The Rapid Intensification of Hurricane Karl (2010): New Remote Sensing Observations of Convective Bursts from the Global Hawk Platform

The evolution of rapidly intensifying Hurricane Karl (2010) is examined from a suite of remote sensing observations during the NASA Genesis and Rapid Intensification Processes (GRIP) field experiment. The novelties of this study are in the analysis of data from the airborne Doppler radar HIWRAP and the new Global Hawk airborne platform that allows long endurance sampling of hurricanes. Supporting data from the HAMSR microwave sounder coincident with HIWRAP and coordinated flights with the NOAA WP-3D aircraft help to provide a comprehensive understanding of the storm. The focus of the analysis is on documenting and understanding the structure and role of small scale, deep convective forcing in the storm intensification process.

Deep convective bursts are sporadically initiated in the down-shear quadrants of the storm and rotate into the up-shear quadrants for a period of ~ 12 h during the rapid intensification. The aircraft data analysis indicates that the bursts are forming through the turbulent transport of warm, buoyant air from the eye to the eyewall at mid-to-low levels as well as convergence with inflowing air. The turbulent mixing between the eye/eyewall and forced convective descent adjacent to the bursts assists in carving out the eye of Karl leading to an asymmetric enhancement of the warm core. The aircraft data reveals that one mechanism for this turbulent mixing is the presence of mesovortices that form as a result of dynamic instability in the vortex.

The Global Hawk aircraft allowed an examination of the vortex response and axisymmetrization period in addition to the burst pulsing phase. A pronounced axisymmetric development of the vortex is observed following the pulsing phase that includes a sloped eyewall structure and formation of a clear, wide eye.