraft air. Later in the life cycle, Michael interacted with more uniform and higher SSTs that were greater than 28°C, while vertical shear imposed asymmetries in Michael’s secondary circulation and distribution of entropy. Midlevel (~4–8 km) outflow downshear, a feature characteristic of hurricanes in shear, transported high-entropy air from the eyewall region outward. This outflow created a cap that reduced entrainment across the boundary layer top, protecting it from dry midtropospheric air out to large radii (i.e., >100 km), and allowing for rapid energy increases from air–sea enthalpy fluxes. Upshear, low-level (~0.5–2 km) outflow transported high-entropy air outward, which aided boundary layer recovery from low-entropy downdraft air. This study underscores the importance of simultaneously measuring atmospheric and oceanographic parameters to understand tropical cyclone structure during rapid intensification.