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

Abstract

he 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 High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) and the new Global Hawk airborne platform that allows long endurance sampling of hurricanes. Supporting data from the High-Altitude Monolithic Microwave Integrated Circuit (MMIC) Sounding Radiometer (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, evolution, and role of small-scale deep convective forcing in the storm intensification process. Deep convective bursts are sporadically initiated in the downshear quadrants of the storm and rotate into the upshear quadrants for a period of ~12 h during the rapid intensification. The aircraft data analysis indicates that the bursts are being formed and maintained through a combination of two main processes: 1) convergence generated from counterrotating mesovortex circulations and the larger vortex-scale flow and 2) the turbulent (scales of ~25 km) transport of anomalously warm, buoyant air from the eye to the eyewall at low levels. The turbulent mixing across the eyewall interface and forced convective descent adjacent to the bursts assists in carving out the eye of Karl, which leads to an asymmetric enhancement of the warm core. The mesovortices play a key role in the evolution of the features described above. 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.