May 23 - July 29
Assessing the Coast’s Influence on Tropical Cyclone Miniature Supercell Mesocyclones
Samuel Hernandez (Saint Louis University), Dr. Addison Alford (OU/CIWRO & NOAA/NSSL, and Thea Sandmael (OU/CIWRO & NOAA/NSSL))
What is already known:
- Miniature supercells embedded in the outer rainbands of tropical cyclones (TCs) are much shallower and smaller in size than their mid-latitude counterparts and are quite difficult to forecast.
- TC miniature supercells are capable of producing tornadoes on land that can strongly impact local communities.
- TC tornadoes are known to form in the right front quadrant of TCs and are often observed <50 km inland in climatological studies.
- The environment <50 km inland is thought to lead to a local enhancement of vertical wind shear as the boundary layer adjusts to land, yet it is unclear how individual storms respond.
What this study adds:
- Mesocyclones within TC miniature supercells are most often identifiable within 20 km of the coastline, both inland and offshore.
- Of those observed, mesocyclones tended to have an increased likelihood of strengthening as a cell moves across the coastal boundary relative to all observed supercells.
- Mesocyclones tended to be the strongest within 20-30 km inland, suggesting the inland coastal region is more conducive for individual mesocyclone strengthening and development.
- The results of this study suggest that individual mesocyclones do indeed intensify as a result of inland changes to vertical wind shear.
Abstract:
Supercells in the outer bands of tropical cyclones (TCs) are often smaller than their midlatitude counterparts. They are often difficult to observe via weather radar due to their small size and rapid evolution. Yet, they are capable of producing damaging winds and tornadoes upon arrival onshore. The potential to develop tornadoes and the production of severe winds are associated with a strong mesocyclone present in an individual supercell. Mesocyclones have been observed to intensify rapidly as they cross the coastal boundary. It is hypothesized that the mesocyclone responds to an increase in vertical wind shear afforded by the onshore change in surface roughness. In order to assess the individual mesocyclone response to the coastal boundary, this study characterizes individual supercells as they move from the ocean onto land. Specifically, the changes in azimuthal shear are examined through a local, linear, least-squares derivative of single-Doppler velocity observations often termed “AzShear.” Relative to more spatially limited dual-Doppler analysis domains, this study affords a large area over which numerous supercells can be examined. Two cases will be examined, including Irene (2011) and Irma (2017), which were characterized by numerous observed supercells and confirmed tornadoes.